‫بسم هللا الرحمن الرحيم‬
‫﴿و ما أوتيتم من العلم إال قليال﴾‬
‫صدق هللا العظيم‬
‫االسراء اية ‪58‬‬
By
Dr. Abdel Aziz M. Hussein
Lecturer of Medical Physiology
Member of American Society of Physiology
Changes that occurs in the
nerve fiber during
propagation of AP
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I- Electrical changes
• In the form of a spike potential and after potential.
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II- Excitability changes
• During conduction of a nerve impulse, the excitability of
the nerve fibers passes in the following phases:
1. Temporal rise of excitability:
2. Absolute refractory period (ARP):
3. Relative refractory period (RRP):
4. Supernormal phase of excitability:
5. Subnormal phase of excitability:
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II- Excitability changes
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III- Chemical changes
• Nerve can generate energy in the form of ATP from
glylcolysis and Kreb s cycle
• At rest, this energy maintains RMP
• During AP, energy production becomes double resting state
• This is manifested by ↑CO2 production, ↑glucose
utilization and ↑heat production
IV- Thermal Changes
• During AP, heat production occurs into 2 phases;
1. Initial heat: due to generation and propagation of nerve
impulse (each impulse ↑ 10-6)
2. Delayed heat: 30 times initial heat and remains for 30
minutes
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• due to reactions needed to reform ATP
Factors affecting the
excitability and conductivity
of the nerve fibers
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1) Physical factors
a)Thermal:
• Cooling →↓es the excitability and conductivity:
• It ↓es the metabolic reactions needed for the Na-K pump
→ accumulation of the Na ions inside the nerve fibers →
loss of the RMP.
• Warming →↑es the excitability and conductivity
b)Mechanical:
Deep pressure ↓es the excitability and conductivity of nerve
fibers.
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2) Chemical factors
i) Local anaesthetic drugs:
e.g. cocaine, novocaine and
xylocaine→block Na
channels→↓the membrane
permeability to Na
ions→↓depolarization.
ii) Ca ions:
↑Ca ions → block Na channels →↓es the membrane
permeability to Na ions and ↑es the threshold of
stimulation→↓es the excitability of nerve fibers.
↓Ca ions → open Na channels →↑es the membrane
permeability to Na ions and ↓es the threshold of
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stimulation→ ↑es the excitability of nerve fibers.
2) Chemical factors
iii) Na ions :
• ↑Na ions→↑es excitability by facilitating the process of
depolarization.
• ↓ Na ions→↓es excitability by delaying the process of
depolarization.
iv) K ions:
•↑ K ions in the extracellular fluid→↓es RMP →↑ es
excitability.
•↓K ions in the extracellular fluid →↑es diffusion of K ions
from inside to outside the nerve fibers producing
hyperpolarization→↓es excitability.
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2) Chemical factors
v) O2 lack and CO2 excess decreases excitability
vi) H ion concentration:
•Alkalinity→↓es free Ca→↑es excitability.
•Acidity →↑es free Ca→↓es excitability.
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3) Electrical factors
Electrotonus means the electrical and excitability changes
which occur in the nerve membrane due to its stimulation
by a constant galvanic current of subthreshold intensity
2) Types:
a) Anelectrotonus: → means the changes which occur
at the region of the anode
•The RMP increases by addition of more +ve charges on
the outer surface of the membrane i.e. localized area of
hyperpolarization.
•It is associated with ↓ed excitability. So, stronger stimuli
(more than threshold) are needed to excite the nerve
fibers.
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3) Electrical factors
2) Types:
a) Anelectrotonus:
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3) Electrical factors
2) Types:
b) Catelectrotonus: → means the changes which occur
at the region of the cathode
•The RMP decreases by addition of negative charges to
the outer surface of the membrane i.e. localized area of
depolarization.
•It is associated with ↑ed excitability.
•So, weaker stimuli (subthreshold( can excite the nerve
fibers.
•Strong anelectrotonus can abolish completely the
excitability and can cause nerve block
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3) Electrical factors
2) Types:
b) Catelectrotonus:
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Nerve block
i)def.
It means failure of conduction of nerve impulses from one
place to another
ii) Causes and types:
a) Physical causes:
Thermal:
- Severe cooling.
Mechanical: - Deep pressure.
- Injury or crushing of the nerve fibers.
b) Chemical causes (membrane stabilizers):
•Local anaesthetic drugs
•Increased Ca ions
•Decreased Na ions
•Decreased K ions
C )Electrical causes→ strong anelectrotonus
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NEURO-MUSCULAR JUNCTION
(NMJ)
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NEURO-MUSCULAR JUNCTION
The skeletal ms fibers are innervated by thick myelinated nerve
fibers (A α) that originate from the motor neurons located in
either the spinal cord or the brain stem.
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Neuromuscular Junction
•Near the surface of the ms, the motor nerve loses its myelin
sheath and divided into many branches.
•Each branch is covered only by the cytoplasm and cell
membrane of Schwann cells which fuses with the ms membrane
or sarcolemma
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Neuromuscular Junction
The NMJ consists of axon terminals, motor end plate, and
synaptic cleft
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Neuromuscular Transmission
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Neuromuscular Transmission
Miniature end plate potential:
•It is a weak depolarization of the MEP caused by
spontaneous rupture of few vesicles at rest.
•It is localized at the MEP and disappears rapidly and
occurs on an average of about one per second.
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Factors affecting NM transmission
1) One way conduction
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Factors affecting NM transmission
2) Synaptic delay
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Factors affecting NM transmission
3) NMT shows fatigue:
•Fatigue of NMT due to depletion of the Ach vesicles due
to rapid repeated stimulation of the motor nerve.
•O2 lack facilitates the onset of fatigue because it ↓es
the metabolic reactions needed to reform Ach
4) Effect of ions:
5) Effect of drugs:
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Myasthenia Gravis
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THANKS