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Chapter 12 Nervous System Cells Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 1 Introduction The function of the nervous system, along with the endocrine system, is to communicate The nervous system is made up of the brain, the spinal cord, and the nerves (Figure 12-1) Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 2 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 12-2) Central nervous system (CNS) • Structural and functional center of entire nervous system • Consists of the brain and the spinal cord • Integrates sensory information, evaluates it, and initiates an outgoing response Peripheral nervous system (PNS) • Nerves that lie in “outer regions” of nervous system • Cranial nerves—originate from brain • Spinal nerves—originate from spinal cord Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 3 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 © 2007, 2003 by Mosby, Inc. Slide 4 Organization of the Nervous System “Systems” according to the types of organs they innervate 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 © 2007, 2003 by Mosby, Inc. Slide 5 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 and glands) 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 • Visceral sensory division—carries feedback information to autonomic integrating centers in the CNS Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 6 Cells of the Nervous System Glia Glial cells support the neurons Five major types of glia (Figure 12-3): • Astrocytes 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) • Microglia Small, usually stationary, cells In inflamed brain tissue, they enlarge, move about, and carry on phagocytosis Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 7 Cells of the Nervous System Five major types of glia (cont.): • Ependymal cells Resemble epithelial cells and form thin sheets that line fluid-filled cavities in the CNS Some produce fluid; others aid in circulation of fluid • Oligodendrocytes Smaller than astrocytes with fewer processes Hold nerve fibers together and produce the myelin sheath Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 8 Cells of the Nervous System Five major types of glia (cont.): • Schwann cells Found only in PNS Support nerve fibers and form myelin sheaths (Figure 12-4) Gaps in the myelin sheath are called nodes of Ranvier Neurilemma is formed by cytoplasm of Schwann cell (neurilemmocyte) wrapped around the myelin sheath; essential for nerve regrowth Satellite cells are Schwann cells that cover and support cell bodies in the PNS Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 9 Cells of the Nervous System Neurons Excitable cells that initiate and conduct impulses that make possible all nervous system functions Components of neurons (Figure 12-5) • Cell body (perikaryon) Ribosomes, rough endoplasmic reticulum (ER), Golgi apparatus – 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 © 2007, 2003 by Mosby, Inc. Slide 10 Cells of the Nervous System 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 knob-like protrusions on dendrites of some brain neurons; serve as connection points for axons of other neurons • Axon A single process extending from the axon hillock, sometimes covered by a fatty layer called a myelin sheath (Figure 12-6) 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 Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 11 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 12-7) – Vesicles (some containing neurotransmitters), mitochondria – Motor molecules shuttle organelles to and from the far ends of a neuron • Functional regions of the neuron (Figure 12-8) 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 © 2007, 2003 by Mosby, Inc. Slide 12 Cells of the Nervous System Classification of neurons Structural classification—classified according to number of processes extending from cell body (Figure 12-9) • 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 © 2007, 2003 by Mosby, Inc. Slide 13 Cells of the Nervous System Classification of neurons (cont.) Functional classification (Figure 12-10) • 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 © 2007, 2003 by Mosby, Inc. Slide 14 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 12-11) • Afferent neurons—conduct impulses to the CNS from the receptors • Efferent neurons—conduct impulses from the CNS to effectors (muscle or glandular tissue) Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 15 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 © 2007, 2003 by Mosby, Inc. Slide 16 Nerves and Tracts Nerves—bundles of peripheral nerve fibers held together by several layers of connective tissue (Figure 12-12) 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 Tracts—within the CNS, bundles of nerve fibers are called tracts rather than nerves Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 17 Nerves and Tracts White matter PNS—myelinated nerves CNS—myelinated tracts Gray matter Made up of cell bodies and unmyelinated fibers CNS—referred to as nuclei PNS—referred to as ganglia Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 18 Nerves and Tracts 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 © 2007, 2003 by Mosby, Inc. Slide 19 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 Nerve fibers can be repaired if the damage is not extensive, the cell body and neurilemma are intact, and scarring has not occurred Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 20 Repair of Nerve Fibers Stages of repair of an axon in a peripheral motor neuron (Figure 12-13) 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 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 © 2007, 2003 by Mosby, Inc. Slide 21 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 outside of the membrane