Cerebellum

Cerebellum anatomy in suites of three

1. 3 lobes: anterior, posterior, flocculonodular 2. 3 cortical layers: molecular, Purkinjie, granular 3. 3 pairs of deep nuclei: fastigial, interposed (globose, emboli form), and dentate. 4. 3 peduncles: superior, midline, inferior 5. 3 functionally distinct subdivisions: vestibulo-, spino- and cerebero-

Ventral Spinocerebellar tract

1. Also deliver proprioceptive input, but it does so only during active (voluntary) movement 2. In addition, it appears to carry internally generated motor information about the movement. 3. It crosses once in the spinal cord and enters through the superior cerebellar peduncle and crosses again in the cerebellar white matter; a double crossed pathway. It's afferents enter via the scp - which is otherwise predominantly efferent.

Principal Circuits

1. Climbing fiber (+) -> Purkinje neuron (-) -> Deep cerebellar nut. or vestibular nuclei 2. Mossy fiber (+) -> Granule neuron (+) -> Purkinje neuron (-) -> Deep cerebellar nuc. or vestibular nuc.

Inferior olive: source of climbing fibers

1. Consists of a gray (cellular) folded lamina arranged in the form of an incomplete capsule, opening medially in an aperture called the hilum. 2. Olivocerebellar fibers are neurons that have their cell bodies in the inferior olivary nucleus. Their axons leave medially through the hilum, cross the midline, and ascend into the cerebellum via the icp. 3. Once they enter the cerebellum, they are referred to as the climbing fibers. 4. Although the IO lies in the medulla oblongata, and receives input from the spinal cord, brainstem, and cerebral cortex, its output goes ENTIRELY to the cerebellum.

Dorsal Spinocerebellar tract

1. Conveys unconscious proprioceptive feedback about the status of muscles during movement. Information flows whether the limbs are moved passively or voluntarily. 2. Information ascends to the cerebellar cortex UNCROSSED via the icp.

Spinocerebellar tracts: unconscious proprioception

1. Dorsal spinocerebellar axons from the lower extremity start in the gracile fasciculus, but split off to synapse in Clarke's nucleus/column, and then ascend (uncrossed) as the dorsal spinocerebellar tract and enter the cerebellum via the icp. Region of innervation: ipsilateral caudal aspect of the body and legs 2. Cuneocerebellar tract; other somatic sensory route from the upper extremity ( ipsilateral arm); it ascends ipsilaterally to the accessory cuneate nucleus and also enters via icp.

Deep Cerebellar nuclei

1. Fastigial nuclei: receive spinocerebellar (vermis) output (trunk) 2. Interposed (globose and emboliform): receive output from lateral spinocerebellum (paravermis) Limbs. 3. Dentate nuclei: receive the output of the cerebrocerebellum 4. Exception: Vestibular nuclei: receive output of the flocculonodular lobe (vestibulocerebellum) and can be thought of as the 4th deep cerebellar nuclei that lie in the medulla rather than in the cerebellum.

Interneuronal circuits

1. Granule (+) ->Basket cell (-) -> Purkinje 2. Granule (+) -> Stellate cell (-) -> Purkinje 3. Granule (+) -> Golgi cell (-) -> Granule neuron

Spinocerebellum

1. Highly organized somatosensory input from the spinal cord - the spinocerebellar tracts are the "subconscious" counterpart to the "conscious" dorsal column system 2. Pathway begins with spinal interneurons and reaches the vermal and paravermal cerebellum as mossy fibers that influence Purkinje cell output via granule cell axons (parallel fibers) 3. Contains topographic somatosensory maps that can be discerned electrophysiologically. 4. Refines movement through proprioceptive and special sensory input 5. Lesions of the interposed nuclei result in positive signs: cerbellar hypotonia, dysmetria, joint ataxia, terminal tremor, and pedular reflexes (slow to stabilize)

