Anatomy and Physiology of the Nervous System
The nervous system coordinates and controls the functioning of the various bodily systems in response to changes in our surroundings. The sensory organs and receptors send information about the body and the environment to the central nervous system. This information is processed by the central nervous system that in turn, sends directions to the various organs to respond and adapt according to the external conditions which affect our body.
Neurons are the basic unit of the nervous tissue. Neurons are cells specialized to conduct nerve impulses and consist of a cell body, axon, and dendrites. The central cell body contains the neuron’s nucleus and associated cytoplasm and organelles. The nerve processes are “finger-like” projections from the cell body that are able to conduct and transmit signals. There are two types of nerve processes: axons and dendrites. Axons typically carry signals away from the cell body, while dendrites typically carry signals toward the cell body. The nervous system comprises the central nervous system (CNS) that includes the brain and spinal cord, and the peripheral nervous system (PNS) comprising cranial nerves and spinal nerves. The brain includes the cerebral hemispheres, brainstem, and cerebellum. The brainstem includes the diencephalon, midbrain, pons, and medulla oblongata. The spinal cord is a thick, whitish cord of nerve tissue that extends from the medulla oblongata down through the spinal column, and from which the spinal nerves branch off to various parts of the body. The peripheral nervous system comprises nerves emerging from the brain (cranial nerves) and nerves emerging from the spinal cord (spinal nerves). The cranial nerves include several nerves that arise in pairs from the brain stem and reach the periphery through openings in the skull. Spinal nerves form a network of pathways for conducting information throughout the body. These nerves are divided into sensory nerves that pass impulses from receptors toward or to the central nervous system, and motor nerves that convey impulses from the central nervous system to muscles or toward glands.
The peripheral nervous system is further divided into the Somatic Nervous System (SNS) and Autonomic Nervous System (ANS). The somatic nervous system is responsible for sensation and control of the skeletal muscles. The somatic nervous system innervates all sensory organs, including the eyes, ears, tongue, and skin, as well as all skeletal muscles used for voluntary movement. For movement, the somatic nervous system carries impulses from the brain to the muscle to be moved. While in its sensory capacity, the somatic nervous system carries impulses from the sensory organs to the brain. There are therefore two portions, or limbs, of the somatic nervous system, the afferent and the efferent. The afferent, or sensory, neurons carry impulses from sense organs into the central nervous system, while the efferent, or motor neurons, carry impulses from the central nervous system to the muscles.
The Autonomic Nervous System
The autonomic nervous system regulates involuntary activity in the body by transmitting motor impulses to cardiac muscles, smooth muscles, and the glands. The muscular activity of the heart and of the circulatory, digestive, respiratory, and urogenital systems are controlled by the autonomic nervous system. The autonomic nervous system is divided into two parts, the sympathetic nervous system and the parasympathetic nervous system, respectively. The sympathetic nervous system is activated especially under conditions of stress and its general action is to help prepare the body for fight-or-flight response. The sympathetic nervous system increases the heart rate and the strength of heart contraction, and causes vasoconstriction of the blood vessels. Furthermore, the sympathetic nervous system regulates the function of the adrenal glands, sweat glands, and decreases secretions of the digestive system, while stimulates the secretion of glucose in the liver. The parasympathetic system is responsible for stimulation of “rest-and-digest” activities that occur when the body is at rest. Thus, the parasympathetic nervous system tends to act in opposition to the sympathetic nervous system, by decreasing the heart rate and dilating the blood vessels. The parasympathetic nervous system regulates the function of many glands, such as those that produce tears and saliva, and stimulates motility and secretions of the digestive system.
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