Cerebellum Flashcards
What is the general function of the cerebellum? What do lesions result in?
Evaluate disparities between intention and action and generate correction signals to the cortex and brainstem.
Lesions results in loss of spatial accuracy and temporal coordination of movement as well as impairment of balance and loss of muscle tone. Sensation and strength of muscle contraction are not impacted.
Describe the general anatomy of the cerebellum.
- Grey matter on the outside, white matter on the inside
- Invaginations are called folia
- Two transverse fissures: primary fissure divides anterior and posterior lobes, posterolateral fissure divides posterior lobe from flocculonodular lobe
- Deep cerebellar nuclei receive input from cerebellar cortex
- Three peduncles containing incoming and outgoing information
- Three mediolateral regions: vermis and two hemispheres
How is the cerebellum divided functionally?
Central vermis and the lateral and intermediate zones in each hemisphere
How is the cerebellum divided anatomically?
Primary fissure divides the anterior and posterior lobes and the posterolateral fissure separates the foculonodular lobe.
Describe the input system from the inferior olive.
The inferior olive projects climbing fibers into the cerebellar cortex via the inferior cerebellar peduncle which wrap around the dendrites of the Purinje cell, making hundreds of synapses. If it fires an action potential, it is guaranteed to stimulate the Purkinje cell. There is little divergence in this pathway.
The inferior olive received inputs from the spino-olivary pathway (travels with spinothalamic) as well as from the cortex. It is responsible for comparing the actual movement with the intended movement.
Describe the how the cerebellum receives the rest of it’s input (non-olivary)
Mossy fibers make up the rest of the input to the cerebellum and consist of axons from the spinocerebellar tracts, pontine nuclei, vestibular nuclei, cortex, etc. This is a largely divergent system in which each mossy fiber terminates on granule cells which send their axons into the Purkinje layer (parallel fibers) to synapse on hundreds of Purkinje cells, however, they only synapse on a single Purkinje cell once or twice (i.e. activation is not guaranteed).
Input from the spinal cord enters through the inferior cerebellar peduncle while input from the cortex enters in the middle cerebellar peduncle.
What is the only output of the cerebellar cortex? Where do these nuclei project?
The cerebellar cortex projects only to deep cerebellar nuclei (except for the vestibulocerebellum)
- Cerebrocerebellum projects to dentate nucleus which projects to the motor and premotor cortices (motor planning)
- The paravermis projects to the interposed nuclei which project to the lateral decending systems for motor execution
- The vermis projects to the fastigial nuclei which project to medial descending systems for motor execution as well
- The vestibulocerebellum (floculonodular lobe) projects to vestibular nuclei which regulate balance and eye movement
Describe the somatosensory map of the spinocerebellum.
The limbs are represented in the intermediate portions of the hemispheres while the axial muscles and the head are represented in the vermis.
What are the functions of the spinocerebellum and cerebrocerebellum? How do they project?
The spinocerebellum compares motor commands with motor execution and corrects them during movement:
- Limbs: interposed nuclei –> VL –> M1 (contralateral projection via SCP)
- Axial: fastigial nuclei –> red nucleus –> rubrospinal/corticospinal tract
The cerebrocerebellum is active during planning prior to movement:
- Dentate –> VL –> association, premotor, M1 (contralateral projeciton via SCP)
The vestibulocerebellum projects to the vestibular nuclei via the ICP
What cerebrocognitive funtions does the cerebellum have?
The dentate nucleus becomes more active when an individual is asked to do sensory discrimination. It is also important in adapting the motor system to a task that requires adaptation.
What are the three types of cerebellar symptoms after a lesion?
- Hypotonia: lack of resistance to passive displacement of limbs
- Ataxia: lack of coordation with resprect to eye movements, walking, initiating movements, dysdiadochokinesia, dysmetria, decomposition of movement
- Intentional tremor
What are the basal ganglia and what do they do?
The basal ganglia are a cluster of neuronal cell bodies at the base of the brain in the CNS which modulate movement, cognition, and behavior.
- Striatum: caudate and putamen
- Pallidum: globus pallidus interna and externa
- Substantia nigra: pars compacta and pars reticularis
- Subthalamic nuclei
How are the basal ganglia connected to the rest of the brain?
Cortex signals to the caudate/putament which projects to the substantia nigra and the globus pallidus. The signal is then sent to the thalamus and back to the cortex.
Nuclei within the basal ganglia modulate the signal and send it back to the cortex via the thalamus.
Describe the direct and indirect circuitry in the basal ganglia.
In the direct pathway, the cortex signals the the striatum which activates the GPi or SNr (normally inhibits VA/VL of the thalamus). The thalamus projects back to the cerebral cortex.
In the indirect pathway, the cortex excites the striatum which inhibits the globus pallidus externa which is normally inhibiting in subthalamic nucleus. STN can now excite the GPi/SNr which inhibits signlaing to the thalamus and subsequent cortical activation.
The substantia nigra pars compacta excites the direct pathway and inhibits the indirect pathway which facilitates movement.
Which neurotransmitters are released by each component of the basal ganglia?
The striatum receives stimuli from the cortex and inhibits the GPi/SNr via GABA in the direct pathway as well as GPe via GABA in the indirect pathway. GPe releases GABA onto the subthalamic nucleus which activates the GPi/SNr using glutamate. The GPi/SNr inhibits VA/VL of the thalamus with GABA. The thalamus projects excitatory inputs back to the cortex. The SNc activates the direct pathway of the striatum (D1 receptor) and inactivates the indirect pathway of the striatum (D2 receptor) using dopamine
Describe the pathology of PD.
In Parkinson’s disease the substantia nigra shows a loss of pigment, glial proliferation, microglial inflammation, and Lewy bodies with alpha-synuclein in neurons (round eosinophilc cytoplasmic inclusions).