Cerebrocerebellum

1. Involved in planning movements, evaluating sensory input and modulating motor output accordingly. 2. relative hypertrophy in great apes and humans is indicative of its recent phylogenic expression 3. Receives ONLY cortical input (via pontine nuclei) through the mcl. 4. Drive by the cortex, the pontine nuc. provide collaterals to the dentate nut. and continue to terminate as mossy fibers in the lateral cerebellar cortex that, (via granule cell axon parallel fibers) innervate Purkinje dendrites. 5. Purkinje cells then, project back to inhibit the dentate nucleus. Lesions disrupt motor planning and prolong reaction time

Cerebellar cortex: Layers

1. Molecular layer: composed of the vast mantle of axons (parallel fibers from granule cells) of the granule cells and the dendrites of the Purkinjie cells. Inhibitory stellate and basket cells are also present. 2. Purkinje layer: contains the Purkinje cell bodies whose dendrites receive the axon terminals of the parallel fibers ( from granule cells) 3. Granule cell layer: enormous population of granule cells with a few Golgi cells

Synaptic relationships - mossy fiber rosette (glomerulus)

1. Mossy fibers are the sensory inputs to the cerebellum 2. Excitatory mossy fiber axon terminal contacting many dendrites of granule cells. Granule axons form the parallel fibers. 3. Golgi cell axons also contact the granule cell dendrites, providing inhibition. 4. Extensive branching in white matter and synapses to multiple granule cells ensures that input from a single mossy fiber axon will influence processing in a very large number of Purkinje cells.

The Circuitry: Afferent inputs - sensory and motor

1. Mossy fibers: from the spinal cord, pons, reticular formation excite granule cells, whose axons(parallel fibers) branch in bundles called "beams" across the folia to excite hundreds of Purkinje cells (simple spieks at high frequency). Sensory 2. Climbing fibers from the inferior olive - branch in the sagittal plane (orthogonal to the parallel fibers) and excite about 10 Purkinje cells creating (complex spikes at very low rates). Motor

Cerebrocerebellum: thalamic and rubral projections

1. Most dentate axons leave the cerebellum via the scp and have 2 principal targets: 1. Contralateral ventrolateral thalamus, pars caudalis (VLc) - which also receives spinocerebellar input from the interposed nuclei. 2. Parvocellular (small-cell) domain of the red nucleus - the part of red nuc. that does NOT get input from the interposed nuc. Thus, the cerebellum projects to the red nucleus, which projects to the IO, which in turn projects back to the cerebellum as climbing fibers - completing a re-emergent motor feedback loop

Excitatory and Inhibitory loop

1. On their way to the cerebellar cortex, climbing and mossy fibers excite the deep cerebellar nuclei via axon collaterals. 2. Their excursion through the cerebellar cortex elicits a descending, inhibitory influence via Purkinje cells onto the same deep nuclei - this inhibition is the sole output of the cerebellar cortex 3. These serial feedforward excitatory and inhibitory loops are the essence of the cerebellar circuit architecture

Afferent inputs - simple and complex spikes

1. Parallel fibers form synapses on distal branches of Purkinje cell dendrites. Simple spikes IIIIII 2. Climbing fibers make some of the most profound excitatory axosomatic contacts in the brain. Complex spikes of climbing fibers. IIIAAII

Nucleus of Clarke - dorsal spinocerebellar tract

1. Seen in lumbar cord - the nucleus (or column) of Clarke is the first relay in the spinocerebellar pathway: the column runs from C8 to about L2. 2. Clarke's nucleus gives rise to the dorsal spinocerebellar tract which enters via the icp.

Cerebello-cortical feedback

1. Spinocerbellum influences motor cortex via the red nucleus and the ventrolateral (VL) nucleus of the thalamus 2. Cerebrocerebellum - driven by corticopontine input, feeds back to motor cortex (also via the VL nucleus of the thalamus).

