1. PHysiology of cell and
By Tsegay. H.
cell signalling
By Tsegay. H.
1

2. What is cell?
 Cells are the smallest structural and functional unit of all living
things,
 Our body is an aggregate of 100 trillions of different cells all
working together for the maintenance of the entire organism.
 A typical cell has two parts: nucleus and cytoplasm.
 The cytoplasm is separated from the surrounding fluid (ECF) by
the plasma membrane.
 The nucleus is separated from the cytoplasm by a nuclear
membrane
2

3. Structural levels of organization of human body
Muscle cells
Nerve cells
Cells: 4 types Epithelial cells
Cells in the connective tissues
Muscle tissue
Tissues 4 types Nerve tissue
Epithelial tissue
connective tissues
Organs: Example: Heart, lungs that are made up of 4 types of
tissues
Organ system: Example: Respiratory system, CVS
Organism: Human organism
3

4.  Cells of different tissues and organs vary in structures
and functions;
However, there are certain characteristics
common to all cells:
Growth and development,
Irritability,
Movement,
Reproduction,
Excretion
4

5. Parts of the Cell
 Typical cell has two parts: nucleus and cytoplasm.
 The nucleus is separated from the cytoplasm by a nuclear
membrane and the cytoplasm is separated from the
surrounding fluid (ECF) by the plasma membrane.
Cytoplasm
 The gel-like material within the cell membrane
 It is a fluid matrix composed of water, salts, organic
molecules and many enzymes that catalyze reactions, along
with dissolved substances such as proteins and nutrients.
5

6. Functions of cytoplasm
 Helping to maintain the shape and consistency of the
cell
 Providing suspension to organelles
The very specialized structures suspended in the
cytoplasm are organelles
 As site of storage for chemical substances
6

7. 7

8. The nucleus
 The nucleus is the control center for the cells.
 It contains the genes, which are hereditary units
 Chemically each gene consists of highly compressed DNA in the
form of chromosomes
 Genes control cellular activity by determining the type of
proteins, enzymes, and other substances that are made by the
cell.
 The nucleus is also the site of RNA synthesis
 There are three kinds of RNA:
 Messenger RNA (mRNA), which carries the instruction from
DNA for protein synthesis to the cytoplasm
 Ribosomal RNA (rRNA), which moves to the cytoplasm
where it becomes the site of protein synthesis
 Transfer RNA (tRNA), serves as an amino acid transporter
system within the cell for protein synthesis.
8

9.  DNA and RNA are made up of nucleotides
 Nucleotides are composed of nitrogen containing bases
purine (A, G) and pyrimidin (C, T) as well as deoxyribose
sugar conjugated by phosphate.
 In RNA, the pyrimidin base T is replaced by U and the 5-
carbon sugar is ribose.
 In addition to the chromatin, the nucleus contains one or two
round bodies called nucleoli.
 It is here that rRNA is synthesized.
 The nuclear contents are surrounded by a double walled
nuclear membrane.
 The pores present in this membrane allow fluids, electrolytes,
RNA, and other materials to move between the nuclear and
cytoplasmic comportments.
9

10. Functions of nucleotides
2.Building units of nucleic acid (DNA, RNA)
3.High energy molecules (ATP, GTP)
4.Biosynthetic mediators (UDP-glycogen)
5.Regulator of chemical reaction in the cell e.g. cAMP
6.Act as coenzyme (NAD, FAD)
10

11. Cellular organelles
 Are ‘specialized structures suspended in the cytoplasm’
 These include the ribosome, endoplasmic reticulum (ER)
Golgi apparatus, mitochondria, lysosomes, and the
cytoskeletal system (microtabules and microfilaments).
 Ribosomes
 Are the sites of protein synthesis in the cell
 Small particles composed of rRNA and proteins
 Found in two forms: attached to the wall of ER or as free
ribosomes.
 Free ribosomes are found in two forms
Scattered in the cytoplasm and
 Clustered (aggregated) to form functional units called
polyribosomes
11

12.  Endoplasmic reticulum (ER)
 It is an extensive membranous structure that connects
various parts of the inner cell.
ER is also connected with the nuclear membrane.
 There are two types of ER: rough ER and smooth ER.
 The rER is studded with ribosomes.
The function of rER is to segregate proteins that are
being exported from the cell.
o rER =protein synthesis.
12

13. Endoplasmic reticulum (ER) cont……d
 The sER is free of ribosome.
 Function of sER varies in
different cells.
 The sarcoplasmic reticulum of
skeletal and cardiac muscle cells
are forms of sER.
 Calcium ions needed for
muscle contraction are stored
and released from the
sarcoplasmic reticulum of
muscle cells.
 In the liver, the sER is involved
in glycogen storage and drug Endoplasmic reticulum (rER and sER)
metabolism.
o sER can synthesize a group
of drug metabolizing
enzymes called microsomal
system. 13

14.  Golgi Complex= delivery center of
the cells
 The Golgi complex consists of
flattened membranous saccules and
cisterns that communicate with the
ER and acts as a receptacle for
hormones and others substances that
the ER produces.
 It then modifies and packages these
substances into secretary granules.
14

15.  Mitochondria
 The mitochondria are literally the “power plants” of the cell,
capable of producing the energy rich compound ATP, which
is required for various cellular activities.
 The mitochondria require oxygen to produce energy (ATP)
from food stuffs.
15

16. Mitochondria
16

17.  Lysosomes
 Lysosomes are sac-like compartments that contain a
number of powerful digestive enzymes.
 They break down harmful cell products and waste
materials, cellular debris, and foreign invaders such as
bacteria, and then force them out of the cell.
 Degrade old dead cells and phagocytosis of microrganisms
17

18.  Cytoskeletal system of the cell
 Composed of microfilament and microtubules
 Rigid threadlike structures dispersed through out the
cytoplasm
 Function of cytoskeletal system:
• Maintain shape of the cells.
• Serve as a transport system for the movement of
compounds and organelles within the cell.
• Provide for the support and movement of cilia and
flagella
• Cell to cell contact: to fasten cell membranes together
18

19. The plasma membrane
 It is a sheet-like structure that surround (enclose) the cell,
separating the cellular contents from the ECF.
 It is entirely composed of proteins and lipids in a ratio of
55:43 respectively, and 3% of carbohydrates.
 Percent proportion
 Proteins: 55 %
Phospholipids 25 %
 Lipids: 42 % Cholesterol 13 %
Neutral fats 4%
 Carbohydrate: 3%
 The level of cholesterol determines rigidity of the membrane
19

