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بسم الله الرحمن الرحيم. ﴿و ما أوتيتم من العلم إلا قليلا﴾. صدق الله العظيم الاسراء اية 58. Nerve and Muscle. 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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بسم الله الرحمن الرحيم ﴿و ما أوتيتم من العلم إلا قليلا﴾ صدق الله العظيم الاسراء اية 58
Nerve and Muscle 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
I- Electrical changes • In the form of a spike potential and after potential.
II- Excitability changes • During conduction of a nerve impulse, the excitability of the nerve fibers passes in the following phases: • Temporal rise of excitability: • Absolute refractory period (ARP): • Relative refractory period (RRP): • Supernormal phase of excitability: • Subnormal phase of excitability:
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; • Initial heat: due to generation and propagation of nerve impulse (each impulse ↑ 10-6) • Delayed heat: 30 times initial heat and remains for 30 minutes • due to reactions needed to reform ATP
Factors affecting the excitability and conductivity of the nerve fibers
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.
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 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.
2) Chemical factors v)O2 lack and CO2 excessdecreases excitability • vi) H ion concentration: • Alkalinity→↓es free Ca→↑es excitability. • Acidity →↑es free Ca→↓es excitability.
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.
3) Electrical factors • 2) Types: • a) Anelectrotonus:
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
3) Electrical factors • 2) Types: • b) Catelectrotonus:
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
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.
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
Neuromuscular Junction The NMJ consists of axon terminals, motor end plate, and synaptic cleft
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.
Factors affecting NM transmission 1) One way conduction
Factors affecting NM transmission 2) Synaptic delay
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:
