PHYSIOLOGY

1. • In order for nerves to transmit impulses, nerve cell need to produce
nerve impulses that are transmitted from the dendrites of on nerve
down to the axon to the axon hillock where it synapses with other
nerves.
• To achieve this, nerves use action potentials.

2. Action potential
Definition: It is the electrical changes which occur in the resting membrane
potential as a result of its stimulation by an effective stimulus
These electrical changes propagate along the nerve fibers to the effector
organ producing the response or action (hence the name action potential).
 The electrical changes of the action potential are:
A- Depolarization.
B- Repolarization.
C- Redistribution of ions.
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3. A) Depolarization:
Definition:  negativity of the membrane potential.
Mechanism: The stimulus  the permeability of the cell membrane
(several hundred fold) to Na+ ions through opening of voltage-activated Na+
channels.
Na+ channels:
Has 2 gating particles:
- an m gate covers the extracellular surface (activation gate) .
- an h gate covers the intracellular surface (inactivation gate).
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4. * Both the m and the h gate must be open for Na+ to flow through the Na+
channels.
* When m gate is open, Na+ ions can pass (the channel is said to be activated).
* When h gate is closed, Na+ ions can not pass the channel is said to be
inactivated.
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5. Na+ diffusion (Na+ influx):
1) At first, Na+ influx is Slow until the threshold potential due to gradual
opening of Na+ channels Change of the membrane potential form
the resting potential (-70 m.v.) to the threshold potential (-55 m.v.)
2) Then, Na+ influx becomes Rapid after the threshold potential due to
sudden opening of most of voltage-gated Na+ channels Changes
the membrane potential to zero.
3) With continuous Na+ influx, the membrane potential becomes positive (+
35 m.v.) causing momentary reversal of polarity or Na+ overshoot.
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6. B) Repolarization:
Definition: Restoration of the resting membrane potential.
Mechanism:
1- Stoppage of Na+ influx:
Due to:
a- Closure of the voltage-activated Na+ channels by closure of h (inactivation)
gate which close at threshold potential but after a certain delay time.
b- Reversal of the electrical gradient as the inside becomes +ve charged which
repel the diffusing Na+.
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7. 2- Opening of voltage-activated K+ channels:
At the threshold potential (-55 m.v), the voltage-activated K+ channels open
but after a slight delay time.
 K+ channels:
* In case of K+ channels, there is only one gate on the intracellular side
called n-gate.
* The n-gate must be open for K+ to flow through the channel.
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8. K+ diffusion (K+ efflux):
1) At first, K+ efflux is rapid due to sudden opening of most (about 70%) of
K channels the membrane is 70% repolarized.
2) Then, K+ efflux becomes slow due to slow opening of the remaining of
K+ channels RMP is restored (-70 m.v).
3) With continuous K efflux due to continuous opening (delayed closure) of
K+ channels the membrane becomes hyperpolarized.
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9. C) Redistribution of ions:
 After passage of an action potential (depolarization and repolarization), the
ionic composition inside and outside the cell membrane is slightly disturbed
(some Na+ ions go inside during depolarization and some K+ ions go outside
during repolarization).
 Redistribution of Na+ and K+ ions to the normal resting condition is
established by the Na+-K+ pump which actively transports sodium out and
potassium into the cell.
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10. PROPAGATION OF THE ACTION
POTENTIAL
-CONDUCTIVITY-
Definition: It is the propagation (transmission) of action potential along the
axon from the region of the initial segment down to the terminal ending.
Significance: The action potential must be propagated in order to transfer
information from one place in the nervous system to the other.
 Direction:
- Inside the body (in vivo): in one direction (unidirectional)
* mostly: away from the cell body (orthodromic)
* to less extent: in the opposite direction (antidromic).
- Outside the body (in vitro): in both directions (bidirectional).
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15.  Mechanism:
The action potential generated at one site on the axon, acts as a stimulus for the
production of another action potential in the adjacent sites of the axon.
Saltatory conduction
Continuous conduction
- It is propagation in myelinated nerve
fibers.
- Mechanism:
1- Stimulation of the nerve fiber by an
effective stimulus generation of
an action potential at the nearest node
of Ranvier.
2- During the action potential, the
nearest node becomes depolarized
(membrane potential becomes
+35m.v).
- It is propagation in unmyelinated
nerve fibers.
- Mechanism:
1- Stimulation of the nerve fiber by an
effective stimulus generation of
an action potential at the site of
stimulation.
