Somatosensation

Lesson 17

Somatosensation

Sensory info from body Cutaneous senses

exteroceptors touch / pain

Kinesthesia interoceptors body position & movement ~

The Cutaneous Senses Physical events next to organism

info about stimuli in direct contact surfaces, objects, energy body position

Feedback critical for skilled movements walking after foot falls asleep talking after a shot of novocaine no feedback from touch ~

The Cutaneous Senses

Primarily mechanical stimulation Several types of receptors

Several submodalities Touch & pressure Kinesthesis Haptic System Temperature Pain ~

Somatosensory Receptors

Several types mechanoreceptors nociceptors thermoreceptors chemoreceptors proprioceptors

Transduction of environmental energy ~

Mechanoreceptors Unmyelinated axon branches Free nerve ending

glabrous & hair regions pain sensation

Basket cells Hairy regions ~

Mechanoreceptors Encapsulated end organs

Glabrous (hairless) skin Non-neural tissue Merkel’s Disks Meissner’s corpuscle Pacinian corpuscles Ruffini endings ~

~

Receptor Adaptation

Different rates of adaptation response to continued stimulation

Slowly adapting (SA) steady pattern of firing

Rapidly adapting (RA) fire only at onset of stimulus ~

Somatosensory Pathways

Touch & Proprioception Dorsal column-medial lemniscal

pathway Pain and Temperature

Spinothalamic system Trigeminal pathway

face & neck cranial nerve V, also others ~

Dorsal

Ventral

dorsal columns

Spinal Cord

lateral columns

Somatosensory cortex

S1 - Postcentral Gyrus Somatotopic Organization

topographic representation of body Distorted Homunculus

disproportionate amount of cortex for body parts

high sensitivity: large cortical area ~

M1S1PPC

Somatosensory Cortex

Topographic representation Body Image – S1 Space around us – PPC Coordinates for movement

Phantom limbs & pain after amputation

Contralateral neglect PPC damage ~

Kinesthesia

Body Position & Movement proprioception

Joint information Pacinian corpuscles & Ruffini endings

Muscle & tendon information changes in tension

Golgi tendon organ

muscle spindle fibers ~

Cutaneous Receptors

Stretching of the skin Limited role in proprioception Knee: anesthesia no affect Mouth, hands, & feet proprioception

significantly reduced by anesthesia Ruffini Endings

slow adapting population of neurons responding

simultaneously ~

Muscle Receptors

Major role in proprioception Stretch receptors

detect changes in tension 2 types of receptors

Muscle spindles & Golgi tendon organs differences in threshold & location ~

Muscle length detectors Parallel with extrafusal fibers Low threshold

Monosynaptic stretch reflex Postural adjustments Muscle tonus

Sensory neuron alpha motor neurons monosynaptic excitation disynaptic inhibition ~

Muscle-Spindle Receptors

Innervated by gamma motor neurons active concurrently with alpha

motor neurons Control tension on muscle spindles

Not force for movement “Load the Spindles” ~

Intrafusal Muscle Fibers

Dorsal

Ventral

+

MS

GammaMotoneuron

+

Golgi Tendon Organ

Gauges muscle tension high threshold

Stretch receptor safety mechanism controlled muscle contraction ~

Dorsal

Ventral

-+

GTO

+Inhibits alphamotor neuron

GTO: Function

Inhibits muscle contraction Control of motor acts

slow contraction as force increases e.g., holding an egg

Autogenic inhibition safety mechanism too much tension damage ~