Motor Control Flashcards
What are the observable signs of decreased quality of movement?
Paralysis, Weakness
Stiffness
Increased reactivity
Ticks, Twitches and Jerks
Fibrillations and fasciculations
Loss of coordination and smoothness
Loss of the effectiveness of movements
What is a “sign”, in relation to disorders of motor control?
Observable or measurable motor abnormalities, the presence of abnormalities, or the absence of normal motor actions.
What is a symptom, in relation to disorders of motor control?
Problems of motor function reported by patients Syndromes: associated or clustered signs and symptoms
Where are motor neuron cell bodies located?
Brainstem (cranial motor neurons) and spinal cord
Why are motor neurons able to produce 1:1 transmission?
Highly myelinated axons
Large synapse spread across muscle fibres
Ruffled post-synaptic membrane for increased SA
Where do motor neurons sit in the spinal cord?
Ventral horn
What are motor columns?
The topographically organised motor neurons in the spinal cord that belong to one muscle.
They tend to be organised into a pool in the ventral spinal cord, spanning multiple segments.
What is a motor unit?
All muscle fibres innervated by a single alpha motor neuron.
What is Henneman’s size principle?
The smallest motor neurons are recruited first, then gradually larger motor units are recruited with the largest being recruited last.
This ensures low force contractions are initially made and large force contractions made last, allowing fine movements to take place.
What types of muscle fibres are recruited first?
Slow fatiguing, because they’re active for all movements, whereas larger motor units are recruited only in large force movements, and therefore are fast fatiguing.
What is myasthenia gravis?
Autoimmune disease whereby the action of the NAChR is blocked.
ACh therefore can’t get to the receptor, blocking contractions.
What happens to muscles if they are un-innervated?
Fibrillations, fasciculations and eventually atrophy and degeneration.
What are fibrillations?
Tiny contraction cause by activity of a single muscle cell - can be due to hypersensitivity, increased AChR expression (including extra-junctional).
What are fasciculations?
Groups of muscle fibres contracting involuntarily, probably a motor unit synaptically activating due to spontaneous activation of a degenerating MN/axon.
Can be measured on surface (cf. fibrillations - need extracellular electrode).
What will happen to a muscle if it is denervated for a long time?
Atrophy and degeneration.
Atrophy can occur in catabolic states (diabetes, Cushing’s disease) but complete denervation will lead progressively to irreversible muscle loss.
When is the only time muscles are naturally inactive?
During REM sleep
What causes involuntary muscle activation?
Lower motor neurons connected to muscle spindles and Golgi tendon organs.
What does the muscle spindle do?
Sensory axons sit within the intrafusal muscle fibres and send signals back to the spinal cord to report on length of the muscle.
What is the Golgi tendon organ?
The Golgi tendon organ is embedded within the collagen matrix of the tendon with nerves that respond to how much force is experienced in the muscle.
What are the signs and symptoms of low motor neuron lesions?
Weakness or paralysis
Decreased superficial reflexes
Hypoactive deep reflexes
Decreased tone
Fasciculations and fibrillations
Severe muscle atrophy
Where can the monosynaptic reflex pathway be tested?
Anywhere there’s an exposed tendon.
This allows elucidation of where the lesion is in the spinal cord.
What is an upper motor neuron?
Any neuron that controls the excitability of lower motor neurons.
What is most of the input to the lower motor neurons via?
Local interneurons
What are the 2 main pathways by which axons get from the brain to the spinal cord?
Ventromedial and lateral pathways.
Which tracts sit in the ventromedial pathways?
Tectospinal
Vestibulospinal
Pontine and medullary reticulospinal.
What types of motor neurons do ventromedial tracts tend to control?
More medial motor neurons
What types of motor neurons do lateral tracts tend to control?
More lateral neurons
What tracts sit in the lateral pathways?
Corticospinal and rubrospinal tracts
Which tract does the reticular formation give rise to?
Reticulospinal tract in the ventromedial pathway.
What is the pathway for axons in the lateral corticospinal tract?
Axons originate from cerebral cortex, travel down through the internal capsule, through the brainstem (some make connections in pons and medulla), decussates at the medullary pyramids and travels down in the lateral white matter.
What type of movements is the lateral corticospinal tract most for?
