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Nerve cells and their connections

6.1 Introduction

The nervous system is adapted to provide rapid and discrete signaling over long distances (from millimeters to a meter or more); this chapter is concerned chiefly with the underlying mechanisms. It will begin with a simple outline of the structure of the central nervous system and peripheral nerves. It will then discuss the cellular events involved in neuronal signaling: action potential generation and synaptic transmission.

The nervous system may be divided into five main parrs:

  1. the brain;

  2. the spinal cord;

  3. the peripheral nerves;

  4. the autonomic nervous system;

  5. the enteric nervous system.

The brain and spinal cord constitute the central nervous system (CNS) while the peripheral nerves, autonomic nervous sys­tem, and enteric nervous system make up the peripheral nervous system. The autonomic nervous system is that part of the nervous system that is concerned with the innervation of the blood vessels and the internal organs. It includes the autonomic ganglia that run parallel to the spinal column (the paravertebral ganglia) and their associated nerves. The enteric nervous system con­trols the activity of the gut. The organization and functions of the autonomic and enteric divisions of the nervous system will be discussed in Chapters 10 and 18.

^ 6.2 The organization of the brain and spinal cord

The brain and spinal cord lie within a bony case formed by the skull and vertebral canal of the spinal column and are covered by three membranes, called the meninges. The space between these membranes is filled with a clear fluid called the cerebrospinal fluid (c.s.f.). Consequently, the CNS floats in a fluid-filled con­tainer which helps to protect it from damage due to mechanical shocks.

The surface of the human brain has many folds, called sulci (singular: sulcus), and the smooth regions of the brain surface that lie between the folds are known as gyri (singular: gyrus). Viewing the brain from the dorsal surface reveals a deep cleft,

Fig. 6.1 A side view of the human brain, showing the cerebellum and the principal divisions of the right cerebral hemisphere.

known as the longitudinal cerebral fissure, which divides the brain into two cerebral hemispheres. Each hemisphere can be broadly divided into four lobes: the frontal, parietal, occipital, and temporal. Posterior to the cerebral hemispheres lies a smaller, highly convoluted structure known as the cerebellum (Fig. 6.1).

Cutting the brain in half along the mid-line between the cerebral hemispheres reveals some details of its internal organ­ization (Fig. 6.2). On the medial surface is a broad white band known as the corpus callosum that interconnects the two hemi­spheres. Immediately below the corpus callosum is a mem­branous structure called the septum pellucidum that separates two internal spaces known as the lateral cerebral ventricles. These ventricles are filled with c.s.f. which probably plays an important role in regulating the extracellular environment of nerve cells. Details of the formation and circulation of the c.s.f. can be found in Section 15.12.

Beneath the septum pellucidum and lateral ventricles is the thalamus, a major site for the processing of information from the sense organs. Lying anterior and ventral ro the thalamus is the hypothalamus which plays a vital role in the regularion of the endocrine system via irs control of the pituitary gland (Chapter 12). Posterior and ventral to the thalamus lies the midbrain (Fig. 6.2). Beneath the midbrain lies the pons, a large swelling

Fig. 6.2 A sagittal view of the right side of the human brain to show the relationships of the principal structures.

containing fibers interconnecting the two halves of the cere­bellum. The pons merges with the medulla which, in turn, con­nects with the spinal cord that runs through the spinal canal of the vertebral column. As it passes down the spinal column, the spinal cord gives rise to a series of paired nerves that connect the CNS to peripheral organs.

If the brain is cut at right angles to the mid-line a coronal section is obtained, as shown diagramatically in Fig. 6.3- The outer part or cerebral cortex is grayish in appearance. The cortex and other parts of the brain that have a similar appearance are

Fig. 6.3 Coronal section through the human brain, showing the spatial relationship of the cerebral cortex, basal ganglia, and thalamus. The small inset diagram shows the plane of section.

Fig. 6.4 A ventral view of the human brain showing the cranial nerves, optic chiasm, the pons, and the decussation of the pyramids (crossing over of the nerve fibers of the pyramidal tracts).

gray matter, which contains large numbers of nerve cell bodies. Inside the gray matter of the cerebral cortex is the white matter, which is composed of bundles of nerve fibers such as those of the corpus callosum and the internal capsule.

Other structures revealed by a coronal section are the caudate nucleus, the putamen, and the globus pallidus, which together form the corpus striatum. Between the caudate nucleus and putamen runs the internal capsule which contains nerve fibers connecting the cerebral cortex to the spinal cord. Beneath the thalamus lies a small region known as the substantia nigra. All of these structures play an important role in the control of move­ment (see Chapter 9 for further details).