20. Function of the plasma membrane
 Separates cellular contents from the ECF
 Regulates the passage of substances in and out.
 It is semi-permeable allowing some subs to pass through it
excluding others.
o This creates unequal distribution of ions on both sides
of the membrane.
 It provides receptors for NTs, hormones and drugs.
 It is a means of cell to cell contact.
 Plays an important role in the generation and transmission of
electrical impulse in nerves muscle.
 Involved in the regulation of cell growth and proliferation.
20

21. Structure of the cell membrane
 A plasma membrane is a fluid in its nature,
 The cell membrane consists of an organized arrangement of
proteins, lipids and CHOs
1. Lipids: The major lipids are phospholipids such as phosphatidyl
choline and phosphatidyl-ethanolamine, and cholesterol.
 Lipids form the basic structure of the membrane.
 The lipid molecules are arranged in two parallel raws, forming a
lipid bilayer.
21

22. Structure of the cell membrane cont….d
 The phospholipids component is organized into a double layer
with their hydrophobic (tail) directed towards the center of
the membrane and polar hydrophilic heads directed out ward
facing ECF and ICF.
 The lipid bilayer portion of the cell membrane is impermeable
to water and water soluble substances such as ions, glucose,
urea and others.
 On the other hand, fat soluble substances such as O2, CO2,
alcohol and drugs can penetrate this portion of the membrane.
22

23. Structure of the cell membrane cont….d
2. Proteins: are two types
• Integral or intrinsic proteins:
 Interdingitated in the hydrophobic center of the lipid
bilayer
 Transmembrane proteins are integral proteins that
span the entire bilayer.
 Transmembrane proteins serve as:
 Channels through which ions pass
 Carriers which actively transports material.
across the bilayer e.g. glucose
 Pumps which actively transport ions
 Receptors for neurotransmitters and hormones
 Integral proteins that are present only on one side of the
membrane, serve primarily as enzymes.
23

24. B. Peripheral or extrinsic proteins:
 Bind to the hydrophilic polar heads of the lipid or on
integral proteins.
 Peripheral proteins that bind to the intracellular surface
contribute to the cytoskeleton.
 Peripheral proteins that bind to the external surface
contribute to the glyco-colgx, a cell coat that is
composed of glycol-lipids and glycol-proteins to
cover the cell membrane
24

25. 3. Carbohydrates
 Attached invariably on the outside surface of the
membrane, binding with protruded integral proteins and
lipid, they form glyco-proteins and glyco-lipid (glycocalyx)
respectively.
 They play a role in
– Immune reaction (antigenical importance),
– Cell to cell attachment and
– Act as receptors for NTs, hormones and drugs
25

26. Transport through the cell membrane
 Substances are
transported through
the cell membrane by:
• Simple diffusion
• Osmosis
ECF
• Facilitated diffusion
• Active transport (1O
and 2O) and
• Vesicular transport ICF
mechanisms.
26

27. 1. Simple Diffusion
 Diffusion is passive movement of substances down their
concentration gradient.
 Factors affecting the net rate of diffusion
 Lipid solubility of the substances
 Membrane permeability
 Concentration difference or Pressure difference
 Electrical potential difference of ions
 Membrane permeability is affected by
 Membrane Thickness
 Lipid solubility
 No of ion channels per unit area
 Temperature: T =  thermal motion of molecule
permeability
 MW
27

28. Simple Diffusion cont….d
 Rate of diffusion is determined by the following factors
summarized in the formula shown below.
S. A. T. C
 Rate of diffusion = D. MW
Where, C = Change of concentration
S = Solubility in lipid
A = Surface area of the membrane
T = Temperature
D = Distance or membrane thickness
MW = Molecular wt of substances
 Examples: Substances that are transported by simple diffusion
are CO2, O2, alcohol, lipid soluble drugs and ions through
specific channels.
28

29. 2. Osmosis
 It is the power of movement of H2O
from an area of higher amount of
water to an area of lower amount of
water through the semi permeable
membrane.
 The direction of movement of water is
governed by the amount of osmoticaly
active particles (solutes).
 The pressure that opposes osmosis of
water is called osmotic pressure
 H2O molecules have very small (0.3
nm) in diameter, so that they can not
traverse the lipid bilayer simply.
Instead they pass through specific
water channels called aquaporins:
 Five aquapurins (AQ1….AQ5) have
been identified in the body.
29

30. 3. Facilitated diffusion
 Carrier mediated transport
 Carriers are saturable, do
not need energy Glucose
 Transports substances
down their concentration
gradient ECF
Cell membrane
 Examples: transport of
ICF
glucose, proteins.
Carrier protein
(Macromolecules)
30

31. 4. Active transport
 Substances are transported
against concentration,
Common examples
electrochemical gradient, up
2. Na+ - K+ ATPase
hill direction.
3. H+ - K+ ATPase
 Used for the transport of Na+, 4. Ca2+ ATPase
K+, Ca2+, Fe2+, H+, Cl-
 Consumes energy in the form
of ATP
Primary active transport
 Carrier protein are involved
 Consumes energy from
ATP
 Carrier protein is anti-
porter
31

32. Secondary active transport
 Carrier protein are involved
 Energy is not from ATP, but
from other ions
 Carrier protein is symporter
Uniport carriers: Carry single substance to one direction
Antiport carriers: Carry two substances in opposite directions
Symport carriers: Carry two substances into the same direction
32

33. 5. Visicular transport
 Vesicles or other bodies in the cytoplasm move macromolecules
or large particles across the plasma membrane.
 Types of vesicular transport include:
 Exocytosis: vesicles fuse with the plasma membrane, then
releasing their contents to the outside of the cell.
33

34.  Endocytosis: capturing of a substance outside the cell, then
the plasma membrane merges to engulf it.
 The substance subsequently enters the cytoplasm enclosed
in a vesicle.
 There are two kinds of endocytosis:
 Phagocytosis or cellular eating occurs when the dissolved
materials enter the cell.
 Pinocytosis or cellular drinking occurs when the plasma
membrane folds inward to form a channel allowing
dissolved substances to enter the cell.
34

35. Intercellular communication
 The intercellular signaling is subdivided into the following
classifications:
 Neurotransmitters are released by axon terminals of neurons
into the synaptic junctions and act locally to control nerve cell
functions.
 Endocrine hormones are released by glands or specialized
cells into the circulating blood and influence the function of
cells at another location in the body.
 Neuroendocrine hormones are secreted by neurons into the
circulating blood and influence the function of cells at another
location in the body.
35

