Introduction to Nervous System.docx

The nervous system is the master control and communication system of the body. Every thought, action, and emotion reflects its activity. It communicates with body cells using electrical impulses which are rapid and specific and cause almost immediate responses. The nervous system does not work alon...

The nervous system is the master control and communication system of the body. Every thought, action, and emotion reflects its activity. It communicates with body cells using electrical impulses which are rapid and specific and cause almost immediate responses. The nervous system does not work alone to regulate and maintain body homeostasis; the endocrine system is a second important regulating system. Whereas the nervous system controls with rapid electrical nerve impulses, the endocrine system produces hormones that are released into the blood. Thus, the endocrine system acts in a more leisurely way. We will discuss the endocrine system in detail in Chapter 9. To carry out its normal role, the nervous system has three overlapping functions: (1) It uses its millions of sensory receptors to monitor changes occurring both inside and outside the body. These changes are called stimuli, and the gathered information is called sensory input. (2) It processes and interprets the sensory input and decides what should be done at each moment---a process called integration. (3) It then causes a response, or effect, by activating muscles or glands (effectors) via motor output. CLASSIFICATIONS OF THE NERVOUS SYSTEM - - Structural Classification\ The structural classification, which includes all nervous system organs, has two subdivisions---the central nervous system and the peripheral nervous system (see Figure 7.2). 1. body cavity and act as the integrating and command centers of the nervous system. They interpret incoming sensory information and issue instructions based on past experience and current conditions. 2. Functional Classification The functional classification scheme is concerned only with PNS structures. It divides them into two principal subdivisions (see Figure 7.2). 1. 2. The autonomic (aw″to-nom′ik) nervous system (ANS) regulates events that are automatic, or involuntary, such as the activity of smooth muscle, cardiac muscle, and glands. This subdivision, commonly called the involuntary nervous system, itself has two parts, the sympathetic and parasympathetic, which typically bring about opposite effects. What one stimulates, the other inhibits. We will describe these later. NERVOUS TISSUE: STRUCTURE AND FUNCTION Even though it is complex, nervous tissue is made up of just two principal types of cells---supporting cells and neurons. - - - - - Supporting cells in the PNS come in two major varieties---Schwann cells and satellite cells (Figure 7.3e). Schwann cells form the myelin sheaths around nerve fibers in the PNS. Satellite cells act as protective, cushioning cells for peripheral neuron cell bodies. NEURONS Neurons, also called nerve cells, are highly specialized to transmit messages (nerve impulses) from one part of the body to another. Although neurons differ structurally from one another, they have many common features (Figure 7.4, p. 256). All have a cell body, which contains the nucleus and one or more slender processes extending from the cell body. - - - Schwann cell cytoplasm is gradually squeezed from between the membrane layers. When the wrapping process is done, a tight coil of wrapped membranes, the myelin sheath, encloses the axon. Most of the Schwann cell cytoplasm ends up just beneath the outermost part of its plasma membrane. This part of the Schwann cell, external to the myelin sheath, is called the neurilemma (nu″r˘ı-lem′mah, "neuron husk"). Because the myelin sheath is formed by many individual Schwann cells, it has gaps, or indentations, called nodes of Ranvier (rahn-vˉer), at regular intervals (see Figure 7.4).\ As mentioned previously, myelinated fibers are also found in the central nervous system. Oligodendrocytes form CNS myelin sheaths (see Figure 7.3d). In the PNS, it takes many Schwann cells to make a single myelin sheath; but in the CNS, the oligodendrocytes with their many flat extensions can coil around as many as 60 different fibers at the same time. Thus, in the CNS, one oligodendrocyte can form many myelin sheaths. Although the myelin sheaths formed by oligodendrocytes and those formed by Schwann cells are similar, the CNS sheaths lack a neurilemma. Because the neurilemma remains intact (for the most part) when a peripheral nerve fiber is damaged, it plays an important role in fiber regeneration, an ability that is largely lacking in the central nervous system. The importance of myelin insulation is best illustrated by observing what happens when myelin is not there. The disease multiple sclerosis (MS) gradually destroys the myelin sheaths around CNS fibers by converting them to hardened sheaths called scleroses. As this happens, the electrical current is short-circuited and may "jump" to another demyelinated neuron. In other words, nerve signals do not always reach the intended target. The affected person may have visual and speech disturbances, lose the ability to control his or her muscles, and become increasingly disabled. Multiple sclerosis is an autoimmune disease in which the person's own immune system attacks a protein component of the sheath. As yet there is no cure, but injections of interferon (a hormonelike substance released by some immune cells) appear to hold the symptoms at bay and provide some relief. Other drugs aimed at slowing the autoimmune response are also being used, though further research is needed to determine their long-term effects. Terminology. Clusters of neuron cell bodies andcollections of nerve fibers are named differently in the CNS and in the PNS. For the most part, cell bodies are found in the CNS in clusters called nuclei. This well-protected location within the bony skull or vertebral column is essential to the well-being of the nervous system---remember that neurons do not routinely undergo cell division after birth. The cell body carries out most of the metabolic functions of a neuron, so if it is damaged, the cell dies and is not replaced. Small collections of cell bodies called ganglia (gang′le-ah; ganglion, singular) are found in a few sites outside the CNS in the PNS. Bundles of nerve fibers (neuron processes) running through the CNS are called tracts, whereas in the PNS they are called nerves. The terms white matter and gray matter refer respectively to myelinated versus unmyelinated regions of the CNS. As a general rule, the white matter consists of dense collections of myelinated fibers (tracts), and gray matter contains mostly unmyelinated fibers and cell bodies. CLASSIFICATION OF NEURONS - - Functional Classification Functionally, neurons are grouped according to the direction the nerve impulse travels relative to the CNS. On this basis, there are sensory, motor, and association neurons (interneurons) (Figure 7.6). 