cerebellum

Question 1

major anatomical features of the cerebellum

Answer 1

hemispheres, vermis, lobes, fissures, cortex, white matter, subcortical nuclei, cerebellar peduncles, blood supply

Question 2

three cerebellar loops

Answer 2

vestibullocerebellar, spinocerebellar, and cerebrocerebellar loops

Question 3

The Cerebellum

Answer 3

(“little” brain or “little” cerebrum):
- constitutes only 10% of the total volume of the brain (~ 150 gr.), but contains more than 50% of neurons
of the entire brain
- is located in the posterior cranial fossa below the tentorium cerebelli (located in transverse fissure)
- tentorium cerebelli supports the occipital lobe & protects cerebellum from the weight of occipital lobe - is located on the posterior part of the brainstem (posterior to pons, medulla, midbrain) & 4th ventricle

Question 4

Tentorium cerebelli:

Answer 4

“tent of cerebellum”

Question 5

Structure of the cerebellum

Answer 5

§The cerebellum contains two hemispheres (right & left) that are separated by the vermis (Latin: “worm”) §The vermis is mostly visible on the posterior cerebellar surface between the two hemispheres §Cerebellum LACKs structure that corresponds to corpus callosum (= LACKS commissural fibres)
§On the anterior surface, prominent structures are the two flocculi (flocculus: Latin for “tuft of wool”)

Question 6

The cerebellar tonsils

Answer 6

are part of the posterior lobe, possess no distinct function but herniation of the tonsils through the foramen magnum may occur owing to higher intracranial pressure
(compression of medulla, letal outcome)

Question 7

Fissures and lobes of the cerebellum

Answer 7

§The Cerebellum contains three lobes (anterior, posterior and flocculonodular lobes)
§Anterior and posterior lobes are separated by the primary fissure
§Posterior and flocculonodular lobes are separated by the posterolateral fissure
§The horizontal fissure divides the cerebellum into equal upper (superior) & lower (inferior) halves
§The nodulus (or nodule) is the most inferior (caudal part) of the vermis

Question 8

Median sagittal features of the Cerebellum

Answer 8

§The cerebellum is composed of outer cerebellar cortex and internal white matter (similar to cerebrum)
§In the median sagital section, the white matter of the the vermis looks like a „tree of life“ (= arbor vitae)
§The superior and inferior medullary vela merge at the fastiguim
§The nodulus (or nodule) is the most inferior (caudal part) of the vermis

Question 9

Blood supply of the cerebellum

Answer 9

§HORIZONTAL FISSURE divides the cerebellum into two equal halves (superior & inferior halves)
§Superior half of the cerebellum is supplied by one superior cerebellar artery
§Inferior half of the cerebellum is supplied by two inferior cerebellar arteries
§Superior & anterior inferior cerebellar arteries are branches of basilar artery
§Posterior inferior cerebellar arteries are branches of the vertebral arteries

Question 10

Cerebellar peduncles

Answer 10

Four pairs of nuclei are embedded within the white matter of the cerebellum:

1. Dentate nuclei (located laterally - teeth like)
2. Interposed nuclei (emboliform nuclei + globose nuclei [2 globose nuclei each side])
3. Fastigial nuclei (located medially)

Question 11

Similar to the cerebrum the cerebellum has cortex & white matter

Answer 11

• the cerebellar cortex uniformly contains only 3 neuronal layers : (1) Molecular, (2) Purkinje, and (3) Granular cell layers
• cerebellum LACKS association fibres; thus cerebellar lobes ARE NOT interconnected and can not share or coordinate information they recieve
• cerebellum LACKS commissural fibres; thus the two cerebellar hemisphres DO NOT communicate with each other

Question 12

1. Molecular Layer

Answer 12

(comprised of parallel axons and dendrites) - afferent (input): parallel fibers of the granule cells axons
- efferent (output): Purkinje cell dendrites

