The Nervous System
The nervous system is present in all higher forms of animal life. It is constituted by specialized cells called 'neurons' or nerve cells. The main function of the nervous system is to collect information from the environment and coordinate it with the internal activity of the organism. Simple animals that lack a nervous system can, however, give some type of response to external stimulation.
The specialization of nerve cells makes possible the transmission of information from one cell to another and the arrangement of cells in nets of great complexity. Some of these cells are adapted to gather information from the surroundings, thus constituting the sensory sub-system. Others integrate the central system, usually conformed by a brain and a spinal cord. Still others form the motor system, which transmits signals to the muscles. The sensory system and the motor system together form the peripheral system.
The information is transmitted by electrical impulses called 'signals' from the sensory cells to the central system, which sends commands through the motor system to the muscles to produce an action. Thus, actions are the result of sensorial perceptions. Muscle activity can be voluntary or involuntary. In the latter case, the nerves form the autonomic nervous system.
A neuron (as found in the human brain) has three principal parts: the cell body, the dendrites, and the axon. The cell body is similar to that of other animal cells. The dendrites are extensions of the cell body that receive signals from neighbor nerve cells. The axon is a long fiber that in big animals can span several meters. It is usually covered with a myelin sheath and its function is to transmit information.
When a neuron is at its rest state, the interior of the cell has a negative potential with respect to the outside of the cell. When the cell is stimulated, there is an inward flux of positive charged ions that first depolarizes the cell, and next produces a positive charge in the cell's interior. This positive charge is propagated along the axon. When the charge reaches the end of the axon, structures known as synaptic bulbs release chemical substances (neurotransmitters) that they store in vesicles.
Neurotransmitters travel the distance that exists between a neuron and the adjacent one, called synapse or synaptic cleft. This space is very small: approximately 200 angstroms. Neurotransmitters, when received by the other cell, cause this one to depolarize and so the nervous impulse is transmitted from one cell to another.
A clear difference between nervous systems of invertebrate and vertebrate animals is the presence in vertebrates of a myelin sheath covering the axons of nerve cells. This sheath produces greater speed and reliability in the transmission of nervous impulses. Without it, nerves would need to be thicker, or they would transmit signals less rapidly.
The organization of nerve cells in nervous systems is another characteristic of higher animals. Lower organisms, such as the jellyfish, do not have nerve cells organized into ganglia, but they constitute a net of nerves. Worms have a more organized system where the cell bodies are grouped into ganglia and axons are assembled to form nerves. Round worms show an organization common to many animals: a cerebral ganglion, a main nervous cord, and bilateral nerves. More sophisticated patterns appear in insects like flies.
Vertebrate animals have nervous systems characterized by their bilaterally symmetrical structure and by the specialization of neurons. Sensory and motor nerves are separated, and the spinal cord segments control different body parts. In the brain, certain regions are specialized in the processing of specific sensory information: visual, auditory, olfactory, and other type of stimuli. In the part of the brain known as the cerebellum, there can be found five different types of neurons that managed delicate tasks. A similarly high specialization is evident in the sense organs as, for example, the eye's retina, where five types of receptor cells are present to process visual stimuli.
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