3. xi
Contents
Preface yu 3. PIa ticitv of Dendrites 58
4. Age-Related Changes
AbbreYiations . . . . . . . . . . . . . . . . . . . xiY in Dendrites 58
D The Axon 59
I. Historical Notes on Neurons
I The Axon Hillock and Axon
and Interneuronal Connections 1 Initial Scgmcnt . . . . . . . . . .. 59
2. The Axon Beyond the
II. Some Evolutionary Aspects
Initial Segment. . . . . . . . . .. 60
and General Features
3. Axonal Transport . . . . . . . .. 69
of Neurons . 6
F The Plasma Membrane 74
III. Shape and Size of Neurons .... 9
J5 VI. Intercellular Junctions
IV. Different Types of Neuron .....
Involving Neurons ........... 80
V. The Structure of Neurons. . . . .. 24 A Interneuronal Adherent
Junction ................. 80
A. The Perikaryon .. . ......... 24
1 The Nissl Substance 24 B InterneJJronal Chemical
2. The Agranular Reticulum . .. 29 Synap e ................. 80
3. The Golgi Apparatus ... . .. 29 1 General Features ......... 80
4 Mitochondria ........... :) 5 2. Number and Density ...... 80
5. Lysosomes and Pcroxisomcs 35 3 Stmctnre 81
6. Neuromelanin Pigment .... 37 4 CorrelatiollS Between
7 Mic[Qrubnles and NCII[Q- Stmct!J[e and Fllnction 86
filaments 37 5. Types of Synaptic Relations 89
8 Centrioles and Cilia 39 6. Reciprocal Synap es. . . . . .. 90
9. Cytoplasmic Inclusion 40 7. Synaptic Glomeruli ....... 90
B. The Nucleus. . . . . . . . . . . . . .. 43 C The Motor End Plate. . . . . . .. 93
1 General Features 43
D. Structural Aspect of Synaptic
2. The Nuclear Envelope ..... 43
Activity . . . . . . . . . . . . . . . . . . 97
3. The Karyoplasm ......... 46
4 The Nucleolus 46 E. Synaptic Plasticity and Age-
5 DNA Content 47 Related Changes in Synapses 102
6 Nucle,u Inclusions 47 1. Synaptic Plasticity ........ 102
2. Age-Related Changes
C Dendrites 49
in Synapses 103
1 General Eeatllrcs 49
2. Dendritic Spines. . . . . . . . .. 53
4. xii Contents
F. Relationship Between Axons 2c Strllcrure and Cbemical
of rhe Autonomic Nervolls Composition of the
System and Effector Cells . . . .. 103 Myelin Sheath ............... 136
2d. Schmidt-Lanterman
G. Electrotonic and Mixed
Junctions ................. 108
Incisures 132
2e. ode of Ranvier ...... 141
H. Synapse-Like Junctions 2£ Nodal AXQIl H8
Involving Neuroglial Cells .... 117 2g. Functional A pect
of the Myelin Sheath 150
2h. Mitotic Activity
VII. The Neuroglial Cells of the
ofScb~'iann Cells ••• I 151
Peripheral Nervous System. . .. 118
2i. PhagoC}~tic Activity
A Hisrorical Notes 118 ofSchwann Cells It" Z" I" 1.H
2j. Contractility of
B. Some Evolutionary Aspects 119
Scbwann Cdls 151
C. The Satellite Cells of Sensory
and Autonomic Ganglia. . . . .. 121
E Classificatioll and Roles of tbe
Neuroglial Cells of the
1. General Organization ..... 121
Peripheral Nervous System .... 151
2. Shape .................. 121
1. Control of the Traffic of
3 Srmcrure 121
M aterial to Neuron ............ 153
4. Relationships Between
Sarellite Cells 124
2 Contml of the I el1e1s of
Nellroac[i~e Amino Acids 154
5. Perikaryal Myelin Sheaths 124
3. Regulation of Ion Concen-
6. Boundari.e Between Neuron
tration in tbe Mi'men~i[Qn-
and Satellite Cell Sheath and
ment ofEacb ~elJ[on 154
Between Satellite Cell Sheath
4. Trophic Role Towards the
and ConnectiveTi ue . .. . . 125
7. Quantitative Relationship
NelJ[on I ••••••••••• lH
Between Nerve and Satellite
Cell ....... . .. . . .. . . . . 126
VIII. The Neuroglial Cells of the
8. Mitotic Activity . . . . . . . . .. 127
Central Nervous System . . ... . 156
9. Phagocytic Activity ....... 127
A. Hi rorica l Note . . . . ..... . . 156
D. Schwann Cells and the Myelin
Sheath. . .. . .. .. ... 128 B. Some Evolutionary A pects . . . 157
1. Unmyelinated Nerve Fibers 128
C. Ependymal Cell ........... 158
la. GeneralOrganization .. 128
1. Shape and Intercellular
1 b. Structure of Schwann
Relationships ......... . .. 158
Cell . ... .... . .. . . . . 130
2. Stru,ture ............... 161
1c. Relationships Ben....een
3. Tanycytes ............... 162
Adjacent Schwann Cells 131
4 Axons and Neurons
1d. Boundaries Between
Associated witb tbe
Axon and Schwann Cells
Ependyma . . . . . . . . . . . . .. 162
and Between Schwann
5. Supraependymal Cells ..... 163
Cells and Connective
6. The Subependymal Layer. .. 163
Ti sue . .. .. . ..... . " 132
7. Functions ............... 164
2. Myelinated Nerve Fiber ... 132
2a . GeneralOrganization .. 132 D. The Choroidal Epithelium . . . . 165
2b. Shape and Structure 1. Cell Shape and Intercellular
ofSchwann Cells .... . . 135 Relation hips .. .... .. ... . 165
5. Contents xiii
2. Structure ........ . . . . . .. 166 IX. Microglial Cells . . . . . . . . . . . . .. 190
3 Kolmer Cells 169
A. Hi rorical Notes .... .. .. . . . 190
4. Functions. . . . . . . . . . . . . .. 169
B. Re tingMicroglia . ......... 190
E. A [rocytes ................ 169
1. Fibrou Astrocyte . . . . .. . . 170 C. Neural Macrophages ........ 194
2. Protoplasmic Astrocytes .... 176
3. Regional Heterogeneity
X. The Cellular Organization
of Asrrocytes ............ 179
ofthe Nervous System . . . . . . .. 196
4. Funcrional Roles ......... 179
F. Oligodendrocyres .......... 181 XI. The Blood Vessels of the
1. Shape and Structure . . . . . . . 181 Central Nervous System ...... 200
2. Funcrional Role . ........ 184
A. Arteries . . . . . . . . . . .. .. .. 200
G Transirional Forms Between
B. Capillaries and the Blood-Brain
Astrocytes and Oligo-
Barrier. . . . . . . . . . . . . . . . .. 200
dendrocyte ............... 188
1. StructLJre . 200
H. Renewal of the Neuroglial Cell 2. The Blood-Brain Barrier 201
Popularion . . . . . . . . . . . . . . .. 189
C. Veins . . . . . . . . . . . . .. .. .. 204
References . . . . . . . . . . . . . . . . . . . . . 206
Index 153
6. xiv
Abbreviations
em, cemimeter nun, millimcter
C 5, central nervou sy tern !-lm, micrometer
D, dalton ms, millisecond
D A, deoxyribonucleic acid mY, mi llivolt
GABA, y-aminoburyric acid nm, nanOmeter
h, hour RNA, ribonucleic acid
kD, kilodalton econd
MAP, S, Svedberg unit of sedimcntarion
MAP microtll bllles-a ociated protein( ) coefficient
7. I. Historical Notes on Neurons and Interneuronal
Connections
Among the components of the nervou ti ue his pupil G.G. Valentin (1810-1883). Valen-
not vi ible to the naked eye, those fir t tin (1836) demon trated that ome Kttgeln
described were nerve fibers. A. Van bore a tail-like proce (schwanzformige Ver-
Leeuwenhoek (1632-1723) ob erved the e langerung) which 1110 t likely corresponded
fibcr in the peripheral nerves (1718) and in- to the proximal segment of a large dendrite.
terpreted them a ' "very minute vessel ", i. e. Remak (1837) and Purkinje (1838) also ob-
as hollow rube with a fluid content. A more served that the e processes proured from
correct de cription wa given by F. Fontana the nerve celi body but the term dendrite
(1730- 1805 ), who interpreted them as thin, was only introdu ed in 1889 by W. His
olid cylinders (1781 ). Important conrribu- (1831-1904).
rions were ubscquently made by R. Remak Wherea Valentin (1836) maintained that
(1815-1 65) and J.E. Purkinje * (1787 to nerve cell bodie and nerve fibers were con-
1869). Remak (1836 183 7) described the un- tiguou but cparate enritie, cell-fiber
myelinated nerve fiber -"hich today bear hi continuity 13 detected by A. Hannover
name and drew attention to rheir lack of a (1814-1894) in vertebrates (1840) and by
white outer layer, which is pre em in other H . L. F. H lmholtz (1821-1894) in inverte-
nerve fibers. In each nerve fiber of the brate (1842) and later received general ac-
peripheral nervous ysrem, Remak recog- ceptance principally due to Koelliker.
nized a flat ribbon, which he called rhe The clear di tinction between the axo n
Primitivband and which corre ponds to the and dendrites is mainly due to R. Wagner
transparent central axi described by Purkinje (1805 -1864), Remak, and O . F. e. Deiter
(1838) in myelinated nerve fibers. This axial (1834-1863). Wagner (1846) identified the
component of the nervc fiber was later (1839) axon and dendrites in the large nerve ceLIs of
given the Latin term cylinder axis by]. F. Ro- the electric lobes of torpedoes. A variable
enthal (1817-1887), a pupil of Purkinje.
