Lec6

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Lec6

  1. 1. Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Neuron structure and classification
  2. 2. Neurons • Neurons = nerve cells – Cells specialized to transmit messages – Major regions of neurons • Cell body – nucleus and metabolic center of the cell • Processes – fibers that extend from the cell body
  3. 3. Neuron Anatomy • Cell body – Nissl substance – specialized rough endoplasmic reticulum – Neurofibrils – intermediate cytoskeleton that maintains cell shape
  4. 4. Neuron Anatomy • Extensions outside the cell body – Dendrites – conduct impulses toward the cell body – Axons – conduct impulses away from the cell body
  5. 5. Axons and Nerve Impulses • Axons end in axonal terminals • Axonal terminals contain vesicles with neurotransmitters • Axonal terminals are separated from the next neuron by a gap – Synaptic cleft – gap between adjacent neurons
  6. 6. Synapses • Site at which neurons communicate • Signals pass across synapse in one direction • Presynaptic neuron – Conducts signal toward a synapse • Postsynaptic neuron – Transmits electrical activity away from a synapse
  7. 7. Structure of a Synapses Synapse
  8. 8. Two Neurons Communicating at a Synapse
  9. 9. How Neurons Communicate at Synapses
  10. 10. Myelin Sheaths • Segmented structures composed of the lipoprotein myelin • Surround the axon except at its endings • Form an insulating layer – Prevent leakage of electrical current • Increase the speed of impulse conduction
  11. 11. Myelin Sheaths • Formed by Schwann cells • Schwann cells wrap in concentric layers around the axon – Cover the axon in a tightly packed coil of membranes
  12. 12. Myelin Sheaths • Thick axons are myelinated – Fast conduction velocity • Thin axons are unmyelinated – Slow conduction velocity
  13. 13. Nerve Fiber Coverings • Schwann cells – produce myelin sheaths in jelly-roll like fashion • Nodes of Ranvier – gaps in myelin sheath along the axon
  14. 14. Nervous System: Functions • Three overlapping functions – Sensory receptors monitor changes inside and outside the body • Change – a stimulus • Gathered information – sensory input – CNS Processes and interprets sensory input • Makes decisions – integration – Dictates a response by activating effector organs • Response – motor output
  15. 15. Neurons Classified by Function
  16. 16. Structural Classification of Neurons Classification based on number of processes – Multipolar – Bipolar – Unipolar (pseudounipolar)
  17. 17. Multipolar Neurons Possess more than two processes Numerous dendrites and one axon
  18. 18. Bipolar Neurons Possess two processes Rare neurons – found in some special sensory organs
  19. 19. Unipolar (Pseudounipolar) Neurons Possess one single process Start as bipolar neurons during development
  20. 20. Structural Classification of the Nervous System • Central nervous system (CNS) – Brain – Spinal cord • Peripheral nervous system (PNS) – Nerve outside the brain and spinal cord
  21. 21. Sensory Input and Motor Output • Sensory signals picked up by sensory receptors – Carried by afferent nerve fibers of PNS to the CNS • Motor signals are carried away from the CNS – Carried by efferent nerve fibers of PNS to effectors – Innervate muscles and glands
  22. 22. Sensory Input and Motor Output • Divided according to region they serve – Somatic body region – Visceral body region • Results in four main subdivisions – Somatic sensory – Visceral sensory – Somatic motor – Visceral motor
  23. 23. Somatic Sensory • Somatic sensory – General somatic senses – receptors are widely spread • Touch, pain, vibration, pressure, and temperature • Body sense – position and movement of body in space
  24. 24. Somatic Motor • Somatic motor – General somatic motor – signals contraction of skeletal muscles • Under voluntary control • Often called “voluntary nervous system”
  25. 25. Visceral Sensory • Visceral sensory – General visceral senses – stretch, pain, temperature, nausea, and hunger • Widely felt in digestive and urinary tracts, reproductive organs
  26. 26. Visceral Motor • Visceral motor – Regulates the contraction of smooth and cardiac muscle and gland secretion – Makes up autonomic nervous system – Controls function of visceral organs – Often called “involuntary nervous system”
  27. 27. Types of Sensory and Motor Information
  28. 28. Nerve Fibers That Transmit Different Types of Signals, and Their Physiologic Classification • Nerve fibers come in all sizes between 0.5 and 20 micrometers in diameter-the larger the diameter, the greater the conducting velocity. • The range of conducting velocities is between 0.5 and 120 m/sec
  29. 29. Physiologic classifications and functions of nerve fibers (Erlanger and Gasser)

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