1. 2. 3. Structural Classification Structural classification is based on the number of processes, including both dendrites and axons, extending from the cell body (Figure 7.8). 1. 2. 3. PHYSIOLOGY: NERVE IMPULSES Neurons have two major functional properties: irritability, the ability to respond to a stimulus and convert it into a nerve impulse, and conductivity, the ability to transmit the impulse to other neurons, muscles, or glands. - The plasma membrane of a resting, or inactive, neuron is polarized, which means that there are fewer positive ions sitting on the inner face of the neuron's plasma membrane than there are on its outer face. The major positive ions inside the cell are potassium (K+), whereas the major positive ions outside the cell are sodium (Na+). As long as the inside remains more negative (fewer positive ions) than the outside, the neuron will stay inactive. - Many different types of stimuli excite neurons to become active and generate an impulse. For example, light excites the eye receptors, sound excites some of the ear receptors, and pressure excites some cutaneous receptors of the skin. However, most neurons in the body are excited by neurotransmitter chemicals released by other neurons, as we will describe shortly. Regardless of the stimulus, the result is always the same---the permeability properties of the cell's plasma membrane change for a very brief period. Normally, sodium ions cannot diffuse through the plasma membrane to any great extent, but when the neuron is adequately stimulated, the "gates" of sodium channels in the membrane open. Because sodium is in much higher concentration outside the cell, it then diffuses quickly into the neuron. (Remember the laws of diffusion?) This inward rush of sodium ions changes the polarity of the neuron's membrane at that site, an event called depolarization. Locally, the inside is now more positive, and the outside is less positive, a local electrical situation called a graded potential. However, if the stimulus is strong enough and the sodium influx is great enough, the local depolarization (graded potential) activates the neuron to initiate and transmit a long-distance signal called an action potential, also called a nerve impulse in neurons.The nerve impulse is an all-or-none response, like starting a car. It is either propagated (conducted, or sent) over the entire axon, or it doesn't happen at all. The nerve impulse never goes partway along an axon's length, nor does it die out with distance, as do graded potentials. Almost immediately after the sodium ions rush into the neuron, the membrane permeability changes again, becoming impermeable to sodium ions but permeable to potassium ions. So potassium ions are allowed to diffuse out of the neuron into the interstitial fluid, and they do so very rapidly. This outflow of positive ions from the cell restores the electrical conditions at the membrane to the polarized, or resting, state, an event called repolarization. After repolarization of the electrical conditions, the sodium-potassium pump restores the initial concentrations of the sodium and potassium ions inside and outside the neuron. This pump uses ATP (cellular energy) to pump excess sodium ions out of the cell and to bring potassium ions back into it. Until repolarization occurs, a neuron cannot conduct another impulse. Once begun, these sequential events spread along the entire neuronal membrane. The events just described explain propagation of a nerve impulse along unmyelinated fibers. Fibers that have myelin sheaths conduct impulses much faster because the nerve impulse literally jumps, or leaps, from node to node along the length of the fiber. This occurs because no electrical current can flow across the axon membrane where there is fatty myelin insulation. This faster type of electrical impulse propagation is called saltatory (sal′tahto″re) conduction (saltare = to dance or leap). - PHYSIOLOGY: REFLEXES Although there are many types of communication between neurons, much of what the body must do every day is programmed as reflexes. Reflexes are rapid, predictable, and involuntary responses to stimuli. They are much like one-way streets--- once a reflex begins, it always goes in the same direction. Reflexes occur over neural pathways called reflex arcs and involve both CNS and PNS structures. Think of a reflex as a preprogrammed response to a given stimulus. The types of reflexes that occur in the body are classed as either somatic or autonomic. 1. a. b. c. d. e. The simple patellar (pah-tel′ar), or knee-jerk, reflex is an example of a two-neuron reflex arc, the simplest type in humans. The patellar reflex (in which the quadriceps muscle attached to the hit tendon is stretched) is familiar to most of us. It is usually tested during a physical exam to determine the general health of the motor portion of our nervous system. Most reflexes are much more complex than the two-neuron reflex, involving synapses between one or more interneurons in the CNS (integration center). The flexor, or withdrawal, reflex is a three-neuron reflex arc in which the limb is withdrawn from a painful stimulus. A three-neuron reflex arc also consists of five elements---receptor, sensory neuron, interneuron, motor neuron, and effector. Because there is always a delay at synapses (it takes time for neurotransmitter to diffuse through the synaptic cleft), the more synapses there are in a reflex pathway, the longer the reflex takes to happen. Many spinal reflexes involve only spinal cord neurons and occur without brain involvement. As long as the spinal cord is functional, spinal reflexes, such as the flexor reflex, will work. By contrast, some reflexes require that the brain become involved because many different types of information have to be evaluated to arrive at the "right" response. The response of the pupils of the eyes to light is a reflex of this type. As noted earlier, reflex testing is an important tool in evaluating the condition of the nervous system. Reflexes that are exaggerated, distorted, or absent indicate damage or disease in the nervous system. Reflex changes often occur before a pathological condition becomes obvious in other ways.