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Anatomy & Physiology Chapter 13: Nervous System Cells Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. Introduction Function of nervous system, along with the endocrine system, is to communicate Nervous system made up of the brain, spinal cord, and nerves (Figure 13-1) Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 2 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 3 Organization of the Nervous System Organized to detect changes in internal and external environments, evaluate the information, and initiate an appropriate response Subdivided into smaller “systems” by location (Figure 13-2) Central nervous system (CNS) Structural and functional center of the entire nervous system Consists of the brain and spinal cord Integrates sensory information, evaluates it, and initiates an outgoing response Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 4 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 5 Organization of the Nervous System Subdivided into smaller “systems” by location (cont) Peripheral nervous system (PNS) Nerves that lie in the “outer regions” of the nervous system Cranial nerves—originate from the brain Spinal nerves—originate from the spinal cord Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 6 Organization of the Nervous System Afferent and efferent divisions Afferent division—consists of all incoming sensory pathways Efferent division—consists of all outgoing motor pathways Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 7 Organization of the Nervous System “Systems” categorized according to types of organs they innervate (Figure 13-2) Somatic nervous system (SNS) Somatic motor division carries information to the somatic effectors (skeletal muscles) Somatic sensory division carries feedback information to somatic integration centers in the CNS Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 8 Organization of the Nervous System “Systems” (cont) Autonomic nervous system (ANS) Efferent division of ANS carries information to the autonomic or visceral effectors (smooth and cardiac muscles, glands, and adipose and other tissues) Sympathetic division—prepares the body to deal with immediate threats to the internal environment; produces “fight-or-flight” response Parasympathetic division—coordinates the body’s normal resting activities; sometimes called the “rest-and-repair” division Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 9 Organization of the Nervous System “Systems” (cont) Autonomic nervous system (cont) Visceral sensory division carries feedback information to autonomic integrating centers in the CNS Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 10 Cells of the Nervous System Glia (neuroglia) Glial cells support the neurons Five major types of glia (Figure 13-3) Astrocytes (in CNS) Star-shaped, largest, and most numerous type of glia Cell extensions connect to both neurons and capillaries Astrocytes transfer nutrients from the blood to the neurons Form tight sheaths around brain capillaries, which, with tight junctions between capillary endothelial cells, constitute the blood-brain barrier (BBB) Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 11 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 12 Cells of the Nervous System Five major types of glia (cont) Microglia (in CNS) Small, usually stationary cells In inflamed brain tissue, they enlarge, move about, and carry on phagocytosis Ependymal cells (in CNS) Resemble epithelial cells and form thin sheets that line fluid-filled cavities in the CNS Some produce fluid; others aid in circulation of fluid Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 13 Cells of the Nervous System Five major types of glia (cont) Oligodendrocytes (in CNS) Smaller than astrocytes with fewer processes Hold nerve fibers together and produce the myelin sheath Schwann cells (in PNS) Found only in peripheral neurons Support nerve fibers and form myelin sheaths (Figure 13-4) Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 14 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 15 Cells of the Nervous System Five major types of glia (cont) Schwann cells (cont) Myelin sheath gaps are often called nodes of Ranvier Neurilemma is formed by cytoplasm of Schwann cell (neurolemmocyte) wrapped around the myelin sheath; essential for nerve regrowth Neuronal sheath is the myelin sheath plus the neurilemma (that is, the whole Schwann wrapping around the axon) Satellite cells are Schwann cells that cover and support cell bodies in the PNS Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 16 Cells of the Nervous System Neurons Excitable cells that initiate and conduct impulses that make possible all nervous system functions Components of neurons (Figure 13-5) Cell body (perikaryon) Ribosomes, rough endoplasmic reticulum (ER), Golgi apparatus Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 17 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 18 Cells of the Nervous System Ribosomes, rough endoplasmic reticulum (ER), Golgi apparatus (Cont) Provide protein molecules (neurotransmitters) needed for transmission of nerve signals from one neuron to another Neurotransmitters are packaged into vesicles Provide proteins for maintaining and regenerating nerve fibers Mitochondria provide energy (ATP) for