0806 - Motor Pathways Flashcards
Summarise the cortical representation of the motor homunculus
Hip is at corner between dorsal surface and median longitudinal fissure. Within median longitudinal fissure is leg and foot. Trunk and out to hands going over onto dorsal surface, then face, then tongue, then swallowing down on lowest aspect of lateral surface before moving into insula.
What elements of the brain are required to perform a motor action?
Ready:
Prefrontal Cx - Make cognitive decision to perform a motor task.
Posterior Parietal Cx and S1 - Provide information about world and body (fridge there, cup here, proprioception)
Set:
Premotor/SMA - Sequence the actions you need to do.
Go:
M1 - Execute the actions required.
S1 and PPC - continue to update based on changes to expected environment.
What is the role of the posterior parietal Cortex?
Converge sensory inputs about own body, immediate space, and environment.
Remains in constant communication with other motor areas during the intended movements.
What is the role of the Lateral Premotor Cortex?
Prime M1 ready for action - sequence what needs to be done following a decision based on external cue and coordinate both sides of the body. Essentially, codes for intention to execute the movements to respond to external event. (e.g. if have lesion, and told to pick up ringing phone, can’t do it, but can pick it up if you want to make a phone call - internal cue).
Ceases firing once it commands M1 to execute the movement.
Controls more complex movements and different body regions to SMA.
Outline the SMA
Movement sequencing/coordination evoked by internal cues (external for LPC) - e.g. ‘I want to make a phone call’
Inputs from frontal eye fields and cingulate sulcus, outputs to M1
May be activated by memory (internal cues)
Controls more complex movements and different body regions to LPC
What is the trajectory of the Corticospinal Tract
Originates from frontal and parietal cortex - 30-40% from M1, just under 1/3 each from either side (S1, supplementary motor area).
From cortex, through corona radiata, forming posterior limb of internal capsule. Run through cerebral peduncle, pass through pons and form pyramids. 80% crosses over after pyramids and then form lateral and corticospinal tract with half the non-crossing fibre (so 80% (contra) +10% (ipsi) of fibres end up in lateral corticospinaltract). Another 10% form ventral corticospinal tract and cross at appropriate level. 90% end up crossing somewhere.
What is the trajectory of Corticobulbar fibres
Motor innervation to head and face (V, VII, IX-XII). Descend from M1 through middle (around genu) part of internal capsule. Enter cerebral peduncles medial to corticospinal tract and synapse bilaterally on motor nuclei of cranial nerves (MV, VII, IX-XII) and in reticular formation.
Exception to bilateral innervation in that they don’t ipsilaterally innervate lower facial motor neurons (everything else is bilaterally innervated).
What are the 4 main extra-pyramidal tracts?
Rubro-spinal* - Auxiliary control of flexors (fine motor)
Reticulo-spinal (medial and lateral) - Anti-gravity, posture and locomotion based on feedback
Tecto-spinal* - Head orientation to external stimuli
Vestibulospinal (medial* and lateral) - Posture and balance based on vestibular inputs.
* - Terminates cervically - upper body/ upper limb control
What is the trajectory/info/functional significance of the Rubro-spinal tract.
Originates from red nucleus in upper mid brain, controls distal flexors. Cross in midbrain or upper medulla. Runs in a discrete bundle in lateral funiculus adjacent to lat CST, terminating mainly in cervical levels (thus is mostly for upper limb flexors).
What is the trajectory/info/functional significance of the Reticulo-spinal tract.
Runs from reticular formation just outside the periaqueductal grey and has 3 functionally distinct fibre systems mediating motor, autonomic, and pain.
Medial (pontine, motor) RS tract runs from large cells in pontine RF (nuclei reticularis pontis oralis and caudalis). Terminates at all levels of spinal cord on ipsilateral alpha and gamma motor neurons. It ACTIVATES extensors to promote upright body, including in locomotion. Medullary/Lateral RS arises in medial medulla and INHIBITS the extensors with same basic layout.
Together, can activate or inhibit extensors to remain upright.
What is the trajectory/info/functional significance of the Tecto-spinal tract
Originate from deep layers of superior colliculus. Cross in the tegmentum and descend C/L in central medulla close to medial lemniscus - in spinal cord, most ventral aspect next to anterior median fissure.
Terminate in intermediate grey in cervical levels of spinal cord.
Orients head reflexes to external stimuli (e.g. looking at a sound)
What is the trajectory/info/functional significance of the Vestibulospinal tract?
Originates from lateral and medial vestibular nuclei of CN VIII. These relay inputs from vestibular ganglion (internal acoustic meatus) and cerebellum.
Fibres from medial VN descend bilateral in medial longitudinal fasciculus to lower medulla and upper cervical spinal cord to control head position and orientation reflexes.
From Lateral VN project to all levels of ipsilateral spinal cord (most ventral aspect), innervating extensors to maintain posture and balance.
What is an upper motor neuron syndrome?
Disruption of central motor pathways resulting from damage to cortex or to fibres originating in the cortex (most commonly internal capsule due to stroke). Damage occurs to cortical fibres that provide input to all the modulators of motor activity, as well as corticospinal fibres.
What are the signs of upper motor neuron syndrome?
Paralysis or paresis of affected muscles (but not wasting)
Increased muscle tone (hypertonia/spasticity) in affected muscles, following an initial period of paralysis.
Abnormal reflexes (babinski, hyper-reflexia, clonus) due to loss of modulatory descending input.
How can you determine if a UMN Lesion is above/below red nucleus?
Above - Flexion of upper limbs, extension of lower due to rubrospinal tract dominance over CST lesion.
Below - Extension of all due to complete lack of RST control.
