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