Graded potential vs action potential graded potentials current

Graded potential vs action potential • graded potentials – current flow, amplitude - post-synaptic potentials • Action potentials – – – Trigger zone threshold voltage All-or-none Channel activity Refractory period Coding for stimulus intensity

Graded potential – amplitude decreases away from point of channel opening

Graded potential • Amplitude is directly proportional to the strength of the stimulating event • Depolarization wave known as local current flow • Strength depends on how much charge enters the cell • Decreases in strength as it travels through the cytoplasm • Can be depolarizing or hyperpolarizing

Graded potential – amplitude decreases away from point of channel opening

Action potential -Initiated if a threshold depolarization is reached at the trigger zone -Also called a spike -Always the same amplitude -Amplitude does not diminish -“All or none”, essential for long distance propogation

overshoot After hyperpolarization (undershoot)

Movement of ions through channels during the action potential (ion conductance)

The voltage gated Na+ channels • Has two gates: an activation gate and an inactivation gate (gate can be opened, gate can be locked)

Refractory period • Another action potential cannot fire when the inactivation gate is latched. • Action potentials cannot overlap • The absolute refractory period • Limits the rate of spike frequency • Important for frequency coding

Resting membrane potential is not restored, Na+ channel gates are unlatched (inactivation Is removed)

Coding for stimulus intensity

Coding for stimulus intensity

NO lecture on Friday • • Today How are action potentials propagated? Factors that affect conduction speed The consequences of abnormal levels of extracellular K+

Coding for stimulus intensity

Coding for stimulus intensity

How does an action potential spread?

What causes adjacent voltage gated sodium channels to open?

Low resistance to current flow in large diameter axons. (wide water pipe)

Mammalian axons are very thin, and action potential are conducted quickly.

Mammalian axons are insulated by myelin sheaths (myelinated axons).

What will abnormally high levels of extracellular K+ cause?

Abnormally high levels of K+ will cause • Resting membrane potential will be shifted toward action potential threshold • Get too much excitation (too many action potentials)

Abnormally low K+ • Membrane resting potential hyperpolarizes • Cell is far away from threshold for action potential