L65. Neuroscience - Cerebellum

• The session aims to enable students to accurately describe the anatomical organization of the cerebellum, explain its functional systems and effects on movement, and predict the signs and symptoms of cerebellar damage.

Anatomy of the Cerebellum

• The cerebellum related posteriorly to the dorsal surfaces of the medulla, pons, and midbrain and covers the fourth ventricle.
• It is located inferior to the occipital cortex.
• The gross location of the cerebellum in the skull is the posterior cranial fossa.
• The cerebrum is 82% of the mass of the brain with 16 billion neurons and 61 billion non-neurons.
• The cerebellum is 10% of the mass of the brain with 69 billion neurons and 16 billion non-neurons.
• The tentorium cerebelli meningeal layer of dura mater separates the cerebellum from the occipital lobe of the cerebrum.
• The brainstem is ventral to the cerebellum and positioned medial to the edges of the tentorial incisure.
• The foramen magnum, an opening in the skull inferior to the cerebellum, serves as a transition from the caudal brainstem (medulla) to the spinal cord.
• Folia are parallel ridges of cerebellar cortex seen on the surface and are analogous to gyri of the cerebral cortex.
• Cerebellar Peduncles are white matter columns that anchor the Cb to the brainstem.
• The anterior lobe is a small region anterior to the primary fissure and comprised of mostly vermis and paravermis regions, with lateral hemispheres contributing minorly.
• The posterior lobe is a large region posterior to the primary fissure, which contains the vermis and paravermis regions but is dominated by the lateral hemispheres.
• The flocculonodular lobe is the oldest part of the cerebellar cortex and consists of the flocculous laterally and the nodulus medially.
• Cerebellar peduncles obscure most of the flocculonodular lobe
• The vermis is the midline region of Cb that resembles a worm and labelled "v".
• The paravermis is the region btween the vermis and lateral hemispheres.
• Cerebellar lateral hemispheres are larger extensions of cerebellar cortex.
• The Cerebellar Tonsil is an inferomedial extension of the paravermal region of the posterior lobe, and in cases of increased intracranial pressure, this tonsil can compress the medulla as it is pushed inferiorly through the foramen magnum, resulting in tonsillar herniation that can be fatal.
• Arbor vitae is the appearance of white matter (axons) deep to the Cb cortex that resembles the branches of a tree in a sagittal section.
• The folia are the gray matter of the cerebellar cortex surrounding each white matter tree branch.
• Important fissures include the primary fissure that divides the anterior lobe from the posterior lobe, and the posterolateral fissure that divides the posterior lobe from the nodulus.
• Lobes related to the fissures are the anterior lobe cerebellum anterior to the primary fissure, the posterior lobe cerebellum between the primary and posterolateral fissures, the flocculonodular lobe which in a medial view of a sagittal section, the nodulus can be viewed.
• Deep Cerebellar Nuclei are four cerebellar nuclei positioned deep to the Cb cortex (folia) and white matter (arbor vitae).
• From medial to lateral they are named Fastigial, Globose, Emboliform and Dentate.
• Dentate is largest and has a wavy appearance.

