1. Unit Nine: The Nervous System: A
Unit Nine: The Nervous System: A. General Principles and Sensory Physiology
Chapter 46: Sensory Receptors, Neuronal Circuits for Processing Information
Guyton and Hall, Textbook of Medical Physiology, 12th edition

2. 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)

3. Types of Sensory Receptors and Their Stimuli
Differential Sensitivity of Receptors
Each receptor type is highly sensitive to one type
of stimulus for which it is designed
Non-responsiveness to other types of sensory
stimuli
Pain receptors do not respond to usual touch or
pressure but will become active when the stimuli
become severe enough to damage the tissues

4. Types of Sensory Receptors and Their Stimuli
Modality of Sensation- The “Labeled Line” Principle-
the specificity of nerve fibers for transmitting only one
modality of sensation
Each receptor type is highly sensitive to one type
of stimulus for which it is designed
Non-responsiveness to other types of sensory
stimuli
Pain receptors do not respond to usual touch or
pressure but will become active when the stimuli
become severe enough to damage the tissues

5. Fig. 46.1 Several types of somatic sensory nerve endings

6. Transduction of Sensory Stimuli into Nerve Impulses
Local Electrical Currents at Nerve Endings—
Receptor Potentials
Mechanisms of receptor potentials
Mechanical deformation of the receptor which
stretches the membrane and opens channels
Application of a chemical to the membrane
Change of the temperature of the membrane
Effects of electromagnetic radiation

7. Transduction of Sensory Stimuli into Nerve Impulses
Local Electrical Currents at Nerve Endings—
Receptor Potentials
b. Maximum receptor potential amplitude (100 mV)
Relation of the receptor potential to APs –the more
the receptor potential rises above the threshold
level, the greater becomes the AP frequency

8. Transduction of Sensory Stimuli into Nerve Impulses
Fig Typical relation between receptor potential and action potentials when the
receptor potential rises above threshold

9. Transduction of Sensory Stimuli into Nerve Impulses
Receptor Potential of the Pacinian Corpuscle
Fig Excitation of a sensory nerve fiber by a receptor potential
produced in a Pacinian corpuscle

10. Transduction of Sensory Stimuli into Nerve Impulses
Relation Between Stimulus Intensity and the
Receptor Potential
Fig Relation of amplitude of receptor potential to strength
of a mechanical stimulus applied to a Pacinian corpuscle

11. Transduction of Sensory Stimuli into Nerve Impulses
Adaptation of Receptors
Fig Adaptation of different types of receptors, showing rapid
adaptation of some receptors and slow adaptation of others

12. Transduction of Sensory Stimuli into Nerve Impulses
Mechanism of Receptor Adaptation- different for
each type of receptor
In the mechanoreceptor the initial compression
causes the receptor potential which disappears
within a fraction of a second even though the
compression continues
Accommodation- slower adaptation and occurs in
the nerve fiber itself; the tip of the nerve gradually
becomes “accommodated” to the stimulus

13. Transduction of Sensory Stimuli into Nerve Impulses
Tonic Receptors (Slow Adapting)
Continue to transmit impulses as long as the stimulus
is present; include the following:
Macula receptors in the vestibular apparatus
Pain receptors
Baroreceptors of the arterial tree
Chemoreceptors of the carotid and aortic bodies

14. Transduction of Sensory Stimuli into Nerve Impulses
Phasic Receptors (Rapidly Adapting)- also called
“rate receptors,” and “movement receptors”
Stimulated only when the stimulus strength changes
React strongly while a change is actually taking place
Cannot be used to transmit a continuous signal

15. Transduction of Sensory Stimuli into Nerve Impulses
Importance of Phasic Receptors- have a predictive
function
General Classification of Nerve Fibers
Type A- large and medium sized myelinated fibers
of spinal nerves (alpha, beta, gamma, delta)
Type C- small, unmyelinated fibers that conduct at
low velocities

16. Transduction of Sensory Stimuli into Nerve Impulses
Fig Physiologic classification and
functions of nerve fibers

17. 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)

18. 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 Translation of signal strength into a
frequency modulated series of nerve
impulses (temporal summation)

19. Transmission and Processing of Signals in Neuronal Pools
Relaying of Signals Through Neuronal Pools-
organization of neurons for relaying signals
The neuron area
stimulated by each
incoming nerve fiber
is the stimulatory
field
Fig Basic organization of a
neuronal pool

20. Transmission and Processing of Signals in Neuronal Pools
Threshold and Sub-threshold Stimuli
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

21. Transmission and Processing of Signals in Neuronal Pools
Threshold and Sub-threshold Stimuli
Fig “Discharge” and “Facilitated” zones of a neuronal pool

22. Transmission and Processing of Signals in Neuronal Pools
Inhibition of a Neuronal Pool
Some incoming fibers inhibit neurons, rather than
excite them
This is the opposite of “facilitation” and is called
the “inhibitory zone”

23. Transmission and Processing of Signals in Neuronal Pools
Divergence of Signals
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

24. Transmission and Processing of Signals in Neuronal Pools
Divergence of Signals
Fig 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.

25. Transmission and Processing of Signals in Neuronal Pools
Convergence of Signals- signals from multiple inputs uniting to excite a single neuron
Convergence from a single source- multiple terminals from a single incoming fiber tract terminate on the same neuron
Convergence from input signals (excitatory or inhibitory) from multiple sources
Allows the summation of information from different sources

26. Transmission and Processing of Signals in Neuronal Pools
Convergence of Signals- signals from multiple inputs uniting to excite a single neuron
Fig Convergence of multiple input fibers onto a single neuron

27. Transmission and Processing of Signals in Neuronal Pools
Neuronal Circuit with both Excitatory and Inhibitory Output Signals
Sometimes an incoming signal causes an excitatory signal going in one direction and an inhibitory signal going elsewhere
Reciprocal inhibition circuit in some reflexes

28. Transmission and Processing of Signals in Neuronal Pools
Neuronal Circuit with both Excitatory and Inhibitory Output Signals
Fig Inhibitory circuit. Neuron 2 is an inhibitory neuron

29. Transmission and Processing of Signals in Neuronal Pools
Prolongation of a Signal by a Neuronal Pool- “Afterdischarge”
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
As long as the potential lasts, it will continue to excite the neuron, causing it to transmit a continuous train or output impulses

30. Transmission and Processing of Signals in Neuronal Pools
Reverberatory (Oscillatory) Circuit
Caused by positive feedback within the neuronal circuit that feeds back to re-excite the input of the same circuit

31. Transmission and Processing of Signals in Neuronal Pools
Reverberatory (Oscillatory) Circuit
Fig Reverberatory circuits of
increasing complexity

32. Transmission and Processing of Signals in Neuronal Pools
Characteristics of Signal Propagation from a Reverberatory Circuit
Fig Typical pattern of the output signal from a reverberatory circuit
after a single input stimulus showing the effects of
facilitation and inhibition

33. Transmission and Processing of Signals in Neuronal Pools
Continuous Signal Output from Some Neuronal Circuits
Continuous discharge caused by intrinsic neuronal excitability
Continuous signals emitted from reverberating circuits

34. Transmission and Processing of Signals in Neuronal Pools
Continuous Signal Output from Some Neuronal Circuits
Fig Continuous output from either a reverberating circuit or a pool of
intrinsic discharging neurons. Also showing the effects of excitatory
or inhibitory input signals

35. Transmission and Processing of Signals in Neuronal Pools
Rhythmical Signal Output
Fig Rhythmical output of summated nerve impulses

36. 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

37. Fig. 46.19 Successive flexor reflexes showing fatigue of conduction through the reflex pathway