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BIOL 2401 : Anatomy/ Physiology
PNS
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Peripheral Nervous System (PNS)
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PNS – all neural structures outside the brain and
spinal cord
Includes sensory receptors, peripheral nerves,
associated ganglia, and motor endings
Provides links to and from the external
environment
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PNS in the Nervous System
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Figure 13.1
Sensory Receptors
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Structures specialized to respond to stimuli
Activation of sensory receptors results in
depolarizations that trigger impulses to the CNS
The realization of these stimuli, sensation and
perception, occur in the brain
Sensory Receptors can be
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specialized cells closely associated with peripheral
endings of sensory neurons
or specialized regions of sensory neurons.
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Receptor Classification by Stimulus Type
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Mechanoreceptors – respond to touch, pressure,
vibration, stretch, and itch
Thermoreceptors – sensitive to changes in
temperature
Photoreceptors – respond to light energy (e.g.,
retina)
Chemoreceptors – respond to chemicals (e.g.,
smell, taste, changes in blood chemistry)
Nociceptors – sensitive to pain-causing stimuli
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Receptor Class by Location: Exteroceptors
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Respond to stimuli arising outside the body
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Found near the body surface
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Sensitive to touch, pressure, pain, and temperature
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Include the special sense organs
Receptor Class by Location: Interoceptors
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Respond to stimuli arising within the body
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Found in internal viscera and blood vessels
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Sensitive to chemical changes, stretch, and temperature changes
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Receptor Class by Location: Proprioceptors
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Respond to degree of stretch of the organs they
occupy
Found in skeletal muscles, tendons, joints,
ligaments, and connective tissue coverings of
bones and muscles
Constantly “advise” the brain of one’s movements
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Receptor Classification by Structural
Complexity
 Receptors are structurally classified as either
simple or complex
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Most receptors are simple and include
encapsulated and unencapsulated varieties
Complex receptors are special sense organs
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Simple Receptors: Unencapsulated
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Free dendritic nerve endings
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Respond chiefly to temperature and pain
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Merkel (tactile) discs
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Hair follicle receptors
Simple Receptors: Encapsulated
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Meissner’s corpuscles (tactile corpuscles)
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Pacinian corpuscles (lamellated corpuscles)
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Muscle spindles, Golgi tendon organs, and Ruffini’s corpuscles
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Joint kinesthetic receptors
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Receptor Density
• Receptors vary in terms of
abundance relative to each other.
• For example, there are far more
pain receptors than temperature
receptors in the body.
• Receptors also vary in terms of
the concentration of their
distribution over the surface of
the body
• The fingertips having far more
touch receptors than the skin of
the back of the hand. The figure
shows the distribution of
temperature receptors in the skin
by area.
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Simple Receptors: Unencapsulated
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Table 13.1.1
Simple Receptors: Encapsulated
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Table 13.1.2
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From Sensation to Perception
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Survival depends upon sensation and perception
Sensation is the awareness of changes in the internal and
external environment
Perception is the conscious interpretation of those stimuli
and of the external world from a pattern of different
sensory nerve impulses via the sensory receptors.
Some perceptions are indeed integrated compound
sensations such as for example “wetness” ( touch, pressure
and thermal input…. there is no such thing as a “wetreceptor”)
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Organization of the
Somatosensory System
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Input comes from exteroceptors,
proprioceptors, and interoceptors
The three main levels of neural
integration in the somatosensory
system are:
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Receptor level – the sensor
receptors
Circuit level – ascending
pathways
Perceptual level – neuronal
circuits in the cerebral cortex
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Processing at the Receptor Lever
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The receptor must have specificity for the stimulus energy
( temperature, touch, pressure, light,…)
The receptor’s receptive field must be stimulated
Stimulus energy must be converted into a graded
potential
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If the receptive field is in the same neuron that generates
the action potential, we call it a generator potential.
If the receptive field is in a separate cell, it is called a
receptor potential. If summed up to reach threshold, hhis
will then release neurotransmitters in order to excite the
associated sensory neuron.
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Processing at the Receptor Lever
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The steps in formation of a generator potential are not known
for every receptor, but where it has been studied the start of the
generator potential usually results from an increase in the
permeability of the membrane of the receptor to all small ions
Usually, the ion furthest from its electrochemical equilibrium
and in greatest concentration, namely sodium, contributes the
greatest current. ( and thus results in EPSP’s)
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Processing at the Receptor Lever
Stimulus (1)
Action
Potential (3)
Generator
Potential (2)
Stimulus (1)
N.T. release (3)
Receptor
Potential (2)
Action
Potential (3)
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Example : Muscle spindle
• Muscle spindles are
composed of 3-10 intrafusal
muscle fibers that lack
myofilaments in their central
regions, are noncontractile,
and serve as receptive
surfaces
• They inform the body of the
muscle tone and length of a
muscle. They become
activated when stretched and
send sensory impulses to the
CNS.
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Example : Muscle spindle
• The figure shows the graded
responses of the muscle
spindles when the muscle is
stretched.
• The amplitude of the
generator potentials increase
with increasing stimulus
strength. Different amounts of
muscle stretch ( as shown by
the heights in the lower trace)
resulted in the graded series
of generator potentials shown
in the upper trace.
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Example : Pacinian Corpuscle
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Pacinian corpuscles are
present in the skin, some
mucous membranes etc. They
are mechanoceptors,
responding to pressure, or any
kind of mechanical stimulus
causing a deformation of the
corpuscle.
The Pacinian corpuscle has a
single afferent nerve fiber. Its
end is covered by a sensitive
receptor membrane whose
sodium channels will open
when the membrane is
deformed in any way.
