Cortical Motor Function, Basal Ganglia and Cerebellum Flashcards
What are areas of higher and lower hierarchy responsible for in motor control?
Higher: more complex tasks (programme and decide on movements, coordinate muscle activity)
Lower: lower level tasks -> execution of movement
Primary motor cortex
- Location: precentral gyrus, anterior to the central sulcus
- Function: control fine, discrete, precise voluntary movement
- Provide descending signals to execute movement
- Somatotopic organisation: Penfield’s motor homunculus
What is the broad function of the different brain parts in motor control?
- Motor cortex (primary and non-primary) receives info from other cortical areas of the brain and then sends commands to the thalamus and brainstem
- The cerebellum and basal ganglia adjust the commands received from other parts of the motor control system. (fine-tuning)
- The brainstem passes the commands from the cortex to the spinal cord.
- > see diagram
Name 3 descending motor pathways
- anterior corticospinal tract
- lateral cotricolspinal tract
- corticobulbar pathway
Lateral corticospinal tract
- upper motor neurone originates at the motor cortex, passes through internal capsule
- Travels down through midbrain, pons
- crosses over to the other side at the medulla (decussation)
- passes down the lateral corticospinal tract in the spinal cord
- once appropriate spinal cord level is reached it synapses in the ventral horn.
- lower motor neurone goes to skeletal muscle in the distal parts of the limbs
=> biggest motor pathway
Anterior corticospinal tract
- Upper motor neurone originates at the motor cortex, passes through internal capsule
- travels down midbrain, pons and medulla to the spinal cord
- travels down the spinal cord in the anterior corticospinal tract
- Once it reaches the appropriate spinal cord level, it crosses over to the other side and synapses in the contralateral ventral horn.
- Lower motor neurone travels to skeletal muscle in the trunk and proximal parts of the limbs.
Corticobulbar pathways
- originates from the cortex
- travels through internal capsule to the medulla where it synapses
- influences the brainstem and innervates several cranial nerves
- e.g. tongue, eyes innervation
Betz cells
- large pyramidal cells
- largest neurones in the CNS
- e.g. upper motor neurone in corticospinal tract, travels very far
Premotor cortex
- Location: frontal lobe anterior to M1
- Function: planning of movements
- Regulates externally cued movements
e. g. seeing an apple and reaching out for it requires moving a body part relative to another body part (intra-personal space) and movement of the body in the environment (extra-personal space)
Supplementary Motor Area
- 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
Association Cortex
- Brain areas not strictly motor areas as their activity does not correlate with motor output/act
- Posterior parietal cortex
- Prefrontal cortex
Posterior parietal cortex function
- ensures movements are targeted accurately to objects in external space
- motor learning, motor planning e.g. dressing yourself
- problem: Apraxia, inability to carry out normal planned movements
Prefrontal cortex function
involved in selection of appropriate movements for a particular course of action
- personality input to movement, based on previous experience, avoids harm
Lower motor neurone
- motor neurones in ventral horn of the spinal cord
- bulbar neurones in the brainstem
Upper motor neurones
- corticospinal: Betz cells and their projections through spinal cord
- corticobulbar: projections from cortex to brainstem (also Betz cells?)
Pyramidal
- lateral corticospinal tract
Extra-pyramidal
- basal ganglia, cerebellum
Upper motor neurone lesions
- 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’s sign
Babinski’s sign
Babinski sign occurs when stimulation of lateral plantar aspect of the foot leads to extension (dorsiflexion or upward movement) of the big toe (hallux). Also, there may be fanning of the other toes.
Apraxia
- A disorder of skilled movement. Patients are not paretic but have lost information about how to perform skilled movements
- Lesion of inferior parietal lobe, the frontal lobe (premotor cortex, supplementary motor area)
- any disease of these areas can cause apraxia
- dementia and stroke are the most common causes
Causes of apraxia
- Lesion of inferior parietal lobe, the frontal lobe (premotor cortex, supplementary motor area)
- Any disease of these areas can cause apraxia
- stroke and dementia are the most common causes
Lower motor neurone lesion
- Weakness
- Hypotonia (reduced muscle tone)
- Hyporeflexia (reduced reflexes)
- Muscle atrophy
- Fasciculations
- Fibrillations
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
Motor Neurone Disease
- Progressive neurodegenerative disorder of the motor system
- Spectrum of disorders
- e.g. Amyotrophic Lateral Sclerosis (ALS)
ALS
Amyotrophic Lateral Sclerosis
What are some upper motor neurone signs?
- Increased muscle tone (spasticity of limbs and tongue)
- Brisk limbs and jaw reflexes
- Babinski’s sign
- Loss of dexterity
- Dysarthria
- Dysphagia
What are some lower motor neurone signs?
- Weakness
- Muscle wasting
- Tongue fasciculations and wasting
- Nasal speech
- Dysphagia
What are the basal ganglia made up of?
