Action Potential Part 1

Reflex arc

Sequence of events where a stimulus activates sensory neurons, sending action potentials to the spinal cord, which then activates motor neurons that cause muscle contraction and withdrawal from the stimulus.

What happens first in a reflex arc

A stimulus (like a tack puncturing the skin) excites sensory fibers.

Where do sensory neurons send signals in a reflex arc

To the spinal cord via sensory axons carrying action potentials.

What happens in the spinal cord during a reflex arc

Interneurons activate motor neurons.

What do motor neurons do in a reflex arc

Carry action potentials to muscles, causing them to contract.

What is the result of muscle contraction in a reflex arc

The body part (e.g., foot) withdraws from the painful stimulus.

What are action potentials

Rapid, transient reversals of membrane potential that allow neurons to send signals over long distances.

What generates an action potential

Ionic currents that make the inside of the neuron positive relative to the outside.

What are other names for action potentials

Spikes, nerve impulses, or discharges.

What are the phases of an action potential

Rising phase, overshoot, falling phase, undershoot, and return to rest.

What principle do action potentials obey

The all-or-none principle (fired once threshold is crossed).

In what direction do action potentials propagate

Along the axon toward the axon terminals without significant decrement.

What is the resting potential

About –65 mV, when the neuron is at rest.

What is the threshold potential

The level of depolarization (around –40 mV) needed to trigger an action potential.

What happens during the rising phase

Rapid depolarization caused by Na⁺ influx through voltage-gated sodium channels.

What is the overshoot phase

The inside of the neuron becomes very positive, around +40 mV.

What happens during the falling phase

Repolarization due to K⁺ efflux through voltage-gated potassium channels.

What is the undershoot or afterhyperpolarization

The membrane becomes more negative than resting potential due to slow K⁺ channel closing.

What happens during the return to rest

Membrane potential gradually restores to –65 mV through leak channels and the Na⁺/K⁺ pump.

What happens when threshold is reached

The neuron fires an action potential in an all-or-none manner; if threshold isn’t reached, no action potential occurs.

What triggers the rising phase

Voltage-gated Na⁺ channels open rapidly, causing massive Na⁺ influx and depolarization.

What causes the overshoot to stop below E_Na

The driving force for Na⁺ decreases and Na⁺ channels inactivate quickly.

What is sodium channel inactivation

A process where the inactivation gate plugs the channel pore, preventing Na⁺ flow until the neuron repolarizes.

What are the four states of the Na⁺ channel

Closed, open, inactivated, and deinactivated (reset).

What causes the falling phase

Na⁺ channels inactivate while delayed voltage-gated K⁺ channels open, allowing K⁺ efflux.

Why are K⁺ channels called delayed rectifiers

They open slowly and help restore the membrane potential to E_K.

What drives repolarization

Outward K⁺ current that overcomes inward Na⁺ current.

What causes hyperpolarization

Continued K⁺ efflux because voltage-gated K⁺ channels close slowly.

What is the relative refractory period

The time during hyperpolarization when a stronger-than-normal stimulus is needed to trigger a new action potential.

What ensures one-way propagation of action potentials

The refractory period prevents backward conduction.

What happens during the return to resting potential

Voltage-gated K⁺ channels close, K⁺ leak channels dominate again, and the Na⁺/K⁺ pump restores ionic balance.

What is the Na⁺/K⁺ pump’s role in resetting potential

Pumps 3 Na⁺ out and 2 K⁺ in, maintaining gradients and stabilizing the membrane around –70 mV.

How long does it take to return to resting potential

Several milliseconds — slower than repolarization but essential for readiness of the next action potential.