Page 245 - Anatomy and Physiology of Farm Animals, 8th Edition

230 / Anatomy and Physiology of Farm Animals

ensory, or afferent, systems are the insects are able to sense geomagnetism of
the earth, information they use for navi­
Smeans by which the nervous system
VetBooks.ir receives information about the external gation. A number of species of fish can
detect and generate electric fields. Other
environment (exteroception), the internal
environment (interoception), and the posi­ animals exploit the traditional sense moda­
tion and movement of the body (proprio­ lities for highly specific (and expressly
ception). The body uses sensory information nonhuman) purposes. For example, ceta­
to generate reflex movements (e.g., a blink ceans and bats both use highly modified
of the eye when it is touched, withdrawal auditory circuits for sonar navigation. In
of a limb from a hot surface, contraction of spite of anecdotal support, however, there
the bowel when it is stretched) without remains no reliable evidence that domestic
the participation of the conscious parts animals can sense an impending earthquake
of the brain. Much (but not all) sensory through a sensory experience unavailable
information is also directed to the cerebral to humans.
cortex for conscious perception.
Strictly speaking, “sensation” is the con­
scious perception of sensory stimuli, which Sensory Receptors
takes place when information reaches
the cortex. It is impossible to know exactly Sensory experiences begin at receptors,
what an animal (or another person, for specialized cells or nerve endings that
that matter) sees, feels, hears, or smells. detect a particular aspect of the internal
We infer what sensations an animal may or external environment. They are the
experience by observing its reaction to mechanism by which the nervous system
various stimuli, by identifying homologies changes some sort of environmental
between human and animal sensory sys­ energy (e.g., heat, pressure, light) into the
tems, and by imagining what we might feel electrical activity of neurons, a process
in similar situations. called transduction.
The experience of a given sensation as it In somatosensory systems, the receptor
is perceived at the cortical level has quali­ is usually a specialized peripheral terminal
ties that make it distinct from other types of the primary afferent neuron (the sen­
of sensations. This perceptual distinction sory neuron extending from the central
defines the sensory modality. For exam­ nervous system [CNS] to the periphery);
ple, the stimulus for the photoreceptors of for the special senses, the receptor is
the retina is light; the sensory modality usually a separate specialized neural cell
that is experienced when photoreceptors that synapses with the primary afferent.
are stimulated is vision. Somatic sensation As described earlier, sensory receptors
or somatosensation describes modalities may be described by the origin of the
that arise primarily from innervation of stimulus: exteroceptors, interoceptors, and
body surfaces and musculoskeletal elements; proprioceptors. They may also be described
it includes pain, touch, temperature, and on a structural basis as encapsulated and
position sense (proprioception). Special nonencapsulated types. Nonencapsulated,
senses include vision, hearing (audition), or free (naked) nerve endings are the
and equilibrium (vestibular sensations). unmodified termini of primary afferent
Chemical senses are smell (olfaction) neurons; these are widely distributed
and taste (gustation). Conscious sensory and sensitive primarily to painful stimuli.
experiences arising from the viscera are Encapsulated receptors, which vary widely
limited primarily to pain, stretch, and in structure, are primarily concerned with
pressure. touch sensations; these receptors are
Certain animals have sensory systems invested with specialized connective tissue
that have no homology in human beings. capsules that impart modality specificity to
For instance, migratory birds and some the receptor (Fig. 12‐1).