brasheranatomynervous

THE NERVOUS SYSTEM The nervous system is the master control system of the body. It communicates with the rest of the body through electrical impulses which cause almost immediate responses. The nervous system works very closely with the endocrine system, which produces hormones, to maintain homeostasis within the body. The nervous system monitors changes, makes decisions, and responds. The //central nervous system//, or CNS, consists of the brain and spinal cord, which occupy the dorsal body cavity. All the nerves that extend from the brain and spinal cord make up the //peripheral nervous system,// or PNS. //Spinal nerves// carry impulses to and from the spinal cord, while //cranial nerves// carry impulses directly to or from the brain. The PNS is divided into two major parts. The //sensory,// or //afferent//, division of the PNS carries impulses toward the CNS. The //motor//, or //efferent,// division carries impulses from the CNS to effector organs such as muscles or glands. There are cells found within the CNS that do not transmit electrical impulses. These cells, called //neuroglia,// can undergo mitosis and reproduce. Because these are the only cells of the nervous system that can divide, they are the cells responsible for all nervous system tumors, called //gliomas.// There are several types of neuroglial cells. //Astrocytes// compose almost 1/2 of all nervous system tissues and anchor neurons to capillaries. //Microglia// are phagocytes which dispose of debris in the nervous system. Within the cavities of the brain and spinal cord are //ependymal cells,// which have cilia that help to circulate //cerebrospinal fluid,// or CSF. Finally, //oligodendrocytes// wrap around nerve fibers to form a flat covering called the //myelin sheath.// Outside of the CNS, the myelin sheath is formed by Schwann cells. The cells of the nervous system that actually carry impulses are called //neurons.// They consist of a cell body and several different processes. Processes that carry impulses toward the cell body are called //dendrites,// while the //axon// carries impulses away from the cell body. The longest processes in the human body extend from the lumbar region of the spine to the big toe. Many neurons are covered by a myelin sheath. Gaps in the sheath, called //nodes of Ranvier,// allow impulses to skip down the axon and speed up the rate of conduction. This jumping action is called //saltatory conduction.// People who have multiple sclerosis, an autoimmune disease, gradually lose the myelin covering on the axon. The body attacks a protein found in the sheath and destroys it.

NEURON PHYSIOLOGY Neurons have the ability to respond to a stimulus and convert it into a nerve impulse. This property is called //irritability.// Neurons have the ability to transmit the impulse to other neurons, muscles, or glands. This is called //conductivity.// A resting or inactive membrane in a neuron is called a //polarized membrane.// When inactive, there are sodium ions outside the membrane and potassium ions inside. Even though both ions are positive ions, the inside of the membrane is more negative than the outside. When a stimulus is received by the neuron, the sodium channels in the membrane open and sodium ions diffuse into the neuron. The neuron is now //depolarized,// which causes the neruon to transmit an //action potential//, or impulse. To compensate for the positive sodium ions which have just flooded the neuron, potassium ions diffuse out. This process is called //repolarization.// After repolarization, the sodium-potassium pump restores the original conditions by breaking down ATP.