11-6. TYPES OF NEURONS
Neurons may be identified according to shape, diameter of their processes, or
a. According to Shape. A pole is the point where a neuron process meets the
cell body. To determine the type according to shape, count the number of poles.
(1) Multipolar neurons. Multipolar neurons have more than two poles (one
axon and two or more dendrites).
(2) Bipolar neurons. Bipolar neurons have two poles (one axon and one
(3) Unipolar neurons. Unipolar neurons have a single process which
branches into a T-shape. One arm is an axon; the other is a dendrite.
b. According to Diameter (Thickness) of Processes. Neurons may be rated
according to the thickness of myelin surrounding the axon. In order of decreasing
thickness, they are rated A (thickest), B, and C (thinnest). The thickness affects the rate
at which impulses are transmitted. The thickest are fastest. The thinnest are slowest.
c. According to Function.
(1) Sensory neurons. In sensory neurons, impulses are transmitted from
receptor organs (for pain, vision, hearing, etc.) to the central nervous system (CNS).
(2) Motor neurons. In motor neurons, impulses are transmitted from the
CNS to muscles and glands (effector organs).
(3) Interneurons. Interneurons transmit information from one neuron to
another. An interneuron "connects" two other neurons.
(4) Others. There are other, more specialized types, for example, in the
11-7. NEURON "CONNECTIONS"
A neuron may "connect" either with another neuron or with a muscle fiber. A
phrase used to describe such "connections" is "continuity without contact." Neurons do
not actually touch. There is just enough space to prevent the electrical transmission
from crossing from the first neuron to the next. This space is called the synaptic cleft.
Information is transferred across the synaptic cleft by chemicals called neurotransmit-
ters. Neurotransmitters are manufactured and stored on only one side of the cleft.
Because of this, information flows in only one direction across the cleft.