Lec. 3 (pain anatomy + physiology)

Question 1

Descartes’ View of Pain

Answer 1

Descartes was the first person that said that there’s a pain pathway that goes from the body to the brain. He compared it to a thread (for example, particles of fire would open the pore of the skin and pull on this thread, which would inform the brain that is at the other side of the thread).

Question 2

Pain anatomy (most important structures)

Answer 2

Pain-relevant loci (for pain below the neck):
* skin/muscle/joint/viscera, except the brain (“periphery”, meaning not the nervous system)
* dorsal root ganglion (DRG)
* dorsal horn of the spinal cord
* brain
Ascending pathways (the “pain matrix”):
* thalamus
* somatosensory cortex
* limbic cortex
* prefrontal cortex
Descending pathways (not motor):
* hypothalamus
* midbrain
* brainstem
* spinal cord

Question 3

Why do we have descending pathways for pain?

Answer 3

The descending pathways are the ones that go from higher to lower areas of the brain, to modulate the pain sensation. For example, with stress-induced analgesia (stress comes from higher brain regions). Ascending and descending pathways meet in the spinal cord (that’s why there’s more pain research focused on the spinal cord than anywhere else).

Question 4

Dorsal vs. ventral pathways in spinal cord

Answer 4

Sensory information (including pain) ascends through the dorsal column of the spinal cord. The information crosses over to the contralateral side as it ascends to the thalamus. Then, the information goes to primary somatosensory cortex in the parietal lobe.
The ventral pathway is reserved to motor information.

Question 5

Skin anatomy

Answer 5

There are 2 types of skin: hairy and glabrous (palm of hand and sole of feet). Both are innervated. Some of the nerve have free endings - those are the ones that encode pain. Sometimes, there are structures that form around the nerve endings (those give the nerves special properties, ex. they encode vibrations vs. stretch). Most of the pain research focuses on the skin, because it’s what easiest to study (compared to joints/organs).

Question 6

Nociceptors

Answer 6

Free nerve endings, sensory receptors for pain. There are more than 1 type of nociceptors. Modern technology allows us to see them individually, and we can see that there are little enlargements at the end of the nerves (but those are still free nerve endings). We can count how many nociceptors a patient has with a biopsy (a number too high or too low could be indicative of a pain disease, although humans have a pretty wide normal range).

Question 7

Types of neurons

Answer 7

• Mulitpolar: many dendrites and one axon. Most common type of neurons (ex. motor neurons, interneurons)
• Bipolar: one dendrite and one axon attachend to the cell body (rare; only found in eye and ear)
• Unipolar: one long axon and a cell body somwhere in the middle (nociceptors and other sesnory neurons are unipolar, with dendrites that lead to the CNS)

Question 8

Afferent fiber classes

Answer 8

• Aα: largest (big myelin sheath, allows for fastest communication), proprioception (muscle control), big enough to see with naked eye
• Aβ: second largest, but largest for touch (and vibration)
• Aδ: largest for pain (a few micrometers), also involved in thermal info
• C: smallest (no myelin sheath, around 1 micrometer, and speed of only around 1 m/s), involved in pain and sweating

Question 9

Myelin sheath

Answer 9

Protective (insulating), fatty coating surrounding nerve fibers. It prevents leaks and allows signals to jump from one gap in the sheath to another, making it go faster than usual.

Question 10

First vs. second pain

Answer 10

• First pain: immediate sharp pain, high intensity but short, due to Aδ fibers
• 2nd pain: different pain (more dull and burning), comes later and lasts longer, due to C fibers
  For example, if you hit your toe, the pain signal travels ~1 meter through the leg. Through the Aδ it travels too fast (takes max. 0.2 seconds) to consciously experience a delay (so the first pain feels instantaneous). However, it will take a full second to get to the spinal cord with the C fiber

Question 11

Nerves

Answer 11

Nerves are bundle of neuron axons. Afferents run in bundles (nerves) together, because every neuron has a very specific area it responds to, so info from all these specific area needs to come together for the brain to interpret it as a general area (it couldn’t compute too precise info). Blood vessels go through the nerve to supply the neurons.

