Touch and Pain

sensory transduction

translation to language the brain can understand - electrical

receptive field

windows through which neurons sense the world

place code

topographical maps — how the brain represents sensory inputs

parallel pathways

processing multiple aspect of input in different pathways

cutaneous senses

provide info from surface of body

proprioception

perception of body position and posture

• submodality of cutaneous sense

• provide info about location of body in space

kinesthesia

submodality of cutaneous sense; provide info about movement of body through space

organic senses

provide info from, in, and around internal organs

cutaneous senses stimuli

• pressure

• vibration

• heating

• cooling

• tissue damage (pain)

coding touch

receptive fields — basis of a place code

• the locations on the skin to which a somatosensory neuron responds to

coding touch ex.

put recording device in somatosensory neuron, and touch diff parts of the body, trying to see which body part will get the neuron to fire

receptive field — resolution of the senses

the two point threshold for any part of the body is determined by the size of the receptive fields and the extent of overlap

• taking a paper clip and bending to make two points that are close together; will feel the two points on the fingers, but not on the shoulder

sensory transduction

mechanoreceptors

mechanoreceptors

neurons that convert mechanical stimulus into ‘receptor potentials’; if this crosses threshold, the neurons will fire

• mechanically gated ion channels (mechanical stimulation opens the channel, causing a change in membrane potential)

• detect stimuli that cause vibration or changes in pressure (tactile stimuli) [also detected by free nerve endings]

four types of somatosensory receptors/mechanoreceptors

• ruffini corpuscles

• pacinian corpuscles

• meissner’s corpuscles

• merkel’s disks

ruffini corpuscles

• vibration sensitive, large receptive fields, slow adapting

• large, diffuse borders (receptive)

• detection of static force against skin; skin stretching; proprioception (function)

pacinian corpuscles

• detects mechanical stimuli, especially vibrations, large receptive fields, fast adapting

• large, diffuse borders (receptive fields)

• detection of vibration; information from end of elongated object being held, such as a tool (function)

meissner’s corpuscles

• touch sensitive, small receptive fields, fast adapting

• small, sharp borders (receptive fields)

• detection of edge contours; braille like stimuli, especially by fingertips (function)

merkel’s disks

• touch sensitive, small receptive fields, slow adapting

• small, sharp borders (receptive field)

• detection of form and roughness, especially by fingertips (function)

glabrous skin

hairless skin of palms and soles of feet

• contains dense complex mixture of receptors (reflecting how we use palms and inside of fingers to explore world)

skin consists of…

subcutaneous tissue, dermis, epidermis, receptors

mechanism of mechanoreceptors

movement of dendrites of mechanoreceptors causes ion channels to open, and the flow of ions into or out of the dendrite causes a change in the membrane potential

location of receptor for merkel’s disk

hairy and glabrous skin

location of receptor for ruffini corpuscles

hairy and glabrous skin

location of receptor for meissner’s corpuscles

glabrous skin

location of receptor for pacinian corpuscles

hairy and glabrous skin

location of receptor for hair follicle ending

base of hair follicle

location of receptor for free nerve ending

hairy and glabrous skin

function of receptor of hair follicle ending

detection of movement of hair

function of receptor of free nerve ending

detection of thermal stimuli, noxious stimuli (pain), tickling, pleasurable touch from gentle stroking with a soft object

pacinian corpuscle characteristics

• hairy and glabrous skin

• bare (no myelin) nerve ending in skin

• touch receptor that detects sudden displacement or high frequency vibrations on the skin (rapidly adapting)

• mechanical pressure bends the membrane

• increases the flow of sodium ions and triggers the receptor potential

meissner’s corpuscle characteristics

• found typically in glabrous skin

• bare nerve ending in skin

• sensitive to light touch and low frequency vibrations

• rapidly adapting (action potentials will stop if touch is continued)

• concentrated in places highly sensitive to touch (lips, fingertips, genitals)

• age related loss

ruffini endings

• slowly adapting

• detected sustained pressure

• glabrous skin (skin with no hair)

• sensitive to skin stretch (ex. joint movement in fingers)

• useful in determining grip on objects

a-beta fibers

• thick myelinated fibers

• fast conducting

• take in information about painless touch into the spinal cord

a-alpha fibers

very thick myelinated fibers for taking in proprioceptive information

fibers for touch information sent to the spinal cord

• a beta fibers

• a alpha fibers

fibers for signaling pain

• a delta

• c-fibers

pain receptors

• nociceptors/nociception

• located all over body

• free nerve endings

• have a high threshold? so need a big stimulus to trigger an action potential

a-delta

• thin myelinated fibers

• acute pain

• ‘good’ pain

‘good’ pain

• ex. sharp object

• tells you where the pain is happening so you can remove your body part and stop the pain

c fibers

• unmyelinated

• dull pain

• ‘bad’ pain

‘bad’ pain

• ex. tissue damage

• pain becomes dull and spreads away from site of injury, hard to tell where exactly the pain is

where are cold receptors located

cold sensors located near skin surface

where are warm sensors located

deeper in skin

what channel do heat and cold receptors open

Na channels

what are the heat and cold receptors

• TRPV4

• TRPV3

• TRPV1

• TRPV1

• TRPV2

• TRPM8

• TRPA1

TRPV4 temperature

warm (~27-34C)

