Introduction

The nervous system is broadly divided into the central nervous system (CNS) and the peripheral nervous system. The CNS consists of the brain and spinal cord, which are enclosed by the meninges and enclosed within bone of the skull and the spinal column, respectively. The brain is composed of the cerebrum, the cerebellum and the brain stem. The cerebrum includes the cerebral hemispheres, basal ganglia and thalamus, while the brain stem consists of the midbrain, pons and medulla oblongata. The spinal cord carries sensory information from the peripheral nervous system to the brain, and motor information from the brain to the muscles and glands.

The brain processes and integrates sensory inputs and controls and co-ordinates motor output. The brain and spinal cord are both broadly comprised of white matter, representing myelinated axons within a matrix of glial cells, and grey matter, representing cell bodies and dendrites with connecting axons and synapses.

The peripheral nervous system, lying outside the dura mater, comprises the somatic or spinal nerves and the visceral or autonomic nerves. The 12 paired cranial nerves may be either part of the central or the peripheral nervous system. There is a close relation between structure and function in the nervous system.

Cells of the nervous system

1. • Neurons are the fundamental structural and functional units of the nervous system. They are specialised for the reception, integration and transmission of information. Neurons can be broadly classed as afferent, efferent and interneurons (integrators and signal changers). There are about 1010 neurons in the body. Neurons act on other neurons, muscle cells and glandular cells via neurotransmitters. The signalling functions of neurons allow the processing of sensory information, and the programming of motor and emotional responses, learning and memory.

2. • Neuroglial cells are the supporting cells of the nervous system, occupying the spaces between the neurons. They are not excitable (not generating action potentials) and do not possess axons or form synapses, thereby not being involved in the transmission of information. They retain the capability to undergo mitosis, and secrete growth promoting molecules. Glial cells form myelin sheaths, provide nutrition to the neurons, contribute to the formation of the blood–brain barrier and are involved in the reuptake of neurotransmitters. The glial cells outnumber neurons 10:1.