Cortical motor function, basal ganglia and cerebellum Flashcards
Describe the hierarchical organisation of motor control
Higher level 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)
What 3 areas make up the motor cortex?
Primary Motor Cortex (M1)
Premotor Cortex
Supplementary Motor Area
Where is the primary motor cortex (M1, Brodmann area 4) located? What is its function?
Precentral gyrus, anterior to the central sulcus
Function: control fine, discrete, precise voluntary movement
Describe the somatotopic organisation and homunculus of the primary motor cortex
Leg, arm more medial; hand, face and tongue more lateral
Penfield’s motor homunculus: more cortical space is devoted to the face, tongue, and hands than other regions, allowing great variety and precision of movements.
The motor cortex gives rise to two lateral pathways for the execution of intentional movements. What are these two motor pathways?
Corticospinal tract and Corticobulbar tract
*The planning of voluntary movements and the elaboration of motor commands for their execution is done by the motor cortex which has its outputs via the lateral motor pathways.
What does the corticospinal tract consist of? Where do these axons project to?
The corticospinal tract consists of the axons of about one million pyramidal cells in layer V of the cortex; over half come from the primary motor cortex (M1, Brodmann area 4) or secondary motor area (MII, Brodmann area 6, premotor cortex and SMA). These axons project to the ventral horns of the spinal cord to alter the activity of alpha and gamma motor neurones.
About 40% of corticospinal tract axons come from the somatosensory cortex (Brodmann areas 1, 2, and 3) or other regions of parietal cortex (Brodmann areas 5 and 7). These axons terminate in the dorsal horns of the spinal cord and regulate sensory input.
What are Betz cells?
Most of the corticospinal tract consists of fine myelinated and unmyelinated axons with conduction velocities between 1 and 25 m/s. However, there are about 30 000 extremely large pyramidal cells in area 4/PMC, called Betz cells, with big myelinated axons that conduct with velocities of 60–120 m/s
Where in the brain do the axons of the corticospinal tract travel down?
Axons of the corticospinal tract pack tightly to pass through the internal capsule which lies between the thalamus and the lentiform nucleus and descend into the brainstem.
How does the corticobulbar tract arise and what do the axons supply?
In the brainstem, the most medial fibres peel off and cross the midline to go to nuclei (trigeminal (V), facial (VII), hypoglossal (XII), and accessory (XI)) of the cranial nerves. These are corticobulbar fibres and are motor to the face, tongue, pharynx, larynx, and sternomastoid and trapezius muscles.
What’s the significance of the pyramidal decussation?
The remaining corticospinal axons descend through the medulla causing a swelling on its ventral surface (the pyramid);
At the caudal medulla 85% of fibres cross the midline as the pyramidal decussation, giving rise to the lateral corticospinal tract. The remaining ipsilateral axons form the anterior corticospinal tract, which crosses over at spinal cord level.
What neurotransmitter do corticospinal tract neurones use? What do they synapse with?
Glutamate (excitatory)
They either synapse directly with a motor neurones supplying distal limb muscles, or synapse with interneurons which make polysynaptic connections with a motor neurones of proximal limb muscles and axial muscles. The corticospinal tract inhibits motor neurons disynaptically via Ia inhibitory interneurons.
Where is the premotor cortex located? What is its function?
Frontal lobe anterior to M1
Function: planning of movements; it is also implicated in movements in response to sensory (mostly visual) cues.
Where is the supplementary motor located? What is its function?
Frontal lobe anterior to M1, medially
Function:
- planning complex movements
- programming sequencing of movements
- regulates internally driven movements (e.g. speech)
- crucial for movements involving both sides of the body, particularly those that have been learnt.
- becomes active when thinking about a movement before executing that movement (shown by increased cerebral blood flow)
Where do SMA and premotor neurones synapse?
The secondary motor cortex contains neurons that fire in a way that correlates with the direction and force of a movement.
- the SMA controls proximal limb muscles directly via its output to the corticospinal tract
- the premotor area neurons synapse with brainstem reticular neurons that go to axial and proximal limb muscles.
- both control distal limb muscles via M1.
What does the association cortex refer to, and what is its purpose?
Brain areas not strictly motor areas as their activity does not correlate with motor output/act;
- Posterior parietal cortex: ensures movements are targeted accurately to objects in external space
- Prefrontal cortex: involved in selection of appropriate movements for a particular course of action
Lesions above/below the pyramidal decussation causes what kinds of deficits?
Corticospinal tract lesions below the pyramidal decussation in primates => ipsilateral motor deficit (below the level of the lesion).
Corticospinal tract lesions above the pyramidal decussation => contralateral motor deficit.
Categories the type of signs that may be seen with an upper motor neurone lesion
Loss of function (negative signs):
- Paresis = graded weakness of movements
- Paralysis (plegia) = complete loss of muscle activity
Increased abnormal motor function (positive signs) due to loss of inhibitory descending inputs:
- Spasticity = increased muscle tone
- Hyper-reflexia = exaggerated reflexes
- Clonus = abnormal oscillatory muscle contraction
- Babinski sign
What’s an apraxia? What is it caused by - what are most common?
A disorder of skilled movement => Patients are not paretic but have lost information about how to perform skilled movements
Caused by lesion of inferior parietal lobe, the frontal lobe (premotor cortex, supplementary motor area); stroke and dementia are the most common causes.
List the the type of signs that may be seen with a lower motor neuron lesion
- Weakness
- Hypotonia (reduced muscle tone)
- Hyporeflexia (reduced reflexes)
- Muscle atrophy
- 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
What is motor neuron disease and what are the associated symptoms?
Progressive neurodegenerative disorder of the motor system (aka Amyotrophic Lateral Sclerosis (ALS))
Spectrum of disorders: UMN signs = Increased muscle tone (spasticity of limbs and tongue) Brisk limbs and jaw reflexes Babinski’s sign Loss of dexterity Dysarthria Dysphagia
LMN signs = Weakness Muscle wasting Tongue fasciculations and wasting Nasal speech Dysphagia
State some overall functions of the basal ganglia?
- Elaborating associated movements (e.g. swinging arms when walking; changing facial expression to match emotions)
- Moderating and coordinating movement (suppressing unwanted movements)
- Performing movements in order
- Motor basal ganglia circuitry is responsible for the execution of appropriate prepro- grammed motor sequences during voluntary movements.
What is the collective term given to the basal ganglia (and cerebellum)?
Classically the basal ganglia constitute part of the extrapyramidal system, on the basis that lesions of the basal ganglia produce quite different symptoms from lesions of the corticospinal tract.