Near the end of the century (1896) A. Koellik-
er u (1817 - 1905) coined the term axon • The real name of this great zech investigator wa
(Neuraxon) for this component. Purkync. Up to 1 50 he u ed the ve r ion Purkinje,
which corre ponded to the pronun iation of his ur-
The objects which were later designated name in German, at th at time the only language of
nerve cell bodies were detected in inverte- cientific di cour cued in ceneral Europe. Purkinje
brates by R.J.H.Dutrochet (1776 - 1847) returned to the Czech ver ion of hi na me when he
who de cribed them a ceflules globuleuses initiated a campaign for the development of cience in
(1824) and subsequently in both inverte- hi countrr and began to en ourage the use of the
brate and vertebrate (1833, 1836) by Czech language in order to make it easier for hi
fellow citizen ' to gain accc to cientific knowledge.
e.G.Ehrenberg (1795-1876), who termed
** In hi publication thi author u ed both form
them Kugeln (club- haped bodie ); however, Koelliker and K6l1iker. Since in hi letter he alwars
their preci e significance was not recognized signed him elf Kocllikcr, I have u ed this form in the
by these ob ervers, bur rarher by Purkinje and book.
8. 2 I. Historical Notes on Neurons and Interneuronal Connections
/ / I .)
~6 ~
-' t ' .
"'~'"
'~. /
, ~
''I;
"
,
/
)
j,
f
.
/
Fig. 1.1 Motoneuron isolated by Deiters from the axon (a) appears unbranched and shows a rather
ventral horn of the spinal cord, probably of ox. In the uniform thickness. Fine axonal processes, which ac-
cell body are evident an accumulation of pigment cording to Deiters originate from the dendrites, are
and the nucleus containing a prominent nucleolus. shown (b). Drawing by Deiters published in 1865,
Dendrites divide repeatedly and become progres- 2 years after his death.
sively thinner towards their ends, whereas the
number of proce es, often branched and con- gran ul ar, unbranched, and of un iform thick-
sisting of the arne granular material a the ness. By exammmg large mammalian
nerve cell body, arose from the latter, but a neurons, Remak (1855) arrived at the ame
single proce in each cell looked different conchl ion as Wagner (1 846). Finally, th e
from the others, being longer, paler, Ie work of Deiters (1865) elevated th is distinc-
9. I. Historical Notes on Neurons and Interneuronal Connections 3
:-/ I /:
IV
I
'~
J, •.
[ J.
. I :"
, I
/
r
: ,~
0
!
,....
--~~ ~
"'-"
....
(
.'"
~-..
-
.":"~ .----~--
/-
, Ii ..
Fig.1.2 Drawing by Goigi showing multipolar neurons impregnated using his black reaction. Ventral horn of
the spinal cord of a mammalian fetus,
tion between axon and dendrite to the tatu nervou y tem. The axon (Fig.I.1), which
of a general law. Dying at 29 year of age, may take origin directly from the nerve cell
Deiter left a manuscript-edited in 1865 by body or from one of the dendrites, appear
hi teach r M. Schultze (1825 - 1874)-in more homogeneou and mor refractive than
which he described the re ult of hi own the latter; it i alway unbranched ha a fairly
tudie unfortunately incomplete, on the uniform thickne ,i mooth- urfac d, and
nervou y tem of man and other mammal. pr ent an unmyelinated proximal egment
Ac ording to Deite the nerve cell po e beyond which it becomes enveloped by the
a body a ingle ( on and everal dendrite '. my lin heath. D it tS al 0 d crib d, and il-
The body con i ts of a ma of cyeopla In of lustrated in hi ' plates. fine axonal proce e
granulofibrillar app arance and a nucl us which, h claimed, sprout d from the den-
containing a prominent nucleolu . The den- drite and interconnected the nerve cell
drites (Fig. I.l), which Deiter sUed Proto- (fig.l.1 ). Almo t certainly, these fine axonal
plasmafortsatze ari e by gradual fran irion proce e were a rually preterminal gm nt
from the cell body and how the arne fin of afferent axons synap ing on the dendrite
granulofibrillar tructure a th ell b dy; ( an der Loo 1967).
they repeatedly divide and become progre - Thank to the work of the e pioneer and
sively thinner towards their end, e enrually to contribution from other inve tigator er-
di appearing into the ground ub tance of the tain fundamental morphological chara teri -
10. 4 I. Historical Notes on Neurons and Interneuronal Connections
technique, other obtained using the Golgt
black reaction and others still obtained from
experimentally induced lesion , led to the for-
mulation of a new conception of the organiza-
tion of nervou ti ue. Several author were
involved in establishing thi theory' among
these, principally His, A. H. Forel (1848 to
1931) and S.Ramon y Cajal (1852 - 1934),
who worked independently of each ocher. Ac-
cording to this theory, nervous ti ue doe not
consist of a yncytial network, but of mor-
phologically, embryologically and function-
ally di tinct unit , which clo ely contact each
ocher but are not in cytopla mic continuity
(Fig. 1.3). The e ba ic unit were termed
neuron (1891) by H. W. G. Waldeyer
(1836-1921). In substance, thi conception,
generally known a the neuron theory, ex-
tended the cell theory to nervou ti sue. The
mo t active propagandist of the neuron
Fig. 1.3 Large motoneuron of the ventral horn of the
cat spinal cord with presynaptic endings of afferent
theory wa certainly Ramon y Cajal.
axons applied to its surface. Drawing published by When it is recalled that the theory that all
Ram6n y Cajal (1934) in the paper which sum- plant and animal organi m are compo ed
marized the evidence that the nervous tissue con- of cell (cell theory) was enunciated in
sists of distinct units which are in contiguity, but not 1838-1839 by M.J.Schleiden (1804 - 1881)
continuity. with one another. and T. Schwann (1810- 1882) it i clear that
there wa a considerable delay before the
theory wa extended to nervou ti ue. Ther
were several rea on for thi . Fir t of all while
ti of the nerve cell came to be defined many ti ue are made up of cell having a
in term which are till valid today. The in- regular hape and micro copic ize, th el-
vention (1873) of Golgi' black reaction ment of nervous tis ue often have extremely
(reazione nera 19i 1 43 - 1926) which irregular hape and arc u ually much larg r
permitted the fir t clear vi ualization of a than typical cell of other ti ue. The indi-
nerve cell body with all its proce e in their vidual cell of many tissue are generally to-
entirety (Fig. 1.2) made it po ible to carry tally contained within one micro copic field,
out detailed and more complete observations wherea the nerve cell i rarely vi ualized in it
on nerve cell morphology and contributed entirety in a ingle hi tological ection, mainly
ub tantially to the advance of under tanding becau e of it axon length. Furthermore,
on the tructural organization of nervou ti- many author were of the opinion that the
u . For a long time nerve cell were thought nerve impul e could pread more ea ily
not to be independent unit: Their bodies through a continuou network than within a
were believed to be at the n d of a complex ti ue con i ting of a great number of di crete
yncytial network U. Gerlach I 20 - 1896) or entitie .
interconnected through fine fibril (neuro- The controver y between the 'neuroni t »
fibril ) running without interruption from [Ramon y Cajal Hi , Forel, K ellik r G. M.
one nerve cell to another (I. Apathy Retziu (1842 -1919), A. Van Gehuchten
1 63 - 1922). number of important find- (1861 - 1914) M.Lenhossek (1863 - 1937),
ing om obtained with the old hi tological E. Tanzi (1856- 1934), E. Lugar (1870 to
11. I. Historical Notes on Neurons and Interneuronal Connections 5
1940), and other ] and the "reticularist " pre- and postsynaptic neuron (Fig. VI.l ),
[Apathy A. Bethe (1 872 - l9~4 ) Golgi A.S. each of which i bounded by it own pia ma
Dogiel (1 852 - ]922), H. Held (1866-1942 ), membrane (see V1.B.3 ) ~' .