Question 13

2. Purkinje Cell Layer

Answer 13

(thin row of large cells)
- 15 000 000 Purkinje cells
- afferent (input): parallel fibers of the granule cells &
climbing fiber from inferior olivary nucleus - efferent (output): deep cortical nuclei

Question 14

3. Granular cell Layer

Answer 14

(closely packed granule cells) - 50 000 000 000 granule Cells

• afferent (input): mossy fiber (mainly from cerebral cortex)
• efferent (output): Purkinje cell dendrites

Question 15

Afferent and efferent cerebellar connections

Answer 15

1. ALL AFFERENT (incoming) fibres entering the cerebellum terminate in the cerebellar cortex
2. Collateral copies of the afferent fibres are ALWAYS given to the cerebellar nuclei
3. ALL EFFERENT (outgoing) fibres that EXIT THE CEREBELLAR CORTEX are INHIBITORY axons of the cortical Purkinje neurones that mostly synapse with neurons of the cerebellar nuclei
4. Efferent fibres that EXIT THE CEREBELLUM are mostly EXCITATORY & originate from the cerebellar nuclei

Question 16

Background info “Feedback” mechanism

Answer 16

muscle, tendon and joint actions > sensory feedback > cns> motor actions ….continuous loop

§Every motor action (command / plan) has a sensory consequence (feedback) that reflects the motor action §
§In other words, sensory feedback is required by motor centers to monitor whether motor actions are
progressing in accordance with the motor plan.
- Sensory feedback is a measure of actual performance of motor plans & is needed to adjust motor actions

Question 17

“Feedback” mechanism of cerebellar function

Answer 17

§Cerebellum compares motor actions & sensory feedbacks (is a comparator)
§Cerebellum can detect disparity (error) between motor actions & sensory feedbacks (is an error detector) §Cerebellum is also involved in the process of error correction of ongoing movements
§Therefore, cerebellum is responsible for coordination of all body movements
§Lesion of the cerebellum leads to lack of coordinated movement - ataxia

Question 18

Cerebellar circuitry

Answer 18

§All mossy fibres terminate at granule cell layer
§Afferent tracts from inferior olivary nucleus are called Climbing fibers and target the Purkinje cell layer
§Each Purkinje cell receives input from about 200000 granule cells (= summated converging input) §Therefore, Purkinje cells have dense dendrites to accommodate axons of the granule cells
§Purkinje cells are flat and have a 2 dimensional (2D) structure to synapse ONLY ONCE with each axon of a granule cell

Question 19

Climbing fibers convey error signals to Purkinje cells

Answer 19

§Afferent tracts from inferior olivary nucleus are called Climbing fibers and target the Purkinje cell layer §Climbing fibres contain information from different sources including the spinal cord, reticular system, red nucleus, cerebral cortex , and even the cerebellum itself!

Question 20

mossy fibres

Answer 20

convey afferent information to cerebellum (eg. Pontine nuclei, spinal cord, vestibular
nerve, reticular formation). All mossy fibres terminate at granule cell layer

Question 21

Lack of proprioceptive information leads to sensory ataxia

Answer 21

§Patient lacks proprioceptive information, e.g. due to proprioceptive neuropathy §Patient cannot perceive position or movement of legs and staggers
§Patient uses vision to monitor his movements, to compensate lack of proprioception

Question 22

Cerebellum is particularly prone to adverse effects of alcohol

Answer 22

§Alcohol consumption transiently (but also permanently) impairs cerebellar function causing:

• ataxic gait (lack of coordinated gait) due to loss of balance (due to lack of error correction in reticulospinal and vestibulospinal tracts)
• involuntary and uncoordinated eye movements (ataxia of the eye), e.g. double vision & nystagmus (due to lack of error correction in vestibulo-ocular reflex)
• Slurry speech owing to the lack of error correction in corresponding cranial nerves (corticonuclear tract) - Loss of ability to show skilled movement (dyspraxia) due to lack of error correction in corticospinal tract