Slide 1

What’s the motor system?

	Parts of  CNS and PNS specialized for control of limb, trunk, and eye movements
	Also holds us together
	From simple reflexes (knee jerk) to voluntary movements (96mph fast ball)
	Remarkable: Muscles only contract

Plan

	Components of the motor systems
	Basic principle of movement control
		What is helpful for understanding basic motor system organization
	Motor programs
	Descending motor pathways

Motor systems

Slide 5

Slide 6

Slide 7

Organization of Movements

	Diverse & adaptive
	Hierarchical: 3 major types
		Reflexes
		Postural adjustments
		Voluntary movements

Organization of Movements

	Hierarchical: 3 major types
		Reflexes Spinal cord circuits
		Postural adjustments Spinal & Brain stem
		Voluntary movements Spinal cord, Brain stem, and cortex

Reflexes

	Stimulus-evoked involuntary muscle contraction
	Monosynaptic (+) reflex
		Knee-jerk
		Jaw-jerk

Knee Jerk

Reflexes

	Stimulus-evoked involuntary motor muscle contraction
	Monosynaptic (+) reflex
		Knee-jerk
		Jaw-jerk
	Disynaptic reflex (+)
		withdrawal

Why Disynaptic?

	Greater control (gate)
		Very simple context
	More complex response

Spinal Circuits

Spinal Circuits

Motor I/O

Postural adjustments

	Context important
		Maintain balance--supported v/s unsupported
	Feedback control-reactive
		Error correction
		Response lags stimulus; sometimes too late
	Feed-forward control-predictive
		Response anticipates stimulus
		More timely, but depends on internal models
		Practice, learning

Voluntary movements

	Organized around purposeful acts
	Flexible input-output relationships
		Limitless
		Price to pay: whole brain

Motor I/O

Voluntary movements

	Organized around purposeful acts
	Flexible input-output relationships
		Limitless
		Price to pay: whole brain

The goal of voluntary movements is represented… somewhere

	Motor equivalence
		Individual motor actions share important characteristics even when performed in different ways
	Abstract representation; effector independent
		Hand writing
		Soccer
	Goal representation

Voluntary movements are
organized by motor programs

	Translate goal into action
		Formation of a movement representation, or motor program
	Program
		To produce the desired goal, which muscles should contract and when
	2 Key movement characteristics
		Spatial (hand path; joint angles) Kinematic plan
		Forces/loads Dynamic plan
	All accomplished by contracting muscles

Kinematic & Dynamic Plans

	Reach to target
		Straight hand path
	Reach up
		Against gravity
		More force to achieve goal
	Reach down
		Gravity assists
		Less force to achieve goal

Summary

	Motor behavior hierarchy
		Reflexes
		Postural adjustments
		Voluntary movements
	Internal representations
		Reflexes and postural adjustments--simple; invariant
		Voluntary movements--complex; flexible
	Voluntary movements
		Kinematic and dynamic representations
		Goal representation

Motor systems

Motor Pathways Hierarchy

Spinal Motor Columns

Ventral Horn
Organization:
Proximal - distal rule

Brain Stem Motor Paths

Brain Stem Pathways

	Lateral
		Rubrospinal tract: distal limb control; crude
	Medial
		Tectospinal tract: eye-head coordination
		Reticulospinal tract: automatic postural adjustments and movements (hip; shoulder)
		Vestibulospinal tract: balance (axial muscles); automatic postural adjustments

Slide 31

Cortical
Motor
Paths

Slide 33

Slide 34

Slide 35

Why bother study the motor pathways?

	Anatomical substrates: How it works
	Multiple parallel paths & diversity of spinal connections
		Damage to 1° motor cortex and pre-motor cortex projections recover some lost functions
		Damage to cortex and brain stem paths recover some lost functions
		Loss of direct connections and alternate paths can recover some lost functions