Types of Nociceptive Fibers

A delta

C fibers

A delta fibers

• nociceptive fiber
• small diameter, fast, good localization
• lightly mylenated
• associated with sharp/"prickling pain" (slap)
• "First Pain" response to a noxious stimulus
• mainly found in the Spinothalmaic tract
• associated with somatic structures - ie: skin, musculoskeletal system

C fibers

• small diameter
• unmyleinated
• associated with dull, aching, throbbing pain
• "second pain" response to noxious stimuli
• Mainly in other ALS tracts (not STT)
• primarily associated with viscera & deep structures
• arrive secondarily to consciousness 

Pain

conscious perception of nociception

destinations for nociceptive info

• cortex - via the spinothalamic tract
• limibic system - indirectly via reticular formation and hypothalamus
• hypothalamus - via spinohyothalamic tract
• brainstem - via spinoreticular & spinomesencephalic tract

Concept of pain in the cortex

arrives via the spinothalamic tract

expressed as conscious perception of pain 

concept of pain in the limbic system

 arrives indirectly via the reticular formation and hypothalamus

is expressed as an emotional response (which initially is an unconscious response)

concept of pain in the hyopthalamus

arrives via the spinohyopthalamic tract

is expressed as autonomic responses 

(initially unconscious responses)

concept of pain in the brainstem

arrives via the spinoreticular and spinomesencephalic tracts

expressed as "alerting" and attention modulating centers

(initially unconscious response)

3 Concepts of Pain

1. Sensory-discriminative aspect
2. Motivational-affective aspect
3. Cognitive-evaluative aspect

Sensory-discriminative aspect

Concept of pain

• in the cerebrum
• function to identify and localize pain - identifying onset, location, intensity and character 
• primarily a function of the Somatosensory Cortex
• via the Spinothalamic tract

Motivational-affective aspect

Concept of Pain 

• emotional meaning of pain (is intrinsicly negative)
• Aversive response (trying to get away from pain or experience the least amount of pain)
• Mediated by: Reticular Formation - Spinoreticular tract (level of alertness); Hypothalamus - Spinohypothalamic tract (autonomic responses); Limbic system - via RF & hypothalamus (emotional response)

Cognitive-evlauative aspect

Concept of Pain

• Learning what it's like to experience pain and how it influences your future experiences
• cultural & experienctial context
• Mediated by: association areas, prefrontal region

Hyperalgesia

increased (perception) pain in response to noxious stimuli

Allodynia

pain perception to a non-noxious stimuli

i.e. someone touches your sunburn

Nociception can be altered by a change in behavior of: 

Nociceptors - naked nerve endings

Posterior Horn - primary afferent synapses

Descending Pathways

Inflammatory mediators

act on nociceptors - change the behavior of nociceptors in the periphery

are released when there is injury of a tissue

are relased by white blood cells

Changes at the Nociceptor

Silent Nociceptors

normally unresponsive to noxious stimuli - w/ high threshold for activation (insensitive to stimuli)

get sensitized by inflamatory mediators which cause the threshold to go down, this adds to the population of receptors responding (receptors are more easily excited) - the response seems greater becuase more receptors are responding

intensity of stimulus related to # of receptors activated

Changes at the Nociceptor

Decrease in Threshold

a decrease in threshold of "regular nociceptors" means a less intense stimuli causes the receptor to fire

produce activity in nociceptive afferents 

Changes at the Nociceptor

Direct Stimulation

direct stimulation of the nociceptor means it becomes part of the adequate stimulus

"Wind UP"

AMPA Receptor 

1. glutamate binds to it

has a fast response

is the normal response of a receptor in the post-synaptic membrane

causes depolarization 

"Wind UP"

NMDA Receptor 

2. binds glutamate - only opens under already depolarized conditions

allows Ca²⁺ to enter, which changes the behavior and membrane sensitivity

dcreases the threshold for activation

is a new 2nd messanger

"Wind Up"

G- protein coupled receptors

3. NT's like substance P and BDNF bind to GPCR

Changes in long term sensitivity

has potential to change genetic expression - possibly permenent

Wind Up 

Spinal Facilitation of Pain

Increases Pain

Gate Control Theory 

non-noxious stimuli can decrease pain perception - reduce pain transmission

Pre-Synaptic Inhibition

occurs in Gate Control Theory at the posterior horn

inhibitory interneuron branches off of the A-alpha (non-noxious) neuron to inhibit the C fiber (noxious stim) before it synapses onto the WDR (wide dynamic range) neuron which ascends the spinal cord via the spinothalamic tract (conscious)

Periaqueductal Grey (PAG)

located in midbrain - area surrounding the mesencephalic aqueduct

neurons there release Endorphins and Opiods (both involved in pain relief) 

neurons descend to Nucleus Raphe Magnus and Spinal Cord (posterior horn) 

reduce the behavior of pain pathways 

Nucleus Raphe Magnus

located in the Pons

neurons release Norepinephrine and Serotonin

neurons extend down to spinal cord (posterior horn)

reduce behavior of pain pathways 

affective component of pain 

when there is a negative emotional reaction to pain that is expressed behavioral due to the connection of the pain pathway to the limbic system

Suffering

the state of emotional distress associated with events that threaten an organism

an emotional state - rooted in the connections between nociception and the limbic system

part of the motivational-affective facet of nociception

WDR neurons

wide-dynamic range neurons

respond to both noxious and non-noxious stimuli

receive info from both somatic and visceral structures - underlines referred pain