Organization of the Motor System A. Closed-loop 1. triggered directly by sensory input 2. reflexive

Organization of the Motor System B. Open-loop 1. triggered by a sensory cue or voluntary desire 2. volitional, originating in cerebral cortex

Organization of the Motor System C. Level 1 1. initiation, planning, programming 2. response to a desire to move 3. basal ganglia and cortical projection areas SMA, PMC 4. prefrontal cortex

Organization of the Motor System C. Level 1 5. damage to basal ganglia and cortical projection sites a. does not produce weakness b. can result in abnormal involuntary movements chorea: irregular, rapid, uncontrolled, involuntary, excessive movement that seems to move randomly from one part of the body to another. dystonia: sustained muscle contractions cause twisting and repetitive movements or abnormal postures. ballismus: jerky or shaking movements of the arms or legs, especially such movements occurring in chorea. c. Parkinson’s disease

Organization of the Motor System D. Level 2 1. cerebellum 2. coordination of movements 3. is a comparator 4. stores a lot of muscle memory 5. damage - so motor pathways can “remember” how to perform actions does not cause weakness loss of coordination

Organization of the Motor System D. Level 3 1. origin of descending motor pathways a. corticospinals/pyramidal tracts b. extrapyramidal tracts - originate from subcortical structures 2. damage - still receive an input from the primary motor cortex (MsI) - weakness - increased tone - hyperreflexia

Organization of the Motor System D. Level 4 1. spinal cord interneurons - some mediate spinal cord reflexes 2. central pattern generators - spinal interneurons capable of generating their own inputs to motor neurons (independent of any input)

Organization of the Motor System D. Level 5 1. lower motor neurons - is the output to the skeletal muscle 2. receives input from the muscle spindle and Golgi tendon organs 3. center for simple stretch reflexes of muscles 4. damage - weakness - wasting - hypotonia - weak or areflexic

Ia afferent nerves (from spindle)

Bag 1: rate change Bag 2: absolute length

Lateral = distal (typically) Ventral = axial & proximal Ventral anterior horn = extensor muscles Dorsal anterior horn = flexor muscles Propriospinals connect the motor neuron pools Propriospinals also connect the CPGs Humans cannot typically locomote in the absence of significant supraspinal inputs (no fictive locomotion)

Remember: almost all motor tracts are crossed.