Motor Pathways: Cortical Motor Function Flashcards
Describe the hierarchical organisation of motor control.
Association Cortex
Motor Cortex
Brainstem
Spinal Cord
What are postural adjustments and unconscious processing?
Postural Adjustments – the motor system has to compensate for changes in the body’s centre of mass during movements
Unconscious Processing – many of the postural adjustments occur without our awareness
What are the three parts of the motor cortex?
Primary Motor Cortex
Premotor Cortex
Supplementary Motor Area
What makes up the association cortex?
Frontal Cortex
Parietal Cortex
NOTE: this is not exactly part of the motor pathway but it influences the planning and execution of movements
What are the two parts of the pyramidal (descending) system?
Corticospinal Tract – starts in the cortex and exits and innervates the muscles in the arms and legs
Corticobulbar Tract – starts in the cortex then exits and innervates the muscles in the face
Describe the side loops of this descending pathway and their role.
The descending pathway also has two side loops that go to the cerebellum and basal ganglia
The cerebellum and basal ganglia check the motor information before ittravels to the muscles and has its effect
What are the most important cells in the primary motor cortex?
Betz Cells (pyramidal cells)
Where are these cells located within the grey matter and which tracts originate from here?
They are found in the 5th layer of grey matter
The corticospinal tracts originate from here
Describe what happens to the upper motor neurones that come from the primary motor cortex.
They travel through the brainstem to the pyramidal decussation in themedulla where 90% of the axons cross the midline.
These axons continue down the spinal cord and synapse with a lower motor neurone and exit into a peripheral nerve to the reach the skeletal muscle.
The pathway of the corticobulbar tract is somewhat similar – upper motor neurones go down into the brainstem and synapse with a lower motor neurone and they exit to the muscles of the face
What are the two examples of descending pathway?
Lateral and Medial (anterior)
What is the function of each of the lateral and anterior corticospinal pathways?
Lateral Control of proximal and distal musculature Voluntary movements or arms and legs Medial/anterior Control of axial muscles Balance and posture
Describe the structure and function of the lateral corticospinal tract.
The lateral corticospinal tract originates in the primary motor cortex from the Betz cells.
Their axons pass down through the brainstem and decussate at the pyramidal decussation in the medulla.
It then passes down the spinal cord and synapses with a lower motor neurone.
It goes onto control mainly the distal musculature.
NOTE: 90% of axons from the primary motor cortex decussate at the medulla (these are the lateral corticospinal tract axons). The 10% that don’t decussate form the anterior corticospinal tract.
Where does the rubrospinal tract originate?
Red nucleus of the midbrain
What is the function of the rubrospinal tract?
It is an alternative pathway that allows voluntary motor commands to be sent down the spinal cord meaning that the body can compensate for a lesion in the primary motor cortex.
It also has a role in movement velocity.
Describe the structure and function of the vestibulospinal tract.
The lateral vestibulospinal tract originates at the lateral vestibular nucleus.
The medial vestibulospinal tract originates at the medial vestibular nucleus.
They mediate postural adjustments and head and eye movements
Describe the structure and function of the reticulospinal tract.
It originates in the reticular formation in the brainstem then goes down the spinal cord to innervate muscle. It is involved in complex actions: Orienting Stretching Maintaining a complex posture
Describe the structure and function of the tectospinal tract.
It originates in the superior colliculus (brainstem)
Its function is not known but is most likely involved in reflexive turning of the head to orient to visual stimuli.
Describe the structure and function of the anterior corticospinal tract.
The anterior corticospinal tract is made up of the upper motor neurone axons coming from the primary motor cortex that do not decussate at the pyramidal decussation.
These fibres cross the midline at the level of the spinal cord It controls proximal musculature.
How can the cortical representation of a muscle in the motor cortex change?
The more we use a muscle, the bigger the representation of that muscle in the cortex.
What is the function of the premotor cortex?
Plans movements and assembles movements into coordinated actions
NOTE: premotor cortex is anterior to the primary motor cortex
What is the function of the supplementary motor area?
Planning complex internally driven voluntary movements e.g. speech
It also becomes active when you are thinking about movement before movement (e.g. rehearsing a dance)
What are the two parts of the association cortex that are involved in motor control? State their functions.
Posterior Parietal Cortex – ensures movements are targeted accurately to objects in external space
Prefrontal Cortex – involved in the selection of appropriate movements for a particular course of action
Describe the features of upper motor neurone lesions.
