Motor Systems Flashcards
Which regions of the cortex are involved in motor control?
The frontal lobe
How does the role of the cortex change as you get more anterior?
More complex/abstract its role in movement is
Where is the primary motor cortex?
Area 4
Immediately anterior to central sulcus
What do lesions to the primary motor cortex result in?
Paralysis
Paresis of specific muscle groups
May be some recovery of function (cortical plasticity) but larger lesions = more muscle groups involved = recovery less likely
What areas do strokes affect?
Always involve multiple cortical areas
What would a stroke occluding the middle cerebral artery affect/result in?
Almost all of one side of frontal lobe
Severe motor disability in contralateral body except lower limb as supplied by anterior cerebral artery
What would infarction of proximal segment of middle cerebral artery affect?
Blood supply to basal ganglia via lenticulostriate arteries
Blood supply to motor cortex
More disabling than stroke affecting distal segment (M3)
What are areas 6&8?
Premotor cortex and supplementary motor cortex
What would damage to areas 6 & 8 result in?
motor apraxia
- normal reflexes
- no muscle weakness
- difficulty performing complex motor tasks
- damage to 1 side (stroke) may produce minimal symptoms as contralateral area can take over some functions
What are areas 7 & 19?
Posterior parietal cortex
What would damage to areas 7 & 19 result in?
Sensory apraxia
- difficulty performing complex motor tasks when triggered by sensory input
- not strictly a motor deficit but difficulty linking sensory input to motor system
What is the role of the frontal eye fields?
Motor control of extraocular eye muscles
What is the role of the Broca’s area?
Motor control of muscles regulating speech
What does damage to Broca’s area lead to?
motor aphasia
difficulty generating speech motor outputs
difficulty linking word strings into complex sentences
What is oculomotor apraxia?
Difficulty moving eyes horizontally and quickly
may have to turn head to compensate for lack of eye movement
- can be caused by bilateral lesions of frontal eye fields which controls complex voluntary eye movements
What is the role of the corticobulbospinal tract?
Axons send commands down to spinal cord
- modulate sensory input
- modulate spinal reflexes
- 40% arise from anterior parietal lobe (somatosensory cortex) and so parietal lobe involved in motor control
What are areas 9 & 10?
Dorsolateral Prefrontal cortex Planning of movement Complex relationship with movement Evaluate different possible future actions and decide which is best Problem solving judgement executive functions
What would damage to areas 9 & 10 result in?
Apathy
Personality changes
Lack of ability to plan/sequence actions or tasks
Poor working memory
What would be the clinical difference between the left hemisphere of areas 9 & 10 being damaged and the right?
Left - poor working memory for verbal info.
Right - poor working memory for spatial info.
How are areas 9 & 10 commonly damaged?
With impact to the frontal bone - in a road traffic accident or blow to the head (contusions)
What is a contusion?
Brain bruising
What is area 11?
Orbitofrontal cortex
What is the function of area 11?
Concerned with control/inhibition of motor responses associated with the limbic system
- responses to hunger, thirst, sexual drives
What does damage to area 11 lead to?
(orbital damage)
- disinhibition of responses to hunger/thirst/sexual drives = pseudopsychopathic behaviour
- impulsiveness, jocular attitude, sexual disinhibition, complete lack of concern for others
- orbital personality
What is the motor hierarchy in the frontal lobe?
- Area 11/ 9 & 10
- Areas 6 & 8
- Area 4
/frontal eye fields/broca’s/primary motor cortex - Corticobulbospinal tract/1,2,3
- Motor neurons in spinal cord
How does the basal ganglia connect to lower motor neurons?
The motor thalamus (VL and VA thalamic nuclei) is only route for motor commands to travel from basal ganglia and cerebellum to CSP tract and LMN
Where is the corticobulbospinal tract particularly vulnerable to damage by stroke?
When it courses through the internal capsule on its way to the brainstem
Where does the corticobulbar component of the corticobulbospinal tract terminate?
- on CN V (trigeminal) and VII (facial) for cortical control of muscles of head
- on oculomotor nuclei : III, IV & VI for eye movement control
- cells of pontine nuclei
- reticular formation
- red nucleus (in midbrain)
Where does the corticospinal component of the corticobulbospinal tract travel?
Continues on to lower medulla -> decussates to opposite side -> forms large lateral corticospinal tract and small medial corticospinal tract
Where does motor decussation occur?
In upper spinal cord
C1-C5
What happens if the brain is injured above the spinal cord/medulla junction?
Motor deficit is on the opposite side
- if injury is in the spinal cord = motor deficit on the same side
Where does the lateral corticospinal tract run?
In dorsolateral cord
Near motor neurons which supply the distal muscles
Where does the anterior corticospinal tract run?
In the medial ventral cord
Only in cervical cord
Controls voluntary movements of the neck
What does the corticospinal tract have connections with?
Monosynaptic connections with motor neurons of thumb and digits only
- other muscles motor actions mediated by CST acting on spinal interneurons
What does damage to the corticospinal tract in the spinal cord cause?
Loss of control of hand and fingers
Posture/locomotion and gait not lost
What is the extrapyramidal system?
Other descending tracts mediating motor functions of posture, locomotion and gait
- originate from groups of cell bodies in the brainstem
What are the main components of the extrapyramidal system?
- lateral vestibulospinal tract
- reticulospinal tracts
What is the function of the lateral vestibulospinal tract?
Controls posture and balance
- tonically active during upright posture
What is the pathway of the lateral vestibulospinal tract?
