488
UNIT 3
Regulation and Integration of the Body
13
(as with free dendrites or the encapsulated receptors of most
general sense receptors), the graded potential is called a
gen-
erator potential
because it generates action potentials in a
sensory neuron.
When the receptor is a separate cell (as in most special
senses), the graded potential is called a
receptor potential
because it occurs in a separate receptor cell. Te receptor po-
tential changes the amount of neurotransmitter released by
the receptor cell onto the sensory neuron. Te neurotransmit-
ters then generate graded potentials in the sensory neuron.
Graded potentials in the first-order sensory neuron must
reach
threshold
so that voltage-gated sodium channels on
the axon are opened and nerve impulses are generated and
propagated to the CNS.
Adaptation
Information about a stimulus—its strength,
duration, and pattern—is encoded in the frequency of nerve
impulses: the greater the frequency, the stronger the stimulus.
Many but not all sensory receptors exhibit
adaptation
, a change
in sensitivity (and nerve impulse generation) in the presence
of a constant stimulus. For example, when you step into bright
sunlight from a darkened room, your eyes are initially dazzled,
but your photoreceptors rapidly adapt, allowing you to see both
bright areas and dark areas in the scene.
Phasic receptors
are
fast adapting
, o±en giving bursts of im-
pulses at the beginning and the end of the stimulus. Phasic re-
ceptors report changes in the internal or external environment.
Examples are lamellar and tactile corpuscles.
Tonic receptors
provide a sustained response with little
or no adaptation. Nociceptors and most proprioceptors are
tonic receptors because of the protective importance of their
information.
Processing at the Circuit Level
At the second level of integration, the circuit level, the task is to
deliver impulses to the appropriate region of the cerebral cortex
for localization and perception of the stimulus (Figure 13.2,
2
).
Recall from Chapter 12 that ascending sensory pathways typ-
ically consist of a chain of three neurons called first-, second-,
and third-order sensory neurons. Te axons of first-order sen-
sory neurons, whose cell bodies are in the dorsal root or cranial
ganglia, link the receptor and circuit levels of processing. Central
processes of first-order neurons branch diffusely when they en-
ter the spinal cord. Some branches take part in local spinal cord
reflexes. Others synapse with second-order sensory neurons,
which then synapse with the third-order sensory neurons that
take the message to the cortex of the cerebrum.
As we saw in Chapter 12, the different ascending pathways
(spinothalamic, dorsal column–medial lemniscal, and spino-
cerebellar) carry various types of information to different des-
tinations in the brain. You may wish to review this information
on p. 469.
Processing at the Perceptual Level
Sensory input is interpreted in the cerebral cortex (Figure 13.2,
3
). Te ability to identify and appreciate sensations depends
on the location of the target neurons in the sensory cortex, not
energy (which is the province of receptors in the eye). Te
more complex the sensory receptor, the more specific it is.
Te stimulus must be applied within a sensory receptor’s
re-
ceptive field
—the area the receptor monitors. ²ypically, the
smaller the receptive field, the greater the ability of the brain
to accurately localize the stimulus site.
Te stimulus energy must be converted into the energy of a
graded potential
, a process called
transduction
. Tis graded
potential may be depolarizing or hyperpolarizing, similar to
the EPSPs or IPSPs generated at postsynaptic membranes
in response to neurotransmitter binding (see Chapter 11,
p. 411).
Receptors can produce one of two types of graded poten-
tials. When the receptor region is part of a sensory neuron
Perceptual level
(processing in cortical sensory centers)
Receptor level
(sensory reception and
transmission to CNS)
Circuit level
(processing in
ascending pathways)
Motor
cortex
Somatosensory
cortex
Thalamus
Spinal cord
Cerebellum
Reticular
formation
Pons
Muscle
spindle
Joint
kinesthetic
receptor
Free nerve
endings (pain,
cold, warmth)
Medulla
3
2
1
Figure 13.2
Three basic levels of neural integration in
sensory systems.
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