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11: Fundamentals of the Nervous System and Nervous Tissue Objectives Functions and Divisions of the Nervous System 1. List the basic functions of the nervous system. 2. Explain the structural and functional divisions of the nervous system. Histology of Nervous Tissue 3. List the types of neuroglia and cite their functions. 4. Define neuron, describe its important structural components, and relate each to a functional role. 5. Differentiate between a nerve and a tract, and between a nucleus and a ganglion. 6. Explain the importance of the myelin sheath and describe how it is formed in the central and peripheral nervous systems. 7. Classify neurons structurally and functionally. Membrane Potentials 8. Define resting membrane potential and describe its electrochemical basis. 9. Compare and contrast graded potentials and action potentials. 10. Explain how action potentials are generated and propagated along neurons. 11. Define absolute and relative refractory periods. 12. Define saltatory conduction and contrast it to conduction along unmyelinated fibers. The Synapse 13. Define synapse. Distinguish between electrical and chemical synapses by structure and by the way they transmit information. 14. Distinguish between excitatory and inhibitory postsynaptic potentials. 15. Describe how synaptic events are integrated and modified. Neurotransmitters and Their Receptors 16. Define neurotransmitter and name several classes of neurotransmitters. Basic Concepts of Neural Integration 17. Describe common patterns of neuronal organization and processing. 18. Distinguish between serial and parallel processing. Chapter 11 Fundamentals of the Nervous System & Nervous Tissue I. Intro A. Chapters 11-15 take us into the world of communication, coordination and integration. While the nervous system accounts for a mere 3% of the total body weight, it is by far the most complex organ system and the most complex aggregate of matter on planet Earth. It may be the last frontier in our understanding of the human body. It is, for sure, the most challenging system for A & P students… B. New unit: regulation & integration [Chapters 11-16] 1. Chapter 11: Fundamentals of the nervous system and nervous tissue II. Functions and Divisions of the Nervous System (pp. 386–387; Figs. 11.1–11.2) A. Functions (Fig. 11.1) 1. Sensory (afferent) input [detect changes (stimuli) in the internal and external environments] 2. Integration [evaluate information] 3. Motor (efferent) output [respond to stimuli usually via muscle contraction and/or glandular secretions] B. Organization [Fig. 11.2; the nervous is subdivided into smaller systems to make it easier to understand; subdivisions can be based on structure, information flow, control of effectors, etc. Here, we categorize the nervous system based on its position in the body.] 1. CNS [Central Nervous System; brain and spinal cord] 2. PNS [Peripheral Nervous System; nervous tissues that lie in the periphery or outer regions of the body] a. Connections between the CNS & PNS: cranial  nerves & spinal  nerves b. Subdivisions of the PNS * Sensory (afferent; "carry towards") * Somatic afferents: from skin, skeletal muscles and joints * Visceral afferents: from your "guts" or organs, mostly in ventral body cavity * Motor (efferent; "carry away") * Somatic motor (voluntary nervous system): skeletal muscles * Autonomic nervous System (ANS, involuntary): smooth muscles, cardiac muscles, glands, adipose tissue & other involuntary tissues * Sympathetic ("fight or flight") * Parasympathetic ("rest & digest" or "rest & repair") III. Histology of Nervous Tissue (pp. 388–395; Figs. 11.3–11.5; Table 11.1) A. Neuroglia (noo-ROG-le-uh), or glial ("glue") cells, are closely associated with neurons, providing a protective and supportive network (pp. 388–389; Fig. 11.3). There are roughly 900 billion glial cells or more than the estimated number of stars in our galaxy...Oh my! Note: gliomas: cancers of glial cells) 1. Astrocytes ("star") are glial cells of the CNS that regulate the chemical environment (take up K+ & neurotransmitters) around neurons and exchange between neurons and capillaries. 2. Microglia are glial cells of the CNS that monitor health and perform immune (defense) functions for neurons. 3. Ependymal cells are glial cells of the CNS that line the central cavities of the brain and spinal cord and help circulate cerebrospinal fluid. May be neural stem cells? 4. Oligodendrocytes are glial cells of the CNS that wrap around neuron fibers, forming myelin sheaths. a. Multiple scelerosis: loss of myelin and injury/death of the oligodendrocytes 5. Satellite cells are glial cells of the PNS whose function is largely unknown. They are found surrounding neuron cell bodies within ganglia. 6. Schwann cells, or neurolemmocytes, are glial cells of the PNS that surround nerve fibers, forming the myelin sheath. B. Neurons are specialized cells that conduct messages in the form of electrical impulses throughout the body (pp. 389–395; Figs. 11.4–11.5; Table 11.1). 1. Nuclei (CNS) & ganglia (PNS): collections of nerve cell bodies/dendrities; tracts (CNS) & nerves (PNS): collections of axons 2. Neurons function optimally for a lifetime, are mostly amitotic, and have an exceptionally high metabolic rate requiring oxygen and glucose. a. The neuron cell body, also called the perikaryon or soma, is the major biosynthetic center containing the usual organelles except for centrioles. b. Dendrites are cell processes that are the receptive regions of the cell. c. Each neuron has a single axon that generates and conducts nerve impulses away from the cell body to the axon terminals. d. The myelin sheath is a whitish, fatty, segmented covering that protects, insulates, and increases conduction velocity of axons. 