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The function of the nervous system is to provide rapid communication and integration between various organs, as well as with the outside environment. It detects changes within the body and in its surroundings, and responds accordingly. Fast communication is achieved by means of electrical signals, known as nerve impulses, which are generated and carried by specialized cells, called neurons.
The major components of the nervous system are the brain, spinal cord and nerves. The brain, enclosed and protected in the cranium, is the central processing center. It receives information, makes decision and coordinates the body response. The spinal cord, enclosed in the spinal column, functions as a communication gateway between the brain and the trunk and limbs. Nerves are cordlike structures that conduct information, similar to electricity-conducting wires. They are composed of axons of neurons, the cell bodies of which are clustered in knot-like structures called ganglia. Ganglia commonly serve as relay centers, where neurons synapse and transmit information to each other.
The brain and spinal cord make up the central nervous system, while nerves and ganglia constitute the peripheral nervous system.
Functionally, a nerve fiber can be sensory or motor. Sensory nerve fibers carry sensory information from sensory receptors to the central nervous system, while motor nerves conduct motor instructions from the central nervous system to effector organs – the muscles and glands. Nerves that contain both sensory and motor fibers are known as mixed nerves.
There are 2 major groups of nerves: cranial nerves and spinal nerves:
– The 12 pairs of cranial nerves emerge from the base of the brain and relay information between the brain and the head and neck regions. The cranial nerve X, named vagus nerve, also communicates with internal organs.
– The 31 pairs of spinal nerves arise from segments of the spinal cord and innervate the trunk and limbs. Spinal nerves communicate with the brain via the spinal cord. All spinal nerves are mixed nerves, they contain both sensory and motor fibers. Typically, sensory receptors send impulses by way of sensory fibers in spinal nerves, to the spinal cord, which relays the information up to the brain. The brain interprets the information and sends back instructions, down the spinal cord, to motor fibers in spinal nerves, to reach effector organs.
The peripheral nervous system can be divided into somatic and visceral subdivisions. The somatic nervous system includes sensory nerves from the skin, muscles, bones and joints; and motor nerves that innervate skeletal muscles. This system controls voluntary muscular contractions, as well as involuntary somatic reflexes. The visceral nervous system, on the other hand, includes sensory division that detects changes in the viscera – the organs in the thoracic and abdominal cavities; and motor division that controls cardiac muscle, smooth muscle of internal organs and glands. It produces, for example, faster heart rate and breathing rate during physical exercise, and slower cardiorespiratory rate during sleep. The visceral motor division is also known as the autonomic nervous system because it is largely autonomous, acting independently of the body’s consciousness and voluntary control.