V. Nervous System

The nervous system has two divisions: the somatic, which allows voluntary control over skeletal muscle, and the autonomic, which is involuntary and controls cardiac and smooth muscle and glands. The autonomic nervous system has two divisions: the sympathetic and the parasympathetic. Many, but not all, of the muscles and glands that distribute nerve impulses to the larger interior organs possess a double nerve supply; in such cases the two divisions may exert opposing effects. Thus, the sympathetic system increases heartbeat, and the parasympathetic system decreases heartbeat. The two nervous systems are not always antagonistic, however. For example, both nerve supplies to the salivary glands excite the cells of secretion. Furthermore, a single division of the autonomic nervous system may both excite and inhibit a single effector, as in the sympathetic supply to the blood vessels of skeletal muscle. Finally, the sweat glands, the muscles that cause involuntary erection or bristling of the hair, the smooth muscle of the spleen, and the blood vessels of the skin and skeletal muscle are actuated only by the sympathetic division.

Voluntary movement of head, limbs, and body is caused by nerve impulses arising in the motor area of the cortex of the brain and carried by cranial nerves or by nerves that emerge from the spinal cord to connect with skeletal muscles. The reaction involves both excitation of nerve cells stimulating the muscles involved and inhibition of the cells that stimulate opposing muscles. A nerve impulse is an electrical change within a nerve cell or fiber; measured in millivolts, it lasts a few milliseconds and can be recorded by electrodes.

Movement may occur also in direct response to an outside stimulus; thus, a tap on the knee causes a jerk, and a light shone into the eye makes the pupil contract. These involuntary responses are called reflexes. Various nerve terminals called receptors constantly send impulses into the central nervous system. These are of three classes: exteroceptors, which are sensitive to pain, temperature, touch, and pressure; interoceptors, which react to changes in the internal environment; and proprioceptors, which respond to variations in movement, position, and tension. These impulses terminate in special areas of the brain, as do those of special receptors concerned with sight, hearing, smell, and taste.

Muscular contractions do not always cause actual movement. A small fraction of the total number of fibers in most muscles are usually contracting. This serves to maintain the posture of a limb and enables the limb to resist passive elongation or stretch. This slight continuous contraction is called muscle tone.

Exercises

1. Render the text into English.

Нервная система регулирует деятельность всех органов и систем органов и, тем самым, обеспечивает функциональное единство организма. Нервная система осуществляет еще одну важную функцию – взаимодействие организма с окружающей его внешней средой. Таким образом, нервная система объединяет отдельные органы и системы органов в целостный организм и обеспечивает единство организма с внешней средой.

Основной структурой и функциональной единицей нервной системы является нервная клетка, или нейрон. Нервные клетки обладают свойством проведения нервного возбуждения – нервного импульса. Нервное возбуждение проводится от дендрита к телу нервной клетки, а по аксону передается следующему нейрону или рабочему органу.