Somatosensory System: Touch, Proprioception, and Pain

Farzana Z Ali

HBY 554 – Principles of Neuroscience

Spring 2013

Learning Objectives for Part I

Part I: Touch and Proprioception

•Classify somatic sensory afferents based on their distinct functional properties

•Characterize mechanoreceptors specialized for tactile information and proprioception

•Understand the central pathways conveying tactile and proprioceptive information from body and face

• Learn about cortical maps and plasticity

Basic Senses

Somatic sensation

Vision

Audition

Vestibular sensation

Chemical senses

Energy from mechanical

forces

Afferent sensory signals

Central neurons

(qualitative + quantitative +

location)

Somatic Sensory System

Somatic - “of the body”

•Relating to body

•Pertaining to body wall in contrast to viscera

•Distinguished from germplasm or psyche

•Touch, pressure, vibration, limb position, heat, cold and pain

Subsystems

Tactile•Cutaneous mechanoreceptors •Fine touch, vibration, and pressure

Proprioceptive•Muscles, tendons, and joints •Position of limbs and other body parts in space

Nociceptive•Painful stimuli, changes in temperature and

coarse touch

Somatic Sensory Afferents

Action potentials: Somatic sensory afferent fibers Cell bodies inDorsal root ganglia ( pseudo-unipolar),Cranial nerve ganglia

Sensory Transduction

Energy of stimulus Electric signal

• Δ Permeability of cation channels • Depolarizing current• Threshold to generate AP

Dermatomes • Dorsal root gangilon

associated spinal nerve• Overlap substantially

(injury)• Pain sensation more

precise

Axon Diameter

• IA: Largest diameter sensory afferents (muscles)

•Aβ: Slightly smaller diameter fibers (touch)

•Aδ, C: Smaller diameter fibers (pain and temperature)

Receptive field

• Sig. Δ in rate of AP• Smaller receptive fields Smaller branching Dense innervation

Two-point discrimination• ↑ receptive fields• Somatic acuity

Temporal Dynamics, Channels and Filters

Channels in sensory afferents• Quality of stimulation• Filter properties of

encapsulating receptors

Stimulus movement, clothes

Spatial attributes of the stimulus: size and shape

Mechanoreceptors for Tactile Information

slow, form + texture, fingertips

spatial resolution

rapid, dense, ↑ sensitive, ↑ fields, grip

rapid, filter ν, ↓threshold, ↑ fields, vibrations

slow, stretching, internal, position/conformation

details

coarse

APs

Mechanoreceptors for Proprioception

Muscle spindles: lengthGroup IA:Velocity + directionGroup II:Sustained, static position

Golgi tendons: tensionGroup Ib:Branched in collagen fibers to form tendons

Joint receptors: finger position

Central Pathways

Cerebral cortex

Third order neurons in thalamus

Second-order neurons in brainstem nuclei

First order neurons in dorsal root and cranial nerve ganglia

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Trigeminothalamic System

Trigeminal nerve trigeminal brainstem complex: i) principal, ii) spinal, iii) mesencephalicLow threshold mechanoreceptors principle nucleus cross midlinetrigeminal lemniscus /trigeminothalamic tract VPM SI and SII

Proprioceptive Pathways for Body• Upper: dorsal columns medulla nuclei cross the midline medial lemniscus VPL

• Lower: medulla outside gracilis decussate and join medial lemniscus VPL

Proprioceptive Pathway for Face

• Cell bodies in mesencephalic trigeminal nucleus• Peripheral processes innervating muscle spindles

+ Golgi tendon organs • Facial musculature + central processes• Brainstem nuclei for reflex control• Thalamus somatosensory cortex• Exact pathway not known

Hemispatial Neglect

Parton A, Malhotra P, and Husain M. Hemispatial neglect. Journal of neurology, neurosurgery, and psychiatry 75: 13-21, 2004.

• Unilateral brain damage• Cerebral infarction or

hemorrhage• Fail to be aware of or

acknowledge items• Right inferior parietal lobe

or nearby temporo-parietal junction

Somatic Sensory Portion of Thalamus

• Ascending: spinal cord and brain stemVentral Posterior Complex of thalamus

• VPL medial lemniscusfrom posterior head + body

• VPM trigeminal lemniscusfrom face

• Muscle spindle/ Golgi tendons

Somatotopic Map

• Homunculus - “Little man”• Face and hands

> torso and proximal limbs• Manipulation,

facial expression and speech

• Cervical spinal cord• Receptor density

Cortical Plasticity

• Reorganization of cortical circuits• Peripheral lesions • Unresponsive responsive to stimulation of

neighboring regions of skin• Central representation of remaining digits• Functional remapping• Thalamus and brainstem

