Physiology of Peripheral Nerves-1 (KING SALMAN INTERNATIONAL UNIVERSITY) PDF

Physiology of peripheral nerves-1 By :Prof. Sherif Wagih Mansour 2024/25  Types of neurons According to the processes: (shape) Unipolar: e.g. cells in dorsal root ganglia. Bipolar: e.g. bipolar cells of the retina. Multipolar: e.g. cells of cerebral cortex....

Physiology of peripheral nerves-1 By :Prof. Sherif Wagih Mansour 2024/25  Types of neurons According to the processes: (shape) Unipolar: e.g. cells in dorsal root ganglia. Bipolar: e.g. bipolar cells of the retina. Multipolar: e.g. cells of cerebral cortex.  According to function: 1) Sensory (Afferent) which carry sensations from organs to the CNS. 2) Motor (Efferent) which arise from CNS to carry orders to organs. According to myelination: 1) Myelinated nerve fiber: e.g. preganglionic neuron. 2) Non-myelinated nerve fiber: e.g. postganglionic neuron. The myelin sheath  The resting membrane potential (RMP) -Definition: it is the potential difference between inside and outside the cell membrane with inside relatively negative to outside. It ranges from –9 to –100 mvolt e.g. in RBCs (- 30) in nerve (-70 in medium sized nerve and –90 in large nerve) and in the muscle –90 mvolt. -Causes of RMP: It is due to Un-equal distribution of ions across the cell membrane [outside: Na+(142mEq./L), Cl-(103mEq./L), HCO 3-(28mEq./L) and inside: K+(140mEq./L)& protein-(40mEq./L)) with more cations outside and more anions inside. -This Unequal distribution is caused by three factors: 1- Selective permeability of the cell membrane K+ ˃ Na+ 50-100 times 2- Na+ - K+ pump 3/2 active 3-The membrane is impermeable to the intracellular protein anions with large molecular weight → more negative charges inside the cell  Action potential It is a property of excitable cells (i.e., nerve, muscle) that consists of a rapid depolarization, or upstroke, followed by repolarization of the membrane potential. Action potentials have stereotypical size and shape, are propagating, and are all-or-none. - Threshold : is the membrane potential at which the action potential is inevitable. At threshold potential, net inward current becomes larger than net outward current. The resulting depolarization becomes self-sustaining and gives rise to the upstroke of the action potential. If net inward current is less than net outward current, no action potential will occur (i.e., all-or-none response).  Ionic basis of the nerve action potential a. Resting membrane potential - It is approximately −70 mV, inside negative. - is the result of the high resting conductance to K+ > Na+ (100 times). At rest, the Na+ channels are closed and Na+ conductance is low. b. Upstroke (depolarization) of the action potential (1) Inward current depolarizes the membrane potential to threshold. (2) Depolarization causes rapid opening of the activation gates of the Na+ channels, and the Na+ conductance of the membrane promptly increases. (3) The Na+ conductance becomes higher than the K+ conductance. Thus, the rapid depolarization during the upstroke is caused by an inward Na+ current. (4) The overshoot is the brief portion at the peak of the action potential when the membrane potential is positive. ( -65 to +35 c. Repolarization of the action potential (1) Depolarization also closes the inactivation gates of the Na+ channels (but more slowly than it opens the activation gates). Closure of the inactivation gates results in closure of the Na+ channels, and the Na+ conductance returns toward zero. (2) Depolarization slowly opens K+ channels and increases K+ conductance to even higher levels than at rest. (3) The combined effect of closing the Na+ channels and greater opening of the K+ channels makes the K+ conductance higher than the Na+ conductance, and the membrane potential is repolarized. Thus, repolarization is caused by an outward K+ current. d. Undershoot (hyperpolarizing after potential) - The K+ conductance remains higher than at rest for some time after closure of the Na+ channels. During this period, the membrane potential is driven very close to the K+ equilibrium potential.  Refractory periods a. Absolute refractory period It is the period during which another action potential cannot be elicited, no matter how large the stimulus. -It coincides with almost the entire duration of the action potential. Explanation: Recall that the inactivation gates of the Na+ channels are closed when the membrane potential is depolarized. They remain closed until repolarization occurs. No action potential can occur until the inactivation gates open. b. Relative refractory period It begins at the end of the absolute refractory period and continues until the membrane potential returns to the resting level. -An action potential can be elicited during this period only if a larger than usual inward current is provided. Explanation: The K+ conductance is higher than at rest, and the membrane potential is closer to the K+ equilibrium potential and, therefore, farther from threshold; more inward current is required to bring the membrane to threshold.  Propagation of action potentials occurs by the spread of local currents to adjacent areas of membrane, which are then depolarized to threshold and generate action potentials. Conduction velocity is increased by: a. ↑ fiber size. Increasing the diameter of a nerve fiber results in decreased internal resistance; thus, conduction velocity down the nerve is faster. b. Myelination. Myelin acts as an insulator around nerve axons and increases conduction velocity. Myelinated nerves exhibit salutatory (jumping) conduction because action potentials can be generated only at the nodes of Ranvier, where there are gaps in the myelin sheath.  Mechanism of Nerve Impulse Conduction A. In the unmyelinated nerve fibers: During rest, membrane is polarized. (+ve outside). At site of stimulation the membrane is depolarized (-ve outside). Then a local current flow occurs between the depolarized area and surroundings areas: In the inner surface: +ve charges migrate from the point of depolarization to the surrounding sites. In the outer surface: +ve charges migrate from the surrounding sites to point of depolarization.  The results are: Point of stimulation begins to repolarize. The surrounding sites being to depolarize partially till they reach the firing level  action potential This is repeated. So, conduction occurs along the nerve fiber. It is called the (Current sink). The speed of propagation is directly proportional to the diameter of the nerve. B. In the Myelinated nerve fibers The same mechanism as in the unmyelinated But the impulse jump from one node of Ranvier to the other because the myelin is insulator for current - So, it is called (Jumping or Saltatory or Node to node) conduction It is characterized by: 1) The rate of conduction in the myelinated nerve is 50 -100 times faster than in the unmyelinated. 2) It occurs with less energy.