Nervous system

What is the central nervous system

1. The Brain
This is the enlarged anterior end of the spinal cord. Like the spinal cord it has two regions called the grey matter and the white matter. In the brain the grey matter makes the outer layer surrounding the inner region of the white matter. There are also spaces called ventricles which are filled with cerebro-spinal fluid. The brain is the receiver and the analyser of the sensory information. It has three main regions: the forebrain, the midbrain and the hind-brain. In man, the cerebrum, which is part of the forebrain, is three quarters of the total and covers the rest. It is divided into two highly folded cerebral hemispheres by deep fissures and these are connected across the fissure by the corpus callosum. The cerebrum is the main analysing centre of the brain. It receives all the sensory data and is the site of memory, will, intelligence and the conscious functions (higher mental activities). Just below the hemispheres is the thalamus. It is used to connect the cerebrum with the rest of the brain. Its base is called the hypothalamus which is closely connected to the hypophysis or pituitary gland. The midbrain is just a passage to the hind-brain. The hind-brain has the cerebellum, also in two hemispheres. The cerebellum co-ordinates muscle movement and maintains the muscle tone. The medulla oblongata is where the spinal cord and the brain join. The involuntary reflex centres controlling heartbeat, breathing, etc. are found here. Between the medulla oblongata and the cerebellum is found the Pons. It contains the nerve fibres joining the various parts of the brain. Paired cranial nerves run from the Pons and other parts of the brain. 2. The Spinal Cord
The spinal cord is a long, hollow tube of neurons travelling to and from the brain with 31 pairs of spinal nerves running from it. Each spinal nerve has a dorsal root where sensory neurons enter and a ventral root where motor neurons leave. The spinal cord has a spinal canal in the centre which is continuous with the brain ventricles and contains cerebro-spinal fluid. The grey matter is on the inside of the spinal cord with the white matter surrounding it on the outside (opposite to the grey/white matter composition in the brain). The spinal nerves, part of the peripheral nervous system, run from the spinal cord and are mixed nerves which link the receptors and effectors to the brain and spinal cord. 3.

The Meninges
The brain and the spinal cord have a set of three membranes that surround them. i The tough, outer dura mater;
ii The middle arachnoid which contains cerebro-spinal fluid; and
iii The inner pia mater which is rich in fine blood vessels.