Sensory Pathways: Lecture

Question 1

How are sensory receptors formed?

Answer 1

Individual axons of sensory nerves have modified terminals

Question 2

What are two broad categories of terminal type?

Answer 2

Free ends - Thermoreceptors, Nociceptors

Enclosed – Mechanoreceptors

Question 3

What are the four classifications of sensory fibres?

Answer 3

Aa - Fastest, Proprioception (Muscle)

Ab - Fast, Mechanoreceptors - Innocuous physical (Skin)

Ad - Quite fast, Thermo and Nociceptors

C - Slow, Aching pain, thermo and Itch

(Noxious mechanical, thermal and chemical stimulation)

Question 4

What do Thermoreceptors consist of?

Answer 4

A delta and C type sensory fibres

Transient Receptor Potential (TRP) Ion channels that respond to temperature

Question 5

What are some Subsets of Transient receptor potential channels?

Answer 5

Heat Responsive
TRPV 1-4

Cold Responsive
TRPM8
TRPA1

Question 6

What are four types of mechanoreceptors?

Answer 6

Meissner’s Corpuscle - Fine, discriminative touch, Low frequency, Vibration

Merkel Cell - Light touch, superficial pressure

Pacinian Corpuscle - Deep pressure, High Frequency vibration, tickling

Ruffini Endings - Continuous Pressure or touch, stretch

Question 7

What is the absolute threshold of a stimulus?

Answer 7

Point of stimulus intensity where it is detected 50% of the time.

Higher intensity = more APs because detected more often, more neurotransmitter release

Question 8

What are two types of receptor in regards to adaptation?

Answer 8

Tonic Receptors - Very Slow or No adaptation, respond for stimulus duration
e.g. Merkel cells respond as long as touch persists

Phasic Receptors - Fast Adapting, respond to CHANGE in stimulus strength
I.E At start and End of GP, e.g. Pacinian Corpuscle respond when pressure applied and released

Question 9

What is a sensory field?

Answer 9

Area of skin that activates SINGLE Neuron when stimulated

Question 10

What is two point discrimination?

Answer 10

Minimum distance at which two point can be perceived as separate when stimulus applied, the smaller the sensory field (e.g hands) the smaller this distance is, compared to places like back.

Question 11

What is a dermatome?

Answer 11

Area of skin innervated by a single spinal nerve

Question 12

Where are the sensory cell bodies of the face and body?

Answer 12

Dorsal Root ganglia - Body

Trigeminal ganglia – Face

Question 13

What are the features of the dorsal horn?

Answer 13

Contains Projection Neurones and Interneurones
Divided into Rexed Laminae (I-VII)
Pain and temperature fibres (A delta and C) terminate in lamina I-II (Superficial)
Innocuous mechanical stimuli fibres (A beta and A alpha) terminate in laminae III-VI (deep)

Glutamate is the main excitatory transmitter for projection neurones

Question 14

What is the purpose of the interneurones in the dorsal horn?

Answer 14

Lateral inhibition – increase the resolution of a stimulus and allow for greater localisation by inhibiting the lateral fibres to the main fibre

Question 15

What are the features of the dorsal columns?

Answer 15

Both dorsal columns transmit innocuous mechanical stimuli through second order nuclei

Below T6 lower limbs ipsilateral gracile (The medial dorsal column for the legs)

Upper limbs ipsilateral cuneate tract (The lateral dorsal columns for the arms)

Question 16

Where do second order sensory fibres terminate?

Answer 16

2nd order CROSS IN MEDULLA - Form contralateral medial lemniscus tract - terminate in Ventral posterior lateral nucleus

Lower limbs are lateral - topographic representation

Question 17

Where do third order sensory fibres terminate?

Answer 17

3rd order neurones from VPL project to somatosensory cortex
Primary somatosensory cortex in postcentral gyrus
Secondary somatosensory cortex in parietal operculum

Question 18

Where do pain, crude touch and temperature sensory fibres travel?

Answer 18

Travel through spinothalamic (anterolateral) tracts
First order terminate in dorsal horn, second order desscutate immediately and ascend contralateral.
Second order terminate in Ventral posterior lateral nucleus in thalamus

Crude touch is mediated by A delta fibres (Free nerve ending).

Question 19

What can loss of only certain sensations suggest?

Answer 19

Area of spinal cord lesion, injury, disease etc can be tested Specific loss of sensation e.g sensation not pain and temp can suggest only dorsal horn affected.

Question 20

What is the definition of pain and how is it transmitted?

Answer 20

Pain - Unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage

Transmitted between neurones via Glutamate synapses

Sensory component - Spinothalamic tract (To thalamus and cortex)

Emotional component - Spinoreticular tract (To hypothalamus and Amygdala)

Question 21

What is the ‘pain signature’?

Answer 21

Brain activity in: 
Cortex:  
SI  
SII  
Insula cortex 
Anterior cingulate cortex  
Prefrontal cortex  

Amygdala
Cerebellum
Brainstem

Question 22

What is the gate control theory?

Answer 22

The gate control theory proposes that stimulating fibres with non-noxious physical sensation facilitates inhibitory interneurones in the dorsal horn (Usually via Ab fibres) – reducing the chance of a painful stimuli being projected to the brain, therefore reducing the intensity of the pain felt

Question 23

What is descending pathway inhibition?

Answer 23

Occurs in Periaqueductal Grey (PAG), via Monoamine inhibition of pain (Noradrenaline more so than Serotonin) preventing nociceptive processing. High conc of mu opioid receptors in PAG and RVM allow the areas to release endogenous opioids to bring about analgesia.

Placebo Analgesia can occur through activation of descending inhibition pathways

Question 24

What is primary hyperalgesia?

Answer 24

Peripheral Sensitization – Often occurs due to inflammation, chemical mediators such as Histamines bind to the nociceptors and can either depolarize them or lower the threshold of a stimulus needed to depolarize them. This leads to more frequent generator potentials and a greater intensity of pain.

It can also cause primary allodynia when non noxious stimuli can now trigger generator potentials.

Question 25

What is secondary hyperalgesia?What is secondary hyperalgesia?

Answer 25

It is Central Sensitization

NMDA receptors activate, this allows Ca2+ in, increases synaptic strength, reduces inhibitory influences, increasing pain sensitivity.

Persistent NMDA activation causes neural plasticity and chronic pain.

Causes allodynia and hyperalgesia and can increase receptive field size

Question 26

What are the three neuropathic pain clusters? How is neuropathic pain placed into ‘phenotypes’?

Answer 26

Sensory loss
Thermal hyperalgesia
Mechanical hyperalgesia

Pain placed into phenotypes over what noxious and non-noxious stimuli cause it and its intensity.

Question 27

What treatments are given for neuropathic pain?

Answer 27

Despite effect of endogenous opioids, exogenous have little effect

SNRI (serotonin noradrenaline reuptake inhibitors) and TCA antidepressants effective

SSRI not very effective, suggest noradrenaline more important in descending inhibition

Some suggestion that Noradrenaline is inhibitory to pain and serotonin stimulatory (or inhibitory to thermal sensation)

Question 28

Describe Neuromodulation in chronic pain patients

Answer 28

Non-invasive primary motor cortex stimulation (transcranial direct current stimulation, tDCS)
Look at:
Activation of endogenous analgesic systems in the brain
PAG
Anterior cingulate cortex

Analgesic mechanisms unclear – however gives insight into which personalised medicines are more likely to effective for each patient.