Movements and actions
Chapter 11
Movements vs actions
•Movements are brief unitary activities of muscle•Reflexes•Postural adjustments•Sensory orientation
•Actions are complex, goal-oriented sets of movements•Walking•Gestures•Acquired skills (speech, tool use, etc.)
There is a complex relationship between movements and actions
Old idea was that actions are collections of reflexes chained together
Counterexamples abound:
Spooner’s ‘Queer old dean’
Control systems theory and movement
1. Closed-loop movements: Information flows from whatever is being controlled back to the device that controls it
2. Open-loop movements: Ballistic movements where once movement is initiated, there is no opportunity for feedback – accuracy is controlled through anticipation of error.
Overview of neural control of movement
Movements are constrained by the skeletal system
Muscles control the actions of the skeletal system
-antagonists
-synergists
The composition of muscles
The neuromuscular junction
Innervation ratios and neural integration
Motor unit=a single motor axon and all muscle fibres it innervates
Innervation ratio = # motorneurons/#muscle fibres
High innervation ratio means each motorneuron innervates only a few muscle fibres (oculomotor or finger muscle)
Low innervation ratio means each motorneuron innervates many muscle fibres (leg)
The importance of sensory feedback
Proprioception=information about body movements and positions
“Pride and a Daily Marathon”-patient with viral infection that attacked parts of dorsal roots that gave kinesthetic information-had to completely relearn to move in a new way that depended on visual feedback (turn off lights – falls down)
Receptors for movement
Muscle spindles signal muscle length
Golgi tendon organs signal muscle tension
John Hughlings-Jackson and the motor system as the Royal Navy
The royal navy is organized as a hierarchy, but each level has some autonomy
-if the admiral dies, all the ships on the sea don’t suddenly stop
The motor system is organized as a hierarchy, but each level in the hierarchy has some autonomy
-if the hierarchy is beheaded, we don’t stop all movement
Spinal reflexes mediate automatic responses
-withdrawal, stretch, scratch are movements organized at the spinal level
-animals with spinal cord transections can support weight and even generate some of the patterned muscular contractions required for walking
Brainstem organization for movement: The extrapyramidal
system
Decorticate animals can still move:
Walking, feeding, grooming, sexual behaviour are all, in some ways, intact
Circuitry involved is mostly in the reticular formation of the brainstem
Overview of neural control of movement
Cortical organization for movement
-Frau Hitzig’s dressing table
-the Jacksonian march of spasm
Orderly representation in motor cortex
Outputs from cortex: The pyramidal system
Beyond MI: Supplementary and premotor cortex
ApraxiaIdeomotor apraxia – inability to carry out a simple motor activity in response to a verbal command
Ideational apraxia – inability to carry out a sequence of actions that are components of a behavioural script
-anatomy is very complex (most strokes cause some degree of apraxia
-may involve disconnection of motor cortical areas from the rest of cortex
Basal ganglia and movement
The basal ganglia are best thought of as a massive feedback loop
-receive huge input from cortex, process this input and then send output to motor cortex
-thought to control amplitude and direction of movments-especially important in producing remembered movements-most known because of involvement in Parkinson’s disease
Cerebellar contributions to movement
1. The cerebellum is a modular structure2. One part of the cerebellum is involved in
posture and balance (ataxia)3. Another part of the cerebellum is involved
in producing precise timing in neural “programs” for the control of skilled movement
Overview of neural control of movement