W1: Sensory pathways and pain physiology

Question 1

What is considered a somatosensation?

Answer 1

Proprioception
Vibration
Light touch

Gross touch
Pain
Temperature

Question 2

What is the common architecture in all somatosensory pathways?

Answer 2

Consists of 3 order ascending neurons:
Sensory Neuron
Spinal cord/brain stem
Thalamus

Question 3

What are the different types of somatosensory receptors?

Answer 3

Mechnoreceptor
Nociceptors
Thermoreceptors

Question 4

What are the different types of mechanoreceptors and what are their functions?

Answer 4

Meissner Corpuscle - fine discriminative touch or tactile touch
Merkel Discs - pressure
Pacinian corpuscle - vibration
Muscle spindles - limb position

Question 5

What are the different types of nociceptors and what is their function?

Answer 5

A-delta mechanical (encapsulated) - pin prick
C-polymodal (free nerve endings) - tissue damage.

Question 6

What are the different types of thermoreceptors and what is their function?

Answer 6

Free nerve endings - cold or warmth

Question 7

What does somatosensory mean?

Answer 7

A sensation that can occur anywhere in the body (rather than in a specific sense organ).
Associated with the primary sensory cortex.

Question 8

What are the different somatosensory nerve fibres?

Answer 8

A-alpha
A-beta
A-dela
C

Question 9

What are the properties of an A-alpha nerve fibre?

Answer 9

Somatosensory
Role in proprioception
Is myelinated
12-20micrometers in diameter
High conductivity at 75-120 m/s

Question 10

What are the features of a A-Beta nerve fibre?

Answer 10

Somatosensory nerve fibre
Role in pressure and vibration
Myelinated
Conduction: 30-75m/s
Diameter: 6-12 micrometers

Question 11

What are the features of an A-delta nerve fibre?

Answer 11

A somatosensory nerve fibre
Role in fast pain, cold.
is myelinated
5-30m/s conduction speed
1-6 micrometers in diameter

Question 12

What are the features of a C-fibre?

Answer 12

Somatosensory nerve fibre
Role in slow pain, warmth
is NOT myelinated
Conduction of 0.5-2 m/s
Diameter of less than 1.5micrometers

Question 13

Compare the conduction velocities of different somatosensory nerve fibres?

Answer 13

Fastest
A-alpha fibre
A-beta
A-delta
C fibres
Slowest

Question 14

What is the difference between pain and nociception?

Answer 14

Pain - an unpleasant sensation (consicous interpretation) of something unpleasant that may or may not correlate with actual tissue damage

Nociception - neural/physiologal response to a noxious stimuli, can be subconscious.

Pain is possible without nociception

Question 15

What are the four different processes in pain physiology?

Answer 15

Transduction
Transmission
Perception
Modulation

Question 16

What is meant by the transduction of pain?

Answer 16

Noxious stumuli causes cell damage which releases sensitizing chemicals
These activate nociceptors and lead to the generation of an action potential.
Therefore - the transformation of noxious stimuli into electrical activity.

Question 17

What does nociceptors are polymodal mean?

Answer 17

Can respond to many different stimuli
Such as:
Temperature extremes
Trauma
Hypoxia
Chemicals

Question 18

What are some of the different substances that can trigger an action potential in a nociceptor?

Answer 18

Prostaglandins
Bradykinins
Seratonin
Substance P
Histamine

Question 19

What are some mechanism that may cause pain transduction?

Answer 19

Stimulus activates:
- Mechanically gated Na and Ca ion channels
- heat sensitive ion channels
- mast cells to release 5HT/histamin
- damaged cells to release substances

Question 20

What are some of the substances released by damaged cells that can cause pain transduction?

Answer 20

Bradykinin: action on G-protein coupled receptors
ATP on ATP- sensitive K channels
Potassium
Prostaglandins

Question 21

What are some of the effects of pain transduction on blood vessels?

Answer 21

Blood vessel typically dilate to allow the inward migration of immune cells or growth factors.

Question 22

What is meant by pain transmission?

Answer 22

The conduction of electrical impulse through the nervous system.
Site of injury - spinal cord - brain stem - cerebrum.

Question 23

What is the pathway in pain transmission?

Answer 23

AP activates the afferent nerve fibre (1st order neuron) this has a cell body in the dorsal root ganglion projects into the dorsal horn of the spinal cord where it synapses and releases a pain neurotransmitter such as substance P.
This activates a 2nd order neuron. Desicates in the anterior white comissure to the contralateral side and travels up the spinal cord in the spinothalamic tract.
Synapses in the thalamus onto a third-order neuron which then projects up into the sensory cortex.

