Chapter 11: Fundamentals of the Nervous System and Nervous Tissue

Transcription

1 Chapter 11: Fundamentals of the Nervous System and Nervous Tissue Objectives: 1. List the basic functions of the nervous system. 2. Explain the structural and functional divisions of the nervous system. 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 tract, and between a nucleus and ganglion. 6. Explain the importance of the myelin sheath and describe how it is formed in the central and peripheral nervous system. 7. Classify neurons structurally and functionally. 8. Define resting membrane potential and describe its electrochemical basis. 9. Compare and contrast graded and action potentials. 10. Explain how action potentials are generated and propagated along neurons. 11. Define absolute and relative refractory periods. 12. Define salutatory conduction and contrast it to conduction along unmyelinated fibers. 13. Define synapse. Distinguish between electrical and chemical synapses structurally and in their mechanisms of information transmission. 14. Distinguish between excitatory and inhibitory postsynaptic potentials. 15. Describe how synaptic events are integrated and modified. 16. Define neurotransmitter and name several classes of neurotransmitters. 17. Describe common patterns of neuronal organization and processing. 18. Distinguish between serial and parallel processing. 19. Describe the role of astrocytes and nerve cell adhesion molecules in neuronal differentiation. I. Nervous System A. Master controlling and communicating system of the body B. Functions 1

2 1. Sensory input a. monitoring stimuli 2. Integration a. interpretation of sensory input 3. Motor output a. response to stimuli II. Organization of the Nervous System A. Central nervous system (CNS) 1. Brain and spinal cord 2. Integration and command center B. Peripheral nervous system (PNS) 1. Paired spinal and cranial nerves 2. Carries messages to and from the spinal cord and brain 3. Two Functional Divisions a. Sensory (afferent) division 1) Sensory afferent fibers a) impulses from skin, skeletal muscles, & joints to brain 2) Visceral afferent fibers a) transmit impulses from visceral organs to brain b. Motor (efferent) division 1) Transmits impulses from the CNS to effector organs C. Motor Division: Two Main Parts 1. Somatic nervous system a. Conscious control of skeletal muscles 2. Autonomic nervous system (ANS) a. Regulates smooth muscle, cardiac muscle, and glands b. Divisions 1) sympathetic 2) parasympathetic III. Histology of Nerve Tissue A. The two principal cell types 1. Neurons a. excitable cells that transmit electrical signals 2. Supporting cells a. cells that surround and wrap neurons B. Supporting Cells: Neuroglia or Glial cells 1. Provide a supportive scaffolding for neurons 2. Segregate and insulate neurons 2

3 3. Guide young neurons to the proper connections 4. Promote health and growth C. Astrocytes 1. Most abundant, versatile, and highly branched glial cells 2. Cling to neurons and their synaptic endings, and cover capillaries 3. Functionally a. Support and brace neurons b. Anchor neurons to their nutrient supplies c. Guide migration of young neurons d. Control the chemical environment D. Microglia and Ependymal Cells 1. Microglia a. Small, ovoid cells with spiny processes b. Phagocytes that monitor the health of neurons 2. Ependymal cells a. Range in shape from squamous to columnar b. Line central cavities of the brain and spinal column E. Oligodendrocytes, Schwann Cells, and Satellite Cells 1. Oligodendrocytes a. Branched cells that wrap CNS nerve fibers 2. Schwann cells (neurolemmocytes) a. Surround fibers of the PNS 3. Satellite cells a. Surround neuron cell bodies with ganglia F. Neurons (Nerve Cells) 1. Structural units of the nervous system 2. Composed of a body, axon, and dendrites 3. Their plasma membrane function in: a. Electrical signaling b. Cell-to-cell signaling during development 4. Nerve Cell Body (Soma) a. Contains the nucleus and a nucleolus b. Major biosynthetic center c. Focal point for the outgrowth of neuronal processes d. Has no centrioles e. Has well-developed Nissl bodies (rough ER) 3

