Introduction to the Nervous System

Introduction to the Nervous System - Sensory receptors...

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Organization of the Nervous System Sensory receptors throughout the body continuously monitor conditions in both the internal and external environments. These receptors send information along afferent , or sensory , neurons to the central nervous system. The central nervous system (CNS) , is the integrating center for neural reflexes. CNS neurons in the brain and spinal cord integrate information and determine if a response is needed. The CNS sends output signals directing a response (if any) from efferent or motor , neurons to the effector cells of the body. Efferent neurons are subdivided into the somatic motor division , which controls skeletal muscles, and the autonomic division , which controls smooth and cardiac muscles, exocrine and some endocrine glands, and some types of adipose tissue. These efferent neurons together with the afferent sensory neurons form the peripheral nervous system (PNS). The autonomic division is also called the visceral nervous system because it controls contraction and secretion in the various internal organs. Autonomic neurons are further divided into sympathetic and parasympathetic branches, which can be distinguished by their anatomical organization and by the chemicals they use to communicate with their target cells. Many internal organs receive innervation from both types of autonomic neurons, and it is a common pattern to find that the two divisions exert antagonistic control over a single target.
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Cells of the Nervous System The nervous system is composed primarily of two cell types: “support” cells known as glial cells (or glia or neuroglia ) and nerve cells , or neurons , the basic signaling units of the nervous system. Neurons are uniquely shaped cells with long appendages that extend outward from the cell body. These appendages, or processes, are usually classified as either dendrites which receive incoming signals or axons which carry outgoing information away from the cell body.
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Neurons that lie entirely within the CNS are known as interneuron's (short for interconnecting neurons). They come in a variety of forms but often have quite complex branching processes that allow them to form synapses with many other neurons.
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The long axons of peripheral neurons are bundled together with connective tissue into cordlike fibers called nerves that extend from the CNS to the targets of the component neurons. Nerves may carry afferent signals only (sensory nerves), efferent signals only (motor nerves), or signals in both directions (mixed nerves). Many nerves are large enough to be seen with the naked eye and have been given anatomical names.
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Glial Cells Glial cells [glia=glue] outnumber neurons 50:1. Glial cell function is an active area of neurophysiology research, and we still do not fully understand what these cells do.
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Introduction to the Nervous System - Sensory receptors...

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