The Nervous System
Key words: Central Nervous System, Peripheral nervous
system, Receptors and Neurons
The central nervous system consists of the brain and spinal cord. It's
sometimes called the "co-ordinator" because it makes sense of the messages it
receives from the sense organs and co-ordinates responses by the muscles and
glands.
The peripheral nervous system consists of the bundles of nerves that relay
messages between the sense organs, the central nervous system and the muscles
and glands. This system is spread out through the entire body, as shown on the
right.
Receptors are cells that allow us to detect different types of stimuli. For
example, olfactory receptor cells located in the nose detect odour molecules in
the air.
There are five primary types of receptors:
 Chemoreceptors are sensitive to chemicals, such as odour
molecules in the air, and are located in the nose and tongue.
 Mechanoreceptors are sensitive to touch, pressure, sound and
motion and are located in the skin, the inner ear and muscles.
 Pain receptors are sensitive to chemical changes in damaged cells
and are located throughout the body, but most are located in the
skin.
 Thermoreceptors are sensitive to temperature changes and are
located in the skin.
 Photoreceptors are sensitive to light and are located in the eyes.
Neurons
Neurons are specialized cells that transmit messages, in the form of electrical
signals, to and from, and within, the central nervous system. They
act something like the wires of an electric circuit. There are a number of different
types of neurons:
 Sensory neurons transm
it messages from
receptors to the central
nervous system.
 Interneurons transmit
messages from sensory
neurons to motor
neurons.
 Motor neurons transmit
messages from the
central nervous system to
"effectors" such as
muscles and glands to
initiate a response.
Although they have different
roles, neurons generally have
the same structure. They are
made up of dendrites (branches
that receive messages), a cell
body (where the nucleus is
found), axons (that transmit a
message away from the cell body) and myelin sheaths (a fatty layer that
insulates the axon and sometimes a dendrite).
Label the diagram below