12.14.2 Human Regulation: The Nervous System and the Action Potential

Question 1

Human Regulation: The Nervous System and the Action Potential

Answer 1

• Electrical changes in the form of action potentials generate nerve impulses.
• At rest, a neuron has a membrane potential of –70 mV. If the membrane potential reaches threshold (–55 to –50 mV), sodium channels open, generating an action potential along the neuron.
• When the sodium channels close while potassium channels stay open, resting potential is restored.
• The sodium channels remain closed for a short time, resulting in a refractory period during which an action potential cannot be generated

Question 2

action potential

Answer 2

• An action potential (nerve impulse) travels down a nerve cell through a series of electrochemical changes. Ions carry electrical charges across the membranes of neurons. The main ions discussed here are sodium (Na+) and potassium (K+). Ion channels (proteins) regulate the flow of ions in and out of the cell.
• At rest, K+ ions diffuse out of the cell through potassium channels. Large molecules with negative charges do not leave the cell. Consequently, the inside of a neuron is 60 to 70 millivolts (mV) more negative than the outside. This negative charge is referred to as the “resting potential” of the neuron. During the resting potential, sodium channels are closed and sodium cannot enter the cell.
• Depolarization occurs when ion channels open in response to a stimulus and allow positive ions into the cell. When the charge inside the cell membrane becomes less negative as the positive ions move in, the membrane is depolarized. If the membrane depolarizes to a threshold of –55 to –50mV, sodium channels open and an action potential results.
• As the membrane potential peaks, sodium inactivation gates close, preventing any further ion flow into the cell. At about the same time, potassium gates slowly open, allowing potassium ions to flow out of the cell. As potassium gates slowly close, membrane potential falls below the resting state; this undershoot is called hyperpolarization.
• Sodium-potassium ATP pumps restore the membrane potential to its resting state during the refractory period.
  During the refractory period, the cell can’t generate a new action potential.
• The time-course of an action potential is very short, moving from the beginning of polarization to the end of
  hyperpolarization in 5–7 milliseconds. Action potentials jump from node to node along myelinated axons.

Question 3

Depolarization is caused by

Answer 3

• an influx of Na+ into the cell.

Question 4

True or false?
During the refractory period, the Na+-K+ ATP pump restores the resting membrane potential across the nerve cell membrane.

Answer 4

• true

Question 5

With regard to the action potential, the term “all or none” refers to

Answer 5

• how many channels open when the threshold potential is reached during an action potential

Question 6

Which of the following is not true about the Na+ and K+ ion channels in a neuron?

Answer 6

• They are both open during the depolarizing phase of the action potential.

Question 7

Schwann cells speed up the propagation of action potentials down an axon by

Answer 7

• limiting the regions on an axon where action potentials can occur.

Question 8

During the _____________ the cell can’t generate a new action potential.

Answer 8

• refractory period