Overview of the nervous system

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Overview of the nervous system

This chapter provides an overview of the main structural and functional components of the nervous system and introduces the topographical anatomy of the brain and its protective coverings. Many of the concepts introduced here will be discussed in more detail in subsequent chapters.

Parts of the nervous system

Central and peripheral nervous systems

The nervous system is divided into central and peripheral parts (Fig. 1.1). The central nervous system (CNS) is made up of the brain and spinal cord, encased within the bones of the skull and vertebral column. The CNS represents the main integrating and decision-making centre of the nervous system. The peripheral nervous system (PNS) includes 31 pairs of spinal nerves which emerge between the vertebrae (Fig. 1.2) and 12 pairs of cranial nerves which arise from the base of the brain (Fig. 1.3). At the roots of the upper and lower limbs, sensory and motor fibres are redistributed in the brachial and lumbosacral plexuses to enter a number of named peripheral nerves (Figs 1.4 and 1.5). The function of the PNS is to transmit nerve impulses to and from the brain and spinal cord.

Fig. 1.1 Central and peripheral components of the nervous system.
The central nervous system (CNS) includes the brain and spinal cord. The peripheral nervous system (PNS) is made up of sensory and motor fibres passing to and from the CNS.

Fig. 1.2 Lateral view of the vertebral column in the lumbosacral region.
The mixed spinal nerves can be seen emerging from the intervertebral foramina.

Fig. 1.3 Ventral aspect of the brain illustrating the cranial nerves, which are conventionally labelled using Roman numerals.
The olfactory nerve (cranial nerve I) is not seen here; it consists of millions of filaments that arise from the nasal cavity and synapse in the olfactory bulb.

Fig. 1.4 The brachial plexus giving rise to the main named peripheral nerves of the upper limb.

Fig. 1.5 The lumbosacral plexus giving rise to the main named peripheral nerves of the lower limb.

The motor component of the peripheral nervous system is further subdivided into somatic and autonomic parts. The somatic nervous system innervates the skeletal musculature and is responsible for ‘voluntary’ (consciously initiated) actions. The autonomic nervous system supplies the internal organs and other visceral structures and operates at an automatic (mainly unconscious) level.

Autonomic nervous system

The autonomic nervous system (ANS) or visceromotor system regulates the activities of the cardiovascular, respiratory, digestive and urogenital systems. It operates autonomously (Greek: autos, self; nomos, governed by law). The ANS depends upon a constant stream of sensory information from the internal organs and blood vessels which it uses to regulate the contraction of cardiac and smooth muscle and the secretory activity of glands. Ultimate control of the autonomic nervous system resides in the hypothalamus, a tiny region at the centre of the brain that is sometimes referred to as the ‘head ganglion’ of the ANS (described later; see also Ch. 3).

Divisions of the ANS

The autonomic nervous system has two divisions that widely innervate the body and tend to have opposing effects on their target structures (Fig. 1.6):

Fig. 1.6 Comparison of the sympathetic and parasympathetic divisions of the autonomic nervous system.
The two limbs of the ANS tend to have opposing or complimentary effects on their target structures.

The sympathetic nervous system is most active during highly stressful situations that provoke intense fear or rage, typified by the ‘fight or flight’ response: e.g. dilated pupils, dry mouth, racing pulse. It is said to act ‘in sympathy’ with the emotions.

The parasympathetic nervous system generally has an antagonistic or complimentary effect on a given organ or tissue and is more associated with vegetative activities (‘resting and digesting’).

It is important to note that most structures are innervated by both divisions of the ANS and can be controlled independently (e.g. it is possible to dilate the pupils without increasing the heart rate); but the ‘fight or flight’ response is a useful way to remember the effects of the sympathetic division.

The two divisions of the ANS have distinct anatomical origins. Sympathetic fibres originate in the thoracic and upper lumbar segments of the spinal cord (T1–L2/3) as the thoracolumbar outflow (Fig. 1.7). This feeds into the sympathetic chain, on either side of the vertebral column, which provides sympathetic innervation to the entire body.

Fig. 1.7 The thoracolumbar outflow gives rise to the sympathetic division of the autonomic nervous system.
Sympathetic fibres from the thoracic and upper lumbar spinal cord (T1–L2/3) innervate the sympathetic chain, which is located on either side of the vertebral column. Superior and inferior extensions of the sympathetic chain provide sympathetic innervation to the entire body.

The parasympathetic fibres originate in cranial nerves III, VII, IX and X and the sacral spinal cord (S2–4). This constitutes the craniosacral outflow (Fig. 1.8). The vagus nerve (cranial nerve X) has a wide territory of distribution that includes most of the thoracic and abdominal viscera (Latin: vagus, wandering).

Fig. 1.8 The craniosacral outflow corresponds to the parasympathetic division of the autonomic nervous system.
The craniosacral outflow includes contributions from cranial nerves III, VII, IX and X and the pelvic splanchnic nerves which arise from segments S2–S4 of the sacral spinal cord. Note that the vagus nerve arises in the brain stem but descends in the neck (dashed blue line) to supply most of the thoracic and abdominal organs.

The term enteric nervous system refers to an extensive network of neurons within the wall of the gastrointestinal tract (Greek: enteron, intestine). This regulates the activity of digestive glands and coordinates the waves of peristalsis that propel food through the bowel lumen. Although it is a component of the ANS, it is sometimes regarded as a separate system since it is capable of coordinated activity, independent of the brain and spinal cord.

Key Points

The central nervous system (CNS) is composed of the brain and spinal cord, encased within the bones of the skull and vertebral column.

