| Structure of a typical neuron.
The above image is from http://eleceng.ukc.ac.uk/~sd5/pics/research/big/neuron.gif.
Three types of neurons occur. Sensory
neurons typically have a long dendrite and short axon, and carry messages
from sensory receptors to the central nervous system. Motor neurons have a
long axon and short dendrites and transmit messages from the central
nervous system to the muscles (or to glands). Interneurons are found only
in the central nervous system where they connect neuron to neuron.
Structure of a neuron and the direction of
nerve message transmission. Image from Purves et al., Life: The Science
of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com)
and WH Freeman (www.whfreeman.com),
used with permission.
Some axons are wrapped in a myelin sheath
formed from the plasma membranes of specialized glial cells known as
Schwann cells. Schwann cells serve as supportive, nutritive, and service
facilities for neurons. The gap between Schwann cells is known as the node
of Ranvier, and serves as points along the neuron for generating a signal.
Signals jumping from node to node travel hundreds of times faster than
signals traveling along the surface of the axon. This allows your brain to
communicate with your toes in a few thousandths of a second.
Cross section of myelin
sheaths that surround axons
(TEM x191,175). This image is copyright Dennis Kunkel at www.DennisKunkel.com,
used with permission.
Structure of a nerve bundle. Image from
Purves et al., Life: The Science of Biology, 4th Edition, by
Sinauer Associates (www.sinauer.com)
and WH Freeman (www.whfreeman.com),
used with permission. |
