Page 325 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Stimulus Reception and Processing higher the sensor potential, the higher the AP
frequency (! C2). This information is decoded
With our senses, we receive huge quantities of at the next synapse: The higher the frequency
9
information from the surroundings (10 bits/ of arriving APs, the higher the excitatory post-
s). Only a small portion of it is consciously per- synaptic potential (EPSP; ! 50ff.). New APs are
1
ceived (10 –10 bits/s); the rest is either sub- fired by the postsynaptic neuron when the
2
consciously processed or not at all. Conversely, EPSP exceeds a certain threshold (! B2).
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we transmit ca. 10 bits/s of information to the
Central Nervous System and Senses byte = 8 bits). The average page of a book contains susceptible to change (and falsification of its infor-
Frequency coding of APs is a more reliable way of
environment through speech and motor activ-
transmitting information over long distances than
ity, especially facial expression (! A).
amplitude coding because the latter is much more
A bit (binary digit) is a single unit of information (1
mation content). At the synapse, however, the signal
must be amplified or attenuated (by other neurons),
roughly 1000 bits, and TV images convey more than
which is better achieved by amplitude coding.
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10 bits/s.
Adaptation. At constant stimulation, most sen-
Stimuli reach the body in different forms of
sors adapt, i.e., their potential decreases. The
energy, e.g., electromagnetic (visual stimuli) or
potential of slowly adapting sensors becomes
mechanical energy (e.g., tactile stimuli).
proportional to stimulus intensity (P sensors or
Various sensory receptors or sensors for these
only at the onset and end of a stimulus. They
organs (eye, ear, skin, tongue, nose) at the body
sense differential changes in the stimulus in-
surface as well as inside the body (e.g., pro-
tensity (D sensors or phasic sensors). PD sensors
12 stimuli are located in the five “classic” sense tonic sensors). Fast-adapting sensors respond
priosensors, vestibular organ). (In this book,
sensory receptors are called sensors to distin- have both characteristics (! p. 314).
guish them from binding sites for hormones Central processing. In a first phase, inhibi-
and transmitters.) The sensory system extracts tory and stimulatory impulses conducted to
four stimulatory elements: modality, intensity, the CNS are integrated—e.g., to increase the
duration, and localization. Each type of sensor contrast of stimuli (! D; see also p. 354). In this
is specific for a unique or adequate stimulus case, stimulatory impulses originating from
that evokes specific sensory modalities such as adjacent sensor are attenuated in the process
sight, sound, touch, vibration, temperature, (lateral inhibition). In a second step, a sensory
pain, taste, smell, as well as the body’s position impression of the stimuli (e.g. “green” or
and movement, etc. Each modality has several “sweet”) takes form in low-level areas of the
submodalities, e.g., taste can be sweet or bitter, sensory cortex. This is the first step of subjec-
etc. tive sensory physiology. Consciousness is a
prerequisite for this process. Sensory impres-
In secondary sensors (e.g., gustatory and auditory sions are followed by their interpretation. The
sensors), sensor and afferent fibers are separated by result of it is called perception, which is based
a synapse, whereas primary sensors (e.g., olfactory on experience and reason, and is subject to in-
sensors and nocisensors) have their own afferent
fibers. dividual interpretation. The impression
“green,” for example, can evoke the perception
A stimulus induces a change in sensor potential “There is a tree” or “This is a meadow.”
(transduction), which results in depolarization Absolute threshold (! pp. 340ff., 352, 362),
of the sensor cell (in most types; ! B1) or hy- difference threshold (! pp. 340ff., 352, 368),
perpolarization as in retinal sensors. The spatial and temporal summation (! pp. 52,
stronger the stimulus, the greater the ampli- 352), receptive field (! p. 354), habituation
tude of the sensor potential (! C1). Once the and sensitization are other important concepts
sensor potential exceeds a certain threshold, it of sensory physiology. The latter two mecha-
is transformed into an action potential, AP nisms play an important role in learning
(! B1; p. 46ff.). processes (! p. 336).
Coding of signals. The stimulus is encoded
312
in AP frequency (impulses/s = Hz), i.e., the
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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