Sensory transduction, pain and local anaesthesia Flashcards
What is pain for and does everyone experience it?
- Tissue protection
- Alerting organism to potentially fatal or serious tissue damage
- Genetic mutation can result in inability to detect pain
What process is used to detect environmental stimuli?
Sensory transduction
What can dysfunction of sensory transduction produce?
Can lead to:
- loss sensation (loss function)
- acute pain
- chronic pain (phantom limb pain)
How can dysfunction and pain arise?
- Spinal injuries (central pain)
- Neuronal damage (neuropathic pain)
- Genetic disorders (e.g. congental anaesthesia- cannot feel physical pain)
- Inflammation
- Cancer (primary symptom in palliative care)
- Adverse effects of drugs
Outline sensory systems
1) Visual (sight)
2) Auditory (hearing)
3) Vestibular (balance)
- Specialised cell receptor
4) Olfactory (smell)
5) Gustatory (taste)
6) Somatosensory (touch, heat, pain)
- modified nerve terminal receptor
Outline 2 types of receptor
1) Sight, sound balance: Receptor is specialised cell which communicates with afferent neuron through release of neurotransmitter
2) Taste, touch and pain: Receptor is modified nerve terminal (part of afferent sensory neuron)
What is the modality- give e.g,? Also what determines specific senation we feel?
Receptors respond to specific energy/ modality
e.g. EM spectrum- vibration, pressure
- Mechanical- vibration, pressure
- Chemical- pheromones, pH
Specific sensation due to type of receptor activated or some sensations due to several receptors
Outline types of mechanoreceptors
- Stretch (muscle spindles)
- Sound energy (hair cells ), hearing and vestibular apparatus
- Physical displacement- pain, subcutaneous (skin) receptors
What does the Pacinian Corpuscle detect? How is this converted into action potential?
- Vibration and rapid movements
- Non myelinated nerve terminal at centre
- Surrounded by lamellae with viscous fluid in between
- Hydraulic pressure transferred to centre of Pacinian corpuscle
- Sensory transduction- process of converting pressure into AP
- Mechanosensitive Na+ channel- in response to pressure this will open and Na will diffuse into neuron
- Generator potential will occur (graded)
- If stimulus strong enough and passes threshold potential this will summate and trigger an action potential
Features of receptor potential/ generator potential (RGP)
- Transduction occurs at naked nerve terminal of sensory neuron
- Graded potential- amplitude proportional to stimulus strength
- Non-propagating decremental passive spread (as does not involve voltage-dependent channels, just positive charge from Na mechanoreceptors in neuron decrementing as it dissipates down neuron)
How much pressure is needed to make an action potential? what is this initiation of an action potential called
Action potential initiation when receptor graded potential exceeds the threshold level
How is propagated information encoded/ how is a more stringer or larger stimulus encoded?
- We will see an increase in the frequency of action potentials
What is high sensitivity? And what limits this?
- High sensitivity- ability to encode and detect a wide range of stimuli
Limited by the refractory period
How do we increase the sensitivity? (2)
- Use neurons with different AP thresholds e.g. Low threshold Merkel discs respond to light touch, intermediate threshold Pacinian corpuscles to pressure and highest threshold nociceptors to pain/ hard knock
- Population encoding- so using a large no. of neurons to detect small stimuli
Greater chance of detection of small stimuli
Larger stimuli excite more neurons
YET this is not very energy efficient so for larger stimuli adaption needed
Explain process of adaption
Allows us to detect stronger stimuli through adaptation-
e.g. application of a set PRESSURE may fire 2/3 AP- depending on strength of stimuli, while pressure is maintained no AP are fired as receptor potential falls then when pressure relieved another2/3 AP fires- this conveys same information (showing us how strong stimuli is and for how long it lasts)
If the stimuli got stronger this
e.g bath getting hotter
Increasing strength of stimuli means whenever stimuli got larger graded potential would increase over threshold level triggering action potential- at first only 1AP may be generated then when stimuli stronger 2 produced, then 3 etc
Outline differences between phasic and tonic receptors
Some receptors respond with a burst of activity when a stimulus is first applied, but then quickly decrease their firing rate—adapt to the stimulus—if the stimulus is maintained. Receptors with this response pattern are called phasic receptors. Receptors that produce a relatively constant rate of firing as long as the stimulus is maintained are known as tonic receptors
Phasic receptors alert us to changes in sensory stimuli and are in part responsible for the fact that we can cease paying attention to constant stimuli. This ability is called sensory adaptation. Odor, touch, and temperature, for example, adapt rapidly; bathwater feels hotter when we first enter it. Sensations of pain, by contrast, adapt little if at all.
Why is the phasic response (which undergoes quick sensory adaptation) useful?
- Improves cellular efficiency
- Increase sensitivity- detecting small changes on large background
What are noxious stimuli and what is our response to them?
Stimuli above the normal range, capable of causing damage
Hence we have reflex action of withdrawing away (withdrawal and aversion behaviour)
Explain 2 types of specialised nerve fibres
- Myelinated A gamma- fast sharp, prickling acute pain due to higher conduction velocity
- Mainly mechanical
- Unmyelinated, C fibres- slow, dull ache
Polymodal e.g. mechanical, thermal
Define polymodality
Sensors can detect more than 1 modality
Explain difference between TRPV1 and Miessner’s Corpuscle
Mechanoreceptors:
TRPV1= polymodality- able to detect different stimuli- pH, temperature above 43 degrees, chemicals e.g. capsaicin, causing burning heat
Meissner’s Corpuscle= detect chemical shanshool, causes tingling/ numbness
Difference between encoding and transducing
Transducing- creation of AP from receptor signals
Encoded- Information encoded to increase sensiticity and increase resolution of info sent to CNS ( not limited by refractory periods)- phasic receptors
In local anaesthetics explain what they do and passive order of block
Block nerve conduction at level of AP
Order of block:
unmyelinated- small myelinated- large myelinated
Limited region of block and used for dental, minor surgery, infection sites and mile inflammatory pain (teething gel)
Explain function of different local anaesthetics
- Weak acids or bases- lipocaine, prilocaine, bupivacaine, cocaine
- Block voltage gated Na channels, acting internally at inner face of channel, hence AP
- Ionised at physiological pH
BH H+ + B
- Removal of H+ making non-polar molecule (B)
- Passes through cell membrane
- Ionised to BH inside cell
- BH blocks Na+ voltage-sensitive ion channels
- No action potentials