Question 12

Shingles and dermatomes

Answer 12

Shingles: reactivated chicken pox virus, produces extremely painful rash. Sometimes the rash goes away but the pain stays (post-herpetic neuralgia). There is fortunately a vaccine (that also helps prevent dementia). The rash from shingles is different from others because it stays within one dermatome (it can spread to the whole dermatome but not cross over to another one). It can happen in several dermatomes at once

Question 13

Dermatomes

Answer 13

Area of skin that is mainly supplied by afferent nerve fibers from the dorsal root of any given spinal nerve. There are 8 cervical nerves (C1 being an exception with no dermatome), 12 thoracic nerves, 5 lumbar nerves and 5 sacral nerves. Each of these nerves relays sensation from a particular region of skin to the brain. Info goes to the brain organized in dermatomes because every dermatome has a dorsal root ganglion associated to it

Question 14

The spinal cord (vertebral column), bones and roots

Answer 14

Info has to come in and out of the spinal cord, going between the bones that make the spine (2 types of bones, of different shapes). The nerve goes between the and dorsal bones, and breaks into 2 roots (ventral root and dorsal root). The dorsal root is fatter than the ventral root, because it contains the spinal (dorsal root) ganglion (a group of neural cell bodies, that’s where the cell bodies of nociceptors are).

Question 15

Dorsal root ganglia (DRG)

Answer 15

The spinal cord is covered by the same 3 membranes as the brain (meninges layers: dura mater, arachnoid membrane, pia mater). The mixed spinal nerve (dorsal for sensory info + ventral for motor info) separates into filaments that then go between the bones into the spinal cord. Inside the spinal cord, the gray matter forms horns (anterior and posterior gray horns, 2 of each). The sensory info comes in through the posterior gray horn (dorsal side)

Question 16

Gray vs. white matter

Answer 16

• Gray matter: neuronal cell bodies
• White matter: myelinated axons (myelin is white)

Question 17

Central canal between horns

Answer 17

The central canal is the equivalent to the ventricles in the brain, it’s filled with CSF (when you do a spinal tap, you put a needle into the central canal to get a CSF sample)

Question 18

Levels of neurons in gray matter

Answer 18

Anatomists have found that neurons are of different sizes/shapes and packed in different densities in different layers. For example, motor neurons in the ventral (anterior) horn are very large.

Question 19

Rexed’s Laminae

Answer 19

Laminae (levels) I-VI are in the dorsolateral fasciculus, posterior (dorsal) gray horn, and substantia gelatinosa.
Laminae VII-XI are in the anterior (ventral) gray horn.

Question 20

Substantia gelatinosa

Answer 20

A not very dense substance, where a lot of afferent sensory fibers come in. The fibers coming in make a hole in the spinal cord, called the dorsolateral fasciculus (made of all white matter, fasciculus is another name for tract). These fibers terminate either in the substantia gelatinosa or deeper. The substantia corresponds to laminae I and II.

Question 21

Neuronal pathway from periphery to spinal cord

Answer 21

The primary afferent neuron picks up a signal from the periphery (skin or deep tissue). Its cell body is in the dorsal root ganglion. The neuron goes through the dorsolateral fasciculus and terminates in one of the upper levels. There, it synapses with a second-order neuron.

Question 22

Second-order neurons

Answer 22

There are 2 possibilities:
- The 2nd neuron goes all the way to the brain stem/thalamus (making it a projection neuron - it crosses side to go to contralateral brain hemisphere)
- The 2nd neuron terminates in a deeper level and synpases to another neuron there (making it an interneuron). This means that the signal is already processed in the spinal cord. The deeper the level, the more complex the processing. Eventually one interneuron synapses with a projection neuron that crosses side to continue processing in contralateral brain hemisphere

Question 23

Termination of Afferent Fibers in the Dorsal Horn

Answer 23

Different afferent nerve fibers terminate in different laminae in the dorsal horn:
- Aβ myelinated fibers: deepest (laminae III-V), touch info
- Aδ myelinated fibers: can go superficial (lamina I) or deep (lamina V)
- Nonpeptidergic C fibers: pretty superficial (inner lamina II)
- Peptidergic C fibers: most superficial (lamina I)

Question 24

Modern staining

Answer 24

New kinds of stains: injecting chemicals (antibodies or fluorescent molecules) that make certain cells autofluorescent.
- NeuN: antibody that stains all neurons
- Nac1.7: stains all nociceptors
And so the idea is, if we can ascribe particular functions to particular subsets of C fibers, we might be able to come up with more targeted interventions.

Question 25

Two Types of C Fibers: “Peptidergic” (CGRP+) and IB4+

Answer 25

• Stains that light up CGRPs (calcitonin gene-related peptide, a transmitter of certain types of C fibers), meaning certain C fibers light up because they contain CGRP. These C fibers are CGRP positive (CGRP+) fibers, or peptidergic fibers.
• Stains that light up IB4, which has no functional role, but isn’t present in C fibers containing CGRP (somehow they’re incompatible).
  These stains are what made us see that peptidergic C fibers are more superficial than IB4+ C fibers (which are in lamina II).

