Nervous system forms an interconnecting fibers of communication network. In the ‘hard-wiring’ of the nerves, the signals travel in the form of a flow of electrical current called nerve impulses. The stimulus-response reactions afford internal constancy in the face of environmental changes.

2.  Overview of the nervous system.
 Neuron doctrine
 Neuron –
discovery, structure, description and
properties.
 Nerve impulse – definition, all or none
property, velocity and refractory period
 Neurotransmission-history, mechanism
and properties.

3.  Nervous system forms an interconnecting fibers of
communication network.
 In the „hard-wiring‟ of the nerves, the signals
travel in the form of a flow of electrical current
called nerve impulses.
 Irritability is the universal property of life which
means the capacity of organisms to respond to
changes in the environment called stimuli.
 The specific reaction elicited by a stimulus is
termed a response.
 The stimulus-response reactions afford internal
constancy in the face of environmental changes.

4. Central
Nervous
System (CNS)
Peripheral
Nervous
nervous
system
Integrating System(PNS)
systems Autonomic
Endocrine
Nervous
system
System(ANS)

5. Neuron Doctrine was
established by Cajal (1852-
1934)
• The neuron is the structural and
functional unit of the nervous system;
• Neurons are individual cells, which are
not continuous.
• The neuron has three parts:
dendrites, soma (cell body) and axon
• Conduction takes place in the
direction from dendrites to soma, to
the end of the axon

6. Camillo Golgi (1843-1926) invented a
specific staining technique for neurons. Cajal
in 1888 identified the networks of nerve cells.
Golgi and Cajal received the Nobel prize in
1906 for Medicine and Physiology.
• Wilhelm His in 1886 showed that the
dendrites and axons grow out progressively
from the immature neurons in the brains
of embryos.
• Henri Forel (1848-1931) in 1886,observed
that when the cell body dies or an axon is
cut, degeneration of the neuron stop at the
junction to another neuron, thus giving
evidence that they are separate.

8.  A neuron consists of a cell body and two kinds of
processes, the dendrites and the axon.
 The cell body has neuroplasm, a nucleus, nissl bodies,
neurofibrils and a cell membrane.
 The dendrites carry impulses towards the cell body.
 The axon carries impulses away from the cell body.
 The axon originates from axon hillock of the cell body.
 The axon is surrounded by two coverings: myelin sheath
and Schwann sheath.
 These two coverings are interrupted at intervals by nodes
of Ranvier.
 The fine branches at the end of axon are called axon
terminals.

9. 1. Excitability –stimulated by suitable stimuli-
mechanical, thermal, chemical, electrical.
2. Conductivity – impulse is conducted similar to cable
conduction and digital in character.
3. All or none law – the stimulus should be in adequate
threshold strength.
4. Refractory period – when the nerve fiber is once
excited, it will not respond to a second stimulus for a brief
period.
5. Indefatigability – nerve is normally not fatigued.
6. Adaptation –the nerve quickly adapts itself.
7. Accommodation – slowly applied stimulus is
accommodated.

10. Nerves/
Environment organism
neuron
Stimulus Impulse Response

11. Nerve impulse
• A nerve impulse is the sum
total of physical and chemical
events associated with the
transmission of a signal along
an axon.
• A wave of physiological
activity- primarily an electrical
phenomenon.

12.  Stimulus is defined as a sudden
change in the environment which is
strong enough to cause a response in
the living organism.
Mechanical Physical Chemicals Electrical
• Sharp • heat • acids • Application
pressure • cold • bases of
electricity

13.  Stimulation can be affected by
strength and duration.
 The weaker the stimulus, the longer it
will have to be applied to produce a
response.
 The nerve takes lesser time to respond
for a stronger stimulus.

14. All or none law indicates the relation between
the stimulus and response.
A stimulus, if it is capable of causing a
response, causes a maximum response. If it is
below the capacity, it will not cause any
response.
The lowest strength of stimulus required to give
rise to an action potential is the threshold
stimulus.
A stimulus which is less than the threshold fails
to induce any response-sub-threshold stimulus.
A stimulus which is greater than optimum is
supra threshold stimulus.

15. Johannes Muller Helmholtz showed that The velocity varies
believed that nerve velocity of conduction from 100 m per sec in
impulse travelled at a was 100m per sec. large fibers to 0.5 m
speed of light-
(about 10 times faster per sec in small non-
1,86,000 miles per
than a man can run) myelinated fibers.
sec.