4 f. Contains an axon hillock 1) cone-shaped area from which axons arise 5. Processes a. Armlike extensions from the soma b. Tracts in the CNS c. Nerves in the PNS d. Two types 1) axons 2) dendrites e. Dendrites of Motor Neurons 1) Short, tapering, and diffusely branched processes 2) Receptive, or input, regions of the neuron f. Axons: Structure 1) Slender processes of uniform diameter arising from the hillock 2) Long axons are called nerve fibers 3) Usually only one unbranched axon per neuron 4) Rare branches, if present, are called axon collaterals 5) Axonal terminal branched terminus of an axon g. Axons: Function 1) Generate and transmit action potentials 2) Secrete neurotransmitters from the axonal terminals 6. Myelin Sheath a. Whitish, fatty (protein-lipoid), segmented sheath around most long axons b. It functions to: 1) Protect the axon 2) Electrically insulate fibers from one another 3) Increase the speed of nerve impulse transmission 4) Formed by Schwann cells in the PNS 7. Schwann cell a. Envelopes an axon in a trough b. Encloses the axon with its plasma membrane c. Has concentric layers of membrane that make up the myelin sheath 8. Neurilemma a. remaining nucleus and cytoplasm of a Schwann cell 9. Nodes of Ranvier (Neurofibral Nodes) a. Gaps in the myelin sheath between adjacent Schwann cells b. Sites where axon collaterals can emerge 4

5 10. Unmyelinated Axons a. Schwann cell surrounds nerve fibers but coiling does not take place G. Axons of the CNS 1. Both myelinated and unmyelinated fibers are present 2. Myelin sheaths are formed by oligodendrocytes 3. Nodes of Ranvier are widely spaced 4. There is no neurilemma H. Regions of the Brain and Spinal Cord 1. White matter a. dense collections of myelinated fibers 2. Gray matter a. mostly soma and unmyelinated fibers I. Neuron Classification 1. Structural: a) Multipolar 1) three or more processes b) Bipolar 1) two processes (axon and dendrite) c) Unipolar 1) single, short process 2. Functional: a) Sensory (afferent) 1) transmit impulses toward the CNS b) Motor (efferent) 1) carry impulses away from the CNS c) Interneurons (association neurons) 1) shuttle signals through CNS pathways 5

6 IV. Neurophysiology A. Neurons are highly irritable B. Action potentials, or nerve impulses, are 1. Electrical impulses carried along the length of axons 2. Always the same regardless of stimulus 3. The underlying functional feature of the nervous system C. Electricity Definitions 1. Voltage (V) a. measure of potential energy generated by separated charge 2. Potential difference a. voltage measured between two points 3. Current (I) a. the flow of electrical charge between two points 4. Resistance (R) a. hindrance to charge flow 5. Insulator a. substance with high electrical resistance 6. Conductor a. substance with low electrical resistance D. Electrical Current and the Body 1. Reflects the flow of ions rather than electrons 2. There is a potential on either side of membranes when a. Number of ions is different across the membrane b. Membrane provides a resistance to ion flow E. Operation of a Gated Channel 1. Example: Na + -K + gated channel a. Closed when a neurotransmitter is not bound to the extracellular receptor b. Na + cannot enter the cell and K + cannot exit the cell c. Open when a neurotransmitter is attached to the receptor d. Na + enters the cell and K + exits the cell e. Example: Na + channel f. Closed when the intracellular environment is negative g. Na + cannot enter the cell h. Open when the intracellular environment is positive i. Na + can enter the cell j. When gated channels are open: 1) Ions move quickly across the membrane 2) Movement is along their electrochemical gradients 3) An electrical current is created 6

