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Nervous System

Nervous System . Structure and Function. Functions. Sensory input Integration Motor output. Classification. Central Nervous System Brain Cranial Nerves Spinal Cord Peripheral Nervous System Spinal nerves Sensory receptors. Divisions of Peripheral Nervous System. Sensory Division

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Nervous System

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  1. Nervous System Structure and Function

  2. Functions • Sensory input • Integration • Motor output

  3. Classification • Central Nervous System • Brain • Cranial Nerves • Spinal Cord • Peripheral Nervous System • Spinal nerves • Sensory receptors

  4. Divisions of Peripheral Nervous System • Sensory Division • Motor Division • muscles and glands • Divisions of the Motor • Somatic • Autonomic

  5. Peripheral nervous system (PNS) Central nervous system (CNS) Cranial nerves and spinal nerves Brain and spinal cord Communication lines between the CNS and the rest of the body Integrative and control centers Sensory (afferent) division Motor (efferent) division Somatic and visceral sensory nerve fibers Motor nerve fibers Conducts impulses from the CNS to effectors (muscles and glands) Conducts impulses from receptors to the CNS Somatic sensory fiber Autonomic nervous system (ANS) Somatic nervous system Skin Visceral motor (involuntary) Somatic motor (voluntary) Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands Conducts impulses from the CNS to skeletal muscles Visceral sensory fiber Stomach Skeletal muscle Motor fiber of somatic nervous system Sympathetic division Parasympathetic division Mobilizes body systems during activity Conserves energy Promotes house- keeping functions during rest Sympathetic motor fiber of ANS Heart Structure Function Sensory (afferent) division of PNS Bladder Parasympathetic motor fiber of ANS Motor (efferent) division of PNS Figure 11.2

  6. How it works

  7. Neurons = nerve cells • Long lived, no mitosis, • Cell body- developed Golgi • Extensions outside the cell body • Dendrites • Axons • Axonal terminals contain vesicles with neurotransmitters • Axonal terminals are separated from other neurons or effectors (muscles or organs) by a gap called the synapse

  8. Nerve Coverings • Myelin- Lipid/Protein • Schwann cells • Nodes of Ranvier

  9. Schwann cell plasma membrane A Schwann cell envelopes an axon. 1 Schwann cell cytoplasm Axon Schwann cell nucleus 2 The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers. The Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath. Neurilemma 3 Myelin sheath (a) Myelination of a nervefiber (axon) Figure 11.5a

  10. Classification of Neurons • Multipolar neurons • Bipolar • Unipolar

  11. Classification Cont.. • Sensory Neurons • afferent • most are unipolar • some are bipolar • Interneurons • multipolar • in CNS • Motor Neurons • efferent • multipolar • carry impulses to effectors, muscle or gland

  12. Table 11.1 (2 of 3)

  13. Neuroglial Cells: Support Cells in CNS • Microglia • Phagocytes that engulf debris, necrotic tissue, invading viruses or bacteria • Astrocytes • Have many processes – look like stars • Perivascular feet wrap around and cover neurons and blood vessels • Form the blood-brain barrier which allows only certain substances to enter neurons from blood vessels

  14. Ependyma • Line the ventricles in the brain and central canal in spinal cord • Form cerebral spinal fluid • Oligodendrocytes • Support CNS cells • Have processes that form myelin sheaths around axons

  15. Neuroglial Cells: Support Cells in PNS • Schwann Cells-PNS • Flattened cells, wrap around the axons • Form the myelin sheath around the axon • Satellite-PNS

  16. Regeneration of Injury (if possible)

  17. Principles of Electricity • Opposite charges attract each other • Energy is required to separate opposite charges across a membrane • Energy is liberated when the charges move toward one another • If opposite charges are separated, the system has potential energy

  18. Definitions • Voltage (V): measure of potential energy generated by separated charge • Potential difference: voltage measured between two points • Current (I): the flow of electrical charge (ions) between two points

  19. Definitions • Resistance (R): hindrance to charge flow (provided by the plasma membrane) • Insulator: substance with high electrical resistance • Conductor: substance with low electrical resistance

  20. Role of Membrane Ion Channels • Leakage (nongated) channels—always open • Gated channels (three types): • Chemically gated (ligand-gated) • Voltage-gated channels • Mechanically gated channels

  21. Generating a Nerve Impulse • polarized membrane: inside is negative relative to the outside under resting conditions • -70 mV

  22. Voltmeter Plasma membrane Ground electrode outside cell Microelectrode inside cell Axon Neuron Figure 11.7

  23. Action Potential (AP) • Brief reversal of membrane potential with a total amplitude of ~100 mV • Occurs in muscle cells and axons of neurons • Does not decrease in magnitude over distance • Principal means of long-distance neural communication

  24. The big picture 1 3 2 Resting state Depolarization Repolarization 3 4 Hyperpolarization Membrane potential (mV) Action potential 2 Threshold 1 1 4 Time (ms) Figure 11.11 (1 of 5)

  25. Generation of an Action Potential • Resting state • Only leakage channels for Na+ and K+ are open • All gated Na+ and K+ channels are closed

  26. Depolarizing Phase • Na+ influx causes more depolarization • At threshold (–55 to –50 mV) positive feedback leads to opening of all Na+ channels, and a reversal of membrane polarity to +30mV (spike of action potential)

