Neurons and activity

Summation→ look over


Action Potentials

  • If threshold is reached (-55mV), an action potential forms down axon to release neurotransmitter

    • Doestn change from distance, intensity

    • look same every time→ all or nothing— either happens or doesn’t

  • Action potential: short complete reversal of action potential

    • Cell becomes positive for short time

    • total amplitute of peak = 100mV

    • Does NOT decrease in strength over distance

    • All or nothing response

  • Primry means of neural communication

  • nerve impulse: action potential in axon

  • Note: Sodium potential highest on outside of cell (wave of positive move down axon—see diagram in slides)


Phases

  • Initially→ at rest (-70mV)

  • Then stimulus→ if positive enough, hit threshold of -75 mV

  • Depolarization→ then top off at +30mV

  • Sodium gates close, potassium gates open

  • Repolarization

  • Hyper-polarization

  • Rest→ then over again


AP: Resting State

  • Na+ and K+ channels are closed

  • Leakage— small movements of Na+ and K+

  • Each Na+ channel as 2 voltage-regulated gates

    • Activation gate— closed in resting state

    • inactivation gate open in resting state

Depolarization

  • Na+ permeability increases: membrane potential reverses

  • Na+ gates open: K+ gates closed

  • Threshold: critcal level of depolarization (-55 to -50 mV)

    • Depolarization becomes self-generating

Depolarization

  • Na inactivation gates close

  • Membrane permeability to Na+ decreases to resting levels

  • as Na gates close, voltage-sensitive K+ gates open

  • K+ exits


Hyper polarization

  • K+ gates stay open, excessive efflux of K+

    • Efflux causes hyperpolarization of the membrane (undershoot)

  • Neuron insensitive to stimulus and depolarization

Refractory Period

  • Absolute

    • time from opening of Na+ activation gates until closing of inactivation gates

    • Prevents generation of AP

  • Relative

    • Interval following absolute refractory period when

      • Na+ gates are closed

      • K+ gates open

      • repolarization is occurring

    • Threshold level is elevated, allowimg strong stimuli


Coding for Stimulus Intesity

  • All or nothing phenomenon— AP occur completely, or not at all



Axon Conduct Velocities

  • Vary widely among neurons

  • rate of impulse propagation determined by:

    • Axon diameter: larger diameter= faster impulse

    • Myelin sheath presence: myelination dramatically increases impulse speed


Myelination

  • Fatty, white, segmented sheath around many long axons

  • Function:

    • Protection

    • Electrical insulation

    • increase speed of electrical impulse

Unmyelinated axons

  • Schwann cells still associate with unmyelinated nerve fibers

    • surround, no coiling

  • One shwann cell can partially enclose 15 or more

  • Q: why are some axons unmyelinated?

Saltatory Conduction

  • Myelinated axon

  • Current only passes at nodes of Ranvier

  • Voltage-gated Na+ channels concentrated at these nodes

  • AP triggered only at Nodes and jump from node to node

  • faster than conduction along unmyelinated axons


Synapses

  • Presynaptic neuron— conducts impulses toward synapse

  • Postsynaptic neuron— transmits impulses away from synapse


release of Neurotransmitters

  • AP arrives

  • voltae gates Ca channels open due to AP

  • Ca enters the cell→ now in cytoplasm of

  • Ca+ binds to calmodulin

  • Activates pKA

  • Phosphorylate synapse protein