neuron; some are transported to end of axon Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 19 Cells of the Nervous System Neurons (cont) Components of neurons (cont) Dendrites Each neuron has one or more dendrites, which branch from the cell body Conduct nerve signals to the cell body of the neuron Distal ends of dendrites of sensory neurons are receptors Dendritic spines—small knoblike protrusions on dendrites of some brain neurons; serve as connection points for axons of other neurons Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 20 Cells of the Nervous System Components of neurons (cont) Axon A single process extending from the axon hillock, sometimes covered by a fatty layer called a myelin sheath (Figure 13-6) Conducts nerve impulses away from the cell body of the neuron Distal tips of axons are telodendria, each of which terminates in a synaptic knob Axon varicosities—swellings that make contact (synapse) with other cells Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 21 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 22 Cells of the Nervous System Components of neurons (cont) Cytoskeleton Microtubules and microfilaments, as well as neurofibrils (bundles of neurofilaments) Allow the rapid transport of small organelles (Figure 13-7) Vesicles (some containing neurotransmitters), mitochondria Motor molecules shuttle organelles to and from the far ends of a neuron Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 23 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 24 Cells of the Nervous System Components of neurons (cont) Functional regions of the neuron (Figure 13-8) Input zone—dendrites and cell body Summation zone—axon hillock Conduction zone—axon Output zone—telodendria and synaptic knobs of axon Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 25 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 26 Cells of the Nervous System Classification of neurons Structural classification—classified according to number of processes extending from cell body (Figure 13-9) Multipolar—one axon and several dendrites Bipolar—only one axon and one dendrite; least numerous kind of neuron Unipolar (pseudounipolar)—one process comes off neuron cell body but divides almost immediately into two fibers: central fiber and peripheral fiber Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 27 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 28 Cells of the Nervous System Classification of neurons (cont) Functional classification (Figure 13-10) Afferent (sensory) neurons— conduct impulses to spinal cord or brain Efferent (motor) neurons—conduct impulses away from spinal cord or brain toward muscles or glandular tissue Interneurons Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 29 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 30 Cells of the Nervous System Reflex arc A signal conduction route to and from the CNS, with the electrical signal beginning in receptors and ending in effectors Three-neuron arc—most common; consists of afferent neurons, interneurons, and efferent neurons (Figure 13-11) Afferent neurons—conduct impulses to the CNS from the receptor Efferent neurons—conduct impulses from the CNS to effectors (muscle or glandular tissue) Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 31 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 32 Cells of the Nervous System Reflex arc (cont) Two-neuron arc—simplest form; consists of afferent and efferent neurons Synapse Where nerve signals are transmitted from one neuron to another Two types: electrical and chemical; chemical synapses are typical in the adult Chemical synapses are located at the junction of the synaptic knob of one neuron and the dendrites or cell body of another neuron Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 33 Nerves and Tracts Nerves—bundles of peripheral nerve fibers held together by several layers of connective tissue (Figure 13-12) Endoneurium—delicate layer of fibrous connective tissue surrounding each nerve fiber Perineurium—connective tissue holding together fascicles (bundles of fibers) Epineurium—fibrous coat surrounding numerous fascicles and blood vessels to form a complete nerve Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 34 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 35 Nerves and Tracts Tracts—bundles of nerve fibers within the CNS Unlike nerves, tracts do not have connective tissue coverings Individual fibers may extend through both a nerve and a tract as it passes into or out of the CNS White matter PNS—myelinated nerves CNS—myelinated tracts Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 36 Nerves and Tracts Gray matter Made up of cell bodies and unmyelinated fibers CNS—referred to as nuclei PNS—referred to as ganglia Mixed nerves Contain sensory and motor neurons Sensory nerves—nerves with predominantly sensory neurons Motor nerves—nerves with predominantly motor neurons Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 37 Repair of Nerve Fibers Mature neurons are incapable of cell division; therefore damage to nervous tissue can be permanent Neurons have limited capacity to repair themselves If the damage is not extensive, the cell body and neurilemma are intact, and scarring has not occurred; nerve fibers can be repaired Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 