In the system
Exploring the nervous system
Like other systems in the body, the nervous system consists of tissue that is made of collections of cells, each of which contains thousands of different molecules.
Different projections from the cell body connect one neuron with others. An axon carries the action potential away from the cell body, while many dendrites carry impulses from other neurons towards the cell body.
Neurons have distinctive shapes that are closely related to their function. Primary sensory neurons, for example, carry information from the body into the spinal cord. They have a single fibre that splits into two. One branch goes out towards the body and the other goes into the spinal cord. Motor neurons in the spinal cord have dendrites that receive signals from other cells, and a long axon that extends to the muscles through peripheral nerves. Interneurons connect sensory and motor neurons towards each other and have short dendrites and a branched axon.
The other cells of the nervous system are called glia. They include Schwann cells, which form the myelin sheaths that wrap around the axons of sensory and motor neurons, principally in the peripheral nervous system (the nerves and nerve cells outside of the brain and spinal cord). The myelin sheath is not continuous but is interrupted by gaps called nodes of Ranvier. Although the myelin sheath is an electrical insulator, the gaps actually speed up nerve conduction because they permit saltatory conduction, where action potentials ‘jump’ from one node to the next, and this increases their conduction speed.Lead image:
Ludovic Collin/Wellcome Images