J. Boeke (1874-1956), P. Stohr Jr. After the neuron dlcory had received al-
(1891-1979), and others) lasted until the ma t general assent, inve tigator of the ner'-
1930. and was at time rather acrimoniou . ou ystem remained harply divided on the
The wealth of evidence which accumulated mechani 111 of ynaptic tran (ni sion. A livel y
over time etded the contrOver y in favor of debate on this topic tOok place during the
the neuron theor}' , Some of the main evidence 1930s and 1940 , Certain authors claimed
in thi context ,",va : (a) the finding of Hi on that tran mi ion wa due to a dirt:ct current
the deve!opment of the nervou ti ue from flow from the pre- to the po tsynaptic
individual cell (neurobla t ); (b) the phy io- neuron; other maintained that tran mis ion
logical ob ervation on the ba i of which 'va mediated by a chemical ubstance re-
C.S. Sherringron (1857-1952) in the seventh lea ed from the presynaptic neuron which ini-
edjtion of Foster' Text Book of Physiology tiated the current flow in the po t ynapti c
(1897) introduced the term ynap e to refer neuron. The advent of intracellular recording
to the region of close contact which i technique etded this controver )' in favor of
pecialized for the tran mis ion of signal chemical tran mi ion . However, ju t when it
from one nerve cell to the next; (c) the dem n- eemed that the electrical tran mis ion hy-
tration th at a nerve impul e caused th re- pothe is had to be buried forever, a refine-
lease of acetylcholine at the neuromu cular ment of intracellular recording technique led
junction (Dale et at. 1936)' (d) the studies by to the di covery of a junction type in inverte-
A. V. Waller (1816-1870) and Fore! on the brate which tran 'mitred electrically ( ee
con equence of ectioning and injuring the VT.G). Later, further exa mple of e1ectrica.1
axon; (e) the data which demon trated that tran mission were described, and accordingly
neurofibrils do not run without interrupti n it wa establi hed that both modes of tran -
from one nerve cell to anod)er and generally mi ion operate in the nervous ystem. In ver-
do not enter the pre ynaptic bouton ; (f) fi- tebrate , mo t interneurona l junction arc
nally, the direct electron micro copi evi- chemical, while ome junction functi n by
dence that there i a di continuity between the an electrical mechani m.
o Other brief hi torical note on individual a pem of
nerve and neuroglial cdls may be found ar rhe begin-
ning of the relevant e tion .
12. 6
II. Some Evolutionary Aspects and General Features
of Neurons
The ability to react to environmental stimuli predators. Hence, nerve cell differentiation
is a general property of all organisms, both was probably an essential precondition for
unicellular and multiceUular. In the latter, size increase in organisms.
specialization of cellular function is the rule, Signal tran mission between the cells in-
and the groups of cells able to react to stimuli volved in the reception of and reaction to
may be relatively distant from the point of environmental stimuli probably first occurred
stimulation. Under such conditions, the abili- by an electrotonic mechanism. In the nervous
ty to react to stimuli ha been considerably system of coelenterates, which is the simple t
enhanced by the development and refinement in the animal kingdom, many neurons are
of device for ignal propagation. electrically coupled. Although an electrotonic
The earliest signal propagation phenome- mechanism of signal transmis ion similar to
na probably aro e in epithelial ti sues (Hor- that found in epithelial tissue till operates in
ridge 1968), where the cells are in clo e con- many neuron ( ee V1.G), even in mammals
tact with each other and would have been comparative tudie on homologous nervous
facilitated by the development of specialized structures of different specie indicate an
intercellular junctions. Coordinated ciliary evolutionary trend towards a decrease in the
movement is one of the bener known con e- number and proportion of electrically cou-
quences of ignal translllis ion through a lay- pled neuron (Shapovalov 1980). It eem
er of epithelial cells. that very early in the course of evolution nerve
Neuron , i. e., cell pecialized for the re- cell developed the capacity to influence the
ception, conduction and transmission of activity of other cell by a chemical mecha-
signal, would have evolved from epithelial ni m, i. e., by the relea e from axon terminals
cells. While an individual epithelial cell can of physiologically active substances ynthe-
only conduct signals over a very short dis- ized by the nerve cells themselve .
tance, a single nerve cell thanks to its elon- Certain neurons which transmit signals via
gated processes can conduct signals rapidly a chemical mechanism synthesize consider-
between distant points. With their appear- able amount of the messenger substance,
ance, therefore, neurons brought a real adv- which are released from axon terminals into
ance in the process of ignal propagation. the extracellular pace of ti sues or into the
While small organi m may not need high general circulation. Via the blood stream,
peed signal propagation, this is an absolute these sub tanees may reach very distant
necessity in larger organisms, for example, for targets. The secretory activity predominates
activitie uch a prey capture or escape from in these nerve cell , whereas impulse conduc-
tion probably serves only to trigger the release
of the mes enger sub tance (Cros 1974) .
* In accordance with Lugaco' (1917) proposal, the
term "conduction " i here employed to indicate the Such cell are in certain respect similar to
intracellular propagation of ignal and the term endocrine gland cell and are known a
"transmi ion " to indicate the intercellular ((an fer- neurosecretory neurons. [0 other neurons, the
cnce of ignal . biosynthesis of messenger ubstances is great-
13. II . Some Evolutionary Aspects and General Features of Neurons 7
Iy diminished in correlation with the estab- electrotonic transmi sion. Compared with
lishment of precise anatomical connections the latter, synaptic transmission i mOre effec-
between neuron and their targets, i. e., with tive and selective and allows the transmjssion
the formation of synaptic junctions. Target of inhibitory influences in a simpler and more
cells are thus directly influenced by me enger efficient manner.
substances, only tiny amounts of which are oncerning the mechanisms by which high
required to exert their effect. Bioelectric activ- speed signal propagation ha been attained
ity has become the dominant feature of such in the cour e of evolution, ee VILB and
neurons. Even primitive invertebrates uch a vn.D.2g.
coelenterate are provided with elementary The appearance of synaptic junction led
ynaptic contacts in addition to electrically to the early establishment of the tVvo-neuron
coupled nerve cell and neurosecretory reflex arc with a con equent divi ion of labor
neurons (Scharrer 1976). between neurons, one of which became
While in more primitive invertebrates pecialized in the reception of stimulj, the
neurosecretory cells constitute a very large other in signal transmission to a non-neuron-
proportion of neurons (e.g., more than 50% al element. A sociation neuron with con-
of all neuron are neurosecretory in the gan- necting function, also called interneurons,
glia of annelids), in the course of evolution the were later inrerpo ed between the ensory and
proportion of neurosecretory cells ha de- effector nerve cell of the two-neuron arcs.
creased considerably. The re pon e evoked With their appearance, one of the main
by neurosecretory signal are not particularly characteri tics of nerve cells, namely their
rapid; moreover, they are diffuse responses tendency to form chains, became evident. The
since they usually involve many cells. On the interposition of an interneuron re ult in a
other hand, the e tablishment of preci e longer time period elapsing between a
anatomica l connections between neurons (or stimulus and the ensuing respon e but allows
between a neuron and a non-neuronal ele- the modification of impu lses before their
ment), i. e. the formation of synaptic junc- transmission to the effector neuron; in other
tions, allows the rap id and preci ely localized words, while two-neuron chain allow gener-
transmission of ignals. In the course of evolu- ally tereotyped responses, the presence of
tion, as the number of neuron increa ed and one or more interneuron enhance the ver-
ever more complex integrative center de- atility and flexibility of the system. Sensory,
veloped, this second mode of signal transmi - effector, and a sociation neurons are the three
sion, which is certainly more efficient than the fundamental classes aI" ay pre ent in meta-
neurosecretory mechanism, became more zoans beginning with the platyhelminths. In
common. However, neurosecretory neuron the cour e of evolution, the proportion of
have not disappeared completely; they still interneurons to sensory and effector neurons
operate even in higher vertebrate, though in gradually increases, so that this ratio may be
much reduced numbers. The development of used as an indicator of the evolutionary stage
an endocrine system proper al 0 contributed of the nervous system of a given specie.
to the reduction in neurosecretory cell num- The fundamental physiological properties
bers. Thi system is absent in the primitive of nerve cells are very similar in the simplest
invertebrates, makes its fir t appearance in and the most complex organi ms. AU neuron
arthropods, and is highly developed in verte- in fact (a) react to various physical and chemi-
brates. Certain functions primitively carried cal stimuli giving rise to ignals (excitability),
out by neuro ecretory neu.rons were later ta- (b) convey these ignal at high peed (con-
ken over by the endocrine ystem. The ap- ductivity), and (c) transmit them to other
pearance of synaptic transmission was a fun- neuron or to non -neuronal elements (e. g.,
damental evolutionary advance not only on muscular or glandular cell ) thereby influenc-
neurosecretory commun ication, but al 0 on ing their activity.
