BIOM 3200 Chapter Notes - Chapter 2: Voltage-Dependent Calcium Channel, Inhibitory Postsynaptic Potential, Resting Potential

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Unit 2: Synaptic Transmission and Muscle physiology
Electrical activity in axons
All cells in the body maintain a potential difference across the membrane (resting membrane potential)
- Inside is – charged in comparison to the outside
- Largely the result of the permibility properties of the membrane
- The membrane traps large, negatively-charged organic molecules within the cell and permits
only limited diffusion of positively-charged inorganic ions
Result is unequal distribution of ions across the membrane
**Na+/K+ pump results in high [Na+] in extracellular fluid, and high [K+] inside the cell
Excitability or irritability
- The ability of neurons muscle cells to produce and conduct changes in membrane potential
Membrane potential to a specific ion results in the diffusion of the ion down its concentration gradient
- Ion currents only occur across limited patches of membranes
- Can be measured using electrodes
Neurons resting membrane potential -70 mV
Heart muscle cell -85 mV
Depolarization + charges flow into the cell
- Potential difference is reduced, excitatory in dendrites/cell body
Repolarization return to resting membrane potential
Hyperpolarization inside of cell is more – than rmp (inhibitory)
Ion gating in axons
Changes in membrane potential are caused by changes in the net flow of ions through ion channels in
the membrane
2 types of channels for K+
1. Gated (closed at rmp)
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2. Not gated- always open, called leakage channels
All Na+ channels are gated and closed at rmp
- Sometime flicker open to allow some Na+ to leak into the resting cell
- Results in rmp slightly less – than equilibrium potential for K+
Depolarization to a certain threshold causes the Na+ channels to open
Plasma membrane is freely permeable to Na+
1. Na+ rushes into the cell
2. Membrane potential moves towards the Na+ equilibrium potential
3. Quickly after it open,s the channels close due to inactivation
4. Just before they close, the depolarization stimulus causes the gated K+ channels to open
5. Makes membrane more permeable to K+ than at rest
6. Membrane potential moves toward K+ equilibrium potential
7. K+ channels close and membrane permeability returns to “at rest”
Na+/K+ channels are voltage-gated
- Close at rmp of -70 mV
- Open in response to depolarization of the membrane to a threshold value
Action potentials
Axon membrane is depolarized to a threshold level
- Nerve impulses caused by changes in Na+ and K+ diffusion and the resulting changes in
membrane potential
Na+ gates open and membrane becomes permeable to Na+
Permits Na+ to enter the axon by diffusion , which further depolarizes the membrane (inside
is less negative)
Na+ channels of the axon membrane is voltage-regulated, so the additional depolarization
opens more Na+ channels and makes the membrane even more permeable to Na+ ions
Positive feedback loopis created causing the rate of Na+ entry and depolarization to
accelerate
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- Results in rapid reversal of membrane potential from -70 to +30
- At +30 mV, the Na+ channels close, causing rapid decrease in permeability
- At the top of the action potential, the voltage does not reach the +66 mV Na+ equilibrium
potential
- As a result of time-delayed effect of depolarization, voltage-gated K+ channels open and K+
diffuses rapidly out of the cell
- Diffusion of K+ out of the cell acts to restore rmp to -70 mV
Repolarization completes a negative feedback loop
The movement of Na+ and K+ ions is sufficient to cause changes in the membrane potential during an
action potential, but does not significantly affect the concentrations of these ions
- Active transport is required to move Na+ out of the axon and to move K+ back into the axon
after an action
Na+/K+ pump isn’t directly involved in action potentials, but are required to maintain the concentratin
gradients needed for the diffusion of Na+ and K+ during action potentials
All-or-none law when membrane potential reaches threshold action potential is irreversibly fired
Stimulus intensity stimulus strength= frequency modulation (fm)
- A weak stimulus will activate only a few axons with low threshold, strong stimuli activate axons
with high thresholds
Recruitment a mechanism by which the nervous system can code for stimulus strength
Refractory periods
Absolute refractory period axon membrane is producing an action potential and is incapable of
responding of further stimulation
Relative refractory period a very strong depolarization is needed for a membrane to respond to a
stimuli
Cable properties of neurons
- High internal resistance of axons
- If the depolarization is below threshold the change in membrane potential will be localized to
within 1-2 mm of the point of stimulation
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