TRPV3 temperature

warmer (~31-39C)

TRPV1 temperature

Hot (greater than or equal to 43C), also activated by capsaicin

• pain produced by burning of the skin, changes in the acid/base balance within the skin

• role in regulation of body temperature

TRPV2 temperature

painfully hot (>52C)

TRPM8 temperature

moderately cold (<25C) [menthol channel]

TRPA1 temperature

<18C

explain the touch parallel pathway

• touch, vibration, two point discrimination, proprioception

• from [right] dorsal root axon (a-alpha, a-beta, a-delta) across dorsal column in spinal cord, to dorsal column nuclei in medulla, to [left] thalamus and to cerebral cortex

• dorsal column-medial lemniscal pathway

explain the pain parallel pathway

• spinothalamic pathway

• from [right] dorsal root axon (A-delta, C), across [left] lateral spinothalamic tract in spinal cord to thalamus, and finally cerebral cortex

• simplified: dorsal root axon —> thalamus —> cereberal cortex

how do we know the location of touch

a place code, cortical magnification; homunculus

3 key neurotransmitters in pain processing

• substance o

• glutamate

• glycine

substance P

• released by c fibers

• signals dull (bad) pain

• more sub p = more pain

what blocks sub p receptors

opiates, morphine

• work because similar chemicals are present in the body (enkephalins and endorphins)

glutamate

released by a-delta, signals acute (good) pain

glycine

an inhibitory neurotransmitter in the SC that reduces activity from Sub P and glutamate

what are two other major regions of opiod mu receptor binding

1. VTA dopamine neurons — addiction aspects

2. NA neurons of locus coeruleus — physiological withdrawal symptoms

thermal stimulation and relativity

• neutral point depends on prior history of thermal stimulation

• increases in temp lower sensitivity of warmth receptors and raise sensitivity of cold receptors, and vice versa

how are pain perception and thermoreception accomplished

networks of free nerve endings in the skin

three types of pain receptors

• high-threshold mechanoreceptors

• free nerve ending that responds to extreme heat, acid, and capsaicin

• fiber containing TRPA1 receptors

high threshold mechanoreceptors

free nerve endings that respond to intense pressure (ex. striking, stretching, pincing)

fiber containing TRPA1 receptors

sensitive to pungent irritants and environmental irritants, provides information about the presence of chemicals that produce inflammation

how do somatosensory axons from the skin, muscles, or internal organs enter CNS

via the spinal nerves

Three primary components of pain perception

• sensory component

• Immediate emotional component

• Long term emotional component

sensory component

pure perception of the intensity of a painful stimulus

sensory component mediated by

mediated by pathway from the spinal cord to ventral posterolateral thalamus to the primary and secondary somatosensory cortex

immediate emotional components of pain

unpleasantness or degree to which the individual is bother by the painful stimulus

immediate emotional components of pain mediated by

pathways that reach the anterior cingulate cortex (ACC) and insular cortex

long term emotional implications of chronic pain

the threat that such pain represents to one’s future comfort and well being

long term emotional implications of chronic pain mediated by

pathways that reach the prefrontal cortex

ACC

• important in emotions of pain

• Activated by stimuli associated with pain (without the stimulus itself) ex. Words indicating pain

• Damage to this results in loss of empathetic reactions to pain

Insula

• stimulation causes sensation of pain without any stimulus (painful burning and stinging)

• Damage to this causes people to still feel pain but not think it’s harmful (so, not avoiding pain)

Romantic couple study

ACC and insula activated when a person watched their romantic partner experience a painful stimulus (amount of activation correlates with how long they have been together)

Hypnosis and pain

Hypnotizing patients and asking them to place their hand in ice caused no change in the somatosensory cortex, but a reduction in ACC activity (reduction in perceived pain)

somatosensory cortex, ACC, and pain

primary somatosensory cortex involved in the perception of pain, and the ACC involved in its immediate emotional effects (unpleasantness)

what in the brain causes phantom limb association

organization of the parietal cortex (which is involved in the awareness of our own bodies)

• people with lesions in right hemisphere of parietal cortex push their own leg out of bed, believing it to be someone elses

action card??? go to slide 24 and draw the diagram

pain and suffering :’)