It is surrounded by several concentric capsules of connective
tissue, with a viscous gel between them.
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Example : Pacinian Corpuscle
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In the resting state, a crosssection through the corpuscle
looks something like this
Now, if the skin over the
corpuscle is touched, it will be
deformed and make a nuisance
of itself:
But the viscous gel between the
capsules will move and allow the
nerve ending to resume its
normal shape:
If the pressure is now released,
the corpuscle as a whole will
resume its original shape, but
the nerve ending will be
deformed in the process:
The viscous gel will then flow
back, and soon we are back at
the beginning.
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Example : Pacinian Corpuscle
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The result is two generator potentials; one when pressure is
applied and one when pressure is released.
This system is thus very good for picking up vibrations.
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Adaptation of Sensory Receptors
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To a certain extent, the duration of the generator potential
depends upon the duration of the stimulus.
However, some receptors have generator potentials that last
only a short time, no matter how long the stimulus is
maintained.
We refer to a decrease in the amplitude of the generator
potential or the frequency of discharge of the sensory fiber in
the face of a persisting, constant stimulus as adaptation.
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Adaptation of Sensory Receptors
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Some receptors are fast adapting, some are slow adapting. Those that adapt fast are
called phasic receptors. Thos that adapt slow or not at all are called tonic receptors.
Receptors responding to pressure, touch, and smell adapt quickly
Receptors responding slowly include Merkel’s discs, Ruffini’s corpuscles, and
interoceptors that respond to chemical levels in the blood
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Pain receptors and proprioceptors do not exhibit adaptation
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There is purpose to these differences.
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Tonic (slowly-adapting) receptors are important in situations where
constant information about a stimulus is important ( they thus send
information about ongoing stimulation)
Phasic (rapidly-adapting) receptors send information related to changing
stimuli. They stop responding to a maintained stimulus, but when the
stimulus is removed, they respond again
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Adaptation of Sensory Receptors
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Tonic and Phasic Receptors
Diagram showing the differences and effects of tonic and
phasic receptors
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Information Coding
Any stimulus contains within it certain features that are of interest to the
body. Stimuli have
• intensities or strengths
• locations or sites of application
• frequencies of application
• rates of application
• modalities
Modality, broadly speaking, is a class of sensations that are referred to a
single type of receptor. Vision, hearing, touch, smell, and taste are all
modalities ( energy forms).
Sensory receptors may be sensitive to different kind of energies. For
example, putting pressure on the eye cause you to see light flashes,
although the function of the eye receptors is to detect light.
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Information Coding
• Doctrine of Specific Nerve Energies, as formulated by Johannus
Müller, says that, although a sense organ may be sensitive to many
forms of stimulus energy other than its real stimulus (called the
adequate stimulus), the sensation evoked is always like that associated
with the adequate stimulus, no matter what kind of energy was applied.
• For example : electrical stimulation of the optic nerve, does not result
in an electric shock; the sensation evoked is one of seeing light.
• The doctrine of specific nerve energies implies that the modality or
submodality of a sensation is determined not by the stimulus, but by
what specific receptor or nerve fiber is stimulated. The doctrine also
implies that the subjective qualities of a modality are determined, not
in the receptors themselves, but in the central nervous system. (in this
case for the optic nerve, it is determined by the visual cortex).
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Processing at the
Circuit Level
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Chains of three (3) neurons conduct
sensory impulses upward to the
brain
First-order neurons – soma reside
in dorsal root or cranial ganglia, and
conduct impulses from the skin to
the spinal cord or brain stem
Second-order neurons – soma
reside in the dorsal horn of the
spinal cord or medullary nuclei and
transmit impulses to the thalamus or
cerebellum
Third-order neurons – located in
the thalamus and conduct impulses
to the somatosensory cortex of the
cerebrum
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Processing at the Circuit Level
• Neuronal signals from skin and deeper structures are segregated in the
spinal cord.
• For pain, temperature and the less discriminative aspects of touch,
neurons in the dorsal horn have axons that cross in the spinal cord and
ascend via the spinothalamic tract
• For discriminative touch and for conscious proprioception, the axons of
primary sensory neurons ascend ipsilaterally ( do not cross over) in the
dorsal funiculus (either gracile or cuneate fasciculus) and end in the
gracile or cuneate nucleus. Fibers arising in these nuclei cross in the
medulla and ascend in the medial lemniscus, which is near the midline in
the medulla and shifts to a lateral location in the midbrain.
• The differences between the two main ascending somatosensory pathways
are important functionally and clinically. Specific lesions within the spinal
cord can thus results in specific loss of sensations in the body.
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Processing at the Circuit Level
Discriminative touch, conscious
proprioception
Simple touch,
temperature, pain
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Processing at the Perceptual Level
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Both spinothalamic tract and the medial lemniscus
terminate in the ventral posterior nucleus of the thalamus.
The thalamus projects fibers to:
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The somatosensory cortex of postcentral gyrus
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Sensory association areas
First one modality is sent, then those considering more
than one
The result is an internal, conscious image of the stimulus
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Main Aspects of Sensory Perception
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Perceptual detection – detecting that a stimulus has
occurred and requires summation
Magnitude estimation – how much of a stimulus is acting
Spatial discrimination – identifying the site or pattern of
the stimulus
Feature abstraction – used to identify a substance that has
specific texture or shape
Quality discrimination – the ability to identify
submodalities of a sensation (e.g., sweet or sour tastes)
Pattern recognition – ability to recognize patterns in
stimuli (e.g., melody, familiar face)
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