The peripheral nervous system consists of nerve endings, peripheral nerve trunks, plexuses and ganglia which link the CNS with other parts of the body. Most of the neurones in the peripheral nervous system are, therefore, either afferent or efferent with respect to the CNS.
Nerve endings include sensory receptors, which detect changes in the internal and external environments, and efferent endings, which control the contraction of muscles and the activity of secretory glands.
Peripheral nerves consist of spinal and cranial nerves and their branches, and the numerous named nerves to which these give rise.
Plexuses (e.g. the brachial plexus and lumbosacral plexus) are structures in which fibres within certain spinal and cranial nerves are redistributed, without synapse, to form other peripheral nerves.
Peripheral ganglia (e.g. dorsal root ganglia and autonomic ganglia) are structures outside the CNS where some nerve cell bodies are located.
All behaviour depends on 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 (Fig. 3.1).
Figure 3.1 (A,B) Transverse sections through striated muscles showing extrafusal muscle fibres and intrafusal muscle fibres (within muscle spindles).
Extrafusal muscle cells 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 brain stem. The axon of an individual alpha motor neurone typically branches within the target muscle to innervate a number of muscle fibres; the combination of a single motor neurone and the muscle fibres that it innervates is known as a motor unit. Motor units consist of relatively small numbers of muscle fibres in those muscles with which delicate, precise movements can be made, such as the muscles of the hand and the extraocular muscles. In contrast, the motor units of large postural muscles – such as the quadriceps, for example – are comprised of relatively larger numbers of muscle cells. Intrafusal muscle fibres are highly specialised cells that act as sensory receptors. They occur in groups known as muscle spindles. Intrafusal muscle fibres bear sensory endings that signal muscle stretch and tension to the CNS. They also 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 brain stem. Gamma motor neurones function to control the sensitivity of the sensory endings.
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 2–3 years of age, the child develops progressive weakness of the arms and legs with muscular contractures. He is wheelchair-bound by the age of 10 years and dies in youth.
Figure 3.2 Myopathy.
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 (effector) 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.
Afferent nerve endings
The sensory systems, or modalities, are broadly divided into the general and the special senses. The special senses are olfaction, vision, hearing, balance and taste and these are considered elsewhere.
There are three types of general sensory ending:
Exteroceptors. These occur superficially in the skin and respond to nociceptive (painful) stimuli, temperature, touch and pressure.
Interoceptors. These occur in viscera and respond principally to mechanical and chemical stimuli.
Proprioceptors. These occur in muscles, joints and tendons and provide awareness of posture and movement (kinaesthesia).
Structurally, sensory nerve endings may be either unencapsulated or encapsulated (Fig. 3.3). Unencapsulated endings, or free nerve endings, consist of the terminal branches of sensory nerve fibres lying freely in the innervated tissue. These are the most abundant type of sensory ending, occurring widely in the integument and also within muscles, joints, viscera and other structures. In the skin, they mediate thermal and painful sensations. The parent nerve fibres are designated physiologically as group Aδ (III) finely myelinated and group C unmyelinated fibres. These are of relatively small diameter and slow conducting. Merkel endings (discs) are located near the border of the epidermis. These are slowly adapting receptors that respond to touch/pressure. The axons of origin are large and myelinated.
Figure 3.3 Sensory nerve endings in skin.
Encapsulated nerve endings are surrounded by a structural specialisation of non-neural tissue, the combination of nerve and its encapsulation often being referred to as a corpuscle. Meissner’s corpusclesoccur in the dermal papillae of the skin and are especially numerous in the fingertips. They respond with great sensitivity to touch. They are rapidly adapting receptors and are responsible for fine, or discriminative, touch. Pacinian corpuscles occur in skin and in deep tissues, e.g. surrounding joints and in mesentery. The largest are a few mm long and they are associated with group Aα (I) large diameter, myelinated axons. Pacinian corpuscles are rapidly adapting and respond to mechanical distortion, especially vibration. Ruffini endings (corpuscles) are slowly adapting mechanoreceptors occurring in the dermis of the skin.
Within skeletal muscles, intrafusal muscle fibres, which in small groups constitute muscle spindles, act as stretch receptors (Fig. 3.4). There are two types of intrafusal muscle fibre, referred to as nuclear bagand nuclear chainfibres. Intrafusal muscle fibres bear two types of sensory ending, which become activated when the muscle in which they lie is stretched. Annulospiral endings (Fig. 3.5) are associated with fast conducting group Ia afferent fibres and flower-spray endings are associated with slower conducting group II afferents. Muscle spindles are particularly abundant in muscles capable of fine, skilled movements. They are important in kinaesthesia and in the control of muscle tone, posture and movement. Their functional importance in motor control is considered in more detail in Chapter 8.