Initially you get loss of function of the motor neurones leading to:
Paresis = graded weakness of movement
Paralysis = complete loss of muscle activity
After a few weeks, the loss of descending inhibitory pathways leads to increased abnormal motor activity such as:
Spasticity (increased muscle tone)
Hyperreflexia (exaggerated reflexes)
Clonus (abnormal oscillatory muscle contraction)
Babinski’s Sign
NO muscle atrophy
What is Babinski’s Sign?
You stroke the plantar surface of the foot and in a normal subject you will see flexion of the toes (they curl downwards)
In the case of upper motor neurone lesions, the patient will show an EXTENSOR PLANTAR RESPONSE where their toes fan out and their big toe lifts up.
Why is muscle atrophy not seen in upper motor neurone lesions?
The lower motor neurones are still in tact and they have a role inproviding nutrients to the muscle.
There will still be partial atrophy due to muscle disuse.
Define apraxia.
A disorder of skilled movement not caused by weakness, abnormal tone or posture or movement disorders (tremors or chorea).
It is caused by the loss of information on how to perform skilled tasks rather than loss of motor command to the muscles.
Lesions in which part of the brain tend to cause apraxia?
Inferior parietal lobe Frontal lobe (premotor cortex and supplementary motor area)
What are the two most common causes of apraxia?
Stroke and Dementia
Describe the features of lower motor neurone lesions.
It is generally the opposite of upper motor neurone lesions.
Hypotonia
Hyporeflexia
Weakness
Muscle Atrophy– the metabolic trophic support to the muscles is lost
FASCICULATIONS– damages motor units produce spontaneous action potentials, resulting in a visible twitch
Fibrillations – twitch of individual muscle fibres (aren’t visible to the naked eye but are picked up on EMG)
What is motor neurone disease?
A progressive neurodegenerative disorder of the motor system – it is a spectrum of disorders.
MND can affect upper motor neurones, lower motor neurones or both
What is the term given for upper AND lower motor neurone disease?
Amyotrophic Lateral Sclerosis (ALS)
Describe how the symptoms of ALS change as the disease progresses.
Some patients may present with only upper motor lesion symptoms or only lower motor lesion symptoms but as the disorder progresses, both upper and lower motor neurone signs will be coexistent.
List some signs of ALS.
Increased muscle tone (spasticity in the limbs and tongue)
Brisk limb and jaw reflexes (hyperreflexia)
BABINSKI’s SIGN
Loss of dexterity
Dyarthria – difficulty speaking
Dysphagia – difficulty swallowing
Which lower motor neurone controls the tongue?
Hypoglossal Nerve (CN XII)
What might you see in the tongue of an MND patient?
Fasciculations and spasticity
Function of primary motor cortex
Controls fine, discrete and precise voluntary movement
Provides descending signals to execute movement
Role of anterior corticospinal tract
Goes to skeletal muscle of trunk and proximal limbs
Role and route of corticobulbar tract
Role is connection of motor cortex to medullary pyramids
Where does basal ganglia adjustment go through
Always the thalamus
Where does cerebellum adjustment information go through
The thalamus or can go directly on to brainstem and then muscles or spinal chord
Location of primary motor cortex
Lies anterior to central sulcus so in posterior part of frontal lobe
Role of primary motor cortex
Control all voluntary movement by providing signals for execution
Somatotopic organisation to motor cortex ( penfield motor homunculus)
Lower limbs most medial (in sulcus)-> upper limbs-> face then lips
Lateral corticospinal motor tract pathway
Begins in motor cortex and one long cell Betts cell passes through basal ganglia to medulla where it decussates to descend in lateral corticospinal tract. When reaches appropriate spinal level enters ventral horn and synapses with lower alpha motor neurone
What do descending motor pathways go past in basal ganglia
Internal capsule
What do descending motor pathways go through in midbrain
Cerebral peduncle
What do descending motor pathways go through in pons
Transverse fibres
Anterior corticospinal tract pathway
Pass from motor cortex all the way down the anterior corticospinal tract ipsilaterally to appropriate spinal level where they decussate
Muscles supplied by motor lateral corticospinal tract
85% of muscles
Those of distal limbs
Musc,es supplied by anterior corticospinal tract
15%
Skeletal muscles of trunk
Corticolbulbar pathway
Passes from appropriate region in motor cortex through internal capsule to a nucleus(depending on cranial nerve) in medulla where leaves and goes to muscle
Premotor cortex location
Frontal lobe anterior to primary motor cortex
