mechanisms of pain perception Flashcards
what is pain?
an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage
what are the 3 components of pain perception?
-sensory component - eg location, intensity and quality of pain
-motivational component - unpleasantness
-cognitive component - distraction, emotional response etc
describe the duration & characteristics of acute pain?
-duration - seconds
-characteristics - associated with the detection of potentially tissue damaging noxious stimuli eg hot surface or pin prick
describe the duration and characteristics of sub acute pain?
-duration - hours to days
-characteristics - associated w/ tissue damage and the infiltration of immune cells; resolves on recovery eg wound inflammation
describe the duration and characteristics of chronic pain
-duration - months - years
-characteristics- pain exceeds resolution of damage, pain may become pathological and then is often chronic eg damage to nervous system
what is dysfunctional chronic pain?
pain that has no known pathophysiology
-hard to treat
what is nociception?
refers to the detection of noxious stimuli by nociceptors followed by the transduction and transmission of the sensory info from the periphery to the brain
describe the characteristics of nociceptors
-located at free nerve endings of primary afferent sensory neurons
-found in skin, muscle joints etc
-respond to mechanical, thermal or chemical stimuli
-info re actual tissue damage or the potential for such damage is relayed to the brain
what are the 4 types of nociceptors?
- mechanical
- chemical
-thermal
-polymodal
what do mechanical nociceptors detect?
-stretch of skin
-pressure on skin
-sharp, prickling pain eg cutting or pinching
what do chemical nociceptors detect?
-noxious chemicals
-factors eg bradykinins, prostaglandins etc
what do thermal nociceptors detect?
-thermal sensations eg slow and burning or cold and sharp
what do polymodal nociceptors detect?
-detect high intensity mechanical, thermal, and chemical stimuli
describe a delta pain fibres - info carried, speed and response
- permit the localisation of pain and form the afferent pathway for the reflexes elicited b y pain
info carried - acute pain eg sharp, well localised
-speed - fast, small myelinated fibres
-response - trigger immediate withdrawal and allow localisation of pain
describe C pain fibres - info carried, speed, response
-C fibres are considered polymodal - they can react to various stimuli
-info carried - dull aching, diffuse pain , temp etc
-speed - slow unmyelinated fibres
-response - suffering, emotional
what is the 3 order neuron system for pain pathway?
-primary pain afferent to spinal cord
-spinothalamic tract to thalamus
-thalamus to sensory cortex
where do the 1st order neurons synapse with the 2nd order neurons?
in the dorsal horn of the spinal cord
describe the pathway of the 2nd order neurons in the pain pathway
-2nd order neurons cross over to opposite side of spinal cord and enter spinal tract
-they ascend the contralateral spinothalamic tract and terminate in the thalamus where they synapse with the 3rd order neurons
through which cranial nerve do 1st order neurons travel with to send information to the brain about pain in the face?
via the trigeminal nerve
what neurotransmitters are released in the ascending nociceptive pain pathway?
-glutamate
-substance P
what is substance P?
small peptide neurotransmitter that is important for the transmission of pain signals from the periphery to the brain
what important NT’s are involved in the modulation descending nociceptive pathway?
-GABA
-Opiates
these act on 1st and 2nd order neurons to modulate pain perception
what are the effects of the NT’s involved in the descending / pain modulation pathway?
-inhibit NT please from 1st order neuron ie substance P and glutamate
-hyperpolarisation of 2nd order neuron, reducing cell excitability
-reduces firing of AP in the 2nd order neuron which blocks the transmission of the pain signal
what happens if opioids bind to the endogenous opioid receptors at the level of the spinal cord in the pain pathway?
-they exert analgesic effects by reducing neurotransmission of substance P and glutamate in the spinal cord both the at the presynaptic and postsynaptic locations
what effects do opioids have @ the presynpatic and postsynaptic neurons in the pain pathway?
-presynaptic = reduce synaptic release of substance P and glutamate by inhibiting Ca2+ influx
-post synaptic - antagonise depolarising effects of substance P & glutamate
what is the gate theory of pain?
Somatic non noxious stimuli can inhibit the transmission of pain signals in the spinal cord
describe the gate theory
-gate is located in the substantial gelatinosa in the dorsal horn of the spinal cod through which pain information must pass on its way to the brain
-large diamater A beta fibres carrying touch pressure vibration etc synapse with interneurons in the dorsal horn and inhibit transmission of info
-interneurons inhibit the transmission of pain from C fibres at the synapse between 1st and 2nd order neurons using GABA etc
what is somatic pain ?
pain felt from skin, muscles & joints
what is visceral pain?
-pain from internal organs
-poorly localised felt in areas removed from site of stimulus
-often perceived in a different location (referred pain)
what is referred pain?
-pain is felt in a location other than the actual source of the pain stimulus
-This occurs because sensory nerves from different parts of the body may converge on the same neural pathways in the spinal cord and brain, leading to confusion in the brain about the origin of the pain signal.
what is allodynia?
a sensation of pain experienced in response to a non-nociceptive stimulus eg something brushing against the skin
what is hyperalgesia?
-phenomenon when there is an enhanced sensation of pain at normal threshold stimulation
what is neuropathic pain?
pain as a result of a disease or injury to the peripheral or central nervous system
what are the causes of neuropathic pain - CNS vs PNS?
- CNS - stroke, spinal cord injury, neurodegenerative diseases
-PNS - diabetic neuropathy, inflammatory disorders
