Pain Pathway Wk1

free nerve endings function

detect potentially damaging stimuli

peripheral nerve organization function

rapid transmission from tissue to CNS

dorsal root ganglion function

house sensory neuron cell bodies

dorsal horn function

first site of sensory processing

ascending spinal pathways function

deliver information to higher brain centers

transduction

Nociceptors detect a noxious stimulus and convert it into an

electrical signal

transmission

The action potential travels along Aδ and C fibers to the

spinal cord

relay

Second-order neurons carry the pain signal through the spinothalamic tract to the thalamus

integration & interpretation

The brain processes the location, intensity, and emotional significance of the pain

modulation

Descending pathways from the brain enhance or inhibit pain

signaling at the spinal cord

what are nociceptors

specialized sensory neurons (i.e., free nerve endings) that detect potentially damaging stimuli and covert this to electrical activity.

what are the 2 types of nociceptors & describe them & type of pain they sense

Aδ fibers

• thinly myelinated

• faster conduction (5–30 m/s)

• sharp, acute, localized pain

C fibers

• unmyelinated

• slower conduction (0.5–2 m/s)

• dull, aching pain

what are the 3 stimuli nociceptors are activated by & describe them and their ion channels

mechanical (pressure, stretching)

• ex.) cut, pinch

• ion channels: mechanosensitive (Piezo family)

thermal (heat, cold)

• ex.) burn, extreme cold

• ion channels: TRVP1, TRPM8

chemical (irritants, inflammation)

• ex.) acid, inflammation

• ion channels: ASICs, TRP

Afferent vs Efferent

• Afferent Arrives → carry sensory signals to the brain

• Efferent Exits → carry motor commands away from the brain

SAME DAVE

• Sensory = Afferent

• Motor = Efferent

• Dorsal (posterior/back) = Afferent

• Ventral (anterior/front)= Efferent

what are the 4 general steps in the transmission of pain

1. Nociceptor activation

2. synapse in the dorsal horn

3. ascending pathways

4. thalamus to cortex

transmission of pain (afferent) step 1: nociceptor activation [what is triggered, where is the next location]

• Noxious stimuli trigger action potentials in Aδ (A-delta) or C fibers

• These fibers enter the dorsal horn of the spinal cord through the dorsal root.

transmission of pain (afferent) step 2: synapse in the dorsal horn [how many levels can primary afferent neurons ascend & what do they do after they ascend, what are the 2 fibers & what neurotransmitters do they release)

• Primary afferent neurons ascend 1 to 2 spinal segments and synapse with second-order neurons

• Neurotransmitters released:

⚬ Aδ fibers: Glutamate (fast transmission)

⚬ C fibers: Substance P (sustained transmission, amplifies

pain)

transmission of pain (afferent) step 3: Ascending pathways [what do the second order neurons do, where does decussation occur, what tract aka direction do the axons ascend)

• Second-order neurons decussate through the anterior white commissure

• Axons ascend in the contralateral spinothalamic

tract

transmission of pain (afferent) step 4: thalamus to cortex [where do 3rd order neurons project from, what happens at the primary somatosensory cortex, function of anterior cingulate cortex & insula]

• Third-order neurons project from the thalamus to:

⚬ Primary somatosensory cortex → localization and

intensity

⚬ Anterior cingulate cortex and insula → emotional

and affective components

1st order neurons [what they are, location, where do they enter, where do they synapse]

First-order neuron

• Cell body in the dorsal root ganglion (DRG)

• Enters the spinal cord through the dorsal root

• Synapses in the dorsal horn (primarily laminae I, II, and V)

2nd order neurons [what they are, location, where they decussate, within how many spinal cord segments of entry, where does it ascend]

Second-order neuron

• Cell body in the dorsal horn

• Decussates through the anterior (ventral) white

commissure

• Usually within 1–2 spinal cord segments of entry

• Ascends in the contralateral spinothalamic tract

3rd order neuron [where does it synapse where does it project]

Third-order neuron

• Synapses in the ventral posterolateral (VPL) nucleus of

the thalamus

• Projects to the primary somatosensory cortex

what are the 2 mechanisms that regulate/modulate pain

• gate control theory

• descending modulation

gate control theory: location, physiology, clinical example

• location: dorsal horn of spinal cord

• physiology: local inhibition of pain transmission by competing sensory input

clinical examples

• rubbing your elbow after bumping it

• TENS (transcutaneous electrical nerve stimulation)

Descending modulation: location/pathway, physiology, clinical examples

• location: brain → brainstem → spinal cord

• physiology: regulates pain by inhibiting or facilitating nociceptive signaling

clinical examples

• stress induced-analgesia

• placebo effect

• chronic stress can enhance pain

gate control theory of pain mechanism

Mechanism

• Pain signals from Aδ and C fibers enter the dorsal

horn

• Rubbing the injured area activates Aβ

mechanoreceptors (touch and pressure)

• Aβ fibers give off collateral branches within the

dorsal horn

• Collaterals activate inhibitory interneurons in the

substantia gelatinosa (Rexed lamina II)

• Interneurons release GABA, reducing transmission

from nociceptive neurons

• Fewer pain signals ascend in the spinothalamic tract,

decreasing pain perception

descending pain pathway mechanism

Mechanism

• Periaqueductal gray (PAG) initiates descending pain

inhibition

• Signals relay through the rostral ventromedial

medulla (RVM) and locus coeruleus

• Descending fibers project to the dorsal horn

(substantia gelatinosa)

• Serotonin (5-HT) and norepinephrine (NE) activate

inhibitory interneurons

• Interneurons release endogenous opioids

(enkephalins, endorphins, dynorphins)

• Pain transmission to second-order neurons is

suppressed, reducing pain perception

what are the 3 nociceptor ion channels & what they are for

• ASIC → mechanical

• TRPV1 → heat

• TRPA1 → chemical/noxious

Somatic reflex arc 5 steps

1. sensory input from the environment

2. info relayed through afferent neuron

3. info passes through integration center

4. info relayed through efferent neuron to neuromuscular junction

5. muscles contracts & causes movement

what are the categories of reflexes

• stretch reflex

• inverse stretch reflex

• withdrawal reflex

what do glutamate & substance P support

• glutamate: fast transmission

• substance P: supports sustained or amplified signaling