Sensory Physiology
193
or through the second-messenger system. The ion channels
are present in a specialized region of the receptor membrane
located at the distal tip of the cell’s single axon or on associated
specialized sensory cells (Figure 7–1). The gating of these ion
channels allows a change in the ion fl uxes across the receptor
membrane, which in turn produces a change in the membrane
potential. This change is a graded potential called a
recep-
tor potential.
The different mechanisms that affect ion chan-
nels in the various types of sensory receptors are described
throughout this chapter.
The specialized receptor membrane region where the
initial ion channel changes occur does not generate action
potentials. Instead, local current fl ows a short distance along
the axon to a region where the membrane has voltage-gated
ion channels and can generate action potentials. In myelin-
ated afferent neurons, this region is usually at the fi rst node
of Ranvier. The receptor potential, like the synaptic potential
discussed in Chapter 6, is a graded response to different stim-
ulus intensities (
Figure 7–2
) and diminishes as it travels along
the membrane.
If the receptor membrane is on a separate cell, the recep-
tor potential there alters the release of neurotransmitter from
that cell. The neurotransmitter diffuses across the extracel-
lular cleft between the receptor cell and the afferent neuron
and binds to receptor proteins on the afferent neuron. Thus,
this junction is a synapse. The combination of neurotransmit-
ter with its binding sites generates a graded potential in the
afferent neuron analogous to either an excitatory postsyn-
aptic potential or, in some cases, an inhibitory postsynaptic
potential.
As is true of all graded potentials, the magnitude of a
receptor potential (or a graded potential in the axon adjacent
to the receptor cell) decreases with distance from its origin.
Most sensory receptors are exquisitely sensitive to their
specifi c stimulus. For example, some olfactory receptors respond
to as few as three or four odor molecules in the inspired air,
and visual receptors can respond to a single photon, the small-
est quantity of light.
Virtually all sensory receptors, however, can be acti-
vated by different types of stimuli if the intensity is suffi ciently
high. For example, the receptors of the eye normally respond
to light, but they can be activated by an intense mechanical
stimulus, like a poke in the eye. Note, however, that the sen-
sation of
light
is still experienced in response to a poke in the
eye. Regardless of how the receptor is stimulated, any given
receptor gives rise to only one sensation.
There are several general classes of receptors that are
characterized by the type of stimulus to which they are sen-
sitive. As the name indicates,
mechanoreceptors
respond to
mechanical stimuli, such as pressure or stretch, and are respon-
sible for many types of sensory information, including touch,
blood pressure, and muscle tension. These stimuli alter the per-
meability of ion channels on the receptor membrane, chang-
ing the membrane potential.
Thermoreceptors
detect both
sensations of cold and warmth, and
photoreceptors
respond
to particular light wavelengths.
Chemoreceptors
respond to
the binding of particular chemicals to the receptor membrane.
This type of receptor provides the senses of smell and taste and
detects blood pH and oxygen concentration.
Nociceptors
are
specialized nerve endings that respond to a number of differ-
ent painful stimuli, such as heat or tissue damage.
The Receptor Potential
The transduction process in all sensory receptors involves
the opening or closing of ion channels that receive informa-
tion about the internal and external world, either directly
Action potentials down the axon
Action potentials at first node of Ranvier
Receptor potentials (mV)
Stimulus intensity
+
Threshold
Receptor
membrane
Myelin
First node
of Ranvier
Into CNS
Axon
terminal with
neurotransmitter
Stimulus
Figure 7–2
Stimulation of an afferent neuron with a receptor
ending. Electrodes measure graded potentials and action
potentials at various points in response to different
stimulus intensities. Action potentials arise at the fi rst
node of Ranvier in response to a suprathreshold stimulus,
and the action potential frequency and neurotransmitter
release increase as the stimulus and receptor potential
become larger.
Figure 7–2
physiological
inquiry
How would this afferent pathway be affected by a
drug that blocks voltage-gated calcium channels?
Answer can be found at end of chapter.
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