Question 26

Modern Molecular Definition of Sensory Neurons

Answer 26

The use RNA sequencing (a way of collecting all RNAs expressed by a cell to see how many copies of that RNA there are) allows us to put them into categories based on gene expression. We don’t need to rely on chance findings from testing all possible stains that we have.
We now have 11 categories of C fibers (in DRG). However, this is still in research, so in the meantime we still use the old “peptidergic” model.

Question 27

Efferent Function of Nociceptors: Neurogenic Inflammation

Answer 27

Nociceptors also have an efferent function: axon reflex. When a neuron fires an AP, it goes both to the CNS and to collateral dendrites that end up near blood vessels. There, NTs are released in the synapse, where it has several possible effects (ex. immune cell stimulation, arteriole dilation…). For example, the neuropeptides SP and CGRP are also potent artery dilators (they make arteries expand and get leaky, so water falls out). The plasma leaking out makes tissue swollen, which causes inflammation. This is not “normal” inflammation, but neurogenic inflammation, directly caused by neural activation.

Question 28

Why do we have neurogenic inflammation?

Answer 28

Inflammation alerts the immune system, which sends cells to kill possible invadors (ex. bacteria). Neurogenic inflammation is extremely fast, that gets the immune response started immediately.

Question 29

Spinal Reflexes

Answer 29

Spinal reflexes are a way to avoid the travel distance and processing time in the brain. The pain signal goes to the spinal cord where it directly stimulates an interneuron that goes directly from the dorsal horn to a motor neuron in the ventral horn. The motor neuron goes directly to the muscle to make the body avoid further pain (ex. removing hand from stove). Some experiments showed this in rats where the spinal cord and brain are not communicating, making them basically a walking spinal cord.

Question 30

Ascending nociceptive pathways

Answer 30

Most pain information goes up to the brain through the antero-lateral column. There are neurosurgery to cut antero-lateral column, gives placebo effect, but the pain comes back, because there are more than 1 way for pain to reach the brain (ex. the dorsal column).

Question 31

Nociceptive tracts

Answer 31

Tracts in the CNS are equivalent to nerves in the PNS (bundles of axons coming from the same place and going to the same place).
- Spinothalamic tract: to the thalamus. Most sensory information goes to the thalamus, and motor information comes out of it. The info is mostly about where the pain is and how it feels like (ex. sharp, dull, burning…)
- Spinoreticular tract: to reticular brain areas (evolutionarily old areas; involved in sleep, breathing, heart rate, etc.). We’re not sure about the function, maybe effect of pain on autonomic functions.
- Spinoparabrachial tract: to parabrachial nucleus in the pons (info about the unpleasantness of a stimulus)

Question 32

Somatotopy

Answer 32

Dr. Penfield found through awake brain surgeries that the somatosensory cortex (S1). He established the somatosensory homonculus. Pain goes to the brain somatotopically (mapping of the body onto the brain). There is somatotopy not only in S1, but also in the thalamus and in the dorsal + antero-lateral columns. Info from the lower part of the body comes in medially through the dorsal column and laterally through the antero-lateral column. Info from higher parts of the body (up until neck) go in laterally through the dorsal column and medially in the antero-lateral column.

Question 33

Trigeminal anatomy

Answer 33

Anatomy above neck is separate from spinal (below the neck) anatomy: sensory info doesn’t go through the spinal cord but the 5th cranial nerve (trigeminal nerve). The vell bodies are in the trigeminal ganglion, they project to the trigeminal spinal tract (not in the spinal cord but in the brainstem, where it then crosses sides to go to the thalamus, cortex etc.). The trigeminal nerve is made of 3 divisions: ophtalmic, maxillary (upper teeth) and mandiublar (jaw + lower teeth).

Question 34

Visceral pain pathways

Answer 34

Visceral pain can take several possible pathways to get to the brain. Sometimes it goes straight from the viscera through a nerve directly into the brain stem, like the vagus nerve, projecting directly to the nucleus tractus solitarius (NTS in the brainstem). But sometimes it goes directly from the visceral organ in question into the spinal cord, ex. through the pelvic nerve. And other times it goes through nerves into ganglia other than the dorsal root ganglia (they’re further laterally, near the organ itself).

Question 35

Afferent Fiber Termination: Somatic vs. Visceral

Answer 35

• Somatic: C fibers terminations are very focused
• Visceral: C fibers terminations are all over the place. If you’re stomach hurts, you can’t really localize it (contrarily to when it hurts on the skin).

