Motor system basics

Development

Motor neurons & Muscle fibers

Local motor control

Muscle spindles enforce the “stretch reflex”: feedback about muscle length.

Golgi tendon provide feedback about level of force.

Spinal chord

Central pattern generators

Taking care of the basic simple functions...

Central pattern generators

Central pattern generators

Decerebrate walking cat

Central pattern generators

Decerebrate walking cat

Cortical control

Postural control

Vestibular and Reticular nuclei (medial motor system).

Adaptation and Anticipation – Sailor “sea legs”

Voluntary controlCortico-spinal and Rubro-spinal pathways (lateral motor system).

Fine motor control.~ 1 Million fibers originating in:• Primary motor cortex (one third)• Premotor cortex (one third)• Somatosensory cortex (one third)

Motor system hierarchy

Motor system hierarchyPerforming actions is complicated…

Incorporate:Visual informationAuditory informationSomatosensory information

Make a decision

Make a motor plan (timing, forces, balance, etc…)

Execute

Somatotopic organization of M1

Encoding of M1 neurons: Force

Encoding of M1 neurons: Direction

Textbook stories describe M1 neurons as responsible for the final motor execution step…

Higher motor levels

Visual – movement performed according to cue.

Internal – movement performed as part of a memorized sequence.

Anterior parietal & Premotor cortex

Visuomotor coordination

Object manipulation

Grasping

Canonical neurons – object specific actionsRegardless of where objects are located

Areas F5 and AIP/PF

Murata (1997, 2000)

MicrostimulationsStory is a bit more complex.Long microstimulations in premotor, anterior parietal, and primary motor cortex generate complicated multi-effector movements.

Like grasp to eat…

Idea of motor primitives

Supplementary motor cortex

Neurons that respond to a specific movement only when it is part of a sequence (a) or to any movement, but only according to its location in the sequence (b).

Functional specializationDamage in M1 creates weakness in the relevant part of the body.

Damage in parietal and premotor cortex creates problems with movement planning and coordination.

Damage in SMA creates problems with learning new movement sequences

Motor control

What is the purpose of the motor system?

Encoding space

Head centered?Hand centered?Eye centered?

Inverse model

You need to translate external space to internal space…

Inverse model

Eventhough you use different joints and muscles for different movements:

Movements remain straight, smooth, with symmetric velocity profile.

Your motor system cares about making smooth efficient movements in external space…

(Morasso 1981)

Motor primitives

Invariance to scale…

Motor primitives

Invariance to effector…Raibert 1977

Inverse model is flexible…

(Brashers-Krug, Shadmehr, and Bizzi 1996)

Inverse model is flexible…

Motor memory

Where does motor learning happen?

How do we study neurophysiology?

Dendrites

Axon

Cell body

Neural activity

In vitro electrophysiology

Control > ASD

In vivo electrophysiology

Control > ASD

In vivo electrophysiology

Control > ASD

Anesthetized or awake

Electrode location

Control > ASD

Magnetic resonance imaging (MRI)

Anatomy - Separating tissues

1T 2T

Anatomy – Cortical thickness

Anatomy – Cortical folding

Anatomy – white matter

TractographyFiber volumeFiber length

Brain function

Neurovascular coupling

Vasculature

Changes in oxygenated blood

Heeger et. al. 2002

זמן

fMRI experiment

Experimental results

In fMRI we always compare measures over time