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Summary Exam 4 Brain and Cognition UvA Year 1
- Instelling
- Universiteit Van Amsterdam (UvA)
Summary Exam 4 Brain and Cognition UvA Year 1
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W1.1MOTOR SYSTEMS – CHAPTER 5
Intermediate Motor control is hierarchical: Complex behaviours are organised at several levels
processing 1. Highest level = Sets of commands to initiate a sequence of movements (motor
programs)
2. Lowest level = Elementary behavioural units that activate muscles (motor execution)
Neural circuits controlling skeletal movements:
1. Upper motor neurons in the brainstem and cortex: Planning of sequences of
movements
- Do not strictly depend on sensory information
- Independent of actual muscle groups that perform the movements
↓
Bernstein’s handwriting task – Motor invariance: Ability to produce the same
motor actions with different body parts – Complex movements preserve distinctive
features even when performed by different muscle groups
- Originate within the central nervous system, rather than arising from sensory
signals
2. Cerebellum: Adjusting ongoing movements and error correction
3. Basal ganglia: Selecting and inhibiting motor action
4. Lower motor neurons in the spinal cord (controls the direct activation of single muscle
fibres and organises reflexes) and brainstem (sends motor signals and corrects posture)
- Axons of the lower motor neurons directly innervate skeletal muscle fibres
- Muscle contractions are directly linked to action potentials in lower motor neurons
– There is no delay between the action potentials and the muscle contractions
- Can connect incoming sensory information to appropriate motor neurons by
connecting the muscle to the spinal cord
- Reflexes (E.g.: stretch reflexes require the activation of the biceps, and the
inhibition of the triceps)
- Spinal cord alone is capable of producing movements (even rhythmic
movement such as walking or swimming) – Cat whose spinal cord has
been separated from the brain can adjust walking speed to the treadmill, but
not when sensory fibres entering the spinal cord are cut through
Muscle agonist: Main muscle that creates a movement
Muscle synergist: Muscles that work together to create a
movement
Muscle antagonist: Muscles that resits movement to keep balance
,Cortical motor structures:
- Primary motor cortex/M1: Initiation of coordinated multijoint
movement and activation of lower motor neurons
- Upper motor neurons from M1 descend to innervate neurons in
the brainstem or spinal cord
- Some neurons branch off in brainstem (E.g.: neurons for
facial expression control)
- Most neurons (the neurons that innervate distal muscles) descend
through the medullary pyramid, cross the midline at the caudal end of
the medulla, enter the lateral corticospinal tract in the spinal cord and
terminate within the grey matter of the spinal cord – Lateral spinal cord
– Lateral corticospinal tract
- Some neurons (the neurons that innervate proximal muscles) remain
uncrossed – Medial spinal cord – Anterior
corticospinal tract
↓
- Damage in the brain: Contralateral side
- Damage below the brain: Ipsilateral side
- Representation of the body – Homunculus – Shows the
parts of the body relative to their represented size in M1 by the complexity of
movement
- Premotor cortex: Learning complex movement from external cues – Requires sensory
input
- Supplementary motor area: Planning of learned sequences of movement from memory
– No sensory input required
- Posterior parietal cortex: Computations for location-based movement, grasping, and
reaching; knowledge on how to use objects
Cortical pathways for motor control:
- Early studies: Stimulation of M1 neurons can lead to simple muscle contractions
- Later studies: Stimulation of M1 neurons produces complex (coordinated, multijoint)
movements
↓
- M1 does not activate individual muscles directly but projects to lower motor regions
(brainstem, spinal cord) through motor programs (= ‘what to do and how to do it’)
- Most M1 neurons fire to a specific subset of movements (e.g., push, pull, or press) and
code direction of movement (not individual muscles)
- Other than the lower motor neurons in the brain stem and spinal cord, M1 neurons fire
well before movement initiation
Population coding of movements:
- Each motor neuron has a preferred direction of movement for which it shows maximal
activation – Other directions also produce activity in the neuron, albeit lower
- Precise movements are encoded by averaging the activity of many coarsely tuned
neurons
, - Population vector: The direction of movement as a result of the brain weighing all the
evidence of all motor neurons → Inactivation of a certain set of motor neurons results
in a shift in the weighted average
- Population coding: The way of neural coding across a set of neurons
Planning movements:
- Anticipatory activity in premotor cortex reflecting the intention to move
- Planning-related activity begins earlier in premotor cortex than in M1 – Supports the
idea that motor areas are hierarchically organised – Premotor areas provide more
abstract planning information, and M1 translates that into the intention to perform
specific movements
- Readiness potential: The measure of activity in the premotor cortex and supplementary
motor area of the brain leading up to voluntary muscle movement – Begins bilaterally
in the premotor areas, but becomes enhanced over M1 contralateral to the movement
- Libet’s experiment: The brain prepares the action before the participant becomes
aware of the intention to act – Information in frontal and parietal cortex up to 10s
before onset of the conscious intention to move
- Anosognosia: Damage to M1 and premotor areas results in the loss of the awareness
of one’s inability to move intentionally – Premotor areas are linked to conscious
awareness
Selecting goals for action:
- High coherence: Most of the dots move into the same direction (easy task) – Monkeys
were almost always correct when about 50% of the dots moved in one direction
- Low coherence: Only slightly more dots move into the left or right direction (hard
task) – Performance fell to chance levels when less than 5% of the dots moved in one
direction
- Activity in motor structures ramps up before the monkey signals the decision
- Reaction time and ramping up of activity is much faster in the case of high coherence
- Evidence accumulates for decision alternatives at some rate, until the evidence for one
alternative reaches some threshold that triggers a decision
Motivational control of goal selection:
- Neurons in the posterior parietal cortex are sensitive to the reward value of making a
particular decision
- Selecting a movement goal involves scaling neuronal responses associated with each
possible movement by that movement’s value – The motor system is biased to produce
a movement that acquires rewards and avoids punishments – Dependent on the
probability and magnitude of the reward that is associated with the target
Sequential movements:
- The supplementary motor area is responsible for generating movements in the absence
of sensory cues – When the supplementary motor area is inactivated, well-learned
movements (from memory) can no longer be performed, and external cues must be
relied on – Uncued
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