Mind, Brain & Behavior

1. Mind, Brain & Behavior Friday February 28, 2003

2. Movement Chapters 26 & 28

3. Three Kinds of Movement • Reflex responses (knee-jerk) – rapid, stereotyped, involuntary responses. • Graded in response to eliciting stimulus. • Rhythmic motor patterns (walking, running, chewing) – part reflex, part voluntary. • Only starting and stopping are voluntary. • Voluntary movements – purposeful (goal-directed) and learned (skilled, practiced).

4. Types of Movement • Extension – takes limb away from body (opens penknife) • Flexion – brings limb toward body (closes penknife) • Muscles can only pull not push so any movement requires coordination • Synergists -- muscles that work together • Agonists – prime movers • Antagonists – muscles that pull in opposite direction to agonists, help brake movement.

5. Parts of the Motor System • Motor control operates at three levels, organized hierarchically, operates in parallel. • Cerebral cortex motor areas – plan and control voluntary movement, affect spinal cord neurons directly & through brain stem. • Brain stem – two systems that regulate spinal cord interneurons, one for posture, one for voluntary movement. • Spinal cord – mediates automatic and stereotyped movements.

6. Parts of the Cortex Motor System • Premotor area – plans the activity • Motor cortex(M1) – initiates motor activity • Basal ganglia loop (near thalamus) gives the “go” signal • Cerebellar loop – tells the motor cortex how to carry out the planned activity • Controls movement direction, timing and force by activating populations of motor neurons in learned programs.

7. Planning Movement • Goal directed movement involves many cortical areas that communicate with Area 6 in Frontal lobe. • Area 6 has two parts: • PMA (premotor area) • SMA (supplemental motor area) • Area 6 plans an action and stays active until it is executed (“go” signal).

8. Direction of Movement • Movement is directed using frequency and population codes: • Motor cortex (M1) neurons fire at different rates depending on the desired direction. • Firing rates are averaged across populations of M1 neurons. • When contributing neurons are inhibited, resultant direction changes. • Cerebellum controls the sequence of movements. See Chapter 29 for details

9. Types of Muscles • Smooth – digestive tract, arteries • Striated: • Cardiac– accelerates or slows heart rate • Skeletal– moves bones around joints, moves eyes, facial expression, respiration, speech • Skeletal muscles are the somatic motor system and are under voluntary control.

10. Motor Units • Each muscle fiber is innervated (controlled) by an alpha motor neuron. • Bundles of fibers form large and small motor units. • Small motor units act first, fine motor movement. • Fast contracting, fast fatiguing white fibers form “fast” motor units (slow ones are red). • Alpha neuron firing rate makes a fiber/motor unit fast or slow.

11. Reflexes • Reciprocal inhibition – cannot flex and extend the same muscle • Myotatic (knee-jerk) • Opposes gravity • Uses spindle sensory feedback • Reverse myotatic (knife-clasp) • Relaxes overloaded muscle • Responds to Golgi tendon organ feedback

12. More Reflexes • Flexor reflex – response to pain • Crossed-extensor reflex – compensates for flexor reflex • One side extends as the other flexes • The circuit for coordinated control of walking resides in the spinal cord. • Circuits called “central pattern generators” give rise to rhythmic motor activity.

13. Two Pathways from the Brain • Two corticospinal pathways: • Lateral tract – voluntary movement, crosses • Ventromedial tract (brain stem pathways) – posture, descends without crossing • Lateral pathways control fractionated movement of distal muscles, especially flexors: • Corticospinal – new (higher mammals) • Rubrospinal – from red nucleus, old

14. Ventral (Medial) Pathways • Tectospinal – orients eyes (fovea) on image • Receives input from superior colliculus • Vestibulospinal – maintains stability of head and turns it, balance • Input from labyrinth of inner ear • Reticulospinal – originate in pons and medulla • Pontine – resists gravity and maintains posture • Medullary – liberates muscles from anti-gravity control