4 - Pain and Temperature Sensation: Anterolateral System

Question 1

What is pain?

Answer 1

An unpleasant SENSORY and EMOTIONAL experience associated with actual or potential tissue damage.

Highly based off of a person’s perceptions and a person’s abstract interpretation of the painful stimulus.

Question 2

Why might two people experience the same painful stimulus differently?

Answer 2

Genetic background: different receptor densities, nociceptor thresholds, density of innervation, pain pathway projections, descending control, or CNS modulation.

Different past experiences, cultures, mental status, anxiety, fear.

Question 3

What is anesthesia? What is analgesia?

Answer 3

Anesthesia: lack of all sensation (light touch, proprioception, pain, temp).

Analgesia: lack of pain

Question 4

What is Athermia? And what is hypoalgesia?

Answer 4

Athermia: lak of thermal sensation

Hypoalgesia: decreased sensitivity to pain

Question 5

What is hyperesthesia? What is Paresthesia?

Answer 5

Hyperesthesia: heightened sensitivity to any stimulus

Parenthesia: unpleasant, abnormal sensation: tingling, pricking, numbing, stinging, “pins and needles”

Question 6

What is pruritus?

Answer 6

Itching

Question 7

What is hyperalgesia?

Answer 7

Increased pain from normally painful stimulus

Question 8

What is allodynia?

Answer 8

Pain from normally non-painful stimulus.

Question 9

What is the purpose of acute pain? What would happen without it?

Answer 9

Protective function; warns that injury should be avoided and/or treated. Critical for daily protection and survival.

Without it: congenital insensitivity to pain resulting in multiple continuous injuries that severely shortens life span.

Question 10

What are some characteristics of chronic pain?

Answer 10

Pain that doesn’t go away or adapt and continues after complete healing.

It’s neuropathic and occurs in the absence of any obvious injury.

Serves no useful purpose.

Question 11

What can inflammation of sort tissues cause? What type of receptors respond?

Answer 11

Activation of nociceptor terminals in skin:

• inflamm chemicals released from blood stream, immune cells.
• chemicals activate receptors on free nerve endings of C fibers
• sensitive nociceptors have a lower threshold and respond more.

Question 12

What are examples of mild and severe inflammation? How can inflammation be treated?

Answer 12

Mild: infection, rash

Severe: rheumatoid arthritis, gout, tumor in soft tissues.

NSAIDs reduce inflammatory pain, opioids also effective.

Question 13

What is neuropathic pain? What does it feel like? Can NSAIDs or opioids treat this?

Answer 13

When there’s direct damage to NERVES in the PNS or CNS from cut, compression, loss of blood supply or oxygen)

Burning, electrical quality; allodynia to light touch is common.

This pain is resistant to NSAIDs and opioids.

Question 14

What is involved in the pain pathway?

Answer 14

The anterolateral system: a combo of several ascending tracts or fibers that convey info about pain and temp to the cortex.

Question 15

What can damage to the anterolateral pathways cause?

Answer 15

Loss of pain and temp sensation BLOW the lesion (but fine discrimination, vibration, and joint position will be ok).

Question 16

What can provide input to the anterolateral system?

Answer 16

Noxious mechanical, thermal, or chemical stim activates AP in free nerve endings of Adelta or C-fiber nociceptors in the skin (muscles, joints, bones too).

Non-painful cooling and warming temperature stimuli/.

Question 17

What mediates pain first? What mediates pain second?

Answer 17

1st: Adelta fibers have a fast, sharp, pricking, short-lasting, protective response that allows you to escape damage.
2nd: C-fibers are delayed, with burning quality, long-lasting, chronic.

Question 18

Where do the central processes of nociceptors go? What happens when they synapse?

Answer 18

They enter the lateral dorsal horn via the dorsal lateral tract of lissauer and synapse on spinal cord neurons in the superficial dorsal horn at lamina I/II (substantia gelatinosa) or V

Synapse: they release Glu and substance P which activates receptors on spinal neurons.

Question 19

What is the action of second order spinal neurons in the anterolateral system?

Answer 19

Send axons across the the CONTRALATERAL side of the spinal cord within 2/3 segments rostral.

Ascend the anterolateral tracts.

Question 20

Lesion of the spinal cord affecting the 2nd order anterolateral system?

Answer 20

Loss of pain and temperature below the lesion on the CONTRALATERAL side.

Loss is complete by 2-3 segments below the lesion because some axons take longer to cross than others.

Question 21

What occurs if theres an anterolateral tract lesion on 1 side? What about if there’s a lesion on 2 sideS?

Answer 21

1 side: lose pain and temp on the CONTRALATERAL side

2 sides: lose pain and temp bilaterally (anterior cord syndrome)

*if dorsal column is spared fine touch and joint position will be intact.

Question 22

What is central cord syndrome? What are some causes? What deficits can occur?

Answer 22

A hole in the center of the spinal cord that can be caused by syringomyelia: a cavity or cyst in the center.

