Physiology of the Nerve fibres, different types of receptors and neurotransmitters is explained in this PPT

1. NERVE FIBRES, RECEPTORS &
NEUROTRANSMITTERS
Sangita Sarma
M.Optometry
Assistant Professor, Jain University

2. Nerves
• Nerves are bundles of axons(or nerve fibers)
• A bundle of fibers that uses electrical and chemical signals to transmit sensory and motor information from
one body part to another.
• Some nerve fibers (axons) and are enveloped in myelin sheath, a lipid and protein rich
layer that aids in increasing the velocity of electrical impulse conduction and others are
non-myelinated.

3. Connective tissue surrounding nerves
• The nerve fibers are organized in nerves with the help of three layers of connective tissue:
• 1. endoneurium-Endoneurium is a thin layer of collagenous connective tissue that surrounds
individual nerve fibers. It allows for the presence of tissue fluid required for the creation of
electrical impulses in between each axon, as well as space for capillaries to provide them with
oxygen and organic nutrients (Henle sheath)
• 2. Perineurim- Perineurium is a sheath of connective tissue surrounding a bundle of nerve fibers
(fascicles) within a nerve.
• 3. Epineurium- According to the number and size of the fascicles in the nerve, the number of
perineurial lalayers. . The composition of the epineurium mainly includes collagen fibers, type I
and type III. The count varies in the body. For example, large fascicles in the sciatic nerve may
have 15

4. CLASSIFICATION OF NERVE FIBRES
• A. DEPENDING UPON STRUCTURE
• i. Myelinated Nerve Fibers Myelinated nerve fibers are the nerve fibers that are covered by myelin
sheath.
• ii. Non-myelinated Nerve Fibers Nonmyelinated nerve fibers are the nerve fibers which are not
covered by myelin sheath

5. • B. DEPENDING UPON DISTRIBUTION
• i. Somatic Nerve Fibers Somatic nerve fibers supply the skeletal
muscles of the body.
• ii. Visceral or Autonomic Nerve Fibers Autonomic nerve fibers supply
the various internal organs of the body

6. • C. DEPENDING UPON ORIGIN
• i. Cranial Nerve Fibers Nerve fibers arising from brain are called
cranial nerve fibers.
• ii. Spinal Nerve Fibers Nerve fibers arising from spinal cord are called
spinal nerve fibers

7. • D. DEPENDING UPON FUNCTION
• i. Sensory Nerve Fibers Sensory nerve fibers carry sensory impulses
from different parts of the body to the central nervous system. These
nerve fibers are also known as afferent nerve fibers.
• ii. Motor Nerve Fibers Motor nerve fibers carry motor impulses from
central nervous system to different parts of the body. These nerve
fibers are also called efferent nerve fibers.

8. • E. DEPENDING UPON SECRETION OF NEUROTRANSMITTER
• i. Adrenergic Nerve Fibers Adrenergic nerve fibers secrete
noradrenaline (sympathetic).
• ii. Cholinergic Nerve Fibers Cholinergic nerve fibers secrete
acetylcholine (parasympathetic).

9. • F. DEPENDING UPON DIAMETER AND CONDUCTION OF IMPULSE
(ERLANGER-GASSER CLASSIFICATION)

10. RECEPTORS

11. Introduction
• Receptors are sensory (afferent) nerve endings that terminate in periphery as
bare unmyelinated endings or in the form of specialized capsulated structures.
• They give response to the stimulus.
• When stimulated, receptors produce a series of impulses, which are transmitted
through the afferent nerves.
• Receptors are often defined as the biological transducers, which convert
(transducer) various forms of energy (stimuli) in the environment into action
potentials in nerve fiber

12. Classification
• A. Exteroceptors- Exteroceptors are the receptors, which give
response to stimuli arising from outside the body.
• B. Interoceptors- Interoceptors are the receptors, which give response
to stimuli arising from within the body

13. Types of Exteroceptors
• 1. Cutaneous Receptors or Mechanoreceptors Receptors situated in the skin are
called the cutaneous receptors. Cutaneous receptors are also called
mechanoreceptors because of their response to mechanical stimuli such as
touch, pressure and pain.
• 2. Chemoreceptors Receptors, which give response to chemical stimuli, are called
the chemoreceptors.
• 3. Telereceptors- These are the receptors that give response to stimuli arising
away from the body. These receptors are also called the distance receptors

17. Types of Interoceptors
• 1. Visceroceptors-Receptors situated in the viscera are called
visceroceptors.
• Proprioceptors-Proprioceptors are the receptors, which give
response to change in the position of different parts of the body.

19. SYNAPSE

20. THE SYNAPSE
• The junction between two neurons is called a synapse.
• It is a specialized junction where transmission of information
takes place between a nerve fibre and another nerve, muscle
or gland cell.
• It is a physiological continuity between two nerve cells.

21. SYNAPSE STRUCTURE
• The synapse consists of:
1. A presynaptic ending that contains neurotransmitters,
mitochondria and other cell organelles.
2. A postsynaptic ending that contains receptor sites for
neurotransmitters.
3. A synaptic cleft or space between the presynaptic and
postsynaptic endings. It is about 20nm wide.

22. SYNAPSE FUNCTIONS
• The main function of the synapse is to transmit the impulses, i.e.
action potential from one neuron to another.
• They allow integration, e.g. an impulse travelling down a neuron
may reach a synapse which has several post synaptic neurons, all
going to different locations. The impulse can thus be dispersed.
This can also work in reverse, where several impulses can converge
at a synapse

23. SYNAPSE TYPES
1. Synapse with another neuron
• It is the junction between two nerve cells. They are of 3 types;
axodendritic, axosomatic, dendrodendritic (rare) & axoaxonic.
2. Neuromuscular
• It is the synapse pf a motor neuron and a muscle.
3. Neuroglandular
• It is the synapse of a neuron and a endo/exocrine gland.