The cranial nerves

On the base of the brain there are a number of nerves that serve the motor and sensory functions of the head (Fig. 6.4). These are the cranial nerves, which are numbered I—XII. Some contribute to the parasympathetic division of the autonomic nervous system (Chapter 10). These are the oculomotor (III), the facial (VII), the glossopharyngeal (IX), and the vagus (X). The names and functions of all the cranial nerves are given in Table 6.1.

The organization of the spinal cord

While the human brain is a large structure, the spinal cord is delicate—scarcely as thick as a pencil for much of its length. A cross-section of the spinal cord shows that it has a central region of gray matter surrounded by white matter (Fig. 6.5). The white matter of the spinal cord is arranged in columns and contains nerve fibers passing between the brain and spinal cord. The gray matter is roughly shaped in the form of an 'H' surrounding a

^ 6.2 The organization of the brain and spinal cord


Table 6.1 The functions of the cranial nerves

Number Name Chief functions

I Olfactory Sensory nerve subserving the sense of smell

^ II Optic Sensory nerve subserving vision (output of the retina) III Oculomotor Chiefly motor control of the extrinsic muscles of the eye and the parasympathetic supply for the intrinsic muscles of the iris and ciliary body

IV Trochlear Chiefly motor control of the extrinsic muscles of the eye

V Trigeminal Sensory and motor—motor control of the jaw and facial sensation

VI Abducens Chiefly motor control of the extrinsic muscles of the eye

VII Facial Sensory and motor—motor control of the facial muscles and parasympathetic supply to the salivary glands. Subserves the sense of taste via the chorda tympani

VIII Vestibulocochlear Sensory—hearing and balance

IX Glossopharyngeal Sensory and motor—control of swallowing and parasympathetic supply to the salivary glands. Subserves the sense of taste from the back of the tongue (bitter sensations)

X Vagus Major parasympathetic outflow to the chest and abdomen. Afferent inputs from the viscera

XI Spinal accessory Motor—control of neck muscles and larynx XII Hypoglossal Motor control of the tongue

central canal, although the exact appearance depends on the level at which the section is made (i.e. on whether the spinal cord has been cut across at the cervical, thoracic, lumbar, or sacral level). This gray matter can be divided broadly into two dorsal horns and two ventral boms. In most individuals, the spinal cord has 31 pairs of spinal nerves. Each pair has a dorsal root and a ventral root. Each dorsal root has an enlargement known as a dorsal roor ganglion that contains the cell bodies of the nerve fibers making up the dorsal root. The fibers of the ventral root originate from nerve cells in the ventral horn of the spinal gray matter. (The dorsal and ventral horns are also known as the posterior and anterior horns.)

Sensory information enters the spinal cord via the dorsal root ganglia. Since the sensory fibers carry information from sense organs to the spinal cord they are known as afferent nerve fibers. The ventral root fibers carry motor information from the spinal cord to muscles and secretory glands (the effectors) and are known as efferent nerve fibers. The nerves that leave the spinal cord to supply the

Fig. 6.5 The structure of the spinal cord at the level of the lumbar enlargement (top) and the arrangement of the spinal roots (bottom). In the lower figure part of the white matter of the spinal cord is cut away to show the direct entry of the spinal roots inro rhe central gray matter.

skeletal muscles are known as somatic nerves, while those that supply the blood vessels and viscera are sympathetic efferent fibers (see Chapter 10 for further details of the sympathetic fibers).

The neuron is the principal functional unit of the nervous system

The CNS is made up of two main types of cell: nerve cells or neurons and glial cells or neuroglia.


Neurons are found throughour the gray matter of the brain and spinal cord. They are very varied in both size and shape but all stain strongly with basic dyes. The material stained within the cell bodies is called Nissl substance, which contains a high pro­portion of RNA. Neurons have extensive branches, called den­drites, that receive information ftom othet cells, while they transmit information to their targets (which may be another neuron) via a thread-like extension of the cell body called an axon (Fig. 6.6). Axons and dendrites are collectively called neuronal processes.


6 Nerve cells and their connections

Fig. 6.6 A diagrammatic representation of a CNS neuron.

The dendrites of neurons within the CNS generally branch extensively and so they are able to receive information from many different sources. Each nerve cell gives rise to a single axon which may branch to contact a number of different targets. The branches are called axon collaterals. As an axon approaches its target it often branches to innervate a number of cells. Each branch ends in a small swelling—an axon terminal (also called a nerve terminal). The contact between an axon terminal and its target is called a synapse. Synapses may be made between nerve cells or between an axon and a non-neuronal cell, such as a muscle fiber.

