physiology of pain 1 Flashcards
What is pain
An unpleasant sensory experience associated with tissue damage, accompanied with an emotional reaction.
Why do we feel pain
It is a warning sign so we can avoid harmful situations, prevents injury or death and tells us to rest following injury,
How is pain classified
Nociceptive- normal functioning of nociceptors in response to tissue injury
neuropathic- pain in response to an injury to the ns
What are nociceptors
they are primary sensory neurons that detect pain, they are pseudo unipolar neurons.
How are sensory nerve fibre s classified?
They are classified by diameter and myelin content
largest; A-alpha and A-beta fibres, myelinated with large diameter and receptive to light touch and proprioception
slightly smaller A-delta fibres- thinly myelinated, medium diameter, light touch, temp and nociception
c fibres- unmyelinated, small diameter, temp and nociception.
Outline the stimuli that can activate nociceptors
pressure, heat, cold, chemical, tissue damage and inflammation.
outline the polymodal nociceptors
most c fibre nociceptors are polymodal, they respond to pressure, temp and chemical.
Outline temp transduction
Involves TRP channels which can have chemical agonists e.g. TRPV1 detects hot temp but also agonistic to capsaicin.
TRPM detects cold and is a menthol agonist
TRPA1 detects V cold and cinnamon
Name chemicals that can directly activate nociceptors
ATP, Protons and seretonin
What effect does inflammation have on nociceptors
inflammation can modulate nociceptors and cause hypersensitivity
What are some important terms?
hyperalgesia: noxious stimuli produce an exaggerated pain response
allodynia: non-noxious stimuli produce a painful response
What are the 2 main mechanisms that lead to hypersensitivity
peripheral sensitization which leads to hyperalgesia
central sensitization leads to allodynia (secondary hyperalgesia)
outline peripheral sensitization
peripheral sensitization is an increase in the responsiveness of the peripheral ends of nociceptors
driven by tissue injury or inflammation.
an example of this is sunburn and then taking a warm shower which will burn more than it should
Outline the action of bradykinin
bradykinin indirectly acts on TRPV1,
bradykinin binds to the receptor, activation of protein kinase which leads to the phosphorylation of TRPV1
What is the spinothalamic tract
pain information ascends the spinothalamic tract
the central axon passes through the dorsal root to reach the dorsal horn and then forms collateral branches and form the tract of Lissauer
on the tip of the dorsal horn, they synapse into second-order neurons in the substantial gelatinosa.
Outline the second-order neurons
the axon of the second-order neurons cross the midline of the spinal cord and those axons enter the anterolateral column and ascend to the thalamus where they synapse onto 3rd order neurons.
Outline referred pain
the convergence of visceral and cutaneous nociceptors on same second-order neurons in the spinal cord.
the brain perceives visceral pain as cutaneous.
Where do third-order neurons project to
They ascend to the primary somatosensory cortex,
Outline the emotional component of pain
Third-order pain also projects to the insula, cingulate cortex and limbic system which are all involved in encoding the emotional part of the pain.
What are the 2 important regions in the descending regulation of pain and how is pain inhibited
The periaqueductal grey matter and rostral ventromedial medulla. PAG grey matter neurons excite serotonergic neurons which excite inhibitory interneurons.
inhibitory interneurons inhibit spinothalamic tract neurons.
