The contraction of skeletal muscle is under the control of the somatic nervous system which is part of the efferent (motor) division of the nervous system.
The nerve cells whose axons innervate skeletal muscle fibres are known as motoneurones. Their cell bodies are located either in the brainstem or in the anterior horns of the spinal cord. The axons of motoneurons are coated in a fatty material known as a myelin sheath, which is formed by specialised cells called Schwann cells. Motoneurons are the largest-diameter axons in the body. This allows them to conduct electrical signals (action potentials) at high velocities from the central nervous system to skeletal muscle fibres with minimum delay.
The myelin sheath is formed during the early years of life, which partly explains why children need time to develop coordinated movement patterns. Individuals affected by certain neurological disorders, such as multiple sclerosis, experience degeneration of the myelin sheath and subsequent loss of coordination.The myelin sheath ends near the surface of the muscle fibre, and the axon divides into a number of short 'finger-like' processes that lie embedded in grooves on the surface of the muscle fibre. The point of contact between axon terminals and the sarcolemma is referred to as the motor end plate. The junction between axon terminals and the motor endplate is known as the neuromuscular junction. In most cases each muscle fibre only receives a contact from one motoneurone. However, each motoneurone (as a consequence of its branching axon) can form a junction with more than one muscle fibre.
| Muscle | Function | Motor Unit Size |
|---|---|---|
| Tensor tympani | Movement of inner ear ossicles | 1 |
| Extraocular muscles | eye movement | 10 |
| Intrinsic hand muscles | finger movement | 100 |
| Tibialis anterior | ankle flexor | 600 |
A single motoneurone innervates many muscle fibres, but each muscle fibre is controlled by an axon terminal from only one motorneurone. One motoneurone together with all the muscle fibres it innervates is referred to as a motor unit.
Motor unit size varies quite significantly from muscle to muscle with the size being typically small in muscles involved in fine or precise movements and very large in muscles producing large forces requiring less precision.
Some examples illustrating the variability in motor unit size in the human skeletal system are given in the table.
The axon terminals are insulated from the surrounding medium by Schwann cells and the terminals themselves are full of small synaptic vesicles containing the neurotransmitter acetylcholine (Ach) as well as a large number of mitochondria.
The membrane of the axon terminal also contains a large number of voltage-gated Ca2+ channels which play an integral role in neuromuscular transmission. The axon terminal is separated from the sarcolemma by a gap of around 60 nm which contains large quantities of enzymes known as acetylcholinesterases which are responsible for the degradation of Ach. Choline molecules released in this process are then transported back into the axon terminals, where they are reused to form new ACh. This recycling of choline allows neuromuscular transmission of action potentials to continue.
Adjacent to the axon terminal, the sarcolemma is highly folded which has the effect of increasing its surface area. Embedded in this membrane are nicotinic acetylcholine receptors which as we will see shortly are critical for effective neuromuscular transmission.
Most of these features are summarised in the diagram opposite.