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  • Revision lecture like a boss
  • Remember we need all the tracts and stuff

Tracts

  • Spinal white matter is made up of different types of fibres:
    • Ascending
    • Descending
    • Intersegmental (around the grey matter)
  • White matter in spinal cord is divided by the dorsal and ventral rootlets into dorsal, lateral and ventral columns (funiculi)
    • Dorsal columns = gracile and cuneate fasciculus (separated by glial septum)
    • Anterior funiculus/column = medial, near the ventral median fissure
  • Several tracts form a column (i.e. several fasciculi form a funiculus)

Ascending tracts

  • Dorsal column + spinothalamic tract
  • Sensory modalities:
    • Tactile = discriminative (fine) touch, crude (light e.g. tissue paper) touch, vibration
  • Pain (nociception)
  • Temperature
  • Proprioception (from muscles and joints)

Dorsal column

  • Somatic sensation = chain of 3-4 neurons from periphery up to cortex
  • 1st neuron on pathway to cortex has cell body in DRG
    • Note after the ML, it synapses in thalamus (VPL), then to the internal capsule (superior thalamic radiation; which contains the ascending fibres to the cortex - thalamocortical fibres)
  • Transmits:
    • Tactile
    • Proprioception
  • Cells of origin: DRG
  • FG fibres from below T6, FC fibres from above T6. Fibres are somatotopically organised IN THE SPINAL CORD (toes etc medially, then out is trunk, then so on)
  • Destination: Gracile and cuneate nuclei (medulla)
    • On ipsilateral side as origin
    • NB: cuneate f. is a wedge-shape
    • intermediate sulcus separates the cuneate and gracile fasciculus and has a glial septum between them
  • Lesion: loss of tactile and proprioceptive sensations on same (ipsilateral) side of body
    • Know the levels they cross at

Medial lemniscus

  • 2nd neuron in the pathway
  • Transmits: tactile, vibration and proprioception
  • Cells of origin
    • Gracile and cuneate nuclei (contralateral side)
  • Crosses in caudal medulla (sensory decussation; internal arcuate fibres)
  • Fibres somatotopically organised
  • Terminates thalamus VP nucleus
  • Turns 90 degrees so that lower limb goes ventral, and upper limb goes dorsal
  • Then as it goes up through pons, it rotates another 90 degrees so upper limb is medial and LL is lateral. It maintains this organisation up to thalamus.
  • Nucleus in thalamus is the ventral posterolateral nucleus of thalamus. This also has a somatotopical organisation. Then the fibres ascend through thalamocortical projection fibres (superior thalamic radiation of IC) to the somatotopically-organised cortex
  • Lesion:
    • Loss of tactile and proprioceptive sensations on contralateral side
Lumbar spinal cord
  • Mainly grey matter (unlike cervical which is mainly white as it has fibres from everything)
  • GF forms the entire dorsal column at this level
  • In cervical cord, you have both CF and GF, and lateral to CF is proprioceptive stuff from above T6, which might stay in dorsal horn or might head ventrally.
  • Tabes dorsalis is a complication of tertiary syphilis, and you lose your CF and GF. You lose ascending proprioceptive information, and get a form of ataxia called sensory ataxia (due to a lack of proprioceptive input to cerebellum)

Spinothalamic tract

2nd neuron on pathway (1st one synapses on say level

  • Transmits pain and temperature, and crude touch.
  • Pain/temperature is 3 neurons
  • Pain and temperature is then relayed from the thalamus to the somatosensory cortex via the superior thalamic radiation (which passes through the posterior limb of the internal capsule)
  • Goes to VPL nucleus of thalamus; which projects somatotopically up to cortex
    • Does so also through the superior thalamic radiation, goes to opposite side
      • Some fibres terminate in the brainstem (reticular formation)
  • Lissauer's tract = pale area in dorsal root tract as it's poorly myelinated.
  • Can go via interneuron from primary neuron to spinothalamic tract, or it can synapse straight away
    • This interneuron sits in substantia gelatinosa (where we can switch off pain)
  • Lesion: loss of pain and temperature sensation in the limbs and trunk on the contralateral side
  • Pain/temperature are transmitted by Ad fibres (relay on direct pathway to VP nucleus; cross midline and go straight up to VP without crossing; fast, well-localised pain) and C fibres (dull + slow pain, extra relay to thalamus; some fibres go to reticular formation)
    • By the spino-reticular-thalamic pathway causes depression/irritation when suffering from chronic pain

Pathways for somatosensory information from the head

  • Recall spinal trigeminal nucleus
  • Pain and temperature fibres from the head relay in the spinal trigeminal nucleus (in medulla) and join the contralateral spinothalamic tract
  • Tactile fibres from the head relay in the main sensory trigeminal nucleus (in the pons, at the level of entry for CN5) and join the contralateral medial lemniscus
  • Pain/temperature fibres turn and go down, travel all the way down to caudal medulla (forming the spinal trigeminal tract), where pain and ascend with the spinothalamic tract on the contralateral side nearby
  • Touch fibres have cell bodies in DRG, and synapse in the main sensory nucleus of V
  • Overall: head fibres are separated into two groups as well, and these bad boys all synapse in the VP as well, before heading up the posterior limb of the internal capsule (via superior thalamic radiation). Lesion would affect all sensory on contralateral side
  • Lesion in superior thalamic radiation of (posterior limb of) internal capsule kills all your sensory information
    • Note that there are some fibres that head via anterior limb of internal capsule to the frontal cortex - so you get some vague awareness of pain, but very poorly localised

