SOMATOSENSORY PATHWAYS

Question 1

What receptors are responsible for the sensory modalities of touch and proprioception?

Answer 1

Mechanoreceptors

Question 2

Describe the structure of mechanoreceptors involved in touchand proprioception.

Answer 2

The receptor is NOT a separate entity but is actually the peripheral terminal of the peripheral axon of the primary sensory neuron.

Question 3

Describe the structure of a Pacinian corpuscle and explain how this structure relates to its function.

Answer 3

There is an axonal ending in the middle and it is wrapped around several concentric circles of epithelial cells – this allows the receptor to be very sensitive to vibration.

This is why mechanoceptors like the pacinian corpuscle are examples of encapsulated nerve endings because they have things surrounding the nerve ending

Question 4

What is the difference between slow adapting and fast adapting receptors?

Answer 4

Slow adapting receptors continue firing impulses for as long as thestimulus is present Fast adapting receptors tend to fire at the start of the stimulus and sometimes when the stimulus switches off but they tend to fade in the middle

Question 5

What type of receptors are mechanoreceptors?

Answer 5

Mixture of slow and fast adapting receptors

Question 6

Describe how sensory neurons vary in their properties.

Answer 6

They vary in SIZE and CONDUCTION VELOCITY

Question 7

What are the two classifications of axons?

Answer 7

Anatomical = based on axon diameter (labelled using LETTERS)

Physiological = based on conduction velocity (labelled using ROMAN NUMERALS) As axon diameter and conduction velocity are related, there is a lot of overlap in the classifications

Question 8

Describe the general structure of sensory neurons that conveytouch and proprioceptive information.

Answer 8

They are LARGE and have a FAST conduction velocity (a beta fibres?)

the receptor at the end of the nerve that detects touch and proprioception is a mechanoreceptor

Question 9

What is a receptive field?

Answer 9

An area of skin that is innervated by one single sensory axon and its branches

Question 10

Describe how the receptive fields in the lips and mouth vary from the receptive fields of the upper arm.

Answer 10

Lips and Mouth – high-density innervation with very small receptive fields. ie a lot of sensory neurons, each with their own individual small receptive fields. this allows much better localisation of the sensory stimulus

Upper arm – larger receptive fields and less dense innervation (less percise perception)

NB The fingers also have many densely packed mechanoreceptors with small receptive fields.

Question 11

Describe how neurons can code for the intensity of a stimulus.

Answer 11

It is coded by the FREQUENCY of the action potentials going down the sensory fibres, NOT AMPLITUDE OF ACTION POTENTIAL

Question 12

Which part of the spinal cord carries sensory axons for touch and proprioception?

Answer 12

Dorsal columns

Question 13

What are the two tracts of the dorsal column system? What type of sensory fibre enters this ascending dorsal column pathway?

Answer 13

Cuneate tract - information from upper limbs + body above T6

Gracile Tract - information from lower limbs + body below T6

Think of the alphabet - Cuneate is above T6

NB these pathways are part of dorsal column system and so are therefore mainly receiving innocuous (non harmful) stimuli incl:

fine discriminative touch

vibration

proprioception

•Aβ fibers enter via the dorsal horn and enter the ascending dorsal column pathways

Question 14

Where do these neurons synapse?

Answer 14

They synapse in the Cuneate and Gracile Nuclei in the caudal medulla

Question 15

Describe what happens after these neurons synapse and the tract that they run in.

Answer 15

First order neurons synapse in the cuneate/gracile nuclei in caudal medulla. The second order neurons then cross the midline (decussation) at the caudal medulla and continue up the brainstem via the MEDIAL LEMNISCUS pathway. This pathway brings the second order neurons from medulla to the ventral posterior lateral nucleus (VPL) of the thalamus

NB pyramidal decussation is for MOTOR fibres and it occurs on the anterior side of the medulla. decussation of the sensory fibres happens on the posterior side of the medulla

Question 16

Which thalamic nucleus is responsible for relaying somatosensory information from the neck down?

Answer 16

Ventral Postero-lateral

Question 17

Describe the passage of the third order sensory neuron.

Answer 17

The third order neurone travels from the ventral postero-lateral nucleus in the thalamus to the primary somatosensory cortex

Question 18

What is the main sensory nerve of the face?

