3. Somatosensory system

Question 1

what are the 2 main types of sensation

Answer 1

1. general
   
   • somatic: conscious
   • visceral: unconscious (e.g. BP, distension, can become conscious in pathology like MI or appendicitis - referred pain)
2. special

Question 2

what are modalities and what are the 2 main groups of these

Answer 2

Modalities = different types of general sensation determined by receptor type

1. Travel to CNS via SPINOTHALAMIC system (evolutionarily older):
   
   • temperature
   • pain
   • crude touch (inc. stretch)
2. Travel to CNS via DORSAL COLUMN system:
   
   • fine touch (inc. 2 point discrimination)
   • vibration
   • proprioception

Question 3

which structures transmit sensory info to the CNS

Answer 3

R + primary sensory neurone (= sensory neurone unit)

Question 4

why do we get used to some stimuli (e.g. pressure when sitting) but not others (e.g. pain)

Answer 4

2 different types of sensory neurone units:

1. RAPIDLY ADAPTING units: PHASIC Rs fire very rapidly originally but firing tapers off over time. Very good at detecting change (e.g. mechanoRs from standing to sitting) but not indicating duration of stimulus.
2. SLOWLY ADAPTING units: TONIC Rs cause constant rate of firing enabling constant awareness of stimulus (e.g. nociceptors)

Question 5

what determines our acuity (ability to detect location of a stimulus)

Answer 5

Acuity of a sensory neurone is inversely related to size of its receptive field (area of skin supplied by multiple Rs converging on a single sensory neurone)

Question 6

why do dermatomes have poorly defined borders

Answer 6

there can be regions of overlap between different receptive fields

Question 7

describe the basic sensory neurone pathway from R to cortex

Answer 7

1. primary sensory neurone (cell body in DRG) transmits info from R to ipsilateral side of CNS…
2. secondary sensory neurone (cell body in dorsal horn or medulla) passes info from ipsilateral CNS to contralateral side thalamus - decussates…
3. tertiary sensory neurone (cell body in thalamus) projects to primary sensory cortex in post-central gyrus

Question 8

what is meant by somatotropic/topographical representation

Answer 8

Point to point correspondance of a body area to a specific point of cortex, with adjacent areas of body represented in adjacent areas of cortex

Means that sensory neurone pathways reorganise/converge from dermatomal pattern to homoncular pattern as get closer to CNS

Question 9

name 2 important sensory neurone pathways and describe their location

Answer 9

1. Dorsal column pathway - fills entire dorsal funiculus

2. Spinothalamic tract - ventrolateral funiculus

Question 10

where do the 3 neurones synapse in the sensory pathway of the DC and ST pathways?

Answer 10

Dorsal column pathway: 1 neurone synapses with 2 neurone at medulla… 2 neurone synapses with 3 neurone at thalamus

Spinothalamic pathway: 1 neurone synapses with 2 neurone at lateral horn of entering spinal level… 2 neurone synapses with 3 neurone at thalamus

Question 11

are lower/upper body neurones represented more lateral or medial

Answer 11

In dorsal column pathway:

• lower body neurones medial
• upper body neurones lateral

In spinothalamic pathway:

• lower body neurones more lateral
• upper body neurones more medial

Question 12

what are the nuclei of the dorsal column pathway called and which neurone cell bodies do these contain

Answer 12

cuneate nucleus: upper body cell neurones

gracile nucleus: lower body cell neurones

Question 13

where does the secondary neurone decussate in the sensory neurone pathways

Answer 13

dorsal column pathway: medulla

spinothalamic pathway: ventral white commissure of entering spinal level