2. INTRODUCTION
• The human CNS contains about 1011(100 billion)
neurons
• Also contains 10–50 times this number of glial cells
• About 40% of the human genes participate, at least
to a degree, in formation of CNS
• The neurons are the basic building blocks of the
nervous system
• Neurons perform the specialized function of
integration & transmission of nerve impulse
3. • Neurons and glial cells along with brain capillaries
form a functional unit that is required for normal
brain function, including synaptic activity, ECL fluid
homeostasis, energy metabolism, and neural
protection
• Disturbances in the interaction of these elements
are the pathophysiological basis for many
neurological disorders (eg, cerebral ischemia,
seizures, neurodegenerative diseases, and cerebral
edema)
4. GLIAL CELLS
• Previously, glial cells (or glia) were viewed as CNS
connective/ supporting tissue; also called neuroglia
or glia (glia = glue)
• Today theses cells are recognized for their role in
communication within the CNS in partnership
with neurons
• non-excitable and do not transmit nerve impulse
(action potential)
• Unlike neurons, glial cells continue to undergo
cell division in adulthood and their ability to
proliferate is particularly noticeable after brain
injury (eg, stroke)
5. • Most commonly, neuroglial cells constitute the
site of tumors in nervous system
• Two major types of glial cells in the nervous system;
microglia and macroglia
• Microglia; smallest neuroglial cells; derived from
macrophages outside of the nervous system; often
called the macrophages of CNS
• Scavenger cells that remove debris resulting from
injury, infection, and disease (eg, multiple sclerosis,
AIDS-related dementia, Parkinson disease, and
Alzheimer disease)
6. • Three types of macroglia: oligodendrocytes,
Schwann cells, and astrocytes
• Oligodendrocytes and Schwann cells are involved in
myelin formation around axons in the CNS and
peripheral nervous system, respectively
• Astrocytes are star-shaped neuroglial cells present
in all the parts of the brain; Two types of astrocytes
are found in human brain:
• i. Fibrous astrocytes- occupy mainly the white
matter
ii. Protoplasmic astrocytes- found in gray matter
and have a granular cytoplasm
7.
8. • Both send processes to blood vessels of brain,
particularly, the capillaries, forming tight junction
with capillary membrane; Tight junction in turn
forms the blood-brain barrier
• Also send processes that envelop synapses and the
surface of nerve cells
• Protoplasmic astrocytes have a membrane potential
that varies with the external K+ concentration but
do not generate propagated potentials
• Produce substances that are tropic to neurons, and
help maintain the appropriate conc. of ions and
neurotransmitters by taking up K+ and the
neurotransmitters glutamate and γ-aminobutyrate
9.
10.
11. Neurones
• Neurons occur in a variety of sizes and shapes
• Most of them contain four parts: (1) a cell body, (2)
dendrites, (3) an axon, and (4) axon terminals
• The cell body(soma) contains the nucleus & ribosome
and is the metabolic center of the neuron
• The dendrites form a series of highly branched
outgrowths from the cell body; receive most of the
inputs from other neurons
• The branching dendrites (some neurons may have as
many as 400,000!) increase the cell’s receptive surface
area and thereby increase its capacity to receive signals
from a myriad of other neurons
12.
13. • Particularly in the cerebral and cerebellar cortex,
the dendrites have small knobby projections called
dendritic spines
• The axon, sometimes also called a nerve fiber, is
a single long process that extends from the cell
body to its target cells (from μm to m in length)
• The part of the cell body where the axon is joined is
known as the initial segment, or axon hillock
• The initial segment is the “trigger zone” where, in
most neurons, the electric signals are generated &
are then propagated away from the cell body along
the axon or, sometimes, back along the dendrites
14.
