The Somatosensory System Csilla Egri, KIN 306 Spring 2012

“Activate my mechanoreceptive free nerve endings and Pacinian corpuscles” Elmo

Outline

Somatosensory system overview Sensory transduction Sensory receptors

Cutaneous mechanoreceptors Thermoreceptors Nociceptors

Central projections Dorsal column medial-lemniscus pathway Anterolateral pathway

Pain sensations Pain disorders

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Somatosensory system: function

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Receptors respond to mechanical, thermal & chemical stimuli within three broad categories: Proprioceptive (lecture 6,10)

Body and limb position Enteroreceptive (KIN 305)

Internal state of the body Exteroreceptive

Touch Temperature Pain (nociception)

Somatosensory fibers4

Afferent fibers carrying info of different modalities are of different sizes

What property of large diameter axons allows for increased speed of

conduction?

Somatosensory transduction 5

two classes: simple or complex mechanical deformation, heat or

chemical stimulus within receptive field opens ion channels causes a local depolarization

= receptor potential propagated by electrotonic

conduction to axon hillock Stimulus intensity coded by

number of receptors activated, and frequency of AP

Receptor potential

Somatosensory transduction 6

How to convey modality of sensation? Labelled line coding

Type of sensation felt when receptor is stimulated determined by where the fiber synapses in the CNS

Examples of “fooled” senses: Chewing minty gum activates cold sensitive

thermoreceptors sensation of cold Seeing stars after being hit on head forceful blow

activates photoreceptors see spots

Cutaneous mechanoreceptors: morphology

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(rapid vibration)

(slow vibration, texture)

(rapid vibration)

(deep pressure)

(movement of hairs)

(sustained touch, pressure)

Cutaneous mechanoreceptors: adaptation

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receptors are classified not only on their morphology, but on their adaptation & receptive fields as well

Cutaneous mechanoreceptors: receptive fields

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small receptive fields permit high resolution of spatial detail (two point discrimination)

Discrimination enhanced by lateral inhibition

Cutaneous mechanoreceptors: adaptation + receptive fields

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Kandel Fig. 21-1

Adaptation

Cutaneous mechanoreceptors: summary

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Receptor Sensation Adaptation rate Receptive field

Free nerve endings Itch, tickle, pain Tonic or phasic Large or small

Ruffini endings Stretching of skin, deep pressure

Tonic Large

Merkel discs Fine touch and pressure

Tonic Small

Meissner corpuscle Fine touch, pressure, slow vibration

Phasic – moderate Small

Hair follicle Crude touch, movement of hairs

Phasic – moderate Small

Krause bulbs Fast vibration Phasic - fast Small

Pacinian corpuscle Pressure, fast vibration, tickling

Phasic - fastest Large

Thermoreceptors12

Free nerve endings with high thermal sensitivity Temperature change activates family of ion channels on the

receptor membrane = TRP (transient receptor potential) channels

Each TRP channel has a unique temperature threshold of firing, and is sensitive to various chemical agonists

Thermoreceptors: tonic responses

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Guyton Fig. 48-10

Warmth receptors: Narrow temperature range Begin firing at 30ºC, rise

steeply with increased temperature, then stop abruptly

Sensation of pain begins >45ºC

Cold receptors: Broader temperature range Maintain steady discharge

rates, with increased firing 20-30ºC

<15ºC neuronal firing ceases

Tonic response of thermoreceptors and thermosensitive pain fibers

Thermoreceptors: phasic responses

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Kandel Fig. 22-9

Thermoreceptors are more responsive to changes in temperature than to constant temperature

A phasic response in both warm and cold receptors occurs when the temperature is changed

Thermoreceptors adapt to a new steady state firing level is stimulus is maintained

Nociceptors15

Free nerve endings that respond to intense stimuli Types:

Mechanical Strong pressure, sharp objects

Thermal Burning heat (>45ºC) Noxious cold (variable)

Chemical pH extremes Environmental irritants Internal neuroactive substances

Polymodal Sensations mediated by Aδ fibers (sharp, intense pain) and C fibers

(persistent, dull pain).

Check: Which fibers are myelinated?

