Anatomy and Physiology of pain 21.02.23 Flashcards
What is the definition of pain?
Pain is an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage
Why do we have pain?
Immediate pain warns of imminent tissue damage so we withdraw from the source of injury
- gives damaged tissues the best chance to heal
What is Nociception?
This describes the neural processes involved in producing the sensation of pain
What is an overview of the nociceptive pathway?
- Transduction in the periphery
- Through transmission to the dorsal horn of the spinal cord
- Then onto the brain
What is acute pain?
Pain for less than 12 weeks
What is chronic pain?
- Continuous pain lasting more than 12 weeks
- Pain that persists beyond the tissue healing
There is a sub classification into chronic non-cancer and cancer pain
What is Nociceptive pain?
Pain that arises from actual or threatened damage to non-neural tissue and is due to the activation of nociceptors
What is neuropathic pain?
Pain caused by a lesion or disease of the somatosensory nervous system
What is nociplastic pain?
pain that arises from altered nociception despite no clear evidence of actual or threatened tissue damage
What are 2 examples of neuropathic pain?
- Trigeminal neuralgia
- glossopharyngeal neuralgia
E.g stinging and burning
What is allodynia?
Pain due to a stimulus that does not normally provoke pain
What is dysesthesia?
An unpleasant or abnormal sensation, whether spontaneous or evoked
What is hyperalgesia?
Increased pain from a stimulus that normally provokes pain
What is hypoalgesia?
Diminished pain in response to a normally painful stimulus
What type of receptor detects pain stimulus?
Peripheral receptor will detect the relevant stimulus
What does the 1st order neuron do?
- Enters spinal cord through spinal nerve through the trigeminal nerve
- Enters ipsilateral to peripheral receptor
- Remains ipsilateral and synapses with 2nd order neuron
What does the 2nd order neuron do?
This crosses to the contralateral cord and ascends to the thalamus (the ‘relay’ station)
What does the 3rd order neuron do?
From the thalamus to the midbrain and higher cortical centres (somatosensory cortex)
What is transduction pathway with pain?
- This is turning a physical stimulus into an action potential
- Most are poly-modal (thermal/ chemical/ mechanical)
What are the primary afferent neurons?
Primary afferents are sensory neurons (axons or nerve fibres) in the peripheral nervous system that transduce information about mechanical, thermal, and chemical states of the body and transmit it to sites in the central nervous system.
What are nociceptors?
These are the free nerve endings of primary afferent neurons and can be:
- A fibres (fast acting): terminals release glutamate
- C fibres (slow acting): terminals release glutamate and substance P
Where are nociceptors found?
- In any area of the body that can sense pain either externally or internally
External: skin, cornea, mucosa
Internal: Viscera, joints, muscles. connective tissue
What ganglion do the cell bodies reside in?
Dorsal root ganglion - body
Trigeminal ganglion - face/ head/ neck
What type of fibres do the ganglions relay?
Sensory (afferent)
What information does the spinothalamic tract carry?
sensory pathway that carries pain, temperature and crude touch information from the body
Which order neurons are part of the spinothalamic tract?
2nd order neurons that originate in the spinal cord cross the midline in the anterior commissure then form the anterolateral tract
So they travel contro-laterally
What are the lateral and anterior STT responsible for?
Lateral: Pain and temperature
Anterior: Crude touch
What is the thalamus fucntion?
- Symmetrical structure in brain where all sensations (except olfactory) relay/pass through
- they have the cell bodies of third order neurons here
- contain the VPL nuclei
What is the function of the insula?
- third order neuron goes to sensory cortex first then to insula
- This is where the degree of pain is judged
- Pain perception
What is the function of the amygdala with pain?
- Key role in learned emotional responses (fear, anxiety, depression)
- Pain modulation
What is the cingulate cortex linked with?
Limbic system which is associated with emotion formation
What is the peri-aqueductal gray?
- grey matter located around cerebral aqueduct
- receives input from cortical and sub-cortical areas
- Projects onto neurons in the dorsal horn
What pathways are in the peri-aqueductal gray?
- Opioid receptors
- Noradrenergic and serotonergic neurones
- Stimulus of the PAG can result in analgesia
What are three ways we can control pain?
- Stimulate descending inhibitory pathways
- Gate control
- Pharmacotherapy
What is the gate control theory of pain?
- The gate control theory of pain asserts that non-painful input closes the nerve “gates” to painful input, which prevents pain sensation from traveling to the central nervous system.
- The gate control theory of pain describes how non-painful sensations can override and reduce painful sensations.
What are 3 drugs commonly used in medicine?
- NSAID
- Paracetamol
- Opioid (good for acute pain)
What are problems with opioids? (3)
- tolerance
- addiction
- immunosuppression
- opioid induced hyperalgesia (abnormal pain sensitivity)
What are 3 causes of cancer pain?
- pain associated with direct tumour
- pain with cancer therapy
- pain unrelated to cancer (headache)
What type of neurons are the first order neurons?
Pseudo-unipolar neurons
Where do the first order neurons terminate?
In the dorsal root horn
What does the dorsal column control?
Fine touch, propioception, vibration
What does the lateral spinothalamic tract carry?
Pain and temperature
What does the ventral spinothalamic tract carry?
Light touch sensations
What is analgesia vs anaesthesia?
Analgesia - selective suppression of pain without effects on consciousness
Anaesthesia - uniform suppression of pain (no pain felt at all) can cause loss of consciousness
