Touch and Pain

What produces a graded potential?

A stimulus to the receptor.

What generates an action potential?

If the graded potential is big enough, the receptor generates an action potential.

How can the intensity of a stimulus be coded?

Remember: all or none action potential
1. A single neuron can convey stimulus intensity by changing the frequency of its action potentials
2. Multiple neurons can act in parallel—as the stimulus strengthens, more neurons are recruited. (More neurons firing, more action potentials per second)
3. Different neurons respond to different ranges (range fractionation)

What is coding?

patterns of action potentials in a sensory system

We use the three strategies of coding in

Vision, hearing, and touch

Range fractionation

Different cells have different thresholds for firing, over a range of stimulus intensities.

What does the somatosensory system detect?

Touch and pain

Four touch systems

Pain, touch, vibration, and stretch

Why is skin so sensitive to pain?

Pain receptors are at the surface of the skin.

What are the five touch receptors

Free nerve endings: naked nerves (pain, temperature), merkel’s disc: precise because trains in Germany leave right on time (touch), meissner’s corpuscle: precise because trains in Germany leave right on time (touch), pacinian corpuscle: not precise because trains in Italy leave later (vibration), ruffini’s ending: not precise because trains in Italy leave later (stretch)

Pain uses two types of neurons:

1. Small unmyelinated (slow) C fibers- dull pain
1. Large myelinated (fast) A- delta fibers- sharp pain

CMR1 free nerve endings respond to

low temperature

TRPV1 free nerve endings respond to

Moderate heat

TRP2 free nerve endings respond to

High temperature

Capsaicin

The hot in chilies, activates C fibers via TRVP1 and is neurotoxic. It kills a lot of the nerves, so people who love spice keep eating more spicy food.

Why is capsaicin banned in equestrian sports?

Hypersensitive horses are more motivated to jump hurdles

The dull, long-lasting pain in toe when you hit it against the bedpost in your dark bedroom is due to activation of
A. Small unmyelinated C fibers
B. Small unmyelinated Adelta fibers
C. Large myelinated Adelta fibers
D. Large myelinated C fibers

A. Small unmyelinated C fibers

What is vibration reception

Moving the skin back and forth (stretching it)

What are the two parallel touch systems

Tonic receptors and phasic receptors

Tonic receptors

Slow or no decline in action potential frequency

Phasic receptors

Adapt quickly by decreased frequency

Receptive field

That part of the world a sensory neuron responds to, property of each single sensory neuron, differ in size and shape

Every sensory neuron in the brain has

A part of the body it responds to

Do receptive fields overlap?

No

Touch receptive field: touch in center

Maximal firing

Touch receptive field: Touch in surround

Decreased firing (also part of the receptive field, neuron knows when this part is touched (excitatory center, inhibitory surrounding))

Touch receptive field: Touch far away

No effect on firing

Most sensory pathways pass though the ______ to end in the ________

Thalamus; sensory cortex

Parallel tactile systems

Dorsal column and spinothalamic

Dorsal column

Touch, vibration

Spinothalamic

Pain, temperature, some touch

Cortical map

Represents the innervation of a body region

A small stroke just medial to you ear would cause loss of sensation of your contralateral
A. Leg
B. Face
C. Arm
D. Genitals

B. Face

What does the somatosensory cortex do after hand amputation?

It reorganizes

Pain perception

Free nerve endings (in relation to pain)

In the skin, have specialized receptors to respond to temperature change, chemicals, and tissue injury (especially substance P- a neurotransmitter)

Congenital insensitivity to pain

Inherited mutated sodium channel in pain fibers in dorsal root ganglia (if the sodium channels in the C fiber don’t work, C fiber cells cannot fire an action potential, and you can’t sense pain)

Man on fire syndrome

Congenital hypersensitivity to pain, severe burning pain

How is pain life saving?

Low levels of pain during everyday tasks tell us when a certain movement or prolonged posture is harmful. (Even during sleep, mild pain makes us toss and turn enough to prevent bedsores or skeletal strain)

Periaqueductal gray

Part of spinothalamic system, located in the midbrain, involved in pain perception; opioid receptors

The major part of the pain pathway that contributes emotion to pain sensation is
A. Periaqueductal gray
B. Spinothalamic tract
C. Somatosensory cortex
D. Cingulate gyrus

D. Cingulate gyrus

Will (cognitive)

Frontal cortex

Emotion (motivation)

Anterior cingulate cortex

Sensation (sensory)

Somatosensory cortex

Pain can be blocked in

Spinal cord

Gate control theory of pain

Gate can be closed by increased activity in A fibers

A-beta fibers produce

Touch sensation

What other factors influence the gate of pain?

Attention and emotional/cognitive factors

What is transcutaneous electrical nerve stimulation (TENS)

It delivers electrical pulses to the skin, and it is low intensity, so it activates the non pain pathways

Non-drug treatments for pain

- Meditation/prayer
- acupuncture
- massage
- physical activity

Chronic pain

Persists even long after an injury has healed and the pain and disability can worsen over time (mostly a disorder in young women)

Reflex sympathetic dystrophy stage 1

- onset of severe pain limited to injury site
- increased sensitivity of skin to touch
- localized swelling
- usually lasts a few weeks then subsides

Reflex sympathetic dystrophy stage 2

- pain becomes more severe and more diffuse
- swelling tends to spread
- hair and nails become brittle
- muscle wasting begins

Vicious cycle of neurogenic chronic pain (RSD)

What surgical treatment would best help a patient with RSD (neurogenic chronic pain)?
A. Cutting the peripheral nerve from the injured area
B. Cutting the sympathetic nerve to the injured area
C. Cutting the spinal cord above the site of injury

B. Cutting the sympathetic nerve to the injured area
Cutting the peripheral nerve would result in you not being able to feel ANYTHING

Neurogenic pain is due to

Inappropriate signaling of pain by neurons

Postsynaptic spinal cord neurons

Take up substance P, released during pain, and remodel their dendrites

Overactivity of sympathetic input

Dorsal horn neurons (light touch) become hyperexcitable (light touch is now felt as pain); GABA inhibitory become excitatory

Chronic pain remodels

The spinal cord and the sensory cortex

Patients in chronic pain have

Overactive somatosensory cortex even at rest

Pain becomes emotional in

Cingulate cortex (ACC)

As pain unpleasantness increases

Activity increases in the cingulate cortex, but not in the somatosensory cortex

Painful heat stimulates

Somatosensory cortex and anterior cingulate cortex

Hypersensitivity to touch

Stimulus avoiders
- hates having face washed or hail or nails cut
- picky eater, only eats certain textures
- refuses to walk barefoot on grass or sand; toe walks

Hyposensitivity to touch

Stimulus seekers
- craves touch, touches everything/everyone
- hurts other children or pets while playing
- many pinch, bite, or bang own head

Synesthesia

A stimulus in one modality creates a sensation in another (this food tastes blue)
Music —> colors
Graphemes (numbered and letters)—>colors.
Taste —> shape
Speech sound —> taste

During synesthesia
A. The prefrontal cortex shuts down
B. Brain areas talk to fewer other areas than normal
C. The amygdala is turned off
D. None of the above

D. None of the above

Causes of synesthesia

- LSD
- posterior temporal lobe seizure
- result of blindness or deafness (visual cells rewire to auditory cell when blind)
- occurs spontaneously

phantom limb pain

pain felt by an amputee that seems to be located in the missing limb