BME 120 Week 3

Time constant (definition)

amount of time to reach 63.2% of the final value

Outward current (hyperpolarizes/depolarizes) the cell, causes a graded response

hyperpolarizes

Inward current (hyperpolarizes/depolarizes) the cell; there is a graded response until (hyperpolarization/depolarization) exceeds threshold and generates an action potential

depolarizes, depolarization

Threshold

membrane potential at which action potential occurs (50% of the time, if it is right at the threshold)

Absolute refractory period

time during which another action potential cannot occur

Relative refractory period

stronger stimulus required to elicit second action potentia1

What happens to the membrane potential (V_m) when the permeability to a specific ion increases?

As the permeability to that ion increases, the membrane potential approaches the equilibrium potential for that ion

Potassium channels (properties)

voltage-gated, activate with depolarization, persistent conductance

Sodium channels (properties)

voltage-gated (to activate), transient conductance (inactivates after a short time)

Summary of conductance changes during the action potential (6 steps)

1. membrane depolarized

2. Na⁺ activation, g_Na increases

3. Na⁺ into cell —> more depolarization, more channel opening

4. K⁺ channels activate, g_K increases

5. Na⁺ closes before K⁺ —> hyperpolarization

6. g_K, g_Na —> baseline; V_m—>E_m; Na⁺ able to open again

Hodgkin-Huxley Neuron

electrical circuit with variable conductances, similar to simple circuit model except it has variable conductances and E for each ion, can model action potential

Hodgkin-Huxley model exhibits complex properties of real neurons:

1. Predicts shape of action potential and conductance changes

2. Exhibits the strength-duration relationship

3. Action potentials have refractory periods

4. Exhibits rate coding (sustained firing with constant current input)

Electrotonic spread

• passive (fast)

• stimulus below threshold

• graded

• attenuated

Action potential

• active (slow)

• stimulus exceeds threshold

• all or none

• regenerative

Nodes of Ranvier: action potential or electrotonic spread?

action potential

lots of ion channels, active spread, resistance decreases while capacitance increases

Myelin sheath: action potential or electrotonic spread?

electrotonic spread

few ion channels, passive spread, resistance increases while capacitance decreases

Saltatory conduction

excitation appears to jump from node to node

What does the cable equation describe?

electrotonic spread of potential over time and space