L9 - Motor Control Flashcards
How is the motor system arranged?
Hierarchically
What is the order of the hierarchy of the motor system?
Highest - primary motor cortex
Middle - brainstem
Lowest - spinal cord
What does the primary motor cortex control?
Projects directly to spinal cord via corticospinal tract
Regulates motor tracts that originate in brainstem
What does the brainstem control?
Lateral descending system that controls distal limbs
Important for goal-directed movements of hand and arm
What does the spinal cord control?
Contains neuronal circuits that mediate reflexes automatisms – e.g. walking
Simplest reflex - monosynaptic-sensory neurone and motor neurone
Most reflexes are polysynaptic with interneurons
Where does the basal ganglia and cerebellum receive information from?
Many areas of the cortex
Where does the basal ganglia and cerebellum project to?
Motor cortex via the thalamus
Where does the basal ganglia feed back to?
Only feeds back to motor cortex - subcortical loop
Where does the cerebellum feed back to?
Mainly feeds back to motor cortex
In an emergency it can by-pass the motor cortex and send signals down the brainstem, spinal cord and onto muscles
What is the important of the basal ganglia and cerebellum being aware of the situation the person is in?
Monitor commands going down to the muscles to ensure they are appropriate for situation
If commands are not appropriate they calculate correction signals
Sent back to the motor cortex for approval before the signal is sent to the muscles
How was the motor cortex discovered?
1870 - electrical stimulus to areas of frontal lobe produced movements on opposite side of body
Mid 20th Century - electrical stimulation used to identify specific motor effects of discrete sites in frontal lobe in different species
What else was produced to aid in the discovery of the motor cortex?
Motor Maps produced and correlated with anatomical and clinical observations on effects of lesions
What is Brodmanns area 4?
Area in which lowest intensity stimulation elicited movement - Primary Motor Cortex
Located just before the central sulcus
Sometimes called precentral gyrus to highlight its position
Motor cortex map
Showed orderly arrangement along the gyrus of control area for leg, trunk, hand
Lesions to specific area of motor cortex lead to?
Degeneration in associated spinal cord area
What was the issue of the motor cortex map for the fingers, hand and face?
Fingers, hand and face had disproportionately large representation for fine movement
Large areas of brain required to control these areas
What causes simple, voluntary movements?
Electrical stimulation of motor cortex does not produce complex or learned movements
- Twitches in individual muscles
- Speech is only simple vowels
What are motor cortex first neurons?
Called upper motor neurones
Carry motor commands down through the brain, brainstem and to the spinal cord
What is the output to upper motor neurons?
Output to lower motor neurons via interneurones
What is the function of upper motor neurons?
Involved in planning, initiating and directing movements
What are some example of other upper motor neurons?
Originate in phylogenetically ancient motor centres of brainstem
Red and vestibular nuclei
Superior colliculus
Reticular formation
What do upper motor neurones regulate?
Muscle tone
Postural muscle
Balance
Head orientation
What are the two upper motor neuron pathways?
Direct and indirect
Both pathways govern production of action potentials in lower motor neuron - final common pathway
- Leads to muscle contraction and movement
What is the indirect motor pathway?
Input to lower motor neurones from motor centres in brainstem
What is the direct motor pathway?
Input to lower motor neurones from axons extending directly from cerebral cortex
What does the basal ganglia contain?
Caudate
Putamen
Substantia nigra
Subthalamic nuclei
What is the function of the basal ganglia?
Provide input to upper motor neurons and connects with motor cortex
Helps initiate and terminate movement
Suppresses unwanted movements
Establishes normal level of tone
What is Tourette’s syndrome?
Damage to basal ganglia so people do not have this level of control
What is the function of the cerebellum?
Control activity of upper motor neurons and connect to cortex via thalamus and to brainstem
Monitors movements for differences in intended and actual movements
- If any discrepancies, sends an error signal and tries to reduce discrepancy
What does proprioception allow?
Allows you to know the orientation of limbs without visual input
Muscle spindles tell the brain about how long the muscle is to work out orientation
Muscle spindle fibres are intrafusal – inside capsule
What do muscle spindles contain?
Bag fibres – nuclei gather in one place
Chain fibres – nuclei all in a straight line
What do Bag and Chain fibred contain?
Sensory fibres – group Ia
Motor fibres – group II
Group Ia fibres
Anulospiral endings
Wrap around equator of bag and nuclear chain fibres
Pulling apart of these coils initiates action potentials in axon
Group II fibres
Endings have a flower spray terminal ending embedded in equatorial region of bag fibres
Anulospiral endings wrap around equator of nuclear chain fibres
What do group Ia fibres relay?
Sensory endings relay information on the dynamic phase of muscle stretch
E.g. as stretch is occurring
What do group II fibres relay?
Sensory endings relay information on the static phase of muscle stretch
E.g. its final length
When are muscle spindles useful?
When under tension
What happens when extrafusal muscles contracts and shorten around the muscle spindle?
Muscle spindles unload and loses tension as intrafusal fibres aren’t doing anything
If no tension - no stretch information going to brain
To solve the problem of a lack of tension of muscle spindles what happens?
As extrafusal muscles contract gamma-motoneurones are co-activated with alpha-motoneurones
- Contract intrafusal fibres
- Spindle shortens in register - Allows brain to continue to get information from group II and group Ia
What is the effect of noradrenaline of gamma motor neuron activity?
Decreases activity
By inhibiting the γ-motoneurones - intrafusal fibres can be made more elastic
Elastic materials don’t transmit stretches well
What is the effect of 5HT of gamma motor neuron activity?
Increases activity
By stimulating the γ-motoneurone - intrafusal fibres can be made slightly stiffer
Stiffer materials transmit stretches with greater fidelity
What are the 3 synaptic relays in the proprioceptive pathway?
- Central processes of DRG cells synapse on neurones in gracile and cuneate nuclei in lower medulla
- Axons from these nuclei synapse on neurones in ventral posterior lateral nucleus of thalamus
- Axons sent to primary somatosensory cortex
Where are somatosensory cortices found?
Primary and secondary somatosensory cortices are in the anterior parietal lobe and the posterior parietal cortex
Where is SII found?
Deep within lateral sulcus
Lesioning S1 does?
Produced proprioceptive deficits
What is the structure of S1?
Divided into four distinct regions cytoarchitectonic regions
