copyright © john wiley & sons, inc. all rights reserved. chapter 16 sensory, motor, and...
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Copyright © John Wiley & Sons, Inc. All rights reserved.
Chapter 16
Sensory, Motor,
and Integrative
Systems
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In this chapter we explore the levels and
components of pathways that convey sensory
nerve impulses from the body to the brain, and
the general sensations (somatic and
visceral) that result. We will
also examine the activation of
motor pathways and movements
In chapter 17 we will look at
the special senses of sight,
hearing, taste, and smell
General Sensations
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General Sensations As sensory impulses reach the CNS, they
become part of a large pool of sensory input
(though not every one
will elicit a response)
Each piece of
incoming information
is combined with other arriving
and previously stored information
in a process called integration
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General Sensations Integration occurs at many places along
pathways in the spinal cord, brain stem,
cerebellum, basal nuclei,
and cerebral cortex. Sensations result in
and evoke a conscious perception
or subconscious awareness that
changes have occurred in the
external or internal environment.
The motor responses are also
modified at several of these levels.
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General Sensations Examples of complex integrative functions
of the brain include wakefulness and sleep,
and learning and memory
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Sensory Modalities Each unique type of sensation is called a sensory
modality, and a given sensory neuron carries
information for only one modality, be it somatic,
visceral, or “special”
Somatic senses include tactile sensations (touch,
pressure, vibration, itch, and tickle), thermal
sensations (warm and cold), pain sensations, and
proprioception (awareness of limb and joint position in
space)
Visceral senses provide information about
conditions within internal organs
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Sensory Modalities The process of sensation begins in a sensory
receptor, which can be either a specialized cell
or the dendrites of a sensory neuron
A particular kind of stimulus (a change in the
environment) activates certain sensory
receptors, while other sensory receptors
respond only weakly or not at all – a
characteristic known as selectivity
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Sensory Modalities For a sensation to arise, four events typically occur:
1. Stimulation of the sensory receptor - an
appropriate stimulus must occur within the
receptor’s receptive field
2. Transduction of the stimulus - a sensory
receptor converts energy in a stimulus into a
graded potential. Recall that graded potentials
(but not APs) vary in amplitude depending on
the strength of the stimulus that causes them,
and are not propagated
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Sensory Modalities The four events that bring about a sensation, cont’d…
3. Generation of nerve impulses – occurs when
the sum of graded potentials reach threshold in
first-order neurons (the first neuron in a specific
tract – in this case from the PNS into the CNS)
4. Integration of sensory input – occurs when a
particular region of the CNS integrates a number
(and even a variety) of sensory nerve impulses
and results in a conscious sensations or
perceptions
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Sensory Modalities
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Sensory Receptors Sensory receptors can be grouped into several
classes
based on structural and functional
characteristics:
Microscopic structure – free nerve endings vs
encapsulated endings, for example
Location…of the receptors and the origin of the
stimuli that activate them
The type of stimulus detected (nociceptors for
pain, mechanoreceptors for pressure, etc.)
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Sensory Receptors Sensory receptors can be grouped into several
classes
based on structural and functional characteristics:
Microscopic structure – free nerve endings vs
encapsulated endings, for example
Location…of the receptors and the origin of the stimuli
that activate them – exteroceptors near the external
surface vs interoceptors (visceroceptors), for example
The type of stimulus detected (nociceptors for pain,
mechanoreceptors for pressure, etc.)
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Sensory Receptors Receptors named according to their location include:
Exteroceptors, which are located at or near the
external surface of the body and respond to external
stimuli
Interoceptors (visceroceptors), which are located
in blood vessels, organs, and muscles and produce
impulses which usually are not consciously perceived
Proprioceptors, which are located in muscles,
tendons, joints, and the inner ear. They provide
information about body position and movement of
joints
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Sensory Receptors Receptors can also denote the type of stimulus
that excites them
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Sensory Receptors Receptors named according to mode of activation are:
Mechanoreceptors, which are sensitive to deformation
Thermoreceptors, which detect changes in
temperature
Nociceptors, which respond to painful stimuli
Photoreceptors, which are activated by photons of
light
Chemoreceptors, which detect chemicals in the mouth
(taste), nose (smell) and body fluids
Osmoreceptors, which detect the osmotic pressure of
body fluids
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Sensory Receptors A characteristic feature of most sensory receptors is
adaptation, in which the generator potential or
receptor potential decreases in amplitude during a
sustained or constant stimulus Because there is an accommodation response at
the receptor level, the frequency of nerve impulses
traveling to the cerebral cortex decreases and the
perception of the sensation fades even though the
stimulus persists
• receptors vary in how quickly they adapt (rapidly
adapting and slowly adapting receptors)
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Mechanoreception Many of the mechanoreceptors and nociceptors
previously described are
located in
the skin
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Mechanoreception This graphic illustrates some representative
examples of general somatic mechanoreceptors and the first-order neurons to which they belong. Receptors for special senses are not shown.
