All About ... Nerve Cells

Nerve cells (or neurones) convey information (messages), as I explained in my blog about the nervous system a while ago.  All living things have to be able to detect changes in their environment and then respond accordingly to survive.  The change is known as a stimulus and this can be from inside the living thing (internally) as well as externally.  Horses, like all mammals, detect external stimuli in the sense organs (eyes, ears, nose, skin, mouth)but then the message has to be conveyed to cause a response.  This is where the nervous system comes in . 

Nerves are tight bundles of nerve fibres.  Nerve cells (neurones) have a body which contains a nucleus and the nerve fibres, which are the long extensions, these transmit the nerve impulses around the body.  The fibres which carry the impulses away from the nerve cell body are called axons and those carrying the impulses to the cell body are called dendrons.  The cell body may also have smaller dendrons called dendrites!

In horses, the sensory neurones transmit messages from the sensory receptors  (in the peripheral sense organs) to the Central Nervous System (CNS).  The motor neurones carry messages from the CNS to the muscles and glands.  

The motor neurones convey information incredibly quickly, a message (or impulse) can be transmitted from the spinal cord to the feet in a few milliseconds.  These neurones are protected along most of their length by a myelin sheath which provides a thick insulating layer.  The sheath is basically a series of cell membranes each of which are formed by a Schwann cell, which are then wrapped around the axon.  Along the membranes of the Schwann cells are gaps (nodes of Ranvier) and these are the key to fast transmission of impulses. 

The nerve impulses are electrical signals, these are generated when something stimulates the nerve membrane.  If this stimulation is above a certain level (threshold) it will cause a change in the membrane's permeability and therefore an exchange of ions across the membrane.  This in turn causes a small current to flow in the cytoplasm of the nerve fibre and in the fluid surrounding the cells (extracellular fluid).  This current will stimulate the nerve membrane of the next neurone which will then alter its permeability again causing an exchange of ions ... and so on along the nerve fibre.  

In nerve fibres which have a myelin sheath (motor neurones) the impulses travel more quickly because they 'jump' from one gap (node of Ranvier) in the cell membrane to the next.  This also uses less energy.   

The impulse then moves along the nerve fibres through the body.  To move from one neurone to another there is either an electrical or a chemical 'message'.  This message has to pass across the gap between cells - the junction (or synapse)!  Most cells will communicate using a chemical 'message' called a neurotransmitter.   

When the impulse reaches the end of the fibre it will cause a change in permeability again, allowing the release of a neurotransmitter across the gap.  This neurotransmitter will then cross and attach itself to the other cell, which will then be stimulated and the ion exchange will continue down that nerve fibre.  When the final nerve fibre interacts with the muscle, gland or organ the neurotransmitter will cause a reaction .. dilate pupils, move a muscle etcetera.  Usually, more than one impulse along a nerve fibre will be needed to cause a reaction.

Stimulation of the nerve membrane can be caused by: light, sound, pressure or other neurones.  The stimulation is also an 'all or nothing' response, there is either an impulse or there is not an impulse, it does not vary in strength.  The stronger the stimulus the more impulses are produced in a given time.  

In sense organs, however, there are many neurones and these have different threshold levels which gives the brain more information about the stimulus.

This is a simplified explanation of how nerves work - it is a complex and amazingly clever system!

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Until next time!

Jo