Quick questions on Motor neuron and skeletal muscle structure: WACE Physical Education Studies Unit 3

A motor neuron is a nerve cell specialised to carry impulses away from the central nervous system to skeletal muscle. The dendrites are short branching projections that receive incoming signals. The cell body, or soma, contains the nucleus and integrates the signal. The axon is the long fibre that conducts the impulse toward the muscle, and it is wrapped in a fatty myelin sheath broken by gaps called nodes of Ranvier, which let the impulse jump from node to node and travel faster.

The point where the axon terminal meets the muscle fibre is the neuromuscular junction. When the impulse arrives, the terminal releases the neurotransmitter acetylcholine into the small gap, the synaptic cleft. Acetylcholine binds to receptors on the muscle membrane and triggers an electrical change that spreads into the fibre. This is the handover point: the electrical signal in the nerve becomes a chemical message, which then becomes an electrical signal in the muscle.

A skeletal muscle fibre is a single long muscle cell. Its outer membrane is the sarcolemma, the equivalent of the cell membrane, and it carries the electrical signal inward through invaginations called transverse (T) tubules. Inside is the sarcoplasm, the cell fluid, which holds the sarcoplasmic reticulum, a network that stores and releases calcium ions. Packed lengthways through the fibre are the myofibrils, the contractile threads.

Each sarcomere contains two protein filaments. The thick filament is myosin, with protruding heads called cross bridges. The thin filament is actin, which carries the regulatory proteins tropomyosin and troponin that block or expose the binding sites. The overlap and sliding of these filaments produce the shortening of the sarcomere, which is the contraction itself.