The Cerebellum

– The cerebellar cortex is folded into numerous, small gyri, making it easy to distinguish from the cerebral hemispheres.

– The cerebellum (CBL) receives its inputs from three major sources: spinocerebellar tracts, pontocerebellar fibers, and olivocerebellar fibers.

– The great majority of CBL output is via its own subcortical nuclei; namely the dentate, interposed, and fastigial nuclei, and the emboliform.

– Phylogenetically, the CBL first appeared in the lowest vertebrates. It enlarged in cartilaginous and bony fish and increased further in Amphibia and Reptilia.

– Birds show the greatest relative development in submammalian species.

– Mammals exhibit the greatest relative enlargement and degree of complexity.

– These phylogenetic differences correlate with the appearance of the three functional subdivisions of the CBL.

– The vestibulocerebellum is oldest, the spinocerebellum is intermediate in evolutionary age, and the cerebrocerebellum is most recent.

Functional Subdivisions of the Cerebellum

– The cerebellum is routinely divided into three, somewhat arbitrary, functional divisions,

1. the vestibulocerebellum (archicerebellum),

2. the spino-cerebellum (paleocerebellum), and

3. the cerebrocerebellum (neocerebellum). – Each division, as its name implies, receives its

predominant afferent input from a different source. – Each division also has somewhat different outputs.

The Vestibulocerebellum– The vestibulocerebellum (archicerebellum) is the oldest

part, first appearing in fish. – It consists of the flocculonodular lobe and, according to

some authors, parts of the vermis.– It receives input mainly from the vestibular system. – It is richly interconnected with UMNs of the medial

brainstem motor pathway, especially those in the vestibular nuclei.

– Some efferent fibers from the vestibulocerebellum also end on reticulospinal neurons.

– The pathways to the vestibular nuclei are via direct projections of Purkinje cell axons from the flocculonodular lobe and via efferent fibers from the fastigial nucleus.

– The vestibulocerebellum provides for coordination of movements involving whole body equilibrium and posture.

The Spinocerebellum– The spinocerebellum (paleocerebellum) is of intermediate

phylogenetic age, first appearing in amphibians. – Its inputs are mainly from proprioceptors and exteroceptors

of the limbs via spinocerebellar pathways. – It is richly interconnected, via efferent fibers from the

globose and emboliform nuclei (together called the interposed nucleus), with UMNs of the lateral brainstem motor pathway (the red nucleus).

– It also connects with reticulospinal neurons and sends information rostrally to the motor cortex via the ventrolateral (VL) nucleus of the thalamus.

– The spinocerebellum provides for coordination of limb movements for posture, progression, and other purposes. It also is important for "updating evolving movements," a role it shares to some extent with the cerebrocerebellum.

The Cerebrocerebellum – The cerebrocerebellum (neocerebellum), the

youngest part, first appears in mammals. – Its inputs arrive mainly from cerebral cortex via

the pontine nuclei. – Because of this fact it is also sometimes called

the pontocerebellum. – It is richly interconnected, mostly via the

ventrolateral (VL) nucleus of the thalamus, with UMNs of the corticospinal motor pathway.

– The cerebrocerebellum provides for coordination of independent limb movement and skilled movement, especially in terms of preprogramming movements.

                                                              

                                                                                            

Most Important Neuronal Connections

The following connections are most important: 1. Mossy fibers are excitatory to granule cells.

2. Climbing fibers are excitatory to Purkinje cells.

3. Both types of afferent fibers send collaterals to the CBL subcortical nuclei.

4. Granule cells, via their axons, the parallel fibers, are excitatory to Purkinje, basket, stellate, and Golgi cells. Glutamate is the neurotransmitter.

5. Each parallel fibers run longitudinally for several millimeters along a cerebellar folium, synapsing on a long strip of Purkinje cells.

6. Basket and stellate cells are inhibitory to Purkinje cells.

7. Axons of basket cells are perpendicular to the parallel fibers in the plane of the CBL cortex.

8. Purkinje cells are inhibitory to nuclear cells.

9. Nuclear cells are excitatory to their target cells (UMNs or thalamic neurons).

The Canonical Circuit– This is the basic circuit,

the canonical circuit, that exists in all parts of the cerebellar cortex.