14. 8 II. Some Evolutionary Aspects and General Features of Neurons
Despite their considerable variet)1 of form receiving impulses, the axon hillock and the
and range of sizes (see Chap. IH), in the great initia l egment of the axon for generating the
majority of vertebrate neurons a body which action potential, and the axon for conducting
accommodates the nucleu and two kind of this to it termina ls. The terminal themsel e
proces e (dendrites and the a on ) can be di - are pecialized for impul e transmi sion. The
tinguished (Fig. 1.1 ). Various partS of the neuron, therefore, show a certain degree
neuron are functionaUy specialized at lea t to of functional polarity, in that information
a certain extent. For example, the cell body i traver e the cell in one direction only. Thi
the metabolic and ynrhetic center of the cell; polarity, which in fact depend on the a ym-
n10 t of the extramitochondrial protein re- metry of the ynap e ( ee VI.B.3 ), is particu-
quired by the neuron are ynthesized there larly evident in neurons with long axon , for
given that the dendrite have very little and example, the motoneuron of the pinal cord
the a on none of the apparatu nece sary for and brain stern.
their ynrhe i . Dendrite ar pecialized for
15. 9
III. Shape and Size of Neurons
Neuronal form and function are do ely re- cour e (Fig. 1.1), bur is richly branched near
lated since the hape of a neuron determine its terminal field. As a rule, axona l branche
its connections with other neuron and in- ari e from their parent tem at right or obtll e
fluence rhe way in which ir proce e synap- angle.
tic information. The e interrelation hip ac- While the morphological and physiologi-
count for the importance of the rudy of cal characteristics discernible with the avail-
neuronal morphology and of the factOr able technique allow u in most ca es to di -
which control it. tingui h clearly between dendrites and axon
While in a non-neuronal ri sue mo t cells in the vertebrate nervou y tern, thi may be
have a roughly similar shape, nerve cell ex- difficult in invertebrates. Dendritic and axon-
hibit a great variet), of form '. To implify, we a l tructure and function are often intermingl-
may say that mo t neuron po e a body and ed in the proces es of invertebrate neuron .
two type of processes, ca ll ed dendrites a nd For this reason , the term neurite has recently
axons (Fig. 1.1). The cell body, also called the come into use to refer to all neuronal proce -
soma, con ist of a nucleu and it urround- se . However, tht: term neurite ha been u ed
ing cytoplasm, which i termed the perika- for over a century with the ame meaning a
ryon. axon, <lnd thu the current trend to u e neurite
In vertebrates mo ·t neuron are provided a a genera l term for all neuronal processes
with several dendrite, but only one axOn . ma y cau e confu ion. The term neurite will
Dendrite, 0 named becau e their ramifica- not be employed in this book, firstly to avoid
tion recall that of the branches of a tree, were confu ion and al 0 because thi book deal
in the past often referred to as protOpla mic mainly with the tructure of the vertebrate
proce e. They are e entially direct exten- nervou y tem, where, a noted above, it i
sions of the perikaryon, have irregular con- generally po ible to di tinguish dendrites
tours, and become n '~rrower as they extend from axollS.
further from the perikaryon, giving off their Over the la t 20 year or 0, numerou ' ex-
branche at acute angle (Fig. 1.1 ). A di CllS - perimenral studies have begun to unravel the
ed later ('ee Chap. V) the tructure of the roles that variOlI factor ' e erci 'e in the con-
dendrite is different to that of the axon. The trol of neuronal shape. Some ba ic feature of
field of dendrite ramification i limited to th e the neuronal form are very likely determined
neighborhood of the cell body. The dendritic by intrin ic factor, a hown by the fact that
tree di play a characteri tic pattern in each nerve cell removed at an early developmental
type of neuron. tage and grown in eli ociated cell culture
The axon may ari e direcrl)1 from the often exhi.bit the main feature of correspond-
perikaryon or from the proximal portion of a ing neuron ' grown in 'ieu (Dichter 1978;
dendritic proce s by way of a mall conical Banker and Cowan 1979). However, the de-
elevation called the axon hillock. The axon rail of dendriric and a ona l form are prob-
has mooth contour, is relatively uniform in ab ly molded by extrin ic facror ( ee Rakic
diameter throughout it length, and u ually 1974 and Jacobson 1978 for review ), a mong
gives off very few collateral branche along its which ynaptic input. eem particularly im-
16. 10 II I. Shape and Size of Neurons
portant. The influence of both incrin ic and motoneuron (Fig. IV.2), pinal ganglion cells
extrin ic factor on dendritic morphology i (Fig.lV.9), Betz ell in the cerebral cortex).
illu trated by the following experiment . If Not only neuron of different type , but al 0
the cerebellar granule cell fail to develop be- neurons of the same type may vary greatly in
cause of a geneti abnormality or are de troy- ize within a ingle organism; for example,
ed during development (e. g., following X-ray the cell body of spinal ganglion p eudounipo-
irradiation), the Purkinje cell dendrites are lar neurons may range from 10 to 80!!m in
deprived of a large per entage of their normal diameter in a rat and from 15 to 120 lAm in a
afferent connections. Under uch condition , human ubject. These size differences become
th e dendrite do not develop fully and dis- mor evident considering the volume of the
play an immature branching pattern in the ceUbody rather than it diameter' it ha been
adult wh ile retajning their unmi takable calculated that in man the soma volume of a
hape (Altman and Ander on 1972; Rakic cerebellar granule cell may be 300 JAm J,
and Sidman 1973 b). The dendritic tree of wherea that of a Betz cell in the cerebral
hippocampal pyramidal cells deprived of cortex may reach 200 000 !!mJ (Haug 1982).
their normal afferent connection during In vertebrate, a considerable ize i
development i similar to that of control reached by the following nerve cell : Mauth-
neuron the only differen e being a reduction n r cell of the medulla of teJeo t , lungfi h,
in the number and ize of the terminal dendri- and amphibian, Muller cells of cydo tome,
tic egment. Mauthner cell deprived of vc - and ome neuro ecretory cell of the spinal
tibular afferent exhibit a reduced d ndritic cord of everal fi h. Th nerve cell of m
arborization in the region which normally re- invertebrate, such a tho e of the visceral
c ive ve tibular terminal.; uperinnervation ganglion of ea hare (Ap/ysia), have a body
of the e cell by v fibular a on i followed which may artain 1 mm in diameter.
by a localized nhancement of dendritic Observations on vertebrate neuron inner-
branching in the region receiving the extra vating rhe periphery have revealed a correla-
terminals (Goodman and Model 1988). The tion between the volume of the nerve cell
neuroglial nvironmcnt aloe m to exert body and the ize of it peripheral field of
ome influence on dendritic morphology a innervation (Levi 1906 1908; Hahn 1912;
hown by in vitro tudie. (Deni -Donini et al. Terni 1914; Donald on and Naga aka 1918;
1984; hamak et al. 1987; John on et al. Netto 1951). Enrique (1908) reached imiLar
1989). Other experiment which reveal that conelu ion concerning the neuron of the
cxtrin ic factor influ nce the pattern and ex- nervous ganglia of invertebrate. The follow-
tent of the d ndritic tree are reported in ing xperiment by T< rni (1920) confirm d
V. . 3. thi correlation. In the lizard rail, amputation
Unlike eU' of other ti u , who e volwne i followed by the regeneration of kin, mu -
varie little with respect to the mean, nerve cle ,and upporring tis ues. The pinal cord
cell differ greatly in ize. Even within a single and pinal ganglia do not regenerate, and the
organi m neuron may di playa very large reg n rated parr of the tail receives en ory
i7.e range. For example, in a ingle human innervation from the last three pairs of pinal
individual th ell bodie of th malle t ganglia left in iru cranial to the plane of
neuron are 5- Ilm in diameter {e.g. cere- amputation. Within these ga nglia, the bodie
bellar granule cell (Fig. IV.G), granul cell of of the nerve cells, who e peripheral fie ld is
the olfactory bulb (Fig. TV.lO), d arf (or thu greatly enlarged, ignificanrly increa e in
arach niform) cell of the human cerebral cor- sIze.
