Sensory transduction, pain and local anaesthesia

Question 1

What is pain for and does everyone experience it?

Answer 1

• Tissue protection
• Alerting organism to potentially fatal or serious tissue damage
• Genetic mutation can result in inability to detect pain

Question 2

What process is used to detect environmental stimuli?

Answer 2

Sensory transduction

Question 3

What can dysfunction of sensory transduction produce?

Answer 3

Can lead to:

• loss sensation (loss function)
• acute pain
• chronic pain (phantom limb pain)

Question 4

How can dysfunction and pain arise?

Answer 4

• 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

Question 5

Outline sensory systems

Answer 5

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

Question 6

Outline 2 types of receptor

Answer 6

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)

Question 7

What is the modality- give e.g,? Also what determines specific senation we feel?

Answer 7

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

Question 8

Outline types of mechanoreceptors

Answer 8

• Stretch (muscle spindles)
• Sound energy (hair cells ), hearing and vestibular apparatus
• Physical displacement- pain, subcutaneous (skin) receptors

Question 9

What does the Pacinian Corpuscle detect? How is this converted into action potential?

Answer 9

• 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

Question 10

Features of receptor potential/ generator potential (RGP)

Answer 10

• 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)

Question 11

How much pressure is needed to make an action potential? what is this initiation of an action potential called

Answer 11

Action potential initiation when receptor graded potential exceeds the threshold level

Question 12

How is propagated information encoded/ how is a more stringer or larger stimulus encoded?

Answer 12

• We will see an increase in the frequency of action potentials

Question 13

What is high sensitivity? And what limits this?

Answer 13

• High sensitivity- ability to encode and detect a wide range of stimuli
  Limited by the refractory period

Question 14

How do we increase the sensitivity? (2)

Answer 14

• 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

Question 15

Explain process of adaption

Answer 15

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

Question 16

Outline differences between phasic and tonic receptors

Answer 16

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.

Question 17

Why is the phasic response (which undergoes quick sensory adaptation) useful?

Answer 17

• Improves cellular efficiency

- Increase sensitivity- detecting small changes on large background

Question 18

What are noxious stimuli and what is our response to them?

Answer 18

Stimuli above the normal range, capable of causing damage

Hence we have reflex action of withdrawing away (withdrawal and aversion behaviour)

Question 19

Explain 2 types of specialised nerve fibres

Answer 19

• 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

Question 20

Define polymodality

Answer 20

Sensors can detect more than 1 modality

Question 21

Explain difference between TRPV1 and Miessner’s Corpuscle

Answer 21

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

Question 22

Difference between encoding and transducing

Answer 22

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

Question 23

In local anaesthetics explain what they do and passive order of block

Answer 23

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)

Question 24

Explain function of different local anaesthetics

Answer 24

• 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