Brain & Cognition, interim 2
CH8: Action
- Parkinson’s disease results from cell death in the substantia nigra. Dopamine
production/activity lowered.
- Perception and action are intimately interwoven.
The anatomy and control of motor structures
Motor system: organized in hierarchical structure with multiple levels of control: spans the
spinal cord, subcortex, and cerebral cortex.
- spinal mechanisms are contactpoint NS and muscles. Capable of producing simple reflexive
movements.
- Hierarchy:
top: premotor and association areas. (planning)
lower: primary motor cortex + brainstem structures (together with cerebellum + basal
ganglia: action goal movement)
lower: spinal cord
lowest: muscles, motor neurons
Muscles, motor neurons, and the spinal cord
Action is generated by stimulating skeletal muscle fibers of an effector (= part of body that
can move). Distal effectors: far from body center – arms, legs, hands. Proximal/central
effectors: close to body center – waist, neck, head.
- all forms of movement result from changes in state of muscles that control an effector/group
of effectors. Muscles: composed of elastic fibers, can change length and tension. Effector can
often either flex or extend.
- muscles activated by motor neurons. Alpha motor neurons innervate muscle fibers and
produce contractions of fibers. Gamma motor neurons part proprioceptive system, sensing
and length regulation fibers. Motor neurons originate in spinal cord, exit through ventral root
and end in muscle fibers.
Action potential motor neuron releases NT, alpha motor neurons: acetylcholine.
-- alpha motor neurons: provide physical basis translating nerve signals mechanical
actions, changing the length and tension of muscles.
- Alpha motor neurons receive peripheral input
(muscle spindles). – they take care of the reflexes,
such as the stretch reflex.
- motor neurons also innervated by spinal
interneurons, lie within spinal cord. Excitatory and
inhibitory signals from higher centers. Excitatory
signal to one muscle, agonist, accompanied by
inhibitory signals to antagonist muscle via
interneurons. This is how reflexes can be overcome
to permit volitional movement.
Subcortical motor structures
Extrapyramidal tracts: motor pathways that send direct projections down the spinal cord. –
primary source of indirect control over spinal activity modulating posture, muscle tone, and
movement speed; receive input from subcortical and cortical structures.
Pyramidal tracts: axons that travel directly from cortex to spinal segments.
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- Cerebellum:
massive, densely packed structure containing more neurons than the rest of CNS combined.
Ipsilateral organization: right side = associated with movements right side body, left with left.
made up of 3 regions:
1) vestibulocerebellum: control balance and coordinate eye movements with body
movements.
2) Spinocerebellum: receives sensory info from visual + auditory systems, as well as
proprioceptive info from spinocerebellar tract. Output of this region: innervates spinal cord +
nuclei extrapyramidal system.
-- balance.
3) Neocerebellum: lesion: ataxia, problems sensory coordination of distal limb movements,
fine coordination disrupted.
- but cerebellum also seems to have other functions than motor functions alone.
- Basal Ganglia:
collection 5 nuclei: caudate nucleus + putamen (striatum), globus pallidus, subthalamic
nucleus, substantia nigra.
Input restricted to striatum nuclei. Output: globus pallidus and part of substantia nigra.
Cortical regions involved in motor control
primary motor cortex, premotor cortex, supplementary motor area, -- motor areas.
posterior and inferior parietal cortex, primary somatosensory cortex. – production movement.
- motor cortex regulates activity spinal neurons in direct and indirect ways.
- corticospinal tract (CST): axons that exit cortex and project directly to spinal cord.
this is the pyramidal tract. Terminate either on spinal interneurons or directly on alpa
motor neurons.
- Primary motor cortex: M1.
receives input almost all cortical areas implicated in motor control. Rostral and caudal part.
Rostral: homologous across all species. Caudal: may terminate directly on alpha motor
neurons.
Somatotopic representation.
Lesion M1: hemiplegia: loss voluntary movements contralateral side body.
- Secondary motor areas: M2.
somatotopic maps. Premotor cortex and supplementary motor area (SMA).
involved with planning and control of movement. Premotor cortex: external sensory-guided
actions. SMA: internally guided personal preferences and goals.
Lesion M2: apraxia: loss skilled action, affects motor planning. Cannot link gestures into
meaningful actions.
- ideomotor apraxia: rough sense desired action, problems execution.
- ideational apraxia: knowledge intent of action is disrupted.
- Association Motor areas
damage often leads to ideational apraxia as well. Broca’s area: production speech movements.
Computational issues in Motor control
- Central pattern generators
- stretch reflex
- central pattern generation: offer powerful mechanism for hierarchical control of
movement. (the cat that even walks without any descending commands or external
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feedback signals) Neurons within the spinal cord can generate an entire sequence of
actions without any external feedback signal.
- Central representations of movement plans
- endpoint control reveals fundamental capability of the motor control system, but
distance and trajectory planning demonstrate additional flexibility in the control
processes.
- Hierarchical representation of action sequences
- hierarchical representation structures organize movement elements into integrated
chunks.
- cortex might be considered as an additional piece of neural machinery superimposed
on a more elementary control system.
Physiological analysis of motor pathways
- Neural coding of movement
- monkeys: central-out task. Motor cell recording. Light goes on in 1 part of table,
monkey moves lever to this position to obtain reward.
>> activity cells correlates much better with movement direction > target location.
most motor area cells: preferred direction. (but complex to interpret the correlation)
- population vector = representation based on combining the activity of many
neurons. > gives reliable signal before movement even begins in what direction the
movement will go. Some cells are involved in planning movements as well as
execution.
- Alternative perspectives on neural representation of movement
- the tuning properties change over the course of an action.
- alternative: we should focus on dynamic properties of neurons, movement arises as
the set of neurons move from one state to another. Neurons may code different
features depending on time and context.
Goal selection and action planning
- Action goals and movement plans
- Affordance competition hypothesis: brain’s
functional architecture evolved to mediate real-time
interactions with the world. Affordances:
opportunities for action defined by environment.
Many interactions don’t allow time for carefully
evaluating goals, considering options, and then
planning the movements. (serial processing)
Hypothesis: processes of action selection and
specification occur simultaneously within an
interactive neural network, and evolve continuously.
- Preferred direction of the cells is
represented along bottom left of
figure and time bottom right. Two
cues appear, firing rate increase in
neuron tuned to either target. When
