4+5) Somatic Sensation and Ascending Tracts Flashcards
What is the function of the somatosensory system?
Carries conscious sensation from body wall
Give examples of some modalities in the somatosensory system:
Temperature, pain, vibration, pressure, stretch, 2-point discrimination, proprioception
Why can we experience different modalities in the somatosensory system?
Travel through different pathways in CNS and have different receptors
Describe phasic receptors and give an example:
Adapt to original stimulus by decreasing rate of firing so eventually become unaware of original stimulus e.g. mechanoreceptors in the feel of clothes
Describe tonic receptors and give an example:
Don’t adapt to stimulus to frequency of AP remains the same e.g. nociceptor in pain
How is sensory acuity related to receptive field?
Acuity is inversely proportional to size of receptive field
Describe how lateral inhibition takes place:
Max stimulation at one neurone and inhibitory interneurones from this neurone laterally inhibit nearby neurones
What is the purpose of lateral inhibition?
Maximise contrast of sensation
What is topographical representation?
For every point of the body surface there is a corresponding point in the neural cortex
What is the stereognosis and what is it used to test?
Test for dorsal columns
Ask someone to close their eyes, place an object in their hand and ask them what it is
What are the functions of the dorsal columns?
Sophisticated aspects of touch: light tough, joint position sense, vibration, two-point discrimination
What are the parts of the dorsal columns?
Fasciculus gracilis
Fasciculus cuneatus
What are ascending tracts?
White matter connection between two areas of grey matter
Describe the general path of a neurones up the DCML pathway:
Primary afferent from body wall and travel up to medulla where they synapse. Second order neurones then travel to thalamus and synapse here, then third order neurones project onto sensory cortex
Where do the different orders of neurones run in the DCML pathway?
1st order in dorsal columns
2nd order in medial lemniscus
3rd order between thalamus and primary sensory cortex
Describe the difference between lower limb and upper limb neurones in the DCML pathway:
Fibres from lower levels travel closer to midline in dorsal columns
Lower limb neurones project medially onto sensory cortex
Where do 1st order neurones in the upper half of the body synapse in dorsal column pathway?
Cuneate nucleus
Where do 1st order neurones in the lower half of the body synapse in dorsal column pathway?
Gracile nucleus
What are the functions of the spinothalamic tracts?
Pain, temperature and crude touch
Describe the general path of a neurones up the spinothalamic pathway:
1st order neurones from body wall synapse at same level in the spinal cord (apart from in leg). 2nd order neurones cross midline and ascends to thalamus. 3rd order from thalamus to primary sensory cortex
Describe the difference between lower limb and upper limb neurones in the spinothalamic pathway:
Lower limb neurones are more lateral in the cord
Lower limb neurones project medially onto sensory cortex
Where are the dorsal columns located?
Posterior spinal cord
Where is the spinothalamic tract located?
Ventral spinal cord
What is the difference between A and C fibres?
A fibres carry impulses from mechanoreceptors in the skin
C fibres carry pain
What is the mechanism behind the rubbing it better phenomenon?
Activation of mechanoreceptors alleviates pain by exciting inhibitory enkepahlinergic interneurones in the cord
Describe the descending pathway controlling pain transmission:
Control from cerebral cortex and peri-aqueductal gray, which send descending fibres to nucleus raphe magnus in medulla causing activation of enkephalinergic interneurones to switch off pain transmission
