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U N I T 1 0
Nervous System
two-pathway system has several advantages. It adds
richness to the sensory input by allowing sensory infor-
mation to be handled in two different ways, and it
ensures that if one pathway is damaged, the other still
can provide input.
Discriminative Pathway.
The
discriminative pathway
is used for the rapid transmission of sensory informa-
tion, such as discriminative touch. It uses only three
types of neurons to transmit information from a sensory
receptor to the somatosensory cortex on the opposite
side of the brain: (1) the dorsal root ganglion neurons,
which project their central axons to the dorsal horn in
the spinal cord; (2) the dorsal column nuclei, which
send their axons upward and across the midline of the
medulla where they form a band of white matter called
the
medial lemniscus
that passes upward to the thala-
mus; and (3) the thalamic neurons, which project to the
primary sensory cortex (Fig. 35-4A). As the fibers in the
medial lemniscus ascend through the brain stem, they
are joined by sensory fibers from the trigeminal nerve
that supplies the head and face. Sensory information
arriving at the sensory cortex by this route can be dis-
cretely localized and discriminated in terms of intensity.
One of the distinct features of the discriminative path-
way is that it relays precise information regarding spatial
orientation. This is the only pathway taken by the sensa-
tions of muscle and joint movement, vibration, and deli-
cate discriminative touch, which is required to correctly
differentiate the location of touch on the skin at two
neighboring points (i.e., two-point discrimination). One
of the important functions of the discriminative pathway
is to integrate the input from multiple receptors. The
sense of shape and size of an object in the absence of visu-
alization, called
stereognosis,
is based on precise afferent
information from muscle, tendon, and joint receptors.
For example, a screwdriver is perceived as being differ-
ent from a knife in terms of its texture (tactile sensibility)
and shape based on the relative position of the fingers
as they move over the object. This complex interpretive
perception requires that both the discriminative pathway
and the somatosensory association area of the cerebral
cortex are functioning properly. If the discriminative
C2
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T12
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Coccyx
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1
FIGURE 35-2.
Cutaneous distribution of spinal
nerves (dermatomes). (From Barr M. The Human
Nervous System. NewYork, NY: Harper & Row;
1993.)
Central
processes
Dorsal
root ganglia
Peripheral
processes
Dermatomes
FIGURE 35-3.
The dermatomes formed by the peripheral
processes of adjacent spinal nerves overlap on the body
surface.The central processes of these fibers also overlap in
their spinal distribution.
