Myelination

Myelination is a process whereby individual axons are wrapped in a sheath of lipid called myelin. The myelin sheath serves as a protective and insulating coating for the axon, and enhances the speed of conduction of electrical impulses along it.

In the PNS, myelination is by cells called Schwann cells: it is achieved by a Schwann cell curling an extension of its cell membrane around the shaft of the axon in a spiral manner (Fig. 1.3A). As the Schwann cell extension wraps around the axon, the cytoplasm in it is squeezed out until only a double layer of the cell membrane of the Schwann cell remains spiralling around the axon (Fig. 1.3A). In this way the myelin sheath is formed by the lipid and protein of the cell membrane of the Schwann cell that surrounds the axon. Peripheral to the myelin sheath, the axon is surrounded by the cytoplasm of the Schwann cell, with the outermost cell membrane of the Schwann cell acting as a second membrane to the axon; this forms the neurolemma (Fig. 1.3A).

Figure 1.3 (A) Myelinated axon in the peripheral nervous system, (B) relationship between unmyelinated axons and Schwann cells.

Along its length, a given axon is ensheathed in series by a large number of Schwann cells, but each Schwann cell is related only to a single axon (Fig. 1.3A). Junction points where the myelin sheath of one Schwann cell ends and the sheath of the next begins are known as Nodes of Ranvier. An axon together with the myelin sheath and the Schwann cells that surround it is referred to as a nerve fibre.

While many axons in the PNS are myelinated a large number remain unmyelinated. Instead of being enveloped by a tightly spiralling sheath of myelin, they run embedded in invaginations of Schwann cell membranes, so that a single Schwann cell may envelop several axons (Fig. 1.3B). Unmyelinated axons are afforded less physical protection than myelinated ones, but the foremost difference between them is that unmyelinated axons conduct impulses at much slower velocities.

In the CNS, myelination is performed by specialized cells called oligodendrocytes. The process is basically similar to that in the PNS, except that a given oligodendrocyte is usually involved in the simultaneous myelination of several separate axons (Fig. 1.4). Unmyelinated axons in the CNS run embedded in cytoplasmic extensions, or processes, of oligodendrocytes.

Figure 1.4 Relationship between an oligodendrocyte and the several myelinated and unmyelinated axons it ensheathes.

Several diseases may affect myelinating cells, including toxic and metabolic diseases, and most notably multiple sclerosis. In these diseases, neurons are not necessarily directly affected, but the loss of their myelin covering affects the way they conduct action potentials resulting in disordered neural function.

Structure of a peripheral nerve

A peripheral nerve is formed by the parallel aggregation of a variable number of myelinated and unmyelinated axons: the greater the number of axons, the larger the size of the nerve. Microscopically, within a peripheral nerve, myelinated axons are surrounded by their individual Schwann cell sheaths and unmyelinated axons run embedded in invaginations of the Schwann cell membrane. The axons are held together by sheaths of fibrous tissue which constitute additional coatings that protect them from external mechanical and chemical insults.

Individual myelinated axons are surrounded by a tubular sheath of fibrous tissue, the endoneurium, whereas clusters of axons are held together by a larger fibrous sheath, the perineurium (Fig. 1.5). Unmyelinated axons are not enclosed by endoneurium but run in isolated bundles parallel to myelinated axons and are enclosed with them in the perineurial sheath.

Figure 1.5 Composition of a peripheral nerve.

A bundle of axons enclosed within a single perineurial sheath is referred to as a nerve fascicle (Fig. 1.5). Axons within a fascicle largely remain within that same fascicle throughout the course of the nerve until their peripheral distribution. However, along the course of a peripheral nerve axons may at times leave one fascicle to enter and continue within an adjacent fascicle.

