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UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function LEARNING OUTCOMES: 10.1 10.2 10.3 10.4 10.5 Overview of the Nervous System 1. Describe the general functions of the nervous system. (p. 360) 2. Identify the two types of cells that comprise nervous tissue. (p. 360) 3. Identify the two major groups of nervous system organs. (p. 361) General Functions of the Nervous System 4. List the functions of sensory receptors. (p. 361) 5. Describe how the nervous system responds to stimuli. (p. 361) Description of Cells of the Nervous System 6. Describe the parts of a neuron. (p. 363) 7. Describe the relationships among myelin, the neurilemma, and nodes of Ranvier. (p. 363) 8. Distinguish between the sources of white matter and gray matter. Classification of Cells of the Nervous System 9. Identify structural and functional differences among neurons. (pp. 363-368) 10. Identify the types of neuroglia in the central nervous system and their functions. (pp. 368-369) 11. Describe the role of Schwann cells in the peripheral nervous system. (p. 370) The Synapse 12. 10.6 Explain how information passes from a presynaptic neuron to a postsynaptic cell. (pp. 371-372) Cell Membrane Potential 13. Explain how a cell membrane becomes polarized. (p. 372) 14. Describe the events leading to the generation of an action potential. (p. 375) 15. Explain how action potentials move down an axon. (pp. 375-377) 10-1 UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function LEARNING OUTCOMES: 16. 10.7 10.8 Compare impulse conduction in myelinated and unmyelinated neurons. (p. 378) Synaptic Transmission 17. Identify the changes in membrane potential associated with excitatory and inhibitory neurotransmitters. (p. 379) 18. Explain what prevents a postsynaptic cell from being continuously stimulated. (p. 381) Impulse Processing 19. Describe the basic ways in which the nervous system processes information. (pp. 382-383). 10-2 UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function 10.1 OVERVIEW OF THE NERVOUS SYSTEM A. The nervous system is a network of cells that sense and respond to stimuli in ways that maintain homeostasis. B. The nervous system is composed of: 1. 2. 3. * C. Neurons have processes that: 1. 2. D. receive information (dendrites) and send (axons) bioelectric signals (neurotransmitters) that cross spaces (synapses) between them. Organs of the nervous system are divided into: 1. 2. 10.2 neural tissue, including a. neurons and b. neuroglial cells blood vessels connective tissue. See Fig 10.1, page 360. central nervous system peripheral nervous system GENERAL FUNCTIONS OF THE NERVOUS SYSTEM The general function of the nervous system is to coordinate all body systems! This is accomplished by the transmission of (electrochemical) signals from body parts to the brain and back to the body parts. A. The organs of the nervous system are divided into two major groups: See Figure 10.2, page 362. 1. Central Nervous System (CNS) = brain & spinal cord. 2. Peripheral Nervous System (PNS) = nerves that extend from the brain (cranial nerves) and spinal cord (spinal nerves). 10-3 UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function 10.2 GENERAL FUNCTIONS OF THE NERVOUS SYSTEM B. Three Major Functions 1. 2. 3. Sensory Input Function a. PNS b. Sensory receptors (located at the ends of peripheral neurons) detect changes occurring in their surroundings (i.e. are stimulated). c. Once stimulated, sensory receptors transmit a sensory impulse to the CNS. d. A sensory impulse is carried on a sensory neuron. Integrative Function a. CNS (brain and/or spinal cord) b. involves interpretation of an incoming sensory impulse (i.e. decision is made concerning what's going to happen next, based on sensory impulse). c. Integration occurs in interneurons. d. A motor impulse begins... Motor Function a. PNS b. involves the response of a body part c. Motor impulses are carried from CNS to responsive body parts called effectors. d. A motor impulse is carried on a motor neuron. e. Effectors = 2 types: o o muscles (that contract) glands 1. that secrete a hormone; endocrine 2. that secrete a substance; exocrine 10-4 UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function 10.2 GENERAL FUNCTIONS OF THE NERVOUS SYSTEM C. Levels of Organization of Nervous System (most will be discussed in Ch 11) NERVOUS SYSTEM CENTRAL NERVOUS SYSTEM (BRAIN & SPINAL CORD) (INTERNEURONS) PERIPHERAL NERVOUS SYSTEM (CRANIAL NERVES & SPINAL NERVES) SENSORY (INPUT INTO CNS) (AFFERENT NEURONS) SOMATIC (EFFECTORS: SKELETAL MUSCLE) (CONSCIOUS CONTROL) PARASYMPATHETIC (HOMEOSTASIS) (NT: ACETYLCHOLINE) MOTOR (OUTPUT FROM CNS) (EFFERENT NEURONS) AUTONOMIC (EFFECTORS: SMOOTH MUSCLE, CARDIAC MUSCLE, GLANDS) (UNCONSCIOUS CONTROL) SYMPATHETIC (FIGHT-OR-FLIGHT) (NT: NOREPINEPHRINE) 10-5 UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function 10.3 DESCRIPTION OF CELLS OF THE NERVOUS SYSTEM A. Neuron = the structural and functional unit of the nervous system; a nerve cell. 1. Neuron Structure See Fig 10.3, page 364. Each neuron is composed of a cell body, dendrites, and an axon. a. Cell Body (soma or perikaryon) = central portion of neuron contains usual organelles, except centrioles o identify: nucleus, prominent nucleolus, and many Nissl bodies (chromatophilic substance) = RER. o A network of fine threads called neurofibrils extends into the axons and supports them. b. Neuron Processes/ Nerve Fibers = extensions arising from cell body. Two types: o Dendrites: o 1. many per neuron 2. short and branched 3. receptive portion of a neuron 4. carry impulses toward cell body Axons: 1. 2. 3. 4. 5. one per neuron long, thin process carry impulses away from cell body Note terminations of axon branch = axonal terminals or synaptic knobs. Axons in CNS (i.e. in brain & spinal cord) a. b. c. Myelin is produced by oligodendrocytes rather than Schwann Cells. A bundle of myelinated nerve fibers = "White Matter". This is in contrast to CNS "Gray Matter" = A bundle of cell bodies (or unmyelinated nerve fibers). 10-6 UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function 10.3 DESCRIPTION OF CELLS OF THE NERVOUS SYSTEM D. Neuron = the structural and functional unit of the nervous system; a nerve cell. 1. Neuron Structure Fig 10.3, page 364. b. Nerve Fibers o Axons (continued) 6. Axons in PNS: See Fig 10.4, page 365. a. Large axons are surrounded by a myelin See Fig 10.5, page 366. sheath produced by many layers of Schwann Cells (neuroglial cells). "myelinated nerve fiber" myelin = lipoprotein Interruptions in the myelin sheath between Schwann cells = Nodes of Ranvier. The neurilemma (or neurolemmal sheath) composed of Schwann cell cytoplasm and nuclei surround the myelin sheath. See Fig 10.5, page 366. b. Small axons do not have a myelin sheath. "unmyelinated nerve fibers" however all axons (in PNS) are associated with Schwann cells. 10.4 CLASSIFICATION OF CELLS OF THE NERVOUS SYSTEM A. Classification of Neurons 1. Neuron Structural Classification: 10.1A, page 368. a. b. c. See Figure 10.6, page 366 and Table Bipolar Neurons o two extensions o one fused dendrite leads toward cell body, one axon leads away from cell body o specialized parts of eyes, nose, ears (i.e. sensory) Unipolar Neurons o one process from cell body o forms central and peripheral processes o only distal ends are dendrites o cell bodies in ganglia outside brain and spinal cord Multipolar Neurons o many extensions o Many dendrites lead toward cell body; one axon leads away from cell body. o most common neuron in brain and spinal cord 10-7 UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function 10.4 CLASSIFICATION OF CELLS OF THE NERVOUS SYSTEM A. Classification of Neurons 2. B. Neuron Functional Classification: See Table 10.1B, page 368. a. Sensory neurons o PNS o afferent neurons o carry sensory impulses from sensory receptors to CNS o input information to CNS o Location of receptors = skin & sense organs. b. Interneurons (Association) o CNS o link other neurons together (i.e. sensory neuron to interneuron to motor neuron) c Motor Neurons o PNS o efferent neurons o carry motor impulses away from CNS and to effectors o output information from CNS o Effectors = muscles & glands. Classification of Neuroglia Recall that neuroglial cells are accessory cells of the nervous system; they form a supporting network for neurons; "nerve glue". 1. 2. Neuroglia of the CNS: 4 types provide bulk of brain and spinal cord tissue: See Table 10.2, page 370, and Fig 10.8, page 369. a. Astrocyte o star-shaped o Function: nourishes neurons and provide structural support b. Oligodendrocyte o looks like eyeball o Function: produces myelin c. Microglia o looks like spider o Function: phagocytosis d. Ependyma o epithelial-like layer with cilia o Function: lines spaces in CNS Neuroglia of the PNS: 2 types a. Schwann cells produces myelin. See Fig 10.4, page 365. b. Satellite cells surround and support cell bodies (in ganglia). 