15. • The main axon may have branches, called
collaterals, along its course
• Near the ends both the main axon and its collaterals
undergo further branching; The greater the degree
of branching of the axon and axon collaterals, the
greater the cell’s sphere of influence
• Each branch ends in an axon presynaptic terminal;
composed of a number of synaptic knobs which are
also called terminal buttons or boutons
• They contain granules or vesicles in which the
synaptic transmitters secreted by the nerves are
stored
16. • The axons of many neurons are myelinated, that is,
they acquire a sheath of myelin, a protein–lipid
complex that is wrapped around the axon
• In the PNS, myelin forms when a Schwann cell wraps its
membrane around an axon up to 100 times
• The myelin is compacted when the extracellular
portions of a membrane protein called protein zero (P0)
lock to the extracellular portions of P0 in the apposing
membrane
• Various mutations in the gene for P0 cause peripheral
neuropathies; 29 different mutations have been
described that cause symptoms ranging from mild to
severe
17. • Myelin sheath is not a continuous sheath; absent at
regular intervals (1-μm constrictions that are about 1
mm apart); the area where myelin sheath is absent is
called node of Ranvier
• Segment of the nerve fiber between two nodes
is called internode
• Myelin sheath is responsible for white color of nerve
fibers
• Unmyelinated neurones are simply surrounded by
Schwann cells without the wrapping of the Schwann
cell membrane that produces myelin around the axon
• In the CNS, most neurons are myelinated; the myelin-
forming cells are the oligodendroglia
18. • Here oligodendrocytes emit multiple processes that
form myelin on many neighboring axons
• In multiple sclerosis, a crippling autoimmune
disease, patchy destruction of myelin occurs in the
CNS
• The loss of myelin is associated with delayed or
blocked conduction in the demyelinated axons
• Neurons are classified by three different methods.
A. Depending upon the number of poles
B. Depending upon the function
C. Depending upon the length of axon
19. • Depending Upon The Number Of Poles:
• Based on the number of poles from which the nerve
fibers arise, neurons are divided into three types:
• 1. Unipolar Neurons: Neurons that have only one
pole; from a single pole, both axon and dendrite
arise; this type of nerve cells is present only in
embryonic stage in human beings
• 2. Bipolar Neurons: Neurons with two poles are
known as bipolar neurons; Axon arises from one
pole and dendrites arise from the other pole
• 3. Multipolar Neurons: Neurons which have many
poles; one of the poles gives rise to axon and all
other poles give rise to dendrites
20.
21.
22. • Depending Upon The Function
• On the basis of function, nerve cells are classified into
two types:
1. Motor or efferent neurons
2. Sensory or afferent neurons
• 1. Motor or Efferent Neurons: Neurons which carry
the motor impulses from CNS to peripheral effector
organs like muscles, glands, blood vessels, etc.
Generally, each motor neuron has a long axon and
short dendrites
• 2. Sensory or Afferent Neurons: Neurons which
carry the sensory impulses from periphery to CNS;
generally, each sensory neuron has a short axon and
long dendrites
23. • Depending Upon The Length Of Axon
• Depending upon the length of axon, neurons are
divided into two types:
1. Golgi type I neurons
2. Golgi type II neurons
• 1. Golgi Type I Neurons: They have long axons; cell
body of these neurons is in different parts of CNS
and their axons reach the remote peripheral organs
• 2. Golgi Type II Neurons: Such neurons have short
axons; present in cerebral cortex and spinal cord
24. AXONAL TRANSPORT
• Neurons-secretory cell; differ from other secretory
cells in that the secretory zone is generally at the
end of the axon, far away from the cell body
• Protein synthesis occurs in cell body & transported
to the axonal ending by axoplasmic flow
• The functional and anatomic integrity of the axon is
very important; if the axon is cut, the part distal to
the cut degenerates (wallerian degeneration)
• Orthograde transport occurs along microtubules
that run along the length of the axon and requires
two molecular motors, dynein and kinesin
25.
26. • Orthograde transport moves from the cell body toward
the axon terminals
• Has both fast and slow components; fast axonal
transport occurs at about 400 mm/day, and slow
axonal transport occurs at 0.5 to 10 mm/day
• Retrograde transport, which is in the opposite
direction (from the nerve ending to the cell body),
occurs along microtubules at about 200 mm/day
• Synaptic vesicles recycle in the membrane, but some
used vesicles are carried back to the cell body and
deposited in lysosomes
• Some materials taken up at the ending by endocytosis,
including nerve growth factor (NGF) and some viruses,
are also transported back to the cell body
