Somatosensation Lesson 17

Somatosensation n Sensory info from body n Cutaneous senses l exteroceptors l touch / pain n Kinesthesia l interoceptors l body position & movement ~

The Cutaneous Senses n Physical events next to organism l info about stimuli in direct contact l surfaces, objects, energy l body position n Feedback critical for skilled movements l walking after foot falls asleep l talking after a shot of novocaine l no feedback from touch ~

The Cutaneous Senses n Primarily mechanical stimulation l Several types of receptors n Several submodalities l Touch & pressure l Kinesthesis l Haptic System l Temperature l Pain ~

Somatosensory Receptors n Several types l mechanoreceptors l nociceptors l thermoreceptors l chemoreceptors l proprioceptors n Transduction of environmental energy ~

Mechanoreceptors n Unmyelinated axon branches n Free nerve ending l glabrous & hair regions l pain sensation n Basket cells Hairy regions ~

Mechanoreceptors n Encapsulated end organs Glabrous (hairless) skin l Non-neural tissue l Merkel’s Disks l Meissner’s corpuscle l Pacinian corpuscles l Ruffini endings ~ ~

Receptor Adaptation n Different rates of adaptation  response to continued stimulation n Slowly adapting (SA) l steady pattern of firing n Rapidly adapting (RA) l fire only at onset of stimulus ~

Somatosensory Pathways n Touch & Proprioception l Dorsal column-medial lemniscal pathway n Pain and Temperature l Spinothalamic system n Trigeminal pathway l face & neck l cranial nerve V, also others ~

Dorsal Ventral dorsal columns Spinal Cord lateral columns

Somatosensory cortex n S1 - Postcentral Gyrus n Somatotopic Organization l topographic representation of body n Distorted Homunculus l disproportionate amount of cortex for body parts l high sensitivity: large cortical area ~

M1 S1 PPC

Somatosensory Cortex n Topographic representation l Body Image – S1 l Space around us – PPC l Coordinates for movement n Phantom limbs & pain l after amputation n Contralateral neglect l PPC damage ~

Kinesthesia n Body Position & Movement l proprioception n Joint information l Pacinian corpuscles & Ruffini endings n Muscle & tendon information l changes in tension Golgi tendon organ muscle spindle fibers ~

Cutaneous Receptors n Stretching of the skin l Limited role in proprioception l Knee: anesthesia no affect l Mouth, hands, & feet proprioception significantly reduced by anesthesia n Ruffini Endings l slow adapting l population of neurons responding simultaneously ~

Muscle Receptors n Major role in proprioception n Stretch receptors l detect changes in tension n 2 types of receptors l Muscle spindles & Golgi tendon organs l differences in threshold & location ~

n Muscle length detectors l Parallel with extrafusal fibers l Low threshold n Monosynaptic stretch reflex l Postural adjustments l Muscle tonus n Sensory neuron  alpha motor neurons l monosynaptic excitation l disynaptic inhibition ~ Muscle-Spindle Receptors

n Innervated by gamma motor neurons l active concurrently with alpha motor neurons n Control tension on muscle spindles l Not force for movement l “Load the Spindles” ~ Intrafusal Muscle Fibers

Dorsal Ventral + MSMS Gamma Motoneuron +

Golgi Tendon Organ n Gauges muscle tension l high threshold n Stretch receptor l safety mechanism l controlled muscle contraction ~

Dorsal Ventral - + GTO + Inhibits alpha motor neuron

GTO: Function n Inhibits muscle contraction n Control of motor acts l slow contraction as force increases l e.g., holding an egg n Autogenic inhibition l safety mechanism l too much tension  damage ~