36.  Paracrines are secreted by cells into the ECF and affect
neighboring cells of a different type.
 Autocrines are secreted by cells into the extracellular fluid
and affect the function of the same cells that produced them
by binding to cell surface receptors.
 Cytokines are peptides secreted by cells into the extracellular
fluid and can function as autocrines, paracrines, or endocrine
hormones.
 Examples interleukins that are secreted by helper T cells
act on other cells of the immune system
Cytokine hormones (e.g., leptin) produced by
adipocytes are sometimes called adipokines.
36

37. 37

38. Cell junctions
 The points of contact between two adjacent plasma
membranes, cell junctions.
 There are 3- types of cell junction:
Tight junctions
Anchoring junctions
Communicating junctions
38

39.  Tight junctions
Cells are attached tightly with belt like
structure so prevent fluid to pass between
Are common among epithelial cells of the
stomach, intestine, and urinary bladder.
They prevent the fluid in a cavity from leaking into
the body by passing between cells.
39

40.  Anchoring junctions
 Are common in tissues subjected to traction and stretching such
as the outer layer of the skin, cardiac muscle, and uterus.
 Anchoring junctions include desmosomes, hemidesmosomes
and adherence junction.
 Adherence junctions connect to microfilaments of the
cytoskeleton and link cells to one another or anchor cells to
extracellular materials.
 Desmosomes form a firm attachment between cells some what
like spotwelds.
40

41.  Communicating junctions (Gap junctions)
 Are narrow channels that directly connect the cytoplasm of
two neighboring cells, consisting of proteins called
connexons.
 These proteins allow only the passage of ions and small
molecules.
o Gap junctions allow communication between cells
through the exchange of materials or the transmission of
electrical impulses.
41

42. Cell junctions
42

43. What tissue mean?
Cells of similar function, come together and form
higher level of body’s organization, tissue.
Cooperative unit of very similar cells that
perform a specific function.
There are four main types of tissues:
o Nervous tissue
o Muscle tissue
o Connective tissue
o Epithelial tissue
43

44. 1.Epithelial Tissue
Cells are tightly fitted together in continuous
layers,
Tight packaging allows tissue to act as a barrier
to protect against mechanical injury, infection,
and fluid loss,
Cover outside of body (skin) and line organs and
internal body cavities (mucous membranes of
digestive, respiratory, and reproductive systems).
44

45. Epithelial Tissue Covers and Lines the Body and its Parts
A. Simple squamous
(Lung air sacs)
D. Statified squamous
(Lining esophagus)
B. Simple cuboidal
(Kidney tubes)
C. Statified columnar
(Lining intestine)
45

46. Epithelial tissues, mucous membranes, absorb
and secrete chemical solutions.
Mucous membranes:
– Digestive tract epithelium (mucous
membranes) secretes mucus and digestive
enzymes.
– Respiratory tract epithelium secretes
mucous that helps trap dust particles
before they reach the lungs.
46

47. 2. Connective Tissue
 Connective tissues are fibrous tissues
 Are made up of cells separated by non-living material,
which is called extracellular matrix.
 Functions:
As the name implies, connective tissue serves a
"connecting" function.
It supports and binds other tissues.
Gives shape to organs and holds them in place.
47

48. Types of connective tissue
A. Loose connective tissue
 Most widespread connective tissue
 Loose matrix with fibers, packing material
 Function: Attaches skin to muscles, binds
and holds tissues and organs in place
B. Adipose (fat) tissue
 Protection
 Insulation
 Energy storage
48

49. Types of connective tissue cont…..d
C. Blood
The only fluid connective tissue
Composed of plasma having water, plasma
proteins, and other substances, and Cellular
elements [RBCs, WBCs and platelets]
Function:
Transporting function
Regulatory function [water, temperature,
pH…..]
Protective function [stoppage of bleeding,
engulfing invaders, development of body
immunity] 49

50. Types of connective tissue cont…..d
D. Fibrous Connective Tissue
Matrix of closely packed collagen fibers,
Strong and non-elastic,
Found in:
Tendons: Attach muscles to bones
Ligaments: Attach bone to bone
50

51. Types of connective tissue cont…..d
E. Cartilage
Hard matrix with collagen fibers
 Found on end of bones, nose, ears, and
between vertebra.
F. Bone
Supports the body
Solid matrix of collagen fibers and calcium,
phosphate, and magnesium salts.
Bone is harder than cartilage, but not brittle
because of collagen.
51

52. Connective Tissue Binds and Provides Support
A. Loose Connective Tissue D. Fibrous Connective Tissue
B. Adipose Tissue E. Cartilage
C. Blood F. Bone 52

53. 3. Muscle Tissue
Muscle, the most abundant type of tissue, is the fleshy
organ of the body that converts potential energy of food
into mechanical energy [produce force and movement] to
perform different activities.
Made up of long cells [sacromers = contractile unit of
muscle] that contract when stimulated by nerve impulses.
Muscle cells have many microfilaments made up of actin
and myosin.
Muscle contraction accounts for much of energy
consuming work.
53

54.  There are three types of muscle tissue
Skeletal (striated) muscle
 Located attached to bones by tendons & moves
skeleton
 They are elongated, cylindrical, multinucleated
cells, and striated
 Innervated by somatic nerve system
 Responsible for voluntary movements.
B. Cardiac muscle
 Striated, uni or bi nucleated muscles found in
walls of the heart forming contractile tissue of
heart.
 Controlled by autonomic nerve system, drugs and 54

55. C. Smooth muscle
 Found in walls of visceral organs [digestive tract, bladder,
arteries, uterus, and…….]
 Non-striated and mono-nucleated cells
 Responsible movement of different activities with in the body.
[food, dust particles, child during delivery, urine, sperm,
ovum……]
 Contract more slowly than skeletal muscle, but can remain
contracted longer.
 Controlled by autonomic nerve system, drugs and hormones [are
involuntary muscles].
 Both cardiac and smooth muscles have authorythamic property,
[generate action potential even with out nerve supply]
55

56. Types of Muscle
B. Cardiac muscle
A. Skeletal muscle
C. Smooth muscle
56

57. 4. Nervous Tissue
 Senses stimuli[receptors], carry sensory information
[sensory =afferent pathway] to CNS where it is
processed and appropriate information is formulated
[association neurons], then transmit the formulated
response[motor =efferent pathway] to the glands and
muscles[ effector organs]
Controls the activity of muscles and glands, and
allows the us to respond to its environment.
57

