3. Somatosensory System Flashcards
What can general sensation be subdivided into?
Somatic sensation - conscious
Visceral sensation - unconscious e.g. homeostasis
What are the different modalities in the spinothalamic system?
Temperature - thermoreceptors
Pain - nocireceptors
Pressure/crude touch - mechanoreceptors
What are the different modalities in the dorsal column-medial lemniscus system?
Vibration - mechanoreceptors
Proprioception (know where limbs are in space) - muscle spindles
Fine touch - mechanoreceptors
Two point discrimination - mechanoreceptors
What does strong receptor activation cause?
High frequency of action potentials in primary sensory neurone
What are rapidly adapting receptors?
E.g. mechanoreceptors
Respond best to changes in strength of stimulation
Adaptation of these receptors explains why you are not aware of clothes on your skin
What are slowly adapting receptors?
E.g. nociceptors
Change their frequency of firing very little after the initial stimulus
Explains why pain can be so persistent
What are receptive fields?
Single primary sensory neurone supplies a given area of skin
Can have many receptors
What happens if an area of skin is supplied by sensory neurones with relatively large receptive fields?
This area will have low sensory acuity
It would have poor two-point discrimination where 2 points would need to be far apart to be distinguished
E.g. skin of the back
Why can dermatomes have ‘fuzzy’ boundaries?
Overlap of receptive fields of primary sensory neurones from adjacent neurones
Describe the sequence of neurones in the somatosensory system
First order sensory neurones - cell bodies in DRG, communicate with receptors, central axon projects ipsilateral to cell body, project onto second order neurones
Second order sensory neurones - cell bodies in spinal cord dorsal horn or medulla, decussate, project onto third order neurones
Third order neurones - cell bodies in thalamus, project to primary sensory cortex
What is somatotopy/topographical representation?
Idea that for every point on the surface of the body, an equivalent point can be identified along the sensory pathway
Adjacent body regions map to adjacent regions of the sensory system
Describe the pathway of first order neurones in the DCML system
Those from lower body (T7 and below) ascend through the gracile fasciculus to the gracile nucleus in medulla
Those from upper half of body (T6 and above) ascend through the cuneate fasciculus in the cuneate nucleus
Describe the pathway of second order neurones of the DCML system
Neurones in gracile nucleus project to the contralateral thalamus in the medial lemniscus
Neurones in the cuneate nucleus project to the contralateral thalamus in the medial lemniscus
Describe the pathway of the third order neurones of the DCML system
Thalamic neurones recovering information ultimately from the lower half of the body project to the medial part of the primary sensory cortex
Thalamic neurones receiving information ultimately from the upper half of the body project to the lateral part of the primary sensory cortex
Describe the topographical organisation of the dorsal columns in DCML
Axons from lower parts of the body run most medially
Axons from progressively superior body segments are added laterally to the dorsal columns
Describe the pathway of the first order neurones in the spinothalamic pathway
Project into second order neurones in the ipsilateral spinal cord dorsal horn in the segment at which they enter the cord through the dorsal root
Describe the pathway of the second order neurones of the spinothalamic pathway
Cell bodies are in the dorsal horn
Axons decussate in the ventral white commissure of the cord and go on to form spinothalamic tract
Spinothalamic tract projects to the thalamus
Describe the third order neurones of the spinothalamic pathway
Thalamic neurones receiving information ultimately from more inferior parts of the body project to the medial part of the primary sensory cortex
Thalamic neurones receiving information ultimately from more superior parts of the body project to the lateral part of the primary sensory cortex
Describe the topographical organisation of the spinothalamic tract
Axons from the lower parts of the body run most laterally/superficial
Axons from progressively superior body segments are added medially/deeper onto the spinothalamic tract
Describe the descending modulation of pain
Second order neurones of spinothalamic system dealing with pain receive nociceptive primary afferent as well as inhibitory interneurons which contain endorphin ecephalin
These encephalinergic interneurons can be activated by incoming pulses from mechanoreceptors
Can also be activated by descending inputs from higher centres such as the periaqueductal grey matter or the nucleus raphe magnus