16. Once a impulse has passed over any
part of the neuron, for a short time it
is unable to conduct any other
stimulus.
This brief period of non-conductivity
is called refractory period.
Under good physiological
conditions, the nerve fiber is
indefatigable.

17. Bioelectricity
• L. Galvani in 1786 discovered the presence
of electrical current in nerves and muscles.
Electrophysiology
• Du Bois Raymond in 1848 concluded that
impulse transmission was electrical by a
wave of relative negativity
Membrane potential hypothesis
• Hodgkin and Huxley in 1939 demonstrated
the electrical and chemical processes
involved in bioelectricity.

18.  In order to confirm the
generation of electricity,
one microelectrode is
placed on the outer
surface of the nerve
membrane and the other
placed inside the nerve
cell.
 When the terminals are
connected to a
galvanometer, the needle
show a deflection
indicating the flow of
electrical current(-70mV).
 In a resting neuron the
electrical potential ranges
form 20 to 100 mV.

19.  According to the membrane potential hypothesis of
Bernstein (1902), the differential concentration of
ions between the inside and outside of the nerve cell
is the basis of resting potential.
 The inner side of the nerve contain large –vely
charged non-diffusible protein ions and smaller
diffusible K+ and Cl- ions.
 Na+ ions are more concentrated on outside of the
nerve cell.
 Radio-isotopic studies showed that potassium and
sodium ions readily diffuse through the nerve cell
membrane.

20.  In a state of physiological rest, the inner side of
a neuron is negative to outside. This difference
in electrical charge is called resting potential.
Biological factors that govern resting potential
 The resting potential is maintained as long as
the cell is alive and active.
 The resting potential depends upon the
selectivity and variable permeability of cell
membranes.
 The resting potential depends upon unequal
distribution of ions.

21.  Active transport
 Concentration gradient
 Membrane permeability
 Electrostatic attraction

22.  This Na+ - K+ pump
maintains unequal
concentration of ions
in the nerve fibers.
 This pump changes
the electrical
character of the
nerve fibers.
 ATP is used as an
energy for the
process.

23.  The active transport
establishes the
concentration gradient.
 The extrusion of sodium
ions is linked with active
uptake of potassium ions.
 The rates of diffusion of
ions depend upon
gradients and membrane
permeability.

24.  “at equilibrium, the product
of the concentration of the
diffusible ions inside the
membrane equals the
product of the concentration
of the diffusible ions outside.”
 (Donnan 1928).

25.  All the plasma
membranes are
selectively
permeable to ions.
 This membrane
selectively allows
inward diffusion of
K+ and prevents
inward diffusion of
Na+ ions.

26.  The inner side of the nerve cell is
negative to the outside.
 There is a growing attraction between
ions of the same charges.
 The parallel diffusion of ions restore
the resting potential.

27.  When a nerve is stimulated, Na+ ions
suddenly move into the cell and causing a
positive potential.
 The influx of Na + ions reaches its peak in
100msec.Sodium permeability causes
depolarization.
 The inward diffusion of Na + ions halted
near the peak of action potential.
 The diffusion K+ ions restores the
membrane potential –called
repolarization.

29. Stimulus
Increased Increased
Sodium Potassium
Sodium Potassium
influx efflux
conductance conductance
Membrane Membrane Membrane
permeability depolarization repolarization

30.  By using radioactive sodium (Na 24)
and potassium (K42) Hodgkin and
Keynes (1955) showed that inward
flux of sodium was increased 20 times
and outward flux of potassium 3-4
times after stimulation of nerve.

31.  The conduction of nerve impulse causes two phases –
biphasic action potential.
 The conduction of nerve impulse in a myelinated fiber is so
rapid because the action potential skips from node to
node – saltatory conduction.
 The magnitude of nerve impulse transmission occurs
without decrement – Non-decremental conduction.
 The velocity of conduction in a myelinated nerve is directly
proportional to its diameter.
 The introduction of myelinization speeded up
conduction velocity, reduced body weight and consumption
of energy.

32.  Dr.B.Victor is a highly experienced
professor, recently retired from the
reputed educational institution- St. Xavier‟
s College, Palayamkottai, India-627001.
 He was the dean of sciences and assistant
controller of examinations.
 He has more than 32 years of teaching
and research experience.
 He has taught a diversity of UG and PG
courses and guided 12 PhDs.
 Send your comments to :
bonfiliusvictor@gmail.com