7 4) Voltage changes across the membrane F. Electrochemical Gradient 1. Ions flow along their chemical gradient when they move from an area of high concentration to an area of low concentration 2. Ions flow along their electrical gradient when they move toward an area of opposite charge 3. Electrochemical gradient a. the electrical and chemical gradients taken together G. Resting Membrane Potential (V r ) 1. The potential difference ( 70 mv) across the membrane of a resting neuron 2. It is generated by different concentrations of Na +, K +, Cl, and protein anions (A ) 3. Ionic differences are the consequence of a. Differential permeability of the neurilemma to Na + and K + b. Operation of the sodium-potassium pump H. Membrane Potentials: Signals 1. Used to integrate, send, and receive information I. Changes in Membrane Potential 1. Changes are caused by three events a. Depolarization 1) inside of the membrane becomes less negative b. Repolarization 1) membrane returns to its resting membrane potential c. Hyperpolarization 1) inside of the membrane becomes more negative than the resting potential J. Action Potentials (APs) 1. A brief reversal of membrane potential with a total amplitude of 100 mv 2. Action potentials are only generated by muscle cells and neurons 3. They do not decrease in strength over distance 4. They are the principal means of neural communication 5. An action potential in the axon of a neuron is a nerve impulse K. Action Potential: Resting State 1. Na + and K + channels are closed 2. Leakage accounts for small movements of Na + and K + 3. Each Na + channel has two voltage-regulated gates L. Action Potential: Depolarization Phase 1. Na + permeability increases; membrane potential reverse 7

8 2. Na + gates are opened; K + gates are closed 3. Threshold a) a critical level of depolarization (-55 to -50 mv) 4. At threshold, depolarization becomes self-generating M. Action Potential: Repolarization Phase 1. Sodium inactivation gates close 2. Membrane permeability to Na + declines to resting level 3. As sodium gates close, voltage-sensitive K + gates ope 4. K + exits the cell and internal negativity of the resting neuron is restored N. Action Potential: Hyperpolarization 1. Potassium gates remain open, causing an excessive efflux of K + O. Repolarization 1. Restores the resting electrical conditions of the neuron 2. Does not restore the resting ionic conditions 3. Ionic redistribution back to resting conditions is restored by the sodium-potassium pump P. Threshold and Action Potentials 1. Threshold a. membrane is depolarized by 15 to 20 mv b. Established by the total amount of current flowing through the membrane c. Weak (subthreshold) stimuli not relayed into action potentials d. Strong (threshold) stimuli are relayed into action potentials 2. All-or-none phenomenon a. action potentials either happen completely, or not at all 3. All action potentials are alike and are independent of stimulus intensity 4. Strong stimuli can generate an action potential more often than weaker stimuli 5. CNS determines stimulus intensity by the frequency of impulse transmission Q. Absolute Refractory Period 1. Time from the opening of the Na + activation gates until the closing of inactivation gates 2. Prevents the neuron from generating an action potential 3. Ensures that each action potential is separate 4. Enforces one-way transmission of nerve impulses R. Conduction Velocities of Axons 1. Conduction velocities vary widely among neurons 8

9 2. Rate of impulse propagation is determined by: 3. Axon diameter a. Larger the diameter, faster the impulse 4. Presence of a myelin sheath a. Myelination dramatically increases impulse speed 5. Saltatory Conduction a. Current passes through a myelinated axon only at the nodes of Ranvier b. Much faster than conduction along unmyelinated axons V. Multiple Sclerosis (MS) A. An autoimmune disease that mainly affects young adults B. Symptoms 1. visual disturbances 2. weakness 3. loss of muscular control 4. urinary incontinence C. Nerve fibers are severed and myelin sheaths in the CNS become nonfunctional scleroses D. Shunting and short-circuiting of nerve impulses occurs E. Treatment 1. The advent of disease-modifying drugs including interferon beta- 1a and -1b, Avonex, Betaseran, and Copazone: 2. Hold symptoms at bay 3. Reduce complications 4. Reduce disability VI. Nerve Fiber Classification A. Nerve fibers are classified according to: 1. Diameter 2. Degree of myelination 3. Speed of conduction B. Synapses 1. A junction that mediates information transfer from one neuron: a. To another neuron b. To an effector cell c. Presynaptic neuron 9