Crushing or cutting an axon prevents a nerve from control­ling irs target cell. In the case of nerves supplying a skeletal muscle, such injuries result in paralysis even though the nerve sheath may remain intact. Control of movement is regained only if the appropriate axons regenerate. Similarly, the death of neurons within the brain following a stroke can cause paralysis even though the peripheral nerves remain intact. (A stroke is a loss of cerebral function due a cerebral hemorrhage caused by rupture of a blood vessel or a cerebral thrombosis, i.e. a blockage of a cerebral blood vessel caused by a blood clot.) These observa­tions demonstrate that the neurons are rhe principal functional units of the nervous system. Their role is to transmit signals to each other and to target cells outside the nervous system.

^ Glial cells

Four main classes of non-neuronal cells occur in the brain and spinal cord:

  1. Astroglia or astrocytes. These are cells with long processes
    that make firm attachments to blood vessels. The ends of
    the astrocytic processes seal closely together and form an
    additional barrier between the blood and the extracellular
    fluid of the brain and spinal cord. This barrier is known as
    the blood—brain barrier and it serves to prevent changes in
    the composition of the blood (such as those following a
    meal) influencing the activity of the nerve cells within the

  2. ^ Oligodendroglia or oligodendrocytes. Oligodendrocytes account
    for about 75 per cent of all glial cells in white matter,
    where they form the myelin sheaths of axons. In the

peripheral nervous system the myelin sheaths are formed by Schwann cells (see below).

  1. Microglia. These are scattered throughout the gray and
    white matter. They are phagocytes and rapidly converge
    on a site of injury or infection within the CNS.

  2. ^ Ependymal cells. These are ciliated cells that line the central
    fluid-filled spaces of the brain (the cerebral ventricles)
    and the central canal of the spinal cord. They form a
    cuboidal—columnar epithelium called the ependyma.

The structure of peripheral nerve trunks

Axons are delicate structures that may need to traverse con­siderable distances to reach their target organs. Outside the CNS they run in peripheral nerve trunks alongside the major blood vessels, where they are protected from damage by layers of con­nective tissue (Fig. 6.7). The outermost layer of a peripheral nerve is a loose aggregate of connective tissue called the epineurium which serves to anchor the nerve trunk to the adjacent tissue. Within the epineurium, axons run in bundles called fas­cicles and each bundle is surrounded by a tough layer of connec­tive tissue called the perineurium. Within the perineural sheath, individual nerve fibers are protected by a thin layer of connective tissue called the endoneurium. Individual axons are covered by specialized cells called Schwann cells. Some nerve trunks transmit information from specific sensory end-organs to the CNS (sensory nerves) while others transmit signals from the CNS to specific

Fig. 6.7 A cross-section through a small peripheral nerve to show the relationship between the nerve fibers and the surrounding layers of connective tissue (the epineurium, perineurium, and endoneurium). Note that at this magnification individual nerve fibers are not clearly resolved.

6.3 The primary function of an axon


Fig. 6.8 Myelinated and unmyelinated nerve fibers and their relationship to the surrounding Schwann cells, (a) A short length of myelinated nerve, (b) The detailed structure of a single node of Ranvier. (c) A cross-section of a myelinated nerve fiber to show how the Schwann cell forms the myelin layers, (d) The arrangement of a number of unmyelinated nerve fibers within a Schwann cell.


1. The nervous system may be divided broadly into five main parts:

  • the brain;

  • the spinal cord;

  • the autonomic nervous system;

  • the enteric nervous system;

  • the peripheral nerves.

  1. Within the CNS two distinct types of tissue can be discerned on the
    basis of their appearance: gray matter and white matter. Gray matter
    contains the cell bodies of the neurons. The space between the
    cell bodies is known as the neuropil and contains the cytoplasmic
    extensions of both neurons and glia. White matter mainly consists of
    myelinated nerve axons and oligodendrocytes.

  2. The neuron is the principal functional unit of the nervous system.
    Neurons are highly differentiated. The cell bodies give rise to two
    types of process: dendrites and axons. The dendrites are highly
    branched and receive information from many other nerve cells. Each
    cell body gives rise to a single axon which may branch to give rise to
    axon collaterals. The axon of a neuron transmits information to other
    neurons or to non-neuronal cells such as muscles (the effectors).

  3. After leaving the CNS, axons run in peripheral nerve trunks which
    provide structural support. Within a nerve trunk each axon is
    protected by three layers of connective tissue: the endoneurium, the
    perineurium, and the epineurium. Individual axons may be either
    myelinated or unmyelinated. Myelin is formed by oligodendrocytes
    in the CNS and by Schwann cells in the periphery.

effectors (motor nerves). Nerve trunks that contain both sensory and motor fibers are called mixed nerves. Peripheral nerve trunks also contain sympathetic postganglionic fibers to the blood vessels and sweat glands (Chapter 10).