t ' bipolar cell f th retina, tellate cell of ince the ize of the peripheral field inner-
the ub tantia gelati no a of the pinal cord], vated by a given typ of neuron i u ually
wherea the cell bodie of the large t neuron greater in large animals than in mall one the
xceed 100 /A III in diameter (e. g., pinal cord bod), ize of homologou neuron innervating
17. III. Shape and Size of Neurons 11
the periphery is usually greater in the former. observations reveal a previously unsuspected
It i also true that certain types of nerve cell plastkity in the NS of adult mammal under
who e axon is confined to the central nervou normal condition. Other hormone al 0
system (eNS) and which do not therefore have eem to have an influence on the ize of
direct connection with the periphery [e. g., pecific nerve cells; for example, the neuron
pyramidal cells of the cerebral cortex of the mesencephalic nucleus of the trigeminal
(Fig. IV.4) and Purkinje cell of the cerebellar nerve in some amphibians increa e in ize dur-
COrtex (Fig. IV.S )] have larger bodies in large ing a critical developmental pha e, probably
animals than in mall ones. Here, too, the ize because of the higher concentration of thy-
difference are probably related to the varying roxine present at that time (Kollros 1977).
extents of the field innervated by the axon. It i to be noted that the volume of the
The correlation between the size of a nerve nerve cell body does not a.lway give an ade-
cell body and the extent of the field innervated quate idea of the true ize of the nerve cell
by it axon may explain why, in some species, ince, especially in large neuron , the volume
neuron enormou ly larger than most of the of the axon and it branche ' may con ider-
other are to be found. The e neuron are abl), exceed that of the c II bod)'. According to
tho e which in fact innervate a particularly e timates by Heidenhain (1911), the volume
large field (e.g., Mauthner cell, Rohon- of the peripheral axonal branch only (exclud-
Beard cells, neurons innervating the electric ing collateral and terminal ramification ) of a
organs of certain fish). large neuron from a human spinal ganglion
Not only the ize of the cell body, but also amount to 125 time the volume of the corre-
the length and complexity of dendrites in- sponding cell body. tn a giant Bet'l pyramidal
crea e a a function of the organism's ize cell th axon volume may b approximately
(Barasa 1960; Purve and Lichtman 1985; 350 times the volume of the corre ponding
Snider 1987). cell body (Haug 1982). A regards the mall-
However, the extent of the peripheral field e 't neuron , on the other hand the volum of
of innervation is probably not the only factor axon and cell body are more clo ely compar-
influencing neuronal ize. Among neuron in- able. For example, th ratio of axon vol.ume
volved in the control of specific aspects of to cell body volume i about 2:1 in a cerebel-
sexual behavior, for in tance some are larger lar granule cell.
in male than in female. Belonging to thi U ually, the axon caliber is po itivel)' cor-
category are certain neurons of the rat preoptic related with the nerve cell body volume (Ra-
area(Gor kieta1.1980},certainoftho eofrhe mon y Cajal 1909· Marine co 1909). Sa ed
nucleus robustus archistriatali ' of the canary on thi fact, it wa thought for many year
(DeVoogd and Nottebohm 1981), and par- that the axonal caliber wa controlled intrin-
ticular neuron of the toad pretrigeminal nu- ically by the neuron. Thi old view eem to
cleus (Schmidt 1982a). Concerning the latter be supported b)' recent morphological ob er-
nucleu , androgen treatment of adult female vations howing that the caliber of an axon i
lead to the enlargement of it neuron , which clo ely correlated with the number of neurofi-
rna)' then reach the characteristic size of the lament present within that axon. Thi indi-
corresponding neuron in the male (Schmidt cate that neurofilament gene expre ion i a
1982 b). The e ob ervations indicate that the primary determinant of the axonal caliber
ize of given neuron ma), be under the influ- (see La ek 1988 for a review).
ence of ex hormone . It i particularly inter- 'Other observation, however, have made
esting that in certain adult mammals the cycli- it evident that not only factor inrrin ic to the
cal variation of the levels of circulating sex neuron, but al 0 external factor play an im-
hormone bring about coincident changes in portant role in the control of the caliber of an
oma size and dendritic extent of particular axon. The whole que tion of the regulation of
neuron ' (Forger and Breedlove 1987). The e axon caliber now appear much more compli-
18. 12 III. Shape and Size of Neurons
cated than previously thought. When a rat tive estimation noted above, the axon ha a
neuron i di connected from its target cells, a llrface area more than 800 times greater than
reduction in axona l caliber takes place, sug- th at of the cell body. Concern ing dendrites
ge ring that the interactions between a neuron on ly, it hould be noted that in the cat the ratio
and its target cells are involved in the control of the urface area of the dendritic tree to that
of axonal caliber (Hoffman et al. 1988 ). In of the cell body i 3: 1 in the neuron of the
turn, the foUowing tudies ugge t that in- lumbosacral region of the pinal cord (A itken
teractions betv,reen an axo n and it upport- and Bridger 1961),5:1 in the giant cell ofthe
ing cells also playa role in thi regulation: reticu lar formation (Ma nnen 1966), and 25 : 1
(a) in a given motoneuron axon of the rat, the in the pyramidal neurons of the omatosen-
peripheral portion (myelinated by Schwann ory cortex (Mungai 1967) .
cell) in the ventral root is thicker than the Th e findings reveal that the perikaryon
central portion (myelinated by oligoden- generally make a mall contribution to the
drocytes) found in the spin al cord (Fraher rotal surface area of rhe neuron. However,
1978 ); (b) when in mice a segment of a Trem- devices which increase the perikaryal surface
bler nerve i grafted onto a normal nerve, area have been de cribed, particularly for
normal axons pas ing through the grafted neurons which do not po ess dendrites. In
egment lack myelin or are hypomyelinated vertebrates, the perikaryon of the pinal gang-
by Trembler Schwann cells and also have re- lion neuron i endowed w ith many slender
duced tran ver e diameters, but their caliber projection (Fig. ill. 1, V.S , VJI.3 ) who e
is restored to normal in the distal nerve tump length range between 0.3 and 3 ~m and
where they are normally myelinated (Aguayo who e tran ver e diameter averages 0.2 Ilm
et al. 1979); (c) in cultures of rat spinal gan- (Panne e er al. 1990a, b). The e projection
glion neuron, myelinated axon have a larger increase the urface area of the perikaryon by
caliber than bare axon (Windebank et al. about 40% in the cat and rabbit and by abou t
1985); (d) in normal lizard axon with both 30% in the gecko and lizard (Panne e et al.
myelinated and unmyelinated portions, the 1983 1985 ). Projections akin to these have
axona l caliber i ignificantly greater in been described in the neuron of the trigemi-
myelinated than in unmyelinated portion nal me encephalic nucleus; in the mouse and
(Panne e et al. 1988d); (c) in human demy- ham ter, they increa e the surface area of the
elinati ng neuropathies, a marked reduction in perikaryon by 12 - 14° (Hinrichsen and Lar-
/.)
axon cali ber occur locally in region of eg- ramendi 1970; Alley 1974) .
mental demyelinatio n (Prinea and · 1cLeod In preparation of ensory ganglia sta ined
1976). The mechani m by which extrin ic using silver impregnation techniques and ex-
factor affect axonal caliber are till un- amined under the li ght micro cope, in addi-
known . It ha been proposed that dley may tion to typical pseudounipolar neurons, nerve
influence the intrin ic mechanisms which pro- cell have been ob erved with horr and thick
duce neurofilaments (Lasek 1988 ). projections, or with long and fine expansions
Due to their highl y irregular hape, neurons ending in club-shaped enlargement, or, final-
u ually have very large urface area . The con- ly, with fene tration ituated in the uperfi-
siderable xtentofthe cell surface area is oneof cial region of the perikaryon (Fig. 111.2). More
the more significant neuronal features. It detailed descriptions of the e structures can
facilitate exchange berween the neuron and be found in Dis e (1893 ), Levi (1908 ), Held
it environment and all ow the estab li hment (1909a), and Ramon y Cajal (1909). They
of numerou connections between nerve cell . were more frequently detected in variou
The axon and dendrite are the major con- pathological stare or under particular experi-
triburor to the extended neuronal urface mental condition s (e. g., in grafted or injured
area. It may be recalled here that ill the neuron sensory ganglia) but, though Ie s commonly,
taken by Heidenha. n (1911 ) for the quantita-
i were al 0 observed under normal conditions.
19. III. Shape and Size of Neurons 13
Fig.1I1.1 Perikaryal projections. Scanning elec- sheaths. The surface is made irregular by the pres-
tron micrograph of the perikaryal surface of a sen- ence of numerous projections. Rabbit spinal gan-
sory neuron exposed after removal of all enveloping glion, x 4300.