Alternating signs result from lesions of medulla

  • Lesions of PICA
  • Lesions of the lateral medulla cause loss of pain and termpature in the contralateral limbs (spinothalamic) and ipsilateral side of the face (spinal trigeminal tract - pain fibres from CN5 descending to reach the spinal lemniscus)
    • Because we've caught the sneaky 2 head fibres at different paths (see CN5 tract divisions in her diagram)

Descending tracts

  • Pyramidal tract: corticospinal (trunk/limbs) and corticobulbar (cranial nerves) tracts (voluntary movement)
  • Extrapyramidal tracts: vestibulospinal (balance) and reticulospinal (postural)
  • Pyramidal lesions usually involve paralysis (or strong paresis), while extrapyramidal tracts give you disorders of movement, but no paralysis per se (e.g. loss of coordination, involuntary movements, etc)

Corticospinal tract

  • Function: control of voluntary movement, particularly skilled movements of the distal limbs
    • 50% of fibres terminate in motor enlargement of cervical cord for arms/upper limb
  • Origin: motor (and somatosensory) areas of Cx. Passes via
    • Internal capsule (posterior limb)
    • Base of cerebral peduncle (midbrain)
    • Base of pons
    • Pyramid of medulla
  • 90% (approx) of fibres cross in pyramidal decussation at lower end of medulla
  • Crossed fibres form the lateral corticospinal tract in the spinal cord
    • This is for the limbs
  • Uncrossed fibres form the ventral corticospinal tract and cross at their level of termination
    • Mainly for trunk muscles (postural)
  • Termination: spinal grey matter, on interneurons or directly on motor neurons
    • Hand = directly, others = via interneuron

Lesions of CS tract

  • Results in spastic paralysis
    • Note hyperreflexia because corticospinal lets you inhibit reflexes
  • If the lesion is above the pyramidal decussation (internal capsule, brainstem) it affects muscles on teh contralateral side
  • If the lesion is below the pyramidal decussation (spinal cord) it affects muscles on the ipsilateral side
  • With rehab, high degree of improvement (other descending tracts can compensate), but fine movements of the hands rarely recover (purely corticospinal).

Corticobulbar

  • Function: controls voluntary movement of muscles in the head (mastication, facial expression, tongue, pharynx, larynx and oeseophagus) but not extraocular mm.
    • Tract: MLF
  • Origin: head area motor areas of cerebral cortex
  • Travels with corticospinal tract (both somatotopically organised)
  • Termination:
    • Trigeminal motor nucleus - both sides
    • Facial motor nucleus
      • Upper face mm - both sides
      • Lower face mm - contra side only
    • Hypoglosssal nucleus - both sides
    • Nucleus ambiguus - both sides (pharynx/larynx)
  • Nucleus ambiguus sends motor fibres to muscles of the pharynx, larynx and upper oesophagus (via CN9 and CN10)
  • Seems like this could be in exam
  • Inputs to the trigeminal motor nucleus and hypoglossal nucleus are bilateral but stronger on the contralateral side
  • Lesion of the corticobulbar tract:
    • Paralysis of contralateral mm of facial expression
    • Weakness of contralateral mm mastication (jaw deviates to side of lesion when opened)
    • Weakness of contra side of tongue (tongue deviates to side of lesion when protruded)
      • NOT paralysis - but weakness
  • Tongue wrinkling - only in LMN (not paralysis), not in UMN
  • Selective weakness of lower facial muscles after cortical damage. This patient suffered stroke that resulted in weakness of his left hand (not whown) and the left side of his face. When he tried to bare his teeth on request (A), weakness of the left lower facial muscle
    • emotional motor goes through a different pathway (reticular formation etc)
  • Face:
    • UMN - lower face only affected
    • LMN - lower face + orb. oculi + temporalis

Other descending tracts

  • Vestibulospinal tract (Purple)
    • Origin - vestibular nuclei
    • Regulates muscle activity to maintain balance
    • Ipsilateral to its cells of origin
    • Terminate interneurons/motor neurons spinal grey matter
  • Reticulospinal tract (blue) - bilateral
    • Control of sterotyped movements (e.g. walking etc) and regulation muscle tone
    • Autonomic control
    • Origin: reticular formation
    • Bilateral (although not shown here)
    • Some fibres terminate in spinal autnomic nuclei e.g. lateral horn
    • Can take over some of the movements of the corticopsinal with rehab after a lesion
    • Reticular formation stimulates nuclei in sacral area etc via this tract
  • Rubrospinal tract - not important in humans (most stuff goes via corticospinal)
    • Doesn't go past cervical region