junctions with other neurons. Note in Figure 6–4 that for
afferent neurons both the cell body and the long axon are out-
side the central nervous system, and only a part of the central
process enters the brain or spinal cord.
Generally, the cell bodies and dendrites of efferent neu-
rons are within the central nervous system, and the axons
extend out to the periphery (a notable exception is the enteric
nervous system of the gastrointestinal tract, described in
Chapter 15). Groups of afferent and efferent neurons form
of the peripheral nervous system. Note that a nerve
ﬁ ber is a single axon, and a nerve is a bundle of axons (ﬁ bers)
bound together by connective tissue.
Interneurons lie entirely within the central nervous system.
They account for over 99 percent of all neurons and have a wide
range of physiological properties, shapes, and functions. The
number of interneurons interposed between speciﬁ c afferent and
Axonal transport along microtubules by dynein and kinesin.
Three classes of neurons. The arrows indicate the direction of transmission of neural activity. Afferent neurons in the peripheral nervous system
generally receive input at sensory receptors. Efferent components of the peripheral nervous system may terminate on muscle, gland, or neuron
effectors. Both afferent and efferent components may consist of two neurons, not one as shown here.
Central nervous system
Peripheral nervous system
Muscle, gland, or neuron