The nervous system is responsible for giving instructions to all the other systems of the body. Some form of nervous system is present in most types of animals and usually includes a central component, with a brain and a spinal cord, and a peripheral component, which includes long sensory and somatic nerves. The nervous system is made up of specialized cells called neurons, which send and receive chemical information between the various parts of the body.
One end of a neuron contains dendrites, which receive signals. The other side contains presynaptic terminals, which send signals to other neurons. In between these two ends there is a long axon, which is covered by myelin and through which the signal travels. Like other cells in the body, neurons are able synthesize energy. Unlike other cells, they are able to send and receive chemical messages.
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In humans and other mammals, the nervous system is divided into a number of subsystems. The central nervous system contains the brain and the spinal cord, which are used to gather and process information retrieved by the neurons in the peripheral nervous system. The brain and spinal cord also send instructions to peripheral neurons. The neurons in the brain are also responsible for learning and memory which allows animals to adapt to their environments.
The spinal cord acts as a go-between from the brain to the peripheral nervous system. It also plays a role in the autonomic nervous system which is a part of the peripheral system. This system functions without conscious awareness and controls many different processes, such as digestion, respiration, sexual desire, and endocrine function. The autonomic system is further broken down into two sub-systems, the sympathetic and parasympathetic nervous systems. The sympathetic nervous system is responsible for increased blood flow, heart rate, respiration, and level of alertness when an animal is under stress, while the parasympathetic nervous system is responsible for calming the organism back down again once the danger has passed.
Aside from regulating the autonomic system, peripheral neurons also control sensory and somatic functions. Sensory neurons are found in the sensory organs, such as the eyes and ears, as well as on the skin, from where they relay information about temperature, pressure and pain back to the brain. Somatic neurons control voluntary movements in the muscles.