Vander et al.: Human
Physiology: The
Mechanism of Body
Function, Eighth Edition
II. Biological Control
Systems
© The McGraw−Hill
Companies, 2001
9. The Sensory Systems
The Sensory Systems CHAPTER NINE
V. Information coming into the nervous system is
subject to control by both ascending and descending
pathways.
SECTION
sensory system
sensory information
sensation
perception
sensory receptor
stimulus
stimulus transduction
adequate stimulus
receptor potential
adaptation
sensory pathway
ascending pathway
sensory unit
receptive field
A
KEY
SECTION
A
REVIEW
QUESTIONS
1. Distinguish between a sensation and a perception.
2. Describe the general process of transduction in a
receptor that is a cell separate from the afferent
neuron. Include in your description the following
terms: specificity, stimulus, receptor potential,
neurotransmitter, graded potential, and action
potential.
3. List several ways in which the magnitude of a
receptor potential can be varied.
4. Describe the relationship between sensory
information processing in the primary cortical
sensory areas and in the cortical association areas.
5. List several ways in which sensory information can
be distorted.
6. How does the nervous system distinguish between
stimuli of different types?
7. How is information about stimulus intensity coded
by the nervous system?
8. Make a diagram showing how a specific ascending
pathway relays information from peripheral
receptors to the cerebral cortex.
TERMS
specific ascending pathway
somatic receptor
somatosensory cortex
visual cortex
auditory cortex
nonspecific ascending pathway
polymodal neuron
cortical association area
modality
recruitment
acuity
lateral inhibition
rapidly adapting receptor
slowly adapting receptor
_
SECTION
SPECIFIC
B
SENSORY
Somatic Sensation
Sensation from the skin, muscles, bones, tendons, and
joints is termed somatic sensation and is initiated by a
variety of somatic receptors (Figure 9–17). Some
respond to mechanical stimulation of the skin, hairs, and
underlying tissues, whereas others respond to temperature or chemical changes. Activation of somatic receptors gives rise to the sensations of touch, pressure,
warmth, cold, pain, and awareness of the position of the
body parts and their movement. The receptors for visceral sensations, which arise in certain organs of the thoracic and abdominal cavities, are the same types as the
receptors that give rise to somatic sensations. Some organs, such as the liver, have no sensory receptors at all.
Each sensation is associated with a specific receptor type. In other words, there are distinct receptors for
heat, cold, touch, pressure, limb position or movement,
and pain. After entering the central nervous system,
the afferent nerve fibers from the somatic receptors
synapse on neurons that form the specific ascending
pathways going primarily to the somatosensory cortex via the brainstem and thalamus. They also synapse
on interneurons that give rise to the nonspecific pathways. For reference, the location of some important ascending pathways is shown in a cross section of the
spinal cord (Figure 9–18a), and two are diagrammed
as examples in Figure 9–18b and c.
SYSTEMS
Note that the pathways cross from the side where
the afferent neurons enter the central nervous system
to the opposite side either in the spinal cord (Figure
9–18b) or brainstem (Figure 9–18c). Thus, the sensory
pathways from somatic receptors on the left side of the
body go to the somatosensory cortex of the right cerebral hemisphere, and vice versa.
In the somatosensory cortex, the endings of the axons of the specific somatic pathways are grouped according to the location of the receptors giving rise to
the pathways (Figure 9–19). The parts of the body that
are most densely innervated—fingers, thumb, and
lips—are represented by the largest areas of the somatosensory cortex. There are qualifications, however,
to this seemingly precise picture: The sizes of the areas can be modified with changing sensory experience,
and there is considerable overlap of the body-part representations.
Touch-Pressure
Stimulation of the variety of mechanoreceptors in the
skin (see Figure 9–17) leads to a wide range of touchpressure experiences—hair bending, deep pressure,
vibrations, and superficial touch, for example. These
mechanoreceptors are highly specialized nerve endings encapsulated in elaborate cellular structures. The
details of the mechanoreceptors vary, but generally
the nerve endings are linked to collagen-fiber networks
within the capsule. These networks transmit the
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