6.1 Essential ideas

6.1.5 Neuron structure and nervous transmission

  • Electrical impulses are transmitted to and from the brain (and central nervous system, CNS) through specialised cells called neurons.
  • Impulses are transmitted in one direction only. Sensory neurons transmit impulses from a sensory tissue to the CNS. Motor neurons transmit signals from the CNS to the effector tissue, which is normally a muscle.

Figure 6.1.5a - A typical neuronFigure 6.1.5a – A typical neuron

The resting potential is generated using ATP

  • Neurons that are not transmitting a signal maintain an average resting potential of –70mV. It is electrochemically more negative inside the cell than outside.

Figure 6.1.5b - Resting potentialFigure 6.1.5b – Resting potential
The slightly negative resting potential is generated mainly by the action of sodium–potassium exchange pumps in the plasma membrane.

Each of the following contribute simultaneously to maintaining a slightly more negative environment in the cytosol:

  1. The cytosol contains many negatively charged organic proteins. They are too large to diffuse across the membrane in response to a concentration gradient.
  2. The sodium–potassium exchange protein uses one ATP molecule for every three Na+ ions pumped out and two K+ ions pumped into the cell. This is the most important way the neuron generates the negative membrane potential.
  3. The plasma membrane is not very permeable to sodium. A negligible number of Na+ ions diffuse into the cell in response to the concentration gradient. Most sodium channels are closed.
  4. The plasma membrane is slightly permeable to potassium. A small number of K+ ions diffuse out of the cell in response to the concentration gradient. Some potassium channels are open.

The action potential involves depolarization and repolarization

  Depolarization Repolarization
Membrane potential (approximate mV) Becomes positive (+30) Becomes negative (–80)
Voltage gated Na+ channels Open, allowing sodium to diffuse into the neuron Closed
Voltage gated K+ channels Closed Open, allowing potassium to diffuse out of the neuron
  • The action potential only occurs if the membrane potential is depolarized past a threshold potential, usually –55mV. When the threshold is reached, the neuron becomes rapidly depolarized to +30mV by the influx of sodium ions.
  • Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential.

Figure 6.1.5c – Propagation of action potentialFigure 6.1.5c – Propagation of action potential
An action potential at one location causes depolarization at an adjacent location.

  • After repolarization, the resting potential must be restored actively. During this ‘refractory period’ of a few milliseconds, no new impulse takes place.

Myelination of nerve fibres allows for saltatory conduction

  • Some neurons are surrounded by Schwann cells, which produce layers of myelin around the axon. Between two Schwann cells, there is a small stretch of unmyelinated axon at the nodes of Ranvier (see Figure 6.1.5a above).
  • Myelin is a fatty material that insulates electricity, preventing the depolarization of the axon and allowing the action potential to ‘jump’ from one node to the next. This is called discontinuous, or saltatory, conduction.
  • Saltatory conduction increases the speed of nervous transmission by up to 100 times. More myelin means bigger, faster ‘jumps’.

Figure 6.1.5d - MyelinFigure 6.1.5d – Myelin
Myelination causes the action potential to occur only at the nodes.

Key concept

Neurons transmit the message. Synapses modulate the message.

Concept help

  • Voltage is a measurement of the electric potential energy difference between two points, measured in volts V.
  • The resting potential of different neurons ranges from –60 to –95mV and depends on the concentration of membrane proteins and permeability of different inorganic ions, including Na+, K+, Ca2+ and Cl–.
  • Once the threshold is reached, the signal is propagated all the way through the neuron. It will never stop halfway. The nervous impulse is an ‘all or nothing’ response.
  • Repolarization should not be confused with hyperpolarization, which occurs when the membrane potential becomes more negative than the resting potential after an action potential. This is sometimes called the ‘refractory period’.
  • The membrane proteins that facilitate the diffusion of sodium and potassium during the resting potential are not voltage-gated.
  • An oscilloscope is an instrument used to measure voltage over time. The physics department at your school probably has one, though it is not advisable to use it to measure action potentials on live animals!

Figure 6.1.5e - OscilloscopeFigure 6.1.5e – Oscilloscope
The screen of an oscilloscope displays changes in membrane potential over time.

Skill

Analyse an oscilloscope trace graph showing resting potentials and action potentials, in [link]6.2.5[end link].

Nature of Science

Cooperation and collaboration between groups of scientists: biologists from around the world contribute to research into memory and learning.

Language help

  • Cations are positively charged ions.
  • Anions are negatively charged ions.

Course link

  • The propagation of an electrical impulse along an axon relies on different types of membrane transport. Review the structure of voltage-gated channels in Page 1.1.4.
  • HL level students will learn more about how different drugs affect nerve impulses in the brain in Option A. See Page 12.1.5.

Did you know?

Multiple sclerosis (MS) is an autoimmune disease that causes degeneration of the myelin in the central nervous system.