What are the 3 divisions of the somatosensory system?
exteroceptive
proprioceptive
enteroceptive
Describe the exteroceptive division?
cutaneous senses - registers information from the surface of the body
Describe the proprioceptive division?
monitors posture and movement of the body
Describe the enteroceptive division?
reports upon the internal state of the body and closely relates to autonomic function
How are stimuli passed on through the nerves?
open a cation selective ion channel in the peripheral terminal of the primary sensory afferent - this elects a depolarising receptor potential
this triggers an action potential
action potentials cause a graded neurotransmitter release
What do low threshold receptors respond to?
low intensity non damaging stimuli
What do high threshold receptors respond to?
high intensity, potentially damaging stimuli
What do thermal nociceptors respond to?
extremes of heat = >45 or <10-15
What do chemical nociceptors respond to?
substances in the tissue eg serotonin, histamine, prostaglandin
What do polymodal receptors respond to?
two or more different stimuli
What is a receptive field?
target area where a sensory unit can be excited
How do receptive fields relate to innervation density and sensory acuity?
inversely proportional
i.e. high receptive field = low innervation density and low sensory acuity
What is two point discrimination?
minimum distance required between two simultaneous stimuli for them to be registered
Do regions with high descriminative capacity have big or small RFs?
small
What stimulates messiners corpuscles?
fine touch
What stimulates merkels discs?
pressure
What stimulates ruffini endings?
stretch
What stimulates pacinian capsules?
pressure and vibration
What is gray matter divided into?
10 laminae of Rexed
What laminae do nociceptors go to?
laminae 1 and 2
What laminae do low threshold mechanoreceptors go to?
laminae 3 - 6
What laminae do properioceptors go to?
laminae 7 - 9
What are the two dorsal columns?
fasciculus gracilis
fasciculus cunate
Which is more medial - fasciculus gracilis or fasciculus cunate?
fasciculus gracilis
What nerves does the fasciculus gracilis carry through the dorsal column?
sensory input from T6 and downwards
eg FROM THE LEGS
What nerves does the fasciculus cunate carry through the dorsal column?
sensory input from T6 upwards
eg FROM THE UPPER BODY
What order does information from the spinal section come in? Laterally to medially?
cervical
thoracic
lumbar
sacral
What is stereognosis?
the ability to recognise an object just by feeling it
What is fine touch?
the ability to recognise the exact location of something just by light touch
What is concious proprioception?
awareness of body movements and posture
What is contrast enhancement?
when information is conveyed from one neuron to the next, the differences in adjacent neurons is amplified - meaning the main signal is wishy washy
How is contrast enhancement combatted?
lateral inhibition
What is lateral inhibition?
works via inhibitory interneurons to sharpen stimulus perception
Where is the somatosensory cortex located?
post central gyrus of the parietal cortex
What makes up the somatosensory cortex?
brodmann areas
What makes up 1, 2, 3a and 3b of brodmann areas?
1 - cutaneous and 3a
2 - 3a,3b,joint afferents, deep tissue, golgi tendon organs
3a - proprioceptors
3b - cutaneous and 3a
What does S1 stand for?
Brodmann areas
What does S2 stand for?
Posterior parietal cortex
What would a lesion in brodmann area 1 present as?
loss of texture discrimination
What would a lesion in brodmann area 2 present as?
impaired grasping and size and shape discrimination
What would a lesion in brodmann area 3a present as?
deficits in texture, shape and discrimination
How is the somatosensory cortex arranged?
in columns - each column consisting of neurons with simlar inputs and responses - fast/slow adapting
What is the role of the posterior parietal cortex (S2)?
recieves and integrates information from S1, other cortical areas (visual, auditory) and subcorticle areas (thalamus) and deciphers the deeper meaning - i.e. inputs the texture size context etc then tells its a key
What can damage to the posterior parietal cortex cause/
neglect syndrome - patients believe that one side of their body doesnt exist