Cerebellum:Anatomy,connections,&functions Dr.Rupjyoti Das
The cerebellum (Latin: "little brain") is a region of the brain that plays an important role in the integration of sensory perception and motor control. The cerebellum is located in the inferior posterior portion of the head (the hindrain), directly dorsal to the pons, and inferior to the occipital lobe & separated from cerebrum by tentoriumcerebelli. Weight=150 gm
On axial & coronal planes,a midline portion,thevermis, and two lateral portions, the cerebellarhemispheres,can be recognized. The vermis is developmetally older & receives mainly spinocerebellar afferents, whereas the hemispheres have more complex fibre connections.
Schematic representation of the major anatomical subdivisions of the cerebellum.
The cortex is convoluted into many folia. Three major transverse divisions (lobes) are recognized— The anterior lobe is most rostral, posterior lobe and flocculo-nodular lobe more caudally. These lobes are divided by the primary fissure and the posterolateral fissure, respectively. Larselldescribed 10 lobules (subdivisions of the lobes) that can be recognized in all animals. Lobules I-V are within anterior lobe, lobules VI-IX are in posterior lobe and lobule X comprises the floccular-nodular lobe.
Because of its large number of tiny granule cells, the cerebellum contains more than 50% of all neurons in the brain, but it only takes up 10% of total brain volume. The cerebellum receives nearly 200 million input fibres.
Anatomical division- : the anterior lobe (rostral to the "primary fissure"), the posterior lobe (dorsal to the "primary fissure") and the flocculonodular lobe, The first two can be further divided in a midline cerebellarvermisand lateral cerebellar hemispheres. The cerebellum can be divided according to three different criteria: gross anatomical, phyologenetical, and functional:
Classification by Phylogenetic and OntogenicDevelopment : Archicerebellum Paleocerebllum Neocerebellum Classification by Afferent Connection Vestibulocerebellum Spinocerebellum Pontocerebellum Classification by Efferent Connection Vermis Paravermal Region Cerebellar Hemisphere
From the phylogenetically oldest to the newest, the three parts are:
During the early stages of embryonic development, the brain starts to form in three distinct segments: the prosencephalon, mesencephalon, and rhombencephalon. The rhombencephalon is the most caudal (toward the tail) segment of the embryonic brain. Development of cerebellum
Along the embryonic rhombencephalic segment develop eight swellings, called rhombomeres. The cerebellum arises from two rhombomeres located in the alar plate of the neural tube, a structure that eventually forms the brain and spinal cord. The specific rhombomeres from which the cerebellum forms are rhombomere 1 (Rh.1) caudally (near the tail) and the "isthmus" rostrally (near the front).
The cerebellum is of archipalliarphylogenetic origin. The pallium is a term for gray matter that forms the cortex. The archipallium is the one of the most primitive brain regions. The circuits in the cerebellar cortex look similar across all classes of vertibrates, including fish, reptiles, birds, and mammals.
Subdivision ofFlocculonodular Lobe NodulusFlocculus Subdivision of Anterior Lobe Vermis Hemisphere Lingula Central Lobule Ala Central Lobule postcentral fissure CulmenQuadriangular Lobule Cerebellum-External configuration
The cerebellar cortex consists of three layers on a core of white matter. 1.Molecular layer Consists mainly of neuropil and is the site of synapses. Contains scanty neurons consisting of stellate and basket cells. 2.Purkinje cell (Piriform) layer Single layer of neurons. Consists of large (25 micrometer) pear-shaped neurons . 3.Granular cell layer Very small(7 micrometer) granular neurons. Very numerous – 3 to 7 million neurons per cubic mm. 4.White matter – forms the core of the foliae.
The white matter of the cerebellum is made up of intrinsic,afferent & efferent fibres. Incoming impulses to the cerebellum reach the dendrites and cell bodies of Purkinje cells. The afferent fibres form the greater part of the cerebellar white matter and on entering the cerebellum,segregate into one of three fibre systems: the climbing,mossy or multilayered.
The climbing fibresare the terminal fibres of the olivocerebellar tracts & make multiple synaptic contacts with one Purkinje cell. The mossy fibresystem includes all other cerebellar afferent tracts.In contrast to the climbing fibre system the mossy fibre system is diffuse,having multiple branches;so a single mossy fibre may stimulate thousands of Purkinje cells through the granule cell.
The multilayered fibresystem includes afferents to the cerebellum from the hypothalamus,raphe nuclei & locus ceruleus & projects into the cerebellar cortex & deep cerebellar nuclei.
In striking contrast to the 100,000-plus inputs from parallel fibers, each Purkinje cell receives input from exactly one climbing fiber; but this single fiber "climbs" the dendrites of the Purkinje cell, winding around them and making a large number of synapses as it goes.
