Somatosensation
Process by which sensory neurons are activated by external stimuli
Nociception
Process by which neural pathways are activated by tissue damaging substances
Pain Pathway
- Transduction–stimuli into action potentials
- Transmission–conduction through the nervous system
- Modulation–alteration of conduction pathways (augments or inhibits pain pathways)
- Perception–action potential into conscious sensation
Peripheral Nerve Pathway
- stimulus activates the nociceptors
- action potential sent to the dorsal root ganglion via afferent sensory (pain) fibers
- action potential travels to the dorsal root & dorsal spinal cord
Afferent Sensory (Pain) Fibers
A-Beta:
- heavily myelinated, large diameter (= fastest)
- light touch & pressure
A-Delta:
- lightly myelinated, smaller diameter
- intense, stabbing pain (“first” pain)
C:
- unmyelinated, small diameter (= slowest)
- Burning pain (“second”, delayed pain)
Central Nerve Pathway
- Located in the dorsal gray of the spinal cord
- Incoming sensory afferent fibers terminate directly or indirectly on cell bodies that send axons into the white of the spinal cord along tracts
- Indirect connections = projection interneurons
Wide Dynamic Range Interneurons
- receives A-beta & A-delta afferent fiber information (greater response w/ noxious, A-delta stimuli)
- faster, spontaneous rate
- high-threshold or nociception-specific cells
- convergence input from the skin, muscle, & viscera (= referred pain)
Nociceptive Specific Interneurons
- receive information from C afferent fibers
- slower, spontaneous rate
Central Sensitization
Prolonged, intense painful stimulus –> increased pain response (larger receptive field or higher discharge rate) due to neuronal plasticity
- MC w/ WDR interneurons
- The basis of chronic pain
Excitatory Neurotransmitters
Glutamate
Aspartate
ATP
Receptors = NMDA
Inhibitory Neurotransmitters
GABA Norepinephrine Serotonine Adenosine Acetylcholine
Excitatory Neuropeptides
Substance P
Neurokinin A
Function: nerve to nerve transmission & affect macrophages & mast cells
Primary role in sensitization & hyperalgesia
Inhibitory Neuropeptides
Somatostatin
“Morphine-like effects”
Enkephalins
Endorphins
