Week 10: Local Anaesthetics Flashcards
What are the 3 types of local anaesthesia?
- Regional anaesthesia –> loss of sensation to a region or part of body
- Local infiltration –> anaesthetic is injected where the injury is
- Topical –> eye, skin (no injection
What is a local anaesthetic?
a drug which reversibly prevents transmission of the nerve impulse in the region to which it is applied, without affecting consciousness
What are the non-pharmacological methods of local anesthesia?
- cold temperature (neuroconduction is blocked if temp falls below 8 degrees)
- pressure
- hypoxia
What are the 2 types of pharmalogical local anaesthesias?
- reversible e.g local anaesthetics block nerve conduction for a variable period of time
- irreversible e.g ethanol, surgical –> once you use these, the nerve conduction is blocked forever
How does an injury cause pain?
- At the site of injury, the Na+ channels open and Na+ moves inside the cell so the membrane depolarises and generates an action potential
- 1st order neuron takes information from site of injury to spinal cord
- 2nd order neuron takes information from spinal cord to thalamus
- 3rd order neuron takes information from the thalamus to the somatosensory cortex
Where is the severity of pain analysed?
the somatosensory cortex
How do local anaesthetics work?
- local anaesthetics block the alpha subunit of the voltage-gated sodium channel so the impulse cannot cross the areas
- so the brain is not receiving information about the pain so you cannot feel it
What is the endoneurium?
each nerve fibre (axon) is surrounded by a layer of connective tissue called the endoneurium
What does the perineurium surround?
- bundles of nerves are called vesicles
- each vesicle is surrounded by perineurium
Many vesicles join together to form a nerve covered by what?
epineurium
Where are LAs injected?
- LAs are injected outside the nerves and then they diffuse through epineurium, perineurium and endoneurium to act on the sodium channels on nerve axons.
- LAs are never injected into the nerves as this will cause damage to the nerves
What two forms does local anaesthetic exist in
- ionised/protonated
- unionised/ unprotinated
Which form of LA can:
- Cross the cell membrane
- Can bind to the sodium channel
- Unionised
2. Protonated
If there is an injury of the hand, explain how the information about pain reaches the somtosensory cortex
- the information will be carried from the site of injury to the spinal cord by the median nerve
- The information will synapse at the grey horn
- then travel in the lateral spinothalamic tract to the thalamus
- then to the somatosnesory cortex
What is the ideal local anaesthetic?
- reversible
- good therapeutic index
- quick onset
- suitable duration
- no local irritation
- no side effects
- no potential to induce allergy
- cheap, stable and soluble