3. There are three structural classes of neurons. a. Multipolar neurons have three or more processes (brain & spinal cord) b. Bipolar neurons have a single axon and dendrite (retina, inner ear, olfactory) c. Unipolar (also called pseudounipolar) neurons have a single process extending from the cell body that is associated with receptors at the distal end (sensory in PNS; the axon and dendrites of these types of neurons fuse into one long process) 4. There are three functional classes of neurons. a. Sensory, or afferent, neurons conduct impulses toward the CNS from receptors. b. Motor, or efferent, neurons conduct impulses from the CNS to effectors. c. Interneurons, or association neurons, conduct impulses between sensory and motor neurons, or in CNS integration pathways. Found in CNS only. VI. Membrane Potentials (pp. 395–406; Figs. 11.6–11.15) A. Basic Principles of Electricity (p. 395) 1. Ions: charged particles; the distribution of charged particles are key to nervous function; opposite charges attract and it takes energy to separate them 2. Voltage is a measure of the amount of difference in electrical charge between two points, called the potential difference (i.e., measure of electrical potential energy; if separated charges of opposite charge are allowed to come together, energy, the ability to do work is released) a. Human body: electrical potentials exist across plasma membranes (hey, let's call them "membrane potentials" and let's measure them in volts); Two main factors determine membrane potential: ion concentration gradients & membrane permeability 3. The flow of electrical charge from point to point is called current, and is dependent on voltage and resistance (hindrance to current flow). 4. In the body, electrical currents are due to the movement of ions across cellular membranes. 5. Resistance: lipids of plasma membrane have high resistance while our salty solutions have low resistance B. The Role of Membrane Ion Channels (basically the only way for ions to cross membranes; p. 395; Fig. 11.6); The 4 main types of channels: Na+, K+, Ca++, & Cl- . Some channels allow both Na+ & K+ movement (Note: The English Channel is highly effective and quite cool, but will not be discussed here...) 1. Passive or leak channels: always open (like 7-11 stores) 2. The cell has many gated ion channels. a. Chemically gated (ligand-gated) channels open when the appropriate chemical binds, such as a neurotransmitter, neuromodulator, or other signal molecules b. Voltage-gated channels open in response to a change in membrane potential. c. Mechanically gated channels open when a membrane receptor is physically deformed; found in sensory neurons and response to things like light or pressure 3. When ion channels are open, ions diffuse across the membrane, creating electrical currents. 4. Electrochemical gradient C. The Resting Membrane Potential (neurons, muscle cells & secretory cells are exitable-- their membrane potentials change in response to stimuli; pp. 396–398; Figs. 11.7–11.8) 1. The neuron cell membrane is polarized, being more negatively charged inside than outside. The degree of this difference in electrical charge is the resting membrane potential. 2. The resting membrane potential is generated by differences in ionic makeup of intracellular and extracellular fluids, and differential membrane permeability to solutes. Highest levels of Na+ & Ca++ are in the extracellular fluid (ECF) with most K+ found in the cell (ICF) 3. Basis of resting membrane potential (RMP): K+ (most important due to membrane permeability), Na+, charged intracellular proteins (designated A-), and the Na+/K+ pump. D. Membrane Potentials That Act as Signals (pp. 398–404; Figs. 11.9–11.14) 1. Neurons use changes in membrane potential as communication signals. These can be brought on by changes in membrane permeability to any ion, or alteration of ion concentrations on the two sides of the membrane. 2. Types of membrane potentials: resting, graded, and action (some introduced later in course: receptor, postsynaptic. end-plate potentials) 3. Changes in membrane potential relative to resting membrane potential can either be depolarizations, in which the interior of the cell becomes less negative, or hyperpolarizations, in which the interior of the cell becomes more negatively charged. Addition of Na+ to inside of cell = depolarization; Loss of K+ from the cell = hyperpolarization; addition of Cl- to cell + hyperpolarization. Changes in membrane potential requires the movement of very, very, very few ions (one or two ions out of 100,000! You got to be kidding me...) 4. Graded potentials are short-lived & short distance local changes in membrane potentials that vary in their size or amplitude. They can either be depolarizations or hyperpolarizations, and are critical to the generation of action potentials. You can think of them as trying to promote or deter action potentials. They start on the cell membrane usually in the dendrites & cell body of a neuron and then move through the cell until they die out or reach the axon hillock or trigger zone. 5. Action potentials, or nerve impulses or spikes, occur on axons and are the principle way neurons communicate. a. Generation of an action potential involves a transient increase in Na + permeability, followed by restoration of Na+ impermeability, and then a short- lived increase in K+ permeability. b. Propagation, or transmission, of an action potential occurs as the local currents of an area undergoing depolarization cause depolarization of the forward adjacent area. c. Repolarization, which restores resting membrane potential, follows depolarization along the membrane. 