Nageotte (1907 a) interpreted the e forma- tri uted a trophic role to the e arypi al pro-
tion a expres ion of the regenerative activi- ce e ; he ugge ted that by amplifying the
ry of the nerve cell. Since the e pro e e do urface area of the neuron the facilitated the
not usually contact orher neuron and, there- metabolic exchange between the neuron and
fore do not playa role in the onduction of it environment. R feren e to paraphyte are
nervou ignal, ageotte (1907 a) named er rarely encountered in th more rec or
them paraphytes to distinguish them from or- literature, probably becau e of the wide-
thophyte (axon and dendrite ) i. e. th pro- pread belief that the e proce largel ari e
ce e along which the conduction of nervous from the hrinkage of the nerve cell bod or
ignal take plac . Levi (1908), however, at- other technical anifa
20. 14 III. Shape and Size of Neurons
Fig. 111.2 Paraphytes. The large sensory neurons ness as well as fenestrations (f) situated in the sup-
shown in these light micrographs display numerous erficial regions of their perikarya. A = axon. Cajal
perikaryal projections of varying length and thick- method, horse spinal ganglion, x 400.
21. 15
IV. Different Types of Neu ron
De pite numerou attempt, the great variety that in which the dendrite of the arne cell
in number, length, and arborization pattern arborize (Fig .IV.2,4,5,6); on tht.: contrary
of proce ses and in the shape, size, and po i- the axon of a type n cell divides repeatedly in
rian of dle cell body has interfered with the the neighborhood of its own neuronal body,
efforts to produce a sati factory cia ification i. e., in the same circum 'cribed field in which
of nerve cells. Here, therefore, a list of the the dendrite ' of that cell ramify (Fig.IV.3 ).
most common types of neuron, distingui hed Example of type 1 cells are the moto-
by the characteristics of their proce se , i neurons of the spinal cord (Fig. rV.2) and
given. brain tem, the large pyramidal cell
Most nerve cells in vertebrate have everal (Fig. IV.4 ), the large cell of the Clarke'
dendrites but only one axon. These multipo- column, Purkinje cell (Fig. IV.S ), and the
lar neuron correspond well to the typical granule cell (Fig. IV.6) of the c.erebellar cor-
nerve cell so often referred to when describing te . All the e cells have axons which termi-
the nerve cell in a general way. nate in field di tinct from those containing
Golgi (1882) made the distinction between the corre ponding bodies and dendrite . The
two types of multipolar neurons on the basis cell body and dendrites of a motoneuron lie in
of the configuration of the axon . The axon of the pinal cord or brain stem, whi le the axon
Golgi type I cells sends out few collateral leaves the S and ends in the periphery. The
branches, retaining its di tinct identity, and cell body and dendrites of a large pyramidal
becomes myelinated, while that of Golgi ty- cell lie in the cerebral cortex, while the axon
pe IT cells, which usually remains unmyeli- leaves the cOrtex and terminates in other re-
nated, divides repeatedly shortly after leaving gions of the C S (c. g., the brain stem or the
the soma, thu giving rise to a complex ramifi- pinal cord). The cell body and dendrite of a
cation *. This simple classification is still valid large cell of the Clarke column lie in the spina l
today, particularly ince the differing mor- gray matter, while its axon terminate in the
phologies of the two types of nerve cell re- cerebellar cortex. The Purkinje ceU ha it
flect their different functional roles. Such a soma and dendrites in the cerebellar cortex,
correlation between shape and function be- while it axon ends in the central nuclei of the
comes clearer by complementing Golgi 's orig- cerebeIJum. A granule celJ of the cerebellar
ina l description with the following remarks. cortex ha its cell body and dendrites in the
The axon of a type I cell Illay vary in length, inner corrical layer, while it axon extends
butit always reaches a territory different from into the outer cortical layer.
Examples of type IT cells are the tellate
ceUin the cerebral cortex, those in the cere-
• It may be recalled here that Golgi believed th at the bellar cortex, and those in the reticular forma-
highly ramjfied axon of type II cell formed a on- tion of the brain stem. The axona l arboriza-
tinuous inter titia l nerwork (what he c.c Ued the rete
,
nerllosa diffllsa Fig. IV.l ) which he recognized in the
tion of each of the e neurons remains in the
gray marrero Goigi believed that the few collateral same field a it dendrites (Fig. rV.3 ).
branches of the axons of ty pe I cells al 0 contributed It is evident from these morphological dif-
to the formation of the arne network. ference that these twO types of neurons have
22. 16 IV. Different Types of Neuron
Fig. IV.1 Drawing by Golgi showing the rete nervosa diffusa impregnated using his black reaction. Fascia
dentata of a mammal.
di ·tinct functional role. The Golgi type I ceU Fig.IV.2 Multipolar neuron (Goigi type I cell). ~
perform mainly integrative and projection Drawing of a motoneuron of the spinal cord as seen
function : It collect information from a great in a Golgi preparation. The cell body and dendrites of
this neuron lie in the ventral horn of the spinal cord,
number of afferent · and, by mean of it while the axon (red) leaves the eNS and terminates
axon, convey its integrated re pon e to in the periphery.
more or Ie distant point. The Golgi type II
cell, whose axon and dendrites arborize in the
same circumscribed field, receives the ame Fig. IV.3 Multipolar neuron (Goigi type II cell). ~
Drawing of a neuron of the lentiform nucleus as seen
input a do the neuron on which its axon
in a Golgi preparation. The axon (red) divides re-
terminates, so that it ha a mainJy local mod- peatedly in the neighborhood of its own neuronal
ulatory role (Palay and Chan-Palay 1977). body, i.e., in the same circumscribed field in which
ot all neuron, however, are multipola r the dendrites of this neuron ramify.
24. 18 IV. Different Types of Neuron
Fig. IV.4 Multipolar neuron (Goigi type I cell).
Drawing of a large pyramidal cell of the cerebral
cortex as seen in a Golgi preparation. The cell body
and dendrites of this neuron lie in the cerebral cortex,
while the axon (red) leaves the cortex and terminates
in the brain stem or spinal cord.
cell . Some neuron lack dendrite and pos- Fig. IV.S), or they may be provided with a
e only axonal proce e, uch a the olfac- ingIe axon which divide into two branche
tory receptor neurons of vertebrate (Fig. (pseudo unipolar en ory neurons, Fig. rv.9).
IV.7), the receptor cell of the retina (rod and Bipolar en ory neuron are numerou io the
cone ), the primary en ory cell of many in- sensory ganglia of cyclo tome, elachian,
vertebrate (Fig. IV.7), who e bodies lie with- and teleosts as well as in the cochlear and
in the integument or directly beneath it, the vestibular ganglia of higher vertebrat while
neuron of the me encephalic nucleu of the pseudounipolar en ory neuron are found in
trigeminal nerve of mammal, and tho e of all the other en ory ganglia of higher verte-
the en ory ganglia. The last neuron may brat . The hape of a en ory ganglion cell
have a globular, oval, or spindle- haped body al 0 faithfully reflect it functional role. For
with two axon (bipolar ensory neurons, xample, a pinal ganglion neuron connect
25. IV. Different Types of Neuron 19
Fig.IV.S Multipolar neuron (Goigi type I cell). drites of this neuron lie in the cerebellar cortex, while
Drawing of a Purkinje cell of the cerebellar cortex as the axon (red) leaves the cortex and terminates in
seen in a Golgi preparation. The cell body and den- the central nuclei of the cerebellum.
the surface of the body or the muscles or the
joints with the spinal cord (Fig.IY.9). This
cell transduces stimuli of various types into
trains of nerve impulses and conveys them to
the eNS with retention of the topographic
order (Palay and Chan-Palay 1977). The
central process of this neuron bifurcates into
ascending and descending branches which
send collaterals into the dorsal gray column
(Fig. IV.9), thus distributing sensory informa-
tion in an orderly way to other neurons.
There are also neurons which lack an ax-
onal process, possessing only dendrites, such
as the amacrine cells of the retina and the
granule cells in the olfactory bulb (Fig. IV.tO).
Fig.IV.6 Multipolar neuron (Goigi type I cell).
Drawing of a granule cell of the cerebellar cortex as
seen in a Goigi preparation. The cell body and den-
drites of this neuron lie in the inner cortical layer,
while the axon (red) extends into the outer cortical
layer.
26. 20 IV. Different Types of Neuron
Flg. IV.7 Neurons which possess only axonal two primary sensory neurons of an invertebrate
processes. Left. Drawing of two olfactory receptor (gastropod) as seen in a Goigi preparation. Axons
neurons as seen in a Golgi preparation. Axons are in are in red. The nuclei of some integumental cells are
red. The nuclei of some supporting cells of the olfac- also shown.
tory epithelium are also shown. Right: Drawing of
Fig. IV.8 Neurons which possess only axonal ganglion, x 1750. Courtesy of S.lurato, S. Colucci,
processes. Bipolar sensory neuron, whose two ax- and A. Zambonln.
ons are marked with arrowheads. Human cochlear
27. IV. Diflerent Types of Neuron 21
t
Fig. IV.9 Neurons which possess only axonal The former terminates in the periphery, while the
processes. Drawing of a pseudounipolar sensory central branch enters the spinal cord where it bifur-
neuron of a spinal ganglion as seen in a Golgi prepa- cates into an ascending and a descending second-
ration. This neuron possesses a single axon (red) ary branch. Each of these sends collaterals into the
which divides into a peripheral and a central branch. dorsal gray column.
