Function of a Neuron
Neurons, or nerve cells, are the functional unit of the nervous system for
transmitting signals through the body. They regulate the contraction of
muscles and secretion of hormones.
Organization of a Neuron
Neurons have three main parts.
- The cell body (yellow) contains the nucleus, endoplasmic
reticulum, mitochondria, nissel bodies etc.
- Dendrites (purple) are short extentions of the cell body. They receive
information, and transmit it to the cell body. Dendrites can be compared
to the hearing part of a telephone-- the dendrite hears a message that is
being sent from sensory receptors or another neuron.
-
The Axon (orange) is a thin tube that can be up to 3 feet long. This part
of the neuron takes impulses away from the cell body. Following the same
metaphor, the axon is like the telephone wire that transmits the message.
Axons are insulated by the myelin sheath (blue) that are the
stacked membranes of Schwann cells. The myelin sheath creates a
layer around the axon and helps to maintain the ionic charge of nerve
impulses during electrical transmission of information (a good metaphor
for this is a coat that keeps you insulated in the winter). The gap that
separates the portions of the myelin sheath formed by two
adjacent Schwann cells is called the node of ranvier. These nodes are the only
regions along the axon where the axonal membrane is in direct contact
with extracellular liquid.
Types of Neurons
Neurons can be different in size, shape, and function. There
are three major classes of nerve cells, each pertaining to the three
functions of the nervous system.
- Sensory neurons communicate information about the internal and
external environment from sensory receptors to the central nervous system
via interneurons. They are the only type of neuron whose dendrites do not
receive information from other neurons.
-
Interneurons combine sensory input with motor output, and can only
make connections with other neurons.
-
Motor neurons bring impulses from the central nervous system to
effector cells. Muscle or gland cells that perform the body?s responses
to stimuli. They are the only type of neuron that does not communicate
with other neurons, and their dendrites are therefore shorter than that
of the other two types of neurons.
|