Axons may either be myelinated or unmyelinated. Myelinated axons are covered by a thick layer of fatty material, called myelin, that is formed by layers of plasma membrane derived from specific satellite cells. In peripheral nerves the myelin is derived from Schwann cells (Fig. 6.8) while in the CNS the myelin is formed by the oligodendrocytes. The myelin extends along the length of the axon and is interrupted at regular intervals by gaps known as the nodes of Ranvier. At the nodes of Ranvier the axon membrane is not covered by myelin and is in direct contact with the extracellular fluid. The distance between adjacent nodes varies with the axon diameter, larger fibers having a greater internodal distance. In peripheral nerves, unmyelinated axons are also covered by Schwann cells but in this case there is no layer of myelin and a number of nerve fibers are covered by a single Schwann cell. Unmyelinated axons are in direct communication with the extracellular fluid via a longitudinal cleft in the Schwann cell called a mesaxon (Fig. 6.8).

^ 6.3 The primary function of an axon is to

transmit information coded as sequence of action


In the late eighteenth century Galvani showed that electrical stimulation of the nerve in a frog's leg caused the muscles to twitch. This key observation led to the discovery that the excita­tion of nerves was accompanied by an electrical wave that passed along the nerve. This wave of excitation is now called the nerve impulse or action potential. To generate an action potential, an axon requires a stimulus of a certain minimum strength, known as the threshold stimulus or threshold. An electrical stimulus that is below the threshold (a subthreshold stimulus) will not elicit an action potential, while a stimulus that is above threshold (a suprathreshold stimulus) will do so. With stimuli above threshold each action potential has approximately the same magnitude and duration irrespective of the strength of the stimulus. This is known as the 'all or none' law of action potential transmission.

In a mammalian nerve each action potential lasts about 1 ms. If a stimulus is given immediately after an action potential has been elicited, a second action potential is not generated. If the time interval between the stimuli is increased progressively, a second action potential can be elicited 1-2 ms after the passage of the first. The interval during which it is impossible to elicit a

particular nerve can transmit in a given period of time.

Following the absolute refractory period there is a further brief period during wnicn tne tnresnoid is iiigner tnan normal, mis is

called the relative refractory period, which lasts for about 10 ms.

What mechanisms are responsible for the generation of the action potential of nerves? The answer to this important ques­tion was provided by a brilliant series of experiments carried out on the giant axon of the squid between 1939 and 1950 by K. C. Cole in the United States of America and by A. L. Hodgkin and A. E Huxley in England. Like mammalian nerve cells, the resting membrane potential of the squid axon is negative (about -70 mV), close to the equilibrium potential for potassium ions. Hodgkin and Huxley found that, during the action potential, the membrane potential briefly reversed in polarity, reaching a peak value of +40 to +50 mV before falling back to its resting level of about —70 mV (Fig. 6.9). This dis­covery provided an important clue about the underlying mechan­ism. At the peak of the action potential the membrane potential is close to the equilibrium potential for sodium, unlike the resting membrane potential which lies close to the equilibrium potential for potassium ions. This suggested that the action potential resulted from a large increase in the permeability of the axon membrane to sodium ions. This was confirmed when it was shown that removal of sodium ions from the extracellular solution prevented the axon from generating an action potential. In addition, if the sodium concentration in the bathing medium was reduced by two-thirds, the action potential was slower and smaller than normal (Fig. 6.9).

What underlies the change in the permeability of the axon membrane to sodium ions during an action potential? The axonal membrane contains ion channels of a specific type called voltage-gated sodium channels. Some of these channels open when the membrane first begins to depolarize and sodium ions

Fig. 6.9 The membrane potential changes that occur during the action potential of the giant axon of the squid in normal sea water (A) and when external sodium is reduced by two-thirds (B). Note that, at the peak of the action potential, the membrane potential is positive and close to the equilibrium potential for sodium (£Na+). Reduction of extracellular sodium both reduces the maximum amplitude of the action potential and slows its time course. The effect is reversible.

so leading to the opening of more sodium channels, leading to a greater influx of sodium which causes a greater depolarization ana so on until, at tne peaK or tne action potential, tne nignest proportion of the available sodium channels is open and the membrane is, for a brief time, highly permeable to sodium. Indeed, at the peak of the action potential, the membrane has become more permeable to sodium than to potassium and this explains the fact that, at the peak of the action potential, the membrane potential is positive (Section 4.4).
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