Motor Unit
comprises:
- -> motor neuron - -> all of the individual skeletal muscle fibers it innervates
- differ in number of muscle fibers
- -> units with fewer fibers have a higher degree of selective motor control (ex. fingers, face)
Muscle contraction
Process
- Motor neuron fires
- ACh is released at neuromuscular junctions
- ACh activates motor-end-plate on each muscle fiber
- Contraction of the fibers
Fast muscle fiber
contracts + relaxes quickly
--> fatigue quickly because poorly vascularized (few blood vessels - pale)
Slow muscle fiber
slower + weaker
--> contraction is more sustained because richly vascularized
Flexor
acts to bend or flex a joint
Extensor
act to straighten or extend a joint
Synergistic Muscles
two muscles whose contraction produces the same movement
Antagonistic Muscles
two muscles whose contraction produces opposite movements
ex.: biceps, triceps
Isometric Contraction
Contraction of a muscle without movement
–> no change in the length of the muscle
ex.: plank, wall sit (no movement)
Dynamic Contraction
Contraction of a muscle resulting in movement
–> change in the length of the muscle
ex.: squats, tricep dips (movement)
Principles of the sensorimotor system
- hierarchally organized
- -> the higher the levels of the hierarchy the more complex functions they perform
- functionally segregated
- -> each level is composed of different units each of which perform different functions
- Information flows down ( Somatosensory S - up )
Structure of the Sensorimotor System
- Association Cortex
- Posterior parietal association cortex
- Dorsolateral prefrontal association cortex
- Secondary motor cortex
- Primary motor cortex
- Brain stem motor nuclei
- Spinal motor circuits
Information transmission in Sensorimotor System
- Posterior parietal AC supplies Dorsolateral prefrontal AC + Secondary MC with information
- Dorsolateral prefrontal AC + Secondary MC issue commands based on that information
- Commands are forwarded to the primary motor cortex
- Commands are now distributed over four main channels to the spinal sensorimotor circuits
- Spinal sensorimotor circuits direct the activities of the muscles
Role of the Posterior parietal association cortex
integrating the information of
1. the original positions of the parts of the body that are to be moved 2. the positions of any external objects the body is going/might interact with
Where does the Posterior parietal AC receive its information from ?
- visual cortex
- auditory cortex
- somatosensory
Where does the Posterior parietal AC send its information to ?
Dorsolateral prefrontal cortex and secondary motor cortex
Appraxia
disorder of voluntary movement
–> damage to Posterior parietal AC
Contralateral Neglect
Disturbance of a patients ability to respond to stimuli on the side opposite to the side of a brain lesion
–> damage to Posterior parietal AC
Role of the Dorsolateral prefrontal association cortex
Decision to initiate voluntary movement
Role of the Secondary motor cortex
involved in the programming of specific patterns of movement after taking general instructions from the dorsolateral prefrontal AC
--> its neurons become active prior to the initiation of the movement and continue the activity throughout the movement
Mirror Neurons
- fire when and individual performs a particular goal directed movement
- or when an individual observes the same goal directed movement performed by others
Role of the primary motor cortex
- Initiating body movements
- moving body parts independently (ex.: fingers, toes)
Major point of
- -> convergence of cortical sensorimotor signals - -> departure of sensorimotor signals from cerebral cortex
Somatotopic organization
organization according to the map of the body
ex.: stimulation of the face elicits face movements
Astereognosia
Inability to identify objects by touch
--> damage to primary motor cortex
