Session 3: Somatosensory Flashcards
What are the classification of the sensation?
- General sensation, referring to the body wall and viscera (including parietal layer of serous membranes and mucosa of pharynx, nasal cavity and anus)
- Special sensation, referring the special senses of vision, hearing, balance, taste and smell
What is general sensation divided into?
- Somatic sensation
- Visceral sensation
Which sensations are controlled by the spinothalmic system?
- Temperature (thermoreceptors)
- Pain (nociceptors)
- Pressure/crude touch (mechanoreceptors)
Which sensation are controlled by dorsal column-medial lemniscus system?
- Vibration (mechanoreceptors)
- Proprioception, or joint position sense, or kinaesthetic sense (detected by a variety of receptors such as muscle spindles and Golgi tendon organs)
- Fine touch/Light touch (mechanoreceptors)
- Two point discrimination (mechanoreceptors
How do primary sensory neurone receive information?
-Each individual primary Nerone receives input from a single receptor type
Where are primary sensory neurone cell bodies found?
-Dorsal root ganglion
Where do dorsal root ganglions receive information?
-Collect information from a single dermatome along their peripheral axon
How do primary sensory neurone communicate with spinal cord?
- Primary sensory neurone project to spinal cord along their central axon
- Strength of receptor activation is converted to analogue signal to a digital signal
- Strong receptor activation causes high frequency of action potentials in the primary sensory neurone
- Weak receptor activation causes a low frequency of action potentials in the primary sensory neurone
What are rapidly adapting receptors?
- Respond best to changes in strength of stimulation. However, their frequency of firing diminishes rapidly after the initial stimulus (i.e. they rapidly adapt).
- Adaptation of these receptors explains why you are not aware of your clothes on your skin
What are slowly adapting receptors?
- Change their frequency of firing very little after the initial stimulus.
- This explains why pain can be so persistent, and you never really get ‘used to’ having pain
What is a receptive field?
Single primary sensory neurone supplies a given area of skin (it’s receptive field)
What happens if an area of skin is supplied by sensory neurones with relatively large receptive fields?
- Area will have low sensory acuity
- Poor two-point discrimination where two points would need to be far apart to be distinguished
- Skin on the back has relatively low acuity
What happens if an area of skin is supplied by sensory neurones with relatively small receptive fields?
- This area will have high sensory acuity
- Great two-point discrimination where two points could be very close together to be distinguished).
- The skin of the fingertip has relatively high acuity
What is the effect of overlap in receptive fields?
-The overlap of receptive fields of primary sensory neurones from adjacent dermatomes is one of the reasons why dermatomes can have ‘fuzzy’ boundaries
What are first order neurones?
- First order sensory neurones
- Have their cell bodies in the Dorsal Root Ganglion
- Communicate with a receptor
- Their central axon projects ipsilateral to the cell body
- Project onto second order neurones
What are second order neurones?
- Have their cell bodies in the spinal cord dorsal horn or medulla
- Decussate
- Project onto third order neurones
What are third order neurones?
- Have their cell bodies in the thalamus
- Project to the primary sensory cortex (postcentral gyrus)
What are the features of the first order neurones of the dorsal column medial lemniscus pathway?
- Those from the lower body (T7 and below) ascend through the gracile fasciculus to the gracile nucleus in the medulla)
- Those from the upper half of the body (T6 and above) ascend through the cuneate fasciculus to the cuneate nucleus in the medulla
What are the features of the second order neurones of the dorsal column medial lemniscus pathway?
- Neurones in the 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
What are the features of the third order neurones of the dorsal column medial lemniscus pathway?
- Thalamic neurones receiving information ultimately from the lower half of the body (via gracile nucleus) project to the medial part of the primary sensory cortex
- Thalamic neurones receiving information ultimately from the upper half of the body (via cuneate nucleus) project to the lateral part of the primary sensory cortex
What is the topographical organisation of the dorsal columns?
- Axons from the lower parts of the body run most medially
- Axons from progressively superior body segments are added laterally to the dorsal columns
Which side of the body is supplied by the spinothalmic tract?
Axons of first order neurones project to the ipsilateral dorsal cord, but the spinothalamic tract supplies the contralateral half of the body
What are the features of the first order neurones of the spinothalamic tract?
-They project onto second order neurones in the ipsilateral spinal cord dorsal horn in the segment at which they enter the cord through the dorsal root (generally)
What are the features of the second order neurones of the spinothalamic tract?
- Their cell bodies are in the dorsal horn
- Their axons decussate in the ventral white commissure of the cord and then go on to form the spinothalamic tract
- The spinothalamic tract projects to the thalamus
What are the features of third order neurones of the spinothalamic tract?
- 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
What is the topographical organisation of the spinothalmic tract?
- Axons from the lower parts of the body run most laterally/superficially
- Axons from progressively superior body segments are added medially/deeper onto the spinothalamic tract
- This is the opposite of the situation for the dorsal columns, and is due to the decussation of the STT second order neurones at the level of entry of the first order neurones
Which structures are destroyed completely unilaterally?
- The dorsal horn
- The ventral horn
- All other cord grey matter
- All white matter pathways
- Dorsal and ventral roots
What are the signs of Brown-Sequard syndrome?
- Ipsilateral complete segmental anaesthesia affecting a single dermatome (due to destruction of dorsal root and dorsal horn
- Ipsilateral loss of dorsal column modalities below the destroyed segment
- Contralateral loss of spinothalamic modalities at and below the destroyed segment
How is pain modulated?
- Second order neurones of the spinothalamic system dealing with pain receive nociceptive primary afferents as well as inhibitory interneurones which contain the endorphin encephalin
- These encephalinergic interneurones can be activated by incoming impulses from mechanoreceptors (hence explaining why rubbing a sore area relieves the pain)
- Additionally, these encephalinergic interneurones can also be activated by descending inputs from higher centres such as the periaqueductal grey matter or the nucleus raphe magnus