10-8 UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function 10.4 CLASSIFICATION OF CELLS OF THE NERVOUS SYSTEM C. 10.5 Neuroglia and Axonal Regeneration: See Fig 10.10, page 371. 1. Cell body injury = death of neuron 2. Damage to an axon may allow for regeneration. THE SYNAPSE (Nerve) impulses are transferred from one neuron to the next through synaptic transmission. A. Synapse = the junction between two neurons where an impulse is transmitted See Fig 10.11, page 372 and Fig 10.12, page 373. 1. 2. 10.6 occurs between the axon of one neuron (the presynaptic neuron) and dendrite or cell body of a second neuron (the postsynaptic neuron). Note that the two neurons do not touch. There is a gap between them = synaptic cleft. CELL MEMBRANE POTENTIAL A. Distribution of Ions 1. A resting neuron's cell membrane is polarized = electrically charged (i.e. the charge inside the cell is different than the charge outside): a. b. 2. Consequently, a potential difference (PD) exists across this resting cell membrane. Definition: Potential Difference (PD) = the difference in electrical charge between 2 points (i.e. across a cell membrane). The resting membrane potential (RMP) of a neuron results from the distribution of ions across the cell membrane. See Fig 10.13, page 373 and 10.14, page 374. a. b. c. d. K+ high inside Na+ high outside Cl- high outside Negatively charged proteins or Anions- high inside. Recall that these ion concentrations are maintained by active transport mechanisms (i.e. mainly the Na+K+-ATPase pump, Chapter 3). 10-9 UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function 10.6 CELL MEMBRANE POTENTIAL B. C. Resting Potential 1. The RMP of a nerve cell is measured to be -70 mV or millivolts (inside / outside). 2. As long as the RMP in a nerve cell is undisturbed, it remains polarized. However, in order for an impulse to be started or propagated in a nerve cell, this resting potential must be disturbed. Local Potential Changes The RMP of – 70 mV can be disrupted or changed in one of two directions: 1. more negative = "hyperpolarization" 2. D. less negative (i.e. towards zero) = "depolarization" a. The cell membrane of a neuron must be depolarized (to approximately -55 mV) in order for certain protein ion channels to open and therefore start an action potential/impulse. See Fig 10.13, page 373. Ion Movement During Action Potentials: See Figs 10.15 -10.18, pages 375378. 1. When the resting membrane potential (RMP) of a neuron is depolarized to -55 mV, threshold potential is reached. a. The threshold potential for a neuron is -55 mV. b. Therefore, a threshold stimulus = +15 mV. 2. When threshold potential is reached, the rapid opening of Na+ channels results in rapid depolarization (and even reversal of the membrane potential [MP] to +30 mV). a. This event is called the action potential. b. The action potential represents the start of an impulse on a neuron. 3. Then K+ channels open, (while Na+ channels close), and repolarization occurs = recovery of the RMP to -70 mV. 4. This all occurs very quickly = 1/1000 sec. 10-10 UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function 10.6 CELL MEMBRANE POTENTIAL D. Ion Movement During Action Potentials: 5. Summary: (Nerve) Impulse Conduction See Table 10.3, page 377. RMP of neuron = -70 mV. + 15 mV stimulus (threshold stimulus) MP of neuron falls to -55 mV = Threshold Potential Na+ channels open (rapid depolarization); Action Potential (impulse) is produced MP of neuron reaches +30 mV = Reversal of MP Na+ channels close; K+ channels open (repolarization) RMP of neuron returns to -70 mV. * ** An action potential represents the start of an impulse in one small portion of the neuron's membrane. How do you think it is transmitted throughout the entire neuron? 6. Impulse Conduction a. An impulse is the propagation of action potentials (AP) along a nerve fiber (i.e. the entire length of the neuron). See Fig 10.18, page 378. b. The impulse is an electrical impulse. c. An impulse is similar to a row of dominos falling (i.e. once the first domino falls, the entire row will fall). d. An impulse begins on a dendrite (or cell body of a neuron), runs toward the cell body, through the cell body, and then down the axon. See Clinical Application 10.3, page 379: Factors Affecting Impulse Conduction. * 10-11 UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function 10.6 10.7 CELL MEMBRANE POTENTIAL E. All-or-None Response = if a nerve cell responds at all, it responds completely. 