Figure 3.4 Innervation of intrafusal muscle fibres. For clarity, only an annulospiral ending is shown on the nuclear bag fibre and only a flower-spray ending on the nuclear chain fibre although, in reality, both types of ending are found on both types of intrafusal fibre.
Figure 3.5 An annulospiral nerve ending on intrafusal muscle fibre.
Golgi tendon organs occur in tendons and respond to tension.
Efferent nerve endings
Efferent endings occur in association with muscle and secretory cells. The endings resemble the synapses that occur between neurones. Transmission is by chemical means, depolarisation of the endings causing release of a neurotransmitter that acts upon receptors in the target cell membrane. In striated muscle, both alpha motor neurones (innervating extrafusal muscle fibres) and gamma motor neurones (innervating intrafusal muscle fibres) end upon muscle cells in synaptic specialisations called neuromuscular junctions or motor end-plates (Figs 3.6, 3.7). The neurotransmitter at all neuomuscular junctions in striated muscle is acetylcholine.
Figure 3.6 Motor end-plates on extrafusal muscle fibres in striated muscle. (A) Ranvier’s gold chloride stain showing efferent nerve fibres terminating in neuromuscular junctions (×600); (B)Acetylcholine esterase stain. The neurotransmitter at the neuromuscular junction is acetylcholine. The action of the transmitter is terminated by the enzyme acetylcholine esterase. The brown stain shows the specific localisation of the enzyme at the neuromuscular junction (×500).
Figure 3.7 A gamma motor neurone ending in a motor end-plate on an intrafusal muscle fibre.
The peripheral nervous system consists of nerve endings, peripheral nerves, plexuses and ganglia.
Nerve endings are broadly classified as afferent (sensory) or efferent (motor).
Sensory endings function as mechanoreceptors, thermoreceptors or chemoreceptors.
In terms of location, they can be subdivided into exteroceptors, interoceptors and proprioceptors.
Structurally, they may consist of unencapsulated (free) nerve endings, or encapsulated endings (such as Meissner’s and Pacinian corpuscles).
Skeletal muscles contain muscle spindles, which are composed of intrafusal muscle fibres. These consist of nuclear bag and nuclear chain types. They possess annulospiral and flower-spray sensory endings.
Efferent nerve endings are either motor end-plates in muscle or endings in association with secretory cells (secretomotor).
Alpha motor neurones innervate extrafusal muscle fibres and gamma motor neurones innervate intrafusal muscle fibres.
Disorders of neuromuscular junction
Myasthenia gravis is an immune disorder; it is the most common disorder of the peripheral neuromuscular junction. It causes weakness and fatigue of cranial muscles (e.g. extraocular, facial and bulbar muscles) and limb (especially proximal) muscles. This occurs without muscular wasting, changes in reflexes or sensation (Fig. 3.8). Treatment with drugs that inhibit acetylcholine esterase (anticholinesterases) potentiates neuromuscular transmission, with relief of symptoms. Intravenous edrophonium (an anticholinesterase; Tensilon) is a diagnostic test for the disease and produces rapid, but brief, return of muscular power. The Eaton–Lambert syndrome causes similar fatigue but is a paraneoplastic syndrome and does not respond to anticholinesterases.
Figure 3.8 Myasthenia gravis.
The term peripheral nerve applies to all the nerve trunks and branches that lie outside the CNS. They are the principal route through which the brain and spinal cord communicate with the rest of the body. A typical peripheral nerve consists of numerous nerve fibres, which may be either afferent or efferent with respect to the CNS.
Some peripheral nerve fibres are myelinated; others are unmyelinated. Within peripheral nerves, the nerve fibres are arranged in bundles and surrounded by sheaths of connective tissue (Fig. 3.9). Between individual fibres is a delicate connective tissue known as endoneurium. Bundles of fibres are surrounded by perineurium and the whole nerve is ensheathed by a tough coat known as epineurium. This configuration provides strength and support for the nerve. The cranial and spinal meninges are continuous with the connective tissue sheaths of spinal and cranial nerves. Thus, the dura mater is continuous with the epineurium, while the arachnoid and pia are continuous with the perineurium and endoneurium.
Figure 3.9 The structure of a peripheral nerve.
Degeneration and regeneration
When a peripheral nerve fibre is transected or otherwise seriously damaged, the portion distal to the transection (furthest from the cell body) undergoes degeneration and dies. This is known as anterograde or Wallerian degeneration. The proximal portion of the neurone, which remains attached to the cell body, may, however, survive and eventually undergo recovery or regeneration. The further from the cell body that transection occurs, the more likely it is that the cell body will survive. Initially, it too usually shows degenerative changes, known as retrograde degeneration. This is characterised by dispersal and loss of staining of Nissl granules (chromatolysis), swelling of the cell body and movement of the normally central nucleus to a peripheral location. If the cell recovers, the distal end of the surviving nerve fibre undergoes sprouting. If the two ends of the peripheral nerve are physically aligned (e.g. by surgery following traumatic injury), the new regenerating fibres may enter the endoneurial tubes that have lost their nerve fibres. Continued growth of the new fibres, at a rate of 1–2 mm/day, may eventually lead to reinnervation of the original structure and recovery of function. Many factors influence the degree to which successful reinnervation and functional recovery take place.
