Vander et al.: Human
Physiology: The
Mechanism of Body
Function, Eighth Edition
238
II. Biological Control
Systems
9. The Sensory Systems
© The McGraw−Hill
Companies, 2001
PART TWO Biological Control Systems
TABLE 9–1 Principles of Sensory System
Organization
1. Specific sensory receptor types are sensitive to certain
modalities and submodalities.
2. A specific sensory pathway codes for a particular modality
or submodality.
3. The ascending pathways are crossed so that sensory
information is generally processed by the side of the
brain opposite the side of the body that was stimulated.
4. In addition to other synaptic relay points, all ascending
pathways, except for those involved in smell, synapse in
the thalamus on their way to the cortex.
5. Information is organized such that initial cortical
processing of the various modalities occurs in different
parts of the brain.
6. Ascending pathways are subject to descending controls.
In some cases (for example, in the pain pathways),
the afferent input is continuously inhibited to some degree. This provides the flexibility of either removing
the inhibition (disinhibition) so as to allow a greater
degree of signal transmission or of increasing the inhibition so as to block the signal more completely.
We conclude our general introduction to sensory
system pathways and coding with a summary of the
general principles of the organization of the sensory
systems (Table 9–1). We now present the individual
systems.
SECTION
A
SUMMARY
I. Sensory processing begins with the transformation of
stimulus energy into graded potentials and then into
action potentials in nerve fibers.
II. Information carried in a sensory system may or may
not lead to a conscious awareness of the stimulus.
Receptors
I. Receptors translate information from the external
world and internal environment into graded
potentials, which then generate action potentials.
a. Receptors may be either specialized endings of
afferent neurons or separate cells at the end of the
neurons.
b. Receptors respond best to one form of stimulus
energy, but they may respond to other energy
forms if the stimulus intensity is abnormally high.
c. Regardless of how a specific receptor is
stimulated, activation of that receptor always
leads to perception of one sensation. Not all
receptor activations lead, however, to conscious
sensations.
II. The transduction process in all sensory receptors
involves—either directly or indirectly—the opening
or closing of ion channels in the receptor. Ions then
flow across the membrane, causing a receptor
potential.
a. Receptor-potential magnitude and action-potential
frequency increase as stimulus strength increases.
b. Receptor-potential magnitude varies with
stimulus strength, rate of change of stimulus
application, temporal summation of successive
receptor potentials, and adaptation.
Neural Pathways in Sensory Systems
I. A single afferent neuron with all its receptor endings
is a sensory unit.
a. Afferent neurons, which usually have more than
one receptor of the same type, are the first
neurons in sensory pathways.
b. The area of the body that, when stimulated,
causes activity in a sensory unit or other neuron
in the ascending pathway of that unit is called the
receptive field for that neuron.
II. Neurons in the specific ascending pathways convey
information to specific primary receiving areas of the
cerebral cortex about only a single type of stimulus.
III. Nonspecific ascending pathways convey information
from more than one type of sensory unit to the
brainstem reticular formation and regions of the
thalamus that are not part of the specific ascending
pathways.
Association Cortex and Perceptual
Processing
I. Information from the primary sensory cortical areas
is elaborated after it is relayed to a cortical
association area.
a. The primary sensory cortical area and the region
of association cortex closest to it process the
information in fairly simple ways and serve basic
sensory-related functions.
b. Regions of association cortex farther from the
primary sensory areas process the sensory
information in more complicated ways.
c. Processing in the association cortex includes input
from areas of the brain serving other sensory
modalities, arousal, attention, memory, language,
and emotions.
Primary Sensory Coding
I. The type of stimulus perceived is determined in part
by the type of receptor activated. All receptors of a
given sensory unit respond to the same stimulus
modality.
II. Stimulus intensity is coded by the rate of firing of
individual sensory units and by the number of
sensory units activated.
III. Perception of the stimulus location depends on the
size of the receptive field covered by a single sensory
unit and on the overlap of nearby receptive fields.
Lateral inhibition is a means by which ascending
pathways emphasize wanted information and
increase sensory acuity.
IV. Stimulus duration is coded by slowly adapting
receptors.