Basal ganglia and cerebellum Flashcards
Basal ganglia: recall the component structures of the basal ganglia
Ataxia: define ataxia and explain how lesions in specific areas of the cerebellum relate to effects in specific parts of the body
Extrapyramidal System
- The basal ganglia and cerebellum are side loop in the hierarchical organisation of the motor system
- The basal ganglia and cerebellum check that the movements selected by the motor cortex are the correct ones
- They both have other functions than just modulating motor control (learning)
- They are considered to be motor structures because clinical signs of their damage are a variety of motor impairments
- They are located in the white matter in the middle of the brain
- It consists of several structures:
- Striatum -‐ consists of the caudate and putamen
- Globus Pallidus -‐ has an external segment (GPext) and an internal segment (GPint)
- GPext is also considered the lateral segment
- GPint is the medial segment
3. Substantia Nigra -‐ consists of the pars compacta (SNc) and the parsreticulata (SNr) (in the midbrain but projecting to basal ganglia)
4.Subthalamic Nucleus
All of these structures work together in the basal ganglion
Describe the functions of the basal ganglia
They are involved in planning and programming movement
- Elaborating associated movements (e.g. swinging the arms when walking; changing facial expression to match emotion)
- Performing movements in order
- They are also involved in moderating and coordinating movements e.g. suppressing unwanted movements
The Circuit of the Basal Ganglia (image)
There are several structures with many fibres between them and they form circuits
We start in the cerebral cortex (primary motor cortex, premotor cortex, supplementary motor area somatosensory cortex and parietal cortex) and these neurones go to the Striatum (caudate and putamen)
From here there are TWO pathways
DIRECT PATHWAY: Putamen -‐-‐-‐> Globus Pallidus Internal Segment + Substantia Nigra
INDIRECT PATHWAY: Putamen -‐-‐-‐> Globus Pallidus External Segment -‐-‐-‐> Subthalamic Nucleus -‐-‐-‐> Globus Pallidus Internal Segment
Direct Pathway = EXCITATORY effect on the motor cortex
Indirect Pathway = INHIBITORY effect on the motor cortex
Normal function of the basal ganglia requires a fine balance between these two pathways
To complicate the matters further, their function is modulated by the nigro-‐ striatal pathway
Once these projections exit the basal ganglion structures they go to the thalamus
From the thalamus they go back to the cortex to the supplementary motor area and primary motor area which are the two regions involved in movement preparation and planning
Basal ganglia: motor and cognitive function
Basal Ganglia -‐ Motor Function
- The basal ganglia and the motor cortex form a processing loop
- The basal ganglia enable the proper motor program stored in the motor cortex circuits via the direct pathway
- It inhibits competing motor programs via the indirect pathway
- Voluntary movements are initiated by the motor cortex and proper functioning of the BG is necessary for the cortex to relay appropriate motor commands to the lower levels of the hierarchy
- In other words, with the involvement of the basal ganglia and its direct
- (excitatory) and indirect (inhibitory) pathways, it can make sure the appropriate motor command gets transmitted down the hierarchy
Basal Ganglia and Cognitive Function
- The basal ganglia are involved in cognitive function as well as motor control
- This is because there are cortical loops that go through the basal ganglia and connects the basal ganglia with the prefrontal association cortex and limbic cortex
- The basal ganglia are involved in selecting and enabling various cognitive, executive and emotional programmes that are stored in these cortical areas
Parkinson’s disease: recall the pathophysiology and clinical signs of Parkinson’s disease
Parkinson’s Disease
- Parkinson’s occurs because of the neuronal degeneration of the Substantia** **Nigra Pars Compacta
- It is caused by the progressive depletion of dopamine neurones
- The clinical signs of Parkinson’s appear when about 80% of the dopamine cells in the substantia nigra have died
- There is loss of nigro-‐striatal dopaminergic neurones in the caudate and putamen
- If the connection between caudate and putamen and the substantia nigra pars compacta is lost then excitation of the motor cortex is REDUCED
- This upsets the fine balance of excitation and inhibition in the basal ganglia and reduces the excitation of the motor cortex
- The lack of excitatory inputs from the basal ganglia interferes with the ability of the motor cortex to generate commands for voluntary movement, resulting in the poverty of movement
Main motor signs of Parkinson’s disease
- 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
Huntington’s disease: recall the pathophysiology and clinical signs of Huntington’s disease
This is sort of the opposite of Parkinson’s
This is a neurodegenerative disorder due to an abnormality on chromosome 4
It is autosomal dominant
It is caused by the degeneration of GABAergic neurones in the STRIATUM (firstly the caudate and then the putamen)
Motor signs of Huntington’s disease
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
The hands and face are affected first
They gradually increase overtime until patients become totally incapacitated by them
Later on the patients will develop cognitive decline and dementia
The excitatory effect of the direct pathway is no longer kept in check by the inhibition of the indirect pathway
If the inhibitory pathway is no longer functioning properly, the cortex will not receive any inhibitory signals so the cortex will be hyperexcitable
These patients will have continuously abnormal movements
