Somatosensory system Flashcards
What classifications can sensation be split into?
- General sensation, referring to body wall and viscera
- This is subdivided into visceral sensation and somatic sensation
- Special sensation - refers to vision, hearing, balance, taste and smell
What are the divisions of somatic sensation?
- Spinothalamic system
- Dorsal column-medial lemniscus system
What is a modality?
- A unit of sensation
- Relies on a distinct receptor type
What modalities is the spinothalamic system responsible for?
- Temperature (thermoreceptors)
- Pain (nociceptors)
- Pressure/crude touch (mechanoreceptors)
What modalities is the dorsal column-medial lemniscus system responsible for?
- Vibration (mechanoreceptors)
- Proprioception/joint position sense/kinaesthetic sense (variety of receptors such as muscle spindles and Golgi tendon organs)
- Fine touch (mechanoreceptors)
- Two point discrimination (mechanoreceptors)
What is the role of primary sensory neurones?
- These receive information from receptors and are responsible for the initial encoding of sensory information
Where do primary sensory neurones receive their information from?
- A single receptor type
Where are primary sensory neurones located in the body?
- Cell body is found in dorsal route ganglion
- Collect information from a single dermatome along their peripheral axon
- Primary sensory neurones project into the spinal cord along their central axon
How is the strength of receptor activation conveyed?
- It is converted from an analogue signal to a digital signal
- Digital signal is equal to the frequency of action potentials in the primary sensory neurones
What does strong receptor activation lead to?
- High frequency of action potentials in the primary sensory neurone
What does weak receptor activation lead to?
- Low frequency of action potentials in the primary sensory neurone
What is meant by a rapidly adapting receptor?
- These respond best to changes in strength of stimulation (e.g. mechanoreceptors)
- Their frequency of firing diminishes rapidly after the initial stimulus
- E.g. this is why we are not aware of the clothes on our skin
What is meant by a slowly adapting receptor?
- These receptors change their frequency of firing very little after the initial stimulus
- This explains why pain can be so persistent and we don’t get used to experiencing pain
What is a receptive field?
- A given area of skin supplied by a single primary sensory neurone
What results in an area of low sensory acuity?
- When an area of skin is supplied by sensory neurones with relatively large receptive fields
- Area will have poor two-point discrimination
- E.g. skin of back
What results in an area of high sensory acuity?
- When an area of skin is supplied by sensory neurones with relatively small receptive fields
- Area will have great two-point discrimination
- E.g. skin of fingertip
Why do dermatomes have ‘fuzzy’ boundaries?
- Due to overlap of receptive fields of primary sensory neurones
Outline the role of first order sensory neurones and where they project to
- 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
Outline the role of second order sensory neurones and where they project to
- Have their cell bodies in the spinal cord dorsal horn or medulla
- Decussate
- Project onto third order neurones
Outline the role of third order sensory neurones and where they project to
- Have their cell bodies in the thalamus
- Project to the primary sensory cortex (post-central gyrus)
What is the principle of somatotopy?
- The idea that for every point on the surface of the body, an equivalent point can be identified along the sensory pathway
- Adjacent body regions tend to map to adjacent regions of the sensory system
- This applies to dermatomal organisation and the homunculus
Why is the sensory cortex organised in a somatotopic fashion?
- This way of organising the pathways minimises the amount of wiring required to transmit sensory information
What happens to modalities at the level of the sensory homunculus?
- They all converge
Outline the organisation of first order neurones of the DCML system
- Ascend ipsilaterally through dorsal columns of the spinal cord
How are the lower body first order neurones of the DCML system organised?
- Those from the lower body (T7 and below) ascend through the gracile fasciculus to the gracile nucleus in the medulla
How are the upper body first order neurones of the DCML system organised?
- Those from the upper body (T6 and above) ascend through the cuneate fasciculus to the cuneate nucleus in the medulla
Outline the organisation of the second order neurones of the DCML system
- 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
Outline the organisation of the third order neurones of the DCML system?
- Thalamic neurones receive information ultimately from the lower half of the body and project to the medial part of the primary sensory cortex
- Thalamic neurones receive information ultimately from the upper half of the body and project to the lateral part of the primary sensory cortex
Outline the topographical organisation of the dorsal columns
- Axons from lower parts of the body run most medially
- Axons from progressively superior body segments are added laterally to the dorsal columns
Outline the organisation of the first order neurones of the spinothalamic tract
- They project to the ipsilateral dorsal cord
- Onto second order neurones in the ipsilateral spinal cord dorsal horn
- In the segment at which they enter the cord through the dorsal root
Outline the organisation of the second order neurones of the spinothalamic tract
- Their cell bodies are in the dorsal horn
- Axons decussate in the ventral white commissure of the cord
- Then form the spinothalamic tract
- The spinothalamic tract projects to the thalamus
Where do the tracts of the DCML decussate?
- In the brain
Outline the organisation of the third order neurones of the spinothalamic tract
- Thalamic neurones receiving information from more inferior parts of the body project to the medial part of the primary sensory cortex
- Thalamic neurones receiving information from more superior parts of the body project to the lateral part of the primary sensory cortex
Outline the topographical organisation of the spinothalamic 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
- Due to the decussation of STT second order neurones at the level of entry of the first order neurones
What is Brown Sequard syndrome?
- A complete cord hemisection causing destruction of one lateral half of a single cord segment
What does Brown Sequard syndrome result from?
- Trauma or ischaemia
Which structures are completely destroyed unilaterally due to Brown Sequard syndrome?
- The dorsal horn
- The ventral horn
- All other 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
Why does Brown Sequard syndrome cause ipsilateral loss of dorsal column modalities below the destroyed segment?
- Because the dorsal columns decussate in the brain
Why does Brown Sequard syndrome cause contralateral loss of spinothalamic modalities at and below the destroyed segment?
- Because the spinothalamic tracts decussate at their level of entry into the spinal cord
Why does rubbing a sore area relieve pain?
- Second order neurones of the spinothalamic system dealing with pain receive nociceptive primary afferents as well as inhibitory interneurons which contain encephalin
- You can activate the inhibitory interneurons by stimulating incoming impulses from mechanoreceptors
What else can activate encephalinergic interneurones?
- Descending inputs from higher centres such as the periaqueductal grey matter or the nucleus raphe magnus
