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Biology LibreTexts

13. 17: Nerve Impulses

How does a nervous system signal move from one cell to the next?

It literally jumps by way of a chemical transmitter. Notice the two cells are not connected, but separated by a small gap. The synapse. The space between a neuron and the next cell.

Nerve Impulses

Nerve impulses are electrical in nature. They result from a difference in electrical charge across the plasma membrane of a neuron. How does this difference in electrical charge come about? The answer involves ions, which are electrically charged atoms or molecules.

Resting Potential

When a neuron is not actively transmitting a nerve impulse, it is in a resting state, ready to transmit a nerve impulse. During the resting state, the sodium-potassium pump maintains a difference in charge across the cell membrane (see Figure below). It uses energy in ATP to pump positive sodium ions (Na+) out of the cell and potassium ions (K+) into the cell. As a result, the inside of the neuron is negatively charged compared to the extracellular fluid surrounding the neuron. This is due to many more positively charged ions outside the cell compared to inside the cell. This difference in electrical charge is called the resting potential.

Sodium Potassium pump

The sodium-potassium pump maintains the resting potential of a neuron.

Action Potential

nerve impulse is a sudden reversal of the electrical charge across the membrane of a resting neuron. The reversal of charge is called an action potential. It begins when the neuron receives a chemical signal from another cell. The signal causes gates in sodium ion channels to open, allowing positive sodium ions to flow back into the cell. As a result, the inside of the cell becomes positively charged compared to the outside of the cell. This reversal of charge ripples down the axon very rapidly as an electric current (see Figure below). 

Action potential

An action potential speeds along an axon in milliseconds.

In neurons with myelin sheaths, ions flow across the membrane only at the nodes between sections of myelin. As a result, the action potential jumps along the axon membrane from node to node, rather than spreading smoothly along the entire membrane. This increases the speed at which it travels.

The place where an axon terminal meets another cell is called a synapse. The axon terminal and other cell are separated by a narrow space known as a synaptic cleft (see Figure below). When an action potential reaches the axon terminal, the axon terminal releases molecules of a chemical called a neurotransmitter. The neurotransmitter molecules travel across the synaptic cleft and bind to receptors on the membrane of the other cell. If the other cell is a neuron, this starts an action potential in the other cell. 

Closeup of synapse during signal transmission

At a synapse, neurotransmitters are released by the axon terminal. They bind with receptors on the other cell.

Summary

  • A nerve impulse begins when a neuron receives a chemical stimulus.
  • The nerve impulse travels down the axon membrane as an electrical action potential to the axon terminal.
  • The axon terminal releases neurotransmitters that carry the nerve impulse to the next cell.

Review

  1. Define resting potential and action potential.
  2. Explain how resting potential is maintained
  3. Describe how an action potential occurs.
  4. What is a synapse?