Functional Systems of the Cerebellum

• Basic Functional Circuitry includes the cerebellar cortex(main processing center) with the deep cerebellar white matter("two-way highway") being a link to the deep cerebellar nuclei(main sender).
• Purkinje cells of the cerebellar cortex project to the deep cerebellar nuclei
• The cerebellar afferents are the input signals, and the cerebellar efferents are the output signals.
• The vestibulocerebellum is the oldest part of Cb that functions with the vestibular system.
• The flocculonodular lobe and fastigial nucleus involved.
• The spinocerebellum functions in unconscious proprioception carried by afferents from the spinal cord.
• The vermis/paravermis regions and globose/emboliform nuclei involved.
• The cerebrocerebellum lateral part of the Cb involved in planning, sequencing, and coordination of movements controlled by the cerebral cortex, and with involvement of the lateral hemispheres and dentate nucleus.
• Main Function of Vestibulocerebellum is the balance and equilibrium, trunk posture, head and eye movements, leading to the facilitation of lower extremity extensors.
• The First Order Neuron is the Vestibular (Scarpa's) Ganglion (CN VIII), with peripheral processes associated with the vestibular apparatus of the inner ear, and central process entering the rostral medulla/caudal pons region of the brainstem as the vestibular root of CN VIII.
• Afferents project to ipsilateral vestibular nucleus or directly to the cerebellum.
• The Pathway to Cerebellum is the ipsilateral juxtarestiform body (medial ICP).
• The Cerebellar Cortex Targets are the flocculonodular lobe & uvula of the posterior lobe with the Fastigial being deeply involved in the Cerebellar Nuclei.
• The brain stem target are the vestibule-spinal which fastigial nucleus projects bilaterally to the vestibular nuclei to affect activity within the medial and lateral vestibulospinal tracts.
• The lateral vestibulospinal tract (LVST) is an ipsilateral descending UMN tract present at all spinal levels, which targets the lumbosacral spinal cord to facilitates lower extremity extensors and postural muscles to affect movements of the vertebral column (axial musculature)
• The medial Vestibulospinal Tract (MVST) is a bilateral descending UMN tract present at cervical and upper thoracic levels, where it especially targets neck and proximal upper extremity muscles in neck.
• Spinocerebellum processes unconscious proprioception important in adjustment of muscle tone, being especially involved in adjusting ongoing movements.
• The main function of spinocerebellum includes the processing of unconscious proprioception critical to muscle tone adjustment and ongoing movements.
• The first order neuron related to Spinocerebellum is the dorsal root ganglion (C2-S5), whose peripheral processes are associated with the muscle spindles or Golgi Tendon Organs .
• Spincerebellum second order relay is the Clarke's Nucleus (Nucleus Dorsalis) in the Spinal Cord which is a cell column present in gray matter of the spine that originates the Dorsal Spinocerebellar Tract(DSCT).
• In medulla-spinal tract 1st order fibres synapse in accessory cuneate nucleus, DSCT and T2-5 unconscious proprioreceptive info moves into interior cerebral peduncle
• Cerebellar efferent neurons located within the globose (G) and emboliform (E) nuclei have received unconscious proprioceptive information from vermal and paravermal regions of the cerebellar cortex or directly from DSCT and CCT collaterals.
• Globus and Emboliform axons project into the SCP, which is directed to the mid brain and can be observed medial to the dentate nucleus.
• The SCP then carries affernt axons to the globose and emboliform nuclei which are in the walls fo the 4th ventricle before moving rostrally towards the midbrain after decussating.
• Fibres of the pons move rostrally to synpase with Ventral Lateral Nucleus in the thalamus.
• The concept of Cerebellum,does not influence motor behaviours by by projecting directly to the spinal cord but rather by alternating the descending UMN pathways.
• The Thalamus relays Cerebellar formation onto the primary area, which influences activity in that area.
• The Red Nucleus originates in the "Rubrospinal Tract" projects to the mono neurons in the spinal cord.
• Vestibular Nucleas is the "Lateral+Medial Vestibulospinal" and Reticular the lateral + medial Reitculospinal tract. All influence mono-neurons on ventral horn.
• The cerebello-cerebellum and cerebral cortex influences by way of nuclear relays connected to the cerebellum. The systems that are related are pontine and red inferior olivary

Effects of Cerebellar Lesions

• The fundamental concept for effects of cerebellar lesions, is that the signs and symptoms of cerebellar damage are most severe when either the deep ceerbllar nuclei or the "SCP" pathways are damaged.
• If damage to deep Cerebellar Nuclei is caused or to teh cerebrallar pathways; symptoms are also evident.
• Contralateral C. lesion affect the same side while lesions distal affect opposite side. In contrast to SCP, these create symptoms on both sides.
• The blood supply is the Superiror middle and Posterior cerebellar arterial. SCA is from entire middle cerebellum while AICIA supplies caudal parts. and posterior ICA is from vermis.
• Muscle symptoms are usually evident on the same side as the damage area.
• The dysfunction of the vestibulocerebellumm causes midline ddamage. The medulloblastoma is the common association and shows bilateral effects + fastigal efferent pathways are alligned.
• Trunk ataxia is caused due to difficulty head till and inability to perform task.
• Anterior lobe syndrome is caused due to alcoholism taht effecs teh cerbeellulm and comprised of "vermis" and "paravermal" that affect spinal cord fxs. Lower and Ataxia limbs and obvious, like having "walking in a wider" gait. Signs/Symptoms of Cerebellar Damage:
  • Hypotonia (decreased muscle tone) and Hyporeflexia (decreased deep tendon reflexes).
  • Muscle Incoordination include:
    • Ataxia (without coordination)- incoordination and weakness of muscles, and tendency to fall toward lesion side.
    • Dysynergia- disturbance of muscular coordination leading to abrupt movements.
    • Dysmetria- the lack of control for distance, speed, and range of motion causing uncoordinated movements.
    • Abnormal heel to shim tests were patients are. unable to co-ordinate.
    • Dysdiadochokinesia and rebound phenomena and are unable rapidly adapt chnages in movement. -Dysarthira- where speech is "slurred", can be slow too.
  • Acton tremor and obvious movements are associated with this type.
  • Nystagmus- The slow phhase will away ,while the higher speed will lean to the lesion.