The nervous system has been divided into tow components: The central nervous system which is composed of the brain and the spinal cord, and the peripheral nervous system, which is composed of ganglia an peripheral nerves that lie outside the brain and spinal cord.

The peripheral nervous system

The peripheral nervous system has been, in turn divided into two subsystem: Somatic and autonomic.

Somatic subsystem: includes sensory neurons of the dorsal root and cranial ganglia that innervate the skin, muscles, and joints and provide sensory information to the central nervous system about muscle and limb position and about the environment.

Autonomic subsystem: it includes the motor system for the viscera, the smooth muscles and exocrine glands. It in turn consist of three segregated subdivisions: the sympathetic system which participates in the response of the body to stress. The parasympathetic system that acts to conserve de body's resources and restore homeostasis. The enteric nervous system which controls the function of smooth muscle of the gut.

The central nervous system

The central nervous system consists of six main regions:

I.- The spinal cord

It extends from the base of the skull through the first lumbar vertebra. The spinal cord receives sensory information from the skin, joints, and muscles of the trunk and limbs, and contains the motor neurons responsible for both voluntary and reflex movements. It also receives sensory information from the internal organs and control many visceral functions. Within the spinal cord there is an orderly arrangement of sensory cell groups that receive input from the periphery and motor cell groups that control specific muscle groups. In addition, the spinal cord contains ascending pathway through which sensory information reaches the brain and descending pathways that relay motor command from the brain to motor neurons.

II.- The medulla

This structure is the direct rostral extension (this means toward the head and nose) of the spinal cord. Together with the pons It participates in regulating blood pressure and respiration. It resembles the spinal cord in both organization and function.

III.- The pons

It lies rostral to the medulla and contains a large number of neurons that relay information from the cerebral hemispheres to the cerebellum. The cerebellum lies dorsal to the pons and medulla. It has a distinctive corrugated surface. The cerebellum receives somatosensory input from the spinal cord, motor information from the cerebral cortex and balance information from the vestibular organs of the inner ear. The cerebellum integrates this information and coordinates the planning, timing and patterning of skeletal muscle contractions during movement. The cerebellum plays a major role in the control of posture, head and eye movements.

IV.- The midbrain

This is the smallest brain stem component which lies rostral to the pons. The midbrain contains essential relay nuclei of the auditory and visual system. Several regions of this structure play an important role in the direct control of eye movement, whereas others are involved in motor control of skeletal muscles.

V.- The Diencephalon

It consists of the thalamus and hypothalamus. Lies between the cerebral hemispheres and the midbrain. The thalamus distributes (and also processes) almost all sensory and motor information going to the cerebral cortex. In addition, it is thought to regulate levels of awareness and some emotional aspects of sensory experiences. The hypothalamus lies ventral to the thalamus and regulates autonomic activity and the hormonal secretion by the pituitary gland. It has extensive connections with the thalamus, midbrain and some cortical areas that process information from the autonomic system.

VI.- The cerebral hemispheres

It form, in humans, the largest region of the brain. They consist of the cerebral cortex, the white matter under the cortex and three deeply located nuclei: the basal ganglia, the hyppocampal formation and the amygdala. The cerebral hemispheres are divided by the hemispheric fissure and are thought to be concerned with perception, cognition, emotion, memory and high motor functions.

The basal ganglia.

The major components of the basal ganglia are the caudate nucleus, the putamen and the globus pallidus. The basal ganglia have an important role in regulation of movement and also contribute to cognitive functions. They receive input from all four lobes (see bellow) of the cerebral cortex but have output (efferent) projections only to the frontal cortex, via the thalamus.

The Hippocampus and amygdala

The hippocampus and amygdala are part of the so called limbic system. The hippocampus is involved in memory storage (at least in the initial stages of memory formation). The amygdala coordinates the actions of the autonomic and endocrine systems and is involved in emotions.

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