58. Types of nervous tissue
Nervous tissue made up of two types of cells:
Neuron
Structural and functional unit of nervous tissue.
Consists of:
Cell body: Contains cell’s nucleus.
Dendrite: Extension that conveys signals
towards the cell body.
Axon: Extension that transmits signals away
from the cell body.
Supporting cells(neuroglial cells)
 Nourish, protect, and insulate neurons.
58

59. Nervous Tissue Forms a Communication Network

60. Excitable Tissues?
 The term excitability refers to an ability of a tissues
to receive stimuli and respond to that stimuli.
Excitable tissues respond to various stimuli by
rapidly changing their resting membrane
potentials and generating electrochemical
impulses (action potential).
 The stimuli can be electrical, chemical, mechanical or
thermal.
 There are two types of excitable tissues:
Nerve
Muscle
60

61. 1. Nervous System
Organization of the Nervous System
61

62. Nervous System
2. Central Nervous System (CNS)
 Spinal cord and brain
 Function: integrate, process and coordinate sensory input
and motor output
3. Peripheral Nervous System (PNS)
 All neural tissue outside CNS
 Function: carry information to and from CNS
 Composed of:
Cranial nerves (12 pairs) to and from brain
Spinal nerves (31 pairs) to and from spinal cord
 Nerve = bundle of axons (nerve fibers) with blood vessels
and connective tissues.
62

63. Divisions of PNS
1. Sensory/Afferent Division
 Carry sensory information to CNS
 Grouped in to:
III.Somatic afferent division
 Carry sensory information of voluntary activities like
of skin, skeletal muscles, joints
IV.Visceral afferent division
o Carry sensory information of involuntary activities
like glands and smooth muscles of internal organs
63

64. 2. Motor/Efferent Division
o Carry information from CNS to effectors
o Grouped to:
B. Somatic Nervous System
 Voluntary nervous system
 Carry to skeletal muscles
B. Autonomic Nervous System (ANS)
 Involuntary nervous system
 Carry to smooth & cardiac muscle, glands
 Could be:
 Sympathetic Division: “fight or flight”
Parasympathetic Division: “rest and digest”
 They antagonize each other
64

65. Nerve tissue
• Two principal cell types that make up the nerve system are
neurons and neuroglial cells
 Neurons are ‘functional units of the nerve system’
o Neurons are specialized for the generation and transmission
of nerve impulse.
Sensory function
Generation of thought
Storage of memory
Integration of idea
Coordination of muscular activities
 Neuroglial cells:
o The neuroglial cells are non excitable cells found in
association with neurons.
o They provide supporting functions to the neuron.
65

66.  Neuroglial cells outnumber neurons
by about 20x
 5 types of supporting cells, 4 are
found in the CNS:
2. Astrocytes
 Star-shaped, abundant, and
versatile cells
 Guide the migration of
developing neurons
 Involved in the formation of
the blood brain barrier
 Function in nutrient
transfer
 Interconnect blood vessels
and nerve fibers.
66

67. Neuroglial cells cont….d
2. Microglial cells
 Specialized immune cells that
act as the macrophages of the
CNS
3. Ependymal Cells
 Line the ventricles of the
brain and the central canal of
the spinal cord
 Some are ciliated which
facilitates the movement of
cerebrospinal fluid
67

68. 4. Oligo-dendrocytes
 Produce the myelin
sheath which provides
the electrical insulation
for neurons in the CNS
5. Schwann cells
 Form myelin sheaths
around the larger nerve
fibers in the PNS.
 Vital for neuronal
regeneration
68

69. Neurons: Functional structures
 Neurons are functional units of the
nervous system.
 Specialized to conduct information from
one part of the body to another
 A typical neuron has three distinct parts.
These are: Dendrites, Cell body &
Axon.
 Dendrites: Collect information and
send it to cell body.
 Cell body: Connect dendrites and
axon, main bio-synthetic and
metabolic center
 Axon: Take information away of cell
body to another cell
69

70. Neurons: morphological
classification
 Structurally neurons are classified
into three classes:
• Multipolar neurons(neurons having
more than 2 processes) found in the
CNS, motor in function
• Bipolar neurons (neurons with 2
processes): found in the retina and
inner ear
• Unipolar neurons (neurons having only
one process): sensory in function
70

71. Functional Classification of Neurons
 Functionally, there are thee classes of neurons:
– Sensory(afferent) neurons: conduct impulses from
periphery to the central nerve system
– Inter-neurons (association or integrative) neuron:
o Receive and interpret sensory information
o Conduct formulated response to effectors via motor
area.
– Motor (efferent) neurons: conduct impulses from
central nerve system (brain & spinal cord) to the
periphery
71

72. Myelin?
 Is electrically insulating material that forms a layer,
the myelin sheath, usually around only the axon of
a neuron.
 Myelin is an outgrowth of glial cell.
 The production of the myelin sheath is called
myelination
 It is essential for the proper functioning of the nervous
system.
 In humans, the production of myelin begins in the 14th
week of fetal development, although little myelin
exists in the brain at the time of birth.
72

73. Myelin cont….d
• During infancy, myelination occurs quickly and
continues through the adolescent stage of life.
• Schwann cells supply the myelin for peripheral
neurons, whereas oligodendrocytes, myelinate the
axons of the central nervous system.
Composition of myelin
• Myelin may be made by different cell types, varies
in chemical composition and configuration, but
performs the same insulating function.
73

74. Composition of myelin cont…..d
 Myelin is about 40 % water;
 The remaining dry mass of myelin is about 70 - 85 % lipids and
about 15 - 30 % proteins.
 Some of the proteins that make up myelin are
 Myelin basic protein (MBP),
 Myelin oligodendrocyte glycoprotein (MOG), and
 Proteolipid protein (PLP).
 The primary lipid of myelin is a glycolipid called
galactocerebroside (GalC).
 The intertwining hydrocarbon chains of sphingomyelin serve to
strengthen the myelin sheath.
74

75. Myelination in the CNS Myelination in the PNS
75

76. Myelin cont….d
The wrapping is never complete
Interspersed along the axon are gaps where
there is no myelin – these are nodes of Ranvier.
Function of myelin sheath
Protects axon
Facilitate rate of action potential conduction
Saves ATP
76

77. The soma
 Is where the signals from the dendrites are joined and passed on
 The soma and the nucleus and organelles do not play an active
role in the transmission of the neural signals.
 Instead, serve to maintain the cell and keep the neurons
functional.
 The supporting structures of the cell include mitochondria,
which provide energy for the cell, and the Golgi apparatus,
which packages products created by the cell and secretes them
outside the cell wall.
77