Cuts the crossing axons of the 2nd order neuron going into the anterolateral tract.

Bilateral loss of pain and temp.

Small hole may spare dorsal columns and anterolateral tracts. Affects only dermatomes where the lesions exist.

Question 23

What are the three major anterolateral pathways? Where do they begin and end?

Answer 23

All begin in the spinal cord.

1. Spinothalamic: terminates in thalamus
2. Spinoreticular: terminates in reticular formation (in medulla and pons)
3. Spinomesencephalic tract: terminates in mesencephalon (midbrain).

Question 24

What is the function of the spinothalamic tract? What does this pathway sense? What are the two nuclei in which the axons from the body terminate?

Answer 24

Most prominent pain pathway that mediates discriminative aspects of pain and temp.

Location, intensity, and duration of noxious stimulus to DC/ML.

VPL nucleus and the central lateral nucleus in the thalamus.

Question 25

What is the function of the VPL in the spinothalamic tract?

Answer 25

Receives pain info from body and sends 3rd order axons to the S1 cortex.

Principle relay nucleus for discriminative pain info from body. Localizes WHERE stimulus occurred and how intense they are.

Question 26

Input from what two tracts go to the VPL? What is an important distinction between the two? How is the VPL organized?

Answer 26

Spinothalamic tract and dorsal column/medial lemniscus inputs to there but they synapse on different neurons.

VPL organized somatotopically: neck and arm is more medial and leg and sacral regions are more lateral.

Question 27

Where do neurons of the spinothalamic tract that synapse in the central lateral nucleus send their 3rd order axons? What is the function of this nucleus? Is it somatotopically organized?

Answer 27

To many areas of the cortex and limbic cortex (cingulate gyrus, hippocampus, amygdala).

Involved in emotional suffering during chronic pain and memory of painful events.

CL is NOT somatotopically organized.

Question 28

What are the functions of the thalamus in pain?

Answer 28

1. Process nociceptor info and begin crude pain, temp sensation, and emotional suffering reactions.
2. Relay info to SI cortex via 3rd order neurons that pass through the posterior limb of the internal capsule and corona radiata.

Question 29

Where do many 2nd order neurons of the spinoreticular tract terminate? What is the function of this location?

Answer 29

In the reticular formation in medulla or pons. (others to thalamus and cortex).

Retic formation processing mediates changes in level of attention to painful stim.

Involved in emotional, arousal, attention, and affective response to noxious stim.

Question 30

Where do 2nd order neurons of the spinomesencephalic tract terminate? What is the funciton of this tract?

Answer 30

Some terminate in the midbrain in the superior colliculus and periaqueductal gray (PAY).

Other neurons in PAG send axons back down cord for descending control to inhibit pain signals coming up.

Question 31

What do all three anterolateral tract pathways have in common? What else does the anterolateral pathway carry?

Answer 31

All 3 pathways are adjacent to each other in the spinal cord.

Anterolateral pathway also carries some crude touch sensation.

Question 32

What is the first place that thalamic neurons from the VPL protect to? What is the function of this?

Answer 32

Somatosensory cortex: SI areas 3b, 1, 2, and then SII cortex.

Helps localize painful stimulus.

Question 33

What is the second place that thalamic neurons project to? What is the purpose of this?

Answer 33

Cingulate gyrus: part of the limbic system.

Processes emotional component of pain: fear, anxiety, depression, angre, attention.

Question 34

What is the third place that thalamic neurons project to? What is the purpose of this?

Answer 34

Insular cortex, which processes info on internal autonomic state of the body.

Also INTEGRATES discriminative affective, emotional, and cognitive components of pain.

Question 35

What can result from lesions to the insular cortex?

Answer 35

“asymbolia for pain”

Patients perceive stimulus as noxious but don’t care. Their emotional response to pain is inappropriate.

Question 36

What are two ways that descending pathways inhibit pain?

Answer 36

Activation by the spinomesencephalic tract and influenced by exogenous opioids.

Question 37

Neurons within the cell bodies of the periaquaductal gray send axons where? Describe this pathway and the result.

Answer 37

To the Raphe nuclei in the medulla and others to the locus ceruleus in the pons.

These send axons to the spinal cord to synapse on inhibitory interneurons or spinothalamic tract neurons.

Suppresses transmission of ascending noxious info to thalamus and cortex.

Question 38

Why do we experience referred pain?

Answer 38

Visceral nociceptors and somatic nociceptors converge and synapse onto the same 2nd order spinothalamis tract neurons.

Brain interprets spinothalamic tract impulses as pain from somatic tissue.

Question 39

What’s a mneumonic to remember the function of the spinothalamis tract, spinoreticular tract, and the spinomesenphalic tract?

Answer 39

Spinothalamic: discrimination-“something sharp is puncturing my right heel”

Spinoreticular: attention, arousal, affect- “ouch! that hurts”

Spinomesencephalic: endogenous pain relief - “ah that feels better”