Primary Motor Cortex- Motor Pathways Flashcards
What are the broad principles of motor control in terms of segregation and hierarchy?
Motor control is functionally segregated: organised in a number of different areas that control different aspects of movement.
Motor control is also organised in a hierarchy:
higher order areas of hierarchy are involved in more complex tasks (programme and decide on movements, coordinate muscle activity)
lower level areas of hierarchy perform lower level tasks (execution of movement).
HIGHER LEVELS= DECIDE, COORDINATE
LOWER LEVELS= EXECUTIVE
Outline the 4 levels of hierarchical organisation, giving examples of each.
Level 4 (highest) – association cortex.
Level 3 – motor cortex.
Level 2 – brain stem and cerebellum (side loop structure (SLS)).
Level 1 (lowest) – basal ganglia (SLS (e.g. caudate nucleus)) and spinal cord.
Where is the PMC located and what is it’s function?
Location: The PMC (M1) is located in the frontal lobe on the pre-central gyrus just anterior to the central sulcus (CS divides front and parietal lobes).
Function: control fine, discrete, precise voluntary movement.
Provide descending signals to execute movement
Betz cells are the largest neurones in the CNS, providing large descending tracts. They are found in the fifth layer of the cerebral cortex grey matter.
Outline the motor system hierarchy:
Motor cortes receives sensory input from cortical areas of the brain, then sends commands to the thalamus and the brainstem.
PMC to thalamus:
PMC communicates with the thalamus via the basal ganglia, which provides more sensory input to the PMC.
PMC to brainstem:
PMC communicates directly with the brainstem which passes commands to the spinal cord.
PMC to cerebellum:
cerebellum is modulating/refining output from M1.
Brainstem and spinal cord= executive branch of hierarchy.
Spinal cord innervates muscles of the body via M1 and brainstem innervation.
Brainstem innervates muscles of face, head and neck.
Explain the homunculus:
Penfield’s motor homunculus is somatatopic maping of motor function.
On M1 it is inverted. Lower limb is supplied by anterior portion of M1.
It is not proprtional, lips/hands are more prominent as they have more motor control due to fine motor movements.
Referring to the homunculus, stroke in the anterior cerebral artery affects what part of the body?
Lower limbs: homunculus is inverted, anterior porttion supplies lower limbs.
Describe the descending motor pathways from the M1 via the lateral and anterior corticospinal tracts:
Corticospinal tract provides innervation to arms, legs and axial structures (trunk and abdomen).
Divided into two separate pathways;
o Lateral corticospinal pathway – distal muscles of limbs.
Right hemisphere cortex –> internal capsule–>midbrain–>medulla oblongata and decussation of the pyramids (crossing over in medulla)
–>ventral horn of the spinal cord and synapse with lower motor neurone–>innervation of distal muscles. (contralateral)
o Anterior corticospinal pathway – proximal limbs and trunk muscles.
Similar pathway but crosses at spinal level needed (ipsilateral)
Describe the descending motor pathways for the corticobulbar pathways:
Motor pathways for corticobulbar tract follow a similar pathway to corticospinal tract.
The upper motor neurone passes through the internal capsule then synapses in the brainstem after crossing over into the lower motor neurone.
Pre-motor cortex: location and function with an example.
Supplementary motor area: location and function with an example.
PreMC
Location: frontal lobe anterior to M1
Function: planning of movements, regulates externally cued movements.
Eg; reaching for an apple requires movement relative to other body parts (intra-personal) and movement relative to the environemnt (extra-personal)
SuppMC
- Location: frontal lobe anterior to M1, medially
- Function: planning complex movements; programming sequencing of movements
- Regulates internally driven movements (e.g. speech)
- SMA becomes active when thinking about a movement before executing that movement. Eg; regulates movement of larynx to allow speech
What is the association cortex, giving examples of its functions:
- Association cortex is areas of the brain not strictly motor areas- activity does not directly correlate with motor input/output.
- Posterior parietal cortex: ensures movements are targeted accurately to objects in external space (apraxia= inability to carry normal planned movements)
- Prefrontal cortex: involved in selection of appropriate movements for a particular course of action. Uses an input of previous of experiences to coordinate movement eg. avoid touching a hot plate after burning hand.
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Define: lower motor neurone, upper motor neruone, pyramidal, extrapyramidal.
•Lower motor neuron
Spinal cord, brainstem
•Upper motor neuron
Corticospinal, corticobulbar
•Pyramidal
Lateral corticospinal tract
•Extrapyramidal
Basal ganglia, cerebellum
What are the positive and negative signs for upper motor neurone lesions?
Upper motor neurone: corticospinal and corticobulbar
Positive signs:
Increased abnormal motor function due to loss of inhibitory descending inputs;
- spasticity- increased muscle tone
- hyper-reflexia= exaggerated reflexes
- clonus- abnormal oscillatory muscle contraction
Negative signs:
Loss of function;
Paresis: graded weakness of movements
Paralysis (plegia): complete loss of muscle activity
What is apraxia?
- A disorder of skilled movement. Patients are not paretic but have lost information about how to perform skilled movements (loss of ability to put actions together)
- Lesion of inferior parietal lobe, the frontal lobe (premotor cortex, supplementary motor area)
- Any disease of these areas can cause apraxia, although stroke and dementia are the most common causes
Fasciculations vs fibrillations?
Fasciculations are damaged motor units that produce sponatneous APs resulting in a visible twitch
Fibrillations: spontaneous twitching of individual muscle fibres- these are recorded during muscle EMG exam.
What are the sympyoms of lower motor neurone lesions?
- Weakness
- Hypotonia (reduced muscle tone)
- Hyporeflexia (reduced reflexes)
- Muscle atrophy- muscle loses innervation and strats to shrink
- Fasciculations: damaged motor units produce spontaneous action potentials, resulting in a visible twitch
- Fibrillations: spontaneous twitching of individual muscle fibres; recorded during needle electromyography examination
- Tongue fasciculations (visible twitch) and wasting
- Nasal speech
- Dysphagia
