The basic unit of the nervous system is the neuron (nerve cell). Reception, conduction, and integration of stimuli are all based on the electrochemical properties of neurons. First, let us understand their structure and the potential in the resting state.
Basic Knowledge
Neuron structure:
① Cell body — The central part containing the nucleus and organelles.
② Dendrites — Short projections extending in large numbers from the cell body. They receive signals from other neurons and receptors.
③ Axon (nerve fiber) — A long projection extending from the cell body. It transmits signals to the next neuron or an effector.
④ Myelin sheath — An insulating layer formed when Schwann cells wrap around the axon. There are myelinated nerves with myelin sheaths and unmyelinated nerves without them.
⑤ Node of Ranvier — A gap in the myelin sheath. It is the site of saltatory conduction.
Resting potential (membrane potential):
When a neuron is at rest and not excited, there is more K⁺ inside the cell and more Na⁺ outside the cell. Across the cell membrane, the inside has a potential about -70 mV relative to the outside (the inside is negative). This is called the resting potential.
This state is maintained by the Na⁺-K⁺ pump (sodium-potassium ATPase), which uses ATP to transport 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell.
Explain why the resting potential is approximately -70 mV from the perspectives of the Na⁺-K⁺ pump and K⁺ permeability.
Answer: The Na⁺-K⁺ pump uses ATP to move Na⁺ out of the cell and K⁺ into the cell, maintaining a high K⁺ concentration inside the cell. At rest, the cell membrane is selectively permeable to K⁺, so K⁺ flows out of the cell along its concentration gradient. As a result, the inside becomes negatively charged and a potential difference of about -70 mV is produced.
- Neuron = cell body, dendrites, axon, myelin sheath, and nodes of Ranvier
- Resting potential = -70 mV (the inside is negative)
- Inside the cell: much K⁺ and little Na⁺ / outside the cell: much Na⁺ and little K⁺
- Na⁺-K⁺ pump = 3 Na⁺ out and 2 K⁺ in (uses ATP)
- Myelin sheath = an insulating layer formed by Schwann cells wrapping around the axon
- Node of Ranvier = site of saltatory conduction in myelinated nerves
Points of Caution
① The resting potential means that “the inside is negative.” During excitation (depolarization), the inside becomes positive. ② The Na⁺-K⁺ pump is active transport that consumes ATP. The asymmetry of 3 Na⁺ out and 2 K⁺ in is important. ③ Myelinated nerves conduct rapidly by saltatory conduction (up to 120 m/s), whereas unmyelinated nerves are slower (about 1 m/s).
Practice
- What is the name of the short projections of a neuron that receive stimuli?
- Which pump is present in the cell membrane to maintain the resting potential? What is the energy source used by the pump?
- What is the name of the site where saltatory conduction occurs in a myelinated nerve?