The fascicles within a peripheral nerve are bound together by an external sheath of fibrous tissue, the epineurium (Fig. 1.5), which forms the external surface of the macroscopic nerve. As a peripheral nerve passes through the tissues of the body, it gives off branches composed of one or more fascicles of the parent nerve, which leave it to reach their particular destination. Along the course of a nerve, this process is repeated until all the fascicles and axons in the nerve have been distributed to their target tissues.

The peripheral nervous system

Nerves that supply the structural tissues, such as bone, muscle and skin, are referred to as somatic nerves. Groups of somatic nerves innervate specific areas or regions and are covered in detail in the respective region.

The nerves that supply the viscera, such as the heart, lungs and digestive tract, are referred to as visceral nerves. However, because the nervous functions concerning viscera are largely automatic and subconscious, that part of the nervous system that innervates viscera is referred to as the autonomic nervous system (ANS). It has components in both the central and PNSs: it is described in detail on page 500.

Constituents of peripheral nerves

Peripheral nerves are composed of different types of axons that are classified according to their size, function or physiological characteristics. The broadest classification of axons recognizes afferent (or sensory) fibres and efferent (or motor) fibres. The terms ‘afferent’ and ‘efferent’ refer to the direction in which axons conduct information: afferent fibres conduct towards, while efferent fibres conduct away from the CNS. The term ‘sensory’ refers to axons that convey information to the CNS about events that occur in the periphery; ‘motor’ fibres cause events in the periphery, usually in the form of contraction of voluntary or smooth muscle.

Axons may also be classified according to their conduction velocities, which in turn are proportional to their sizes (Fig. 1.6). If a peripheral nerve is stimulated electrically and recordings of the evoked activity are taken some distance along the nerve, a wave of electrical activity can be recorded. The wave is generated by the summation of the electrical activity in each of the axons in the nerve, its size being proportional to the number of axons in the nerve, while its shape reflects the type of axons present.

Figure 1.6 Neurogram of an idealized peripheral nerve. Activity in different types of nerve fibres is reflected in depolarizations that occur at different times after the delivery of an electrical stimulus to the nerve.

The waveform obtained from a nerve containing every known type of axon is represented in Fig. 1.6. It has three principal peaks, named the A, B and C waves. The A wave occurs very soon after the triggering stimulus, as it is produced by rapidly conducting axons with velocities in the range 12–120 ms^−l: these are classified as A fibres. The A wave is broken down into several secondary peaks called Aα, Aβ, Aγ and Aδ waves, each being produced by a particular subgroup of rapidly conducting axons.

The B wave is produced by more slowly conducting, but nevertheless fast, fibres classified as B fibres. The C wave arrives at the recording electrode much later as it is produced by slowly conducting axons classified as C fibres with conduction velocities in the range 0.5–2 ms^−l.

Not all peripheral nerves contain every type of fibre, therefore not all nerves will exhibit A, B and C waves. For example, B fibres are not regularly present in all nerves; B waves are therefore not always recordable.

Myelinated axons conduct impulses faster than unmyelinated ones, and among myelinated axons those with larger diameters conduct impulses faster than those with smaller diameters. The conduction velocity of a myelinated axon is directly proportional to the total diameter of the axon and its myelin sheath. A and B fibres are myelinated axons of various diameters and fast conduction velocities, while C fibres are unmyelinated axons with small diameters and slow conduction velocities.

Functionally, Aα and Aγ fibres represent motor fibres connected to voluntary muscles and also include certain sensory fibres that transmit position sensation from skeletal muscles. Aβ fibres mediate the sensations of touch, vibration and pressure from skin; Aδ fibres are sensory fibres that mediate pressure, pain and temperature sensations from skin, pain and pressure from muscle, and pain, pressure and position sensation from ligaments and joints. B fibres are preganglionic sympathetic efferent fibres. C fibres are largely unmyelinated sensory fibres that arise in virtually all tissues of the body and transmit pain, temperature and pressure sensations, however, some are postganglionic sympathetic neurons. Because Aβ fibres have large diameters they are sometimes referred to as large diameter afferent fibres, while Aδ and C fibres are referred to collectively as small diameter afferent fibres.

