Title: The Nervous System

Key words: Neurone, glial cells, brain, limbic system, afferent, efferent, motor system, somatic, autonomic, sympathetic, parasympathetic, hypothalamus, myelin, synapse

Date: Aug 2000

Category: 6. The Body

Type: Article

Author: DJE Candlish

 

The Nervous System

Introduction

The function of the nervous system is communication: both between the external environment and the body and within the body, controlling and co-ordinating all the body systems. In human beings, the nervous system is also responsible for thought, creativity and other higher functions of the mind. The basic unit of the nervous system is the nerve cell or neurone.

The nervous system functions as an integrated whole, but structurally it can be subdivided into separate components. The first division is into the central and peripheral systems. The central nervous system (CNS) consists of the brain and spinal cord. The CNS is protected by being totally enclosed in bone by the skull and spinal column.

The peripheral nervous system consists of all the other nerves, which carry information to and from the CNS from the rest of the body. Afferent (sensory) nerves take information to the CNS and efferent (motor) nerves control the CNS response to the incoming information. Peripheral nerves connect with the CNS in pairs (1 afferent and 1 efferent). There are 12 pairs of cranial nerves connecting with the brainstem and 31 pairs of spinal nerves connecting with the spinal cord.

The peripheral system is further divided into the somatic (voluntary) and autonomic systems, while the autonomic system subdivides into the parasympathetic and sympathetic systems.

The somatic system controls conscious activities, while the autonomic controls all the functions of the body that are not under conscious control, like heartbeat, digestion and so on.

The parasympathetic system controls routine functions, like heartbeat and breathing, even while we sleep, and maintaining the slow regular movements of the gut walls (peristalsis) that move food through the digestive tract.

The sympathetic system is more like an emergency system. It takes over when action is needed, stimulating the activity of the heart and lungs and preparing the muscles for vigorous activity.

The human brain

Our brains may be the most complex structures in the universe. Around 12 million nerves with an almost infinite number of interconnections or synapses give us our intelligence, language and other mental attributes, as well as maintaining and co-ordinating the various functions of our body.

The brain consists of three main structures:

These structures are the result of human evolution, with the brain stem the most primitive part and the cerebrum the most recently developed. They are protected, not only by the bony skull but also by three inner membranes or meninges which also extend down the spinal column.

Between the inner and middle membranes is a cushioning layer of fluid - the cerebrospinal fluid (CSF). This flows around the brain and spinal cord to act as a shock absorber and also to supply nutrients, especially glucose.

The cerebrum is the largest part of the brain. It consists of two hemispheres linked by a thick bundle of nerve fibres called the corpus callosum. The surface of these cerebral hemispheres is convoluted and deeply fissured. It is covered by a thin layer of highly specialised cells known as the cerebral cortex. This is the most recent evolutionary development of our brains and is where our higher mental faculties, such as thought, are carried out. The cerebral cortex is divided functionally into four sections or lobes, each responsible for a specific type of activity:

A specific area within the parietal lobe known as the motor cortex is dedicated to the control and co-ordination of movement.

Beneath the cerebral cortex, there are other structures and groups of nerves, each with their own specific functions. The basal ganglia, for example, work with the motor cortex to control movement. The thalamus, deep within the cerebrum, receives and transmits sensory information, while the hypothalamus co-ordinates the hormonal system and regulates body temperature.

The limbic system is a system of nerve pathways linking the cerebral hemispheres, the thalamus, hypothalamus and brain stem. It plays a major role in our moods and emotions and is the site of action for drugs like tranquilisers.

The cerebellum is a smaller structure than the cerebrum. It too has a deeply fissured surface. It controls balance and motor co-ordination in response to direct input from the ear and other sensory organs, via the spinal column. Reflexes like withdrawing the hands from hot objects take place in the cerebellum, as this is faster than involving the cerebrum. In fact, the movement is usually completed before our minds become aware of the pain.

The brain stem is the oldest part of the brain in evolutionary terms. It connects the rest of the brain to the spinal cord and is mainly involved in autonomic processes like breathing and digestion.

