Somatosensory System

Question 1

Define sensation

Answer 1

A physical feeling from stimulation of a sense organ

Question 2

How are the senses classified?

Answer 2

Special = CN: hearing, taste, sight, smell

General = 1) somatic (body wall, skin, mucosa) 2) visceral

Question 3

What are the modalities of the senses?

Answer 3

Somatic = pain, temp, pressure, stretch, vibration, joint position, 2-point discrimination

Each modality has diff qualities associated with it (temp: hot, cold)

Question 4

How are the modalities determined?

Answer 4

By the type of receptor being activated

Question 5

How is the quality of the modality determined?

Answer 5

How the receptor is activated

Question 6

Describe the basic layout of the somatosensory system

Answer 6

Primary sensory neurone – 1st order sensory neurone in the dorsal root ganglion – enters spinal cord

(Properties of the sensory neurone and the receptor determine the modality)

Synapses with 2nd order neurone

2nd order neurone sends axon decussates the midline and up to the thalamus

Synapses with 3rd order neurone in the thalamus

3rd order neurone projects to the cortex

Question 7

Give some examples of cutaneous receptors in the skin

Answer 7

Pacininian corpuscles

Ruffini endings

Question 8

Outline how the generator potential is produced

Answer 8

Mechanoreceptors get physical deformation = open ion channels = Na rushes in

Small deformation = small Na (and vis-versa)

If deformation is large enough = large Na influx = AP generated

Question 9

Your perception of the strength of the stimulus is proportional to what?

Answer 9

The frequency of APs along the first order neurone

Question 10

How are the receptors from which the generator potential is produced classed?

Answer 10

Tonic receptors – slowly adapting (pain) = AP frequency remain firing at a very high rate

Phasic receptors – rapidly adapting (pressure) = AP frequency slow down though adaption

Question 11

Discuss acuity

Answer 11

The larger the receptive field = poorer acuity

Acuity is inversely proportional to the size of the receptive field

Lateral inhibition increase acuity

Question 12

What clinical response occurs from the overlapping of primary receptive fields?

Answer 12

Means dermatome boundaries are not clear lines

Question 13

Explain lateral inhibition

Answer 13

Associated with every 1st order neurone are inhibitory neurones

Excitatory AP stimulate the inhibitory neurones to send inhibitory signals to the neighbouring axons

Localising the epicentre of sensation

Question 14

Where are the locations of the somatosensory neurones?

Answer 14

1st order = dorsal root ganglion

2nd order = grey matter in the cord, brainstem- decussates

3rd order = thalamus (projects to cortex)

Question 15

Outline the dorsal-column medial lemniscus pathway

Answer 15

Light touch, 2-point discrimination, vibration, joint position sense

Medial to lateral = sacral, lumbar, thoracic, cervical = to stop crossing over

Lumbar 1st order synapse in medulla grey mater - gracile nucleus 2nd order dips down decussates, synapse in thalamus with 3rd order neurone - project medially

Cervical 1st order synapse in medulla grey mater - cuneate nucleus 2nd order dips down decussates, synapse in thalamus with 3rd order neurone - project laterally

White mater pathway created by 2nd order neurones = medial lemniscus

Question 16

What are the internal arcuate fibres?

Answer 16

Axons of second-order sensory neurones that compose the gracile and cuneate nuclei of the medulla oblongata

Question 17

Which nucleus handles touch, vibration, 2-point discrimination, joint position from T6 downwards?

Answer 17

Gracile nucleus

Question 18

Outline the spinothalamic pathway

Answer 18

Pain, crude touch, temp

Medial to lateral = cervical, thoracic, lumbar, sacral

Lumbar 1st order neurone synapses at entry level, dorsal horn 2nd order neurone decussates, anterior-lateral spinothalamic tract, to thalamus, synapse with 3rd order neurone – project medially

Cervical 1st order neurone synapses at entry level, dorsal horn 2nd order neurone decussates, anterior-lateral spinothalamic tract, to thalamus, synapse with 3rd order neurone – project laterally

Question 19

Why does rubbing a painful area alleviate pain?

Answer 19

Activation of mechanoreceptors = activation of A-beta neurones that project into the dorsal horn = exciting inhibitory enkephalinergic interneurones = inhibits 2nd order pain neurone = switching off pain transmission

Question 20

Which fibres carry pain?

Answer 20

C fibres – slow conducting

Question 21

Outline the inhibitory pain pathway created by hypnosis

Answer 21

Excitatory cortical neurones project down to midbrain

2nd neurone sits in periaqueductal grey (midbrain), projects down to medulla

Excites nucleus raphe magnus (reticular formation) project down the cord

Excites inhibitory enkaphalinergic interneurones

Inhibit 2nd order sensory neurones