Neuromuscular Junctions

NEURO MUSCULAR JUNCTIONS It is the junction between the motor nerve and skeletal muscle fiber. STRUCTURE The axon of nerve fiber losses its myelin sheath and divide into terminal buttons that come in intense contact with a specialized part of muscle cytoplasm known as motor end plate One nerve fiber ends on one motor end plate. Electron microscopy 1. Terminal nerve endings contain minute vesicles, synaptic vesicles containing chemical transmitter acetylcholine responsible for synaptic transmission. 2. The muscle membrane of motor end plate is thrown into folds, palisades that increase surface area for the neurotransmission ton act. 3. the nerve membrane is called presynaptic membrane and muscle membrane is called post synaptic membrane 4. Space between two pre and post synaptic membrane is called synaptic cleft, 50-100 nm wide, filled with CSF. 5. Nicotinic aCh receptors are present on post synaptic membrane that are stimulated by both nicotine and acetylcholine and inhibited by curare. 6. Enzyme acetylcholine esterase is present in postsynaptic cleft in high concentrations that destroys acetylcholine.

SYNTHESIS OF ACETYLCHOLINE Free choline+ acetyl choline enzyme A + ATP + glucose (present in mitochondria) ↓ Enzyme choline acetyltransferase ↓ Acetylcholine ↓ Acetylcholine esterase ↓ Choline + acetate

Sequence of events during transmission of nerve impulse 1. Nerve impulse reaches the presynaptic nerve endings 2. Acetylcholine is released in synaptic cleft. 3. Acetylcholine diffuses rapidly to the post synaptic membrane, motor end plate. 4. Acetylcholine binds to nicotinic acetylcholine receptors and increases the permeability of motor end plate to sodium ions, potassium, and calcium. 5. Depolarization of post synaptic membrane and generation of local action potential, end plate potential. 6. The spike potential sets a propagated muscle action potential that travels in both the directions along muscle membranes that leads to muscle contraction. IMPORTANT POINTS • Motor end plates Contains approximately 50 million nicotinic acetylcholine receptors

• A propagated nerve impulse causes release of approximately 106 molecules of acetylcholine at a single motor nerve ending. • Blockage of neuromuscular junction Through contraction, muscle performs three important functions: 1. Motion 2. Maintenance 3. Heat production All movements such as walking, running, grasping a pencil etc. rely on the integrated functioning of the bones, joints and skeletal muscles attached to the bones. In addition to the movement function, muscular system also enables the body to maintain posture. The contraction of skeletal muscle holds the body in stationary position, such as standing and sitting. Skeletal muscle produces most of the heat generated in the body and is thereby important in maintaining normal body temperature.