Somatosensation - Pain & Temperature

What are the 3 subsystems of the somatosensory system?

• fine touch, vibration, pressure

• proprioception

• pain, temperature, non-disciminative affective touch

What 3 receptors are capable of conveying somatosensory info?

• mechanoreceptors

• nociceptors

• thermoreceptors

Where are nociceptors located?

the skin

Where are mechanoreceptors located?

the skin

Where are proprioceptors located?

muscles, tendons, joints

What do mechanoreceptors detect?

changes in pressure or movement

What do nociceptors detect?

painful stimuli

What do thermoreceptors detect?

changes in temperature

What is a nociceptor also known as?

free nerve ending

What is nociception?

the sensory process that gives signals to trigger pain

What is pain?

the perception of uncomfortable sensations following nociceptor activation (top-down processing)

What two general brain structures will pain sensory info go to?

• somatosensory cortex

• cortical parts linked with emotional processing

3 reasons why pain also has an emotional component?

• sensation alone may not keep you away from the stimulus

• emotional processing helps you remember the pain

Why is it good to feel pain?

keep body tissue protected

What 3 factors influence the subjective feeling of pain?

• biological factors

• psychological factors

• social factors

What are 3 reasons why pain is categorized based on duration?

• better communication between doctor and patient

• better quantification of pain in a lab setting

• medical malpractice suits

How long does acute pain last?

3 or 4 weeks

How long does persistent/subacute pain last?

6 weeks to 3 months

How long does chronic pain last?

more than 3 months

Which type of pain can be helpful for making clinical diagnoses?

persistent/subacute

Which type of pain is not useful for diagnosing and just causes misery?

chronic pain

What is nociceptive pain?

injury to tissue (caused by dammage/inflammation)

What is neuropathic pain?

injury to nerves

What does neuropathic pain feel like?

burning or electric sensation

As the temperature goes from cool to warm, how does AP firing rate change in a nociceptor?

• cool temp = no firing

• transition from cool to warm = steadily increasing APs

• very hot temp = exponentially more rapid APs

As the temperature goes from cool to warm, how does AP firing rate change in a thermoreceptor?

• cool temp = some firing

• transition from cool to warm = steadily increasing APs

• very hot temp = ff has plateaued

Nociceptors ________ __ ____ exponentially at super high temperatures because high temperatures are painful.

continue to fire

Once the _________ temperature is reached, the thermoreceptor ______ out its ff and no more info about heat is conveyed.

threshold, maxxes

What are the 5 ways in which stimulation can cause tissue damage?

• mechanical stimulation

• temperature extremes

• oxygen deprivation of tissue

• noxious chemicals released into tissue

• damaged cells release things

What type of stimulation that causes tissue damage is a crush, cut, or puncture?

mechanical stimulation

What type of stimulation that causes tissue damage is anaerobic metabolism during long distance running?

oxygen deprivation

What type of stimulation that causes tissue damage is a bee sting?

damaged cells releasing molecules

What 4 molecules do damaged cells release that signal tissue damage?

• proteases

• ATP

• K+ ions

• histamine

What does stimulation that causes tissue damage lead to?

inflammation soup

What is the pathway of inflammation soup?

• release of NTs, peptides, lipids, proteases, neurotrophins, cytokines, chemokines, ions, etc.

• —> inflammation

• immune cells show up and process signals

• macrophage releases bradykinin, cytokines

• mast cell releases histamine

• substance P and CGRP (neuropeptides) released by sensory afferents into blood vessels

• —> nociceptor channels activated by all of these stimuli

What do damaged cells release to trigger inflammation?

NTs, peptides, lipids, proteases, neurotrophins, cytokines, chemokines, ions

What do macrophages release?

bradykinin and cytokines

What do mast cells release?

histamine

What’s released by sensory afferents into the blood vessel?

substance P and CGRP (neuropeptides)

What are substance P and CGRP?

neuropeptides

Nociceptor channels are activated by the stimuli from which cells?

damaged cells, macrophages, mast cells

What are the 5 types of nociceptors?

• polymodal

• silent

• mechanical

• thermal

• chemical

What do polymodal nociceptors detect?

multiple types of stimuli

What do silent nociceptors detect?

tissue damage

Silent receptors have a ____ threshold and are typically ________.

high, inactive

Which type of nociceptor is involved in chronic pain?

silent receptor

What do mechanical nociceptors detect?

strong pressure, sharp objects

What do thermal nociceptors detect?

extreme temps (burning heat, noxious cold)

What do chemical nociceptors detect?

pH extremes, environmental irritants, neuroactive substances

Oftentimes when detecting pain, are one or more nociceptors active at a time?

more than one

What is the only part of the body that doesn’t have nociceptors?

brain

Do the meninges have nociceptors?

yes

What are the 4 different types of ion channels on nociceptors?

