The Nervous System - Advanced Version / What is the Nervous System?

The nervous system of the human being is responsible for sending, receiving, and processing nerve impulses throughout the body. All the organs and muscles inside your body rely upon these nerve impulses to function. It could be considered as the master control unit inside your body. Sense organs provide the nervous system with information about the environment by means of such senses as sight, hearing, smell, taste, tough, pressure, and pain. Nerves are connected throughout the whole body to the brain. They carry the information throughout the body in the form of electrochemical signals called impulses. These impulses travel from the brain and spinal cord to the nerves located throughout the body.

For example, if we touch something, impulses travel through the nerve network to the brain at a rate of 350 feet per second. This message is sent along the functional component of the nervous system called the neuron or nerve cell. It takes the cooperation of three sub-divisions of the nervous system to carry out the mission of the nervous system . They are the central, the peripheral, and the autonomic nervous systems.

Although divided for discussion purposes, these systems are connected and function together. OK, the nervous system is divided into:

• The Central Nervous System which has has the responsibility for issuing nerve impulses and analyzing sensory data, and includes the brain and spinal cord, the
   
• Peripheral Nervous System which is responsible for carrying these nerve impulses to and from the body and many structures, including the many craniospinal nerves which branch off the brain and the spinal cord, and the
   
• Autonomic nervous system which is composed of the sympathetic and parasympathetic systems and is responsible for regulating and coordinating the functions of vital organs in the body.

Almost all neurons are generated during prenatal life, and they are not replaced by new neurons during postnatal life. Structurally, the nervous system first appears about 18 days after conception. Functionally, it appears with the first sign of reflex activity during the second prenatal month, when stimulation by touch of the upper lip has been shown to evoke an avoidance-withdrawal response of the head. Many reflexes of the head, trunk, and extremities can be elicited in the third month.

During its development the nervous system undergoes remarkable changes to attain its complex organization. In order to produce the estimated one trillion neurons present in the mature brain, an average of 2.5 million neurons must be generated per minute during the entire prenatal life. This includes the formation of neuronal circuits comprising 100 trillion synapses, as each potential neuron is ultimately connected with either a selected set of other neurons or specific targets such as sensory endings.

The nervous system can be divided into "systems".

They release different transmitter substances (neurotransmitters), usually triggering opposite reactions.

The vegetative nervous system is controlled by the hypothalamus in the brain (cerebrum), where the information arrives, coordinating the interaction and functions of the organs.

If, for example, increased physical exertion leads to an oxygen deficiency, the depth and rate of the breaths taken automatically increase. This leads to an increase in cardiac activity and an increased supply of oxygenated blood.

Now we'll move to the nerves, which are the functional unit of the nervous system and discuss them first.

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