2- During the action potential, the
stimulated area becomes depolarized
(membrane potential becomes
+35m.v).
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16. Saltatory conduction
Continuous conduction
3- This creates a potential difference
between the depolarized (active) node
(+ 35 mv) and the next polarized
(resting) node (- 70 m.v).
4- Because of this potential difference,
local circuits of current flows between
the two nodes (in which the charges
jump) causing the polarized (resting)
node to become depolarized to the
threshold level.
5- This generates an action potential at
the resting node, which by turn
becomes the stimulus for the adjacent
nodes & so on.
3- This creates a potential difference
between the depolarized (active) area
(+ 35 mv) and the adjacent polarized
(resting) area (- 70 m.v).
4- Because of this potential difference,
local circuits of current flows between
the two areas (in which the charges
move) causing the polarized (resting)
area to become depolarized to the
threshold level.
5- This generates an action potential at
the resting area, which by turn becomes
the stimulus for the adjacent region &
so on.
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17. Saltatory conduction
Continuous conduction
 Velocity of conduction:
fast (may reach up to 120 met/sec).
The greater the distance between nodes
of Ranvier, the greater the velocity of
conduction of the action potential.
 Velocity of conduction:
slow (0.5-2.0 meter/sec)
Significance of Saltatory conduction and myelin sheath :
a) It increases the velocity of conduction because the action potential occurs
only at the nodes of Ranvier which is transmitted by jumping (saltatory
conduction).
b) It decreases the energy needed for the Na+ - K+ pump which is restricted to
the nodes of Ranvier. Myelinated fibers use about 1% of the energy used by
the unmyelinated fibers.
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18. Monophasic action potential
 Definition: It is the action potential recorded when one micro electrode
(recording electrode) is introduced inside the nerve fiber and the other
electrode (reference electrode) is placed in the extracellular fluid away form
the excited region.
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20. 1- Latent period:
Definition: It is the time passed between the stimulation of the nerve and
the start of the action potential.
 Cause: It represents the time taken by the impulse to travel from the site
of stimulation to the site of recording electrodes.
 Duration: is affected by:
- The distance between the stimulating and recording electrodes.
- The velocity of conduction of the nerve fibers.
Thus, the velocity of conduction of a nerve fiber can be calculated as follow:
Velocity of conduction =
Distance between the stimulating and recording electrodes
Duration of the latent period
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21. 2- Spike potential:
 Definition: It is large wave of a short duration (Its magnitude & duration
depends on the type of the nerve fiber).
It consists of:
Descending limb
Ascending limb
- Represents 70% of
repolarization.
- Represents the process of depolarization.
- Due to:
* Stoppage of Na+ influx
(see before).
* K+ efflux which occurs
at first rapid due to the
sudden opening of most
(about 70%) of the voltage
activated K+ channels.
- Due to Na+ influx which occurs in two stages:
2-Rapid after the threshold potential
1- Slow until the threshold potential
* Due to rapid Na+ influx due
to sudden opening of most of
voltage activated Na channels.
* Changes the membrane
potential to zero & with
continuous Na+ influx, the
membrane potential becomes
positive (+ 35 m.v.). This is
known as reversal of polarity
or Na+ overshoot.
* Due to gradual Na+
influx due to slow
opening of some Na
channels.
* Changes the membrane
potential form the resting
potential (-70 m.v.) to the
threshold potential (-55
m.v.)
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22. 3- After potentials:
Definition: They are small waves with relatively longer durations.
b) Positive after potential
(after hyperpolarization)
a) Negative after potential
(after depolarization)
- relatively prolonged (40 m. sec)
- Relatively short. (4 m. sec)
- Caused by prolonged opening of the
K+ channels (delayed closure) which
cause a continuous K+ efflux.
- Caused by slow opening of the
remaining voltage-activated K channels.
- The membrane is hyperpolarized.
- The membrane is partially depolarized.
- known as positive after potential due to
the presence of more positive charges on
the outer surface of the membrane.
- known as negative after potential due to
presence of the some negative charges on
outer surface of the membrane.
- The excess K+ ions return back again
inside the nerve fiber by Na+-K+ pump.
- the negative charges are gradually
neutralized by the outward diffusion of
K+ ions at the end of this phase.
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24. Excitability changes
i) Temporal rise of excitability:
* Corresponds to the slow depolarization of the nerve fiber before firing level which is
called the “local response”.
* The nerve can respond to another Subminimal stimulus applied to it during this phase.
