Motor System

Three types of movements (antialphabetic): - [blank_start]Voluntary[blank_end] movements - [blank_start]Rhythmic[blank_end] movements - [blank_start]Reflexes[blank_end]

The visuomotor delay is approximately [blank_start]200[blank_end] ms. The proprioceptive delay is approximately [blank_start]50[blank_end] ms.

Three things that make motor control difficult (alphabetic): - [blank_start]environmental[blank_end] uncertainty - [blank_start]motor[blank_end] noise - [blank_start]sensory[blank_end] noise

The human body has "only" 600 muscles. Nonetheless motor control is very difficult, since the number of possible movement patterns increases [blank_start]exponentially[blank_end] with the [blank_start]degrees of freedom[blank_end].

The principle of [blank_start]redundancy[blank_end] means that there is a [blank_start]gap[blank_end] between high- and low-level specification. Any high level task can be achieved in [blank_start]infinitely[blank_end] many low level ways.

[blank_start]Fitt's[blank_end] law describes the [blank_start]speed-accuracy[blank_end] trade-off.

In simple grasping tasks the paths of hand movements are usually [blank_start]straight[blank_end] and the speed profiles are [blank_start]bell-shaped[blank_end].

The two-thirds power law: [blank_start]angular[blank_end] velocity ~ [blank_start]curvature[blank_end]^(2/3)

Three levels of analysis: (1) [blank_start]Computational[blank_end]: What is the [blank_start]problem[blank_end] the brain is trying to solve? (2) [blank_start]Algorithmic[blank_end]: What is the [blank_start]strategy[blank_end] to solve this problem? (3) [blank_start]Neuronal[blank_end]: How is it done by the nervous system?

The forward model [blank_start]predicts behaviour[blank_end] based on [blank_start]motor commands[blank_end]. The inverse model [blank_start]calculates motor commands[blank_end] from [blank_start]desired behaviour[blank_end].

Feedforward ([blank_start]open[blank_end]-loop) control: Because of the delays in the sensorimotor system, open-loop control is used for [blank_start]short[blank_end] movements. Disadvantages: - [blank_start]Inaccuracies[blank_end] cannot be corrected - [blank_start]Unexpected changes[blank_end] will not be taken into account

In order for feedforward control to be sufficient, the [blank_start]inverse[blank_end] model would have to be perfect.

Feedback (closed-loop) control can compensate for errors due to

In an experiment, subjects are given 400g blocks and practice to lift them. After training the weight is surprisingly increased to 800g. The experiment shows the subjects use

The [blank_start]gain[blank_end] factor is the amount by which the [blank_start]corrective[blank_end] motor command is increased or decreased per unit of error.

[blank_start]High[blank_end] gain and [blank_start]high[blank_end] sensorimotor delay can lead to [blank_start]overcompensation[blank_end] and [blank_start]instabilities[blank_end].

An [blank_start]efference[blank_end] copy is an internal copy of an outflowing ([blank_start]efferent[blank_end]), movement-producing signal generated by the motor system.

Two strategies to compensate for sensory delays: (1) [blank_start]Intermittency[blank_end] of movements (2) [blank_start]Prediction[blank_end]

The experiment where you move your eyeball with your hand and the world moves shows that the brain uses [blank_start]motor commands to the eye[blank_end] in order to predict the eye's position.

What's the problem with using a forward model in feedback control to predict the position of a limb.

Which are the main structures of the CNS involved in motor control?

A motor unit consists of [blank_start]a spinal motor neuron[blank_end] and [blank_start]the muscle fibres that it innervates.[blank_end]

A muscle is typically innervated by [blank_start]a few hundred[blank_end] spinal motor neurons.

The [blank_start]innervation number[blank_end] tells how many muscle fibres a motor neuron innervates.

The innervation number

The muscle force is controlled by

Motor units differ in [blank_start]speed[blank_end] and maximum [blank_start]force[blank_end].

Recruitment: [blank_start]Small[blank_end] motor neurons are recruited first because of the [blank_start]high[blank_end] resistance within their axon. They have a [blank_start]lower[blank_end] innervation number and therefore produce [blank_start]less[blank_end] force.

Spinal reflexes are [blank_start]involuntary[blank_end] and triggered by [blank_start]external stimuli[blank_end]. Despite classical theory they can be quite flexible.

The withdrawal from a painful stimulus is called [blank_start]flexion-withdrawal[blank_end] reflex. The [blank_start]flexor[blank_end] muscle is activated and the [blank_start]extensor[blank_end] muscle is inhibited. This is called [blank_start]reciprocal innervation[blank_end]. The opposite effect happens on the [blank_start]contralateral[blank_end] limb. The force [blank_start]does[blank_end] (does or does not) depend on the stimulus intensity.

The stretch reflex is a [blank_start]muscle contraction[blank_end] in response to stretching within the muscle. The antagonist muscle is inhibited, this is called [blank_start]reciprocal innervation[blank_end]. The stretch reflex increases the [blank_start]stiffness[blank_end] of the muscle.

The areas of the cerebral cortex that are used in motor control: - [blank_start]Primary motor cortex[blank_end] (M1) - [blank_start]Premotor cortex[blank_end] (PM) - [blank_start]Supplementary motor area[blank_end] (SMA)

The motor cortices receive input from

The motor homunculus is most detailed for the [blank_start]primary motor cortex[blank_end]. It disappears if you [blank_start]zoom on[blank_end].

The motor pathway to lateral motorneurons runs [blank_start]contralateral[blank_end] and controls mainly [blank_start]distal muscles[blank_end]. It includes the [blank_start]corticospinal tract[blank_end]. The patway to medial motorneurons runs [blank_start]ipsilateral[blank_end] and controls mainly [blank_start]stance and posture[blank_end].

The corticospinal tract The connections come from the cortical layer [blank_start]V[blank_end]. Many of these connections terminate on spinal [blank_start]interneurons[blank_end]. Only [blank_start]M1[blank_end] contains neurons projecting directly to spinal motor neurons. These cortical neurons are called [blank_start]corticomotorneurons[blank_end]. Those neurons have only one synapse.

The axons of corticomotorneurons terminate on [blank_start]spinal motor neurons[blank_end]. The can also influence other muscles via spinal [blank_start]interneurons[blank_end]. Most of the time the excite agonist muscles and inhibit antagonist ones.

Reading information about the direction of a movement from several motor neurons is done via [blank_start]population codes[blank_end].

Neuronal tuning with respect to movement means

Different digits of the hand are controlled by sharply separated areas in M1.

Four symptoms of cerebellar disorders: [blank_start]Hypotonia[blank_end]: A diminished resistance to passive limb displacements. [blank_start]Astasia-abasia[blank_end]: An inability to stand or walk. [blank_start]Ataxia[blank_end]: The abnormal execution of multijointed voluntary movements, characterized by lack of coordination. [blank_start]Action tremor[blank_end]: A form of tremor at the end of a movement, when the patient attempts to stop the movement by using antagonist muscles.

The basal ganglia are involved in

Relation between function and areas of the brain (hypothesis)

The motor control loop

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Optimal feedback control

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The forward-model is used at the [blank_start]beginning[blank_end] of movements.

There are slow- and fast-[blank_start]twitch[blank_end] muscle fibers.

[blank_start]Supraspinal[blank_end] centers play an important role in modulating and adapting spinal reflexes, even to the extent of reversing movements when appropriate.

Stretch reflex: In addition to the muscle that is stretched [blank_start]synergistic[blank_end] muscles are contracted.

M1: Brodman [blank_start]4[blank_end] PM & SMA: Brodman [blank_start]6[blank_end]

Motor areas