What are Nerve Cells?

Nerve Cell

Nerve cells carry impulses in one direction only. Impulses travel from sense organs such as the eye, ear and skin into central nervous system (the brain and spinal cord) along sensory nerve cells. Impulses then travel along connecting nerve cells to motor nerve cells, which carry them to muscles. A reflex action is a simple example of nerve cells in action.

Receptors

Information about your surrounding is collected by receptor cells.These are usually grouped together in sense organs. Each type of receptors is sensitive to a different kind of change or stimulus. The table below shows the human sense organs, the senses, and the stimulus to which they are sensitive.

Sense Organs	Sense	Stimulus
Skin	Touch	Pressure, pain, hot and cold temperature
Tongue	Taste	Chemicals in food and drink
Nose	Smell	Chemicals in the air
Eyes	Sight	Light and color
Ears	Hearing Balance	Sound movement and position of the head

ReflexesThe spinal cord is more than just a high speed route for electrical impulses to and from the brain. The spinal cord can control some of your actions without the brain being used at all. Sometimes you do not have time to think - in an emergency you must act right away. If for example you touch something hot, you quickly pull your hand away without stopping to think what to do.  

Reflex ActionsReflex Action protects us from injury. What happens is this: impulses travel from the pain receptors in you finger along a sensory nerve cell to spinal cord. Here they pass along connections which lead them to a motor nerve cell. This carries the impulses to the muscles, which pull the finger away from the painful stimulus. A similar spinal reflex operates when you stand on something sharp.
There are also reflexes in which the impulses are sent straight to the brain. These are called cranial reflexes. Examples include blinking when a piece of dust lands in your eyes, sneezing and producing saliva when you smell food.