Another system of axon classification relates specifically to sensory fibres. In this system, sensory fibres are classified according to their conduction velocities into groups I, II, III and IV. These groups have the same conduction velocities as type Aα, Aβ, Aδ and C fibres respectively, as shown in Fig. 1.6, but do not include the Aα and Aβ motor fibres. The fibres of groups III and IV largely mediate pain and temperature sensations. Group II constitutes fibres that mediate pressure and touch, and fibres that form spray endings in muscle spindles (p. 15). Group I is divided into groups Ia and Ib. Group Ia fibres are slightly larger and innervate muscle spindles, while group Ib fibres innervate Golgi tendon organs (p. 15).

Nerve endings

The terminals of axons in peripheral nerves have unique structures depending on their function. Motor axons have terminals designed to deliver a stimulus to muscle cells (see p. 16), while sensory axons have terminals designed to detect particular types of stimuli. These terminals are called receptors, and a diversity of morphological types can be found (see p. 26).

Section summary

Nervous system

• Consists of specialized cells (neurons) concerned with the transmission of information throughout the body.

• Neurons have a cell body (expanded part of the cell), an axon (single long process arising from the cell body) and dendrites (multi-branching radiating processes arising from the cell body).

• Two parts: CNS comprises brain, spinal cord and spinal nerves; PNS comprises nerves (peripheral and cranial) and sensory receptors.

• Connections (synapses) between neurons allow integration of information from several sources. Resultant effect may be excitatory or inhibitory depending on the type of neurotransmitter and type of receptors on the postsynaptic membrane.

• Axons may be myelinated or unmyelinated. In CNS myelination is by oligodendrocytes, in PNS by Schwann cells. Myelinated axons have faster conduction velocities.

Components of the musculoskeletal system

As this book is concerned essentially with the musculoskeletal system a brief account of the major tissues of the system, i.e. connective, skeletal and muscular tissue, and of the type of joints which enable varying degrees of movement to occur, is given as it will aid in understanding the mobility and inherent stability of various segments. The initiation and coordination of movement is the responsibility of the nervous system.

Connective tissue

Connective tissue is of mesodermal origin and in the adult has many forms; the character of the tissue depends on the organization of its constituent cells and fibres.

Fat

Fat is a packing and insulating material; however, in some circumstances it can act as a shock absorber, an important function as far as the musculoskeletal system is concerned. Under the heel, in the buttock and palm of the hand, the fat is divided into lobules by fibrous tissue septa, thereby stiffening it for the demands placed upon it.

Fibrous tissue

Fibrous tissue is of two types. In white fibrous tissue there is an abundance of collagen bundles, whereas in yellow fibrous tissue there is a preponderance of elastic fibres.

White fibrous tissue is dense, providing considerable strength without being rigid or elastic. It forms: (1) ligaments, which pass from one bone to another in the region of joints, uniting the bones and limiting joint movement; (2) tendons for attaching muscles to bones; and (3) protective membranes around muscle (perimysium), bone (periosteum) and many other structures.

Yellow fibrous tissue, on the other hand, is highly specialized, being capable of considerable deformation yet returning to its original shape. It is found in the ligamenta flava associated with the vertebral column as well as in the walls of arteries.

Skeletal tissue

Skeletal tissues are modified connective tissues, in which the cells and fibres have a particular organization and become condensed so that the tissue is rigid.

The internal architecture of bone reveals a system of struts and plates (trabeculae) running in many directions (Fig. 1.7), which are organized to resist compressive, tensile and shearing stresses. Surrounding these trabecular systems, which tend to be found at the ends of long bones, is a thin layer of condensed or compact bone (Fig. 1.7). The network of the trabeculae, because of its appearance, is known as cancellous or spongy bone. In the shaft of a long bone there is an outer, relatively thick ring of compact bone surrounding a cavity, which in life contains bone marrow.