78. Soma cont….d
 Contains nucleus and normal organelles
 Biosynthetic center of the neuron
 Contains a very active and developed rough endoplasmic
reticulum which is responsible for the synthesis of NTs.
 The neuronal rough ER is referred to as the Nissl body.
 Contains many bundles of protein filaments (neurofibrils)
maintain the shape, structure, and integrity of the cell.
78

79. Soma cont….d
 Clusters of soma in the CNS are known as nuclei
 Clusters of soma in the PNS are known as ganglia
79

80. Nerve fiber
 Is a threadlike extension of a nerve cell
 A nerve fiber may be myelinated and/or
unmyelinated.
 Found both in the central and peripheral nervous
system.
 Consists of an axon and myelin sheath (if present) in
the nervous system.
80

81. Nerve fiber cont…….d
 In the central nervous system (CNS), myelin is produced
by oligodendroglia cells.
 Schwann cells form myelin sheath in the peripheral
nervous system (PNS).
Schwann cells can also make a thin covering for an
axon which does not consist of myelin (in the
PNS)= endoneurium
A peripheral nerve fiber consists of an axon, myelin
sheath, Schwann cells and its endoneurium.
There are no endoneurium and Schwann cells in the
central nervous system.
81

82. Classification of nerve fibers
1. Central nerve fibers
 In CNS, nerve fibers differ in terms of size, conduction
velocity, and presence or absence of myelin.
 For example, the olfactory nerve fibers are short and
unmyelinated, but the optic nerve fibers are myelinated
 A bundle of nerve fibers in CNS constitutes = tract
 The pyramidal tract and extra pyramidal tracts have long
nerve fibers that descend from the brain to the spinal cord.
 These fibers have an important role in motor control, and are
known as descending tracts.
82

83.  There are other bundles of nerve fibers in the CNS are called
ascending tracts.
 These carry sensory information from the periphery to
the different areas of the brain.
Peripheral nerve fiber types
 A nerve may be sensory, motor or sensory-motor (mixed).
 There are three types of nerve fibers in a mixed nerve:
 Sensory nerve fibers (afferent fibers)
 Motor nerve fibers (efferent fibers)
Autonomic nerve fibers
Somatic nerve fibers
83

84. Components of peripheral nerve fiber
 Each peripheral nerve fiber contains:
An axon
Axolemma
Myelin sheath (if present)
Schwann's sheath (neurolemma)
Endoneurium, [endoneurial channel, sheath or tube], is
a layer of weak connective tissue made up of
endoneurial cells that encloses the myelin sheath of
peripheral nerve fiber.
84

85. Classification of peripheral nerve fibers
 There are three types of peripheral nerve fibers based on
their diameter and myelination:
 A group
 B group
 C group
A group
 Have a large diameter and are myelinated fibers, high
conduction velocity, and.
 The A group consists of four types of nerve fibers:
A alpha fibers (afferent or efferent fibers)
A beta fibers (afferent or efferent fibers)
A gamma fibers (efferent fibers)
A delta fibers (afferent fibers)
85

86. Motor fibers of the A group
A alpha fibers
 High conduction velocity.
 Alpha motor neurons innervate extrafusal muscle fibers
A beta fibers
 Beta motor neurons innervate intrafusal muscle fibers of
muscle spindles (nuclear bag and nuclear chain fibers),
with collaterals to extrafusal muscle fibers.
A gamma fibers
 Gamma motor neurons innervate intrafusal muscle fibers
of muscle spindles (nuclear bag and nuclear chain fibers).
86

87. Sensory fibers of the A group
A alpha fibers (Ia fiber or Ib fibers)
 Characteristics:
 High conduction velocity
 Ia fibers are related to muscle spindle primary endings
(muscle sense)
 Ib fibers are related to golgi tendon organs (muscle sense)
A beta fibers (II fibers)
 II fibers carry sensory information related to muscle spindle
secondary endings, touch, and kinesthesia.
A delta fibers (III fibers)
 III fibers carry sensory information related to pain,
temperature, crude touch, and pressure.
87

88. B group
 Are myelinated with a small diameter
 They are the preganglionic fibers of the autonomic nervous
system.
 Have low conduction velocity.
C group
 Are unmyelinated and as the B group fibers have a small
diameter and low conduction velocity.
 These fibers include:
 Postganglionic fibers in the autonomic nervous system
(ANS)
 Nerve fibers at the dorsal roots
 These fibers carry the following sensory information:
pain, temperature, touch, pressure and itch
88

89. Neuronal Processes
• Axons: myelinated/unmylinated
• Most neurons have a single axon –
a long (up to 1m) process designed
to convey information away from
the cell body.
• Originates from a special region of
the cell body called the axon
hillock.
• Transmit APs from the soma
toward the end of the axon where
they cause NT release.
• Often branch sparsely, forming
collaterals.
• Each collateral may split into
telodendria which end in a synaptic
knob, which contains synaptic
vesicles – membranous bags of
89
NTs.

90. Classification of Ion Channels
I. Leak channel
II. Gated Channel
 Na+ channels
 There are three major types;
 Na+ leak channels
 Voltage-gated Na+ channels
 Ligand (chemical –gated) Na+ channels
 K+ Channels
 There are four major classes:
K+- leak channels
Voltage-gated K+ channels
Ligand- gated K+ channels
90
G-prorein- gated K+ channels

91.  Ca+ Cahnnels
 Are the following:
 Voltage- gated Ca+ channels with subtypes
L-type Ca+- gated channels
T- type Ca+- gated channels
N-type Ca+- gated channels
 Ligand- gated Ca+- gated channels
 G- protein-gated Ca+- gated channels
91

92.  The Na+-K+ channel causes large concentration gradients for
sodium and potassium across the resting nerve membrane.
 These gradients are the following:
 Na+ outside =142mEq/L, Na+ inside =14mEq.L
 K+ outside = 4mEq/L, K+ inside = 140mEq/L
 The ratios of these two respective ions from the inside to
the outside are
Na+ inside/Na+ outside =0.1
K+ inside/K+ outside =35
92

93. • All cells have a voltage difference across their plasma
membrane.
 This is called membrane potential.
• The membrane potential (VM) at rest is called resting
membrane potential (RMP)
• The RMP of a typical neuron is -90 mv
 Meaning, ‘at rest there is more electro-positivity out and
electro-negativity inside the cell membrane of the neuron.’
93

94. Origin of Resting Membrane Potential
1. Leakage of Potassium and Sodium through the Nerve
Membrane
 The nerve membrane contains channel protein through
which potassium and sodium ions can leak, called a
potassium-sodium (K+-Na+) "leak" channel.
o On average, the channels are far more permeable to
potassium than to sodium, normally about 100 times
as permeable and hence K+ plays a major role than Na+.
94

