class 5 somatosensory

7/27/2019 Class 5 Somatosensory

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The sensory modalitySensory pathway

Pain Definition

Classification

Gate theory of pain

Visceral pain and referred pain

Somatovisceral Sensory Physiology


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The General Model for the Translation

of A Stimulus into a Perception


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Modalities of Somatic

Sensations

Touchsensing the physical properties of

surfaces in contact with the skin.

Pressure

Proprioceptionsensing the position and

movement (kinesthesis) of body parts.

TemperatureSensing the warmness and

coldness Pain


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Four Modalities of Somatic

Sensation

Tactile: pressure, touch, vibration

Thermal: sensing temperature (warmness and

coldness)

Pain

Proprioceptive: sensing the position and

movement (kinesthesis) of body parts


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Modality Sub Modality Sub-Sub ModalitySomatosensory Pathway

(Body)

Somatosensory Pathway

(Face)

Pain

sharp cutting pain Neospinothalamic

Spinal Trigeminal

dull burning pain Paleospinothalamicdeep aching pain Archispinothalamic

Temperaturewarm/hot Paleospinothalamic

cool/cold Neospinothalamic

Touch

itch/tickle & crude touch Paleospinothalamic

discriminative touch

touch

Medial Lemniscal Main Sensory Trigeminal

pressure

flutter

vibration

Propriocepti

on

Position: Static Forces

muscle length

muscle tension

joint pressure

Movement: Dynamic Forces

muscle length

muscle tension

joint pressure

joint angle

The Sensory Modalities Represented by the Somatosensory Systems

Cl ifi ti f S t i l


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Classification of Somatovisceral

Sensory Receptors according to

Modality Mechanoreceptors

Mechanoreceptors are activated following physical

deformation of the skin, muscles, tendons, ligaments, and

joint capsules in which they reside

Thermoreceptors

Cold receptors (free nerve endings of lightly myelinated A

fibers); warmth receptors (free nerve endings of unmyelinated

C fibers); temperature-sensitive nociceptors (sensitive to

excessive heat or cold). Nociceptors

Nociceptors are rapidly adapting receptors that are sensitive

to noxious or painful stimuli; they are the free endings of A or

C fibers.


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Mechanoreceptors


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Thermoreceptors


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Nociceptors (pain receptors)

Nociceptors are the free endings of A

and C type fibers.

According to their locations, nociceptorsare divided into:

Skin nociceptors

Joint nociceptorsVisceral nociceptors


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Somatovisceral Sensory Process

Sensory receptors (skin, muscles, joints,

viscera)

First-order neuron (dorsal root ganglia)

Second-order neuron (spinal cord or brainstem)

Third-order neuron (thalamus except olfaction)

Somatosensory areas of the cortex


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Major Ascending Pathways that Carry

Sensory Information


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Anterolateral (spinothalamic)

tract located in the ventral

spinal cord is for somatic pain,

temperature, crude touch and

deep pressure


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The Dorsal ColumnMedial Lemniscal

Pathway

Medial lemniscus pathway

located in the dorsal dorsal

spinal cord is for fine touch,

vibration, proprioception,

visceral distension, and visceral

pain.


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The Dorsal ColumnMedial Lemniscal

Pathway

Medial lemniscus pathway

located in the dorsal dorsal

spinal cord is for fine touch,

vibration, proprioception,

visceral distension, and visceral

pain.


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The Thalamus in Sensory

Processing

The thalamus is responsible for sensory

processing and movement and is the relay

station for all sensory impulses except for

the olfaction, the sense of smell. Thethalamus also receives sensory

information and then relays this

information to the cerebral cortex. Thecerebral cortex also sends information to

the thalamus, which then is transmitted

into other areas of the brain and spinal


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A Summary of Ascending Pathway


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Pain is a sensory experience that is

accompanied by emotional responses and

by somatic and autonomic motor

adjustments.

What is Pain?


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Pain Stars with Pain Receptors

(nociceptors)

Pain receptors (nociceptors)are the

free endings of A and C fibers that

respond to noxious stimuli.

Adequate StimulationTemperature

Mechanical damage

Chemicals stimuli (Bradykinin,

serotonin, histamine, K+

, acids,acetylcholine, and proteolytic

enzymes etc.)

prostaglandins and substance P

enhance the sensitivity of pain endings

but do not directly excite them.


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Types of Pain

Superficial somatic pain arises from skin

areas.

Deep somatic pain arises from muscle,

joints, tendons & fascia.

Visceral pain arises from nociceptors in

visceral organs.


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Fast Pain vs. Slow Pain

Fast Pain

Thinly myelinated A (fast-conducting) fibers, which

relay sharp, short-term, well-localized pain (such as

that resulting from a pinprick). These fibers transmitsensations that do not elicit an affective component

associated with the experience.

Slow pain Unmyelinated C (slow-conducting) fibers, which relay

dull, persistent, poorly localized pain (such as that

resulting from excessive stretching of a tendon).

These fibers transmit sensations that elicit an affectiveres onse.


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A Comparison between Fast Pain and

Slow Pain

Fast Pain Slow Pain

Well localized yes no

Nerve fibers

involvedA C fiber

Description Sharp, pinprick Dull

Duration short PersistentAffective

responseNo Yes


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Gate Control Theory of Pain


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Examples of Gate Theory Application

Stimulation of touch for pain relief

Transcutaneous electrical nerve stimulation (TENS)

Acupuncture

Massage

Release of endogenous opioids

Hypnosis

Placebo effect


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Visceral Pain


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Afferent Innervation of the Viscera

Often anatomical separation nociceptive innervation (in sympathetic

nerves) from non-nociceptive (predominantly in vagus).

Many visceral afferents are specialized nociceptors (A and Cfibers), as in other tissues.

Large numbers of silent/sleeping nociceptors, awakened by

inflammation.

Nociceptor sensitization well developed in all visceral nociceptors.


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Features of Visceral Pain

There are no fast and slow components.

Pain is often poorly localized, deep and dull.

Injury is not always required to result visceral pain.

Excessive distension (for example, of the colon),

ischemia and inflammation are common causes toinduce visceral pain.

May be referred to certain body areas


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Pain Can be Referred

A pain that is localized in one

part of the body remote form itssource.

Visceral pain arising from

ischemia or pressure on viscerais felt as somatic pain.


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Convergence Theory for Referred Pain

The convergence of

somatic and visceral

nociceptive afferents upon

the same sets of somaticneurons has been

proposed as a mechanism

of referred pain.

It is conjectured that

reference is due to more

frequent activation of the

somatic input in ordinary

life and learned place

class 5 somatosensory

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