Anaxonic neuron arc present aloin inverte- ' c termed the neuropi l wh ile the axon leave
brat . the ga nglio n a nd ends in ide a muscle.
The motoneurons of the ventral gangli a Bipo lar and mul tipolar neuron are al 0
of certai n in vertebrate ' (e. g., po lychaete fo und in the peripheral pie use. of inverte-
leeches, and cru tacean ) pre ent si ngular brate . It i doubtful , however, whether these
characteri ·tic . They have a globu la r or c1 ub- cwo cell type in hi ch a clear di tinction
haped body which co ntain ' the nucleu and between a o n and dendrite cannot be made,
ends out on ly one process; the latter run for correspond to the bipolar and multipolar
a variab le di tance (up to several hundred neuron of vertebrate.
micro meters) and then expa nds in diameter Some of th e exa mples reported above illu -
and give rise to evera l d ndritic branche and trate how useful the morphology of a neuron
to a ingle axon (Fig. IV.11 ). The dendrites can be in the interpretation of it functional
ramify in the centra l core of the ga nglion, role, particularly when it connection are
participating in a den 'e me hwork of proces- known in detail. The fo llowing peculative
28. 22 IV. Different Types of Neuron
Fig.IV.10 Neurons lacking axonal processes. granule cell of the olfactory bulb as seen in a Golgi
Left Drawing of an amacrine cell of the retina as preparation. Note that many spines (gem mules) pro-
seen in a Golgi preparation. Right Drawing of a ject from the dendrites of this neuron.
con iderations are al a relevant to the eluci- (Fig.IV.11) and so are less affected by the
dation of the relationships between shape and chemical and electrical event a ociated with
function in neurons. As is known, the cell propagation. The longer and thinner the seg-
bodies of multipolar neurons lie directly in the ment linking the neuronal body to the point of
path of impulse propagation (Fig. IV.4). origin of the dendritic tree, the morc com-
Thus, on the a sumption that their genetic plctelyisolated from the rest of the neuron is
apparatus and that for macromolecular syn- that body together with its genetic apparatus
thesi (both located within the cell body, see and the one for macromolecular synthesi .
Chap. U) are directly exposed, more than For this rea on, it has been proposed that
those of other neurons, to the chemical and these neurons are particularly suited to the
electrical events associated with signal pro- control of stereotyped behavior (Cohen
ces ing, the hypothesis was advanced that 1970). The pseudounipolar neurons of verte-
their function i influenced by their past activ- brate (Fig. IV.9) have characteristics which
ity and hence that they are involved, more are in some respects similar to those discussed
than other nerve cell type, in learning proce - here.
ses (Cohen 1970). Conversely, the cell bodies An anempt to classify neurons on the basis
of the motoneurons occurring in the ventral of dendrite configuration was made by
ganglia of certain invertebrate are removed Ramon-Moliner (1968), who recognized
from the path of impulse propagation isodendritic, allodendritjc, and idiodendritic
29. IV. Different Types of Neuron 23
Fig. IV.11 Motoneuron from a ventral ganglion of a single axon (red) . The dendrites ramify in the cen-
an invertebrate (crustacean). Only one process tral core of the ganglion, while the axon leaves the
originates from the body of this neuron; the process ganglion and terminates inside a muscle.
in tum gives rise to several dendritic branches and to
neuron . The first kind possesses relatively mitral cells of the olfactory bulb, neurons
straight dendrite, radiating in aU directions with extremely wavy dendrite ' of the inferior
and bearing few spines (e. g. motoneurons of olivary nucleus, a nd neuron ' with highly
the spinal cord and brain stem, neurons of the tufted dendrite from the ventral cochlear nu-
vestibular nuclei, and large neuron of the cleus). T hi cia sification al 0 attempted to
reticular formation of the brain tern ). Al- interpret dendritic patterns in evolutionary
lodendritic neurons are characterized by terms: I odendritic neurons a re considered
wavy, rather short, highly branched den- phyLogenetically mOre primitive and idioden-
drites, which ramify in a fairly re tricted field dritic neurons more high ly evolved. This cla -
(e. g., pyramidal cells of tht:: cerebral cortex sification doe not appear atisfacrory, how-
many neurons in the sensory relay nuclei, and ever particularly becau e it groups highly di-
many thala mic neurons ). ldiodend riti c ver e neuron imo the same category, a i
neuron are rho e whose dendritic pattern i ev ident from the large variety of type in-
peculiar enough to allow immediate identifi- cluded in the idiodendritic cla s (Peter et al.
cation of the region to w hich they belong 1976).
(e. g. , Purkinje cells of the cerebellar cortex,
30. 24
V. The Structure of Neurons
A. The Perikaryon bodie een in tained ection a artifacr due
to the precipitation of ome ub tance nor-
mally di p r ed within th cytopla m ( ,
1. The Nissl Substance
among other, Held 1895). However i 51
Thi material, adier ob erved by Arndt bodie were al 0 detected in living neuron
(1 7 ) Key and Retziu (1876), and Flem- (Palay and Wi ig 1953· Hild 1954; Deitch
ming (1882), Va carefully described by Ni sl and Murray 1956) whereupon their precxi -
(18 9, 1 94)· , who e tabli hed the impor- tence in vivo could no longer be doubted.
tance of alcohol fixation for it vi ualization. U ing cytochemical technique it ha been
Under rhe lighr micro cope thi cytopla roic shown that ribonucleoprotein are among the
component OCCur in the form of bodi main components of Ni I bodi (Brachet
which are tained inten ely by mean of ba ic 1940; Gersh and Bodian 1943); this accounts
dye (e.g., merhylene blu , toluidine blu , for their trong ba ophilia.
thionin, and cre yl violer). The matcrial, al 0 In electron micrograph (Fig. V.2, 3) each
known a chromophili ub rance, chromi- Nis I body appears to con i t of (a) a number
dial ub tanc , rigroid bodie tc., exhibit of ci ternae who e membran are tlIdd d
di tincti,'e and con i rent characteri ti in with polysome (i. e., ci ternae of the granular
different types of neuron ; for example, it endopla mic redcululll) and (b) free poly-
appear a large, rhomboid rna e epa rated orne occurring in the cytopla mic matrix
by light channels in the motoneuron of the between the ci ternae (Palay and Palade
pinal rd (Fig. V.l ) and brain tem and a 1955). Mo t of the ribo om in ea h Ni L
mall granul in orne neuron of the en ory body are et into the free poly ome occurring
ganglia (Fig. Y.l ) and in the Mauthner c 11 of in the cytopla roic matrix betw en the ci ter-
t leo t.. Under th light micro cope it eems nae. Such poly orne u ua lly con i t of a few
to be ab em from certain mall neuron ,such (5 or 6) ribo orne) whereas membrane-at-
a ', for example, cerebellar corte granule tached poly om arc compo ed of a greater
cell, retina bipolar cell, and ub taoria number (up to 30). In the motoneuron of the
gciatino a mall neuron . pinal cord and brain tem, each Ni I body
Since fixation and po tmort m phenome- con i t of numerou ,broad and ometime
na may influence the appearance of the Ni I fene trated ci ternae of the granular endo-
ub ranee ome author regarded the Ni I pia mic reticulum which are ori nted paeall 1
to each other (Fig. V.2) thereby forming a
highly regular tack; ana5tomo e bet een
• [II reality i I' fir t ob ervation on the objc ci ternae are frequent. In addition to th
now known a i I bodie were reported in an es ay well-ordered Ni I bodie, i olated ci ternae
on the pathological ch:mge of nerve ell in the cereb-
ral cortex. For this es ay F. issl (1860- 1919) wa and mall group of randomly di tributed ci -
awarded 3 prize in a cientific competition el up in tern a can al 0 be found. In the pinal gan-
1884 by the Medical Fa ulty of Munich niver ity glion neuron, the ci ternae of the Ni I
(Kreutl.bcrg J9 4). TIli text remain unpubli hed. bodi often la k a pref rential ori ntation
31. A. The Perikaryon 25
Fig. V.l Nissl substance. In motoneurons (/ef~ A large, deeply stained nucleolus can be seen in the
Nissl bodies appear as large. rhomboid masses nucleus of both types of neuron. Light micrographs
within the perikaryon and as elongated bodies in the of sections stained with creysl violet. spinal oord
initial portions of dendrites. while in sensory neurons motoneurons and spinal ganglion neurons of man.
(righ~, most Nissl bodies appear as small granules. x 675.