and slight deficiency of positively charged ions on inside of membrane (Figure 12-14) 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 © 2007, 2003 by Mosby, Inc. Slide 22 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 12-15) Sodium-potassium pump (Figure 12-16) • 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 © 2007, 2003 by Mosby, Inc. Slide 23 Nerve Impulses Local potentials Local potentials—slight shift away from the resting membrane in a specific region of the plasma membrane (Figure 12-17) Excitation—when a stimulus triggers the opening of additional Na+ channels, allowing the membrane potential to move toward zero (depolarization) 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 © 2007, 2003 by Mosby, Inc. Slide 24 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 12-18 and 12-19) An adequate stimulus triggers stimulus-gated Na+ channels to open, allowing Na+ to diffuse rapidly into the cell, producing a local depolarization As threshold potential is reached, voltage-gated Na+ channels open and more Na+ enters the cell, causing further depolarization The action potential is an all-or-none response Voltage-gated Na+ channels stay open for only about 1 millisecond before they automatically close 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 sodium-potassium pumps Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 25 Action Potential Refractory period (Figure 12-20) Absolute refractory period—brief period (lasting approximately half a millisecond) during which a local area of a neuron’s membrane resists restimulation and will not respond to a stimulus, no matter how strong Relative refractory period—time during which the membrane is repolarized and is restoring the resting membrane potential; the few milliseconds after the absolute refractory period; membrane will respond only to a very strong stimulus Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 26 Action Potential Conduction of the action potential 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 12-21) This cycle continues to repeat The action potential never moves backward, as a consequence 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 12-22) Speed of nerve conduction depends on diameter and on the presence or absence of a myelin sheath Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 27 Synaptic Transmission Two types of synapses (junctions) (Figure 12-23): 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 © 2007, 2003 by Mosby, Inc. Slide 28 Synaptic Transmission Structure of the chemical synapse (Figure 12-21) 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 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 postynaptic axon Plasma membrane of a postsynaptic neuron—has protein molecules that serve as receptors for the neurotransmitters Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 29 Synaptic Transmission Mechanism of synaptic transmission (Figure 12-25)— sequence of events is the following: Action potential reaches a synaptic knob, causing calcium ions to diffuse into the knob rapidly Increased calcium concentration triggers the release of neurotransmitter via exocytosis Neurotransmitter molecules diffuse across the synaptic cleft and bind to receptor molecules, causing ion channels to open 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 either 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 12-26) Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 30 Synaptic Transmission Summation (Figure 12-28) 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 © 2007, 2003 by Mosby, Inc. Slide 31 Neurotransmitters Neurotransmitters—means by which neurons communicate with one another; there are more than 30 compounds known to be neurotransmitters, and dozens of others are suspected Classification of neurotransmitters—commonly classified by these features: Function—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 receptor directly opens a channel or instead uses a second messenger mechanism involving G proteins and intracellular signals (Figures 12-29 and 12-30) Chemical structure—mechanism by which neurotransmitters cause a change; there are four main classes; since the functions of specific neurotransmitters vary by location, they are usually classified according to chemical structure Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 32 Neurotransmitters Small-molecule neurotransmitters (Figure 12-31) 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 and at other times, an inhibitory one Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 33 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, etc. 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 Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 34 Neurotransmitters Small-molecule neurotransmitters (cont.) Other small 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 © 2007, 2003 by Mosby, Inc. Slide 35 Neurotransmitters Large-molecule neurotransmitters—neuropeptides (Figure 12-32) Peptides made up of 2 or more amino acids May be secreted by themselves or in conjunction with a second or third neurotransmitter; in this case, neuropeptides act as a neuromodulator, a “cotransmitter” 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 © 2007, 2003 by Mosby, Inc. Slide 36 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 Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 37 Cycle of Life: Nervous System Cells Synapses Form and reform 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 © 2007, 2003 by Mosby, Inc. Slide 38 The Big Picture: Nervous System 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 © 2007, 2003 by Mosby, Inc. Slide 39 The Big Picture: Nervous System 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, etc. Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 40