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What are motor control pathways, what are the 4 major ones, and where are they located?
- Descending axons that connect motor neurones to the spinal cord
- Corticospinal (pyramidal): cortex
- Vestibulospinal: vestibular nuclei
- Reticulospinal: reticular nuclei
- Rubrospinal: red nucleus
- Tectospinal: superior colliculus
What type of inputs handle control of distal limbs?
- Corticospinal and rubrospinal inputs are mainly to nuclei in the lateral part of the spinal cord, confined to only a few segments and one side
What type of inputs handles posture?
- Vestibulospinal and reticulospinal inputs mainly to nuclei in the medial part of the spinal cord, spanning multiple segments and sometimes bilateral.
- Reticular nuclei are under cortical control
What pathways are important for precision grip, and how do we know this?
- Lawrence and Kuypers (1968)
- Cutting the pathway leaving the medulla impairs power grip but has less effect on precision grip
- Implication is that direct pathways from cortex important for precision grip (fractionated grip)
How do we know which pathways are important for posture?
- Lawrence and Kuypers (1968b)
- Lesioning to monkey medial brainstem pathways led to an almost complete loss of posture and balance (had to be supported in chair)
Which pathway handles face and head movement?
- Corticobulbar pathway
- Also receives signals from the cerebellum and basal ganglia
Where does moment planning take place?
Where do perceptions of space and the location of limbs take place?
Where does actual control of muscles come from?
- Premotor cortex
- Supplementary motor area
- Primary motor cortex
Draw the homunculus of the primary motor cortex
How doe the layers of the motor and sensory cortex differ, and why?
- Motor cortex is thicker than sensory
- cortex, but also less densely packed with the cell bodies of neurones
- This leaves more room for the formation of connections between neurones, which may be important in the coordination of novel, or task dependent movements
What are betz cells?
- The cells in motor cortex that send axons to the spinal cord.
- Located in layer 5 they are some of the largest neurons in the brain
Why can't you say motor neurones code for the force of a muscle?
- Each neurone in M1 controls several muscles (divergence)
- Each muscle is controlled by many neurones in M1 (convergence)
- So motor neurones do not encode the force of a single muscle, but rather a movement
What is M1 primary concerned with?
Direction of limb movement, and the force applied (also position and velocity)
Why can M1 be said to have topographical maps, and why are these biased?
- Movement of electrodes across M1 shows orderly sequence of body movement, indicating a topographical map (like in the somatosensory cortex)
- This is over-represented in areas which involve more fine movement (e.g. hands, tongue)
How can motor neurones be said to employ 'population coding'?
- Motor cortex neurons encode global aspects of movement (direction, distance etc)
- Each neuron has a direction, and the movement is basically the result of tallying up the direction of a population of neurons
- Adding up each neuron's vector gives a population vector.
- If all neurons are active, it will cancel out to 0 (no movement).
- If only some are, the population vector will have direction and length/distance which predicts the body movement.
Where do the supplementary motor area (SMA) and premotor cortex (PMC) send signals to, and where do they get signals from?
- SMA, M1 and PMC send output to overlapping bits of the brainstem and SC
- BUT: they receive v. different input from one another.
- M1 gets input from PMC and SMA, and parietal areas
- PMC gets input from parietal areas and PFC.
What is (probably) the neural pathway for grasping?
- AIP - F5 - F1 (motor cortex) is a visuomotor pathway through the ‘dorsal stream’. This pathway helps allow the direct processing of objects that afford actions.
- Similar areas have been identified as playing a role in grasping in human, but we cannot yet draw the arrows
What is alien hand syndrome, and what causes it?
- Hand movement occurs without the person being aware of it
- Can come about after lesioning to the supplementary motor area
- This are is also important for complex sequences of actions, and is active prior to them
What are mirror neurones?
- Single-neurone recordings in ventral premotor cortex of macaque (area F5) suggest that it provides a ‘motor vocabulary’ of possible movements for reaching, grasping, holding and bringing to the mouth
- Some neurones in F5 (~17%) active when monkey executes a specific action (usually grasping) and observes same (strictly congruent) or similar action with similar goal (broadly congruent)
- These neurones not active to objects, or to observation of action without a target object.
- Similar neurones found in parietal areas (including LIP, VIP, AIP and PFG), which get input from higher-order visual centres in STS. F5 is connected to AIP and PFG, and also ventral prefrontal cortex and pre supplementary motor areas.
- Similar response to observed actions also found in primary motor cortex of monkeys, and inferred in M1 of humans, though in M1 activity during executed movements is much greater than that during observed movements
Are mirror neurones fully understood in humans?
- Not really
- fMRI says observing action increases activity in PMC (where activity is unaffected by whether the action is goal-directed or not) and parietal areas (where activity is affected by the goal-direction of the action).
Why are motor neurones controversial?
- “If this mirror mechanism is fundamental to understanding actions and intentions, the classical view that the motor system has a role only in movement generation has to be rejected and replaced by the view that the motor system is also one of the major players in cognitive functions.” Rizzolatti and Sinigaglia, 2010
- “...when the observed action impinges on the motor system through the mirror mechanism, that action is not only visually labelled but also understood, because the motor representation of its goal is shared by the observer and the agent. In other words, the observed action is understood from the inside as a motor possibility and not just from the outside as a mere visual experience” Rizzolatti and Sinigaglia, 2010
- “A null hypothesis is that F5 is fundamentally a motor area that is capable of supporting sensory–motor associations [relevant to action selection].” Hickok 2009
What is the mirror model of action understanding?
Objects are understood by ventral stream, and actions are understood by dorsal stream
What is the classic model of action understanding?
Both objects and actions are understood in ventral stream; both objects and actions are used in selecting appropriate action in dorsal stream