Motor Pathways: Cortical Motor Function Flashcards
Describe the hierarchical organisation of motor control.
High order areas: involved in more complex tasks (programme + decide on movements, coordinate muscle activity)
Lower level areas: perform lower level tasks (execution of movement)
What are the three parts of the motor cortex?
Primary Motor Cortex
Premotor Cortex
Supplementary Motor Area
What are the two parts of the pyramidal (descending) system?
Corticospinal Tract: starts in the cortex + exits to innervate the muscles in the arms + legs
Corticobulbar Tract: starts in the cortex then exits to innervate muscles in the face
Describe the side loops of the pyramidal system and their role.
2 side loops that go to the cerebellum + basal ganglia
Cerebellum + basal ganglia adjust the commands received from other parts of the motor system
Which lobe are the three parts of the motor cortex found in? Describe their arrangement.
Frontal (anterior to the central sulcus)
Primary motor cortex is on the precentral gyrus
Premotor cortex + Supplementary motor area are anterior to the primary motor area
What is another name given to the three parts of the motor cortex?
Primary Motor Cortex = Brodmann’s Area 4
Premotor + Supplementary Motor Area = Brodmann’s Area 6
What are the most important cells in the primary motor cortex?
Betz Cells (pyramidal cells)
Where are Betz cells located within the grey matter and which tracts originate from here?
5th layer of grey matter
Corticospinal tracts originate from here
Describe what happens to the upper motor neurones that come from the primary motor cortex.
Travel through the brainstem to the pyramidal decussation in the medulla where 90% of the axons decussate.
These axons continue down the spinal cord + synapse with a lower motor neurone + exit into a peripheral nerve to the reach the skeletal muscle.
Pathway of the corticobulbar tract is somewhat similar – upper motor neurones go down into the brainstem + synapse with a lower motor neurone, then exit to muscles of the face
What are the two types of descending pathway?
Lateral
Anterior
Which tracts fall into each of these 2 types of descending pathway?
Lateral: Lateral corticospinal tract + Rubrospinal tract
Anterior: Anterior corticospinal tract, Reticulospinal tract, Vestibulospinal tract + Tectospinal tract
What is the function of each of the 2 main descending motor pathways?
Lateral: Control of proximal + distal musculature, Voluntary movements or arms and legs
Anterior: Control of axial muscles, Balance + posture
Describe the structure and function of the lateral corticospinal tract.
Originates in the primary motor cortex from the Betz cells.
Their axons pass down through the brainstem + decussate at the pyramidal decussation in the medulla.
It then passes down the spinal cord + synapses with a lower motor neurone.
It goes on to control mainly the distal musculature.
Describe the structure and function of the anterior corticospinal tract.
Made up of upper motor neurone axons coming from the primary motor cortex that do not decussate at pyramidal decussation.
These fibres decussate at the level of the spinal cord
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 supplementary motor area?
Planning complex movements
Programming sequencing of movements
Regulates internally driven movements e.g. speech
Becomes active when thinking about movement before executing movement
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.
Loss of function (negative signs):
Paresis = graded weakness of movement
Paralysis = complete loss of muscle activity
Increased abnormal motor activity (positive signs) due to loss of inhibitory descending inputs:
Spasticity (increased muscle tone)
Hyperreflexia (exaggerated reflexes)
Clonus (abnormal oscillatory muscle contraction)
Babinski’s Sign
What is Babinski’s Sign?
Stroke the plantar surface of the foot
Normal subject: flexion of toes (curl downwards)
Upper motor neurone lesions: Toes fan out + big toe lifts up.
Define apraxia.
A disorder of skilled movement
Patients aren’t paretic but have lost information about how
to perform skilled movements
Lesions in which part of the brain tend to cause apraxia?
Inferior parietal lobe Frontal lobe (premotor cortex + supplementary motor area)
What are the two most common causes of apraxia?
Stroke
Dementia
Describe 6 features of lower motor neurone lesions.
Hypotonia
Hyporeflexia
Weakness
Muscle Atrophy: metabolic trophic support to muscles is lost
FASCICULATIONS– damaged motor units produce spontaneous AP’s, resulting in a visible twitch
Fibrillations: spontaneous twitching of individual muscle fibres (seen on needle electromyography)
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
