Upper Motor Circuits

What are voluntary movements?

in reflexes there tends to be a one to one mapping with respect to stimulus and response where was with voluntary movements there are multiple neurons of many brain regions that coordinate muscles to move

• you decide how to respond

• adaptable response

What are the motor control centres in the brainstem and what are they important for?

they are important for postural control

• vestibular nucleus (vestibulospinal tract)

• reticular formation (reticulospinal tract)

• superior colliculus (colliculospinal tract)

• Red nucleus (rubospinal tract)

what are the two sources of upper motor neurons with functional specialization?

motor cortex → planning, initiating and directing voluntary movements

brainstem centres → rhythmic, stereotyped movements and postural control

what is the intermediate zone?

where you have interneurons that receive information from the cortex and then their output goes to the alpha motor neurons

what do the medial axons control in the cortex? lateral in the brainstem?

medial control proximal muscles (goes through the cortex)

lateral control distal muscles (goes through brainstem)

where do cortical motor areas receive inputs from?

• basal ganglia

• cerebellum

• parietal lobe

where is the primary motor cortex located?

pre central gyrus

What is the pyramidal tract?

highway that carries voluntary motor commands from the cortex to the muscles via the spinal cord or brainstem

• they contain Betz cells

What are Betz cells?

• largest cell soma in the CNS

• 5% of them project to the spinal cord

• important for distal muscle control

• only found in primary motor cortex

what is cortobulbar tract?

the corticobulbar tract gives rise to bilateral collaterals the innervate multiple brainstem nuclei and terminate on local circuit neurons in the middle medulla

• controls muscles in head, face, neck and mediating their functions

• innervates trigeminal nerve

• bilateral

• we want coordination here

What is the cortical spinal tract?

divides into two parts

• 90% of the fibres go to the other side when they reach the caudal part of the medulla to form the lateral cortiocospinal tract (distal)

• 10% of the fibres do not cross when they reach the caudal medulla and instead continue terminating bilaterally forming the ventral cortiospinal tract (proximal)

what is the topographic map of movement in the primary motor cortex?

• Body parts are represented systematically along the cortex, but not in proportion to their size in the body.

• The layout is roughly:

  • Medial cortex (near the longitudinal fissure) → leg and foot

  • Middle cortex → trunk

  • Lateral cortex → arm, hand, face, tongue

• Some parts, like hands and face, take up much more cortical area because they require fine motor control.

What is intracortical microsimulation?

• showed the upper motor neurons in layer 5 of the motor cortex that projected to lower motor neuron can be stimulated focally

• small currents excited excitation of several muscles suggesting that movements must be organized rather than muscles

• local circuits are involved in controlling movements

• but people don’t like this stimulate because its not the natural thing that happens in the brain so they feel its not as accurate

what was the evidence that force is produced by cortical motor neurons?

• as firing of neurons increase the force they produce also increases

• it was proposed that motor cortex contributes to the early phase of movement generation and planning

• recorded via intracortical recordings

What is the natural version of intracortical microsimulation?

• Spike triggered averaging

• allows you to correlate the activity of an individual neuron in the motor cortex with muscle activity

• it is shown that a peripheral muscle group is activated by one motor neuron

• confirms that single upper motor neurons connect several motor neuron pools

how does spike triggered averaging work?

1. identify a motor even

2. measure muscle activity and neuron firing during each event

3. align all trials to the trigger → all responses are lined up so the event occurs at the same point in time

4. average the responses so that constant patterns emerge clearly → reveals typical effect of the neuron on the muscle

what does prolonged stimulation of the primary motor cortex do in monkey’s?

it elicits coordinated movements of the hand to mouth as if to feed → its a purposeful movement that is generated

What evidence is there that shows that there are neurons that encode the direction of movement?

they get an animal to move a joystick in the different directions and they start recording from specific neurons → record how neurons respond

• they see that some neurons have a preference and fire more strongly in specific directions rather than others

• encode movement in certain direction (tuned to specific direction)

• population average will determine the direction of the movement

What are the indirect and direct influences on motor behaviour by the premotor cortex mediated by?

indirect → reciprocal projections to the primary motor cortex

direct → mediated by axons that project to the corticobulbar and corticospinal tracts

What is the role of the lateral premotor cortex?

• lateral

  • especially important in closed loop motor tasks → tasks that require precision and adjustment so feedback is esential

  • neurons fire at the appearance of the cue and increasing firing rates increase between cue and signal to move

  • neurons encode the intent to move

  • ventrolateral also contains neurons that respond to observed movement made by another individual

  • involved in planning → no muscle movement when firing occurs but its preparation to move muscles

What is a mirror motor neuron?

• occurs with neurons in a ventral-anterior lateral part of the premotor cortex

• if the monkey is being shown to pick up the food from the plate → the firing rate increases from the baseline

• then when the monkey picks it up firing increases even more

• however if the money is shown to pick it up with pliers, the same neuron won’t respond because the observed movement no longer matches the money’’s internal motor representation of grasping with the had

• mirror motor neurons are motor matched neurons

What was done to see if the premotor cortex mirror neurons are involved in direct control of movement?

• they get the animal to remove the food pieces but they can’t see their own movement → a barrier blocks so there is no observation

• if they are only associated with observation then they wouldn’t fire because theres no observation with the barrier BUT they do fire

  • meaning involved in action itself

  • encodes observation action of others and imitation/observational learning

What does the lateral premotor cortex require?

external cues for selection of movement

if their is a lesson in monkeys that imparts closed loop tasks the monkey loses the ability perform visual cue condition tasks even though they can see the visual cue

what is the medial premotor cortex?

• mediates selection or initiation of movements but is involved in open loop conditions

• requires internal cues

• lesions in the medial premotor cortex in monkeys reduces spontaneous movements but preserves the ability to initiate movement in response to visual cues

where do the pathways that influence lower motor neurons in the medial ventral form originate from?

in the upper motor neurons in the vestibular complex, reticular formation and the superior colliculus

what are the vestibular nuclei?

• give rise to medial vestibulospinal tract

  • bilateral

  • mediates feedback or responds to a disturbance of body posture and then stability signalled by the semicircular canals since they detect angular head movements

• gives rise to lateral vestibulospinal tract

  • ipsilateral

  • involved in proximal muscle responses for stable balance and upright posture in response to signals from the otolith organs

What is reticular formation neurons?

neurons project in the reticulospinal tract to the medial ventral horn and modulate reflexes for stereotyped movement

• these neurons receive input from cortex, hypothalamus or the brainstem and initiate feedforward adjustments to stabilize posture during ongoing movements

what is an example of reticular formation?

1. there is lever on the wall that you wanna pull towards yourself

2. when you pull the lever, the lever also pulls you towards the wall → if your leg muscles don’t produce any activity, you will be pulled towards the wall

3. to make sure you don’t get pulled towards the wall and keep your stability → you flex your elbow and stabilize your legs (adjustments made)

what is the superior colliculus?

• contains direct pathway neurons to the spinal cord and indirect pathway through the reticular formation → inputs to the resticulospinal tract to control axial muscles in the neck and functions in head orientation

What is the red nucleus?

control of arms and hands in animals