Peripheral nervous system

Muscle

All behaviour depends upon the ability to control the activity of skeletal muscles, which maintain posture and permit movement. Such control is subserved by a rich innervation of muscle with both motor and sensory neurones. Individual muscle cells (fibres) run parallel to the main axis of the muscle and fall into two main functional groups: namely extrafusal and intrafusal muscle fibres ( Fig. 3.1 ).

Extrafusal muscle fibres are by far the more numerous, constituting the bulk of the muscle and conferring its contractile strength. Extrafusal muscle fibres are innervated by alpha motor neurones , the cell bodies of which lie in the ventral horn of the spinal cord grey matter and in the motor cranial nerve nuclei of the brainstem. The axon of an individual alpha motor neurone typically branches within the target muscle to innervate a number of muscle fibres and these fibres contract simultaneously when activated by the motor neurone. The combination of a single motor neurone and the muscle fibres that it innervates is known as a motor unit . In those muscles capable of controlling delicate, precise movements, such as the muscles of the hand, the muscles of facial expression and the extraocular muscles, individual motor units are comprised of relatively small numbers of muscle fibres. In contrast, the motor units of large postural muscles such as the quadriceps, for example, are comprised of relatively large numbers of muscle fibres.

Intrafusal muscle fibres are specialised muscle cells that act as sensory receptors. They occur in groups known as muscle spindles , interspersed amongst the extrafusal fibres ( Fig. 3.1 ). Intrafusal muscle fibres bear sensory endings that signal muscle stretch and tension to the CNS. Their function is fundamental to the monosynaptic stretch reflex and the control of muscle tone ( pp. 75–76 ), to the coordination of movement by the cerebellum ( p. 121 ) and to the conscious perception and control of movement of the body in space (praxis). Intrafusal muscle fibres receive a motor innervation from gamma motor neurones , whose cells of origin, like those of alpha motor neurones, lie in the ventral horn of the spinal cord and the motor cranial nerve nuclei of the brainstem. Gamma motor neurones function to control the sensitivity of the sensory endings on intrafusal muscle fibres. Various disorders of the CNS can affect gamma motor neurone activity and, thus, cause abnormalities of the stretch reflex and muscle tone ( pp. 75–76 ).

Myopathies

• Myopathy is characterised by weakness and muscle wasting (of facial, bulbar and proximal limb muscles) with preservation of tendon reflexes and sensation ( Fig. 3.2 ).

• Polymyositis is an immune disorder of muscle causing a painful or painless myopathy. When it occurs in the elderly, there is often a primary neoplasm elsewhere (paraneoplastic syndrome). In children, the skin is also inflamed, causing a rash. This is referred to as dermatomyositis.

• Duchenne muscular dystrophy is an inherited degenerative disorder of male children (X-linked inheritance). After 2 to 3 years of age, the child develops progressive weakness of the arms and legs with muscular contractures, is wheelchair-bound by the age of 10 years and dies in youth.

Nerve endings

There are various overlapping conventions for the classification of nerve endings. Overall, they may be classified as either afferent or efferent. Afferent nerve endings respond to mechanical, thermal or chemical stimulation (mechanoreceptors, thermoreceptors or chemoreceptors, respectively). The nerve fibres to which they belong conduct action potentials to the CNS. If the afferent information reaches a conscious level, then the pathway is termed sensory. Efferent nerve endings innervate muscle or secretory cells and, under control from the CNS, influence muscular contraction or cellular secretion. Nerve endings that induce movement are called motor; those that induce secretion are sometimes called secretomotor.