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Transmission of Nerve Impulses
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  1. Transmission of Nerve Impulses Honors Anatomy & Physiology for copy

  2. Electrical Signals in Neurons • neurons are electrically excitable • 2 types of electrical signals: • graded potentials • only for short distance communication • action potentials • allow communication over both short & long distances

  3. Graded & Action Potentials • both depend on 2 basic features of plasma membrane in excitable cells: • existence of a resting membrane potential • presence of specific ion channels

  4. Membrane Potentials • electrical voltage difference across the membrane • flow of ions thru ion channels => flow of current

  5. Ion Channels • when open: allow specific ions to move across plasma membrane down their electrochemical gradient by diffusion • ions move down their concentration gradient & (+) charged cations move toward (-) charged anions

  6. Ion Channels • 4 types: • leakage channels • voltage-gated channels • ligand-gated channels • mechanically gated channels

  7. Leakage Ion Channels • channels randomly alternate (open/close) • typically are more K+ leakage channels than Na+ & K+ channels are leakier than Na+ • (so membranes more permeable to K+ than Na+)

  8. Voltage-Gated Channels • open in response to change in membrane potential • important in generation & conduction of action potentials

  9. Ligand-Gated Channels • open/ close in response to a specific chemical stimulus • ligands include: • neurotransmitters • hormones • particular ions • work in 1 of 2 ways: • directly: ligand molecule itself opens/closes gate • indirectly: ligand activates another molecule which in turn opens/closes gate

  10. Mechanically Gated Channels • gates open/close in response to mechanical stimulation in form of: • vibration (sound waves) • pressure (touch) • tissue stretching

  11. Resting Membrane Potential • exists because there is small build-up of (-) charge just inside plasma membrane  small build-up of (+) charge just outside membrane • separation of charge source of PE • greater the difference in charge the larger the membrane potential (voltage)

  12. Resting Membrane Potential • neurons: membrane potentials range from • -90 mv to -40 mv • (-) indicates the inside of cell is (-) relative to the outside • cells with membrane potential are said to be polarized • most body cells are polarized with membrane potentials vary from +5 mv to -100 mv

  13. Generation of Action Potentials (AP) • 2 phases: (lasts ~ 1msec) • Depolarizing Phase • (-) membrane becomes less (-) • Repolarizing Phase • normal membrane potential restored

  14. AP Voltage-Gated Channels • 2 types of voltage-gated channels open & then close: • present mainly in axon plasma membrane & axon terminals • 1st to open: Na+ • Na+ rushes into cell • causes depolarization • then K+ channels open • K+ flows out of cell • produces repolarizing phase

  15. Action Potential Characteristics • “all-or-none” principle • threshold: (~ -55mv) must be reached for AP to occur

  16. Refractory Period • period of time after an AP begins during which an excitable cell cannot generate another action potential

  17. Propagation of Nerve Impulses • AP must travel from trigger zone  axon terminal • this mode of travel called propagation or conduction • uses (+) feedback • when Na+ flows into cell it causes voltage-gated Na+ channels in adjacent segments to open

  18. Local Anesthetics • drugs that block pain & other somatic sensations • cold applied to area also produces anesthetic effect: axons propagate slower • ice partially blocks axon propagation of pain • examples: • Lidocaine, Novacaine • act by blocking the opening of voltage-gated Na+ channels: • sensory nerve impulses cannot travel past obstructed region so pain signals never make it to the brain

  19. Effect of Axon Diameter • larger diameter axons propagate impulses faster than smaller diameter ones due to larger surface area • Classified: • A-fibers: (5-20 μm) travel 12-130 m/s • used by somatic sensory neurons & motor neurons • B-fibers: (2-3 μm) travel 15 m/s • found in sensory neurons going from viscera  brain • C-fibers: ( .5-1.5 μm) travel 0.5 – 2 m/s • all unmyelinated, found in some sensory for pain from skin and viscera & in autonomic motor fibers

  20. Signal Transmission @ Synapses • presynaptic neuron: neuron sending the AP • postsynaptic neuron: neuron receiving the AP • synapse: space between the 2 • synapses can be: • electrical • chemical

  21. Electrical Gap Junctions • common in visceral smooth muscle, cardiac muscle, & in developing embryo • 2 advantages to electrical synapses • faster communication • AP passes directly from presynaptic neuron  postsynaptic neuron • synchronization • electrical synapses can coordinate the activity of a group of neurons or muscle fibers

  22. Chemical Synapses • synaptic cleft: space between filled with interstitial fluid • in response to AP axon terminal of presynaptic neuron releases neurotransmitters that diffuse across the synaptic cleft bind to receptors in plasma membrane of postsynaptic neuron  producing a postsynaptic potential • electrical signal (AP)  chemical signal (neurotransmitters)  electrical signal (AP)

  23. Removal of Neurotransmitter • occurs in 3 ways: • diffusion • enzymatic degradation • uptake by cells

  24. Neurotransmitters • 2 classes: • small-molecule neurotransmitters • neuropeptides

  25. Plasticity in Nervous System • nervous system exhibits plasticity: capability to change based on experience • @ level of individual neurons: • sprouting new dendrites • synthesis of new proteins • changes in synaptic contacts with other neurons

  26. Regeneration in Nervous System • limited capacity to replicate or repair itself • until recently: thought was little or no repair done to damage in CNS: have found neuroglial cells more capable of this than previously appreciated • PNS: as long as cell body intact & Schwann cell active new axon can be regenerated

  27. Homeostatic Imbalances in the Nervous System • Multiple Sclerosis (MS) • progressive destruction of myelin sheaths of neurons in the CNS • autoimmune disorder • afflicts ~ 350,000 in USA (estimated) with: • >2x female/male • white > other races • diagnosis difficult but most diagnosed ages 20 - 50

  28. MS Causes? • unknown but do see: • genetic susceptibility: having 1st degree relative with it increases your chances several-fold • possible association to living farther away from equator (?) so maybe something to do with vitamin D • possible association with history of herpes-6, Epstein Barr virus, chlamydia

  29. Epilepsy • characterized by short, recurrent attacks of motor, sensory, or psychological malfunction • afflict ~1% of world’s population • http://www.dailymotion.com/video/xaigxc_brain-animation-of-epileptic-seizu_creation#.UaPsb0B-8TY

  30. Causes of Epilepsy • brain damage • most frequently from birth trauma • metabolic disturbances • hypoglycemia, hypocalcemia, uremia, hypoxia • infections • encephalitis or meningitis • toxins • alcohol, tranquilizers, hallucinogens • head injuries • tumors & abcesses of the brain

  31. Medical Terminology • Guillain-Barre Syndrome (GBS) • acute demyelinating disorder in which macrophages strip axons of myelin in PNS • may be response of immune system to bacterial infection • Neuroblastoma • most common tumor in infants • immature neurons (neuroblasts) grow in abdomen or adrenals • Neuropathy • any disorder that affects the nervous system but particularly a disorder of a cranial or spinal nerve example: Bell’s palsy