The degenerative processes that follow nerve cell injury are essentially similar in the CNS. Sprouting of surviving neurones also occurs in the CNS, but the re-establishment of previous connections does not, unfortunately, take place to any significant extent.
Peripheral sensorimotor neuropathies
Peripheral sensorimotor neuropathies are characterised by muscular weakness and wasting (especially of distal muscles), distal areflexia and a ‘glove and stocking’ distribution of sensory loss (Fig. 3.10). Peripheral neuropathies may be caused by systemic disease, vascular disease, heredo-degenerative disorders, infection, immune disorders and paraneoplastic syndromes.
Figure 3.10 Peripheral sensorimotor neuropathy.
In general, there are two pathological types. Demyelinating neuropathies predominantly damage Schwann cells and myelin sheaths. Axonal neuropathies primarily cause axonal degeneration. Recovery from neuropathy requires remyelination and regeneration of axons.
Distribution of spinal and peripheral nerves
At the roots of the upper and lower limbs are located the brachial plexus (Figs 3.11, 3.12) and the lumbosacral plexus (Fig. 3.13), respectively. Here, the nerve fibres present in spinal nerves become redistributed to form named peripheral nerves, which then run distally to their targets. The distribution of peripheral nerves is, therefore, different from that of spinal nerves.
Figure 3.11 The brachial plexus.
Figure 3.12 Lesion of the brachial plexus (lower cord; C8–T1).
Figure 3.13 The lumbosacral plexus.
Each spinal nerve carries the sensory innervation for a part of the body surface. The area of skin that is supplied by a particular spinal nerve is known as a dermatome. Dermatome maps are given in Figure 3.14. These are only approximate since, in reality, the cutaneous territories of adjacent spinal nerves overlap considerably. There is, however, little overlap where adjacent areas are served by non-contiguous nerves, such boundaries being referred to as axial lines. The cutaneous distribution of important peripheral nerves is also illustrated in Figure 3.14.
Figure 3.14 The cutaneous distribution of spinal nerves (dermatomes) and named peripheral nerves. Axial lines are in bold.
The group of skeletal muscles innervated by a particular spinal nerve is collectively known as a myotome. These muscles are usually functionally related and are responsible for particular patterns of movement. The segmental spinal nerve values of some important movements are shown in Figure 3.15.
Figure 3.15 The segmental innervation of limb movements.
Brachial plexus lesions
In motorcycle accidents, trauma to the shoulder and neck may cause avulsion of the brachial plexus, causing immediate weakness and loss of feeling in one upper limb. Later, the arm wastes and becomes painful.
A tumour of the apex of the lung may infiltrate the lower part of the brachial plexus, producing severe pain in the arm, weakness and wasting of the hand and sensory loss on the inner aspect of the forearm and hand (Pancoast’s syndrome).
Lumbosacral plexus lesions
Malignant disease and surgical procedures for cancer can damage the lumbosacral plexus in its course through the pelvis, causing pain, weakness and wasting of the muscles and numbness of the leg(s), with bladder and bowel incontinence.
Compression and entrapment neuropathies
Peripheral nerves are vulnerable to extrinsic compression (e.g. excessive pressure on a recumbent limb) and to chronic entrapment by normal or diseased anatomical structures adjacent to them. The most common examples are radial nerve compression in the spiral groove of the humerus, and chronic entrapment of the ulnar nerve at the elbow and of the median nerve at the wrist (carpal tunnel syndrome) in the upper limb. Entrapment of the lateral cutaneous nerve of the thigh and compression of the common peroneal nerve at the head of the fibula occur in the lower limb (Fig. 3.16).
Figure 3.16 Sensory and motor deficits in peripheral nerve lesions.
Many of the nerve fibres in cranial and spinal nerves become rearranged as they course peripherally, passing through the brachial or lumbosacral plexuses, to emerge as named peripheral nerves.
Peripheral nerves consist of variable numbers of bundles, or fascicles, of nerve fibres.
The nerve fibres are ensheathed in three connective tissue coverings: endoneurium, perineurium and epineurium.
The endoneurial tubes within which individual axons lie are important for successful regeneration and reinnervation by nerves following nerve injury.
The area of skin supplied by a spinal nerve is called a dermatome.
The group of muscles innervated by a spinal nerve is called a myotome.