  27. Repolarizing Phase • Repolarizing phase • Na+ channel slow inactivation gates close • Membrane permeability to Na+ declines to resting levels • Slow voltage-sensitive K+ gates open • K+ exits the cell and internal negativity is restored

  28. Hyperpolarization • Hyperpolarization • Some K+ channels remain open, allowing excessive K+ efflux • This causes after-hyperpolarization of the membrane (undershoot)

  29. The AP is caused by permeability changes in the plasma membrane 3 Action potential Membrane potential (mV) Na+ permeability 2 Relative membrane permeability K+ permeability 1 1 4 Time (ms) Figure 11.11 (2 of 5)

  30. Voltage at 0 ms Recording electrode (a) Time = 0 ms. Action potential has not yet reached the recording electrode. Resting potential Peak of action potential Hyperpolarization Figure 11.12a

  31. Voltage at 2 ms (b) Time = 2 ms. Action potential peak is at the recording electrode. Figure 11.12b

  32. Voltage at 4 ms (c) Time = 4 ms. Action potential peak is past the recording electrode. Membrane at the recording electrode is still hyperpolarized. PLAY A&P Flix™: Propagation of an Action Potential Figure 11.12c

  33. Impulse Conduction

  34. Coding for Stimulus Intensity • All action potentials are alike and are independent of stimulus intensity • How does the CNS tell the difference between a weak stimulus and a strong one? • Strong stimuli can generate action potentials more often than weaker stimuli • The CNS determines stimulus intensity by the frequency of impulses

  35. Saltatory Conduction • Appear the jump from node to node. • Speed of impulses is much faster on myelinated nerves then unmyelinated ones. Speed also increases with increase in diameter. Ex.) 120m/s skeletal muscle .5m/s skin.

  36. Conduction Velocity • Conduction velocities of neurons vary widely • Effect of axon diameter • Effect of myelination • Myelin sheaths insulate and prevent leakage of charge • Saltatory conduction in myelinated axons is about 30 times faster

  37. Nerve Fiber Classification • Group A fibers • Large diameter, myelinated somatic sensory and motor fibers • Group B fibers • Intermediate diameter, lightly myelinated ANS fibers • Group C fibers • Smallest diameter, unmyelinated ANS fibers

  38. The Synapse • Presynaptic neuron—conducts impulses toward the synapse • Postsynaptic neuron—transmits impulses away from the synapse • Axodendritic • Axosomatic • Some electrical, most chemical • Cleft = gap

  39. Axodendritic synapses Dendrites Axosomatic synapses Cell body Axoaxonic synapses (a) Axon Axon Axosomatic synapses Cell body (soma) of postsynaptic neuron (b) Figure 11.16

  40. Chemical synapsestransmit signals fromone neuron to anotherusing neurotransmitters. Presynapticneuron Presynapticneuron Postsynapticneuron 1 Action potentialarrives at axon terminal. 2 Voltage-gated Ca2+channels open and Ca2+enters the axon terminal. Mitochondrion Ca2+ Ca2+ Ca2+ Ca2+ Synapticcleft 3 Ca2+ entry causesneurotransmitter-containing synapticvesicles to release theircontents by exocytosis. Axonterminal Synapticvesicles 4 Neurotransmitterdiffuses across the synapticcleft and binds to specificreceptors on thepostsynaptic membrane. Postsynapticneuron Ion movement Enzymaticdegradation Graded potential Reuptake Diffusion awayfrom synapse 5 Binding of neurotransmitteropens ion channels, resulting ingraded potentials. 6 Neurotransmitter effects areterminated by reuptake throughtransport proteins, enzymaticdegradation, or diffusion awayfrom the synapse. Figure 11.17

  41. An EPSP is a local depolarization of the postsynaptic membrane that brings the neuron closer to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowing the simultaneous pas- sage of Na+ and K+. Membrane potential (mV) Threshold Stimulus Time (ms) (a) Excitatory postsynaptic potential (EPSP) Figure 11.18a

  42. An IPSP is a local hyperpolarization of the postsynaptic membrane and drives the neuron away from AP threshold. Neurotransmitter binding opens K+ or Cl– channels. Membrane potential (mV) Threshold Stimulus Time (ms) (b) Inhibitory postsynaptic potential (IPSP) Figure 11.18b

  43. Integration: Summation • A single EPSP cannot induce an action potential • EPSPs can summate to reach threshold • IPSPs can also summate with EPSPs, canceling each other out

  44. Neurotransmitters • Most neurons make two or more neurotransmitters, which are released at different stimulation frequencies • 50 or more neurotransmitters have been identified • Classified by chemical structure and by function • Some excite and some inhibit • Can be nucleotides, gas, protein, amino acid, lipoprotein

  45. Neurotransmitters

  46. Ion flow blocked Ions flow Ligand Closed ion channel Open ion channel (a) Channel-linked receptors open in response to binding of ligand (ACh in this case). Figure 11.20a

  47. Closed ion channel Open ion channel 1 Neurotransmitter (1st messenger) binds and activates receptor. Adenylate cyclase Receptor G protein cAMP changes membrane permeability by opening or closing ion channels. 5a cAMP activates specific genes. 5c GDP 5b cAMP activates enzymes. 2 3 4 Receptor activates G protein. G protein activates adenylate cyclase. Adenylate cyclase converts ATP to cAMP (2nd messenger). Nucleus Active enzyme (b) G-protein linked receptors cause formation of an intracellular second messenger (cyclic AMP in this case) that brings about the cell’s response. Figure 11.17b

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