MVC - Nerve Fibres in Nociception Flashcards
What can cause action potentials in free nerve endings?
Activation of ion channels if sufficient depolarisation occurs
What are some features of A-delta fibres? (5)
Mechanosensitive or mechanothermal nociceptors (mechanical and thermal)
- Myelinated
- Myelin (Shwann cells) insulates/protects/nourishes fibre
- Rapid/saltatory conduction (sharp well localised pain). 5-40m/s
Respond to first pain
What are some features of C fibres? (4)
Polymodal nociceptors (mechanical, thermal and chemical)
- Unmyelinated
- Dull/aching pain. Slower 0.5-2m/s
Respond to second pain
What are the features of nociceptors? (2)
- Small diameter
- Slowly conducting (Ad,C), compared with other sensory fibres
How are harmful stimuli detected?
Transcutaneous recordings using microneurography show that there are specialised nerve fibres that detect noxious stimulation
Are pain signals directed to a single “pain center” in the brain?
No
- Pain signals are integrated from different brain structures into a conscious experience of pain
Discriminative component vs Affective-motivational component
The discriminative component:
- Pathways that target the traditional somatosensory areas of cortex,
The affective-motivational component
- Depends on additional cortical and brainstem pathways. (Paleospinothalamic)
What are the components of the nociceptive ascending systems? (2)
Body (and posterior portion of the head)
Pain and temperature:
- Spinothalamic tract a.k.a. anterolateral system
Face (and anterior portion of the head)
Pain and temperature:
- Trigeminothalamic tract (face, oral)
- Through the spinal nucleous of the trigeminal complex
What are 3 examples of painkiller types?
Painkillers: insight into pain modulation
1) Local anaesthetic, also known as nerve block
eg. lidocaine, novocain
- Inhibit sodium channel molecules directly and prevents the generation and propagation of action potentials
2) Non-steroidal anti-inflammatory drugs (NSAIDs)
Paracetamol
- Target the production of prostaglandins by inhibiting the enzyme COX
3) Opioid analgesics
Ketamine
What are
Inflammatory chemicals/mediators and 3 examples of them?
Cause an increase in Neuronal Excitability
- Prostaglandins
- Bradykinin
- Substance P
How do sunburn nociceptors function?
Sunburn Nociceptors are bathed in inflammatory mediators
→ Increase sensitivity of nociceptors
→ Lower the depolarisation threshold
What is the first level of modulation in the processing of nociceptive signals?
Dorsal root of the spinal cord
Segmental controls of spinal origin:
- A gating mechanism in the spinal cord (substantia gelatinosa of dorsal horn) can be opened or closed in varying degrees thereby modulating incoming signals before they reach the brain
How could a first pain mask a second pain?
Descending pathways diffuse inhibitory controls induced by nociceptive stimuli
- Ascending nociceptors make connections within the brainstem, e.g. in the periaqueductal gray matter.
- These brain stem structures can return descending signals to inhibit nociceptors.
What controls acute stress?
Descending controls of the brain associated with psychological factors
How does inhibition by enkephalin work?
Pre-synaptically
- Makes the action potential narrower → limits neurotransmitter release
Post-synaptically
- Generates an inhibitory postsynaptic potential driving the cell away from the action potential firing threshold