Answer 18

Trigeminal Nerve (CN V)

Question 19

Where does the trigeminal nerve enter the brainstem and where does it synapse with a second order neuron?

Answer 19

Pons

It synapses at the trigeminal cranial nucleus with a second order neuron

axon of the second order neuron then crosses the midline and joins the medial end of the medial lemniscus towards the VPL nuclei in the thalamus to synapse with a third order sensory neuron

Question 20

Describe the passage of this second order neuron.

Answer 20

The second order neuron crosses the midline (decussation) and joins the medial part of the median lemniscus

Question 21

Which thalamic nucleus is responsible for relaying sensory information from the face?

Answer 21

Ventral Postero-medial

Question 22

What is lateral inhibition?

Answer 22

Overlapping of receptive fields may make it difficult to distinguish between 2 stimulus locations (because some locations are occupied by adjacent receptive fields). Lateral inhibition helps to pinpoint the location of stimulus by preventing the overlapping areas

Lateral inhibition takes place in the dorsal horn of spinal cord, Mediated by inhibitory interneurons

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Question 23

What is the purpose of lateral inhibition?

Answer 23

Improves the resolution of localising the stimulus

Question 24

Name the three parts of the somatosensory cortex.

Answer 24

Primary Somatosensory Cortex (SI)

Secondary Somatosensory Cortex (SII)

Posterior Parietal Cortex

All 3 of these areas are involved in taking in sensory information. S2 and posterior parietal cortex tend to receive the info after it has been analysed to a certain extent from S1

Question 25

What is the posterior parietal cortex mainly involved in?

Answer 25

Spatial relationships

Question 26

What is the most common cause of peripheral neuropathy?

Answer 26

Diabetes mellitus

chronic high blood sugar damages nerves

Question 27

What are the 5 main somatosensory modalities

Answer 27

Discriminative touch

Temperature

Proprioception

Visceral functions

Nociception (pain etc, nociception basically means harmful stimuli)

Question 28

Define stimulus threshold

Answer 28

•= the level of intensity of a stimulus (stimulus strength) needed to produce a positive response of detection 50% of the time. Ie the strength of stimulus needed such that 50% of the time the stimulus at this strength will be detected by sensory fibres

Question 29

How does increased stimulus strength and duration lead to greater stimulus intensity?

Answer 29

more strength = more frquent action potentials

duration= how long it has been firing for

Increased stimulus strength and duration = increased neurotransmitter release = greater intensity

greater strength gives u higher frequency of action potentials and if the stimulus lasts long then it is likely that the action potentials will go on for longer and hence lead to more neurotransmitter release

Question 30

2 types of adaptation

Answer 30

•Slow Adapting (tonic receptors)

•continue to fire impulses for as long as the stimulus is present

•Fast Adapting (phasic receptors)

•tend to fire at the start of the stimulus and sometimes when the stimulus switches off but they tend to fade in the middle

•

Question 31

describe the somatotopic organisation of the primary somatosensory cortex

Answer 31

point to point correspondence

Areas with the body with densely packed sensory receptors will equal to a greater ‘area’ occupied in the primary somatosensory cortex

Question 32

other than the primary somatosensory cortex, where else has this somatotopic organisation

Answer 32

Dorsal column nuclei in the brainstem (medulla)

dorsal column nuclei = gracile nucleus + cuneate nucleus

Question 33

2 types of neurons in the dorsal horn?

Answer 33

• Those with axons that project to the brain (projection neurons)
• Those with axons that remain in the spinal cord (interneurons)

Question 34

what receptor types have free nerve endings and which ones have enclosed nerve endings

Answer 34

free nerve endings: thermoreceptors and nociceptor

enclosed nerve ending: mechanoreceptor

Question 35

3 classifications of sensory neuron fibres, what do they respond to and what is the speed of transmission

Answer 35

• Aβ-fibres: innocuous (non harmful) mechanical stimulation, very fast at transmitting
• Aδ-fibres: noxious(harmful) mechanical and thermal stimulation, fast but not as fast as AB
• C-fibers: noxious (harmful) mechanical, thermal and chemical stimulation, slow

Question 36

How precise is the localisation of pain and temperature stimuli?