1. subthreshold stimulus (5 mV) = no AP; no impulse. 2. threshold stimulus (15 mV) = yes AP; yes impulse. 3. > threshold stimulus (20 mV) = yes AP; yes impulse, but no greater intensity than above. E. Refractory Period = the period following an impulse when a threshold stimulus cannot produce another impulse. 1. The RMP has to be restored before it can be depolarized again (i.e. dominos must be set back up, in order to be knocked down again) F. Impulse Conduction = the manner in which the impulse runs down the neuron/nerve fiber 1. unmyelinated nerve fibers: impulse must travel the length of the nerve fiber; slow. See Fig 10.18, page 378. 2. myelinated nerve fiber: "Saltatory Conduction". a. Impulse appears to jump from node of Ranvier to node of Ranvier. b. Very fast transmission. See Fig 10.19, page 379. SYNAPTIC TRANSMISSION A. Steps in Synaptic Transmission page to draw illustration. 1. 2. 3. 4. 5. Presynaptic Neuron (axon) Fig 10.12, page 373. Use space at bottom of The impulse reaches axonal terminal of presynaptic neuron causing depolarization of axonal terminal/synaptic knob. Ca2+ channels open and calcium ions rush into axonal terminal causing synaptic vesicles (filled with neurotransmitter/NT) to release NT via exocytosis into the synaptic cleft. NT diffuses across synaptic cleft and depolarizes the postsynaptic neuron's membrane. An action potential (AP) is triggered and an Impulse begins in the postsynaptic neuron. Postsynaptic Neuron (cell body or dendrite) 10-12 UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function 10.7 SYNAPTIC TRANSMISSION B. Synaptic Potentials 1. 2. C. Postsynaptic neurons respond to neurotransmitter binding. a. May be depolarization = Excitatory Post-Synaptic Potential (EPSP) b. May be hyperpolarization = Inhibitory Post-Synaptic Potential (IPSP) Summation = many subthreshold stimuli received one after another may allow threshold potential to be reached, and trigger an AP, which in turn begins an impulse on a neuron. a. +15 mV = threshold = AP = impulse b. +5, +5, +5, = +15 mV = threshold = AP = impulse. Neurotransmitters (NT) See Table 10.4 page 380. 1. at least 30 different produced by CNS 2. Some neurons produce/release only one, while others release many. 3. Most typical NT is Acetylcholine (ACh). a. ACh is released by o all motor neurons (i.e. those that stimulate skeletal muscle) o some CNS neurons 4. Other NTs include Biogenic Amines and Amino Acids: a. Biogenic Amines (i.e. monoamines; modified amino acids) o are widely distributed in the brain where they play a role in 1. emotional behavior and 2. circadian rhythm o are present in some motor neurons of the ANS o include: 1. epinephrine 2. norepinephrine 3. dopamine 4. serotonin 5. histamine b. Amino Acids (unmodified amino acids) o glutamate o GABA (gamma aminobutyric acid) 10-13 UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function 10.7 SYNAPTIC TRANSMISSION C. D. Neurotransmitters (NT) 5. Events Leading to Neurotransmitter Release: See Table 10.7, page 382. 6. Fate of Neurotransmitter in Synaptic Cleft: Destruction of Neurotransmitter: o Enzymes that are present in the synaptic cleft destroy NT. 1. For example, acetylcholinesterase destroys ACh. b. Reuptake of Neurotransmitter: o NT is transported back into presynaptic knob. o The enzyme monoamine oxidase inactivates epinephrine and norepineprine after reuptake. 1. Monoamine Oxidase (MAO) Inhibitors are used to treat some psychological disorders. * Both of the above processes prevent continual stimulation of the postsynaptic membrane! Neuropeptides 1. act as NTs or neuromodulators that either: a. alter a neuron's response to a NT b. block the release of a NT 2. include enkephalins (CNS): a. synthesis is increased during painful stress b. bind to the same receptors in the brain as the narcotic morphine c. relieve pain include endorphins (CNS): a. same as above, but with a more potent and longer lasting effect. 3. 4. E. a. include Substance P (PNS) Gases 1. Nitric Oxide a. CNS = memory b. PNS = vasodilation * See Clinical Application 10.4, page 382: Opiates in the Human Body. * See Clinical Application 10.5, page 383: Drug Addiction. 10-14 UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function 10.7 SYNAPTIC TRANSMISSION G. Disorders Associated With Neurotransmitter Imbalances. See Table 10.5, page 381. 