Sensory Systems

Question 1

Sensory receptors may be simple free nerve endings or complex structures, give some examples of both?

Answer 1

Nociceptors & cold receptors are free nerve endings

Pacinian Corpuscles (vibration) and Meissner’s Corpuscles (Touch) are more complex structures

Question 2

How is tactile acuity determined?

Answer 2

Larger number of smaller receptive fields (i.e. nerve ending density) allows greater acuity as you can isolate a signal to a smaller area.

Question 3

What is a receptive field?

Answer 3

The area of which a receptor will detect a stimulus

Question 4

What does an “adequate Stimulus” mean?

Answer 4

The specific mode of stimulus the receptor responds to

Question 5

Signal transduction occurs differently for mechanoreceptors and nociceptors, how do mechanoreceptors transduce a signal?

Answer 5

The receptor transduces the adequate stimulus into a depolarisation called the receptor or generator potential which then evokes an AP

Question 6

How is the intensity of a stimulus encoded by mechanoreceptors?

Answer 6

In the size of the receptor or generator potential

Then in the frequency of the APs

Question 7

What are the various types of sensory primary afferent fibres?

Answer 7

Aalpha & Abeta:

• Large myelinated fibres
• Touch, pressure, vibration (all the innocuous mechanoreceptive modalities)
• Also carry proprioception from muscle spindles & GTOs

Adelta:

• Small myelinated fibres
• Carry cold and “fast” pain

C:

• Small Unmyelinated fibres (slowest)
• Carries Warmth & “slow” pain

Question 8

Describe the pathway of mechanoreceptive fibres (Aalpha & Abeta)

Answer 8

They all enter through the dorsal root via the DRG

• 1st order neuron projects striaght up through the ipsilateral dorsal column
• 2nd order neuron decussates in the brainstem
• Projects up to reticular formation –> Thalamus –> Cortex

Question 9

Describe the pathway of thermoreceptive & nociceptive fibres (Adelta & C)

Answer 9

Enters through dorsal root via DRG

• 1st order neuron synapses in the dorsal horn
• 2nd order decussates in spinal cord
• Then projects up the contralateral spinothalamic (anterolateral) tract to the reticular formation –> Thalamus –> Cortex

Adelta- divides into anterior (crude touch) and lateral
C- just goes into lateral

Question 10

What happens if you damage the dorsal column?

Answer 10

You lose ipsilateral mechanoreception (Touch, vibration, proprioception)

Question 11

What happens if you damage the spinothalamic tract?

Answer 11

You lose contralateral thermoreception & nociception

Question 12

Define adaptation?

Answer 12

Receptors can be divided into slow and fast adapting.

Fast adapting receptors such as pressure receptors adapt to a sustained stimulus, only telling you when the stimulus changes

Slow adapting receptors such as muscle spindles maintain the signal until the stimulus is removed

Question 13

Define Convergance?

Answer 13

Multiple receptive fields synapsing onto one neuron, this saves on neurons but reduces acuity

Question 14

What is the difference between a specific and non-specific pathway per convergence?

Answer 14

A specific pathway has receptors of the same modality converging on one neuron

A non-specific pathway has receptors of different modalities converging on one neuron, usually pain and touch. These can’t be distinguished

Question 15

Explain the concept of lateral inhibition?

Answer 15

The neuron closest to the stimulus inhibits the synapses of the neighbouring neurons.

This cleans up sensory info giving you a better definition of the stimulus boundary

(So the central neuron has a higher than tonic frequency of APs and the neighbouring ones have a lower than tonic level)

Question 16

What are the different types of pain?

Answer 16

Fast (initial) pain - Sharp & Stabbing - Uses Adelta fibres

Slow (Delayed) pain - Diffuse & Throbbing - Uses C fibres

Question 17

What is a possible explanation for referred pain?

Answer 17

Convergence of neurons from the viscera and other tissues e.g. the skin

Question 18

How are nociceptors activated?

Answer 18

• Low pH activates an ASIC receptor
• Heat activates a TRPV1 receptor
• Also by local chemical mediators such as prostaglandins and bradykinin (B2 Braykinin receptor) released on tissue injury

Question 19

How do prostoglandins affect pain?

Answer 19

They stimulate nociceptors themselves but more importantly sensitize nociceptors to bradykinin

Question 20

What is the Gate control theory?

Answer 20

The ability to “gate” pain signals, preventing them from crossing the 1st-2nd order synapse

Question 21

How is pain gated by segmental control?

Answer 21

Mechanoreceptive fibres (Aalpha/beta) activity in the same region

• -> Activates an inhibitory internueron
• -> Releases opioid peptides (endorphins)
• -> Inhibit the nociceptive synapse

(Probably the basis for TENS & rubbing it better)

Question 22

How is pain gated by Descending Control?

Answer 22

Peri-aqueductal grey (PAG) matter in cortex gives off a descending fibre

• > Synapses in Nucleus Raphe Magnus (NRM)
• > Travels down to the spinal level of nociceptive fibres
• > Activates the inhibitory interneuron
• > Releases opioid peptides (endorphins) that inhibit the nociceptive synapse`

Question 23

What does TENS stand for and what does it do?

Answer 23

Transcutaneous Electrical Nerve Stimulation

Analgesic – It activates innocuous mechanoreceptors and so inhibits the nociceptive synapse. (Segmental Gate control)

Question 24

How do NSAIDs work as analgesics?

Answer 24

Remember prostaglandins sensitize nociceptors to bradykinin

So NSAIDs prevent the formation of prostaglandins by inhibiting the cyclo-oxygenase enzyme (Arachidonic acid –> Prostaglandins)

Therefore they work best with inflammation associated pain

Question 25

How do local anaesthetics work as analgesics?

Answer 25

The block Na+ channels preventing AP transmission

Question 26

How do opiates work as analgesics?

Answer 26

• Directly reduce nociceptor sensitivity
• Block neurotransmitter release at the 1st-2nd order synapse in the dorsal horn (Pain Gate Control)
• Activate descending pathways in the peri-acqueductal Grey (PAG) matter (Descending Gate control) to inhibit the 1st-2nd order synapse

Question 27

What is visual acuity determined by?

Answer 27

Depends on:

1- Two-point discrimination

Determined by the size of the receptor field and the density of innervation
Receptor field describes the size of the area of a neurone which can be stimulated to trigger firing of that neurone. The greater the receptor field, the less precise the perception is
Density of innervation: describes the number of neurones present in a given area. The greater the density, the more accurate the firing.

SO: small receptive field + high density of innervation = high acuity

2-Convergence and divergence:

Convergence:When multiple 1st order neurones stimulate 1 2nd order neurone
Divergence: When 1 1st order neurone stimulates multiple 2nd order neurones

SO: convergence= reduced acuity and Divergence= higher acuity

3-Lateral inhibition