Neuromuscular junction

Each muscle fibre receives a nerve branch from the motor cell body in the anterior horn of the spinal cord or cranial nerve motor nuclei.

When a nerve fibre reaches the muscle it loses its myelin sheath and its neurilemma then merges with the sarcolemma under which the axon spreads out to form the motor endplate. The axon fibre with its endings and muscle fibres it supplies is called the MOTOR UNIT. The number of muscle fibres in a motor unit varies: in the eye muscles it is small (5-10), whereas in the limb muscles the number is large (in the gastrocnemius about 1800). Each motor unit contains only one type of muscle fibre, i.e. type I or type II. The neuromuscular junction is the point at which neuromuscular transmission is effected. The motor endplate is separated from the sarcoplasm by the synaptic cleft.

Physiology

Muscle contraction results from the following: -

1) A depolarisation wave arrives at the axon terminus and opens voltage sensitive Ca2 ' channels

2) Ca2+ influx results in acetylcholine release from ihe synaptic vesicles into the synaptic cleft.

3) Acetylcholine attaches to end-plate receptors with Na + entry into muscle. Post synaptic depolarisation initiates an action potential that spreads along the sarcolemmal membrane.

4) Release of Ca ''' from the sarcoplasmic reticulum and the interaction of actin and myosin result in muscle contraction.

The enzyme cholinesterase, found in high concentration at motor endplates, destroys acetylcholine so that normally a single nerve impulse only gives rise to a single muscle contraction. Biochemistry

Muscle contraction requires energy in the form of adenosine triphosphate (ATP). This is produced by breakdown of glucose and glycogen to pyruvate (glycolysis) and breakdown of lipid by beta-oxidation, both through the Krebs cycle.

Schwann cell

Neurilemma

Sarcolemma

Schwann cell

Neurilemma

Sarcolemma

Sarcoplasmic reticulum

Synaptic cleft

Synaptic vesicles

Mitochondria

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