95. 2. Contribution of the Potassium Diffusion Potential
 The diffusion potential level across a membrane that exactly
opposes the net diffusion of a particular ion through the
membrane is called the Nernst potential for that ion.
 The magnitude of Nernst potential is determined by the
ratio of the concentration of specific ion on the two sides
of the membrane.
 The greater this ratio, the greater the tendency for the ion
to diffuse in one direction, and therefore the greater the
Nernst potential required to prevent additional net diffusion.
95

96. • The following equation, called the Nernst equation, can be
used to calculate the Nernst potential for ion at normal body
temperature of 37°C:
• EMF (in mv) is electromotive force
EMF=+ 61 log concentration inside
concentration out side
When using this formula, it is usually assumed that the
potential in the extracellular fluid, outside the membrane
remains at zero potential, and the Nernst potential is the
potential inside the membrane.
Thus, when the concentration of positive potassium ions on
the inside is 10 times that of the outside, the log of 10 is 1, so
that the Nernst potential calculates to be -61 millivolts inside
the membrane
96

97.  We make the assumption that the only movement of ions
through the membrane is diffusion of potassium ions
between inside and outside the membrane.
 Because of the high ratio of potassium ions inside to
outside, 35:1, the Nernst potential = the logarithm of 35 is
1.54, and times -61 millivolts is -94 millivolts.
 Therefore, if potassium ions were the only factor causing
the resting potential, the resting potential inside the fiber
would be equal to -94 millivolts.
97

98. 3. Contribution of Sodium Diffusion through the Nerve
Membrane.
 There is slight permeability of the nerve membrane to sodium
ions caused by the minute diffusion of sodium ions through
the K+-Na+ leak channels.
 The ratio of sodium ions from inside to outside the membrane
is 0.1, and this gives a calculated Nernst potential for the
inside of the membrane of +61 millivolts.
 But the Nernst potential for potassium diffusion is -94
millivolts.
 How do these interact with each other, and what will be the
summated potential?
98

99. Calculation of the Diffusion Potential When the Membrane Is
Permeable to Several Different Ions
• When a membrane is permeable to several different ions, the
diffusion potential that develops depends on three factors: (1)
the polarity of the electrical charge of each ion, (2) the
permeability of the membrane (P) to each ion, and (3) the
concentrations (C) of the respective ions on the inside (i) and
outside (o) of the membrane.
• Thus, the following formula, called the Goldman equation, or
the Goldman-Hodgkin-Katz equation, gives the calculated
membrane potential on the inside of the membrane when two
univalent positive ions, sodium (Na+) and potassium (K+), and
one univalent negative ion, chloride (Cl-), are involved.
EMF=-61 log C Na+i P Na+ + C K+i P K+
C Na+o P Na+ + C K+o P K +
99

100. • This can be answered by using the Goldman equation
described previously.
• Intuitively, one can see that if the membrane is highly
permeable to potassium but only slightly permeable to sodium,
it is logical that the diffusion of potassium contributes far
more to the membrane potential than does the diffusion of
sodium.
• In the normal nerve fiber, the permeability of the membrane to
potassium is about 100 times as great as its permeability to
sodium.
• Using this value in the Goldman equation gives a potential
inside the membrane of -86 millivolts, which is near the
potassium potential
100

101. 4. Contribution of the Na+-K+ Pump.
 The Na+-K+ pump provides an additional contribution to the
resting potential.
 There is continuous pumping of three sodium ions to the
outside for each two potassium ions pumped to the inside
of the membrane by hydrolyzing one ATP
101

102.  This creates an additional degree of negativity (about -4
millivolts additional) on the inside beyond that which can
be accounted for by diffusion alone.
 There are also negatively charged non-diffusible proteins
within the ICF that cannot travel through the membrane.
 What this adds up to is the fact that the inside of the cell is
negative with respect to the outside.
 The interior has less positive charge than the exterior.
 Therefore, the net membrane potential with all these factors
operative at the same time is about -90 millivolts.
102

103. In summary,
• The diffusion potentials alone caused by potassium
and sodium diffusion would give a membrane
potential of about -86 millivolts, almost all of this
being determined by potassium diffusion.
• Then, an additional -4 millivolts is contributed to the
membrane potential by the continuously acting
electrogenic Na+-K+ pump, giving a net membrane
potential of -90 millivolts.
• There are also negatively charged non-
diffusible proteins within the ICF that cannot
travel through the membrane
103

104. Nerve Action Potential
• Nerve action potentials are rapid
changes in the resting membrane
potential that spread rapidly
along the nerve fiber membrane.
• Each action potential begins with
a sudden change from the normal
resting negative membrane
potential to a positive potential
and then ends with an almost
equally rapid change back to the
negative potential.
• To conduct a nerve signal, the
action potential moves along the
nerve fiber until it comes to the
fiber's end.
104

105. The successive stages of the action potential are as follows:
• Resting Stage:
 This is the resting membrane potential before the action
potential begins.
 The membrane is said to be "polarized" during this stage
because of the -90 mv negative membrane potential that is
present.
 To explain more fully the factors that cause both
depolarization and repolarization, we need to describe the
special characteristics of two other types of transport
channels through the nerve membrane: the voltage-gated
sodium and potassium channels.
105

106. 2. Depolarization Stage:
 At this time, the membrane suddenly becomes very permeable
to sodium ions, allowing tremendous numbers of positively
charged sodium ions to diffuse to the interior of the axon.
 The normal "polarized" state of -90 mv is immediately
neutralized by the inflowing positively charged sodium ions,
with the potential rising rapidly in the positive direction.
 This is called depolarization.
 In large nerve fibers, the great excess of positive sodium ions
moving to the inside causes the membrane potential to actually
"overshoot" beyond the zero level and to become somewhat
positive.
 In some smaller fibers, as well as in many central nervous
system neurons, the potential merely approaches the zero level
and does not overshoot to the positive state.
106

107. 3. Repolarization Stage.:
 Within a few 10,000ths of a second after the membrane
becomes highly permeable to sodium ions, the sodium
channels begin to close and the potassium channels open more
than normal.
 Then, rapid diffusion of potassium ions to the exterior re-
establishes the normal negative resting membrane potential.
 This is called repolarization of the membrane.
 K+ channels are slow to open and slow to close. This causes
the VM to take a brief dip below resting VM. This dip is the
undershoot and is an example of hyperpolarization
107

