Neuromuscular junction

                  Neuro muscular junction

Neuromuscular junction

junction between terminal branch of the nerve fiber and muscle fiber.

¨  The neuromuscular junction is the point of synaptic contact between axon terminal of the motor neuron and the muscle fiber it control

¨  Action potential in the motor neuron cause acetylcholine release into the NMJ

¨  Muscle contraction follows the delivery of Ach to the muscle fiber

¨  A single motor unit consists of a motor neuron and all of the muscles fibers it controls

¨  The smallest part of the muscle that can contract individually is known as motor unit

Structure of neuromuscular junction

Ø  Synaptic tough →axon terminal

Ø  Synaptic cleft →is the space between axon terminal and sarcolemma

Ø  Subneural cleft →fold present in muscle fiber

Ø  Synaptic vesicle →presence in axon terminal and its function is to store Ach

Neuromuscular transmission

¨  Defined as the transfer of information from motor nerve ending to the muscle fiber through NMJ

¨  Transmission takes place by following  steps:

                        1. release of Ach

                        2.action of Ach

                        3.development of Epp

                        4.destruction of Ach

1.Release of acetylcholine

¨  When AP reaches to axon terminal, it increases permeability of presynaptic membrane for ca⁺⁺ by opening of voltage gated calcium channel which is +nce in membrane of axon terminal

¨  Calcium ion enters to axon terminal from ECF

¨  Excess of ca⁺⁺ releases the acetylcholine from syanaptic vesicle through exocytosis as “quanta” or “packets”

¨  The Ach diffuses across through the presynaptic membrane & enter the synaptic cleft

2.Action of acetylcholine

¨  Ach binds with nicotinic receptor +nce in the post synaptic membrane

¨  It opens ligand gated channels for sodium

¨  Sodium ion enter from ECF but potassium cannot release out from muscle because potassium & ligand gated channel both have negative charge, so there is excess of sodium ion which produce an electrical potential called Endplate potential

3.End plate potential

¨  EPP is the change in the resting membrane potential when an impulse reaches the NMJ. The resting membrane potential at the NMJ is -90 mv. When sodium ions enters inside, slightly depolarization occurs up to -60 mv which is called EPP

¨  The EPP is not propagative. But it causes the development of APin the muscle fiber

4.Fate of acetylcholine

¨  Ach is released into the synaptic cleft is destroyed very quickly.

¨  With in 1 millisecond after the release into the synaptic cleft, it is destroyed by the acetylcholinesterase.

¨  However, the Ach line so potent, that even this short duration of 1 millisecond is sufficient to excite the muscle fiber

¨  Rapid destruction of Ach prevents the repeat excitation of the muscle fiber &allow the muscle to relax

Myasthenia gravis

¨  Antibodies to Ach receptor at NMJ

¨  This block ach receptors of NMJ

¨  Destroys Ach receptors & triggers endocytosis of Ach receptor.

¨  Neuromuscular transmission is blocked

¨  Easy fatigability on exercise

¨  May lead to paralysis

Lambert-Eaton syndrome

¨  Condition that resembles the myasthenia gravis

¨  Rare condition

¨  Antibodies to the voltage-gated calcium ion channels on nerves endings

¨  Impaired release of Ach at NMJ

¨  Proximal muscles of lower extremities are affected