Question 24

What specific area in the dorsal horn are 1st order sensory neurons synapsing in?

Answer 24

May be called the substantia gelatinosa or the nucleus propius.

Specifically Laminae 1-5

Question 25

What is meant by the integrating centre in pain transmission?

Answer 25

The spinal cord As receives afferent information, and can trigger multiple efferent pathways by interneurons

Question 26

What are the different efferent pathways that can be activated in the spinal cord in response to pain?

Answer 26

1 - efferent to brain/ascending tract - spinothalamic tract - role in memory and interpretation etc 2 - the reflex ark Spilt into an excitatory internuron to a contracting muscle And in inhibitory interneuron to a muscle you want to relax

Question 27

What is meant by pain perception?

Answer 27

The subjective sensation of pain

Question 28

What are the main two types of pain fibres?

Answer 28

A fibres - myelinated respons to mechanical pressure and produce sharp, localised and fast pain, further split into beta (pressure nd vibration) and delta (temperature) C fibres - chronic, slow, dull or knawing pain, often delayed after the onset of injury

Question 29

What are the different regions of the brain that contribute to pain perception?

Answer 29

Thalamus/sesnory cortex - localisation Limbic cortex - emotional experience Reticular formation - alertness. Autonomic interconnection

Question 30

What is meant by pain modulation?

Answer 30

The Process of altering the pain signals along the transmission pathway of pain.

Question 31

What are the different methods of pain modulation?

Answer 31

Gate theory Descending Inhibitory Pathway Cortical

Question 32

What is the Gate theory of pain modulation?

Answer 32

Pain signals can be interrupted in the substantial gelitanosa of the spinal cord By this non-noxious stimuli can suppress noxious stimuli and decrease impulse transmission up the spinothalamic tract.

Question 33

By what mechanism can rubbing a wound alleviate pain?

Answer 33

Painful stimuli activates the alpha delta and c fibres, impulse travels along primary neuron to the dorsal horn Deep touch is detected by pacinian corpuscle and projects along the DCML also into the dorsal horn via alpha beta neuron. 1. the impulse from deep touch activates an inhibitory internueron which inhibits the secondary neuron in the pain pathway so pain not transmitted (this is seen in substantia gelatinosa) 2. Neuron has an axonal branch that directly inhibts the synapse Deep touch continues to travel up the the brain (no secondary neuron in the spinal cord) Therefore only deep touch is perceived not pain * note pain fibres may also modulate deep touch but normally to a lesser extend

Question 34

What is the mechanism of descending inhibitory pathway of pain modulation?

Answer 34

Periaqueductal gray - primary control centre for descending pain modulation Descending pain inhibitory tracts originate here Synapse in the nucleus raphe magnus (seratonin or enkephalin neurotransmister) Neuron projects into.. Can act in the dorsal horn to inhibit the synapse between the primary and secondary neurons in the pain pathway 1: releases serotonin which activates enkephalin releasing neurons to inhibit the transmission to secondary neuron 2: Releases serotonin or norepinephrine to act directly on and inhibit the synapse.

Question 35

What are the different excitatory ubstances in neurotransmission of pain?

Answer 35

NT - glutamate NP - Substance P

Question 36

What are the different inhibitory substances in the neurotransmission of pain?

Answer 36

NT - GABA, norepinephrine, 5HT NP - enkephalins

Question 37

What anaglesics can be used to modify pain at the site of tissue injury?

Answer 37

Local anesthetics (wound infiltration field block) NSAIDs/Cox-2 inhibitors Opioid agonists

Question 38

What anaglesics can be used to stop peripheral nerve input?

Answer 38

Local anesthetics (peripheral nerve blocks)

Question 39

What anaglescis are effective at the spinal cord level?

Answer 39

These effect synapises between the primary and secondary neurons. Are neuromodulators Includes: Local anesthetics (epidural) Opiods Agonists NMDA antagonists

Question 40

What anaglesics are effective in the cerebral cortex?

Answer 40

Opiod agonists Acetominophen ketamine

Question 41

What are some limitations of anaesthetics?

Answer 41

Local anaesthetics are very pH sensitive, with infection the pH may drop and the environment becomes acidic - this will prevent the local anaestethic from working.

Question 42

What is the cortical method of pain modulation?**

Answer 42

Changing the affective-motivational aspects of pain, so it looses its emotional and aversive components By changing the way we think about pain we can reduce the feeling of pain.