How is a motor neuron and a skeletal muscle fibre structured to receive a signal and produce a contraction?
Describe the structure of the motor neuron and skeletal muscle fibre, and explain the role of each component in stimulating a contraction
A focused answer to the WACE Year 12 Physical Education Studies Unit 3 content on the structure of the motor neuron and skeletal muscle fibre. The dendrites, cell body, axon and motor end plate, plus the sarcolemma, sarcoplasm, myofibrils and sarcomere, and how each part carries a signal toward contraction.
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What this dot point is asking
WACE wants you to label the parts of a motor neuron and a skeletal muscle fibre and, more importantly, to explain what each part does in the chain that turns a decision to move into an actual contraction. Marks come from linking structure to function, not from a bare list.
Structure of the motor neuron
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. At its end the axon branches into terminals that finish at the motor end plate.
The neuromuscular junction
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.
Structure of the skeletal muscle fibre
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 myofibril is divided into repeating units called sarcomeres, the functional contractile unit, bounded by Z lines.
Inside the sarcomere
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. You do not need the full sliding filament mechanism here, only to know which structures house it.
How this maps to the exam
Expect a labelled diagram or a description question. State each structure in the order the signal travels, from dendrites through to the sarcomere, and attach a one line function to each. Examiners reward the logical sequence because it proves you understand the pathway, not just the vocabulary.