The cortex will continuously send involuntary commands for movements
Main Signs of Huntington’s Disease
Choreic Movements (Chorea) -‐ rapid, jerky, involuntary movements of the body
These are choreic hand movements
If the chorea affects their legs then they wont be able to walk
Cerebellum: explain how the cerebellum contributes to coordination of movement, recognise the relevance of pathways into and out of the cerebellum, recognise how cellular organisation of the cerebellum relates to its functioning
The cerebellum is divided horizontally into THREE lobes:
o Anterior
o Posterior
o Flocculonodular
NOTE: flocculonodular lobe is the little part that’s near the brainstem
Coronally, it is divided into THREE zones:
- Vermis -‐ in the middle
- Intermediate Hemisphere
- Lateral Hemisphere
NOTE: the hemispheres are on either side of the vermis and they are subdivided into the intermediate hemisphere (closest to the vermis) and the lateral hemisphere (outside the intermediate hemisphere)
Cerebellar Cortex Histology
Histologically there are THREE layers in the cerebellar cortex
- Innermost Layer: Granule Cells
- Middle Layer: Purkinje Cells
- Outer Layer: Molecular Layer
This contains axons of the granule cells and dendrites of the Purkinje cells
Inferior olive projects to Purkinje cells via climbing fibres
All other input to granule cells via mossy fibres and then onwards via parallel fibres
All output from Purkinje cells via deep nuclei
Connections
There are THREE deep nuclei:
Fastigial -‐ involved in control of balance and connected with vestibular nuclei
Interposed and Dentate -‐ both are involved in voluntary movement and are connected to the thalamus and the red nucleus
There are THREE sources of input to the cerebellum:
1. Mossy Fibres -‐ brings information from the cerebral cortex and pons (these are called corticopontine fibres)
2. Mossy Fibres from the spinocerebellar tract
3. Climbing Fibres from the inferior olive (inferior olivary nucleus)
Divisions of the Cerebellum
1. Vestibulocerebellum
Functionally, the cerebellum is divided into THREE
The vestibulocerebellum (or flocculonodular lobe) is connected to the vestibular nuclei so its main function is its involvement with balance, posture and regulation of gait
It is also involved in coordination of head movements with eye movements
2. Spinocerebellum
This division of the cerebellum involves the vermis and the intermediate hemisphere
This division receive information from the axial portions of the body (trunk) and visual and auditory input -‐ these project to the VERMIS
The spinal afferents from the limbs project to the INTERMEDIATE HEMISPHERE
All this information gets processed and the output is sent back to the spinocerebellar tract
The main role of this part of the cerebellum are:
- Coordination of Speech
- Adjustment of Muscle Tone
- Coordination of Limb Movement
3. Cerebrocerebellum
The third division is the lateral hemisphere
This receives projections from the cortex
Functions of the cerebrocerebellum:
- Coordination of skilled movements
- Cognitive function o Attention
- Processing of language
- Emotional control
Function of the Cerebellum
MAINTENANCE OF BALANCE AND POSTURE
Makes postural adjustments in order to maintain balance
Through its inputs from vestibular receptors and proprioceptors, modulates commands to motor neurons to compensate for shifts in body position
COORDINATION OF VOLUNTARY MOVEMENTS
Coordinates the timing and force of different muscle groups to produce smooth body movements
MOTOR LEARNING
The cerebellum is also involved in motor learning -‐ it can fine tune motor programmes to make accurate movements through a trial and error process
COGNITIVE FUNCTIONS
Its functions extend beyond motor control in ways that are not well understood yet
Cerebellar dysfunction
Localisation of Cerebellar Dysfunction
Vestibulocerebellar or Flocculonodular Lobe Syndrome
Damage (tumour) causes syndrome similar to vestibular disease
Patients tend to lose their balance with gait ataxia and a tendency to fall
Spinocerebellar Syndrome
Damage (degeneration and atrophy associated with chronic alcoholism)
Mainly affects the legs
It causes an abnormal gait and a wide-‐based stance
Cerebrocerebral or Lateral Cerebellar Syndrome
Damage mainly affects arms
It affects coordinated movements (tremor)
Speech becomes very hesitant and slow -‐ staccato
Main Signs of Cerebellar Disorders
Deficits apparent only upon movement
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)
Dysdiadochokinesia
Inability to perform rapidly alternating movements, (rapidly pronating and supinating hands and forearms)
Scanning speech
Staccato, due to impaired coordination of speech muscles
Intention tremor is assessed clinically with nose-‐finger tracking
You ask the patient to move their finger from their nose to a target and they will show an oscillatory movement that doesn’t go directly to the target
Apraxia
A disorder in skilled movement NOT caused by weakness, abnormal tone or posture or movement disorders (tremors or chorea)
Patients are NOT paretic (partial motor paralysis) but have lost information about how to perform skilled movements
This is not because they’ve lost motor command to the muscle but is instead because they have lost the information on how to perform the skilled movements
This happens with lesions of the inferior parietal lobe and the frontal lobe (premotor cortex and supplementary motor area)
Any disease of these areas can cause apraxia, but stroke and dementia are the most common causes
- MRI of a person who suffered a bilateral supplementary motor area infarct
- The motor command from the primary motor cortex is still there but this person will have apraxia
- They will not be able to perform coordinated movement
