Motor system: Side loops (Cerebellum) Flashcards
What is the role of the cerebellum in the motor system?
- Operates at an unconscious level
- Controls maintainenance of equilibrium (balance)
- Influences posture and muscle tone
- Coordinates movements
- Detects errors (compares intended movements to actual movements)
- Plays a major role in attention and planning of motor learning (automaticity)
- It does not output directly to the lower motor neruones , but modifies the firing patterns of the upper motor neurones
What is the structure of the cerebellum?
Structure
* Large areas of cortex (grey matter)
* Deep nuclei = output nuclei from cerebellum
* Cortex = makes fine adjustments to deep nuclei
and therefore to motor functions
Tentorium cerebelli
- Co-ordination of planned movements
- Maintenance of an upright posture in respect to position
in space - Maintenance of tension or firmness of muscle (tone)
What are the three main functional regions of the cerebellum?
**Spinocerebellum **
Midline vermis and surrounding
paravermis
Receives major spinal cord inputs
Principallyfrom spinocerebellartracts
* Regulates axial muscle tone and
posture
* Somatotopically organised
Neocerebellum:
Paleocerebellum (spinocerebellum)
**Neocerebellum **
(cerebrocerebellum)
* Remainder (and vast majority) of cerebral
hemispheres)
* Receive major pontocerebellarfibres
Muscular coordination, trajectory, speed
and force
**Vestibulocerebellum **
* Flocculonodularlobe (& posterior vermis)
* Connections with vestibular & reticular nuclei
Balance/equilibrium
Status of head position & control of eyes
Axial muscle control
Flocculonodular lobe
(vesti bulocerebellum)
What are the cerebellar pundcles and deep nuclei?
- The inputs and outputs of the cerebellar cortex
- Superior peduncle :
- Mainly efferent fibres
- Main output route of cerebellum
- Emerge from cerebellar nuclei
- Synapse with contralateral red nucleus and ventrolateral nucleus of thalamus
Middle peduncle :
- Mainly afferent fibres
- 2nd limb of di-synaptic pathway linking cerebral cortex with cerebellar cortex
- Fibres originate in contralateral pontine nuclei
Inferior peduncle :
- Mainly afferent fibres
- Bring information from medulla and spinal cord
- Terminate in cerebellar cortex
- Deep nuclei
- Dentate nucleus : projects to ventrolateral nucleus of thalamus
- Interposed nuclei : globose nucleus andn emboliform nucleus - project to red nucleus
- Fastigial nucleus : projects to reticular formation and vestibular neurones
What are the principal inputs to the cerebellum?
Contralateral cerebral cortex to pons to cerebellum
Vestibular inputs, inferior olive, spinal cord to cerebellum
- also relies upon sensory feedback from periphery to modulate ongoing (or planned) motor activity - unconscious proprioception, visual, vestibular inputs
What are the principal outputs of the cerebellum?
- Cerebellar cortex TO
- Deep nuclei
- Contralateral thalamus TO contralateral cerebral cortex
- Vestibular nuclei
- Inferior olive
- Contralateral red nucleus
How does cerebellar disease/ damage cause ipsilateral symptoms?
**
Ataxia. **
Disturbance of voluntary movement
Tremor (no resting tremor) when carrying out motor tasks
Erro**rs in direction, range, rate and force of movernent
Hypotonia: reduced muscle tone
Dysdiadochokinesia: no rapidly alternating movements
Pendular reflexes: no limit by stretch reflexes
Nystagmus: rhythmical eye movements (linked to vestibular
system)
Intention tremor: tremor when coming to the end of a
determined and visually directed movement
Gait ataxia: wide stance gait
Dysmetria: lack of coordination — overshoot/undershoot of
intended position
What is tonsillar herniation?
Tonsillar herniation
Descent of cerebellar tonsils (+/- brainstem) below the foramen magnum
Cause?
Secondary sign of intra-cranial mass effect (e.g. tumour,
haemorrhage, abscess etc). Will displace cranial fossa structures
inferiorly.
Outcome?
If brainstem is compressed respiratory and cardiac centres will be
interrupted in medulla and pons (life threatening)
Cause?
Chiari malformations
Skull not large enough
Displaces structures inferiorly
* Interrupts CSF flow
Outcome?
Chiari type l: Displaces cerebellum -
headaches, visual disturba nces, nystagmus,
ataxia (usually non-life threatening)
Chiari Il: Displaces cerebellum & brainstem
— life threatening
What are the basal ganglia?
- Group of highly interconnected brain nuclei
- Involved in motor, cognitive and limbic functions
- composed of:
the caudate nucleus and putamen
(collectively known as the striatum)
the globus pallidus
the subthalamic nucleus
the substantia nigra
What are the functions of the basal ganglia?
- Selecting appropriate movements (habitual
and goal-directed) - Initiating internally generated movements
- Planning and executing complex motor
strategies - Non motor- cognition and emotional
behaviour
What are the nuclei of the basal ganglia?
Nuclei of the basal ganglia
Caudate Nucleus and Putamen
Globus Pallidus (two divisions):
* Internal Globus Pallidus (GPi)
* External Globus Pallidus (GPe)
Substantia Nigra (two divisions):
Substantia Nigra pars reticulata (SNr)
Substantia Nigra pars compacta (SNc)
Subthalamic nucleus (STN)
What is the substantia nigra (pars compacta)?
- Black substance in neuromelanin, a by-product of dopamine production
How is rhe balance maintained between the direct and indirect pathways in the basal ganglia pathways?
- Dopamine maintains the balance through its actions at different dopamine receptors on striatal projections neurones :
- Dopamine increases transmission along the indirect pathway through activation of D2 receptors
- Makes the thalamus and cortex more easily excited
- Loss of dopamine results in pathological increases in basal ganglia output and excessive inhibition of targets
- The indirect pathway predominate making the thalamus and cortex less easily activated
What does basal ganglia dysfunction lead to?
- Abnormal motor control
- Alterations in posture and muscular tone
- Abnormal involuntary or voluntary movements
- The combination of symptoms depends on nature and site of the lesion
What is Parkinson’s disease?
- A BG disorder
- presenting signs :
- T - tremor
- R- rigidity
- A- Akinesia (impairment voluntary movement)
- M - Mask
- P - postural instability
- 2 OR MORE - strongly suggestive of PD
- D ue to a decreased nigral neuromelanin