Answer 36

Not as precise as the localisation of the stimuli that travel in the dorsal column pathway such as discriminative touch

Question 37

what subpathways does the spinothalamic pathway consist of and what stimuli goes through these pathways

Answer 37

The spinothalamic pathway – anterior and lateral pathway subtypes

Anterior spinothalamic tract = crude touch

Lateral spinothalamic tract= pain + temperature

NB Crude touch is mediated by Adelta fibres (Free nerve ending). Fine discriminative touch is mediated by A beta fibres (Meissner’s corpuscles)

Question 38

Pathway for the spinothalmaic tracts?

Answer 38

1st order afferent axons terminate in spinal cord, synapse and the 2nd order neurons decussate immediately to form the spinothalamic tract.

2nd order neurons ascend and reach the VPL nucleus in the thalamus and synapses with 3rd order neuron. 3rd order neuron then goes to primary somatosensory cortex

Question 39

compare spinothalamic tract with dorsal column pathway

Answer 39

slide 38 and 12

Question 40

2 point discrimination will go through what sensory pathway

Answer 40

2 point discrimination requires fine touch sensation to distinguish two locations so it would be the dorsal columns pathway

NB crude touch= you can feel someone touched you but u cant localise it and this is transmitted via the spinothalamic pathway

Question 41

how can you test the integrity/patency of the ascending pathways (spinothalamic and dorsal columns)?

Answer 41

QST quantitative sensory testing

These tests are non invase and they test multiple sensory modalities eg heat/temperature for spinothalamic tract and 2 point discrimination/fine discriminative touch for dorsal columns tract (slide 41)

Question 42

What are the effects of anterior spinal cord lesion ?

Answer 42

• Spinothalamic tract damage causes pain and temperature loss below the level of the lesion
• Retained light touch, vibration and 2-point discrimination due to intact dorsal columns

Question 43

Which two fibres carry nociception?

Answer 43

•Aδ fibers that mediate sharp, intense or first pain (the immediate pain from banging your foot)

–Type 1: noxious mechanical

–Type 2: noxious heat

•C-fibres mediate dull, aching or second pain (the pain after the first pain which causes you to be careful with your feet while it is healing)

–Noxious thermal, mechanical and chemical stimuli (polymodal)

Question 44

What tract is involved in the emotional component of pain?

Answer 44

Spinoreticular tract

(involved in emotionalcomponent of pain) this tract is not part of the spinothalamic tract but it is very close to it and is involved with emotional pain slide 47

Question 45

What is the gate control theory

Answer 45

Pain could be inhibited in the spinal cord before it reached the brain. We now know that activation of A-beta fibres can inhibit pain signals in the dorsal horn by activating inhibitory interneurons which block the signal transmitted by A-delta or C-fibres.

NB Abeta fibres are the innocouous stimuli fibres and A delta and C fibres are the noxious fibres

eg rubbing the place when you banged your feet, rubbing stimulates AB fibres because rubbing is a non harmful stimuli and this could potentially inhibit A delta and C fibres from transmitting pain and noxious stimuli up the brain

Question 46

•Explain how nociceptive input can be gated by central mechanisms. Mention some of the areas involved in this central niceceptive modulation and what neurotransmitters are involved.

Answer 46

There are descending control pathways which could modulate pain ie make it more painful or less painful.

Areas involved:

The periaqueductal grey (PAG) in the midbrain and the rostral ventromedial medulla (RVM) are two important areas of the brain involved in descending inhibitory modulation.

Both these centres contain high concentrations of opioid receptors and endogenous opioids, which helps explain why opioids are analgesic (relieve pain).

Neurotransmitters involved: NA and serotonin

Descending pathways project to the dorsal horn and can inhibit pain pain transmission.

These pathways are monoaminergic, utilising noradrenaline and serotonin as neurotransmitters.

Question 47

2 broad types of pain?

Answer 47

Nociceptive (ie you have stimulated a pain receptor somewhere either in the periphery or in the viscera)

Neuropathic ( you damaged the nerves)

slide 55

Question 48

Define hyperalgesia and allodynia

Answer 48

• Allodynia: pain due to a stimulus that does not normally provoke pain
• Hyperalgesia: increased pain from a stimulus that normally provokes pain iemore sensitive to pain

Question 49

what happens to the descending control systems of patients suffering from chronic pain

Answer 49

• Loss of protective descending adrenergic pathways (NA = inhibit pain, serotonin= makes pain stimuli more painful) can cause the development of chronic pain because patients become more susceptible to pain essentially