– All the redish colored cells, the Golgi cell, the Purkinje cell, the stellate cell, and the basket cell, are inhibitory.

– They release GABA. – Only the granule cell and

the nuclear cells are excitatory.

                                                                                                                        

                               

Mossy Fibers Excite a Strip of Purkinje Cells

Climbing Fibers Climbing fibers originate from cells in the inferior olivary

nucleus and terminate with numerous, strong excitatory synapses on the cell body and dendrites of Purkinje cells.

Although the exact role of the climbing fibers is not understood, they are believed to play a role in motor learning. The frequency of firing of climbing fibers (as evidenced by an increased frequency of firing of complex spikes) increases when a subject encounters an unexpected load during a movement. Once the subject adapts to the load, or learns the new movement, the frequency of complex spikes returns to baseline level.

Function of the Cerebellum – All movements, postural or voluntary, of limbs, eyes, or

speech, require an appropriate pattern of activation of agonist and antagonist muscles or muscle groups. The relative timing of the bursts of activity of the agonists and antagonists determines the smoothness and coordination of the movement.

– The function of the cerebellum is to provide for smooth, coordinated, synegistic movement by adjusting the timing of the bursts of activity of agonist and antagonist muscles.

– The notion of the CBL as a regulator of timing is entirely compatible with the major clinical signs of CBL dysfunction, incoordination and loss of muscle synergy. These signs are quite different from the weakness and spasticity that occur with UMN problems.

Function of the Cerebellum – The neocerebellum adjusts (sets) this timing signal prior to a

voluntary movement and is said to be involved in "preprogramming" of movement parameters

– The spinocerebellum adjusts this signal as a consequence of information it receives about the movement via spinocerebellar pathways. Since the spinocerebellum also receives information ("efference copy") about the intended movement via the pontine nuclei, it is often considered to be a comparator.

– According to this formulation, the spinocerebellum compares the intended movement (information from cerebral cortex via the pontine nuclei) with the actual movement (information from the spinal cord via spinocerebellar tracts) and adjusts its clock signal to update the evolving movement to make it smooth and accurate.

Clinical Signs of Cerebellar Dysfunction

                                         

With one exception, all cerebellar signs reflect inaccurate, inappropriate, or absent clock signals necessary for smooth, coordinated, synergistic movement. The following common signs of CBL dysfunction are due to inappropriate timing signals:

1.Delay in movement initiation.

2.Dysmetria: alterations in the rate and force of a movement.

3.Asynergia: decomposition of movement.

4.Past Pointing.

5.Intention Tremor.

6.Dysarthria.

7.Dysdiadochokinesis. Another important sign of CBL dysfunction is hypotonia. This is due to a decreased firing frequency of the gamma motor neurons innervating the muscle spindles of the affected limb.

Depending on the distribution of CBL signs, the following two syndromes are often recognized:

Archicerebellar Syndrome: Incoordination of the muscles of equilibrium; dysequilibrium with wide-based gate; disturbances of stance and gait. Heel-to-toe walking is impaired.

Neocerebellar Syndrome: Incoordination of muscles for voluntary movements. Heel-to-shin movements and rapid alternating movements (diadochokinesis) are impaired.

Clinical Signs of Cerebellar Dysfunction• With one exception, all cerebellar signs reflect inaccurate,

inappropriate, or absent clock signals necessary for smooth, coordinated, synergistic movement.

• The following common signs of CBL dysfunction are due to inappropriate timing signals:

• Delay in movement initiation.

• Dysmetria: alterations in the rate and force of a movement.

• Asynergia: decomposition of movement.

• Past Pointing.

• Intention Tremor.

• Dysarthria.

• Dysdiadochokinesis.

• Another important sign of CBL dysfunction is hypotonia. This is due to a decreased firing frequency of the gamma motor neurons innervating the muscle spindles of the affected limb.

Depending on the distribution of CBL signs, the following two syndromes are often recognized:

Archicerebellar Syndrome: Incoordination of the muscles of equilibrium; dysequilibrium with wide-based gate; disturbances of stance and gait. Heel-to-toe walking is impaired.

Neocerebellar Syndrome: Incoordination of muscles for voluntary movements. Heel-to-shin movements and rapid alternating movements (diadochokinesis) are impaired.