The Basics of Human Nervous System
ByThe nervous system has two main parts: a central one consisting of the brain and spinal cord, and a peripheral one composed of the nerves connecting the various parts of the body with the brain and spinal cord.
The peripheral nerves receive information about events both inside and outside the body and relay reports to the brain or spinal cord. From the latter, quick reflex commands can be transmitted back along the nerves to instruct a part of the body to respond. Alternatively, the information can be transmitted to the brain where it is interpreted, considered and acted upon. Some information is stored for future reference.
Tiny nerve endings from specific tissues in each area join to form bundles of white nerve fibres. Information travels along nerve fibres in the form of electrical impulses at a speed of 50-80 metres (160-260 ft) per second. There are separate fibres to carry messages to and from the central nervous system.
The brain is protected by the hard rigid skull. The spinal cord runs through a canal in the vertebral bones in the spine. Both brain and spinal cord are covered with three layers of tissue called the meninges. Between two of these layers is cerebrospinal fluid which cushions the underlying nervous tissue and acts as a reservoir to maintain normal pressure in the brain.
The brain is the main co-ordinating centre of the nervous system. It has sensory input from the twelve pairs of cranial nerves (which receive information from the head, including the ears, eyes, nose and mouth) and also from the spinal cord, which relays messages from the rest of the body.
The bulkiest part of the brain is the cerebrum in the front, top and back of the head. The human cerebrum is proportionately larger than that in other animals and this accounts for the difference in intelligence. The cerebrum is composed of two halves, the cerebral hemispheres. These are concerned with the control of voluntary movement, consciousness, memory, learning, reasoning, judgement, emotions, dreaming and creativity. The right hemisphere controls the muscles of the left side of the body and vice versa.
Inside the cerebrum lies the wedge-shaped thalamus which conveys incoming sensory messages to the cerebral hemispheres. The hypothalamus is connected to the underside of the thalamus and makes several hormones which regulate the production of hormones by the pituitary gland. It also functions as an integrating centre for all the involuntary or automatic activities of the body which are normally not under conscious control, such as breathing, the heartbeat, the regulation of blood pressure and the movements of the gut.
The midbrain connects the cerebrum with the cerebellum, which is responsible for balance and the co-ordination of movements, the pons, the junction box of all the nervous pathways of the brain, and the medulla, which is the continuation of the spinal cord into the brain.
When things go wrong
Sometimes pain isn’t felt in the part of the body it comes from, but is felt in the skin somewhere else. This is known as referred pain, because the body ‘refers’ the pain from the injured or diseased part to a specific area of skin elsewhere. This happens because the body of the foetus develops in segments. The skin, organs and other structures in each segment share a common sensory nerve supply. As the baby grows, the organs and structures may move to occupy positions far away from the skin originating from the same segment, but pain may be interpreted by the nervous system as coming from the area of skin which developed from the same segment.
One of the commonest examples of referred pain is earache. The skin of the outer and middle ear is supplied by six different nerves, each of which also carries sensory information from other areas to the brain. Pain in one of these areas may be referred to the part of the ear which is supplied by the same nerve. Three to six days after a tonsillectomy, pain is often referred to the middle ear and to the crease behind the ear. The same can happen with tonsillitis, an abscess in the back of the throat (a quinsy), or with something stuck in the throat. Dental decay in the lower molars can cause pain in the ear and pain from mumps, wounds or swollen lymph nodes (glands) in the neck may also be felt there. In early appendicitis, pain may be felt around the tummy button.
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