anatomy and physiology of pain Flashcards
nociceptive
damage to non-neural tissue – activation of nociceptors
neuropathic
lesion or disease of the somatosensory nervous system
nociplastic
arises from altered nociception no clear evidence of actual or threatened tissue damage causing the activation of peripheral nociceptors or evidence for disease or lesion of the somatosensory system causing the pain
allodynia
pain due to a stimulus that does not normally provoke pain
dysesthesia
unpleasant abnormal sensation, whether spontaneous or evoked
hyperalgesia
increased pain from a stimulus that normally provokes pain
hypoesthesia
decreased sensitivity to stimulation, excluding the special senses
acute pain
less then 12 weeks
chronic pain
continuous more than 12 weeks, pain that persists beyond the tissue healing time – split into cancer and non cancer
first order/ primary afferent neurone
- Enters the spinal cord through a spinal nerve, or the brainstem through the trigeminal nerve, on the same side of the body (ipsilaterally) as the peripheral receptor is located
- Remains ipsilateral and synapses with a second order neurone within the CNS
second order neurone
- Cell body is located in the spinal cord/brainstem (exact location depends where sensory receptor is e.g. brain stem if face)
- Its axons cross over (decussates) to the other side of the CNS and ascends to the thalamus where it terminates
third order neurone
• Cell body is located within the thalamus and its axon projects to the somatosensory cortex – located in the post central gyrus of the parietal lobe of the cerebral hemisphere
nociceptors
- Sensory neurons found in any area of body that can sense pain
- Nociception – perception of pain
- External e.g skin, mucosa, cornea
- Internal e.g. viscera, joints, muscles and connective tissue ect.
- Cell bodies of neurons reside in either dorsal root ganglia or trigeminal ganglion
- Most are polymodal (thermal, chemical, mechanical)
- Where there is tissue damage, bradykinin & prostaglandin E2 – these substances both reduce the nociceptive action potential threshold thereby increasing their sensitivity to stimuli – known as hyperalgesia
afferent fibres: alpha delta fibres
o Thinly myelinated
o Carried: touch, temperature & fast pain information
o Small diameter (1-5 micrometers)
o Conduction speed is medium (5-40m/s)
afferent fibres: C fibre
o Unmyelinated
o Carried: slow pain, temperature, touch, pressure, itch, postganglionic autonomic fires information
o Smallest diameter (0.2-1.5 micrometers)
o Conduction speed is slowest (0.5-2m/s)
what do nociceptors synapse with
- Nociceptors (A delta and C fibres – first order neurones) synapse with the secondary afferent neurones (second order) in the grey matter of the dorsal horn of the spinal cord (which is divided up into Rexed laminae)
- The A delta (primary afferent neurone) terminals release glutamate (fast acting) as their neurotransmitter
- C fibres (primary afferent neurone) terminals release glutamate and substance P (slow acting – involved in the mediation of dull aching pain) as their neurotransmitters
- First order/ primary afferent neurones transmit information into the dorsal root ganglion where they synapse with second order neurones at the substantia gelatinosa
second order neurones transmit pain impulses up the::
o Spinothalamic tract (carries pain, temp and crude touch information from the body) – enters the spinal cord at Lissauer’s fasciculus
o Trigemino-thalamic tract (carries pain, temperature and crude touch information from face/head/ neck)
where to first order neurones enter
First order neurones enter at the pons and then descend to the medulla forming the spinal trigeminal tract
Receives contributions from the trigeminal, facial, vagus and glossopharyngeal nerves
where do the spinothalamic and trigemino-thalamic tracts terminate
terminate at the thalamus (ventral posterior lateral nucleus)
what can the spinothalamic tract be divided into
can be divided into the anterolateral tract which consists of:
o Lateral spinothalamic tract (pain & temperature)
o Anterior spinothalamic tract (crude touch)
where do third order neurones ascend
• Third order neurones then ascend from the thalamus to terminate in the somatosensory cortex on the post-central gyrus of the parietal lobe
thalamus
- Midline, paired symmetrical structure in the brain
- All sensation (except olfactory) relay/pass through it
- Contains multiple nuclei: relay function, association function
insula
- Lies within the brain via the sylvian fissure
- Where the degree of pain is judged
- Contributes to the subjective aspect of pain perception
cingulate gyrus
- Located on the medial aspect of the cerebral hemispheres
- Linked with the limbic system which is associated with emotion formation and processing learning and memory
- Involved in the emotional response to pain
periaqueductal grey
- Located in the midbrain – grey matter around cerebral aqueduct
- Receives input from the somatosensory cortex
- Part of descending pain pathway
- Contains high concentration of opioid receptors & endogenous opioids
- Under situations of extreme stress this pathway can be activated
- Resulting in the modulation of afferent noxious transmission
- It projects to the dorsal horn
periaqueductal grey and opioids
- Once activated, opioid receptors are activated resulting in a reduction in pre-synaptic neuronal sensitivity (thereby reducing substance P release) which in turn results in reduced pain sensation
- Meaning less impulses travel up the first, second and third order neurons to the somatosensory cortex – meaning less pain is felt
- Opioids such as morphine, methadone, codeine & oxycodone mimic this effect by binding to the opioid receptors in the periaqueductal grey thereby conferring profound analgesia
analgesia
the selective suppression of pain with-out effects on consciousness or other sensations
anaesthesia
the uniform suppression of pain – no pain is felt at all, and sometimes consciousness is lost (general anaesthesia)
melzack-wall pain gate
states that non-painful input closes the gate to painful input, thereby preventing pain sensation from travelling to the somatosensory cortex to be perceived and thus felt
treatments for pain
- Analgesics
- Acupuncture
- Exercise
- Hypnotherapy
substance P
- Peptide neurotransmitter involved in pain transmission
- Also a vasodilator
- Remains bound to receptors for longer time thereby transmitting long-lasting pain