Voluntary movements
What 3 tracts originating in the brainstem give rise to the ventromedial pathway?
Vestibulospinal, reticulospinal and colliculospinal.
What is the main function of the vestibulospinal tract?
Postural control with inputs from the vestibular apparatus at the vestibular nuclei.
These nuclei have a large role in postural maintenance.
What is the main function of the reticulospinal tract?
Maintaining muscles of the midline, the axial muscles.
These affect postural control.
What is the main function of the colliculospinal tract?
The superior colliculus is a structure related to our orientation in space and receives input from auditory and visual systems.
It subserves orienting reflexes – if a sound is hear or a flash of light seen, movement towards the source is done by the superior colliculus.
It’s also important in visual looming – if something is getting bigger, you’re falling towards it.
What are most medial motor tracts for?
Posture
What are most lateral tracts for?
Voluntary, skilled movements
True or false: when the brain can no longer influence the spinal cord, the spinal locomotor circuits can still be activated.
False.
In humans, the spinal locomotor circuits cannot be activated, even by activation of the muscle sensory inputs which can initiate locomotion in spinal quadrupeds.
What happens to the excitability of lower motor neurons when upper motor neuron modulation is lost?
They become more excitable, because upper motor neurons normally inhibit.
What are the signs and symptoms of upper motor neuron lesions?
Weakness
Spasticity - increased tone, hyperactive deep reflexes, clonus
Babinski’s sign
Loss of fine voluntary movements.
Why do gait and reflex control systems reveal more than the immediate causes of upper motor neuron syndromes?
Because these circuits are inexorably enmeshed in a motor control system that comprises significant proportion of the brain, spinal cord and the periphery, these most simple of movements interrogate the entire nervous system.
What are the postural signs of decerebrate rigidity?
Extension of the upper and lower limbs. Extensor muscles dominate.
What are the postural signs of decorticate rigidity?
Upper limbs flex, lower limbs extend.
Where are the motor control parts of the reticular formation?
In the pons and medulla
Where is the red nucleus?
In the midbrain.
What does the red nucleus control?
More lateral spinal neurons
What does the reticular formation excite?
Extensors
What does the red nucleus excite?
Flexors
What form of modulation do the reticular formation and red nucleus receive from the cortex?
Inhibitory inputs
What happens if cortical inhibition to the red nucleus is released?
The red nucleus will excite the flexors
What is the rubrospinal tract?
The short tract between the red nucleus to motor neurons at cervical levels of the spinal cord.
What must be released for the rubrospinal tract to excite flexors?
Cortical inhibition
What is decorticate rigidity?
Removal of cortical input to all levels of the brainstem (red nucleus and reticular nucleus), resulting in a free running reticular nucleus and red nucleus, leading to dominance of extensors, but with flexion as well in the arms because flexors dominate over extenors.
What is decerebrate rigidity?
The level of disruption is below of the midbrain, meaning only the red nucleus has been compromised. Therefore, extensors dominate.
What is rostrocaudal demise?
If there’s progression from the decorticate state to the decerebrate, it means the lesion is affecting more and more of the brainstem.
As it progresses towards the medulla, death is imminent as it will inevitably affect the CV and respiratory centres.
What are the only cranial nerves of the face and tongue that receive contralateral innervation?
Hypoglossal and lower facial nerve.
How will an UMN lesion present in the face?
Loss of innervation to a quadrant of the face, i.e. lower left.
How wll a LMN lesion present in the face?
Loss innervation to half of face - weakness of superior and inferior facial muscle.
Where is the motor cortex?
Immediately anterior/rostral to the central sulcus
What effect will an occlusion of the middle cerebral artery have on movement?
The lower limb and trunk will be affected resulting in, for example, circumduction.
Why are parietal cortical areas important for motor tasks?
Because they synthesise the model of the space the body is in, i.e. where everything is in relation to you.
How do the “where” and “what” pathways of the primary visual cortex feed into the motor cortex?
The where stream controls reaching towards an object and the what stream controls grasping of the object itself.
Why are cerebellar lesions ipsilateral deficits, not contralateral?
Axons decussate twice after leaving the cerebellum - once at the VL nucleus of the thalamus or red nucleus and then again at the pyramidal decussation.
