Somatic sensation Flashcards
What nuclei are exclusively found in the thoracic spinal cord?
- Intermediolateral nucleus: Preganglionic sympathetic fibres
- Clarke’s nucleus: Relay for proprioceptive fibres from the lower limb
What are the columns of white matter in the spinal cord?
- Dorsal column
- Ventral column
- Lateral column
What is the general pattern of grey and white matter as one ascends the spinal column?
White matter is generally thicker as one ascends the spinal cord as more ascending axons have entered, and fewer descending axons have left.
What are the features of the cervical spinal cord?
Pronounced oval shape with thick white matter
What are the features of the thoracic and upper lumbar spinal cord?
Circular shape with thin ‘H’ of grey matter
What are the features of the lumbosacral spinal cord?
Circular shape with thick ‘H’ of grey matter and thin white matter
What are the features of the lower sacral spinal cord?
Circular shape with thick grey matter and very thin white matter
What are the features of the spinal cord?
- Gracile fascicle (dorsal column): DC-ML lower body fibres
- Cuneate fascicle (dorsal column): DC-ML upper body fibres
- Dorsolateral column: Descending motor fibres
- Anterolateral column: Pain and temperature fibres
- Dorsal horn: Sensory processing
- Ventral horn: Motor fibres to axial muscles
- Ventral horn: Motor fibres to limb muscles
- Clarke’s nucleus (thoracic/upper lumbar only): Proprioception relay for lower limbs
- Intermediolateral nucleus: Contains pre-ganglionic sympathetic fibres
- Ventral column: Motor fibres
- Lissauer’s tract: Afferent nociceptive factors ascend/descend to adjacent levels of spinal cord before entering grey matter.
What are the layers of the spinal cord grey matter called?
Rexed’s laminae
What is the substantia gelatinosa?
Rexed’s layers I - III
What are the functions of the substantia gelatinosa?
- Processing of pain information
- Initiation of defensive reflexes to pain
What are the functions of Rexed layers IV and V?
Relay neurones that cross the midline at the ventral commissure and carry pain/temperature fibres.
What are the functions of Rexed layer VII?
Spinal interneurones mediating spinal processing (e.g. reflexes)
What are the features of the lower caudal brainstem (spinal cord/medulla junction)?
- Dorsal column: Somatosensory fibres ascend in this tract up to the brainstem (in gracile and cuneate fascicles and terminate in nuclei of the medulla)
- Anterolateral tract: Pain and temperature fibres ascend in this tract to the medulla.
- Trigeminal nucleus: Where pain/temperature fibres of the trigeminal system ascend to.
What are the features of the medulla (level of sensory decussation)?
- Dorsal column nuclei: Gracile/cuneate nuclei (medial/lateral) into which the somatosensory fibres of the gracile/cuneate fascicles enter.
- Medial lemniscus: Sensory fibres leaving the dorsal column nuclei decussate at the sensory decussation and cross to contralateral side of brainstem to ascend in these tracts.
- Trigeminal nucleus
What are the features of the medulla (rostral)?
- Medial lemniscus
- Trigeminal nucleus
What are the features of the upper pons?
- Ventral part: Appears as large swelling in brainstem and is grooved in the midline by the basilar artery
- Dorsal part is hidden due to cerebellum
- Medial lemniscus
What are the features of the lower midbrain?
- Tectum (swellings in roof of midbrain dorsal to cerebral aqueduct): Inferior colliculus
- Medial lemniscus: Becomes more lateral from their previously central positions and is joined by fibres of the anterolateral tract.
What are the features of the upper midbrain?
- Tectum (swellings in roof of midbrain dorsal to cerebral aqueduct): Superior colliculus
- Cerebral aqueduct
- Medial lemniscus
What are the contents of the internal capsule?
- Fibres between the thalamus and cortex
- Fibres between the cortex and spinal cord
- Fibres between different nuclei in the thalamus
- Fibres between different areas of the cortex
What is the structure of the DC-ML system?
- As Aβ fibre enters the spinal cord, it bifurcates into a short branch entering the dorsal horn and a long branch ascending in the dorsal column.
- Below the mid-thoracic level (T7), nerve fibres enter the gracile fascicle on the medial aspects of the dorsal column.
- Above the mid-thoracic level, the dorsal column is divided by a sulcus and subsequent spinal nerve entering the column enters a more lateral cuneate fascicle.
- Nerve fibres in the cuneate/gracile fascicles synapse in the cuneate/gracile nuclei of the medulla oblongata respectively, together forming the dorsal column nuclei.
- Axons from the dorsal column nuclei cross the midline to ascend as the medial lemniscus contralaterally, being joined by fibres from the trigeminal nuclei (receiving sensory afferents from head).
- Fibres synapse in the ventroposterior medial/lateral nuclei of the thalamus before projecting into the primary/secondary somatosensory cortex.
What is the structure of the ST system?
- Nociceptive fibres entering spinal cord give both ascending and descending branches into Lissauer’s tract.
- Branches from Lissauer’s tract enter the grey matter and synapse with second order cells principally in substantia gelatinosa. This synapse allows for gating mechanisms (e.g. gate-control of pain).
- Fibres originating from lamina I ascend in the lateral spinothalamic tract while fibres originating from lamina V ascend in the ventral spinothalamic tract; both crossing over at the midline to ascend on the contralateral side. Together, these 2 tracts form the anterolateral system.
- At the medial lemniscus, fibres from the trigeminal nucleus cross the midline to join the tract.
- From the medial lemniscus, fibres in the spinothalamic tract synapse in the ventral posterior lateral/medial nuclei of the thalamus before projecting various areas of the brain.
What are the projections of the ST system into the cortex?
- S1 (post-central gyrus): Conscious perception and localisation of pain.
- Insula (deep in lateral fissure): Involved in effector aspects of pain (e.g. homeostatic changes).
- Anterior cingulate cortex (medial part of frontal lobe, inside longitudinal fissure): Involved in processing nociceptive information (possibly emotional aspects).
What are the projections of the ST system into the brainstem?
- Reticular formation (medulla): Sympathetic stimulation
- Reticular formation (midbrain): Arousal
- Periaqueductal grey: Descending analgesia