6. A critical minimum, or threshold, depolarization is defined by the amount of influx of Na+ that at least equals the amount of efflux of K+. 7. Action potentials are all-or-none phenomena: they either happen completely, in the case of a threshold stimulus, or not at all, in the event of a subthreshold stimulus; all action potentials are identical. 8. Stimulus intensity is coded in the frequency of action potentials. 9. The refractory period of an axon is related to the period of time required so that a neuron can generate another action potential. Absolute refractory period: once an AP has begun, a second AP cannot be triggered (approx. 1 msec). Relative refractory period: time after an AP during which a stronger than normal graded potential can start another AP. E. Conduction Velocity (pp. 404–406; Fig. 11.15) 1. Axons with larger diameters conduct impulses faster than axons with smaller diameters. 2. Unmyelinated axons conduct impulses relatively slowly (continuous conduction), while myelinated axons have a high conduction velocity (saltatory conduction). VII. The Synapse (pp. 406–413; Figs. 11.16–11.19; Table 11.2) A. A synapse is a junction that mediates information transfer between neurons or between a neuron and an effector cell (p. 406; Fig. 11.16). B. Neurons conducting impulses toward the synapse are presynaptic cells, and neurons carrying impulses away from the synapse are postsynaptic cells (p. 406). C. Electrical synapses have neurons that are electrically coupled via protein channels and allow direct exchange of ions from cell to cell (p. 406). D. Chemical synapses are specialized for release and reception of chemical neurotransmitters (pp. 407–408; Fig. 11.17). E. Neurotransmitter effects are terminated in three ways: degradation by enzymes from the postsynaptic cell or within the synaptic cleft; reuptake by astrocytes or the presynaptic cell; or diffusion away from the synapse (p. 408). F. Synaptic delay is related to the period of time required for release and binding of neurotransmitters (p. 408). G. Postsynaptic Potentials and Synaptic Integration (pp. 408–413; Figs. 11.18–11.19; Table 11.2) 1. Neurotransmitters mediate graded potentials on the postsynaptic cell that may be excitatory or inhibitory. 2. Summation by the postsynaptic neuron is accomplished in two ways: temporal summation, which occurs in response to several successive releases of neurotransmitter, and spatial summation, which occurs when the postsynaptic cell is stimulated at the same time by multiple terminals. 3. Synaptic potentiation results when a presynaptic cell is stimulated repeatedly or continuously, resulting in an enhanced release of neurotransmitter. 4. Presynaptic inhibition results when another neuron inhibits the release of excitatory neurotransmitter from a presynaptic cell. 5. Neuromodulation occurs when a neurotransmitter acts via slow changes in target cell metabolism, or when chemicals other than neurotransmitter modify neuronal activity. V. Neurotransmitters and Their Receptors (pp. 413–421; Fig. 11.20; Table 11.3) A. Neurotransmitters are one of the ways neurons communicate, and they have several chemical classes (pp. 413–419; Table 11.3). B. Functional classifications of neurotransmitters consider whether the effects are excitatory or inhibitory, and whether the effects are direct or indirect (pp. 419–420). C. There are two main types of neurotransmitter receptors: channel-linked receptors mediate direct transmitter action and result in brief, localized changes; and G protein– linked receptors mediate indirect transmitter action resulting in slow, persistent, and often diffuse changes (pp. 420–421; Fig. 11.20). VI. Basic Concepts of Neural Integration (pp. 421–423; Figs. 11.21–11.23) A. Organization of Neurons: Neuronal Pools (pp. 421–422; Fig. 11.21) 1. Neuronal pools are functional groups of neurons that integrate incoming information from receptors or other neuronal pools and relay the information to other areas. B. Types of Circuits (p. 422; Fig. 11.22) 1. Diverging, or amplifying, circuits are common in sensory and motor pathways. They are characterized by an incoming fiber that triggers responses in ever-increasing numbers of fibers along the circuit. 2. Converging circuits are common in sensory and motor pathways. They are characterized by reception of input from many sources, and a funneling to a given circuit, resulting in strong stimulation or inhibition. 3. Reverberating, or oscillating, circuits are characterized by feedback by axon collaterals to previous points in the pathway, resulting in ongoing stimulation of the pathway. 4. Parallel after-discharge circuits may be involved in complex activities, and are characterized by stimulation of several neurons arranged in parallel arrays by the stimulating neuron. C. Patterns of Neural Processing (pp. 422–423; Fig. 11.23) 1. Serial processing is exemplified by spinal reflexes, and involves sequential stimulation of the neurons in a circuit. 2. Parallel processing results in inputs stimulating many pathways simultaneously, and is vital to higher level mental functioning.
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