The Motor System Flashcards
What does motor control involve
Motor control involves a dynamically changing mix of conscious and unconscious regulation of muscle force, informed by continuous and complex sensory feedback, operating in a framework sculpted by evolutionary pressures
Examples of voluntary motor control
Running
Walking
Talking
Examples of goal directed motor control
Conscious
Controlled
Examples of involuntary motor control
Eye movement
Facial expressions
Postural muscles
How is motor control governed
By upper and lower neurons
Where are Lower motor neuron cell bodies found
In the brain stem or spinal cord
Where do lower motor neurons project to
Muscles
Where do upper motor neurons originate
In higher centres in the cerebral cortex
Where do upper motor neurons project to
They project to the lower motor neurons
Describe a basic descending control system
Association cortex
Motor cortex
Brainstem circuits
Spinal circuits
Motor unit
Effect
What is control of muscle force depend on
Individual muscle fibres act in an all or one manner and so the control of muscle force depends on the way in which lower motor neurons activate different types of muscle fibre
What are the 3 types of muscle
Skeletal
Smooth
Cardiac
What percentage of muscle makes up or body weight
40%
What is an example of antagonistic arrangement of muscles
Bicep/tricep/eye
What is an antagonistic arrangement of muscles
Combined co-ordinated actions
What is recruitment of muscle fibres
Fast/slow twitch
What is a fasciculi
A group of muscle fibres
What is a muscle fasciculus
Several muscle fibres which make up a muscle cell
What makes a muscle fibre
Several myofibrils
What do myofibrils contain
Actin and myosin myofilaments
What is a motor unit
A single alpha motor neuron + all the muscle fibres it innervates
If a motor neuron innervates a fewer number of fibres causes what
A greater movement resolution eg-finger tips
What does activation of the alpha motor neuron cause
The alpha motor neuron depolarises and causes contraction of all the muscle fibres in that unit going along with the all or none theory
How do we increase the power of the muscle
More motor units are activated
The number of muscle fibres innervated by a single motor neuron depends on
The functional requirements of that muscle
- level of control
- strength
What are lower motor neurons
They originate in the grey matter of the spinal. Cord or in the brain stem
What is includes in the unit of control of muscle force
The lower motor neuron and the muscle fibres it connects with
What do all lower motor neurons innervate
They all innervate a single muscle
What does the motor pool contain
Alpha and gamma motor neurons
How are motor pools arranged
Motor pools are arranged in a rod like shape within the ventral horn of the spinal cord
How are cell bodies in the ventral horn activated
They are activated by sensory information from the muscle and descending information from the brain
How much tension is on the muscle is sensed by what
Golgi tendon organs sense tension
What senses stretch of muscle fibres
Muscle spindles
Where is the Golgi tendon organs found
They are found within the tendon where the muscle joins to the bone
What is the function of the Golgi tendon organ
It sends ascending sensory information to the brain via the spinal cord about how much force there is in the muscle
What is the Golgi tendon organ critical for
Pro-perception
When a muscle is under extreme tension what does the Golgi tendon organ do
During extreme tension it is possible that the Golgi tendon organ can act to inhibit muscel fibres via the circuit in the spinal cord to prevent damage
What are reflexes
Reflexes can be quite simple or quite complex
They can operate without engaging with the brain and are critical for the avoidance of injury and effective motor control
What is the most simple reflex
Monosynaptic
What is an example of a monosynaptic reflex
The patellar tendon reflex
What are intrafusal fibres innervated by
They are innervated separately by gamma motor neurons
What is needed for an efficient motor control system
An efficient motor control system needs to know how much each muscle is stretching which is provided by the muscle spindles
Where are muscle sensory receptors found
In the muscle spindles
What are coiled around the intrafusal fibres
Sensory fibres
What does reciprocal innervation explain
Reciprocal innervation of antagonistic muscles explains why the contraction of one muscle induces the relaxation of the other
What does reciprocal innervation allow
Execution of smooth movements
Where do motor commands originate
In the motor cortex pyramidal cells in layers 5-6 of the grey matter
What type of motor neurons are found in the motor pyramidal cells
Upper motor neurons
Where do axons from the pyramidal cells project to
Directly or indirectly via the brainstem to the spinal cord
In the spinal cord upper motor neurons synapse with what
Lower motor neurons
What do axons of the upper motor neurons make
The pyramidal tracts
What is the motor homunculus
The homunculus is a reasonable representation of what areas of the motor cortex control what in the body
What do both dorsolateral tracts and ventromedial tracts both contain
They both contain a direct corticospinal route
They both contain an indirect route via brainstem nuclei
What is the brain stem nuclei that the dorsolateral tract travel by
The red nucleus
What are the brainstem nuclei that the ventromedial tracts travel through
They travel through the tectum, vestibular nuclei, reticular formation and cranial nuclei
What do the dorsolateral tracts innervate
They innervate the contralateral side of one segment of the spinal cord
What does the ventromedial tracts innervate
The ventromedial tracts diffuse there innervation which projects to both sides and multiple segments of the spinal cord
Where do the dorsolateral tract projections go to
They sometimes project directly to alpha motor neurons
Project to the distal muscles eg— fingers
Where do the projections of the ventromedial tracts go to
Proximal muscles muscles of trunk and limbs
What is the basal ganglia
A group of structures beneath the cortex that act as a gate keeper for control of the motor system
What is the cerebellum
The cerebellum is a large brain structure that acts as a parallel processor enabling smooth coordinated movements and important in a range of cognitive tasks
What does the cerebellum do in relation to motor neurons
The cerebellum has no direct projections to lower motor neurons but does modulate activity of upper motor neurons
What inputs does the cerebellum get from the cortical tract
Mostly from the motor cortex which are copies of the motor commands and somatosensory and visual areas of parietal cortex
What inputs does the cerebellum get from to spinal tract
Proprioceptive information about limb position and movement
What inputs do the cerebellum get from the vestibular tract
Rotational and acceleratory head movements
What are the function of the cerebellum
It knows what the current motor command is
It knows about actual body positions
It projects back to the motor cortex
Computes motor error and adjusts cortical commands accordingly
