PSYC-2900 Chapter 7.2

somatosenses

senses that provide info about body’s surface and inside. includes:

• cutaneous sense, proprioception & kinesthesia, and organic senses

cutaneous senses

a type of somatosense that detects touch, temperature, and pain. receptors in the skin respond to different stimuli:

• pressure, vibration, temperature, nociception (pain)

proprioception and kinesthesia

a type of somatosense that detects body position and movement

• skeletal muscles contain stretch receptors that respond to changes in muscle length

• receptors in joints respond to limb movement

• receptors that respond to skin stretching during joint/muscle movement

organic senses

a type of somatosense that detects visceral sensory info and internal organs

• aware of: stomach ache, heart burn, gall bladder attacks, uterine cramps

• unaware of: nutrients & minerals within digestive system

skin

contains 3 layers

• epidermis (outer layer)

• dermis

• subcutaneous tissue

different types present at different surfaces of the body

• glabrous

• hairy

• mucous membranes

receptors are scattered throughout

glabrous skin

a type of skin found on the palms of hands and soles of feet. it has the highest density of receptors and no hair

hairy skin

a type of exposed skin found on most of the body

mucous membrane

a type of skin found on the inside of the nose

sensing touch

4 types of mechanoreceptors sensitive to vibration or changes in pressure convey info:

• Ruffini corpuscles, Merkel’s disks, Meissner’s corpuscle, Pacinian corpuscle

Merkel’s disks

a type of mechanoreceptor

• small, sharp borders

• found in hair & glabrous skin

• detects form & roughness, especially by fingertips

Ruffini corpuscles

a type of mechanoreceptor

• large, diffuse borders

• found in hair & glabrous skin

• detects static force, skin stretching, proprioception

Meissner’s corpuscles

a type of mechanoreceptor

• small, sharp borders

• found in glabrous skin

• detects edge contours, Braille-like stimuli, especially by fingertips

Pacinian corpuscles

a type of mechanoreceptor

• large, diffuse borders

• found in hair & glabrous skin

• detects vibration, info from the end of a long object being held (e.g., tool, pencil)

sensing temperature

• detected as relative change rather than absolute (except at extremes)

• neutral point depends on history

• detected by 2 types of free nerve endings: cold & warm

cold nerve endings

a type of temperature-detecting free nerve ending, located more superficially

• TRPM8, menthol, detects temperature below 28C

warm nerve endings

a type of temperature-detecting free nerve ending, located more deeply

• TRPV1, capsaicin: detects heat above 43C

• TRPV2: detects noxious heat above 52C

• TRPV3: detects warmth above 31C

• TRPV4: detects warmth above 25C

nociception

describes the perceptual experience that occurs in the brain. sensory signals produce the experience of pain and trigger escape/withdrawal responses. 3 types of pain-detecting free nerve endings:

• high-threshold mechanoreceptors: respond to intense pressure

• TRPV1: respond to heat, acids, and capsaicin

• TRPA1: responds to pungent irritants that produce inflammation

sensing itch

a sensation caused by skin irritation. (e.g., histamine, chemical irritants, allergies) scratching inhibits spinothalamic tract activity (overlap with pain pathways)

• opiates used to treat pain can stimulate

• naloxone blocks opiate receptors, reducing sensation

somatosensory pathways

receptors → (action potential) → axons of primary sensory neurons (skin, muscle, organ)

• limbs & trunk → spinal nerves

• face & head → trigeminal nerve (CN V)

cell bodies located in dorsal root ganglion or cranial nerve ganglion

ascending somatosensory pathways

differ depending on modality

• touch → dorsal root columns → dorsal column nuclei (medulla) → (decussate) → medial lemniscus → ventral posterior nuclei (thalamus) → primary somatosensory cortex

nociceptive/temperature somatosensory pathways

signals synapse in dorsal horn, axon on secondary neuron → (crosses) → spinothalamic tract → ventral posterior nuclei (thalamus) → primary sensory cortex

somatosensory cortex

• highly organised in mapping and column organisation

  • neurons in columns respond to particular stimuli

  • columnar organisation in this cortex discovered before visual & auditory cortex

  • primary and secondary cortices mapped into 5-10 maps of body surface relating to use of inputs

tactile agnosia

a deficit in recognition based on touch due to damage to the somatosensory association cortex

strains of pain

serve a constructive role

• protective function: inflammation increases sensitivity of region following injury

multidimensional experience having different perceptual & behavioural effects with different neural mechanisms

• sensory component: intensity

• immediate emotional component: unpleasantness

• long term emotional component with chronic pain

pain perception

involves the primary somatosensory cortex

• anterior cingulate cortex (ACC) involved in immediate emotional effects

• burning/stinging sensation from stimulation of ACC and insula, damage decreases emotional response

• prefrontal cortex involved in emotional consequences of chronic pain

emotional consequences of chronic pain

appears to involve the prefrontal cortex

• damage to prefrontal cortex associated with deficits in planning, recognition

phantom limb pain

chronic pain resulting from activity in the axons of an amputated or absent limb. the cut ends of nerves form neuromas

• association with parietal lobe explains sensation from missing limbs at birth