Nerve and Neuromuscular Control

A neuron has a cell body, branching dendrites that receive signals, and a single axon that carries the signal away. Many axons are wrapped in a myelin sheath, formed in the peripheral nervous system (PNS) by Schwann cells and interrupted at intervals by gaps called the nodes of Ranvier. In a peripheral nerve the axons are grouped into fascicles and wrapped by connective tissue: the endoneurium around each fibre, the perineurium around each fascicle, and the epineurium around the whole nerve. Nerves may be motor, sensory or, most commonly, mixed.

Signals travel as action potentials, a brief reversal of the membrane voltage as sodium ions flow into the axon and then potassium ions flow out to restore the resting state. In a myelinated axon the impulse jumps from node to node, saltatory conduction, which is much faster than conduction along an unmyelinated fibre, and the largest, most heavily myelinated fibres, such as the motor and proprioceptive fibres, conduct fastest. Conduction speed therefore depends on both the diameter of the fibre and the thickness of its myelin, and different fibre types, from large fast motor fibres to fine slow pain fibres, are specialised for different jobs. Where a motor nerve meets muscle, at the neuromuscular junction (NMJ), the nerve releases acetylcholine, which triggers the muscle action potential, and a motor neuron with the fibres it supplies forms a motor unit.

Control of movement depends on constant sensory feedback. Proprioception, the sense of joint position and movement, comes mainly from muscle spindles, which detect muscle length and the speed of stretch, together with Golgi tendon organs, which detect tension, and receptors within the joints. This lets the central nervous system (CNS) know where the limbs are and adjust accordingly. Some responses are reflex: in the stretch reflex, stretching a muscle excites its spindle, whose sensory (Ia) fibre signals directly across a single synapse to an alpha motor neuron in the spinal cord, making the muscle contract. Only this central Ia afferent to alpha motor neuron connection is monosynaptic; relaxation of the opposing muscle is arranged separately through an inhibitory interneuron. The Golgi tendon organs sense the force a muscle is generating and feed this back to the spinal cord, where the effect is context-dependent, able to facilitate or damp activity rather than simply switching contraction off at high tension.

Voluntary movement is organised in a hierarchy. The motor cortex plans and drives movement through the descending corticospinal tract, the cerebellum coordinates and fine-tunes it, and the spinal cord handles reflexes and patterns, the whole system using both feedforward commands and feedback correction. This blend of fast reflexes, continuous sensory feedback and planned commands is what makes skilled, stable movement possible.

The nervous system is trainable. Much of the early improvement in strength and skill comes from better neuromuscular control, as the brain and spinal cord learn to recruit and coordinate motor units more efficiently, and proprioceptive and balance training sharpens the feedback that protects a joint. When a peripheral nerve is injured, the effect depends on severity, described by the Seddon classification. In neurapraxia the axon is intact and only conduction is blocked, and recovery is good. In axonotmesis the axon is disrupted while the surrounding connective tissue framework is sufficiently preserved to guide regrowth, so the nerve beyond the injury undergoes Wallerian degeneration and then regrows slowly along that framework, at roughly a millimetre a day, though the rate varies and recovery depends on the distance to the muscle and whether that muscle is still viable. In neurotmesis the nerve is fully divided and recovers poorly without surgical repair. Even when an axon does regrow, the reconnection is often imperfect, which is part of why recovery from a significant nerve injury can be slow and incomplete.

This physiology shapes everyday practice. A peripheral nerve injury or entrapment causes numbness, tingling or weakness in the territory of that nerve, such as foot drop from a common peroneal nerve problem. After a joint injury, blunted proprioception often shows as a joint that feels unstable or gives way, particularly the ankle or the knee after an anterior cruciate ligament injury, and neuromuscular and balance work is used to improve sensorimotor control, with the strongest evidence for reducing recurrence after a lateral ankle sprain. Some features are red flags: sudden focal weakness with other new neurological signs, rapidly progressive or symmetrical weakness, or saddle numbness with bladder or bowel disturbance all need urgent assessment rather than rehabilitation.

• •A neuron has a cell body, dendrites and an axon, often myelinated with nodes of Ranvier.
• •Saltatory conduction jumps between nodes and is faster than unmyelinated conduction.
• •Muscle spindles sense length and stretch; Golgi tendon organs sense tension.
• •The stretch reflex is a fast, monosynaptic spindle-to-motor-neuron loop.
• •Joint injury impairs proprioception, which neuromuscular training helps improve sensorimotor control.
• •Nerve injury grades are neurapraxia, axonotmesis and neurotmesis, from conduction block to a divided nerve.