Chapter 46: Sensory Receptors,
Neuronal Circuits for Processing
Information
Guyton and Hall, Textbook of Medical Physiology, 12th edition
Types of Sensory Receptors and Their Stimuli
• Mechanoreceptors- detect mechanical compression
or stretching
• Thermoreceptors- detect changes in temperature
• Nociceptors- pain receptors (damage to tissues)
• Electromagnetic receptors- detect light on the retina
• Chemoreceptors- detect taste, smell, oxygen level,
osmolality, etc.
(See Table 46.1 in the text)
Types of Sensory Receptors and Their Stimuli
• Differential Sensitivity of Receptors
a. Each receptor type is highly sensitive to one type
of stimulus for which it is designed
b. Non-responsiveness to other types of sensory
stimuli
c. Pain receptors do not respond to usual touch or
pressure but will become active when the stimuli
become severe enough to damage the tissues
Types of Sensory Receptors and Their Stimuli
• Modality of Sensation- The “Labeled Line” Principlethe specificity of nerve fibers for transmitting only one
modality of sensation
a. Each receptor type is highly sensitive to one type
of stimulus for which it is designed
b. Non-responsiveness to other types of sensory
stimuli
c. Pain receptors do not respond to usual touch or
pressure but will become active when the stimuli
become severe enough to damage the tissues
Fig. 46.1 Several types of somatic sensory nerve endings
Transduction of Sensory Stimuli into Nerve Impulses
• Local Electrical Currents at Nerve Endings—
Receptor Potentials
a. Mechanisms of receptor potentials
1. Mechanical deformation of the receptor which
stretches the membrane and opens channels
2. Application of a chemical to the membrane
3. Change of the temperature of the membrane
4. Effects of electromagnetic radiation
Transduction of Sensory Stimuli into Nerve Impulses
• Local Electrical Currents at Nerve Endings—
Receptor Potentials
b. Maximum receptor potential amplitude (100 mV)
c. Relation of the receptor potential to APs –the more
the receptor potential rises above the threshold
level, the greater becomes the AP frequency
Transduction of Sensory Stimuli into Nerve Impulses
Fig. 46.2 Typical relation between receptor potential and action potentials when the
receptor potential rises above threshold
Transduction of Sensory Stimuli into Nerve Impulses
• Receptor Potential of the Pacinian Corpuscle
Fig. 46.3 Excitation of a sensory nerve fiber by a receptor potential
produced in a Pacinian corpuscle
Transduction of Sensory Stimuli into Nerve Impulses
• Relation Between Stimulus Intensity and the
Receptor Potential
Fig. 46.4 Relation of amplitude of receptor potential to strength
of a mechanical stimulus applied to a Pacinian corpuscle
Transduction of Sensory Stimuli into Nerve Impulses
• Adaptation of Receptors
Fig. 46.5 Adaptation of different types of receptors, showing rapid
adaptation of some receptors and slow adaptation of others
Transduction of Sensory Stimuli into Nerve Impulses
• Mechanism of Receptor Adaptation- different for
each type of receptor
a. In the mechanoreceptor the initial compression
causes the receptor potential which disappears
within a fraction of a second even though the
compression continues
b. Accommodation- slower adaptation and occurs in
the nerve fiber itself; the tip of the nerve gradually
becomes “accommodated” to the stimulus
Transduction of Sensory Stimuli into Nerve Impulses
• Tonic Receptors (Slow Adapting)
a. Continue to transmit impulses as long as the stimulus
is present; include the following:
1.
2.
3.
4.
Macula receptors in the vestibular apparatus
Pain receptors
Baroreceptors of the arterial tree
Chemoreceptors of the carotid and aortic bodies
Transduction of Sensory Stimuli into Nerve Impulses
• Phasic Receptors (Rapidly Adapting)- also called
“rate receptors,” and “movement receptors”
a. Stimulated only when the stimulus strength changes
b. React strongly while a change is actually taking place
c. Cannot be used to transmit a continuous signal
Transduction of Sensory Stimuli into Nerve Impulses
• Importance of Phasic Receptors- have a predictive
function
• General Classification of Nerve Fibers
a. Type A- large and medium sized myelinated fibers
of spinal nerves (alpha, beta, gamma, delta)
b. Type C- small, unmyelinated fibers that conduct at
low velocities
Transduction of Sensory Stimuli into Nerve Impulses
Fig. 46.6 Physiologic classification and
functions of nerve fibers
Transmission of Signals of Different Intensity in Nerve Tracts
• Spatial Summation- increased signal strength by
using progressively larger numbers of fibers;
stronger signals spread to more and more
fibers (Fig. 46.7)
• Temporal Summation- increased signal strength
by increasing the frequency of nerve impulses
in each fiber (Fig. 46.8)
Fig. 46.7 Pattern of stimulation of pain fibers in
a nerve leading from an area of skin
pricked by a pin (spatial summation)
Fig. 46.8 Translation of signal strength into a
frequency modulated series of nerve
impulses (temporal summation)
Transmission and Processing of Signals in Neuronal Pools