The peripheral nervous system (PNS) includes 12 pairs of cranial nerves and 31 pairs of spinal nerves which carry sensory and motor fibres to and from the CNS.

At the roots of the upper and lower limbs, sensory and motor fibres are redistributed in the brachial and lumbosacral plexuses to form a large number of mixed peripheral nerves.

The motor component of the peripheral nervous system can be separated into somatic (somatomotor) and autonomic (visceromotor) divisions.

The somatic nervous system innervates the voluntary (skeletal) musculature.

The autonomic nervous system (ANS) innervates smooth muscle, cardiac muscle and glands within the internal organs (viscera) and blood vessels. It is controlled by the hypothalamus.

The physiological actions of the sympathetic division of the ANS (originating from the thoracolumbar outflow) are typified by the ‘fight or flight’ response.

The parasympathetic division of the ANS (represented by the craniosacral outflow) generally has an opposing action on a given organ or tissue.

Cells of the nervous system

Neural tissue contains two specialized cell types: neurons and glia. These will be introduced here and discussed further in Chapter 5.

Nerve cells (neurons)

The basic structural and functional unit of the nervous system is the neuron or nerve cell. Neurons are electrically excitable cells that are highly specialized for the receipt, integration and transmission of information via rapid electrochemical impulses. A typical neuron is illustrated in Fig. 1.9.

The cell body ranges from 5–120 µm in diameter and contains the nuclear DNA and the biological machinery for protein synthesis and other housekeeping functions. Two types of process (or neurite) arise from the cell body. A profusely branching ‘tree’ of dendrites (Greek: dendron, tree) is specialized to receive and integrate information and may receive projections from many thousands of other neurons.

Nerve impulses are triggered in the cell body and transmitted away from the neuron along the slender nerve fibre or axon. A typical nerve cell has a single axon which may be up to two metres long in humans. Axons make contact with their target cells (and may branch to influence more than one target cell) at swellings called axon terminals. Axons often give rise to branches (or collaterals) which typically arise at right angles to the long axis of the nerve fibre.

The point of contact between two neurons is called a synapse (Greek: sunapsis, point of contact). Neurons influence effector structures such as muscle fibres and glandular tissue at neuroeffector junctions. The specific junction between a somatic motor neuron and a skeletal muscle fibre is the neuromuscular junction (NMJ) (see Ch. 4). The electrical activity of the target cell is influenced by neurotransmitters, chemical mediators that are released at the axon terminal in response to the arrival of a nerve impulse. Neurotransmitters are stored in membrane-bound organelles within the axon terminal called synaptic vesicles (see Ch. 7).

Neurons can be classified into three functional types (Fig. 1.10):

Fig. 1.10 Afferent neurons, interneurons and efferent neurons.
The vast majority of nerve cells in the central nervous system are interneurons (association neurons).

Afferent neurons carry nerve impulses towards the central nervous system. The term afferent fibre (or simply ‘afferent’) is also used to refer to any nerve process carrying impulses towards a particular structure. For example, ‘cortical afferents’ are fibres carrying impulses to the cerebral cortex. The term sensory neuron is usually reserved for afferent neurons conveying information that is to be consciously perceived.

Efferent neurons carry nerve impulses away from the central nervous system. Again, the term efferent fibre (or ‘efferent’) can be used to refer to any process carrying impulses away from a particular structure. The term motor neuron is best reserved for efferent neurons that are involved in pathways concerned with voluntary movement.

Association neurons (or interneurons) make up the vast network of interconnections within the brain and spinal cord. They have an integrative function, transforming sensory inputs into appropriate motor responses. The majority of neurons therefore fall into this category.

Neurons can also be classified by the number of processes that they have. The example shown in Fig. 1.9 is a multipolar neuron because it has several processes. Neurons with one or two processes are termed unipolar and bipolar respectively. The vast majority of neurons in the CNS are multipolar.

Grey and white matter

CNS tissue can be divided into grey and white matter (Fig. 1.11). Grey matter is composed mainly of neuronal cell bodies, dendrites and synapses. It is sharply demarcated from the adjacent white matter, which is made up of nerve fibres travelling to other parts of the nervous system. The pale colour of white matter is due to the lipid-rich myelin sheath that surrounds axons and enhances their conduction velocity (see Fig. 1.9; see also Chs 5 & 6). Discrete groups of neurons within the central nervous system are referred to as nuclei (singular: nucleus). In the peripheral nervous system, collections of neuronal cell bodies form aggregates that resemble knots on a piece of string. These are referred to as ganglia (singular: ganglion).

Neuroglial cells

In addition to neurons, the nervous system contains a variety of support cells that are known collectively as glia. These cells were originally thought to offer mainly physical support, literally holding nervous tissue together (Greek: glia, glue) but they are now known to carry out a wide range of important functions. The four main types of glial cell (discussed further in Chapter 5) are:

Astrocytes, which provide structural and metabolic support to neurons and help to regulate the exchange of molecules between the bloodstream and the brain.

Oligodendrocytes, which invest axons with a myelin sheath in the brain and spinal cord.

Schwann cells, the peripheral counterparts of oligodendrocytes.

Microglia, the resident phagocytic and immunocompetent cells of the central nervous system.

Although glial cells are relatively small, with an average diameter of around 4–8 µm, they outnumber neurons in a ratio of 10 : 1 and make up approximately 50% of the total volume of the CNS. Glial cells are able to undergo cell division (unlike neurons, which are post-mitotic cells).

Key Points

The neuron is the basic structural and functional unit of the nervous system. The main types are afferent (sensory), efferent (motor) and association (interneurons).