Learning Objectives for Part II

Part II: Pain

• Learn about nociceptive receptors and signal transduction

• Distinguish pain pathways from mechanosensory pathways

• Understand the pathway for visceral pain

• Characterize peripheral and central sensitization

• Gain insight into different aspects of the modulation of pain perception

Nociception

• Nocere – “to hurt”• Nociceptors• Aδ group: myelinated, dangerously intense mechanical/ both intense

mechanical and thermal stimuli

• C fiber group: unmyelinated, majority respond to thermal,

mechanical and chemical stimuli

Nociceptor Afferents

• Aδ mechanosensitive, Aδ mechanothermal, polymodal C fibers

• Sharp first pain: Aδ activated when stimulus intensity ↑

• More delayed, diffuse, and longer-lasting second pain: small-diameter C fibers activated when simulation ↑ even farther

Signal Transduction

• Aδ and C: VR-1 or TRPV1 45°C + capsaicin+ acid• Aδ: VRL-1 or TRPV2 ↑ threshold, 52°C, not capsaicin• TRP channel family: resembles voltage gated K+ / cyclic

nucleotide gated• Influx of Na+ and Ca2+ initiates AP generation

Anterolateral System

Brainstem and thalamus in anterolateral/ventrolateral quadrant

of contralateral half of spinal cord

Cross midline

Second-order neurons in Rexed’slaminae 1 and 5

Gray matter of dorsal horn

Dorsolateral tract of Lissauer

Sensory neurons in dorsal root ganglia

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Anterolateral vs. Dorsal Column Medial Lemniscal

• Medial lemniscus enter spinal cord, ipsilateral dorsal columnmedulla synapse on dorsal column nuclei cross midline ascend to contralateral thalamus

• Anterolateral system information crosses in spinal cord• Unilateral spinal cord lesion

Parallel Pain Pathways

Sensory discriminative: • Location, intensity, quality• VPL/ VPM

somatosensory cortex• Small receptive fields

Affective-motivational: • Unpleasant feeling,

fear and anxiety• Autonomic activation

Visceral Pain

Referred pain

Punctate

Pain from the Face

Discriminative: projections

to the contralateral ventral

posterior medial nucleus via

trigeminothalamic tract

primary and secondary

somatosensory cortex

Affective/motivational:

targets in reticular formation

and midbrain + midline

nuclei of thalamus which

supplies cingulate and

insular regions of cortex

Peripheral Sensitization

Sensitization: Neuronal sensitivityHyperalgesia: Slightly painfulSignificantly painful↑sensitivity to temperature

after a sunburnPeripheral sensitization: • “Inflammatory soup”• Vasodilation, swelling, histamine• Protect, promote healing and

guard against infection• ↑ blood flow and migration of white blood cells

Central Sensitization

Allodynia: Innocuous stimulus to the skin

• ↑ Activity in nociceptive afferents AP in dorsal horn neurons↑ pain sensitivity

Windup: ↑ Discharge rate of dorsal horn neurons from repeated ↓ ν activation of nociceptive afferents

• ∑ All slow synaptic potentials in dorsal horn neurons

sustained depolarization

• Voltage dependent L-type calcium channels

• Removing Mg block of NMDA receptor (glutamate)

Phantom Pain

Neuropathic pain:• Afferent fibers or central pathways damaged

Amputation of extremity:• Illusion that missing limb is still present• Functional reorganization of somatotopic maps

in primary somatosensory cortex• Neurons lost their original inputs from remote limb

tactile stimulation of other body parts

Placebo Effect

•Placebo - “I will please” •Physiological response following the administration

of a pharmacologically inert remedy

1. Sedative: >2/3 reportedly felt drowsy

2. Stimulant: Majority ↓ tired•1/3 headaches, dizziness, tingling extremities,

staggering gait

•Not “imagining” it•Acupuncture anesthesia, analgesia through hypnosis

Pain Modulation

• Electrical/ pharmacological stimulation midbrain

• Descending pain modulating pathways+ spinal trigeminal nucleus

• Regulated transmission of information to higher centers

• Periaqueductal gray of midbrain• Descending pathways arise from brainstem sites

Gate Theory of Pain

• Local interneurons b/w mechanoreceptiveafferent and neural circuit within dorsal horn nociceptive information to higher centers

• Rub the site of injury after stubbing a toe

• ↓ Sensational sharp pain by activating ↓ threshold mechanoreceptors