motor control Flashcards
upper motor system (cerebral)
complex movements
learning
lower motor system (spinal)
reflexes
pattern generation
lower motor neurons
- located in the ventral horn of the spinal cord
- innervate muscles: alpha - extrafusal muscle, generates force (motor unit); gamma - intrafusal fibers in spindles (sensing stretch)
3 types of motor units
- fast fatigable (FF) - large force, but become fatigued after a couple mins; running, jumping
- fast fatigue-resistant (FR) - intermediate force, don’t fatigue as rapidly; jogging and finger movements
- slow (S) - low force, but don’t fatigue due to many mitochondria; posture and other sustained actions like walking
motor units undergo plasticity with training
- muscle fibers increase
- motor neurons increase in size and dendritic complexity, and change excitability
size recruitment principle
motor units are activated so that small force units are activated first (due to their low threshold for activation), if more force is required the FR are activated, and if maximal force is needed, FF units are activated
spinal stretch reflex loop
allows for stabilization of sudden loads, which stretch muscle
unexpected stretch activates spindles, which activate alpha motor neurons causing tension in muscle to counteract stretch
negative feedback from Golgi tendon organs
regulates the force generated by the muscle, preventing excessive force that could cause damage
flexion-crossed extension reflex
strong cutaneous stimuli cause withdraw of ipsilateral leg (hazard avoidance) and extension of contralateral leg (for balance)
reflexes can be overridden by
top-down control
descending commands from the brain via gamma motor neurons and/or spinal interneurons
central pattern generation
neurons in periphery (spinal) that are able to generate patterns for typical rhythmic coordinated movement of torso and extremities
- can be refined and modulated by inputs from sensory systems and the CNS
efference copy
a copy of the muscle command that is used by the brain to anticipate movement and posture without having to wait for feedback from Golgi tendon organs and muscle spindles
feed forward controller
using a model of the body/world, the controller can anticipate future states and act accordingly before sensory info arrives
cerebral cortex involvement in movement
- multiple pre-motor areas that have a variety of complex functions for planning movement
- primary motor cortex (M1) which has a rough topographic map of the contralateral body and initiates movement
M1 neurons typically fire most rigorously for movement in
one direction
mirror neurons
neurons in premotor cortex that respond to the observation of a movement and the execution of the movement
afferent (inputs) projections to cerebellum
- frontal cortex (motor and sensory signals)
- parietal cortex (sensory signals)
- inferior olive (teaching signal for errors)
- vestibular nucleus (balance)
- spinal cord (proprioception)
efferent (output) projections from cerebellum
- projections go to the deep cerebellar nuclei, which project to the thalamus/cerebral cortex involved in motor control, and to brainstem structures involved in motor control
cortico-cerebellar loops
cerebellum forms a loop with premotor, motor, and parietal regions of cerebral cortex - with additional postural info from the brainstem
- helps fine tune motor output and is part of the feed forward model for motor control
structure of cortex of the cerebellum
- purkinje cells have a fan-like dendritic structure that are in parallel with other purkinje neurons
- parallel fibers pass through the fans
- climbing fibers wind around the dendrites -> activate during motor error and drive synaptic plasticity so as to modify future motor programs and avoid object
