Upper MN Flashcards
Describe medial motor neuron and interneuron groups
Receive descending input from pathways that originate mainly in the brainstem
Course through the anterior-medial white matter of the spinal cord and terminate bilaterally
Describe the lateral motor neuron and interneuron groups
Receive major descending projection from the contralateral motor cortex via the main lateral division of the corticospinal tract which runs in the lateral white matter of the Spinal cord
Where are upper MN in the cerebral cortex?
Posterior frontal lobe
Upper MN do what?
Mediate planning and initiation of complex temporal sequences of voluntary movements
Upper MN are modulated by?
Basal ganglia
Cerebellum (via thalamus)
Sensory inputs from the parietal regions
Motor cortex refers to?
The primary motor cortex (M1)
The primary motor cortex is located where?
Anterior bank of the central sulcus
Paramedian lobule
The primary motor cortex can be distinguished from the premotor regions by?
Cytoarchitecture (brodmann’s area 4)
Low electrical threshold for eliciting movements due to direct connections to brainstem and spinal cord alpha motor neurons
Describe giant pyramidal neurons
Located in layer 5 (Betz cells)
Represent 5% of the total upper motor neurons of M1
Giant pyramidal neurons project to?
Via the corticospinal tract (to spinal cord controlling movements from the neck down) and corticobulbar tracts (to brainstem controlling face movements)
Non-Betz pyramidal neurons?
Remaining upper motor neurons in layer 5 that are found in M1 and the premotor corticies
Neurons in the motor and premotor coritices give rise to axons that? Where from there?
Travel through the internal capsule and coalesce on the ventral surface of the midbrain within the cerebral peduncle
Continue through the pons and come to lie on the ventral surface of the medulla giving rise to the medullary pyramids
Collaterals from the motor and premotor corticies?
As axons course through the brainstem, corticobulbar axons give rise to collaterals that innervate brainstem targets (cranial nerve nuclei, reticular formation, and red nuclei)
Most of the corticospinal tract nerves cross hwre?
In the caudal part of the medulla to form the lateral CST in the spinal cord (90%)
Those that do not cross form the ventral CS tract that terminates bilaterally in the lower motor neurons innervating proximal muscles
Axons that form the ventral CS tract project where?
Some directly to alpha motor neurons like in the hand but most contact interneuronal pools
Some components of the cortico-fugal projections from the layer 5 of the parietal regions (somatosensory) terminate?
What do these do?
In the interneurons near the sensory trigeminal and dorsal column nuclei in brainstem and in the dorsal horn in the spinal cord
Modulate transmission of proprioceptive and mechanosensory signals relevant to monitoring body movements
Describe the functional organization of M1
Post central gyrus
Fine motor control of face and hands are represented by a greater area of motor cortex than is the musculature requiring less precise motor control
Also somatotropically orgnaized
Contralteral to body intervated
Describe pathway somatotropy
Keep somtatotropic organiztion down to targets
Axons from the face are located?
In the genu of the internal capsule
Axons for motor commands from the arm region are located where relativel to the trunk and leg?
Posteriorly
Somatropic arrangement for the corticofugal pathways are seen where?
Cerebral peduncle
Pons
Medulla
Spinal cord
Individiual pryamidal tract axons termiante?
On sets of spinal motor neurons that innervate different muscles
Describe topographic representation of movement in M1
Organized around different categories of motor behaviors
Seen in premotor areas as well
Lower MN damage in the face affects all muscles…
Ipsilateral to the side of the lesion
Superior and inferior facial muscles
Upper MN damage in the face tends to spare?
The forehead
Mostly affects the inferior portion of the CONTRALATERAL face
Why does upper MN damage affect the contralteral face? Why spare forehead?
Corticobulbar projections from M1 are directed to the lateral cell columns in the contralateral facial motor nucleus
Dorsal cell columns that innervate the superior facial muscles are governed by bilateral premotor areas in M! and anterior cingulate gyrus
Why is injury to upper MN common?
