1. Sensory, Motor, and Integrative Systems
Chapter 16
Sensory, Motor, and Integrative Systems

2. General Sensations
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

3. 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

4. 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.

5. General Sensations
Examples of complex integrative functions of the brain include wakefulness and sleep, and learning and memory

6. 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

7. 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

8. Sensory Modalities
For a sensation to arise, four events typically occur:
Stimulation of the sensory receptor - an appropriate stimulus must occur within the receptor’s receptive field
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
Sensory receptors produce two different kinds of graded potentials—generator potentials and receptor potentials—in response to a stimulus. When stimulated, the dendrites of free nerve endings, encapsulated nerve endings, and the receptive part of olfactory receptors produce a generator potential (Figure 16.1a, b). When a generator potential is large enough to reach threshold, it triggers one or more nerve impulses in the axon of a first-order sensory neuron. The resulting nerve impulse propagates along the axon into the CNS. Thus, generator potentials generate action potentials.
By contrast, sensory receptors that are separate cells produce graded potentials termed receptor potentials.

9. Sensory Modalities
The four events that bring about a sensation, cont’d…
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)
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

10. Sensory Modalities

11. 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.)
Interoceptors (visceroceptors) are located in blood vessels, visceral organs, muscles, and the nervous system and monitor conditions in the internal environment. The nerve impulses produced by interoceptors usually are not consciously perceived; occasionally, however, activation of interoceptors by strong stimuli may be felt as pain or pressure.

12. 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.)
Interoceptors (visceroceptors) are located in blood vessels, visceral organs, muscles, and the nervous system and monitor conditions in the internal environment. The nerve impulses produced by interoceptors usually are not consciously perceived; occasionally, however, activation of interoceptors by strong stimuli may be felt as pain or pressure.

13. 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
Exteroceptors provide information about the external environment.
Interoceptors monitor the internal environment. Occasionally activation of interoceptors by strong stimuli may be felt as pain or pressure.

14. Sensory Receptors
Receptors can also denote the type of stimulus that excites them

15. 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
Mechanoreceptors provide sensations of touch, pressure, vibration, proprioception, and hearing and equilibrium. They also monitor the stretching of blood vessels and internal organs.

16. 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)
Receptors associated with pressure, touch, and smell are rapidly adapting.

17. Mechanoreception
Many of the mechanoreceptors and nociceptors previously described are
located in
the skin

18. 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.

19. Nociception
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

20. 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
An example of slow pain is the pain associated with a toothache.

21. 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

22. Nociception
Common patterns of referred visceral pain are shown in this graphic

23. 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)

24. Proprioception
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 fine
movements and much more
sparse in those that control
course or forceful movements
Free nerve endings and Ruffini corpuscles in the capsules of joints respond to pressure.
Pacinian corpuscles respond to acceleration and deceleration of joints during movement.

25. 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
Mechanoreceptors provide sensations of touch, pressure, vibration, proprioception, and hearing and equilibrium. They also monitor the stretching of blood vessels and internal organs.

26. Somatic Sensory Pathways
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

27. Somatic Sensory Pathways
Third-order neurons conduct impulses from the thalamus to the primary somatosensory area of the cortex on the same side.

28. 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

29. Somatic Sensory Pathways
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

30. Somatic Motor Pathways
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
The basal nuclei and cerebellum influence movement through their effects on upper motor neurons.

31. Somatic Motor Pathways
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

32. Somatic Motor Pathways
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
Only LMNs provide output from the CNS to skeletal muscle fibers. For this reason, they are also called the final common pathway.

33. Sensory and Motor Pathways (Interactions Animation)
Somatic Sensory Pathways
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34. End of Chapter 16
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