Motor neuron and skeletal muscle structure: WACE Physical Education Studies Unit 3
“Describe the structure of the motor neuron and skeletal muscle fibre, and explain the role of each component in stimulating a contraction”
A motor neuron carries the nerve impulse from the spinal cord to muscle through its dendrites, cell body, axon and motor end plate, releasing acetylcholine at the neuromuscular junction. The signal then spreads through the muscle fibre via the sarcolemma and T tubules to the myofibrils and sarcomeres, where the contraction is actually produced.
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.
A motor unit is a single motor neuron and all the muscle fibres it controls. When the neuron fires, every fibre in that unit contracts together, obeying the all or none law. Muscles needing fine control (the eye, the hand) have small motor units, while large power muscles such as the quadriceps have huge motor units. This is why recruiting more or larger motor units increases force.
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.
These three sound alike and students swap them under exam pressure. The sarcolemma is the outer membrane that carries the signal in. The sarcoplasm is the internal fluid. The sarcoplasmic reticulum is the internal store that releases calcium to start the contraction. Learn them as membrane, fluid and calcium store so you can place each one correctly when describing how a signal reaches the myofibrils.
Step 1: start in the motor neuron
Begin at the cell body, then the axon, naming the myelin sheath and nodes of Ranvier that speed conduction toward the axon terminals at the motor end plate.
Step 2: cross the neuromuscular junction
State that the terminal releases acetylcholine into the synaptic cleft, where it binds receptors on the muscle membrane. This is the electrical-to-chemical-to-electrical handover.
Step 3: spread the signal into the fibre
The change travels along the sarcolemma and down the T tubules, carrying the signal deep into the cell rather than just across its surface.
Step 4: release calcium and reach the sarcomere
The T tubules trigger the sarcoplasmic reticulum to release calcium into the sarcoplasm, which reaches the myofibrils and their sarcomeres, where actin and myosin interact to shorten the unit. Attaching a one-line function to each structure in this order is what earns the marks.
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.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2021-style6 marksDescribe the pathway of a nerve impulse from the motor neuron to the myofibrils of a skeletal muscle fibre, naming the structures involved and stating the role of each.Show worked answer →
A 6 mark describe answer needs the structures in order, each with its function.
- Along the motor neuron
- The impulse passes from the cell body along the axon (insulated by the myelin sheath, with nodes of Ranvier speeding conduction) to the axon terminals at the motor end plate.
- Neuromuscular junction
- At the junction the terminal releases acetylcholine into the synaptic cleft; it binds to receptors on the muscle membrane, converting the nerve signal into an electrical change in the muscle.
- Across the fibre membrane
- The signal spreads along the sarcolemma and inward through the T tubules.
- To the calcium store
- The T tubules signal the sarcoplasmic reticulum to release calcium ions into the sarcoplasm.
- To the contractile units
- Calcium reaches the myofibrils and their sarcomeres, where it allows the actin and myosin to interact and the sarcomere to shorten.
Markers reward the correct order, acetylcholine at the junction, the sarcolemma and T tubules carrying the signal in, and the sarcoplasmic reticulum releasing calcium to the myofibrils.
WACE 2023-style4 marksExplain what is meant by a motor unit and how the size and number of motor units recruited affects the force a muscle can produce.Show worked answer →
A 4 mark explain answer needs the definition plus the force relationship.
- Definition
- A motor unit is a single motor neuron and all the muscle fibres it stimulates; when the neuron fires, all those fibres contract together (the all-or-none law).
- Unit size and control
- Muscles needing fine control (such as those of the eye or hand) have small motor units; large power muscles (such as the quadriceps) have large motor units controlling many fibres.
- Force
- Greater force is produced by recruiting more motor units and by recruiting larger motor units, so more fibres contract simultaneously. Increasing the firing rate also increases force.
Markers reward the definition, the all-or-none response of a unit, and force rising with the number and size of units recruited.