Download PHYSIOLOGY OF THE NERVE

PHYSIOLOGY OF THE NERVE
dendrites
Presynaptic terminals
soma
nucleus
Nod of
Ranvier
Myelin sheath
Schwann
cell nucleus
Axon terminals
Typical nerve cell has cell body (soma) with 5-7 short projections (dendrites)
and a longer fibrous axon. The axon ends in a number of synaptic knobs
(terminal buttons) which stores the neurotransmitter . Axons of some nerve
fibers have a myelin sheath, (a protein-lipid insulator formed by Schwann cell
wrapping around the axon.) The sheath envelops the axon except at its ends and
periodic constrictions of 1 mm distance (node of Ranvier), these are called
myelinated nerve fibers
Genesis Of Resting Membrane
Potential (RMP)
RMP is an Electrical potential exists across the
membranes of all body cells with a negative
charge inside the cell relative to the positive
charge outside .it due to
1-passive diffusion of Na, and K, ions through
leak channels (passive channels, are always
open, allowing the passage of sodium ions (Na+)
and k but more permeable to k)this create (-86
mv)
2-Na-K pump: this is an
electrogenic pump because
more positive charges are
pumped to the outside than
to the inside
leaving a net deficit of
positive ions on the inside.
This will add (-4 mv) making
a total of (-90 mv)
This -90 mv is the RMP for
large neuron
NERVE ACTION POTENTIAL (AP)
Nerve signal (impulse) is transmitted by action potential .
sudden change from the normal negative resting potential inside the cell to a
positive potential (depolarization) followed by rapid return back to the
negative potential(repolarization).
Action potential moves along the nerve fiber to its end in a constant rate and
amplitude
Over shoot
Resting
stage
STAGES OF ACTION POTENTIAL
1. Resting stage: The membrane is said to be “polarized” –90 millivolts .
2. Depolarization:
a. increased Na permeability by opening voltage-gated Na channels. inside becomes positive.
And the membrane potential to “overshoot” beyond the zero
3. Repolarization:
a. closure of voltage-gated Na channels.
b. increasd K permeability by opening voltage-gated K channels K move out
c. inside cell returns back to negative.
Action potential lasts for 1 millisecond in large myelinated nerve fiber.
Stimulus For Nerve and Muscle
Excitation
1. Chemical
2. Mechanical pressure.
3. Electrical current.
all above factors increases membrane
permeability to Na (leak channels) shifting the
membrane potential toward the firing
level(open voltage gated Na channels) producing
an AP
CATHODE-RAY OSCILLOSCOPE (CRO)
An electrical instrument used to record very small electrical events (mv) which occurs very
rapidly (ms) in the living tissues. CRO has two electrodes applied to the nerve, a stimulating
electrode and a recording one
ACTION POTENTIAL AS RECORDED BY
CRO
1. latent period: the time taken by an impulse
to travel from the stimulating electrode to the
recording electrode. From the duration of the
latent period the velocity of nerve conduction
can be calculated.
2. Depolarization: slow at first but after initial
15 mv of depolarization the rate increases
sharply(firing level) due to sudden opening of
fast Na channels.
3. Repolarization: this stage starts by closure
of fast Na channels and opening of K channels
retuning membrane potential back to the
polarized state.
4.
After
depolarization:
when
the
repolarization is 70% completed the rate is
decreased due to built up of large amounts of
K outside the cell which resist K outflow.
5. After hyperpolarization: when membrane
potential reaches the resting level, it becomes
somewhat more negative than normal
because:
2
3
4
6
1
5
REFRACTORY PERIOD
Refractory period :
1. Absolute refractory period: the period from firing level until repolarization is
about half completed. During Absolute refractory period, a second stimulus will
not produce a second action potential
2. Relative refractory period: from end of absolute refractory period . Another
action potential can be produced, but only if the stimulus is very strong
With increasing stimulus strength, subsequent action potentials occur
earlier during the relative refractory period of the preceding action
potentials.
Threshold stimulus
The minimum intensity of stimulus that will just
produce a response (AP) .