Large amount of cortex occupied by motor areas
Also pathways extend all the way from cerebral cortex down to the lower end of the spinal cord
Initial manifestation of upper MN damage?
Most severe in arms and legs
Hypotonia (spinal shock) due to decrease in spinal activity by decrease in spinal activity by the interruption of descending commands
Trunk muscle control in upper MN syndrome?
Usually preserved because of bilateral projections of the corticospinal pathway to local circuits that control midline musculature
Several days after upper MN damage?
Spinal cord circuits regain much of their function
Thereafter Babinski sign is present
What is babinski sign?
Stroking foot should evoke flexion of big toe
Following damage to the descending upper MN stimulates extension of big toe and fanning of other toes
Occurs in human infants before maturation of the corticospinal pathway
What is spasticity?
Increased muscle tone and hyperactive stretch reflexes
Spasticity is prominent with?
With lesions to the cortex or internal capsule
Caused by removal of inhibitory influences from the cortex
Extensive upper motor neuron lesions can be accomanpied by?
Rigidity of the extensor muscles of teh leg and flexor muscles of the arm (decerebrate rigitidy)
Due to influence from the remnant vesticular n and reticular formation
What is clonus?
Oscillatory motor response due to muscle stretching
Consequence of hyperreflexivity (movement doesn’t stop)
Loss of ability to perform fine movements?
Lesion involves descending pathways that control lower MN to the upper limbs
Describe the premotor regions
Mosaic of interconnected frontal lobe areas that lie rostral to M1 and contribute to motor function
List premotor regions
Lateral premotor dorsal and ventral areas (BA 6l, 44 and 45), supplementary motor area (BA 6m) and cingulate motor regions (BA 23 and 24).
Premotor regions receive multisensory inputs from?
Parietal and prefrontal region
Premotor regions influence?
Motor behavior indirectly by modulating M1 and with direct projections via the CST and the corticobulbar tracts terminating in the spinal and brainstem interneuronal pools
Function of premotor dorsal region?
Associated with selection of appropriate movements before they occur (motor planning)
These neurons encode the intention to perform a movement
Supplementary motor region is involved in?
Coordination fo bimanual movements
Cingulate motor regions are associated with>
Modulation of emotionally triggered movements
Ventral premotor cortex is made up of?
Inferior portion of the precentral gyrus
Broca’s areas
What are mirror neurons?
In the ventral premotor cortex
Active during the execution of skilled goal oriented actions and discharge at the same rate during observation of the same movement performed by others
Imitation and motor learning
Describe the vestibulo-spinal pathway
Remember that these are for balance control through the VSR and VCR
Describe neurons of the reticular formation?
Scattered among axon bundles that course through the medial portion of the midbrain, pons, and medulla.
Reticular formation is found?
In the core region of the brainstem or the brianstem tegmentum
Diffuse fibers in the tegmentum are known as?
Reticular formation
Describe the function of the reticulo-spinal pathways
Involved in temporal and spatial coordination of limb and trunk movements
Reticulo-spinal pathways terminate?
Medial parts of the spinal grey matter
Like those from the vestibular nuclei
How is anticipatory body posture maintained?
Cued movement of the arm-hand is accomopanied by significant increase in proximal musculature that occurs before contraction of corresponding upper limb muscles
What is feedforward mechanism?
Predicts the resulting disturbance in body stability and generates an appropriate stabilizing response
Neurons in the motor cortex that supply the lateral part of the ventral horn do what?
Initiate movements of the distal limbs and also terminate on neurons in the reticular formation to mediate postural adjustments to support that movement.
Reticulospinal pathway terminates?
In the more medial parts of the ventral horn wehre the lower MN that innervate axial and proximal muscles are located
Motor corticies can influence the activity of spinal cord neurons via?
Direct and indirect routes
Tectospinal tract is from where and does what?
From superior colliculi and terminates int eh cervical spinal cord
Controls axial muscles of the neck (orienting movements of the head)
Mostly indirect since major output from superior colliculs is to the reticular formation and from there to the SC