38 Repair of Nerve Fibers Stages of repair of an axon in a peripheral motor neuron (Figure 13-13) Following injury, distal portion of axon and myelin sheath degenerates Macrophages remove the debris Remaining neurilemma and endoneurium form a tunnel from the point of injury to the effector New Schwann cells grow in the tunnel to maintain a path for regrowth of the axon Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 39 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 40 Repair of Nerve Fibers Stages of repair of an axon in a peripheral motor neuron (cont) Cell body reorganizes its Nissl bodies to provide the needed proteins to extend the remaining healthy portion of the axon Axon “sprouts” appear When “sprout” reaches tunnel, its growth rate increases The skeletal muscle cell atrophies until the nervous connection is reestablished In CNS, similar repair of damaged nerve fibers is unlikely Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 41 Nerve Impulses Membrane potentials All living cells maintain a difference in the concentration of ions across their membranes Membrane potential—slight excess of positively charged ions on the outside of the membrane and slight deficiency of positively charged ions on the inside of the membrane (Figure 13-14) Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 42 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 43 Nerve Impulses Membrane potentials (cont) Difference in electrical charge is called potential because it is a type of stored energy Polarized membrane—a membrane that exhibits a membrane potential Magnitude of potential difference between the two sides of a polarized membrane is measured in volts (V) or millivolts (mV); the sign of a membrane’s voltage indicates the charge on the inside surface of a polarized membrane Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 44 Nerve Impulses Resting membrane potential (RMP) Membrane potential maintained by a nonconducting neuron’s plasma membrane; typically 70 mV The slight excess of positive ions on a membrane’s outer surface is produced by ion transport mechanisms and the membrane’s permeability characteristics The membrane’s selective permeability characteristics help maintain a slight excess of positive ions on the outer surface of the membrane (Figure 13-15) Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 45 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 46 Nerve Impulses Resting membrane potential (cont) Sodium-potassium pump (Figure 13-16) Active transport mechanism in plasma membrane that transports Na+ and K+ in opposite directions and at different rates Maintains an imbalance in the distribution of positive ions, resulting in the inside surface becoming slightly negative with respect to its outer surface Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 47 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 48 Nerve Impulses Local potentials Defined as slight shift away from the resting membrane in a specific region of the plasma membrane (Figure 13-17) Excitation—when a stimulus triggers the opening of additional Na+ channels, allowing the membrane potential to move toward zero (depolarization) Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 49 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 50 Nerve Impulses Local potentials (cont) Inhibition—when a stimulus triggers the opening of additional K+ channels, increasing the membrane potential (hyperpolarization) Local potentials are called graded potentials because the magnitude of deviation from the resting membrane potential is proportional to the magnitude of the stimulus Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 51 Action Potential Action potential—the membrane potential of a neuron that is conducting an impulse; also known as a nerve impulse Mechanism that produces the action potential (Figures 13-18 and 13-19) When an adequate stimulus triggers stimulusgated Na+ channels to open, allowing Na+ to diffuse rapidly into the cell, a local depolarization is produced As threshold potential is reached, voltagegated Na+ channels open and more Na+ enters the cell, causing further depolarization Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 52 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 53 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 54 Action Potential Mechanism (cont) As more Na+ rushes into cell, the membrane moves rapidly toward and continues in a positive direction to the peak of the action potential Voltage-gated Na+ channels stay open for only about 1 msec before they automatically close; action potential is an all-or-none response After action potential peaks, membrane begins to move back toward the resting membrane potential when K+ channels open, allowing outward diffusion of K+; process is known as repolarization Brief period of hyperpolarization occurs, and then the resting membrane potential is restored by the sodiumpotassium pumps Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 55 Action Potential Refractory period (Figure 13-20) Absolute refractory period—brief period (lasting approximately 0.5 msec) during which a local area of a neuron’s membrane resists re-stimulation and will not respond to a stimulus, no matter how strong Relative refractory period—time during which the membrane is