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25. v) Subnormal
phase of
excitability
iv) Supernormal
phase of
excitability
iii) Relative
refractory period
(RRP)
ii) Absolute
refractory period
(ARP)
- the excitability is
below normal.
- the excitability is
above normal.
- the excitability of
the nerve is partially
recovered (but still
below normal)
- the excitability of
the nerve fiber is
completely lost.
i.e., the nerve is
refractory to further
stimulation
- stronger stimuli
are needed to
excite the nerve.
- weaker stimuli
can excite the
nerve.
- Stronger stimuli are
needed to excite the
nerve.
- no other stimulus
whatever its strength
can excite the nerve.
- corresponds to
the positive after
potential.
- corresponds to
the negative after
potential.
- corresponds to the
late part of the
descending limb of
the spike potential
till the start of the
negative after
potential.
- corresponds to: the
ascending limb of the
spike potential (after
the firing level) and
the early part of the
descending limb
(initial 1/3 of
repolarization).
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26. v) Subnormal
phase of
excitability
iv)
Supernormal
phase of
excitability
iii) Relative
refractory period
(RRP)
ii) Absolute refractory
period (ARP)
Mechanism:
- The
membrane is
hyperpolarized
& away from
the threshold
level.
Mechanism:
- The membrane
is still partially
depolarized &
near to the
threshold level.
Mechanism:
- During this period,
the membrane is
partially repolarized &
Strong stimuli can
reopen many (not all)
of the gates of the
Na+ channels.
- This leads to
depolarization of the
membrane and
production of a
second waeker action
potential (not all Na+
gates are opened).
Mechanism:
- During the ascending
limb of the spike: the
gates of the voltage
activated Na+ channels
are already opened (by
the first stimulus) . If a
second stimulus is
applied, it can not have
any effect (the gates are
opened).
- During the early part
of the descending limb:
the gates are just closed
& need a sufficient
period of repolarization
to be re-opened.
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27. Types of Nerve Fibers, function and susceptibility
to hypoxia, pressure and local anaesthetics
• “A” fibers: Largest diameter myelinated fibers with the
fastest saltatory conduction (12-130 m/sec) and a
brief absolute refractory period. Axons of motor
neurons and axons of sensory neurons that conduct
touch, pressure, and thermal sensations.
• “B” fibers: intermediate diameter myelinated fibers
With slower saltatory conduction than “A” fibers and
longer absolute refractory periods. Dendrites of visceral
sensory neurons and axons of presynaptic neurons of
the ANS.
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28. Types of Nerve Fibers
• “C” fibers: Smallest diameter unmyelinated fibers with
slow continuous conduction (.5 – 2 m/sec.) and the longest
absolute refractory periods. Axons of some somatic
sensory neuron that carry pain, touch, pressure and
thermal sensation, neuron that carry visceral pain
sensations, and postsynaptic neurons of the ANS
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29. Classification of Nerve fiber
• General features of nerve:
• Greater the diameter of nerve fiber
• Greater speed of conduction
• Greater magnitude of spike potential
• Smaller duration of spike
• Lesser threshold of excitation
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30. CONTI
• Speed of conduction
• Myelinated fibers
• Approximately 6 times fiber diameter
• Myelinated fiber diameter ranges from 1-20µ m
• Therefore conduction velocity varies from 6-120 mts/sec
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31. • Nonmyelinated fibers:
• Speed of conduction proportional to square root of diameter
• Largest unmyelinated fiber approxi 1µm in diameter
• Therefore max conduction velocity 1 mt/sec
• Long axon mainly concerned with proprioceptive, pressure and touch
sensation and somatic motor functions
• Small axons concerned with pain (slow or chronic) and temp (hot)
sensation and autonomic functions
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32. Classification of Nerve fibers
1. Depending upon structure
• Myelinated nerve fibers
• Non myelinated nerve fibers
2. Depending upon distribution
• Somatic nerve fibers (supply skeletal muscles)
• Visceral or autonomic (supply internal organs)
• Depending upon origin
• Cranial nerve (arising from brain)
• Spinal nerve (arising from spinal cord)
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33. • Depending upon functions:
• Sensory nerve fibers (afferent nerve fiber)
• Motor nerve fibers (efferent nerve fibers)
• Depending upon secretion of neurotransmitter
• Adrenergic nerve fibers
• Cholinergic nerve fibers
• Depending upon diameter and conductions of impulse (Erlanger- gasser
classification)
• Classified into three major groups:
• Type A nerve fibers
• Type B nerve fibers
• Type C nerve fibers
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