There are two types of movement:
- Automatic movements (reflexes)
- Volitional movements (self-controlled)
The motor system has a hierarchical organization:
- Peripheral
- Nerves from spinal cord to muscles
- Somatic (voluntary) and autonomic (involuntary)
- Central
- Spinal cord
- Brainstem
- Subcortical (basal ganglia)
- Cortex (primary motor cortex, premotor cortex, PCC and SMA
- Cerebellum
Motor sensor loop:
- Sense
- Move
- Sense & feedback
- Move
- Sense & feedback
- Adjust or strengthen internal motor plan
- New movement etc,
For rapid targeted movement, there is a primary fast ballistic movement, and then an error-correction movement.
Activating muscles through acetylcholine release. Then the muscle contracts.
Motor neurons are controlled by the central nervous system through the spinal cord. They are relatively large and are arranged in antagonistic (opposite) pairs.
Reflex arc: simple and short circuit for fast response. A reflex happens in the spinal cord and spinal neurons can generate an entire sequence of movements without feedback or input.
What happens when the sensory nerves are destroyed, but motor nerves are spared? No error-correction --> errors accumulate during sequences of automatic actions.
Summary peripheral motor system:
- Mechanism on its own in non-primates
- Strong dependence on central nervous system in humans
Central nervous system: there are different tracts responsible for different motoric functions:
Primary motor cortex:
- Simple movements, mapping body parts (overlap)
- Large Betz cells direct connections to alpha motor neurons in spinal cord. Only get activated in response to a very specific type of movement
- Damage: Relatively common. Paralysis to contralateral lesion sire. Reflexes become stronger.
- Is interconnected to surrounding regions, such as pre-motor cortex
Pre-motor cortex:
- Closely linked to primary motor cortex. Sends signal to primary motor cortex to achieve moment
- Also closely linked to higher-order areas (frontal). Sends signal to adapt movement depending on context.
- Closely linked to thalamus.
- Activity reflects planning and goal depending on context
- Damage: no response to cues, anosognosia (unaware of inability).
Posterior Parietal Cortex
- Important for sensory feedback and control of movements
- Controls externally-generated movement: perception action coupling
- Attention & gaze are coded in the parietal cortex
- Damage: difficulty with vision-based motor adjustment (apraxia)
- Ideomotor apraxia: (imitation of) action after instruction is disrupted. Unable to create an action concept
- Ideational apraxia: know what the desired sequence of actions is, but can’t execute it correctly
Supplementary Motor Area (SMA)
- Planning internally-generated sequence movements (specifically learned movements, such as playing the piano).
- Damage: defects in motor sequence planning. Not using affected limb when appropriate. Alien-limb syndrome
Internally vs externally generated actions:
- External: premotor & PPC
- Internal: SMA
Basal Ganglia
- Multiple subcortical structures (nuclei)
- Timing action: controlling when to start a planned action or sequence
- Inhibitory neurons (GABA). It inhibits the potential movement until needed
- The reward value manipulates the degree of inhibition
- Damage: Parkinson disease. Can include negative symptoms or positive symptoms
Cerebellum
- Contains the basis of automatic movements
- Damage: initiation of movements is normal, but no smooth control of movement. Timing is off.
- Coordinates highly practiced and fine movements, and is crucial for error-detection.
Perception & action are highly intertwined processes. Automatic imitation is an example of strong P&A coupling. A disadvantage of automation is that bad behavior is also imitated.
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