The nervous system is a complex network of nerves and cells that carry messages to and from the brain and spinal cord to various parts of the body.
The nervous system consists of the brain, spinal cord and peripheral nerves. It is made up of nerve cells, called neurons, and supporting cells called glial cells.
There are three main kinds of neurons.
Sensory neurons are coupled to receptors specialised to detect and respond to different attributes of the internal and external environment. The receptors sensitive to changes in light, sound, mechanical and chemical stimuli subserve the sensory modalities of vision, hearing, touch, smell and taste. When mechanical, thermal or chemical stimuli to the skin exceed a certain intensity, they can cause tissue damage and a special set of receptors called nociceptors are activated; these give rise both to protective reflexes and to the sensation of pain.
Motor neurons, which control the activity of muscles, are responsible for all forms of behaviour including speech. Interposed between sensory and motor neurons are Interneurons. These are by far the most numerous (in the human brain). Interneurons mediate simple reflexes as well as being responsible for the highest functions of the brain.
Glial cells, long thought to have a purely supporting function to the neurons, are now known to make an important contribution to the development of the nervous system and to its function in the adult brain. While much more numerous, they do not transmit information in the way that neurons do.
Neurons have an architecture that consists of a cell body and two sets of additional compartments called ‘processes’. One of these sets are called axons; their job is to transmit information from the neuron on to others to which it is connected. The other set are called dendrites their job is to receive the information being transmitted by the axons of other neurons. Both of these processes participate in the specialised contacts called synapses. Neurons are organised into complex chains and networks that are the pathways through which information in the nervous system is transmitted.
The brain and spinal cord are connected to sensory receptors and muscles through long axons that make up the peripheral nerves. The spinal cord has two functions: it is the seat of simple reflexes such as the knee jerk and the rapid withdrawal of a limb from a hot object or a pinprick, as well as more complex reflexes, and it forms a highway between the body and the brain for information travelling in both directions.
These basic structures of the nervous system are the same in all vertebrates. What distinguishes the human brain is its large size in relation to body size. This is due to an enormous increase in the number of interneurons over the course of evolution, providing humans with an immeasurably wide choice of reactions to the environment.
Anatomy of the Brain
The brain consists of the brain stem and the cerebral hemispheres.
The brain stem is divided into hind-brain, mid-brain and a ‘between-brain’ called the diencephalon. The hind-brain is an extension of the spinal cord. It contains networks of neurons that constitute centres for the control of vital functions such as breathing and blood pressure. Within these are networks of neurons whose activity controls these functions. Arising from the roof of the hind-brain is the cerebellum, which plays an absolutely central role in the control and timing of movements (See Chapters on Movement and Dyslexia).
The midbrain contains groups of neurons, each of which seem to use predominantly a particular type of chemical messenger, but all of which project up to cerebral hemispheres. It is thought that these can modulate the activity of neurons in the higher centres of the brain to mediate such functions as sleep, attention or reward.
The diencephalon is divided into two very different areas called the thalamus and the hypothalamus: The thalamus relays impulses from all sensory systems to the cerebral cortex, which in turn sends messages back to the thalamus. This back-and-forward aspect of connectivity in the brain is intriguing – information doesn’t just travel one way. The hypothalamus controls functions such as eating and drinking, and it also regulates the release of hormones involved in sexual functions.
The cerebral hemispheres consist of a core, the basal ganglia, and an extensive but thin surrounding sheet of neurons making up the grey matter of the cerebral cortex. The basal ganglia play a central role in the initiation and control of movement. Packed into the limited space of the skull, the cerebral cortex is thrown into folds that weave in and out to enable a much larger surface area for the sheet of neurons than would otherwise be possible.
This cortical tissue is the most highly developed area of the brain in humans – four times bigger than in gorillas. It is divided into a large number of discrete areas, each distinguishable in terms of its layers and connections. The functions of many of these areas are known – such as the visual, auditory, and olfactory areas, the sensory areas receiving from the skin (called the somaesthetic areas) and various motor areas. The pathways from the sensory receptors to the cortex and from cortex to the muscles cross over from one side to the other.
Thus movements of the right side of the body are controlled by the left side of the cortex (and vice versa). Similarly, the left half of the body sends sensory signals to the right hemisphere such that, for example, sounds in the left ear mainly reach the right cortex. However, the two halves of the brain do not work in isolation – for the left and right cerebral cortex are connected by a large fibre tract called the corpus callosum.
The cerebral cortex is required for voluntary actions, language, speech and higher functions such as thinking and remembering. Many of these functions are carried out by both sides of the brain, but some are largely lateralised to one cerebral hemisphere or the other. Areas concerned with some of these higher functions, such as speech (which is lateralised in the left hemisphere in most people), have been identified. However there is much still to be learned, particularly about such fascinating issues as consciousness, and so the study of the functions of the cerebral cortex is one of the most exciting and active areas of research in Neuroscience.