CEREBELLUM

Functional anatomy

Location of cerebellum:

• Largest part of hindbrain• Occupies most of posterior cranial fossa• Lies behind pons & medulla– forming roof of 4th ventricle

• Separated from posterior part of cerebrum– by tentorium cerebelli

Important facts

• The hemispheres, vermis, flocculus, nodule, and tonsil are major landmarks of the cerebellar cortex.

• Afferent fibers end in the three-layered cerebellar cortex. The Purkinje cells have axons that end in the cerebellar nuclei.

• Motor part of the brain, serving to maintain equilibrium and coordinate muscle contractions.

• Makes a special contribution to synergy of muscle action (i.e., to the synchronized contractions and relaxations of different muscles that make up a useful movement).

• Ensures that contraction of the proper muscles occurs at the appropriate time, each with the correct force.

• Participates in learning patterns of neuronal activity needed for carrying out movements and in the execution of the encoded instructions.

• Imagined movements are accompanied by an increase in cerebellar blood flow that is larger than the increase detected in the motor areas of the cerebral cortex.

• Evidence also suggests that the cerebellum has sensory and cognitive functions.

• The cerebellum consists of a cortex, or surface layer, of gray matter contained in transverse folds or folia plus a central body of white matter.

• Four pairs of central nuclei are embedded in the cerebellar white matter.

• Three pairs of cerebellar peduncles, composed of myelinated axons, connect the cerebellum with the brain stem.

Cerebral peduncles:

• Joined to the brain stem via:• Superior cerebellar peduncle –> Midbrain• Middle cerebellar peduncle –> Pons• Inferior cerebellar peduncle –> Medulla

Major contents of the cerebellar peduncles. For simplicity, the inferior cerebellar peduncle is depicted

as containing only climbing fibers

Contents of Cerebellar Peduncles

• The superior cerebellar peduncle contains cerebellar efferent fibers, the ventral spinocerebellar tract, and tectocerebellar fibers.

• The middle cerebellar peduncle consists of fibers from the contralateral pontine nuclei

Contents of Cerebellar Peduncles

• Inferior cerebellar peduncle contains olivocerebellar and dorsal spinocerebellar fibers and the vestibulocerebellar and fastigiobulbar connections.

Microstructure

Microscopic structure of cerebellar cortex (3 layers

• External Molecular layer• Middle Purkinje layer– large flask shaped neurons– arranged in single tier in a plane transverse to

folium– dendrites produce profuse branching– receive communications afferent fibres entering

cerebellum

– axons run to DEEP CEREBELLAR NUCLEI• sole output of cerebellar cortex

– functionally• inhibitory (GABA)–Stimulate Purkinje cells, inhibits deep nuclei

• Internal Granular layer– closely packed small neurons

All afferent input via 2 different fibres

• Mossy fibres– entire inflow of cerebellum• except from inferior olive• particularly from cerebral cortex via pons

– they synapse in expanded MOSS-like appearance with granule & Golgi cells in granular layer

– fast-firing• rapid adjustment for ongoing movement

• Climbing fibres– inflow from inferior olive– run up and synapse with dendrites of single

Purkinje cell– slow-firing• helps in learning muscle habits

Origin of climbing fibers from the inferior olivary nucleus. The axon of each olivary neuron has several branches, each of which

forms the single climbing fiber of one Purkinje cell.

• The fastigial, interposed, and dentate nuclei receive branches of all cerebellar afferent fibers and the output of the cortex.

• These nuclei contain the cerebellar efferent neurons.

• The vestibular system is connected ipsilaterally with the vestibulocerebellum, which comprises the flocculonodular lobe and the fastigial nucleus.

• This nucleus projects to the ipsilateral vestibular nuclei and to the reticular formation

• Proprioceptive signals are carried ipsilaterally to the spinocerebellum, which consists of vermis, paravermal zones, and interposed nuclei.

• These nuclei project to the contralateral red nucleus and to the posterior division of the contralateral ventrolateral (VLp) thalamic nucleus. The VLp projects to the primary motor cortex.

Connections of vestibulocerebellum and vestibular nuclei. Afferents-blue; efferents-red;neurons-black

Connections of spinocerebellum and vestibular nuclei. Afferents-blue; efferents-red;neurons-black

Connections of pontocerebellum. Afferents-blue; efferents-red;neurons-black