repolarized and restoring the resting membrane potential; the few milliseconds after the absolute refractory period; will respond only to a very strong stimulus Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 56 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 57 Nerve Impulses At the peak of the action potential, the plasma membrane’s polarity is now the reverse of the RMP The reversal in polarity causes electrical current to flow between the site of the action potential and the adjacent regions of membrane and triggers voltage-gated Na+ channels in the next segment to open; this next segment exhibits an action potential (Figure 13-21) This cycle continues to repeat, producing continuous conduction Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 58 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 59 Nerve Impulses The action potential never moves backward because of the refractory period In myelinated fibers, action potentials in the membrane only occur at the nodes of Ranvier; this type of impulse conduction is called saltatory conduction (Figure 13-22) Speed of nerve conduction depends on diameter and on the presence or absence of a myelin sheath Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 60 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 61 Synaptic Transmission Two types of synapses (junctions) (Figure 13-23) Electrical synapses occur where cells joined by gap junctions allow an action potential to simply continue along postsynaptic membrane Chemical synapses occur where presynaptic cells release chemical transmitters (neurotransmitters) across a tiny gap to the postsynaptic cell, possibly inducing an action potential there Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 62 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 63 Synaptic Transmission Structure of the chemical synapse (Figure 13-25) Synaptic knob—tiny bulge at the end of a terminal branch of a presynaptic neuron’s axon that contains vesicles housing neurotransmitters Synaptic cleft—space between a synaptic knob and the plasma membrane of a postsynaptic neuron Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 64 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 65 Synaptic Transmission Structure of the chemical synapse (cont) Arrangements of synapses Axodendritic—axon signals postsynaptic dendrite; common Axosomatic—axon signals postsynaptic soma; common Axoaxonic—axon signals postsynaptic axon; may regulate action potential of postsynaptic axon Plasma membrane of a postsynaptic neuron—has protein molecules that serve as receptors for the neurotransmitters Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 66 Synaptic Transmission Mechanism of synaptic transmission (Figure 1325)—sequence of events is as follows: Action potential reaches a synaptic knob, causing calcium ions to diffuse into the knob rapidly Increased calcium concentration triggers the release of neurotransmitter by way of exocytosis Neurotransmitter molecules diffuse across the synaptic cleft and bind to receptor molecules, causing ion channels to open Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 67 Synaptic Transmission Mechanism of synaptic transmission (cont) Opening of ion channels produces a postsynaptic potential, either an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP) The neurotransmitter’s action is quickly terminated by neurotransmitter molecules being transported back into the synaptic knob (reuptake) and/or metabolized into inactive compounds by enzymes and/or diffused and taken up by nearby glia (Figure 13-26) Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 68 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 69 Synaptic Transmission Summation (Figure 13-28) Spatial summation—adding together the effects of several knobs being activated simultaneously and stimulating different locations on the postsynaptic membrane, producing an action potential Temporal summation—when synaptic knobs stimulate a postsynaptic neuron in rapid succession, their effects can summate over a brief period of time to produce an action potential Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 70 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 71 Synaptic Transmission Synapses and memory Memories are stored by facilitating (or inhibiting) synaptic transmission Short-term memories (seconds or minutes) may result from axoaxonic facilitation or inhibition of the presynaptic axon terminal Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 72 Synaptic Transmission Synapses and memory Intermediate long-term memory (minutes to weeks) happens when serotonin blocks potassium channels in the presynaptic terminal—thus prolonging the action potential and increasing the amount of neurotransmitter released Long-term memories (months or years) require structural changes at the synapse—for example, more vesicles, more vesicle release sites, more presynaptic terminals, more sensitive postsynaptic membranes Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 73 Neurotransmitters Neurotransmitters—means by which neurons communicate with one another; there are more than 50 compounds known to be neurotransmitters, and dozens of others are suspected Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 74 Neurotransmitters Classification of neurotransmitters— commonly