1. Clinical Depression = deficient norepinephrine/ serotonin 2. Epilepsy = Excess GABA leads to excess norepinephrine & dopamine 3. Huntington Disease = deficient GABA 4. Hypersomnia = excess serotonin 5. Insomnia = deficient serotonin 6. Mania = excess norepinephrine 7. Parkinson's disease = deficient dopamine 8. Schizophrenia = deficient GABA leads to excess dopamine 9. Tardive dyskinesia (uncontrollable movements of facial muscles) = deficient dopamine. F. Drugs that Alter Neurotransmitter Level: See Table 10.6, page 381. DRUG Tryptophan Reserpine Curare Valium Nicotine Cocaine Tricyclic Antidepressants Monoamine oxidase inhibitors Selective serotonin reuptake inhibitors Dual reuptake inhibitors G. NT AFFECTED Serotonin Norepi MECHANISM OF ACTION stimulates NT synthesis decreases packaging of NT into vesicles blocks receptor binding enhances receptor binding Activates receptors elevates levels blocks reuptake blocks reuptake blocks reuptake blocks enzymatic degradation of NT in presynaptic cell EFFECT Serotonin blocks reuptake Serotonin Norepi blocks reuptake Antidepressant, anti-anxiety agent mood elevation Ach GABA Ach Dopamine Dopamine Norepi Serotonin Norepi sleepiness decreases blood pressure muscle paralysis decreased anxiety increased alertness sense of pleasure euphoria antidepressant antidepressant antidepressant See Table 10.7, page 381: Events Leading to Neurotransmitter Release. 10-15 UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function 10.8 IMPULSE PROCESSING: See Fig 10.21, page 384. A. Neuronal Pools – neurons that synapse and work together. 1. B. Convergence 1. 2. 3. C. Working together results in facilitation – a general excitation that makes stimulation easier to achieve many neurons come together on fewer neurons (summation occurs) typical of motor pathways many inputs from brain, but usually only one motor response Divergence 1. 2. 3. fewer neurons spread out to signal many neurons (signal amplifies) typical of sensory pathways reason that a stimulus (i.e. odor) can cause many responses 10-16 UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function NERVE PATHWAY SUMMARY (keyed at the end of this outline) NS Division CNS OR PNS? TYPE OF NEURON? RECEPTIVE PORTION OF NEURON STIMULATED BY WHAT? TRANSMISSION AND DIRECTION OF IMPULSE THROUGH NEURON TRANSMISSION OF IMPULSE TO NEXT NEURON (OR EFFECTOR) 10-17 UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function OTHER INTERESTING TOPICS: A. B. C. D. E. F. G. THE WHOLE PICTIRE. See page 359. CAREER CORNER: Pharmacist, see page 361. Clinical Application 10.1, Migraine. See page 362. Tay-Sachs Disease. See box on page 363. Clinical Application 10.2, Multiple Sclerosis (MS). See page 367. Neuroma. See box on page 370. Neural Stem Cells. See box on page 371. INNERCONNECTIONS of the Nervous System – see page 385. CHAPTER SUMMARY – see pages 384, 386-387. CHAPTER ASSESSMENTS - see page 387. INTEGRATIVE ASSESSMENTS/CRITICAL THINKING - see page 388. 10-18 UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function Nerve Pathway Summary NS Division CNS OR PNS? TYPE OF NEURON? sensory PNS sensory (afferent) neuron Integrative CNS Interneuron motor PNS motor (efferent) neuron RECEPTIVE PORTION OF NEURON sensory receptors at the ends of dendrites dendritic ends (or cell body) dendritic ends (or cell body) STIMULATED BY WHAT? A stimulus (i.e. light, temperature change, etc) neurotransmitter (NT) released by sensory neuron neurotransmitter released by interneuron TRANSMISSION AND DIRECTION OF IMPULSE THROUGH NEURON from sensory receptors of dendrites down dendrites through cell body down axon* and into axonal terminal (synaptic knobs) *if axon is myelinated = saltatory conduction down dendrites through cell body (where integration occurs). Impulse is passed down axon* and into axonal terminal (synaptic knobs) *if axon is myelinated = saltatory conduction down dendrites through cell body down axon* and into axonal terminal (synaptic knobs) *if axon is myelinated = saltatory conduction TRANSMISSION OF IMPULSE TO NEXT NEURON (OR EFFECTOR) from electrical within sensory neuron to chemical between neurons. NT released from axonal terminals of sensory neuron stimulates the dendrite (or cell body) of an interneuron from electrical within interneuron to chemical between neurons. NT released from axonal terminals of interneuron stimulates the dendrite (or cell body) of a motor neuron from electrical within motor neuron to chemical between motor neuron and effector. NT/Acetylcholine released from axonal terminals of motor neuron stimulates an effector to respond. Effectors = muscle (contract) or glands (secretion of hormones) 10-19