- Caudate nucleus
- Lentiform nucleus (putamen + external globus pallidus)
- Subthalamic nucleus
- Substantia nigra
- Ventral pallidum, claustrum, nucleus accumbens, nucleus basalis of Meynert
What are the 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
Be able to locate the basal ganglia on images
x
Neuropathology of PD
Classically the primary pathology involves the neurodegeneration of the dopaminergic neurons that originate in the substantia nigra and project to the striatum
Main motor signs of PD
- Bradykinesia: slowness of (small) movements (doing up buttons, handling a knife)
- Hypomimic face: expressionless, mask-like (absence of movements that normally animate the face)
- Akinesia: difficulty in the initiation of movements because cannot initiate movements internally
- Rigidity : muscle tone increase, causing resistance to externally imposed joint movements
- Tremor at rest: 4-7 Hz, starts in one hand (“pill-rolling tremor”); with time spreads to other parts of the body
Akinesia
difficulty in the initiation of movements
Huntington’s disease
- Genetic neurodegenerative disorder
- Chromosome 4, autosomal dominant (Huntington gene)
- CAG repeat
- Degeneration of GABAergic neurons in the striatum, caudate and then putamen
- progressive disorder
- with generations the onset is earlier and the disease is more severe (term for this?
How many CAG repeats are needed to establish HD?
35
Motor signs of HD
- Choreic movements (Chorea): rapid jerky involuntary movements of the body; hands and face affected first; then legs and rest of body
- Speech impairment
- Difficulty swallowing
- Unsteady gait
- Later stages, cognitive decline and dementia
Cerebellum
- sits in the posterior cranial fossa
- 3 layer structure
- finer sulci and gyri
What is the fold of dura above the cerebellum called?
Tentorium cerebelli
How many layers do the cerebral hemispheres have?
6
What are the layers of the cerebellum?
- molecular layer
- piriform layer
- granular layer
Circuitry in the cerebellum
- Inferior olive projects to Purkinje cells via climbing fibres and synapses with the dendritic fibers of purkinje cells
- All other input to granule cells via mossy fibres and then onwards via parallel fibres
- All output from Purkinje cells via deep nuclei
-> 2 main inputs, 1 output!
Molecular layer of the cerebellum
- outermost
- not too many neurones there
Piriform layer of the cerebellum
- contains the main processing cells of the cerebellum
- contains Purkinje cells
- very large cells
- these cells have large dendritic trees, lots of input
- project into the nuclei in the white matter (i.e. central nucelus)
Granular layer of the cerebellum
- lots of very small neurones involved in processing
What are the 3 main divisions of the cerebellum?
- vestibulocerebellum
- spinocerebellum
- cerebrocerebellum
Vestibulocerebellum
- Regulation of gait, posture and equilibrium
- Coordination of head movements with eye movements
- close connections with superior colliculi in/and midbrain
Spinocerebellum
- Coordination of speech
- Adjustment of muscle tone
- Coordination of limb movements
Inferior cerebellar peduncle, fibres into the cerebellum
Cerebrocerebellum
- Coordination of skilled movements
- Cognitive function, attention, processing of language
- Emotional control
- evolutionarily more recent
- this is the bit that makes us more human.
Vestibulocerebellar Syndrome
Damage (tumour) causes syndrome similar to vestibular disease leading to gait ataxia and tendency to fall (even when patient sitting and eyes open)
Spinocerebellar syndrome
Damage (degeneration and atrophy associated with chronic alcoholism) affects mainly legs, causes abnormal gait and stance (wide-based)
Cerebrocerebellar or Lateral Cerebellar Syndrome
Damage affects mainly arms/skilled coordinated movements (tremor) and speech
What are the main signs of cerebellar dysfunction?
- Ataxia
- Dysmetria
- Intention Tremor
- Dysdiadochokinesia
- scanning speech
Ataxia
General impairments in movement coordination and accuracy. Disturbances of posture or gait: wide-based, staggering (“drunken”) gait
Dysmetria
Inappropriate force and distance for target-directed movements (knocking over a cup rather than grabbing it)
Intention Tremor
Increasingly oscillatory trajectory of a limb in a target-directed movement (nose-finger tracking)
Dysdiadochinesia
Inability to perform rapidly alternating movements, (rapidly pronating and supinating hands and forearms)
Scanning speech
Staccato, due to impaired coordination of speech muscles
Summary (primary motor cortex, premotor cortex, SMA, basal ganglia + cerebellum)
- Primary motor cortex issues descending commands to lower motor neurons in order to execute basic movements
- Premotor cortex involved with planning and preparation of complex movements (externally guided movements)
- Supplementary motor area also responsible for planning and preparation of complex movements (internally generated e.g. speech)
- Basal ganglia and cerebellum are responsible for monitoring and coordination of movements by modifying the output from the motor cortex
What is the difference between the supplementary motor area and the premotor cortex?
- SMA is anatomically above/superior (to) the premotor cortex
- premotor cortex: Planning of movements; externally cued movements e.g. considering intra- and extrapersonal space)
- SMA: active when thinking about a movement before executing it; planning complex movements and programming sequencing of movements; regulates internally driven movements such as speech.
What is the tentorium cerebelli?
The folds dura above the cerebellum
When are cerebellar problems seen?
Only apparent upon movement.