108. Phases of action potential
108

109. Phases of action potential
B. RMP: causes
D. Depolarization: ionic causes
F. Repolarisation: ionic causes
109

110. Action potential cont…..d
• If membrane potential(VM)reaches
threshold, Na+ channels open and Na+
influx =depolarizing the cell and
causing the VM to increase.
 This is the rising phase of an AP.
• Eventually, the Na+ channel will have
inactivated and the K+ channels will
be open.
• Now, K+ effluxes and repolarization
occurs.
 This is the falling phase.
110

111. Events during an action potential
111

112. Role of action potentials in the transmission of a nerve
impulse.
A STMULUS causes the Na+ gated channel
proteins to open which allows Na+ ions to diffuse
down the concentration gradient across the
membrane into the cell and so set off an action
potential.
112

113. 113

114. 114

115. Voltage-Gated Sodium and Potassium Channels
 The necessary actor in causing both depolarization and
repolarization of the nerve membrane during the action
potential is the voltage-gated sodium channel.
 A voltage-gated potassium channel also plays an important
role in increasing the rapidity of repolarization of the
membrane.
Voltage-Gate
 The voltage-gated sodium channel has two gates-one near
the outside of the channel called the activation gate, and
another near the inside called the inactivation gate.
115

116. They have 2 gates
 At rest, one is closed (the activation gate) and the other is
opened (the inactivation gate).
 Suprathreshold depolarization affects both of them.
1
2
116

117. 3 4
5
117

118. Activation of the Sodium Channel
 When the membrane potential becomes less negative than
during the resting state, rising from -90 millivolts toward zero,
it finally reaches a voltage-usually somewhere between -70
and -50 millivolts-that causes a sudden conformational change
in the activation gate, flipping it all the way to the open
position.
 This is called the activated state; during this state, sodium ions
can pour inward through the channel, increasing the sodium
permeability of the membrane as much as 500- to 5000-fold.
118

119. Initiation of the Action Potential
A Positive-Feedback Cycle Opens the Sodium Channels.
 First, as long as the membrane of the nerve fiber remains
undisturbed, no action potential occurs in the normal
nerve.
 However, if any event causes enough initial rise in the
membrane potential from -90 mv toward the zero level, the
rising voltage itself causes many voltage-gated sodium
channels to begin opening.
119

120. Initiation of the Action Potential cont……d
 This allows rapid inflow of sodium ions, which causes a
further rise in the membrane potential, thus opening still more
sodium channels and allowing more streaming of sodium ions
to the interior of the fiber.
 This process is a positive-feedback cycle ,
 Continues until all the voltage-gated sodium channels
have become opened.
 Then, within another fraction of a millisecond, the rising
membrane potential causes closure of the sodium channels as
well as opening of potassium channels, and the action
potential soon terminates.
120

121. Threshold for Initiation of the Action Potential:
• An action potential will not occur until the initial rise in
membrane potential is great enough to create the vicious cycle
described in the preceding paragraph.
• This occurs when the number of Na+ ions entering the fiber
becomes greater than the number of K+ ions leaving the fiber.
• A sudden rise in membrane potential of 15 to 30 millivolts
usually is required.
• Therefore, a sudden increase in the membrane potential in a
large nerve fiber from -90 millivolts up to about -65 millivolts
usually causes the explosive development of an action
potential.
• This level of -65 millivolts is said to be the threshold for
stimulation.
121

122. Propagation of the Action Potential
• An action potential elicited at any one point on an excitable
membrane usually excites adjacent portions of the membrane,
resulting in propagation of the action potential along the
membrane.
• When a nerve fiber is excited in its mid-portion-that is, the
mid-portion suddenly develops increased permeability to
sodium, and there will be current flow from the depolarized
areas of the membrane to the adjacent resting membrane areas.
• That is, positive electrical charges are carried by the inward-
diffusing sodium ions through the depolarized membrane and
then for several millimeters in both directions along the core
of the axon.
• This transmission of the depolarization process along a nerve
or muscle fiber is called a nerve or muscle impulse.
122

123. Propagation of Action Potential
• Continuous (sweeping) conduction (occurs in unmyelinated
axons)
 In this situation, the wave of de- and repolarization simply
travels from one patch of membrane to the next adjacent
patch.
 APs moved in this fashion along the sarcolemma of a
muscle fiber as well.
123

124. 2. Saltatory (jumping) conduction (occurs in myelinated
axons)
• Recall that the myelin sheath is not completed.
 There exist myelin free regions along the axon, the nodes
of Ranvier.
• The wave of depolarization and repolarization jump from
nodes of Ranvier to nodes of Ranvier
Advantages
 Increase speed of transmission by 100 folds.
 Conserve energy as sodium-potassium pump only has to
operate at the nodes and fewer ions have to be transported
124

125. Rates of AP Conduction Depends Upon
• Level of myelination
 Faster in mylinated than in unmyelinated
2. Size of nerve fiber
 Faster in large sized than in smaller ones
3. Age slower in babies and in elderly
 Maximum b/n the age 5-15 years
4. Temperature
 which affects the rate of diffusion and the rate of energy
release by respiration for active transport (since it is
controlled by enzymes)
 the consequence is that nerve impulse transmission is faster
in endothermic animals which maintain a high body
temperature.
125

126. Properties of the action potential
 Action potentials have a threshold:
 This is the minimum level of stimulus necessary to cause
depolarisation (i.e. open the ion channels)
 Action potentials have all -or -nothing principle:
 Once an action potential has been produced at any
point on the membrane of a normal fiber, the
depolarization process travels over the entire membrane if
conditions are right, or it does not travel at all if conditions
are not right.
This is called the all-or-none principle, and it applies to
all normal excitable tissues.
126

127. Properties of the action potential cont....d
 Has refractory Periods
 During the time interval between the opening of the Na+
channel activation gate and the opening of the inactivation
gate, a Na+ channel cannot be stimulated. This is called
refractory period.
 There are two types of refractory period:
5. Absolute refractory period (ARP)
6. Relative refractory period (RRP)
Absolute refractory period (ARP):
 Interval b/n the opening of the Na+ channel activation
gate and the opening of the inactivation gate.
 2nd action potential can not be generated regardless of
the strength of stimulus.
127

128. • ARP begins at the start of up stroke (the activated Na+
channels start as fast as possible) and extends into the
downward stroke (Na+ channels are inactivated) .
• A Na+ channel cannot be involved in another AP until the
inactivation gate has been reset.
Relative Refractory Period(RRP)
 New action potential can occur in an excitable fiber if the
stimulus is supra threshold
 The stimulus should be greater than normal b/c there are still
inactivated sodium channels and more K+ channels than
normal are still open.
 The RRP begins when the ARP ends.
 Reason:
Number of inactivated Na+ channels and
Activated K+ channels during RRP.
128