24. Classification- A. Anatomical classification
• 1. Axoaxonic synapse in which axon of one neuron terminates on axon
of another neuron
• 2. Axodendritic synapse in which the axon of one neuron terminates
on dendrite of another neuron
• 3. Axosomatic synapse in which axon of one neuron ends on soma
(cell body) of another neuron

25. B. Functional classification
• It is based on the process which nerve cells communicate
among themselves or with muscles and glands.
• The synapse is the anatomic site where this
communication occurs.
• It can be of 2 types:
A. Electrical transmission
B. Chemical transmission

26. ELECTRICAL TRANSMISSION
• In these synapses the membranes of the two cells actually touch, and they
share proteins. This allows the action potential to pass directly from one
membrane to the next.
• They are very fast, but are quite rare, found only in the heart and the
eye.
CHEMICAL TRANSMISSION
• In a chemical synapse, electrical activity in the presynaptic neuron is
converted into the release of a chemical called a neurotransmitter that binds
to receptors located in the plasma membrane of the postsynaptic cell.

27. • On the basis of functions, synapses are divided into two types:
• 1. Excitatory synapses, which transmit the impulses (excitatory
function)
• 2. Inhibitory synapses, which inhibit the transmission of impulses
(inhibitory function

28. PROPERTIES OF SYNAPSE
• 1. One way Conduction–
• BELL-MAGENDIE LAW According to Bell Magendie law, the impulses
are transmitted only in one direction in synapse, i.e. from presynaptic
neuron to postsynaptic neuron.

29. • 2. SYNAPTIC DELAY It is a short delay that occurs during the
transmission of impulses through the synapse. It is due to the time
taken for:
• i. Release of neurotransmitter
• ii. Passage of neurotransmitter from axon terminal to postsynaptic
membrane
• iii. Action of the neurotransmitter to open the ionic channels in
postsynaptic membrane. Normal duration of synaptic delay is 0.3 to
0.5 millisecond. Synaptic delay is one of the causes for reaction time
of reflex activity

30. • 3. Summation
• Summation is the fusion of effects or progressive increase in the
excitatory postsynaptic potential in post synaptic neuron when many
presynaptic excitatory terminals are stimulated simultaneously or
when single presynaptic terminal is stimulated repeatedly.

31. • i. Spatial Summation Spatial summation occurs when many
presynaptic terminals are stimulated simultaneously
• ii. Temporal Summation Temporal summation occurs when one
presynaptic terminal is stimulated repeatedly.
• Both spatial summation and temporal summation play an important
role in facilitation of response.

32. • NEUROTRANSMITTER

33. • Neurotransmitter is a chemical substance that acts as a mediator for
the transmission of nerve impulse from one neuron to another
neuron through a synapse.
• Existence of neurotransmitter was first discovered by an Austrian
scientist named Otto Loewi in 1921.

34. Properties-
• To consider a substance as a neurotransmitter, it should fulfill certain
criteria as given below:
• 1. It must be found in a neuron
• 2. It must be produced by a neuron
• 3. It must be released by a neuron
• 4. After release, it must act on a target area and produce some
biological effect
• 5. After the action, it must be inactivated

35. Classification-
• DEPENDING UPON CHEMICAL NATURE
• 1. Amino Acids Neurotransmitters of this group are involved in fast
synaptic transmission and are inhibitory and excitatory in action.
GABA, glycine, glutamate (glutamic acid) and aspartate (aspartic acid)
• 2. Amines- Amines are the modified amino acids. These
neurotransmitters involve in slow synaptic transmission. These
neurotransmitters are also inhibitory and excitatory in action.
Noradrenaline, adrenaline, dopamine, serotonin and histamine.

36. • 3. Others- Some neurotransmitters do not fit into any of these
categories. One such substance is acetylcholine.

37. Types-Based on function
• 1. Excitatory neurotransmitters
• Excitatory neurotransmitter is a chemical substance, which is responsible
for the conduction of impulse from presynaptic neuron to postsynaptic
neuron.
• Neurotransmitter released from the presynaptic axon terminal does not
cause development of action potential in the postsynaptic neuron. Rather,
it causes slight depolarization by the opening of sodium channels in the
postsynaptic membrane and the influx of sodium ions from ECF.
• This slight depolarization is called excitatory postsynaptic potential (EPSP).
EPSP in turn causes development of action potential in the initial segment
of the axon of the postsynaptic neuron

38. • 2. Inhibitory neurotransmitters
• Inhibitory neurotransmitter is a chemical substance, which inhibits the
conduction of impulse from the presynaptic neuron to the postsynaptic
neuron
• . When it is released from the presynaptic axon terminal due to the arrival
of action potential, it causes opening of potassium channels in the
postsynaptic membrane and efflux of potassium ions.
• This leads to hyperpolarization, which is called the inhibitory postsynaptic
potential (IPSP). When IPSP is developed, the action potential is not
generated in the postsynaptic neuron.
• Common inhibitory neurotransmitters are gamma aminobutyric acid
(GABA) and dopamine.

39. Re-uptake of NJeurotransmitters
• Reuptake is a process by which the neurotransmitter is taken back
from synaptic cleft into the axon terminal after execution of its action.

40. Examples

42. References
• 1. Guyton and Hall’s textbook of Medical Physiology
• 2. K Simbulingum, Prema Simbulingam, Essentials of medical
Physiology