Human Physiology, 14th edition (2016)

Sensory Physiology 271

Neural Pathways for

Somatesthetic Sensations

The conduction pathways for the somatesthetic senses —which

include sensations from cutaneous receptors and proprioceptors—

are shown in chapter 8 (see fig. 8.24). These pathways involve

three orders of neurons in series. Sensory information from

proprioceptors and pressure receptors is first carried by large,

myelinated axons (classified as A-beta fibers ) that ascend in the

dorsal columns of the spinal cord on the same (ipsilateral) side.

These fibers do not synapse until they reach the medulla oblon-

gata of the brain stem; hence, fibers that carry these sensations

from the feet are remarkably long. After the fibers synapse in the

medulla with other second-order sensory neurons, information

in the latter neurons crosses over to the contralateral side as it

ascends via a fiber tract, called the medial lemniscus, to the thal-

amus (chapter 8, fig. 8.24). Third-order sensory neurons in the

thalamus receive this input and in turn project to the postcentral

gyrus (the somatosensory cortex; see fig. 8.6).

Mechanical sensations are conducted into the spinal cord

on the thickest, most myelinated sensory axons, classified

as A-alpha and A-beta fibers. Thinner, less myelinated, and

slower-conducting A-delta fibers convey noxious heat and

cold sensations. Most painful sensations, as well as the sen-

sations evoked by capsaicin and some sensations of cold, are

Analogous to the way that capsaicin evokes the sensation
of heat, menthol can produce the sensation of cold. Scientists
have recently identified a membrane ion channel on sensory
neurons that responds both to menthol and to cold in the 8 8
to 28 8 C range. This cold and menthol receptor responds to
either stimulus by producing a depolarization. The cold/men-
thol receptors and the heat/capsaicin receptors are members of
the same family of cation (Na^1 and Ca^2 1 ) channels, called the
transient receptor potential (TRP) channels. Scientists have
recently identified a particular TRP channel that functions as
the principal receptor for the sensation of both coolness (at
temperatures below 30 8 C) and painful cold (at temperatures
below 15 8 ).
The sensation of itch (pruritus), evoking a desire to scratch,
is distinct from that of pain. Receptors for acute itch, as in a
mosquito bite, are stimulated by histamine released by mast
cells and basophils. Because of this, the sensation of acute itch
can respond to antihistamines. The itch induced by histamine
is usually accompanied by the localized redness and swelling
of an inflammation (chapter 15, section 15.1). By contrast,
receptors for chronic itch mostly respond to molecules other
than histamine; antihistamines therefore do not relieve chronic
itch. Receptors for itch stimulate unmyelinated sensory axons
that conduct to the spinal cord. From there, ascending axons
follow the pathways described in the next section.

Figure 10.4 The cutaneous sensory receptors. Each of these structures is associated with a sensory (afferent) neuron.
Free nerve endings are naked, dendritic branches that serve a variety of cutaneous sensations, including that of heat. Some cutaneous
receptors are dendritic branches encapsulated within associated structures. Examples of this type include the pacinian (lamellated)
corpuscles, which provide a sense of deep pressure, and the Meissner’s corpuscles, which provide cutaneous information related to
changes in texture.

Pacinian corpuscle

(vibration; deep pressure)

Ruffini endings

(skin stretch)

Root hair plexus

(light brushing) Free nerve ending

(touch, pain, hot, cold)

Merkel’s discs

(indentation)

Meissner’s corpuscle

(changes in texture)