129. Graded Potentials also called receptor potentials
 Local changes in membrane potential
 Upon being stimulated, the dendrites of a neuron produce a
graded potential.
 Stimulation can occur in many ways, including chemical
stimulation (neurotransmitters, etc.), mechanical stimulation
(certain pain receptors, hair receptor, etc.), light stimulation
(photoreceptors) and a few other methods.
 Magnitude of change is related to magnitude of triggering
event
129

130. Triggering event
 Triggering event causes a flow of ions across the membrane
 Leads to localized change in membrane potential
 Without constant triggering event, graded potentials will die
out
 Current is local – does not spread very far; and is small,
<10mV change
 Depending on the strength of the stimulus, can be changed to
AP usually on summation.
130

131. Comparison between Graded potential and action potentials
Graded potential Action potential
 Graded responses-Amplitude varies  All or none response; once a
with condition of the initiating membrane is depolarized to
events threshold, amplitude is independent
of the initiating event.
Graded responses can be summated Action potential can not be summated
Has no refractory period Has refractory period
Is conducted decrementally; amplitude Not affected by distance
decreases with distance
Duration varies Duration is constant with a specific cell
under constant condition
Can be depolarization or repolariztion Is depolarization with an overshoot
Initiated by environmental stimulus Initiated by membrane depolarization.
(receptor) ,by NTs, or spontaneously
131

132. Synaptic Transmission
 Synapse is the junction b/n two cells in which one must be a
neuron.
 It is the site of transmission from one neuron to the next.
 Is site where neuron communicates with another cell:
(neuron or effectors)
 There 3 types of synapses
1. Neuroneuronal junction (presynaptic and postsynaptic
neurons)
2. Neuromuscular junction
3. Neuroglandualr junction
 There 3 types of neuroneuronal junctions i.e.
o Axo-dendritic,
o Axosomatic and
o Axo-axonic junctions
132

133.  In autonomic NS, they are called ganglia (ganglion singular)
 Pre- synaptic structure is always a nerve
 Post- synaptic structure can be:
• A nerve
• Muscle
• Gland
• Skin
 The post- synaptic structures are collectively called effector
organs or simply effectors.
133

134. 134

135. Synaptic Transmission
 Begins with the stimulation of a neuron.
 One neuron may be stimulated by another, by a receptor
cell, or even by some physical event such as pressure.
 Once stimulated, a neuron will communicate information
about the causative event.
 Such neurons are sensory neurons and they provide info
about both the internal and external environments.
 Sensory neurons (afferent neurons) will send info to
neurons in the brain and spinal cord.
 There, association neurons (interneurons) will integrate the
information and then perhaps send commands to motor
neurons (efferent neurons) which synapse with muscles or
glands.
135

136.  Thus, neurons need to be able to conduct information in 2
ways:
 From one end of a neuron to the other end (pre and post
synaptic neurons).
Accomplished electrically via APs
 Across the minute space separating one neuron from
another (synapse)
Accomplished chemically via neurotransmitters.
 One neuron will transmit information to another neuron or to a
muscle or gland cell by releasing chemicals called
neurotransmitters.
136

137.  The site of this chemical interplay is known as the synapse.
 An axon terminal (synaptic knob) will adjoin another cell, a
neuron, muscle fiber, skin, or gland cell.
 This is the site of transduction – the conversion of an
electrical signal into a chemical signal.
Mechanism of transmission
 An AP reaches the axon terminal →open VG-Ca2+ channels
→Ca2+ rushes in and binds to regulatory proteins → initiation of
NT exocytosis.
 NTs diffuse across the synaptic cleft bind to receptors on the
postsynaptic membrane → initiation of some sort of response on
the postsynaptic cell.
137

138. Mechanism of transmission
138

139. Mechanism of Synaptic Transmission
• An AP reaches the presynaptic axon terminal of the
presynaptic cell and causes V-gated Ca2+ channels to open.
• Ca2+ rushes in, binds to regulatory proteins & initiates NT
release by exocytosis.
• NTs diffuse across the synaptic cleft and then bind to specific
receptors on the postsynaptic membrane and initiate
postsynaptic potentials.
• NT-receptor interaction results in either EPSP or IPSP
 Different neurons can contain different NTs.
 Different postsynaptic cells may contain different
receptors.
 Thus, the effects of NT can vary.
139

140. Mechanism of Synaptic Transmission
 When the NT-R combination triggers the opening of
ligand-gated Na-channels, this leads to membrane
depolarization, EPSP.
E.g. Ach on Nicotinic receptor
 When the NT-R combination triggers the opening of
ligand gated K+ or Cl- channels, this leads to
membrane hyperpolarization, IPSP
e.g. GABA on GABAb receptor
140

141. EPSPs & IPSPs
Graded hyperpolarization
Graded depolarization
Bring the neuronal VM closer to Bring the neuronal VM farther
threshold. away from
Thus, it’s often referred to as an
excitatory postsynaptic potential or threshold and
EPSP. thus are referred to as
inhibitory postsynaptic
potentials or
IPSPs.
141

142. Summation
• One EPSP is usually not strong enough to cause an AP
• However, EPSPs may be summed
A. Temporal summation
 The same pre-synaptic neuron stimulates the postsynaptic
neuron multiple times in a brief period.
 The depolarization resulting from the combination of all
the EPSPs may be able to cause an AP.
B. Spatial summation
 Multiple neurons all stimulate a postsynaptic neuron
resulting in a combination of EPSPs which may yield an
AP
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143. 143

144. 144

145. Neurotransmitter Removal
 NTs are removed from the synaptic cleft via:
 Enzymatic degradation
 Diffusion
 Reuptake
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146. Properties of synaptic transmission
4. Unidirectional conduction
5. Synaptic delay (0.5 -1.0m/s)
6. Fatigue -↓in response of postsynaptic neurons after repetitive
stimulation by the presynaptic neurons
7. Synaptic potentiation (facilitation):– persistence of out put
signals after the stoppage of in put signal
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147. Factors Affecting Synaptic transmission
 pH
 Alkalosois ↑ Synaptic transmission
 Acidosis ↓ Synaptic transmission
 Hypoxia ↓ Synaptic transmission
 Drugs
 Caffeine, theophylline, theobromine ↑ Synaptic
transmission
 Strychinine ↑ Synaptic transmission
 Hypnotics, anesthtics, tranquilizers ↓ Synaptic transmission
147