The Purkinje cells are central neurons (everything else converges on them) They consist of= A large dendritic tree in the molecular layer, which is elaborately branched and fan-shaped (branches are all in one plane) and has dendritic spines at the sites of synapses. A large cell body. An axon which forms the efferent pathway from the cerebellum , and sends collaterals in the granular layer. GABA is the main neurotransmitter.
The Granule cells: Very numerous: 3-7 million / mm3 Very small (7mm), closely packed neurons. Heterochromatic nuclei, scanty cytoplasm. Small dendritic tree in granule layer. An unmyelinated axon. Directed to molecular layer (centrifugal). *Splits in T-shape manner to form parallel fibre. Parallel fibers run longitudinally along folia Cross dendrites of many Purkinje cells. Have glutamate as neurotransmitter.
Cerebellar Neurons are Stimulatory or Inhibitory to Purkinje Cells. Climbing fibres are strongly excitatory. Mossy fibres stimulate granule cells. Parallel fibres of granule cells stimulate several Purkinje cells simultaneously. Basket cells strongly inhibit Purkinje cells. Stellate cells inhibit Purkinje cell dendrites. Golgi Type II cells inhibit directly the mossy fibre input.
Embedded within the white matter—which is known as the arbor vitae (Tree of Life) in the cerebellum due to its branched, treelike appearance—are four deep cerebellar nuclei: From lateral to medial, they are the dentate, emboliform, globose, and fastigial.
Dentate n. Emboliform n. Globose n. Fastigial n.
These nuclei receive inhibitory (GABAergic) inputs from Purkinje cells in the cerebellar cortex and excitatory (glutamatergic) inputs from mossy fibre pathways. Most output fibers of the cerebellum originate from these nuclei.
The fastigial nucleus assists stance & gait & controls muscles only in the modes of sitting,standing & walking. The nucleus interposed assists segmental reflexes & speeds the initiation of movement triggered by somatosensory cues. The dentate nucleus assists in tasks requiring fine dextirity. Each nucleus controls a diff.type of movement as follows:
The cerebellum follows the trend of "threes", with three major input and output peduncles (fiber bundles). These are the superior (brachium conjunctivum), middle (brachium pontis), and inferior (restiform body) cerebellar peduncles.
Main Connections of the Vestibulocerebellum Vestibular Organ Floculonodular Lobe Vermis VESTIBULAR NUCLEUS vestibulospinal tract MLF FASTIGIAL NUCLEUS lower motor neuron ARCHICEREBELLUM LMN
Main Connections of the Paleocerebellum NUCLEUS INTERPOSITUS RED NUCLEUS RED NUCLEUS Rubro spinal tract ANTERIOR LOBE PARAVERMAL ZONE Inferior Olivry Nucleus PALEOCEREBELLUM Lower motor neuron SPINAL CORD spinocerebellar tract
Main Connections of the Neocerebellum CEREBRAL CORTEX THALAMUS DENTATE NUCLEUS Pontine Nucleus POSTERIOR LOBE CEREBELLAR HEMISPHERE Pyramidal tract NEOCEREBELLUM Lower motor neuron LMN
Cerebellum and Automatic Motor Control MOTOR CORTEX CEREBELLUM RED NUCLEUS VESTIBULAR NUCLEUS RETICULAR FORMATION LOWER MOTOR NEURON Proprioceptors
Corticonuclear Connections A zone ---------- fastigial nucleus medial vestibular nucleus B zone ---------- lateral vestibular nucleus C1, C3 zone --- emboliform nucleus C2 ---------------- globose nucleus D1 ---------------- parvocellular portion of dentate nucleus D2 ---------------- magnocellular portion of dentate nucleus Voogd originally described 4 zones, from medial to lateral--
The inferior olivary nucleus or inferior olive comprises 3 major divisions – the principal olive (PO), the dorsal accessory olive (DAO) and the medial accessory olive (MAO). Different divisions of the olive project to different cortical zones. The inferior olive is the only source of climbing fibre inputs to the cerebellum. Inputs from all other sources are as mossy fibres.
Olivocerebellar Connections Caudal portion of medial and dorsal accessory olivary nucleus ----------------- vermis of cerebellar cortex (A and B) fastigial nucleus vestibular nucleus Rostral portion of medial and dorsal accessory olivary nucleus ----------------- paravermal region (C1, C2, C3) nucleus interpositus Principal Inferior Olivary Nucleus ----------------- cerebellar hemisphere (D1, D2) dentate nucleus
Caudal portion Rostral portion Principal inferior olivary nucleus medial and dorsal accessory olivary nucleus
Vascular supply of the cerebellum: The posterior inferior cerebellar artery (PICA): supplies- Lat.medullarytegmentum, inferior cerebellar peduncle The ipsilat.portion of the inferior vermis & the inferior surface of the cerebellar hemisphere. The medial br. of the PICA supplies the medial cerebellum & the dorsolat. Medulla oblongata. The lateral br.supplies the inferoposterolat. aspect of the cerebellum.
