Page 249 - Anatomy and Physiology of Farm Animals, 8th Edition

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Ascending proprioceptive pathways consciousness but is instead important in
directing the autonomic activity in viscera.
project both to the cerebral cortex and the
VetBooks.ir cerebellum (see Fig. 10‐15). The cerebral Receptors of the viscera are confined to
mechanoreceptors and chemoreceptors.
cortex uses proprioception to help formu­
late voluntary motor plans; the cerebellum The latter as a rule do not project to
uses it to adjust ongoing motor movements cortical levels, so perceived sensations
so that they are smooth and accurate. The are primarily limited to pain and pressure.
information carried in the separate tracts Since unmyelinated C fibers are the
to these targets arise from the same predominant type of sensory fiber inner­
peripheral receptors and primary afferent vating the viscera, visceral pain has a
neurons; it is only the ultimate destination burning, aching quality.
(and therefore use) that is different. Remarkably, the viscera tend to be
For the cerebral cortex and cerebellum relatively insensitive to stimuli such as
to make effective use of feedback on body crushing, cutting, and thermal injury.
position to guide movements, the proprio­ Surgical manipulation, therefore, tends
ceptive information must be delivered very to produce surprisingly little activity in
rapidly to these brain regions. Consequently, sensory systems. Visceral afferents do
proprioceptive tracts typically have few respond vigorously to stretch, dilation,
synapses and are composed of very large tension, and ischemia (reduced blood
diameter, highly myelinated axons (called flow), however. For this reason, cramp-
Aα fibers). In fact, the very fastest (up to ing (increased muscular tension in the
120 m/s) axons of the entire nervous system wall of a viscus) and stretching due to
transmit proprioceptive information. gas accumulation are quite painful. The
We, and presumably animals, do not cramping, stretching, and/or ischemia
generally focus conscious attention on that occur when the equine large intes-
proprioception, but it is nonetheless critically tine twists or is displaced can produce
important in the execution of accurate, severe abdominal pain, called colic, in
well‐coordinated movements. Injury to the horses.
proprioceptive pathways results in awkward,
inaccurate, uncoordinated gait and move­
ment. The incoordination typical of propri­ Chemical Senses
oceptive deficits is referred to as ataxia.
Chemical senses are those that detect

Touch particular molecules in the external or
internal environment. Chemical senses
that detect molecules outside the body
Touch is the modality associated with include gustation (taste) and olfaction
non‐noxious mechanical contact with the (smell). Within the body, the chemical
body. Touch receptors are encapsulated, senses include the detection of blood pH
and the axons that transmit touch informa­ and carbon dioxide concentration. These
tion to the brain are typically medium in latter afferents are associated with auto­
diameter and degree of myelination. Spinal nomic reflexes and do not project to the
cord tracts associated with touch are found cerebral cortex for perception.
in all the funiculi of the cord.

Gustation
Visceral Sensations
Taste, or gustation, is the modality asso­
Visceral sensations involve structures ciated with dissolved substances contacting
within the body cavities. Most visceral specialized receptor cells on the tongue
afferent information is not available to and throat region. The receptors, simply