It varies according to the type of axon
At the level of single nerve axon, any stimulus with subthreshold
intensity will not produce an AP. Again, increasing the stimulus
intensity above threshold level will produce no change in
response, thus the AP of a single nerve axon obey the "all- ornone law“ ( all-or-nothing principle)
PROPERTIES OF MIXED NERVES
Each peripheral nerve consists of a number of
neurons bound together by fibrous sheath
1. Different neurons have different thresholds:
Subthreshold stimulus produce no response.
When threshold stimulus is applied, some
neurons with low threshold intensity will
respond first producing an AP.
As the stimulus intensity is increased
(submaximal stimulus), more and more neurons
will be brought about into action (recruitment)
producing larger AP. The stimulus which excites
all neurons is called maximal stimulus. After
that, increasing stimulus intensity
(supramaximal stimulus) will produce no change
in response, therefore mixed nerves don't obey
all or none law.
2. Different neurons have different speed of
conduction giving rise to compound AP on
recording
SUMMATION OF NERVE IMPULSES
Nerve impulses obey all- or- none law, however
stronger nerve signals can be obtained by two
means:
1. Spatial summation: many fibers discharge
impulses at the same time.
2. Temporal summation: the same fiber
discharge impulses rapidly and repeatedly
PROPAGATION OF AP
Nerve cell membranes are polarized at
rest with outside positive. When
stimulus of enough strength is applied
on axon, an AP will be generated at the
site of stimulation causing polarity to be
reversed (inside becomes positive).
Positive charges then move from the
adjacent part of membrane to the area
of negativity creating a local circuit of
current flow between the depolarized
area and the adjacent resting area of
the membrane
This spontaneous sequence of events
will move along the unmyelinated axon
in both directions to its end
In myelinated axons, AP will jump
from one node of Ranvier to another .
This type of conduction is called
saltatory conduction which is about 50
times faster than the unmyelinated
fiber
electrical current flows through the
surrounding
extra-cellular
fluid
outside the myelin sheath as well as
through the axoplasm inside the axon
from node to node, exciting successive
nodes one after another
FACTORS WHICH AFFECT THE
CONDUCTION VELOCITY
1-Myelination
2- Axon diameter: in unmyelinated nerve axon,
the conduction velocity is directly proportional
to the square root of axon diameter while in the
myelinated
neuron
conduction
velocity
increases directly with axon diameter
plateau
PLATEAU IN SOME AP
In some excitable tissues, repolarization does not occur immediately after
depolarization, instead the membrane potential remains near the peak of spike for
many milliseconds(plateau), therefore plateau greatly prolongs the period of
depolarization. This type of AP is seen mainly in cardiac muscle where it lasts for as
long as 0.3 seconds thus prolonging the time for cardiac contraction.
Plateau is due to:
1. Opening of slow Ca –Na channels,
2. Delayed opening of voltage-gated K channels.
SPONTANEOUS RHYTHMICITY
Spontaneous discharge occurs normally in cardiac (heart beating) and smooth
muscle (intestinal peristalsis) and many neurons in the CNS (rhythmic control
of breathing).
Spontaneous repetitive discharge occurs due to Na leak during the resting
state. The RMP of such cells is about - 60 to -70 mv which is not enough to
keep Na or Ca channels closed so there will be Na or Ca leak to inside the cell
making the membrane potential less negative, this will cause more channels
to open allowing more Na and Ca inflow then opening more new channels
and so on.
This regenerative process is repeated until the firing level is reached and
another AP is produced. At the end of AP, the membrane repolarizes again but
shortly then after a new AP is generated spontaneously.
THE EFFECT OF CALCIUM IONS ON
NEURON EXCITABILITY
A decrease Ca ion concentration in extra cellular
fluid (hypocalcemia) leads to increased neuron
excitability due to the easily opening the
voltage-gated Na channels .
it leads to spontaneous discharge in many
peripheral nerves causing muscle tetany .
Factors Which Inhibit Nerve Exitability
1. Hypercalcemia(high ECF Ca).
2. Hypokalemia (low ECF K)
3. Local anesthesia
ORTHODROMIC AND ANTIDROMIC CONDUCTION
An axon can conduct in both directions, however in living animals'
impulses pass in one direction only from synaptic junction or
receptors then along the axon to their termination. Such conduction
is called orthodromic
while conduction in the opposite direction is called antidromic.
Synapse permits conduction in one direction only.