• Relaying of Signals Through Neuronal Poolsorganization of neurons for relaying signals
The neuron area
stimulated by each
incoming nerve fiber
is the stimulatory
field
Fig. 46.9 Basic organization of a
neuronal pool
Transmission and Processing of Signals in Neuronal Pools
• Threshold and Sub-threshold Stimuli
a. The discharge of a single excitatory presynaptic
terminal almost never causes an action
potential in a postsynaptic neuron
b. Instead, large numbers of input terminals must
discharge on the same neuron either at the same
time or in rapid succession to cause excitation
Transmission and Processing of Signals in Neuronal Pools
• Threshold and Sub-threshold Stimuli
Fig. 46.10 “Discharge” and “Facilitated” zones of a neuronal pool
Transmission and Processing of Signals in Neuronal Pools
• Inhibition of a Neuronal Pool
a. Some incoming fibers inhibit neurons, rather than
excite them
b. This is the opposite of “facilitation” and is called
the “inhibitory zone”
Transmission and Processing of Signals in Neuronal Pools
• Divergence of Signals
a. Amplifying-an input signal spreads to an
increasing number of neurons as it passes through
successive orders of neurons in its path
b. Divergence in multiple tracts- the signal is
transmitted into two directions from the pool;
information transmitted up the dorsal column
from the spinal cord takes two courses (a) into the
cerebellum, and (2) on through the lower regions
of the brain to the thalamus and cerebral cortex
Transmission and Processing of Signals in Neuronal Pools
• Divergence of Signals
Fig. 46.11 Divergence in neuronal pathways. A: Divergence within a pathway to cause
amplification of the signal, B: Divergence into multiple tracts to transmit the
signal to separate areas.
Transmission and Processing of Signals in Neuronal Pools
• Convergence of Signals- signals from multiple
inputs uniting to excite a single neuron
a. Convergence from a single source- multiple
terminals from a single incoming fiber tract
terminate on the same neuron
b. Convergence from input signals (excitatory or
inhibitory) from multiple sources
Allows the summation of information from
different sources
Transmission and Processing of Signals in Neuronal Pools
• Convergence of Signals- signals from multiple
inputs uniting to excite a single neuron
Fig. 46.12 Convergence of multiple input fibers onto a single neuron
Transmission and Processing of Signals in Neuronal Pools
• Neuronal Circuit with both Excitatory and
Inhibitory Output Signals
a. Sometimes an incoming signal causes an
excitatory signal going in one direction and an
inhibitory signal going elsewhere
b. Reciprocal inhibition circuit in some reflexes
Transmission and Processing of Signals in Neuronal Pools
• Neuronal Circuit with both Excitatory and
Inhibitory Output Signals
Fig. 46.13 Inhibitory circuit. Neuron 2 is an inhibitory neuron
Transmission and Processing of Signals in Neuronal Pools
• Prolongation of a Signal by a Neuronal Pool“Afterdischarge”
a. Synaptic Afterdischarge- when excitatory synapses
discharge on a dendrite or on the soma, a postsynaptic
electrical potential develops in the neuron and lasts for
msec
b. As long as the potential lasts, it will continue to excite
the neuron, causing it to transmit a continuous train or
output impulses
Transmission and Processing of Signals in Neuronal Pools
• Reverberatory (Oscillatory) Circuit
a. Caused by positive feedback within the neuronal circuit
that feeds back to re-excite the input of the same circuit
Transmission and Processing of Signals in Neuronal Pools
• Reverberatory (Oscillatory) Circuit
Fig. 46.14 Reverberatory circuits of
increasing complexity
Transmission and Processing of Signals in Neuronal Pools
• Characteristics of Signal Propagation from a
Reverberatory Circuit
Fig. 46.15 Typical pattern of the output signal from a reverberatory circuit
after a single input stimulus showing the effects of
facilitation and inhibition
Transmission and Processing of Signals in Neuronal Pools
• Continuous Signal Output from Some Neuronal Circuits
a. Continuous discharge caused by intrinsic neuronal
excitability
b. Continuous signals emitted from reverberating circuits
Transmission and Processing of Signals in Neuronal Pools
• Continuous Signal Output from Some Neuronal Circuits
Fig. 46.16 Continuous output from either a reverberating circuit or a pool of
intrinsic discharging neurons. Also showing the effects of excitatory
or inhibitory input signals
Transmission and Processing of Signals in Neuronal Pools
• Rhythmical Signal Output
Fig. 46.17 Rhythmical output of summated nerve impulses
Instability and Stability of Neuronal Circuits
• Inhibitory Circuits as a Mechanism for Stabilizing
Nervous System Function
• Synaptic Fatigue as a Means of Stabilizing the Nervous
System
• Automatic Short-Term Adjustment of Pathway
Sensitivity by the Fatigue Mechanism
• Long-Term Changes in Synaptic Sensitivity by Automatic
Down Regulation or Up Regulation of Synaptic
Receptors
Fig. 46.19 Successive flexor reflexes showing fatigue of conduction through the reflex pathway