The ant.inf.cerebellar artery (AICA) supplies- Ant.petrosal surface of the cerebellar hemisphere, Flocculus, Lower portion of the middle cerebellar peduncle & Lat.pontomedullarytegmentum
The superior cerebellar artery (SCA) supplies- The upper surface of the cerebellar hemisphere, Ipsilat.portion of the superior vermis, Most of the dentate nucleus, Upper portion of the MCP,SCP & lat.pontinetegmentum.
Maintenance of Equilibrium - balance, posture, eye movement Coordination of half-automatic movement of walking and posture maintenace - posture, gait Adjustment of Muscle Tone Motor Leaning – Motor Skills Cognitive Function Cerebellum:functions
Archicerebelum Floculonodular lobe- Vestibulocerebelum Function: Maintenance of equilibrium Suppress Vestibulo Ocular Reflex Muscle tone in relation to head posture Animal ablation Disorder of equilibrium Positional nystagmus Human: Meduloblastoma Trunkal ataxia Vestibular nystagmus (fast component towards the side of lesion) Positional nystagmus does not fatigue Vertigo 28 October 2010
Palleo-cerebelum Anterior lobe + Vermis Afferent: Spinocerebellar tract Spino-> olivo->cerebellar Spino -> reticulo -> cerebellar Efferent Festigio -> vestibulo -> spinal Festigio -> Reticulo -> Spinal Function Tone control Posture of axial muscle Equilibrium and locomotion Animal ablation Increased lengthening and shortening Increased tendon reflex Exagerated postural reflex (positive supporting reflex) Human Alcoholic degeneration Cerebellar degeneration Gait ataxia Rarely mild hypotonia, dysmetria and dysarthria 28 October 2010
Neocerebelum Ablation in Dog and Cat inconstant result Monkey ablation Hypotonia Clumsiness of ipsilateral limb Dentate nucleus ablation -> more enduring effect intension tremor 28 October 2010
Cerebellar zones Vermis zone: control posture, tone, locomotion, equilibrium Vestibular connection project to festigial nucleus Control position of the head in relation to trunk and extraocularmovments Intermediate zone Affrent- proprioceptive from limb Sensorimotor cortex Collateral from corticospinal tract Efferent globos and embodiform nucleus ->VL thalamus to motor cortex Function: Regulation of movment via sensory feedback from the corticospinal muscle Control velocity, force, pattern of movement 28 October 2010
Cerebellar zones: Lateral Afferent: motor and sensory association cortex Efferent: Dantate -> thalamus -> motor cortex (open loop) Function: Programing of movement before initiation Animal: coordination of ipsilateralmovment Human: Hypotonia- flabby muscle, abnormal posture (slopping of shoulder) increased excursion of outstretched hand tapping. Hyperflexibility of joint, pendular reflex knee Cerebellum control separately the activity of alpha and gamma motor neuron Reduced fusimotor activity from abnormal long loop reflex through precentral cortex 28 October 2010
Cerebellar dysfunction Ataxia: Limb, gait and speech Speech: Scanning Tremor: Postural, intention, limb, trunk and head Tone: Hypotonia -> Hyperextensibility, pendular knee jerk, rebound phenomenon Voluntary movement: Dysmetria, dysynergia, disdidokokinesia Gait: ataxic, truncal ataxia Head tilt Postural abnormality: due to unequal hypotonia of truncal muscle -> scoliosis, elevation or depression of shoulder, pelvic tilt 28 October 2010
Voluntary movement abnormality Gorden Holms : Rate, range and force Dyssynergia: Disruption of the normal smooth control of movement provided by gradual contraction of synergic muscle and relaxation of their antagonist Voluntary movement- longer to start and longer to stop Prolongation of interval between the command and triphasic agonist- antagonist and motor sequence Agonist burst may be too long or short or continue into the antagonist burst -> dysmetria and dysenergia Rebound phenomenon (abnormal check reflex) Dysdiadokokinesia Decomposition of movement Scanning speech Cogwheel eye moevment 28 October 2010
Voluntary movement abnormality Dysmetria Inability of the sensorimotor apparatus to measure distance in the course of movement Hypometria and hypermetria of the limb and eye Tremor : Postural and intention Ataxia of gait: falling towards the side of lesion Nystagmus: Gaze paretic (evoked) deviation Downbeat Rebound Sustained horizontal Opsoclonus Skew deviation Weakness, faitibability and loss of associted movement 28 October 2010