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All of our sensory modalities are important,
but pain serves a protective function and is
indispensable for survival
Nociceptors are chemoreceptive free nerve
endings activated by tissue damage from
intense thermal, mechanical, or chemical
stimuli - they’re found in every tissue of the
body except the brain
Nociception
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Nociception There are two types of pain: fast and slow
The perception of fast pain (acute, well localized)
occurs rapidly because the nerve impulses
propagate along medium-diameter, myelinated A
fibers
By contrast, slow pain begins after a stimulus is
applied and gradually increases in intensity over a
period of several seconds or minutes. Impulses for
slow pain conduct along small-diameter,
unmyelinated C fibers. This type of pain may be
excruciating and often has a burning, aching, or
throbbing quality
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Nociception Pain that arises from stimulation of receptors in
the skin is called superficial somatic pain;
stimulation of receptors in skeletal muscles, joints,
tendons, and fascia causes deep somatic pain Visceral pain results from stimulation of
nociceptors in visceral organs
In many instances of visceral pain, the pain is
felt in or just deep to the skin that overlies the
stimulated organ, or in a surface area far from
the stimulated organ. This phenomenon is called
referred pain
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Nociception Common patterns of referred visceral pain are
shown in this graphic
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Proprioception Muscle spindles are the proprioceptors in
skeletal muscles that monitor changes in the
muscle length and participate in stretch reflexes
By adjusting how vigorously a muscle spindle
responds to stretching of a skeletal muscle,
the brain sets an overall level of muscle tone
(the small degree of contraction that is
present while the muscle is
at rest)
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Each muscle spindle consists of several slowly adapting sensory nerve endings that wrap around 3 to 10 specialized muscle fibers. A connective tissue capsule encloses the sensory nerve endings and anchors the spindle to the endomysium
and perimysium Muscle spindles are plentiful
in muscles that control finemovements and much more sparse in those that controlcourse or forceful movements
Proprioception
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Somatic Sensory Pathways No matter the type of receptor on the receiving
end (where the generator potential is set up),
first-order somatosensory neurons are unipolar
in structure
This means that their cell body is located in the
dorsal root ganglia (DRG) just outside the CNS
Their other end terminates
nearby in the posterior
gray horns of the
cord, usually at the
level where they enter
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Second-order neurons conduct ascending
impulses from the brain stem where their
axons decussate (cross over to
the opposite side) before
ascending to the thalamus
Thus, all somatic sensory
information from one side
of the body reaches the
thalamus on the opposite side
Somatic Sensory Pathways
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Somatic Sensory Pathways
Third-order
neurons conduct
impulses from the
thalamus to the
primary
somatosensory area
of the cortex on the
same side.
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Somatic Sensory Pathways Somatic sensory neurons (and their axons that
convey somatic sensations) are not distributed
evenly in the body
The peripheral areas with the highest density
are represented in the brain with the
largest amount of gray matter in the
sensory homunculus. The most
sensitive areas in the body are
therefore the tip of the tongue,
lips, and fingertips
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There are two major spinocerebellar tracts in
the spinal cord that carry proprioceptive
impulses to the cerebellum Although they are not
consciously perceived,
sensory impulses sent
to the cerebellum
along these two pathways
are critical for
posture, balance, and
coordination of skilled movements
Somatic Sensory Pathways
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Motor activity begins in the primary motor
areas of the precentral gyrus and other cerebral
integrative centers
Any motor neuron that is not directly
responsible for stimulating target
muscles is called an upper
motor neuron (UMN)
•UMNs connect the brain to
the appropriate level in the
spinal cord
Somatic Motor Pathways
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From there, all excitatory and inhibitory signals
that control movement converge on second-
order motor neurons known as lower motor
neurons (LMNs) that descend
to innervate skeletal muscle
Since only LMNs provide
output from the CNS to
skeletal muscle fibers they are also
called the final common pathway
Somatic Motor Pathways
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Axons of LMNs extend through cranial nerves
to the skeletal muscles of the face and head,
and through spinal nerves to innervate
skeletal muscles
of the limbs and trunk
Two of the major
LMN tracts are the
lateral and anterior
corticospinal tracts
Somatic Motor Pathways
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Sensory and Motor Pathways(Interactions Animation)
Somatic Sensory Pathways
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