Motor control 2 (UMNs, subcortical and cortical control) Flashcards
Where is the primary motor cortex located?
Precentral gyrus and paracentral lobule; Brodmann 4
What types of cells are the UMNs of the pre+primary motor cortex?
Pyramidal cells of cortical layer 5
Betz cells also play a role in LMN activation of most distal muscles
What is the common pathway of the corticobulbar and cortical spinal tracts?
Posterior limb of internal capsule-cerebral peduncle (midbrain)-base of pons-scattered amongst nuclei of the basal pontine grey matter-coalesce again on ventral surface of the medulla (medullary pyramids)
Suffix = place of termination ie brainstem (bulbar) vs spinal cord
Where do corticobulbar tracts terminate?
Cranial nerve nuclei: mostly bilaterally = unilateral damage means less significant functional deficits; contralateral exceptions = hypoglossal, trigeminal and (lower) facial motor nuclei
Reticular formation, Red nucleus Pontine nuclei then cerebellar nuclei
What is the pathway of corticospinal tracts?
Medullary pyramids - 90% decussate = lateral CST, 10% bilateral = ventral/anterior CST
What does the ventral CST govern?
From the dorsal-medial regions of the motor cortex - trunk + proximal limb muscles (region also gives rise to projects to the reticular formation)
Where do neurons of the lateral CST terminate?
Terminates mostly in the lateral parts of ventral horn and intermediate grey matter
Some synapse directly with alpha motor neurons of forearm and hand - direct control over some discrete finger movements - links to poor recovery of certain movements following motor cortex injury
Most terminate in pools of local circuit neurons
How are motor actions represented in the cortex?
Similar homunculus to somatosensory cortex but not as discrete; proportions depending on what degree of control required ie high in hands and face; not individual muscles represented but groups of coordinated movements - substantially overlapping regions
There are also directional correlates in neuronal populations - some neurons firing most strongly when an action is performed in a certain way/direction
This was discovered through cortical stimulation using microelectrodes
What is spike triggered averaging?
Correlating timing of single cortical neuron discharge with contracts of muscles it innervates can see the relationship between two - gave rise to ‘muscle fields’ of UMNs = all the muscles stimulated by a particular cortical neuron
How have studies on cortical neurons during visually guided reaching movements of the arm and hand provided insight into motor control?
Directions of arm movements in monkeys could be predicted by calculating a ‘neuronal population vector’ derived from activation of the UMNs that are ‘broadly tuned’ = each neuron discharges to movements in multiple directions
Suggests that individual UMNs cannot specify the direction of an arm movement - instead movement encoded by concurrent discharge of population of linked neurons
What is the basic structure and function of the premotor cortex?
Frontal lobe - Brodmanns areas 6,8,44/45 (lateral), 23, 34 (medial)
Receives input from inferior and superior parietal lobules + prefrontal lobe
Influence motor control directly - axons projecting through corticobulbar, and spinal paths (>30%); and indirectly - reciprocal connections with primary motor cortex
Use variety of inputs to select context and goal appropriate movement
What are some other differences between the role of primary and premotor cortices in the control of movement?
Primary = >number of monosynapses with alpha motor neurons (LMNs)
Action goals encoded by primary tend to be localised to within arms length; premotor = beyond arms length
What is the function of the lateral premotor cortex?
65% of lateral - timelocked to occurrence of movements ie firing most strongly before + during specific movement made; also key for conditional/closed loop motor tasks = when learning to pair stimulus with movement, as strength of conditioning increases, these neurons fire following cue presentation and before action - appear to be encoding ‘intention’
Lesions in monkeys impair performance on visually or verbally cued tasks; similar to frontal lobe patients
What is the function of the ventrolateral premotor cortex?
Locations of so called ‘mirror motor neurons’ - fire when action is observed
Respond even if final stage of action is hidden
Fire less when action is pantomimed without explicit presence of an action goal (ie pretending to grasp an object when there’s no object)
Seem to be responsive to intention to make movements but also, might all be a pile of wank
What is the function of the rostral lateral premotor cortex?
Especially in the left
Broca’s area (Brodmann 44/45) - speech production
