THE SOMATO-SENSORY SYSTEM

SOMATIC SENSATIONEnables body to feel, ache, chill. Responsible for feeling of touch and painDifferent from other systems because receptors are widely distributed throughout all the body and responds to different kinds of stimuli

Types and layers of skinHairy and glabrous (hairless)Epidermis (outer) and dermis (inner)

Functions of skinProtective functionPrevents evaporation of body fluidsProvides direct contact with world

MechanoreceptorsMost somatosensory receptors are mechanoreceptors.Pacinian corpusclesRuffini's endingsMeissner's corpuscles Merkel's disksKrause end bulbs

TOUCH RECEPTORS

TOUCH RECEPTORSTwo-point discrimination varies across the body surface (Importance of fingertips over elbow). Difference in density of receptors, size of receptive fields, brain tissue devolved in processing the information

Big toesole

calf

back

lip

forearm

thumbIndex finger

PRIMARY AFFERENT AXONS

Big toe

lip

Gray matterwhite matter

Dorsal root

Dorsal root ganglion

Dorsal rootganglion cell

receptor

Spinalnerve

Dorsalroot

Primary Afferent AxonsAα, Aβ, Aδ, CC fibers mediate pain and temperatureA β mediates touch sensations

PRIMARY AFFERENT AXONS

THE SPINAL CORDDivided in spinal segments (30)- spinal nerves within 4 divisions Dermatomes (area of the skin innervate by the R and L dorsal roots of a single spinal segment) have 1-to-1 correspondence with segments

THE SPINAL CORD

Division of spinal gray matter: Dorsal horn; Intermediate zone; Ventral horn

Myelinated Aβ axons (touch-sensitive) mainly synapses in the dorsal horn with the second order sensory neurons

Dorsal Column–Medial Lemniscal PathwayTouch information ascends through dorsal column, dorsal nuclei, medial lemniscus, and ventral posterior nucleus to primary somatosensory cortex

ASCENDING PATHWAYS

The Trigeminal Touch PathwayTrigeminal nervesCranial nerves

Medial lemniscus

dorsal column nuclei

dorsal column

VPN

S1

trigeminal nucleus VPN

S1

From face

SOMATOSENSORY CORTEX

Primary is area 3bReceives dense input from VP nucleus of the thalamusLesions impair somatic sensationsElectrical stimulation evokes sensory experiences

Area 3a receive information from vestibular systemArea 1 receive information from 3b and code for textureArea 2 receive information from 3b and code for size and shape

Other areasPosterior Parietal Cortex (5,7)

SOMATOSENSORY CORTEX

Cortical Somatotopy (Homunculus)

Cortical Map PlasticityRemove digits or overstimulate – examine somatotopy before and after

Showed reorganization of cortical maps

SOMATOSENSORY CORTEX

SOMATOSENSORY CORTEX

The Posterior Parietal CortexInvolved in somatic sensation, visual stimuli, and movement planningLesion has been associated to: Agnosia, Astereoagnosia and Neglect syndrome

Pain - feeling associated to nociceptionNociception - sensory process, provides signals that trigger pain

Nociceptors: Transduction of Pain Bradykinin , Mast cell activation: Release of histamineTypes of Nociceptors: Polymodal, Mechanical, Thermal and Chemical

PAIN

Hyperalgesia: higher sensitivity to pain in tissue already damaged

Primary occurs in the damaged tissues and secondary hyperalgesia in the surroundings

Bradykinin, prostaglandins, and substance P (secondary hyperalgesia)

PAIN

Primary Afferents First pain mediated by fast axons and second pain by slower C fibers

Spinal mechanisms

brain

Dorsal root

Ventral root

PAIN ASCENDING PATHWAYS

Main differences between touch and pain pathwayNerve endings in the skinDiameter of axonsConnections in spinal cordTouch – Ascends IpsilaterallyPain – Ascends Contralaterally

Two pathways: 1) Spinothalamic Pain Pathway2) The Trigeminal Pain Pathway

Spinothalamic Pain Pathway

PAIN ASCENDING PATHWAYS

REGULATION OF PAINAfferent Regulation: gate theory of pain

Dorsalhorn

To dorsal column

To spinothalamic tract

Primary auditory cortex

Secondary auditory cortex

Descending pain control pathway. Use of serotoninStimulation of the PAG cause deep analgesia

REGULATION OF PAIN

The endogenuos opiatesOpioids and endomorphins

TEMPERATURE

Thermoreceptors“Hot” and “cold” receptors. Varying sensitivities

The Temperature PathwayIdentical to pain pathway

Cold receptors coupled to Aδand CHot receptors coupled to C