The motor system Flashcards
What are the types of motor control?
Voluntary - running , talking, playing music
Goal-directed - conscious, explicit, controlled
Habit - unconscious, implicit, automatic
Involuntary - eye movements, facial expressions, cardiac, diaphragm
What is the difference in the role of the upper motor neurons and the lower motor neurons?
Upper - originate in higher centres and project down to meet lower motor neurons
Lower - begins in brainstem or spinal cord and projects to muscles
What do the basal ganglia and cerebellum play a part in the sensorimotor system?
WHAT to do - basal ganglia (inhibitory)
HOW to do it - cerebellum (excitatory)
Kinda outside the system
Descending control system
With lots of ascending feedback along the way
How is muscle force controlled?
Not about how much a muscle fibre is activated - all or nothing - contracted or relaxed
Its about how many muscle fibres are activated
And what type of muscle fibres are activated
Antagonistic arrangement
Size of motor units
What does antagonistic arrangement mean?
Allows for much better precision of positioning of different body parts e.g. eye, arm
How does muscle fibre number differ across different people?
Doesn’t necessarily have more fibres when muscles are bigger
Just means fibres diameter are wider
Genetically determined, changed slightly by training
What is the build up of a skeletal muscle?
Muscle fasciculus - several muscle fibres
Muscle fibre - several myofibrils
Myofibrils - protein filaments actin and myosin
What does a motor unit consist of?
single alpha motor neuron and all the muscle fibres it innervates
Different motor neurons - different numbers of motor fibres
When do motor units have fewer fibres?
Greater movement resolution - fine motor control e.g. those innervating lips and tongue
What dictates the average number of muscle fibres innervated by a single motor neuron?
functional requirements of the muscle
- Level of control
- strength
What is the size principle
How are the intrafusal fibres innervated?
Gamma neurons
Detects muscle stretch
Important for running on uneven surfaces - adjust when limbs aren’t where you expect them to be
Withdrawal reflex?
One leg tenses in preparation for taking the weight of the other
Powerpoint 2
Why is the sensorimotor system so complex?
There can be simple reflex circuits
But a complex array ascending sensory feedback affecting the descending control system
What sort of movements are controlled by the brainstem?
Balance
Posture control
Orienting
Gross loimb movement
Affecting occulomotor control
Roles in speech
Respiration
How can simple movements be controlled by motor cortex?
Pimary cortex exerts quite direct, top down control over muscular activity
With as few as one synapse (in the spine) between a cortical neurone and innervation of muscle cells.
How do projections descend from the motor cortex?
Originate in layers 5-6 grey matter
Pyramidal cells here are upper motor neurones
Axons project directly OR indirectly to spinal cord, where they synapse with lower motor neurons
There descending axons form the pyramidal tract
Most cortical projections innervate contralateral motor units
How do basal ganglia and cerebellum affect cortical motor control?
Cerebellum - excitatory
Basal ganglia - inhibitory
What do the dorsolateral tracts do?
Control distal limb muscles (contralateral)
Cross over in the pyramids
What do the ventromedial tracts do?
Control trunk and proximal limb muscles
Cross over at synapse with lower motor neuron
Compare the paths that the dorsolateral and ventromedial tracts take?
What path does the dorsolateral corticospinal tract take?
Ventro
How do pathways in the basal ganglia work?
Substantia nigra can inhibit the inhibitory effect of basal ganglia
Excitatory input received through Glutamate
What are the basal ganglia?
Group of nuclei lying deep within cerebral hemispheres
Role in motor control not fully understood
Affects the thalamus and so affects the cortex
Mainly inhibitory
Where are 50% CNS neurons?
Cerebellum
Where does the cerebellum receive input from?
Cortex -PONS- motor cortex mostly(copies of muscle commoands), also somatosensory and visual areas of parietal cortex
Spinal cord - info about limb position and movement
Vestibular system - rotational and acceleratory head movement
What is the gross function of the cerebellum?
Computes motor error and adjusts cortical moto commands accordingly
Provides precise control, fine adjustment and coordination of muscle activity based on continual sensory feedback