- originates in the vestibular nuclei in upper medulla/lower pons
- projects ipsilaterally to antigravity muscles
What is the function of the reticulospinal tract?
- general arousal of spinal cord
- autonomical control driving sympathetic preganglionic neurons
- drive to respiration via phrenic nerve
What is the pathway of the reticulospinal tract?
Arises in the reticular formation of the pons and medulla
- projects bilaterally down spinal cord
What is the function of the rubrospinal cord?
Carries cerebellar commands to the spinal cord
What is the pathway of the rubrospinal cord?
Orign in the red nucleus of the brainstem
Receives main input from the cerebellum
What is different about the rubrospinal tract in males?
It is vestigial - atrophied, remnant left
What is the red nucelus?
- large nucleus in midbrain
- gives rise to rubrospinal tract and large ascending projection to motor thalamus
What are the minor extrapyramidal pathways?
- tectospinal tract
- medial vestibulospinal tract
What is the function of the tectospinal tract?
Coordinates voluntary head and eye movements
- activates reflex movements of the head in response to visual and auditory stimuli
What is the pathway of the tectospinal tract?
Originates in the superior colliculus
- projects to the contralateral cervical spinal cord
- terminates in rexed laminae VI, VII, VIII
What is the function of the superior colliculus?
- also called the optic tectum
- receives afferents from the retina
What is the function of the medial vestibulospinal tract?
Continuation of the DCML
- mediates reflex co-ordination of the head and neck muscles with the extraocular eye muscles to maintain objects in view despite body movements
Which upper motor neurons act directly on lower motor neurons in the cord?
Neurons driving the muscles of the thumb and fingers (and lips and tongue)
- major descending motor tracts act on interneurons in cord to modulate strength and activity of reflex pathways instead
What is spasticity?
Abnormally increased muscle tone
- muscles have increased tendon reflexes
- characteristsic of UMN lesion
What is an example of an UMN lesion?
Damage to motor cortex or any descending tract
What is clonus?
Series of jerky contractions of a particular muscle following sudden stretching of it
What is hyper-reflexia?
Abnormally/pathologically brisk tendon reflex seen in one or more muscles
What are some severe signs of motor system damage?
Decorticate and decerebate posturing
What are the features of decorticate posturing?
Arms are adducted and flexed
Wrists and fingers flexed on the chest
Legs may be internally rotated and stiffly extended
Feet may be plantar flexed
What does decorticate posture indicate?
Damage to the corticospinal tract in the midbrain
- more favourable than decerebrate but still very severe
- seen in unconscious patients
- may progress to decerebrate or may alternate
- can occur on one or both sides of the body
What are the features of decerebrate posturing?
Arms adducted and extended Wrists pronated Fingers flexed Legs may be internally rotated and stiffly extended Feet may be plantar flexed
What would decerebate posturing indicate?
Severe injury to the brain at the level of the brainstem
- damage to corticospinal and rubrospinal tracts
- due to excessive activity (disinhibition) in the extrapyramidal system
- mainly due to vestibulospinal tract
- only in unconscious patients
How can vestibulospinal tract disinhibition cause decerebrate posture?
Normally vestibulospinal tract is under tonic inhibition by corticobulbospinal tract and red nucleus
- red nucleus damage by severe midbrain injury = decerebrate posturing
What does a discrete acute lesion lead to?
Initial paralysis
Followed by variable degree of recovery
During recovering - weakness, clumsiness, fatigue of movements
What is plasticity in the cortex?
Allows recovery
After lesion muscles may be driven by cells from a different part of the cortex as the homunculus has changed
What do larger lesions lead to?
Slower recovery
Permanent loss of certain movement
Increased weakness, clumsiness and fatigue
What do small lesions lead to?
Good recovery of motor skills
Motor weakness and quick fatigue always present
What is hemiplegic dystonia?
Persisting spasticity following motor cortex lesion
Combined with motor weakness
Persistent flexion of arms and extension of legs
What is the clasp knife reflex characteristic of?
Chronic cerebral motor lesions
What is spinal shock?
Condition which occurs after an acute damage to the cord including descending tract damage
What are the acute effects of spinal shock?
Paralysis/paresis
Reduced reflex responses in all muscles below injury region
If severe - all reflexes below lesion level are inactive
What are the chronic effects of spinal shock?
Eventually wears off Weak monosynaptic reflexes appear Crossed extensor reflexes may recover Severe - reflexes not controlled by brain so become exaggerated and hyperactive (hyper-reflexia) Clonus Babinski sign present
What is the difference between an UMN lesion and a LMN lesion?
UMN lesion:
- in CNS
- motor cortex, corticospinal tract or other motor tracts involved
- groups of muscles affected
- paralysis of voluntary movements
- increased muscle tone
- hyperactive reflexes
- positive Babinski
- atrophy
- spastic paralysis
LMN lesion:
- CNS or PNS
- involves spinal cord, brainstem, alpha motor neurons, peripheral motor axons
- affects muscles innervated by damaged motorneuron/axon
- paralysis of voluntary movements
- decreased muscle tone
- decreased/absent reflexes
- decreased/absent Babinski
- atrophy of muscles
- flaccid paralysis
In spinal injury which tracts are responsible for which symptoms?
Injury to:
- corticospinal: paralysis/weakness of voluntary movement, hyperactive tendon reflexes
- reticulospinal: loss of bladder/bowel control, poor gait, loss of temp. regulation, loss of blood pressure regulation
Vestibulospinal: loss of balance, poor gait