3 Functional Organization

1. Spinocerebellum: "paleo" vermis and intermediate hemisphere 2. Cerebrocerebellum: "neo" lateral hemispheres 3. Vestibulocerebellum: "archi" flocculonodular

Cerebellar Peduncles

1. Superior (scp) - mostly efferent axons; originating in the deep cerebellar nuclei - targeting midbrain, thalamus and forebrain motor centers. Afferent axons: proprioceptive afferents of the ventral spinocerebellar tract run in the scp 2. Middle (mcp) - only afferent axons (cortico-pontocerebellar fibers) 3. Inferior (icp) - afferent axons: (olivo-, vestibulo-, dorsal spino- and cuneocerebellar fibers). Efferent axons to the vestibular nuclei

Vestibulocerebellum (gravity and balance)

1. The flocculonodular lobe receives input from the vestibular labyrinth (semicircular canals and otolith organs) of the inner ear via mossy fibers from the vestibular nuclei through the inferior cerebellar peduncle. 2. this informs the CNS of the position of the head with respect to gravity and about its relative motion 3. Visual input via mossy fibers from the superior colliculus and visual corticopontine projectsions converge to coordinate eye movements 4. Purkinje cell output inhibits the lateral and medial vestibular nuclei 5. Enables postural adjustments via extensor muscle groups in the lower extremity 6. Lesions of these pathways disrupts the ability to use vestibular feedback to sustain balance and coordinate gaze - patients compensate with a wide stance and often stumble or fall.

Sensory: Spinocerebellar System

1. While both the dorsal and ventral spinocerebellar tracts convey proprioceptive inputs from the trunk and lower extremity to the cerebellum, there are significant differences. In addition: cuneocerebellar and rostral spinocerebellar. 2. These parallel (unconscious) pathways to the cerebellum with proprioceptive afferents provide sensory feedback and enable refinements to movement.

Vestibulocerebellum

1. flocculonodular lobe 2. receives vestibular, proprioceptive, and visual input and helps control balance and eye movements 3. via the vestibular nuclei

Cerebellum Conundrum

1. function is not thoroughly understood 2. not essential for movement nor critical for survival 3. receives inputs from all modalities except taste, olfaction 4. receive massive input from the spinal cord but does not directly project to it 5. projects to subcortical midbrain, thalamic and forebrain motor nuclei 6. all below conscious perception 7. Its output is entirely inhibitory 8. Damage to cerebellar circuits results in positive signs: -resting tremor -rigidity -inaccurate motor targeting

Cerebrocerebellum

1. lateral hemispheres 2. receives only corticopontine (cerebral) input and is involved in motor planning and sensory testing 3. projects to dentate nuclei

Red Nucleus (rubrospinal system)

1. thought to be largely vestigial, having given most of its motor control responsibilities to the corticospinal tract. Magno and parvo part - mango div. projects as far as the mid-thoracic spine. 2. Arm swinging while walking and the arm movements of a baby crawling are thought to be rubrospinal pattern mvmt. 3. Red nuc. receives many inputs from the contralateral cerebellum (interposed nucleus and lateral cerebellar nucleus) and an input from the ipsilateral motor cortex. 4. Most red nuc. axons do not project to the spinal cord, but instead (via its parvocellular part) relay info from the motor cortex to the cerebellum through the IO complex of the medulla.

Spinocerebellum

1.Vermis and Paravermis - (intermediate hemisphere) lies in the midline 2. Receives spinal, trigeminal as well as auditory and visual input and regulates the precision of body movements 3. via fastigial nuclei

Rostral Spinocerebellar

Golgi tendon organ input only - much smaller than the ventral tract (uncrossed).

Cerebellar cortex: 3 Layers + 5 cell types

Layers: molecular, Purkinjie, granule Cells: 1. Granule 2. Stellate 3. Basket cells 4. Golgi cells 5. Purkinjie cells Granule cells are very small excitatory interneurons - the other 4 are all inhibitory. Granule sells make up HALF of the neurons in the brain

Spinocerebellar tracts, uncrossed, and double crossed

Uncrossed: Cuneocerebellar, Rostral spinocerebellar, dorsal spinocerebellar Double crossed: Ventral spinocerebellar.

Cuneocerebellar

Upper limb mechanoreceptor input is primarily muscle stretch but some tendon strain receptors (uncrossed).