• TRPV1

• TRPM8

• NaVs

• GPCRs

What does TRPV1 detect?

heat pain

What 3 things stimulate TRPV1?

• capsaicin

• high temperatures

• H+ ions (acidic substances)

What does TRPM8 detect?

cold hypersensitivity

What stimulates TRPM8?

menthol

Why does capsaicin stimulate TRPV1?

• capsaicin is hydrophobic

• diffuses across membrane

• binds to inside of TRPV1

• opens ion channel

• influx of Ca2+/Na+

• depolarization

What happens when TRPV1 ion channel opens?

• Na+ and Ca2+ influx

• depolarization

Which fiber carries fast, sharp pain?

Aδ fiber

Which fiber carries slow, dull pain?

C fiber

Which fiber carries the first pain?

Aδ fiber

Which fiber carries the second pain?

C fiber

Are Aδ fibers myelinated?

lightly myelinated

Are C fibers myelinated?

no

What does the first pain feel like?

fast, sharp

What does the second pain feel like?

slow, dull

Thermoreceptos can express _____ type of channel at at time.

one

Different parts of the skin are sensitive to different temperatures because…

each TRP is responsive to a different temperature range, and only one TRP can be expressed at a time.

Which TRP receptors are active in lower temperatures?

• TRPA1

• TRPM8

Which TRP receptors are active in higher temperatures?

• TRPV4

• TRPV3

• TRPV1

• TRPV2

Which type of receptor processes extreme cold?

TRPA1

Which type of receptor processes extreme cold all the way up to room temp?

TRPM8

As the temperature goes from warm to cool, how does AP firing rate change in a cool receptor?

• warm temp = slow firing

• transition from warm to cool = very rapid APs

• stable cool temp = steady, fast APs

As the temperature goes from cool to warm, how does AP firing rate change in a warm receptor?

• cool temp = slow firing

• transition from cool to warm = very rapid APs

• stable warm temp = steady, fast AP

Where are the cell bodies for Aδ and C fibers?

in the DRG

What type of fibers do cool receptors use?

Aδ and C fibers

What type of fibers do warm receptors use?

C fibers

Describe the 4 neurons of the spinothalamic pathway.

• 1st order neuron = terminates in spinal cord, decussation, ascends via anterolateral system

• 2nd order neuron = terminates in VPL of thalamus

• 3rd order neuron = to somatosensory cortex

• 4th order neuron = within somatosensory cortex

What is referred pain?

visceral nociceptor activation is perceived as cutaneous sensation

Why does referred pain happen?

pain fibers from the viscera enter the spinal cord via the same route as cutaneous nociceptors

Due to referred pain, when you’re having a heart attack, you might feel pain in your…

left arm

What info does the dorsal column-medial lemniscal pathway transmit?

tactile and proprioceptive info

Touch sensation ascends on the ______ side as the stimulus.

same

Where does decussation occur in the dorsal column-medial leminiscal pathway?

medulla

What info does the spinothalamic tract transmit?

pain and temperature

Pain sensation ascends on the ______ side as the stimulus.

opposite

Where does decussation occur in the spinothalamic tract?

spinal cord

What is the spinothalamic tract also known as?

anterolateral system

If the left side of the spinal cord was lesioned…

• R leg can’t feel pain

• L leg can’t feel touch

Why might lesioning the spinal cord cause reduced sensation instead of a complete loss?

• extent of spinal cord damage (if it ain’t too severe, you can still feel a lil)

• some fibers may not cross over

What are the two distinct aspects of the experience of pain?

• discriminative/sensory component

• affective/motivational component

Pathway of discriminative/sensory pain:

• anterolateral system

• VPL

• somatosensory cortex

Pathway of affective/motivational pain:

• anterolateral system

• areas within the brainstem and brain

What areas within the brain and brainstem are involved with emotional processing?

• brainstem

  • reticular formation

  • periaqueductal gray (PAG)

  • superior colliculus

• brain

  • medial thalamic nuclei

  • hypothalamus

  • amygdala

  • anterior cingulate cortex

  • insula

Where is the anterior cingulate cortex located?

parallel to and above the corpus callosum

The same level of nociceptor activation might produce more or less pain depending on…

other non-painful sensory input that enters the CNS with the painful input

What is ascending regulation of pain?

pain modulation occurs as nociceptive signals enter the spinal cord

What is descending regulation of pain?

higher brain regions project down to influence/mediate nociception

What are the 4 neurons involved in the gate control pathway?

• mechanoreceptive fiber

• nociceptive fiber

• inhibitory interneuron

• projection neuron