(Fig. V.3), while in the mall and medium ize n rve ell mo t poly ome 0 cur fre in the
pyramidal neuron of the erebral cortex, rhe cyropla mic matrix and only a mall pr por-
ci rernae of rhe granular endopla mi re- tion are attached to the i ternae f the end -
ticulum u uaJly occur i ala red or in malJ pIa mic reticulum' and ( ) nl the quantiti
group. In the granule cell of rhe cerebellar of b th pol ome and i rerna and the or-
COrtex, in which the Ni I ub tance cannot be ganizational pattern of the latt r can differ
appreciared by light micro copy, only di crete from one type of neuron to another. The fail-
tubule of the granular endopla mic re- ure to detecr the i I ub ranee in certain
ti ulum and free poly orne an be found neuron under the light micr pe i du to
(Fig. VA). the in ufficienr amount of ba ic dye being
Electron micro cope rudie have clarified retained by th ir free and membrane-arra hed
why the Ni I ub ranee annat be een in rh poly m .
light micro cope preparation of certain It hould b noted rhar there are a number
neuron. They have e tabli hed that (a) th of importanr differ nc b tween the 'rru -
granular endopla mic reticulum and free rure of neuronal Ni I bodie and thar of the
poly ome , i. e., the omponent of the i sl erga topla m of the prorein- ynthe izing
ub ran e, are pr ent in all neuron . (b) in all gland ell . For example in rhe e ' crin pan-
32. 26 V. The Structure of Neurons
Fig. V.2 Nissl substance. In the Nissl body of a to the outer surface of the membranes limiting the
motoneuron shown here, the cisternae of the granu- cisternae, there are many polysomes which occur
lar endoplasmic reticulum are arranged in a fairly free in the cytoplasmic matrix between the cisternae.
ordered array. In addition to the polysomes attached Ventral horn of the rat spinal cord, x 50000.
crea tic cells the membranes limiting the erga - minology) a yet unintegrated into a neuronal
topla mic cisternae are uniformly tudded circuit and the nerve cell engaged in axonal
with attached poly orne, and only a few regeneration both contain free polysome al-
polysome occur free in the cytopla mic ma- mo t exclu ively (Pannese 1963 a, 1968,
trix, wherea in the i I bodie large areas of 1974). Since in both of these ituation the
the ci ternal membrane are devoid of nerve cell i mainly engaged in the synthe i of
ribo orne , and the mall poly o rne occur- new protopla m and it ynaptic acti vity i
ring free in the cytopla mic matrix between mi ing or deficient, the free polysome were
the ci ternae are very numerous. a umed to synthe ize mostly structural pro-
Re ult of tudie carried out in the course tein , i. e., protein for the maintenance and
of neuronal development and during axona l renewal of the protopla m in both the peri-
regeneration ugge t that the two classes of karyon and cell proce ses (Panne e 1963 a).
polysomes in th e nerve cell may ynthesize Thi hypothe i ha received upport from in
proteins playing different role. The imma- vitr tudie carried ou t in cell-free y tern
ture neuron (the neurobla t of cla ica l ter- containing either free polysomes or mel11-
33. A. The Perikaryon 27
Fig. V.3 Nissl substance. In the Nissl body of a trix between cisternae. Microtubules (some indi-
sensory neuron shown here. the cisternae of the cated by arrowheads) and neurofilaments (some ar-
granular endoplasmic reticulum lack a preferential rowed) can be seen in the cytoplasm surrounding the
orientation. As in the Nissl body shown in Fig. V.2. Nissl body. Rabbit spinal ganglion. x 53000.
many polysomes occur free in the cytoplasmic ma-
brane-attached polysome . By thi technique, been localized by mean of cytochemical tech-
it was shown that neurofilament protein are niques within the ci ternae of the granular
synrhesized on the free polysome population endoplasmic reticulum. It is relevant to note
and that there is no detectable synrhe is of here that the rate of protein synthesis is
uch proteins on the membrane-attached thought to be very high in neurons and it
poly ames (Strocchi et al. 1982). These latte r has been estimated that a large perikaryon
might be engaged a usual in the synthesis of may renew one-third of its protein every day
enzymes, since some enzymes (e.g., acetyl- (Peters et al. 1976).
cholinesterase and acid pho phatase) have
35. A. The Perikaryon 29
2. The Agranular Reticulum and are in conrinuity with more deeply situ-
ated element of both the granular and
Within the perikaryon thi organelle con ists
agran ular reticulum, may be sites where ions
of irregularly branching and anasromo ing
are taken up from the extracellular space
tubules and cisternae devoid of polysomes
and channeled direcdy into the endoplasmic
(Fig. V.S ), which are continuous at many sites
reticulum.
with elements of the granular endoplasmic
reticulum. In the reticulum of the neuronal
perikaryon, therefore, orne region have at-
3. The Goigi Apparatus
tached polysomes, whereas others lack poly-
omes. The agranular tubules and cisternae The internal reticular apparatus was original-
are apparently dispersed at random. ly described by Golgi (1898 a, b) in nerve cells
Flattened cisternae lying clo e to and par- (Fig. V.6 ). When examined under the light
allel with the plasma membrane (Fig. VII.2) micro cope after silver impregnation, in thick
are present in many neurons (Ro enbluth sections it appear' a a continuous, loose-
1962b; Kaiserman-Abramof and Palay meshed network of irregular, rortuou rib-
1969). No organelle is ever found interpo ed bon distributed through most of the perika-
between these cisternae and the plasma mem- ryon, except near the cell urface. Numerol!
brane (Fig. Vli.2), they being only between 10 authors questioned the reality of the ap-
and 60 nm apart. The e cisternae, which have paratus de cribed by olgi, and the ensuing
been named subsurface cisternae by Ro enb- debate gave ri e to long-I a ting and at time
luth (1962 b) and hypolcmmal ci ternae by acrimoniou controver ies regarding its exi t-
Kaiserman-Abramof and Palay (1969), often ence. It is now universally recognized that th e
appear in continuity with the element of the Golgi apparatus i not the re ult of technical
granu lar or agranular reticulum lying more artifacts, but a genuine organelle present in
deeply in the neuron al perikaryon. Sometime nearly all eukaryotic cells.
polysomes are attached to the deep, but never Xfhen a nerve cell i examined in emithin
to the external, aspect of the subsurface cister- sections by light microscopy (Fig. V.7) or in
na. In most neurons, ubsurface cisternae are thin sections under the electron micro cope,
neither very numerous nor very large, but in only a few segments of the continuOlls net-
Purkinje cell they are so highly developed, work detectable in thick ection by light mi-
not only in the perikaryon, but also in the cro copy can be een. The e egment appear
dendrites (Fig. V.14 ) and axon, that they can a discrete entitie . However, when the same
be used to identify i olated profiles of the e type of neuron i examjned in thick ection
neuron in thin section (Patay and Chan- under the high voltage electron micro cope,
Palay 1974). such segment appear to be connected to each
Very litde information is at present avail - other (Rambourg et al. 1973 ). Likewise, th e
able on the role of the agranular reticulum in tructure resulting from the recon truction of
the perikaryon. As regards its possible role a a erie of thin ection i a continuous net-
a Ca 2 + reservoir, see V.D.2. A current view is work.
that the subsurface cisternae, which arc in In electron micrograph of thin ections,
clo c relationship with the plasma membrane each segment of the Golgi apparatu appears
• Fig. V.4 Granule cells of the cerebellum. The nu- ated by light microscopy, consists of a small number
cleus, which occupies most of the cell body, displays of discrete tubules of the granular endoplasmic re-
dense clumps of chromatin, some of which lie adja- ticulum (some arrowed) and free polysomes. Golgi
cent to the nuclear envelope. Note that the Nissl complexes (G) and mitochondria can also be seen in
substance, which in these cells cannot be appreci- the cytoplasmic rim. Rat cerebellar cortex. x 20000.
36. 30 V. The Structure of Neurons
Fig. V.S Perikaryal portion of a spinal ganglion jections which arise from the neuronal perikaryon at
neuron. Nissl bodies (N) , profiles of the agranular other levels; they thus appear as discrete entities
endoplasmic reticulum (some of which are indicated completely embedded in the satellite cell sheath
with an arrowhead) , mitochondria, and dense bodies (Sc) . Arrows point to pinocytotic vesicles along the
can be seen in the neuronal perikaryon. Microtubu- neuronal plasma membrane. ct = connective tissue.
les and neurofilaments are interposed between the Rabbit spinal ganglion, x 28000.
other organelles. * Indicates cross sections of pro-
to con i t of a tack of five to even fl attened, distinguishing feature of the Golgi complex i
smooth- urfaced cisternae and of a cluster of its lack of either free or attached ribosomes.
associated vesicles (Fig. V.7). This whole The cisternae are thin in th eir central portion
structu re i usually called a Golgi complex. A and slightly expanded at thei r periphery and
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