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CHAPTER 11. The Body Senses and Movement The Body Senses. The Body Senses. Information about our body is processed by somatosensory system vestibular system. Somatosenses include Proprioception: spatial location of body via touch

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CHAPTER 11

The Body Senses and Movement

The Body Senses


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The Body Senses

Information about our body is processed by

somatosensory system

vestibular system.

Somatosenses include

Proprioception: spatial location of body via touch

Skin senses: tell us about conditions at the surface of our body,

Interoceptive system: concerned with sensations in our internal organs.

Vestibular system informs the brain about

Body position

Body movement.


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Proprioception: sense that informs us about:

positionof limbs and body

movementof our limbs and body.

Skin senses include:

Touch

warmth, cold

pain.

The skin Senses


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Four types of skin receptors

Meissner’s corpuscles

Merkel’s discs

Pacinian corpuscles

Ruffini endings

Two general types of skin receptors.

Free nerve endings:

simply processes input at the ends of neurons.

They detect warmth, cold, and pain.

Encapsulated receptors:

form all other receptors

more complex structures enclosed in a membrane.

Their role is to detect touch.

The skin receptors


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Meissner’s corpuscles:

respond to brief burst of impulses

distributed on various areas of the skin,

concentrated in areas especially sensitive to light touch, (e.g., fingers and lips)

Merkel’s discs:

respond to sustained response to mechanical deflection of the tissue, particularly low vibrations

wide distribution in superficial skin layers

clustered beneath the ridges of the fingertips that make up fingerprints and in specialized "touch domes" or "hair disks“ of hairy skin

responsible for the ability to feel fine detailed surface patterns (e.g. for reading Braille).

Both meissner’s and merkel’s detect

texture, fine detail of objects

detect movement

use both when examining texture, shape of object

Superficial layers skin:


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2 kinds

Pacinian corpuscles

Ruffini endings

Both contribute to perception of shape of grasped object

Pacinian corpuscles

Larger, fewer in number than both Merken cells or Meissner's corpuscles

detect gross pressure changes/vibrations

rapidly adapting (phasic) receptors.

large receptive field

Deeper skin receptors


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Ruffini endings

sensitive to skin stretch

Important for kinesthetic sense of and control of finger position and movement

register mechanical deformation within joints (more specifically angle change up to 2 degrees)

Also detect continuous pressure states

Deeper skin receptors


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Sensitivity varies greatly with concentration of receptors

most in fingertips, tongue

fewer in upper arms, calves of legs, back

Different firing rates for different cells:

Particularly warmth, cold pain

cold receptors near skin’s surface:

warmth receptors are deeper

pain receptors separate from warmth receptors

The skin receptors


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Vestibular sense:

helps maintain balance

provides information about head position and movement.

Vestibular organs in the inner ear

semicircular canals,

the utricle

the saccule

The vestibular system sends projections to the cerebellum and the brain stem.

Parieto-insular-vestibular cortex: Pathway to a cortical area

The vestibular Senses


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Vestibular sense:

helps maintain balance

provides information about head position and movement.

Vestibular organs in the inner ear

semicircular canals,

the utricle

the saccule

The vestibular system sends projections to the cerebellum and the brain stem.

Parieto-insular-vestibular cortex: Pathway to a cortical area

The vestibular Senses


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Somatosensory Pathway

First neuron: Free nerve endings or encapsulated receptors

Connect to cell body on dorsal root ganglion of the spinal nerve or cranial nerves

Thus form the first link in the chain

Second neuron: Cell body of spinal cord or brainstem.

Second neuron's ascending axons cross (decussate) to the opposite side either in the spinal cord or in the brainstem.

Axons of these neurons terminate in

Thalamus

reticular system

cerebellum.

For Touch/Some types of pain: THIRD neuron

Third neuron has cell body in the ventral posterior nucleus or VPN of the thalamus

Ends in the postcentralgyrus of the parietal lobe


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Pathway into brain

From thalamus, body sense neurons go to their projection area: somatosensory cortex

located in the parietal lobes (remember?)

just behind the primary motor cortex and the central sulcus.

Most of the neurons cross from one side of the body to the other side of the brain

Contralateral = crossing

Ipsilateral= not crossing; stays on same side

touch of an object held in the right hand registered mostly in left hemisphere.


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The Body Senses

  • Dermatomes:

    • Body is divided into segments

    • each segment served by a spinal nerve.

    • These segments are called dermatomes

  • Divided into several subdivisions:

    • Cervical: head, upper neck

    • Thoracic: lower neck to chest

    • Lumbar: middle

    • Sacral or coccygeal: tail


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Identifying nerve position

  • The labels identify the nerve.

    • Letters = part of the spinal cord where the nerve located

    • Numbers = nerve’s position within that section.

    • For example: Areas I, II,and III on the face innervated by branches of the trigeminal (fifth) cranial nerve.


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The cranial nerves

  • I-Olfactory nerve

  • II-Optic nerve

  • III-Oculomotor nerve

  • IV-Trochlear nerve

  • V-Trigeminal nerve

  • VI-Abducens nerve

  • VII-Facial nerve

  • VIII-Vestibulocochlear nerve/Auditory nerve

  • IX-Glossopharyngeal nerve

  • X-Vagus nerve

  • XI-Accessory nerve/Spinal accessory nerve and

  • XII-Hypoglossal nerve.

