Motor system basics

Motor 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 De-cerebrate walking cat

Postural control Vestibular and Reticular nuclei (medial motor system). Adaptation and Anticipation – Sailor “sea legs”

Voluntary control Cortico-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)

Somatotopic organization of M1

Motor system hierarchy

Motor system hierarchy Performing actions is complicated… Incorporate: Visual information Auditory information Somatosensory information Make a decision Make a motor plan (timing, forces, balance, etc…) Execute

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

Areas F5 and AIP/PF Canonical neurons – object specific actions Regardless of where objects are located Murata (1997, 2000)

Microstimulations Story 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 specialization Damage 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 So far we have talked about physiology. What is the purpose of the motor system?

Encoding space Head centered? Hand centered? Eye centered?

You need to translate external space to internal space… 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 Invariance to effector… Similar brain solution to very different movements

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 Cell body Axon חומר אפור ולבן, תאים וקישורים בין תאים - מיילין. Axon

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

Anatomy – Cortical thickness

Anatomy – Cortical folding

Anatomy – white matter Tractography Fiber volume Fiber length

Brain function Neurovascular coupling

Vasculature

Changes in oxygenated blood זמן Heeger et. al. 2002

fMRI experiment

In fMRI we always compare measures over time Experimental results In fMRI we always compare measures over time

Converging evidence Good science is achieved by generating hypotheses that can be tested with different types of experiments in order to generate converging evidence.