The Nodes Of Ranvier…

The human nervous system consists of the brain, spinal cord and a network of nerves travelling all over the body. The brain and spinal cord together are known as the central nervous system (CNS). This system controls the voluntary and involuntary actions of the body. By voluntary I mean the sort of actions that you are doing now to actively read and understand this sentence; moving the mouse, tilting your head, frowning in irritation etc. There are also involuntary actions going on as your body regulates temperature, blood pressure, digestion etc.; things that you don’t need to think about in order to function.

A nerve cell (or neuron (or neurone)) is a good example of a specialised animal cell. Whilst it shares features such as a cell membrane and cytoplasm with other animal cells, its shape and structure make it quite different. There are two basic kinds of neuron. Sensory neurons collect information and transmit it via sensory organs to the CNS and motor neurons send messages from the CNS to glands and muscles. There is a third kind of neuron called an interneuron, which as the name implies connects bundles of neurons together in the brain and spinal column.

A neuron is made up of the main body of the cell called the soma, wispy dendrites that connect the soma to the rest of the nervous system, the axon and finally the branching axon terminals. The axon can be up to a metre in length (or longer in other species) and it is the part of the cell that carries the nerve signal. The signal itself is an electrical impulse created chemically; similar to the process that takes place in a battery. Special regions of the axon called ion pumps and ion channels ensure that there is a difference in the concentration of certain ions (Na⁺, K⁺, Ca²⁺ & Cl^–) inside and outside the cell. This difference produces a voltage. If the voltage reaches a high enough level there is an ‘all or nothing’ electrical pulse called an action potential that travels down the axon. This action potential will trigger other neurons to fire, muscles to contract or glands to release hormones. By ‘all or nothing’ I mean that there are not different levels of action potential, there is just firing and not firing. (I think Yoda would approve.)

Most vertebrate nerves have a coating of myelin on the axon. This prevents the electrical charge from leaking away as it travels down the axon. Furthermore, nodes of Ranvier have clusters of ion channels so that the electrical pulse can jump from node to node like a spark jumping a gap. This greatly increases the speed at which the neuron can transmit its signal. (This gap-jumping action potential is called saltatory conduction!) Any classicists reading this may like to mention in the comments how it got this name.

In the body, nerves are made up of bundles of neurons and named according to which direction they are carrying their signals. Afferent nerves carry signals from receptors to the brain. Efferent nerves carry impulses from the brain to muscles and glands. There are also nerves that contain mixed bundles of afferent and efferent neurons. Most sensory neurons do not travel all the way to the brain but connect to the central nervous system via the spinal cord.

When a receptor has been triggered (for example if you touch a hot object) an action potential occurs. The pathway of the signal is known as the reflex arc. Sensory neurons carry the signal to the spinal column (it is not usually necessary for the signal to travel all the way to the brain) before a return signal is sent down a motor neuron to a muscle to make you take your hand away. Your brain will be informed of the incident but it does not have to directly respond to it; most often the response is automatic.