NERVOUS SYSTEM
Для продолжающих
The Nerve is an element of the central or peripheral nervous system that carries impulses from the brain or spinal cord to the eyes, ears, muscles, glands and other parts of the body. Nerves consist of bundles of fibers covered with a sheath of connective tissue and sometimes by a layer of fatty cells (myelin). Nerve impulses are transmitted by a weak electrical current that results from chemical changes taking place through the nerve wall. The final transmission from one nerve to another or from a nerve to another structure is carried out by a chemical reaction via substances known as neurotransmitters.
The main nerves are named according to the regions to which they branch off. There are 12 pairs of cranial nerves and 31 pairs of spinal nerves. The spinal nerves consist of 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumber nerves, 5 pairs of sacral nerves and 1 pair of coccygeal nerves.
The Nervous system is a network of billions of interconnected nerve cells (neurons) that receive stimuli, coordinate this sensory information and cause the body to respond appropriately. The nervous system is comprised of 2 main divisions: the central nervous system (CNS) which consists of the brain and the spinal cord; and the peripheral nervous system (PNS) which consists of spinal nerves and cranial nerves that link the CNS with the body’s receptors and effectors. The receptors include sensory cells and sense organs whose function is to sense and respond to various types of stimulation. For example, eyes respond to light, ears respond to sound. The effectors are all of the parts of the body, such as muscles and glands that respond to nerve impulse instructions from CNS.
The CNS receives the information from the PNS and sends instructions to various parts of the body to respond to continually changing conditions. The brain is involved in the processes of thinking, learning, memory and intelligence.
The PNS spreads out from the brain and spinal cord all over the body. It carries information to and from the CNS.
The autonomic nervous system controls involuntary functions in the body. These functions include gland activity, intestinal movements, beating of the heart, sweating. Within the autonomic nervous system there are two divisions: the sympathetic and parasympathetic systems.
The sympathetic system prepares the body for action by increasing blood pressure and heart rate, increasing glucose production by the liver, reducing the secretion of saliva, causing the erection of hairs on the skin, dilating the pupils of the eyes. The parasympathetic nervous system calms down the body after the sympathetic nervous system has speeded up its responses. The effects of the parasympathetic system include constriction of the pupils, slowing of the heart rate, contraction of the bladder, increase in the rate of digestion, constriction of the air passages.