Question 36

Visceral Pain: Somatic-Visceral Convergence

Answer 36

Afferent info from viscera terminates from same second-order neuron as info from the skin, that’s why visceral pain refers (it feels like it’s hurting in a place where it’s not really hurting; ex. heart attacks hurts in arm or jaw and not as much in the chest). The brain interprets visceral input as skin input that’s converging.
Visceral and somatic pains also respond to different stimuli. For example, the heart responds to ischaemia (restricted blood flow). The colon doesn’t feel hot temperatures, but it responds to distension (ex. when blockage). The colon doesn’t have to feel temperatures, because if it’s burning everything else is burning, and the food doesn’t get too hot in the colon.

Question 37

Referred Pain

Answer 37

Referred means, in general, feeling pain in one place different than where the injury or cause of pain is. Skin pain is almost always localized, but muscle pain can be referred (as skin or muscle pain) and visceral pain is almost always referred (as skin, mucle, or visceral pain). In 1909, James Mackenzie opened a conscious patient and saw that he felt the pain several inches away from where he was operating. Nowadays, intramusculuar injections of saline (hypertonic, causing cells to shrink) into an anesthetiszed area can cause pain in another (unanesthesized area).

Question 38

Brain Mapping Techniques

Answer 38

• Lesions: natural (disease, trauma) or induced (TMS, surgery). This was the first method used by classical psychologists.
• Stimulation and recordings: direct (electrodes, optical imaging), indirect (EEG, magnetoencephalography)
• Hemodynamic response (PET, fMRI, SPECT). These are the most recent methods. They measure brain activity with a little delay (measure blood flow where in different areas, more blood flow = recent neuronal activity in that area). Overall, fMRI is the most popular, because it has the biggest spatial extent (from cortical layers to whole brain) and the biggest time span (from microseconds to hours).

Question 39

Cortical Areas Involved in Pain: The “Pain Matrix”

Answer 39

The pain matrix involves many different brain regions: M1, S1, the thalamus, the PFC, the insula, the basal ganglia, the anterior cingulate cortex (ACC)… The concept of the pain matrix is in doubt: it’s true that these areas light up in pain, but other things than pain can light them up. Maybe it’s just a salience matrix (pain is important to the body). If people are experiencing emotional pain, the pain matrix lights up, but not exactly in the same way as physical pain (it’s a lumping/splitting problem: how specific do we want to get?)

Question 40

Frequency of brain areas (of pain matrix) active during acute vs. chronic pain

Answer 40

• Acute pain (tested on normal subjects): none of these areas are active in 100% of participants. The one that comes closest is the insula (insular cortex, IC), active in 94% of people.
• Chronic pain: the IC is active in only 58% of patients. Most of the areas see lower rates of activation, except the PFC (goes from 55% in acute pain to 81% in chronic pain). So, maybe the pain matrix is only for acute experimental pain; not for clinical pain. This is yet another splitting problem: is clinical pain different than acute pain?

Question 41

Sensory-Discriminative vs. Motivational-Affective Aspects of Pain

Answer 41

• Sensory-discriminative: localization of pain, quality of pain, intensity of pain (activates S1)
• Motivational-affective. unpleasantness, drive to escape or attend to pain (activates ACC)
  In experiments, the use of hypnosis to either drive the S-D or M-A aspects up/down show the different areas involved in each aspect. But do these streams of information really end up in different places? Most of the time, sensory signals circulate around the brain, without a real endpoint, and conscious perception somehow emerges from that.

Question 42

A modern view of the aspects of pain

Answer 42

S-D vs. M-A is probably not enough of a split, we should split further: Motivational only, Sensory only, Affective only, Cognitive only (what does the pain mean? What should I do about it?), descending modulation, etc. The view of the pain matrix is evolving and getting more complicated with time as more research is done on the subject

Question 43

Descending pain-modulatory pathways

Answer 43

Descending pathways go from the midbrain to the spinal cord. There are at least 2 precise pathways. They both go through the periaqueductal gray (PAG) in the midbrain, and then:
- to the locus coeruleus (LC, in pons), then to the spinal cord through the ventrolateral funiculus (VLF) tract
- ro the rostroventral medulla (RVM) and then enters the spinal cord through the dorsolateral funiculus (DLF)

Question 44

How do descending pain-modulatory pathways work?

