Sensory Physiology
199
cortex responsible for conscious recognition of the informa-
tion. Sensory pathways are also called
ascending pathways
because they go “up” to the brain.
Ascending Pathways
The central processes of the afferent neurons enter the brain
or spinal cord and synapse upon interneurons there. The cen-
tral processes may diverge to terminate on several, or many,
interneurons (
Figure 7–13a
) or converge so that the processes
of many afferent neurons terminate upon a single interneuron
(
Figure 7–13b
). The interneurons upon which the afferent
neurons synapse are termed second-order neurons, and these
in turn synapse with third-order neurons, and so on, until
the information (coded action potentials) reaches the cerebral
cortex.
Most sensory pathways convey information about only a
single type of sensory information. Thus, one pathway conveys
information only from mechanoreceptors, whereas another is
infl
uenced by information only from thermoreceptors. This
allows the brain to distinguish the different types of sensory
information even though all of it is being transmitted by
essentially the same signal, the action potential. The ascend-
ing pathways in the spinal cord and brain that carry informa-
tion about single types of stimuli are known as the
specifi
c
ascending pathways.
The specifi c ascending pathways pass to
the brainstem and thalamus, and the fi nal neurons in the path-
ways go from there to specifi c sensory areas of the cerebral cor-
tex (
Figure 7–14
). (The olfactory pathways send some branches
to terminate in parts of the limbic system rather than to the thal-
amus.) For the most part, the specifi c pathways cross to the side
of the central nervous system that is opposite to the location of
their sensory receptors. Thus, information from receptors on the
right side of the body is transmitted to the left cerebral hemi-
sphere, and vice versa.
Rapidly adapting
Slowly adapting
Stimulus
intensity
Stimulus
intensity
Action
potentials
Action
potentials
Time
Time
Figure 7–11
Rapidly and slowly adapting receptors. The top line in each graph
indicates the action potential fi ring of the afferent nerve fi ber from
the receptor, and the bottom line, application of the stimulus.
Some rapidly adapting receptors also generate a brief burst of
action potentials when a stimulus ceases—an “off” response (not
shown here).
Excitatory
neuron
Inhibitory
neuron
+
+
+
Sensory
endings
Skin
Higher brain centers
Figure 7–12
Descending pathways may infl uence sensory information by directly
inhibiting the central terminals of the afferent neuron (an example
of presynaptic inhibition) or via an interneuron that affects the
ascending pathway by inhibitory synapses. Arrows indicate the
direction of action potential transmission.
Interneurons
(a)
Divergence
(b)
Convergence
Afferent neurons
Direction of
action potential
propagation
Direction of
action potential
propagation
Afferent neuron
Central nervous system
Figure 7–13
(a) Divergence of an afferent neuron on to many interneurons.
(b) Convergence of input from several afferent neurons onto single
interneurons.
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