  • On Old Olympus' Towering Top AFinn And German Viewed Some Hops


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Somatosensory cortices

Primary somatosensory cortex

each contains a map of the body

Each plays a role in processing sensory information for the body.

Secondary somatosensory cortex

receives input from the left and the right primary somatosensory cortices,

combines information from both sides of the body.

Neurons in this area particularly responsive to stimuli that have acquired meaning (e.g., association with reward).

Connects to temporal lobe and hippocampus

Hippocampus critical for learning,

may determine whether a stimulus is committed to memory.


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Posterior parietal cortex

  • The primary somatosensory cortex also projects to the posterior parietal cortex

  • Posterior parietal cortex:

    • association area

    • brings together the body senses, vision, and audition.

    • determines

      • body’s orientation in space,

      • the location of the limbs,

      • the location in space of objects detected by touch, sight, and sound.

    • it integrates the body with the world.


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Pain

  • Pain processing:

    • begins when free nerve endings stimulated by

      • intense pressure

      • temperature

      • damage to tissue.

  • There are three types of pain receptors.

    • Thermal pain receptors: respond to extreme heat/cold.

    • Mechanical pain receptors: respond to intense stimulation like pinching/cutting.

    • Polymodal pain receptors: activated by

      • both thermal and mechanical stimuli

      • chemicals released when tissue is injured.


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Spinal cord

response to pain

  • In the spinal cord: Pain neurons release:

    • glutamate

    • substance P: neuropeptide involved in pain signaling.

    • Substance P released only during intense pain stimulation.

  • Gate control theory:

    • (Ronald Melzack and Patrick Wall)

    • hypothesized that pressure signals arriving to brain trigger an inhibitory message

    • This inhibitory message travels back down spinal cord

    • Result: closes a neural “gate” in the pain pathway.


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endorphins

  • Endorphins function both as:

    • neurotransmitters and as

    • hormones

    • act at opiate receptors in many parts of the nervous system.

  • Pain = one of stimuli that release endorphins

    • Only releases under certain conditions.

    • physical stress

    • acupuncture

    • vaginal stimulation in rats and women.


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Brain response to pain

  • Periaqueductal gray area: PAG

    • Brain stem structure

    • Contains large number of endorphin synapses.

    • Stimuli like pain and stress cause the release of endorphins in PAG

    • Endorphin release inhibits the release of substance P, closing the pain “gate” in the spinal cord.

  • Activation of the endorphin circuit has multiple neural origins:

    • cingulate cortex during placebo analgesia

    • amygdala in the case of fear-induced analgesia.


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Cannabinoid receptors

  • Cannabinoid receptors respond to active ingredient in marijuana

  • In rats: blocking cannabinoid receptors in periaqueductal gray reduces analgesia produced by brief foot shock.

  • This suggests that cannabinoids also serve as

    • internal pain relievers

    • share the neural gating system used by endorphins

    • May explain ‘pleasure’ sensation


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Phantom pain

  • Phantom pain: pain that is experienced as located in the missing (amputated) limb.

  • 70% of amputees experience

  • Phantom pain real sensation: brain not know that limb is missing

  • Significant problem in post-amputation pain management.


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Phantom pain

  • originates in the brain.

  • With loss of limb: awareness of the details of limb's shape/perceived ability to move it tend to fade with time.

  • Most amputees report continuing to feel some phantom sensations throughout the remainder of their lives.


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Phantom pain

  • Good news: Neural mechanisms which permit perception of phantom limbs well understood.

  • Major muscles in residual limb tense up several seconds before cramping

    • This coincides with phantom limb pain begins

    • Muscles remain tense for much of duration of episode.

  • Burning phantom limb pain also closely associated with reduced blood flow in residual limb

  • Brain acting like limb still there because other muscles “cue” brain to act


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What is brain doing?

  • Researchers noted that stimulating face often produces sensations in a phantom arm (huh?)

  • used brain imaging to map face and hand somatosensory areas in upper-limb amputees to determine this relationship.

  • Found that neurons interpreting facial areas moving into and forming pathways in other areas….missing limb areas.


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In Phantom Limp patients

neurons from the face area appear to invade area that normally receives input from the missing hand.

Thus, as face moves, brain processes this as movement of limb, and pain reaction to movement

Facial movement produces sensation of missing limb “hurting”


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Phantom pain: Treatment

Temperature biofeedback may be helpful

teach amputees with burning/tingling phantom pain to habitually and unconsciously keep their residual limbs as warm as the intact limb.

For cramping pain: teach relaxation in related muscles to prevent onset of the muscle tension “cues” in residual limb which lead to pain.


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Phantom pain: Treatment

  • How do this? Several stages:

  • Subjects shown the relationship between residual limb's temperature or muscular activity and the onset and intensity of phantom pain

  • Given muscle tension and temperature awareness training

    • begin increasing their awareness of changes in limb temperature and tension patterns

    • begin to learn to control these parameters.

  • After several weeks, patients begin doing exercise at home and in their normal work environment.

  • Generalize awareness of changes to their normal environment.


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