The nerve cell or neurone

The nervous system contains millions of nerve cells or neurones. These form a network throughout the body collecting and transmitting sensory and motor information from every organ and muscle in the body. Although they are typical cells in some ways, having a nucleus, cytoplasm and a cell membrane, nerve cells have a distinctive appearance and function.

Neurones do differ from other cells in one important way; they cannot reproduce. This ensures that the integrated structure of the nervous system is not disrupted by the formation of any new circuits, but it also means that damaged nerves cannot be replaced.

Neuronal structure

Neurones have a small nucleus but a relatively large number of energy-producing mitochondria and an extensive network of protein-producing endoplasmic reticulum. Every nerve cell has a cell body with hundreds of short, branching extensions or dendrites that receive information from other neurones and a single long extension or axon that conducts outgoing information. The longest axons in the body can reach a metre or more in length, such as those from neurones in the spinal cord to muscles in the feet.

The end of the axon (the axon terminal) is adapted to transmit impulses to other neurones or target cells such as muscle fibres. These connections or synapses are not direct, as electrical connections have to be. Instead there is a tiny gap called a synaptic cleft that separates the cells.

There are two types of nerve - myelinated and unmyelinated. This refers to the presence or absence of a protective sheath of white insulating material or myelin. Myelin is not continuous, like the insulation on electrical cable, but has gaps at regular intervals, called Nodes of Ranvier. Certain areas of the CNS contain only myelinated nerves. These areas are described as 'white matter'. 'Grey matter' only contains unmyelinated nerves

Nerve impulse transmission

A nerve impulse is essentially electrical. The cell membrane is 'excitable' and, when stimulated by an incoming message or triggered by a receptor or nerve ending it generates an impulse or a series of them, depending on how strong the stimulation is. The unmyelinated nerves act like electrical cable. An impulse travels along the full length of the axon. In myelinated nerves, the impulse travels faster, by jumping from one Node of Ranvier to the next.

The impulse itself is produced by means of rapid changes in the concentration of sodium and potassium ions inside and outside the neurone. Reversing these changes to get ready for the next impulse takes time and uses energy.

Although transmission along the axon involves electrical energy, a chemical messenger or neurotransmitter is released to cross the synaptic cleft and carry the impulse to the next cell. Specialised proteins in the cell membrane act as receptors for these chemical messengers.

 

Synaptic transmission

A neurotransmitter is involved in every receptor to nerve, nerve to nerve or nerve to muscle transmission, or action potential. There are several neurotransmitters in the human nervous system but they all operate in a similar way. When the nerve impulse reaches the end of the presynaptic axon, it releases the neurotransmitter from storage vesicles. The neurotransmitter molecules then diffuse across the synaptic cleft and interact with receptors in the postsynaptic cell, to stimulate a new action potential and continue the impulse. Transmission of impulses is always one way only.

After triggering the new impulse, neurotransmitters are either actively transported back to the presynaptic membrane and stored again in vesicles or are metabolised by enzymes.

Neurotransmitters

In different areas of the brain and in different neuronal pathways around the body, different neurotransmitters are involved. These include

Acetyl choline is the only neurotransmitter used by the parasympathetic system while the amines noradrenaline and dopamine mainly occur in the sympathetic system.

Imbalances or disturbances in neurotransmitter function are the underlying cause of illnesses such as depression (noradrenaline and 5-HT) and Parkinson's disease (dopamine). Fortunately, neurotransmitter imbalances are relatively easy to alter and increasingly effective drugs have been developed in recent years for many illnesses.

Glial cells (neuroglia)

These cells are indispensable helpers for the neurones. There are roughly ten times as many glial cells as neurones, divided into three types

The astrocytes are star shaped and about the same size as neurones. They have many radiating, branched extensions and fit between the neurones and the capillaries supplying them. Astrocytes are involved in the exchange of substances between neurone and capillary.

Oligodendrites are smaller than astrocytes and have fewer, shorter extensions. They are involved in the formation of the myelin sheath around neurones.

Microglial cells are the smallest of these helper cells. They use their cell membrane extensions to move through nerve tissue, rather like spiders. These cells remove any damaged or dead neurones and other tissue.