Answer 44

All the information going up went through the spinal cord, and all the information coming down is impinging on those same projection neurons, those same 2nd-order neurons. So the brain is in effect controlling its own input. It can tell the spinal cord what it does or doesn’t want to hear. It can presumably prevent the ascending information from ever ascending, in which case it would not be there anymore. This is important because many of the treatments that we have for pain are not working by blocking the ascending system, but by activating this descending system. That includes opioids and all the antidepressant drugs that are used for chronic pain.

Question 45

Pain Physiology: Ed Perl

Answer 45

• Specificity theory: if there’s an innocuous stimulus and the neurons are specific to pain, then they shouldn’t fire at all, and they should start firing precisely when the stimulus becomes noxious. And the more noxious it is, the more they should fire.
• Intensity theory: once it gets into the noxious range, the neurons are going to fire more. There is a linear relationship between the intensity of the noxious stimulus and the firing of the dorsal horn projection neurons. So, the neurons can be activated by innocuous stimulation, but will be more activated with noxious stimulus.

Question 46

Pain physiology: Pat Wall + Ron Melzack

Answer 46

• Pattern theory: primary afferents are activated by everything but in different combinations with different strengths. And then somewhere in the spinal cord (or maybe higher) the coding of it is figured out. Different cells with different impulses, and somewhere higher up, that is turned into a perception, but it’s not directly related to the stimulus.
• Gate-control theory: a type of pattern theory, with a particular circuit.

Question 47

Gate-control theory

Answer 47

This is not the way it actually works, but it’s still an important theory. It’s about the ratios of inputs from large (Aβ touch fibers) and small (Aδ and C pain fibers). Small fibers inhibit the substantia gelatinosa (with contains interneurons inhibiting the projection neuron) and excite the projection neuron. So small fibers make sure the projection neuron is activated. Large fibers also excite the projection neuron but excite the substantia gelatinosa, which inhibits the projection neuron. So, the more input from large fibers, the less likely the pain signals will go through. If only small fibers fire, pain signals go through. This theory could explain why rubbing your arm when it hurts help reduce the pain. The idea is that the gate is closed when large fibers fire more and open when small fibers fire more.

Question 48

Microneurography (in humans)

Answer 48

Microneurography records (electrophysiological) impulses from primary afferent fibers. The concept is to use electrodes placed in the nerve. You can tell from the pattern that you get what kind of afferent fibers you’re recording from (ex. a C nociceptor). We can use it in people, but only in the limbs (doing it directly in the spinal cord would be too painful and unethical). Also, it is very technically challenging, so it’s quite rare.

Question 49

Electrophysiological Recording of Dorsal Horn Cells (in mice)

Answer 49

Most often, the animals are anesthetized (you need to find the right balance between too much anesthesia and then the neuron doesn’t fire and not enough anesthesia where the animal is in too much pain). To get the neuron to fire, you need to stimulate the animal in the receptive field of the neuron.
For example, for a neuron that is tracking temperature, the higher the temperature you apply to its receptive field, the more the neuron fires.

Question 50

Types of 2nd-order projection neurons relevant to pain (in spinothalamic tract)

Answer 50

• Wide dynamic range (WDR): gets input from Aβ, Aδ, and C fibers. This could be proof for intensity theory
• Nociceptive specific (NS): gets input from only Aδ and C fibers (pain neurons). This could be proof for specificity theory (there are some dorsal root neurons that only get info from pain stimuli)
• Low threshold mechanosensitive (LTM, “silent nociceptors”): these are normally silent for pain signals, but ahve the ability to become nociceptors. There is evidence that LTMs can change their properties after an injury, and that this is the cause of sensitisation (maybe also chronic pain if they don’t revert back to touch-only, Aβ receptors)

Question 51

Electrophysiological Recording of Anterior Cingulate Cells

Answer 51

Recording from a rats show activation no matter where you apply the noxious stimulus. This means that the whole body is the recorded cell’s receptive field. the anterior cingulate does not care about the sensory discriminative aspects of pain. It is not there to tell you where the pain is. It is there to tell you how unpleasant the pain is. And if you’re making a distinction on how unpleasant A stimulus is, it barely matters where that stimulus is (however, visceral pain is much more unpleasant than somatic pain, for the same pain intensity. This is probably because it’s alarming and disturbing, and you can’t see your injury).

Question 52

Counterirritation

Answer 52

Pain in one place can relieve pain in another area. This is also called diffuse noxious inhibitory controls, conditioned pain modulation, and heterotopic noxious conditioning stimulation. The most common term nowadays is CPM. Experiences have shown that ratings of pain from a (test) stimulus decrease if the stimulus is applied at the same time or shortly after another noxious (conditioning) stimulus.