classified by the following: Function—the function of a neurotransmitter is determined by the postsynaptic receptor; two major functional classifications are excitatory neurotransmitters and inhibitory neurotransmitters; can also be classified according to whether the receptor directly opens a channel or instead uses a second messenger mechanism involving G-protein– coupled receptors (GPCRs) and intracellular signals (Figures 13-29 and 13-30) Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 75 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 76 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 77 Neurotransmitters Classification of neurotransmitters— commonly classified by the following (cont): Chemical structure—the mechanism by which neurotransmitters cause a change; there are four main classes; because the functions of specific neurotransmitters vary by location, they are usually classified according to chemical structure Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 78 Neurotransmitters Small-molecule neurotransmitters (Figure 1331) Acetylcholine Unique chemical structure; acetate (acetyl coenzyme A) with choline Acetylcholine is deactivated by acetylcholinesterase, with the choline molecules being released and transported back to presynaptic neuron to combine with acetate Present at various locations, sometimes in an excitatory role; other times, inhibitory Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 79 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 80 Neurotransmitters Small-molecule neurotransmitters (cont) Amines Synthesized from amino acid molecules Two categories: monoamines and catecholamines Found in various regions of the brain, affecting learning, emotions, motor control, and so on Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 81 Neurotransmitters Small-molecule neurotransmitters (cont) Amino acids Believed to be among the most common neurotransmitters of the CNS In the PNS, amino acids are stored in synaptic vesicles and used as neurotransmitters Other small-molecule transmitters Nitric oxide (NO) derived from an amino acid NO from a postsynaptic cell signals the presynaptic neuron, providing feedback in a neural pathway Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 82 Neurotransmitters Neuropeptides—large-molecule neurotransmitters Neuropeptides are short strands of amino acids called polypeptides or peptides (Figure 13-32) Peptides first discovered to have regulatory effects in the digestive tract; also act as neurotransmitters in the brain Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 83 Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 84 Neurotransmitters Neuropeptides (cont) May be secreted by themselves or in conjunction with a second or third neurotransmitter; in this case, neuropeptides act as a neuromodulator, a “co-transmitter” that regulates the effects of the neurotransmitter released along with it Neurotrophins (neurotrophic [nerve growth] factors) stimulate neuron development but also can act as neurotransmitters or neuromodulators Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 85 Role of Nervous System Cells Neuron doctrine—proposes neuron is basic structural and functional unit of the nervous system and neurons are independent units connected by chemical synapses Reticular theory—proposes nervous system is best understood as a large integrated network Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 86 Role of Nervous System Cells Today, the dominant neuron doctrine has expanded to include concepts of the reticular theory—neurons are distinct units but also participate in a network; some neurons are functionally connected by electrical synapses, and chemical signals can move in two directions at the same synapse Because neuroglia also participate in the neural network, the whole concept of nervous system cell function is rapidly expanding Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 87 Cycle of Life: Nervous System Cells Nerve tissue development Begins in ectoderm Occurs most rapidly in womb and in first 2 years Nervous cells organize into body network Synapses Form and re-form until nervous system is intact Formation of new synapses and strengthening or elimination of old synapses stimulate learning and memory Aging causes degeneration of the nervous system, which may lead to senility Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 88 The Big Picture: Nervous System Cells and the Whole Body Neurons act as the “wiring” that connects structures needed to maintain homeostasis Sensory neurons—act as receptors to detect changes in the internal and external environment; relay information to integrator mechanisms in the CNS Information is processed, and a response is relayed to the appropriate effectors through the motor neurons Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 89 The Big Picture: Nervous System Cells and the Whole Body At the effector, neurotransmitter triggers a response to restore homeostasis Neurotransmitters released into the bloodstream are called hormones Neurons are responsible for more than just responding to stimuli; circuits are capable of remembering or learning new responses, generation of thought, and so on Mosby items and derived items © 2013, 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc. 90