Sensory Pain and Local Anaesthesia

Question 1

What is pain for

Answer 1

Tissue protection; alerting the organism to potentially fatal or serious tissue damage

Question 2

Sensory Transduction

Answer 2

How we detect environmental stimuli

Question 3

What can be produced by dysfunction of sensory transduction

Answer 3

Loss of sensation/function
Acute pain
Chronic pain (phantom limb)

Question 4

How can sensory and pain dysfunction arise

Answer 4

Cancer
Drugs
Spinal/neuronal damage
Genetic disorders

Question 5

6th Sensory System

Answer 5

Vestibular (balance)

Question 6

What is a better way to refer to the ‘touch’ sensory system

Answer 6

Somatosensory (touch, heat, pain)

Question 7

How is sight, sound and balanced processed differently to taste, touch and pain

Answer 7

Taste, touch and pain have no specialised cells and instead only have modified nerve terminals

Question 8

Modalities responded to by sensory receptors

Answer 8

Electromagnetic spectrum (light, thermal)
Mechanical (Vibration, pressure)
Chemical (pheromones, pH)

Specific sensation due to type of receptor activated

Question 9

Polymodal Sensation

Answer 9

Sensations that arise from more than one modality

e.g. ‘wetness’ due to mechanical and thermal receptoes

Question 10

Types of Mechanoreceptors

Answer 10

(These are touch receptors)

Those that feel for:
Stretch (muscle spindles)
Sound energy (hair cells)
Physical displacement (skin and pain receptors)

Question 11

Pacinian Corpuscules

Answer 11

They detect vibration and rapid movements (e.g. texture and tickle)

They are on a naked end of the the modified sensory end of the afferent neuron

Onion like cell arrangements

Question 12

Electron Micrograph of Pacinian Corpuscule in skin

Answer 12

Left is transverse, right is longitudinal

Question 13

How are mechanical stimuli converted into action potentials at the pacinian corpuscule

Answer 13

Mechanosensitive Na+ channel is tethered to the cytoskeleton of the affarent neuron via cytoskeletal anchorages

Application of pressure to outside via pacinian corpuscule, nerve terminal is deformed and anchorages detatch from Na+ channel and the ion rushes in

This generates electrical activity depending on the severity

Question 14

How are mechanical stimuli converted into action potentials at the pacinian corpuscule

Answer 14

Mechanosensitive Na+ channel is tethered to the cytoskeleton of the affarent neuron via cytoskeletal anchorages

Application of pressure to outside via pacinian corpuscule, nerve terminal is deformed and anchorages detatch from Na+ channel and the ion rushes in

This generates electrical activity depending on the severity

Question 15

RGP

Answer 15

Receptor Generated Potential

Question 16

Where does transduction event occur at for sensory neurons

Answer 16

At naked nerve terminal and involves changes in ion channel activity

Question 17

Is amplitude of RGP proportional to stimulus strength

Answer 17

Yes but it is non-propagating so there is a deceremental passive spread across membrane (localised)

Question 18

Where is the presence of voltage-gated Na+ and K+ channels the highest in the detecting part of sensory nerve cells

Answer 18

Question 19

How does a mechanosensitive nerve receptor potential lead to nerve propagation

Answer 19

Question 20

Pressure needed to result in action potential encoding potential on mechanosensitive nerve; how are suprathreshold stimuli encoded (e.g. detection of even hotter water)

Answer 20

Enough to cross the threshold potential

Since an action potential is an all or nothing event, we do not vary the magnitude of the impulse and instead vary the frequency of such impulses based upon intensity

Question 21

Sensitivity

Answer 21

Ability to encode and detect a wide range of stimuli strengths

Maximum AP frequency limited by refractory period (usually 3ms, as low as 1); upper limit (300Hz-1kHz)

Question 22

How are stimuli where RGP is < AP threshold detected

Answer 22

Different neurons have different AP thresholds across the skin

Question 23

Different threshold neuron molecules

Answer 23

Question 24

Population encoding

Answer 24

The use of a larger number of neurons to detect smaller stimuli to increase chance of detection of smaller stimuli

(Stronger stimuli activates more neurons)

Question 25

Which two body parts have greatest number of nerve receptors compared to torso, forearm, leg and foot

Answer 25

Head and especially Hands

Question 26

Why is a hot bath quite painful at first but after a bit of time becomes more comfortable

Answer 26

Fast adaptation

Action potential is not maintained; there are some to indicate the potential pain but they decay quite rapidly

Question 27

How do we detect stepwise increases in temperature of a bath

Answer 27

If the frequency were simply continually increased, it would be hard to detect specific differences

Instead, adaptation leads to this kind of behaviour

Question 28

Purposes of Adaptation

Answer 28

Detecting temporal change in output in response to stimulus

Transient sensory information

Encodes rate of change of stimulus

Question 29

Which stimuli do pain nerve fibres respond to

Answer 29

They respond to noxious stimuli (stimuli above normal range)

Process known as nociception

Question 30

Compare Aδ vs C pain nerve fibres (Consider stubbing toe)

Answer 30

Aδ fibres are faster and myelinated; for fast sharp, pricking acute pain
Mainly mechanical

C fibres are slow and unmyelinated; for dull aches
Polymodal - e.g. mechanical, thermal

For toe stubbing, the sharp pain is from Aδ fibres then the slower and longer dullness if from C fibres

Question 31

TRPV1 Receptors

Answer 31

Polymodal sensory receptors capable of detecting: H+ (pH), temp > 43C, chemicals like capsaicin

Generate burning sensation

Responsive to Na+, Ca2+ cations to generate action potentials

Question 32

Analgesics and order of block

Answer 32

Very localised anaesthetics that block nervous conduction at the level of the action potential

Block in order of:
Unmyelinated > Small myelinated > Large Myelinated
(Pain blocked first)

Question 33

How to identify local anaesthtics by name with examples

Answer 33

They end in -aine

Lidocaine, Novocaine, Cocaine

Question 34

How do analgesics work

Answer 34

Block voltage gated Na+ channels to block AP conduction

Analgesics are ionised at physiological pH so they get into the neuron cytosol by being pH dependent

Non-polarised blocks get into cell at equilibrium but are inactive as such; once within cytosol they ionise once again and act as a block

Question 35

Innocuous sensory signals

Answer 35

Non-noxious (so not pain) sensory signals