University:
Bradley University
Course:
BIO 466 | Human Anatomy
Academic year:

2021
Views:

327

Pages:

4
Author:

Maverick Martin

Mechanism of Nerve impulse conduction There are 2 types of impulse conduction depending on types of the nerve fiber. 1- Unmyelinated nerve fibers (Current sink): - The impulse is propagated as a wave of A.P i.e. reversal of polarity. - A local circular current flow between activated point and neighboring inactive point. - The +ve charges move towards the -ve one produced by A.P (area of sink) leading to decreased polarity in the inactive areas (electrotonic depolarization) producing local response which when reach the firing level initiate A.P which depolarizes the membrane. The event is repeated till the end. So, the nerve impulse is a self-propagated depolarization fallowed by repolarization. 2- Myelinated nerve (saltatory conduction): - The myelin sheath is interrupted by node of Ranvier. - It is an insulator i.e. negligible conduction occurs through it. - The current flow in the node of Ranvier is more due to its Na+ high permeability. - The circular current occurs by jumping of +ve charge from the inactive area to the active one (node) leading to electrotonic response which when reaching the firing level, a propagated action potentials are produced. The jumping of depolarization is a rapid process has a rapid rate of conduction (50 time faster than current sink) and preserve energy (need smaller energy than unmyelinated fibers). During impulse transmission it occurs more rapidly. Unmyelinated nerves Not present Myelin sheath Myelinated nerves Present Loca7on of voltage gate Na⁺ and K⁺ channels Allover the membrane Only at nodes of Ranvier Propaga7on of ac7on poten7al Sweeping conduc7on Salutatory conduc7on Mechanism of neuromuscular transmission: - Stimulation of a nerve fiber leads to production of impulse. - Arrival of nerve impulse to the nerve terminals leads to opening of voltage gated Ca++ channels that allow movement of Ca++ from E.C.F to the nerve terminal followed by rupture of the vesicle and release of A..Ch. in the synaptic cleft. - The released A. Ch. Cross the synaptic cleft and combine with the nicotinic receptor in the muscle membrane leading to opening of ac. ch. channels (ligand-gated channels) which causes increased permeability to Na+ which in turn causes increased Na+ influx and strong depolarization at the M.E.P (motor end plate potential) which on reaching the firing level causes action potential in the muscle. * A. Ch. once released into the synaptic cleft, it continue to activate its receptors as long as it persist. However, it is rapidly hydrolyzed (removed) by cholinesterase enzyme, so no further stimulation of the muscle. So that, single stimulation leads to single excitation. Properties of neuromuscular transmission 1- Unidirectional: means one way direction i.e. the impulse pass from the nerve to the muscle not in the opposite way. 2- Delay: it represents the time needed for neuromuscular transmission. It is about 0.5 - 0.7 millisecond. This delay is needed for; a- Release of A. Ch. b- Passage to of ac. ch to the synaptic cleft and the muscle. c- Binding of A. Ch to its receptors on the membrane of the muscle fibers. d- Increased Na+ permeability and increased Na+ influx. 3- Fatigue: MEP is the first site of the neuromuscular system undergoing fatigue. - It occurs rapidly due to exhaustion (depletion) of the chemical transmitter ac. ch. as a result of repeated stimulation.