Sensory Systems, Pain And Local Anaesthesia

Question 1

What is pain for?

Answer 1

Tissue protection;
Alerts the organism to potentially fatal or serious tissue damage;
Genetic mutation may result in inability to detect pain - major consequences;

Question 2

How do detect environmental stimuli?

Answer 2

Process called sensory transduction;

Question 3

Dysfunction of sensory transduction can produce…

Answer 3

Loss of sensation (loss of function);
Acute pain;
Chronic pain (phantom limb pain);

Question 4

Dysfunction and pain can arise from…

Answer 4

Spinal injuries (central pain);
Neuronal damage;
Inflammation;
Cancer;
Adverse effects of drugs;
Genetic disorders;

Question 5

Sensory systems

Answer 5

Visual (sight);
Auditory (hearing);
Olfactory (smell);
Vestibular (balance);
Gustatory (taste);
Somatosensory (touch, heat and pain);

Question 6

Receptors for sight, sound, balance

Answer 6

Specialised cell - neurotransmitter transmitted to afferent neuron

Question 7

Receptors for taste, touch, pain

Answer 7

Modified nerve terminal - sensory afferent neuron

Question 8

Receptors response to specific energy, also known as?

Answer 8

Modality

Question 9

3 types of modality

Answer 9

Electromagnetic - light, thermal;
Mechanical - vibration, pressure;
Chemical - pH, pheromones;

Question 10

Specific sensation due to type of receptor activated

Answer 10

Some sensations require many receptors;

Eg - wet sensation - thermal and mechanical receptors;

Question 11

Touch receptors known as…

Answer 11

Mechanoreceptors

Question 12

3 types of touch receptors

Answer 12

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

Question 13

Pacinian corpuscule detects …

Answer 13

Vibration and rapid movements;

Sensations of texture and tickle;

Question 14

How are mechanical stimuli converted to action potential?

Answer 14

Transduction

Question 15

Sensory transduction involves…

Answer 15

Ion channel events at the nerve terminal

Question 16

Electrical events at sensory nerve terminal

Answer 16

Receptor potential; a graded potential

Question 17

Properties of receptor potential for sensory neurons

Answer 17

Transduction always occurs at naked nerve terminal;
Always involves changes in ion channel activity;
Receptor potentials also known as RGP - graded potential; amplitude of RGP proportional to strength of stimulus; non-propagated;

Question 18

Initiation of sensory action potentials

Answer 18

Encoding - how much pressure is needed to give an action potential;
Electrical events at 1st node - initiation when RGP more than threshold

Question 19

How are suprathreshold stimuli encoded? Sensitivity (detection of hot water getting hotter)

Answer 19

Propagated info encoded by AP frequency not their amplitude (all or none);

Question 20

What is sensitivity?

Answer 20

Ability to detect and encode a wide range of stimuli strengths (high sensitivity);
Max AP frequency limited by refractory period;

Question 21

How is sensitivity increased to detect weaker stimuli (RGP less than threshold)?

Answer 21

Use neurons with different AP thresholds
Merkel’s discs - Low threshold - respond to light touch;
Ruffini corpuscles - high threshold - respond to pressure;
Noci receptors - highest threshold - respond to hard knock (pain);

Population encoding (range fractionation) -
Use a larger number of neurons to detect small stimuli;
Increases chance of detection of stimuli;
Larger stimuli excite more neurones;

Question 22

How do we detect stronger stimuli?

Answer 22

Adaptation - temporal change in output in response to stimulus

Question 23

Adaptation

Answer 23

Phasic response - transient sensory information; encodes rate of change (velocity) of stimulus;
Increases cellular efficiency;
Detects small changes in large background;

Question 24

Nerves responsive to noxious stimuli

Answer 24

Pain nerve fibres j

Question 25

Noxious stimuli

Answer 25

Stimuli above normal range;
Stimuli capable of causing damage;
Withdrawal and aversion behaviour;

Question 26

Myelinated nerve fibres

Answer 26

Fast, sharp, prickling, acute pain;

Mainly mechanical;

Question 27

Unmyelinated nerve fibres

Answer 27

Slow dull ache;

Polymodal - thermal, mechanical;

Question 28

Transduction of different sensory modalites

Answer 28

Polymodality

Question 29

Analgesics

Answer 29

Local anaesthetics;
Block nervous condition at the level of AP;
Specific order of block - unmyelinated, small myelinated, large myelinated;
Limited region of block - nerve trunk for Dentist;
Used for minor surgery, Dentist, injection site etc;
Not used for major operations (general);

Question 30

Structure and function of anaesthetics

Answer 30

Weak acids and bases - lidocaine;
Block voltage gated Na channels - act internally on the channel; block AP conduction;
Ionised at physiological pH - membrane impermeable; action is pH dependent;

Question 31

Gating of ion channels - synopsis

Answer 31

Spontaneously active
Voltage gated
Ligand gated
Indirectly ligand gated
Mechanosensitive
Polymodal