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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