The nervous system coordinates behavior and helps to maintain the internal stability of animals. It may be as simple as the nerve net of Cnidarians or as complex as the centralized system of mammals. In all nervous systems the functional unit is the nerve cell or neuron, a cell specialized to transmit and receive a stimulus.
To survive, animals have to respond to changes in their internal and external environment. General responses are found in animals that have a simple nervous system and can only process information in a limited way. An example of this type of nervous system is found in the common freshwater Hydra, a cylinder-shaped inverebrate. It has a nerve net of neurons between the outer and inner layers of a s…
The nervous system shows the greatest development in vertebrates. There is an increase in centralization with increasing elaboration of the brain with areas with specific functions. The central nervous system includes the brain and a dorsal (upper) spinal cord encased and protected by the skeletal system. The central nervous system is connected to the rest of the body through a peripheral nervous …
In humans, centralization has reached the greatest degree of specialization. The brain and spinal cord are formed early in embryonic development. At the beginning of the third week of gestation, the embryo has already formed a neural plate on the dorsal surface which eventually folds together to form a hollow tube from which the brain and spinal cord develop. During this time the 100 billion neuro…
The central nervous system operates through the peripheral nervous system which is the "roadway" that links the central nervous system to the rest of the body. The nerves that carry information to the central nervous system from sensory receptors such as the eye are called sensory nerves or afferent nerves; those that carry impulses away from the central nervous system to effector or…
The functional unit of the nervous system is the neuron, a cell specialized to receive and transmit impulses. The types and functions of neurons found in organisms seem to be directed by several regulatory genes and by certain cues that occur during development. Once neurons mature, they lose the ability to divide. An exception is the olfactory neurons which are replaced every 60 days from stem ce…
When a stimulus is strong enough, a nerve impulse is generated in an "all or none" response which means that a stimulus strong enough to generate a nerve impulse has been given. The stimulus triggers chemical and electrical changes in the neuron. Before an impulse is received, a resting neuron is polarized with different charges on either side of the cell membrane. The exterior of th…
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