10 1) conducts impulses toward the synapse d. Postsynaptic neuron 1) transmits impulses away from the synapse 2. Synaptic Cleft a. Fluid-filled space separating the presynaptic and postsynaptic neurons b. Prevents nerve impulses from directly passing from one neuron to the next c. Transmission across the synaptic cleft is a chemical event (as opposed to an electrical one) d. Ensures unidirectional communication between neurons 3. Synaptic Cleft: Information Transfer a. Nerve impulses reach the axonal terminal of the presynaptic neuron and open Ca 2+ channels b. Neurotransmitter is released into the synaptic cleft via exocytosis in response to synaptotagmin c. Neurotransmitter crosses the synaptic cleft and binds to receptors on the postsynaptic neuron d. Postsynaptic membrane permeability changes, causing an excitatory or inhibitory effect 4. Termination of Neurotransmitter Effects a. Neurotransmitter bound to a postsynaptic neuron 1) Produces a continuous postsynaptic effect 2) Blocks reception of additional messages 3) Must be removed from its receptor b. Removal of neurotransmitters occurs when they: 1) Are degraded by enzymes 2) Are reabsorbed by astrocytes or the presynaptic terminals 3) Diffuse from the synaptic cleft VII. Neurotransmitters A. Chemicals used for neuronal communication with the body and the brain B. 50 different neurotransmitters have been identified C. Classified chemically and functionally D. Chemical Neurotransmitters 1. Acetylcholine (ACh) 2. Biogenic amines 3. Amino acids 4. Peptides 10

11 E. Acetylcholine 1. First neurotransmitter identified, and best understood 2. Released at the neuromuscular junction 3. Synthesized and enclosed in synaptic vesicles 4. Degraded by the enzyme acetylcholinesterase (AChE) 5. Released by: a. All neurons that stimulate skeletal muscle b. Some neurons in the autonomic nervous system F. Biogenic Amines 1. Include a. Catecholamines 1) dopamine 2) norepinephrine 3) epinephrine b. Indolamines 1) serotonin 2) histamine 2. Broadly distributed in the brain 3. Play roles in emotional behaviors and our biological clock G. Amino Acids 1. Include a. GABA Gamma (γ)-aminobutyric acid b. Glycine c. Aspartate d. Glutamate 2. Found only in the CNS H. Peptides 1. Include a. Substance P 1) mediator of pain signals b. Beta endorphin c. dynorphin d. enkephalins 2. Reduce pain perception I. Functional Classification of Neurotransmitters 1. Some neurotransmitters have both excitatory and inhibitory effects 2. Determined by the receptor type of the postsynaptic neuron 3. Example: acetylcholine a. Excitatory at neuromuscular junctions with skeletal muscle 11

12 b. Inhibitory in cardiac muscle VIII. Patterns of Neural Processing A. Serial Processing 1. Input travels along one pathway to a specific destination 2. Works in an all-or-none manner 3. Example: spinal reflexes B. Parallel Processing 1. Input travels along several pathways 2. Pathways are integrated in different CNS systems 3. One stimulus promotes numerous responses 4. Example: a smell may remind one of the odor and associated experiences IX. Development of Neurons A. The nervous system originates from the neural tube and neural crest B. The neural tube becomes the CNS C. Axonal Growth 1. Guided by a) Scaffold laid down by older neurons b) Orienting glial fibers 2. Release of nerve growth factor by astrocytes 3. Neurotropins released by other neurons 4. Repulsion guiding molecules 5. Attractants released by target cells a. N-CAM nerve cell adhesion molecule b. Important in establishing neural pathways c. Without N-CAM, neural function is impaired 12