Ch 11 - Fundamentals of Nervous System (2)

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  1. membrane potentials
    • neurons are highly irritable
    • electrical charge (ions) is separated by the cell membrane
    • in the human body, electrical charge is carried by ions (Na+, K+, etc) --> not electrons
    • undisturbed neuron has more negative charge (ions) on the inside than on the outside
    • membrane potential = potential difference between the outside and the inside of a cell (-70 mV)
  2. role of membrane ion channels:

    2 main types
    • large proteins serve as selective membrane ion channels
    • ((ex -> potassium ion channel only allows potential ion to pass))

    • Two main types of ion channels:
    • 1) Leakage (nongated) channels -- always open
    • 2) Gated
    • - part of protein changes shape to open/close channel
    • - normally closed
    • - when gated channels are open: ions diffuse quickly across membrane along electromchemical gradients (along chemical concentration gradients from higher concentration to lower concentration; along electrical gradients toward opposite electrical charge)
    • - ion flow creates an electrical current and coltage changes across membrane
  3. differences in plasma membrane permeability
    • impermeable large anionic proteins
    • slightly permeable to Na+ (through leakage channels -- sodium diffuses into cell down concentration gradient)
    • 25x more permeable to K+ than sodium (more leakage channels -- potassium diffuses out of cell down concentration gradient)
    • quite permeable to Cl-
  4. resting membrane potential
    • more potassium diffuses out than sodium diffuses in (cell more negative inside; establishes resting membrane potential)
    • sodium-potassium pump stabilizes resting membrane potential (maintains concentration gradients for Na+ and K+; three Na+ pumped out of cell; two K+ pumped in)
    • membrane is likely polarized
  5. membrane potential changes: used as communication signals
    • membrane potential changes when:
    • - concentrations of ions across membrane change
    • - membrane permeability to ions changes

    • action potentials:
    • - long-distance signals of axons

    changes in membrane potential are used as signals to receive, integrate, and send info
  6. Changes in membrane potential
    terms describing membrane potential changes relative to resting membrane potential

    • Depolarization:
    • decrease in membrane potential (toward zero and above)
    • inside of membrane becomes less negative than resting membrane potential
    • Hyperpolarization:
    • an increase in membrane potential (away from zero)
    • inside of cell more negative than resting membrane potential
  7. action potentials (AP)
    • principle way neurons send signals
    • principle means of long-distance neural communication
    • occur in the axons of neurons
    • brief reversal of membrane potential with a change in voltage of up to 100 mV
  8. generation of an Action Potential:  4 steps
    • 1) Resting State
    • all gated Na+ and K+ channels are closed
    • only leakage channels for Na+ and K+ are open (this maintains the resting membrane potential)
    • 2) Depolarizing Phase
    • depolarizing local currents open voltage-gated Na+ channels (Na+ rushes into cell)
    • Na+ influx causes more depolarization which opens more Na+ channels -> ICF less negative
    • at threshold (-55 to -50 mV) positive feedback causes opening of all Na+ channels -> a reversal of membrane polarity to +30mV (spike of action potential)
    • 3) Repolarizing Phase
    • Na+ channel slow inactivation gates close
    • membrane permeability to Na+ declines to resting state (AP spike stops rising)
    • slow voltage-gated K+ channels open (K+ exits the cell and internal negativity is restored)
    • 4) Hyperpolarization
    • some K+ channels remain open, allowing excessive K efflux (inside of membrane more negative than resting state)
    • this causes hyperpolarization of the membrane (slight dip below resting voltage)
    • Na+ channels begin to reset
  9. role of the sodium-potassium pump
    • repolarization resets electrical conditions, not ionic conditions
    • after repolarization Na+/K+ pumps (thousands of them in an axon) restore ionic conditions
  10. the all-or-none phenomenon
    • an AP either happens completely, or it does not happen at all
    • specific to action potential
  11. propagation of an Action Potential
    once initiated an AP is self-propagating (in myelinated axons each successive segment of membrane depolarizes, then repolarizes; propagation in myelinated axons differs)

    degree of myelination: continuous conduction in unmyelinated axons is slower than saltatory (leaping) conduction in myelinated axons
  12. continuous conduction in unmyelinated axons
    depolarization spreads: opposite charges attract each other. This creates local currents that depolarize adjacent membrane areas, spreading the wave of depolarization
  13. coding for stimulus intensity
    • all action potentials are alike and are independent of stimulus intensity (how does CNS tell difference b/w a weak stimulus and a strong one?)
    • strong stimuli cause action potentials to occur more frequently (number of impulses per second or frequency of APs)
    • CNS determines stimulus intensity by the frequency of impulses (higher frequency means stronger stimulus)
  14. conduction velocity
    conduction velocities of neurons vary widely

    • rate of AP propagation depends on:
    • axon diameter -
    • larger diameter fibers have less resistance to local current flow so faster impulse conduction
    • degree of myelination -
    • continuous conduction in unmyelinated axons is slower than saltatory conduction in myelinated axons
  15. conduction velocity: effects of myelination
    • myelin sheaths insulate and prevent leakage of charge
    • saltatory conduction (possible only in myelinated axons) is about 30x faster:
    • -- voltage-gated Na+ channels are located at myelin sheath gaps
    • -- APs generated only at gaps
    • -- electrical signal appears to jump rapidly from gap to gap
  16. importance of myelin sheaths: multiple sclerosis (MS)
    • autoimmune disease affecting primarily young adults
    • myelin sheaths in CNS destroyed
    • --immune system attacks myelin; turns it to hardened lesions called scleroses
    • --impulse conduction slows and eventually ceases
    • --unaffected axons increase N+ channels; causes cycles of relapse and remission
    • Symptoms:
    • visual disturbances, weakness, loss of muscular control, speech disturbances, and urinary incontinence
    • treatment:
    • drugs that modify immune system's activity improve lives
    • prevention?:
    • high blood levels of Vitamin D reduce risk of development
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Ch 11 - Fundamentals of Nervous System (2)

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