The nervous system communicates information by sending nerve impulses. In the last article we saw how impulses are generated at receptors in response to a stimulus. Now the impulses must be transmitted by neurones. In this article we will look at some of the different types of neurones found in the nervous system, and see how the neurone cell membranes are polarised at rest.
Sensory neurones
Sensory neurones carry nerve impulses from receptors to the central nervous system (CNS). They connect to receptors using short dendrites, and the impulse is carried towards the cell body along the dendron. The cell body contains the nucleus and other organelles. The impulse is then carried away from the cell body along a short axon to the axon terminal.
Relay neurones
Relay neurones quite literally relay the nerve impulse between a sensory neurone and a motor neurone. They are found in the CNS. The many dendrites branching off the cell body connect to the axon terminals of sensory neurones via synapses (another article!). The impulse is carried along the axon to the axon terminal.
Motor neurones
Motor neurones carry nerve impulses from the CNS to an effector (e.g. a muscle or gland) which will bring about the response. The cell body is found at the start of the neurone and has many short dendrites branching off it to receive impulses from relay neurones. It has one long axon ending in the axon terminal, which connects to the effector via synapses.
Resting potential
When a neurone is not stimulated the cell membrane is polarised, which means that the outside of the cell is more positively charged than the inside. This is due to the movement of positive ions across the membrane. Sodium (Na+) ions are pumped out of the neurone by active transport using sodium-potassium pumps. The Na+ ions cannot diffuse back into the neurone because the sodium ion channel proteins are closed, so an electrochemical gradient of sodium ions is maintained. However, the potassium (K+) ions which are pumped into the neurone (by the sodium-potassium pump) are able to diffuse back out of the neurone through potassium ion channel proteins (facilitated diffusion) down the concentration gradient. We could say that the membrane is permeable to K+ ions but impermeable to Na+ ions when in the resting state. During action potential that all changes – we will take a look at action potential in the next article. The voltage across the neurone cell membrane during at resting potential is -70mV.
Summary
- Sensory neurones carry nerve impulses from receptors to relay neurones.
- Relay neurones are found in the central nervous system.
- Motor neurones carry nerve impulses from relay neurones to effectors.
- Resting potential is maintained by active transport of sodium and potassium ions, and diffusion of potassium ions. The voltage across the cell membrane is -70mV.
