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
What is the function of the medial premotor cortex?
Part of it also known as supplementary motor area
Appear to be involved in response to internal rather than external cues (open loop); carrying out of motor tasks from memory
Lesions lead to reduction in spontaneous movements
What the fuck am I doing with my life?
……
What is the medial vestibulospinal tract?
From medial vestibular nucleus (from 8th cranial nerve) - semicircular canals/otoliths input = rotational and translational movements
Terminates bilaterally in medial ventral horn of cervical spinal cord
Mediates reflex activation of neck muscles
What is the lateral vestibulospinal tract?
Lateral vestibular nucleus
Ipsialteral control
Terminates amongst medial LMN pools that govern proximal + trunk muscles involved in balance and posture
What is the reticular formation?
Brainstem circuits from rostral midbrain to caudal medulla
Mediate fuck-tonnes - cardiovascular and respiratory systems, sensorimotor reflexes, eye movement coordination, initiation of locomotion (mesencephalic locomotor region) sleep/wake, temporal + spatial coordination of limb/trunk movements that are involved in locomotion
Reticulospinal projects are typically bilateral - to both medial ventral horns
Involved in lots of preparatory motor behaviours prior to the completion of a voluntary action ie stabilisation of the trunk
so many lesioned animals…
What is the function of the superior colliculus?
Dorsal midbrain
Input from eyes n shit (more info elsewhere)
Some direct output to UMNs in spinal cord (tectospinal tract) but most output via reticular formation
Important for head orienting movements
What is the rubrospinal tract?
Red nucleus (in tegmentum of midbrain) - lateral white matter (unlike other brainstem projections) - lateral ventral cervical spinal cord
Distal muscles of upper extremities but significance is dubious (great…)
What is spinal shock?
An initial period of contralateral distal hypotonia + hyporeflexia following upper motor neuron injury (ie cortex/internal capsule)
Trunk muscles usually preserved due to remaining brainstem pathways or bilateral corticospinal projections
After a period of several days, spinal cord circuits often regain lots of function (?strengthening of remaining connection, new connections etc)
What are the symptoms of upper motor lesions?
Babinski sign +ve = extension of big toe and fanning of remaining toes; also just general emergence of primitive reflexes
Hyperreflexia
Spasticity - increased muscle tone, clonus - from an increase in the gain of spinal cord reflexes due to the loss of descending suppression; more severe in central UMN lesions compared to spinal - greatest extensor tone if lesion between level of vestibular nuclei and red nucleus = decerebrate rigidity
Loss of fine motor control
