Neuron Anatomy
Neurons are classified according to 2 different characteristics: their
morphology (anatomical or structural features) and their function.
Although we describe both here, the functional classification will be
used predominantly in later units.
Functional classification of neurons is based on the direction of
information flow along axons relative to the CNS. Based on this
criterion, there are 3 types of neurons: sensory neurons, interneurons,
and motor neurons.
Sensory (afferent) neurons are specialized for detection of sensory
information (e.g. light, pressure, vibration, temperature, chemicals,
etc.). They transduce physical and chemical stimuli into electrical
signals and transfer this information from the periphery towards the
central nervous system for processing. In many cases, sensory neurons
have their dendrites, soma and a part of their axon residing outside the
CNS with axon terminals forming connections (synapses) with other
neurons within the CNS.
Interneurons (association neurons) are located entirely within
the central nervous system (with the dendrites, soma and axons of the
cell all residing within the CNS). Interneurons are also referred to as
association neurons, in part because they are sandwiched between
sensory and motor neurons where they integrate and distribute sensory
information and coordinate motor output. Interneurons account for
90% of all neurons of the CNS and therefore are the most numerous
neurons in the body. Almost all interneurons are multipolar (see below).
Motor (efferent) neurons carry impulses or motor commands away
from the central nervous system to effectors/target organs (e.g. muscles
and glands). Most motor neurons have dendrites and cell bodies in the
CNS and axons that exit the CNS to form peripheral nerves that travel to
effectors (targets).
The structural classification of neurons is based on the number of
processes that extend from the soma. There are 4 basic neuronal
structures like those shown in the figure below though there are many
subtle variations on each theme.
Bipolar neurons have a single dendrite extending from one side of
the cell body and a single axon extending from the other side. Bipolar
neurons are small cells, typically extending for less than 30 microns
from dendrite to axon terminal. There are not many true bipolar cells in
the body. A few examples are found in the special sense organs for vision
and olfaction (smell).
Unipolar or pseudounipolar neurons have a single process that
emanates from the cell body. The single process has dendrites on one
end and the rest of the process is an axon. Most sensory neurons of the
peripheral nervous system are unipolar neurons. The dendrites are
located in the periphery, where stimuli are detected. The sensory
information travels on the dendrite toward the soma (usually located
ganglia just outside the CNS). The axon stretches into the CNS at the
spinal cord.
Multipolar neurons have two or more dendrites on one side and a
single axon on the other side of the soma. Multipolar neurons are the
most common neurons in the CNS. One example are motor neurons
which have dendrites and somas located in the spinal cord and axons
that leave the CNS to innervate skeletal muscles.
Anaxonic neurons are small, stellate (star-shaped) cells with
processes that all look alike with no apparent axon. Anaxonic neurons
can be found in the central nervous system, the retina, and in the
adrenal medulla. Their functions are not well understood.