4 - Somatosensory System Flashcards
What are the different types of general somatic sensation and what is the unit of sensation called?
Modalities: each one has it’s own type of receptor
Spinothalmic: temperature, pain, crude touch
Dorsal Column: vibration, proprioception, fine touch, 2 point discrimination
(learn areas on picture)
How do primary sensory neurones (1st order) convert analogue signals from receptors into digital signals that the brain can interpret?
- Low stimulation then low frequency of action potentials
- High stimulation then an increased frequency of action potentials
What are the two different types of sensory receptors?
- Rapidly adapting: emit a high frequency of a.p’s but then the rate slows down. e.g mechanoreceptors when you sit down
- Slowly adapting: frequency of action potentials doesn’t change, e.g pain/nocireceptors
What is the receptive field?
- Region of skin containing receptors to supply the spinal nerve root
- Dermatomes can overlap when there is a larger receptive field
- Large receptive field then low acuity (cannot localise the sense detected e.g on the back). but large number of sensory neurones then high sensory acuity
- Large number of sensory neurones with small receptive fields means high acuity
When testing a patient’s dermatome, what part of the dermatome should you test?
Autonomous zone where there is no overlapping receptive fields
What are the neurones involved in a somatosensory pathway?
- 1st order: takes information from the receptor along a spinal nerve, cell body in the DRG apart from if in the head and neck, project ipsilaterally into cord on 2nd order
- 2nd order: cell body in dorsal horn or medulla depending on the tract, decussate, project onto 3rd order
- 3rd order: cell body in thalamus, project onto primary sensory cortex at post central gyrus via the internal capsule
What is somatotopy/topographical representation?
- Name to describe the homunculus
- How the body reorganises the fibres so the different areas of cortex recieve all modalitiess. Translation from dermatomes to homunculus
- Lower regions are always more medial in the cortex
Describe how the dorsal column medial lemniscal tract relays sensory information from a receptor to the cortex.
- Primary neurone enters spinal cord, ascends ipsilaterally in the dorsal column and synapses in medulla
- Gracile nucleus T7 and below, Cuneate nucleus T6 and above
- Secondary neurone decussates across the midline and ascends to the thalamus via the medial lemniscus pathway
- Tertiary neurone projects to the cortex, laterally if upper limb, medially if lower limb. Via internal capsule
- Upper regions enter the spinal cord and add on LATERALLY
What is the function of the dorsal column medial lemniscus tract?
- Allows sensation of light touch, proprioception and vibration and 2 point discrimination
- Decussation in medulla
- Third neurone in internal capsule
- Higher fibres added more laterally
Describe how the spinothalmic/anterolateral tract relays sensory information from a receptor to the cortex.
- Primary neurone enters the cord and synapses in the dorsal horn
- Secondary neurone decussates at ventral white commisure and travels up cord
- Upper fibres added medially
- Secondary neurones synapse at the thalamus and then tertiary neurones project onto cortex
What is the function of the spinothalmic tract?
- Allows sensation of pain, temperature and crude touch/pressure
- First synapse in dorsal horn at point of entry
- Decussation in the ventral white commisure
- Higher fibres are added more medially
What is Lissauer’s tract?
- Pathway formed from the proximal end of small unmyelinated and poorly myelinated fibers in peripheral nerves from primary neurones, which enter at the lateral aspect of the dorsal horn and ascend and descend up to two segments before synapsing
- Means sensory loss with spinothalmic tract lesion is not as much as you think
What are the functions and the first point of decussation of the following:
- Dorsal columns
- Anterior spinothalmic
- Lateral spinothalmic
- Ventral and Dorsal spinocerebellar
- Proprioception, discriminative touch and decussates at medulla
- Touch and pressure and decussates at level of spinal nerve
- Pain and temperature and decussates at level of spinal nerve
- Posture and coordination of movement. Ventral decussates at level of spinal nerve but has two decussations so cancels self out, dorsal doesn’t decussate at all
In the spinal cord sensory tracts, where can the part carrying fibres from the lower limb be found?
- DC lower limb is medial
- ST lower limb is lateral/superficial
(draw a picture)
What would happen if you had an ipsilateral lesion to the DC or ST tract?
- DC: loss of certain sensory on ipsilateral side below lesion
- ST: loss of certain sensory on contralateral side below lesion
THINK ABOUT DECUSSATION
What is Brown-Sequard Syndrome?
Complete hemisection of spinal cord segments from trauma or ischaemia leading to:
- Ipsilateral complete segmental anaesthesia of single dermatome as loss of dorsal root and horn
- Ipsilateral loss of dorsal column modalities from that segment and below
- Contralateral loss of spinothalmic modalities below the lesion
- Spastic paralysis ipsilaterally as corticospinal tract decussates in medulla
What types of receptors detect each modality in the dorsal column?
What structures are destroyed in Brown-Sequard Syndrome?
How can pain be modulated by rubbing the area of pain?
- Spinothalmic tract
- C fibres carry pain
- If you rub site of pain you acitvate mechanoreceptors and A fibres which excite inhibitory enkephalinergic interneurones that inhibit 2nd order neurone in the dorsal horn
How can pain be modulated by the brain itself and opiates?
- Descending cortical neurone from cortex to midbrain, synapsing in the periaqueductal grey
- Periaqueductal grey neurones excite nucleus raphe magnus neurones which go on to excite inhibitory neurones, to inactivate 2nd order neurone
- Neurotransmitter from N.Raphe Magnus neurones is endorphin called encephalin
What are the two ways that pain can be modulated?
- Rubbing the area
- Hypnosis causing cortical neurones to activate peri-aqueductal grey neurones
- Both work by activating inhibitory neurones against the 2nd order neurone in the dorsal horn
When resecting a tumour from the cerebral cortex, where is sensory function likely to be impaired?
- In the hands as this area occupies a large area of the cortex, will affect all modalities as convergence in the cortex, not just ST or DC tracts
This is the MRI of a vegan man who presented with sensory ataxia, what is the diagnosis and what may be found on clincial examination?
- Dorsal column of C2-C3 has undergone subacute combined degeneration of the spinal cord due to B12 deficiency as B12 is needed for myelination
What does a positive romberg test mean?
- Need at least 2 of proprioception, vision and vestibular system for balance
- If positive suggests sensory ataxia and the issue is in the dorsal column due to lack of proprioception
- If negative issue in cerebellum
Which sensory system is affected and what cyst can this be explained by?
- Spinothalmic tract
- Syringiomyelia
- Cyst pushes on ventral white commisure leading to bilateral sensory symptoms starting in the upper limbs as upper limbs more medial in ST tract. As gets bigger can affect dorsal column
In the dorsal column and the spinothalmic tract, where are cervical, lumbar and thoracic nerves located?
Why does diabetes lead to an insensitivity to pain?
Neuropathy caused by…
When do you get flaccid and spastic paralysis?
- Flaccid: lower motor neurone lesion
- Spastic: upper motor neurone lesion
DCN -> doral column nuclei in medulla, can’t tell if upper or lower so cannot specify if gracilis or cuneate