Nociceptors: hyperalgesia of inflammation

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Nociceptors are non adapting receptors Primary hyperalgesia: damaged tissue

has increased sensitivity to pain Reduced threshold to pain

If normally non painful stimuli felt as painful: allodynia

Increased intensity of sensation Spontaneous pain

Inflammatory response releases bradykinin, prostaglandins, serotonin, substance P, K+, H+

Substance P activates mast cells release histamine activates nociceptors

B&B Fig. 13-26

Somatosensory projections: dermatomes

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Sensory neurons (dorsal root ganglion cells) enter the spinal cord through the dorsal roots

Each dorsal root innervates a field of skin called a dermatome

Dermatomal map used to determine level of lesion of spinal injury

Epidural analgesia blocks sensations thru out several dermatomes

B&L Fig. 7-4

Somatosensory projections: tracts

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2. Anterolateral pathway1. Dorsal column-medial lemniscus pathway

1. Dorsal column-medial lemniscus pathway

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Carry signals from mechanoreceptors of the skin, joints & muscle fine, discriminatory touch

Information has high spatial & temporal resolution Large, myelinated fibers with rapid conduction velocities 1º afferents terminate & form synaptic connections with

2nd order neurons in the dorsal column nuclei within the medulla

2nd order neurons cross over at the medulla and continue to the thalamus via the medial lemniscus pathway

After thalamic processing, 3rd order neurons project to the primary somatosensory cortex

2. Anterolateral pathway20

Pain, temperature, crude touch, tickle, itch, sexual sensations

Low spatial or temporal resolution Small, myelinated/unmyelinated fibers with slower

conduction velocities 1° afferents terminate upon entering spinal cord &

synapse on 2nd order neurons 2nd order neurons cross to contralateral side, ascend to

brain via anterolateral quadrant in spinal cord & project to thalamic nuclei

After thalamic processing, 3rd order neurons project to the primary somatosensory cortex and other cortical areas

Somatosensory cortex: somatotopy

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B&B Fig. 14-11

Spatial orientation of signals form different parts of the body: somatotopy

Size of somatotopic areas is proportional to density of sensory receptors in that body region

Map is plastic (modifiable) size of cortical region representing

particular portion of body surface can expand or contract depending on use of that body region

Pain and temperature localization not as precise Integration probably happens more in

the reticular formation and thalamus

Pain sensation: referred pain22

Pain from visceral nociceptors is poorly localized, can be felt as pain on surface areas Knowledge of referred pain maps important in clinical diagnosis Somatic and visceral afferents may converge on same 2nd order neuron

Guyton Fig. 48-6Kandel Fig. 24-3

Pain sensation23

Sensation of pain intensity not necessarily linked to activation of nociceptors; under CNS control Perception of pain and pain tolerance is subjective Gate control theory of pain

Activation of non-painful fibers (Aα) sends inhibitory signals to nociceptive afferents traveling to the CNS

Mechanism of acupressure analgesia? Phantom limb pain

Pain felt even though nociceptors no longer present in missing limb Peripheral sensitization Somatosensory reorganization

Neuropathic pain Damage to Aδ or C fibers may increase sensitivity or cause

spontaneous AP firing

Pain disorders: CRPS24

Complex regional pain syndrome (CRPS) Neuropathic pain disorder involving peripheral

and central mechanisms (autonomic nervous system)

Changes in somatosensory systems processing thermal, tactile, noxious stimuli Local edema, altered sweating, redness,

skin temperature changes, burning pain, hyperalgesia, allodynia

Acute: warm, red extremities Chronic: cool, bluish extremities

CRPS I – no documented nerve injury CRPS II – presence of nerve injury

Surgery, fracture, crush injury, sprains, but can develop even after minimal injury

WebCT readings: Complex Pain Syndrome

Left arm affected by CRPS

Right foot affected by CRPS

Pain disorders: CRPS pathophysiology

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Peripheral and central sensitization Tissue injury release of substance P and

bradykinin increased excitability of nociceptive neurons in periphery and spinal cord

Increased local, systemic, and CSF inflammatory factors

Reduced density of Aδ and C fibers Altered SNS function and sympatho-afferent

coupling Expression of adrenergic receptors on

nociceptors Symptoms worsened by emotional arousal Reduced representation of affected limb in

somatosensory cortex Genetic predisposition

Increased susceptibility to above mentioned changes?

Bruehl S. Anesthesiology 2010WebCT readings: Complex Pain Syndrome

ObjectivesAfter this lecture you should be able to: List the structure and function of the various cutaneous

mechanoreceptors Describe the mechanism of somatosensory transduction,

including modality coding and receptor adaptation Differentiate between tonic and phasic responses of

thermoreceptors Compare and contrast the function and anatomy of the dorsal

column-medial lemniscus pathway with the anterolateral pathway

List the factors affecting pain sensation Relate what we’ve learned in this course so far to the different

theories of CRPS pathophysiology

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1. Predict on which side of the body and what sensations would be impaired if there was a spinal cord transection at T4 of:

a) The left dorsal column-medial lemniscus pathwayb) The left anterolateral pathway

2. Describe what would happen to both cold and warm thermoreceptors in response to an increase in temperature from 35ºC to 40ºC , sustained for 5 min, and removed.

3. Meissners corpuscles are __________________ adapting receptors with __________________ receptive fields.

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