头晕相关脑解剖(小脑).doc

头晕相关的脑解剖Cerebellum Anatomy Relevant to DizzinessTimothy C. Hain, MD Marcello Cherchi, M.D., Ph.D. Page last modified: June 14, 2009 This page is meant to provide a general outline of cerebellar function. It is adapted from a clinical neuroanatomy lecture given on a yearly basis to Northwestern PT students. This page is under construction and should not be relied upon.The cerebellum with surrounding skull and spinal fluid occupies the bottom 1/3 of this axial MRI image.What is the cerebellum and what does it do ?The cerebellum is part of the brain. It lies under the cerebrum, towards the back, behind the brainstem and above the brainstem. The cerebellum is largely involved in coordination. Persons whose cerebellum doesnt work well are generally clumsy and unsteady. They may look like they are drunk even when they are not.Gross Anatomy: Cerebellar Hemispheres and Vermis Cerebellar hemispheres, each containing a lateral and an intermediate portion Vermis, situated between the two cerebellar hemispheres; is basically circular (interrupted by fourth ventricle ventrally) Gross Anatomy: Lobes & Lobules Anterior lobe: o 1. Lingula o 2. Central lobule o 3. Culmen Posterior lobe: o 4. Declive o 5. Folium o 6. Tuber o 7. Pyramis o 8. Uvula o 9. Tonsil Flocculonodular lobe: o (10.) Flocculus & nodulus Gross Anatomy: Fissures Primary fissure: separates anterior and posterior lobes Horizontal fissure: separates posterior lobes rostral lobules (declive, folium) from its caudal lobules (tuber, pyramis, uvula, tonsil) Posterolateral fissure: separates posterior and flocculonodular lobes Connections with brainstem: Superior cerebellar peduncle connects to midbrain Middle cerebellar peduncle connects to pons Inferior cerebellar peduncle connects to medulla Sections of the CerebellumVestibulocerebellum or archicerebellum Comprises the flocculonodular lobe Extensive connections with the vestibular system Phylogenetically oldest Spinocerebellum or paleocerebellum Comprises the vermis (medial) & paravermal (intermediate) region Extensive connections with the spinal cord & brainstem Phylogenetically of intermediate age Cerebrocerebellum or neocerebellum Comprises lateral portions of cerebellar hemispheres (excluding paravermal regions) Extensive connections with cerebral cortex through relay stations in cerebellar nuclei and dorsal thalamus Phylogenetically youngest CEREBELLAR PEDUNCLESSuperior cerebellar peduncle (brachium conjunctivum) Connects to midbrain Afferents: only ventral spinocerebellar tract Efferents: Most of the efferents from the cerebellum All of the efferents from three (out of four) pairs of nuclei: dentate, emboliform, and globose Middle cerebellar peduncle (brachium pontis) Connects to pons Afferents: from cerebral cortex (“corticoponto-cerebellar system”). Corticopontine projections (originating in the cerebral cortex) synapse in ipsilateral basal pons. From there, most pontocerebellar projections decussate, pass through middle cerebellar peduncle and enter cerebellum. A small number remain ipsilateral. Efferents: none. Inferior cerebellar peduncle (“corpus restiform” or “restiform body”)Connects to medulla Two components: Restiform body o Afferents: Ascending spinal proprioceptive fibers from three of the spinocerebellar tracts (dorsal, rostral, and cuneocerebellar) Ascending fibers from contralateral inferior olivary nuclei to cerebellar cortex (olivocerebellar projections) Reciprocal connections with motor reticular formation and spinocerebellum (paleocerebellum): reticulocerebellar and cerebelloreticular projections Juxtarestiform body o Mostly contains reciprocal connections to and from vestibulocerebellum (archicerebellum) and vermal portion of spinocerebellum (paleocerebellum): vestibulocerebellar and cerebellovestibular fibers Cerebellar NucleiFastigial nucleus Location: Roof of the 4th ventricle Phylogenetically oldest Associated with the vestibulocerebellum (archicerebellum) Afferents: o Vestibulocerebellum o Vermis Efferents: o Via inferior cerebellar peduncle o To the vestibular nuclei o Some project directly to the ipsilateral side o Some decussate, loop around superior cerebellar peduncle and reach the vestibular and reticular formation via the uncinate bundle of Russell Function: Controls antigravity muscles Controls proximal muscles of stance (station) & walking (ambulation) Globose and emboliform (the “interposed nuclei”) Location: Roof of the 4th ventricle, slightly lateral to the fastigial nuclei Afferents: Paravermian region of spinocerebellum (paleocerbellum) Efferents: Via superior cerebellar peduncle to contralateral red nucleus Function: o Mainly involved in modulation of stretch reflexes Distal muscles Dentate nucleus Location: Deep within cerebellar white matter Largest cerebellar nuclei Afferents: o Purkinje cells of entire cerebrocerebellum (neocerebellum) and part of spinocerebellum (paleocerebellum) o Premotor & supplementary motor cortices via pontocerbellar system Efferents: o Through superior cerebellar peduncle; cross to contralateral red nucleus & then on ventrolateral nucleus of thalamus Function: o Helps to initiate and control volitional movements Vestibular nucleus “Displaced” cerebellar nuclei Closely associated with vestibulocerebellum (archicerebellum) Function: Maintain gaze fixation Maintain upright posture CEREBELLAR Histology: Cortical Layers Molecular layer (outermost layer) contains: o Cell bodies: stellate cells, basket cells o Cell processes: dendrites of the Purkinje cells, dendrites of Golgi cells, axons (parallel fibers) of granule cells, axons of stellate cells o Purkinje layer (middle layer) contains: o Cell bodies: only Purkinje cells o Cell processes: dendrites of Golgi cells, axons of basket cells Granular cell layer (innermost layer) contains: o 1 x 1011 neurons (more neurons than entire cerebral cortex!) o Cell bodies: granule cells, Golgi cells o Cell processes: dendrites of Golgi cells, axons (Purkinje cell, granule cell, Golgi cell, climbing fibers, mossy fibers), glomeruli (bulbous expansions of mossy fibers where synaptic contact with granule cells and Golgi cells is made Histology: Cell Types Purkinje Granule Golgi (interneuron) Basket (interneuron) Stellate (interneuron) Purkinje CellsLargest cells in CNS (cell body = 60-90 m in diameter)Cell body In Purkinje cell layer Inhibitory synapse (GABA) from basket cells Excitatory synapse (aspartate) from a single climbing fiber Dendrite In molecular layer Dendritic arbor is “planar,” oriented in a rostrocaudal plane Inhibitory synapses (GABA & taurine) from stellate cells Excitatory synapses (glutamate) from granule cells parallel fibers. Over 200,000 mossy fibers indirectly (through Golgi cell, then granule cell) excite each Purkinje cell Axon The only projections that exit the cerebellar cortex Mostly project to cerebellar nuclei (to all four); some fibers project to vestibular nuclei in brainstem Forms inhibitory synapses (GABA) Often also releases a peptide cotransmitter Granule Cells1 x 1011 granule cells (more neurons than entire cerebral cortex!)Cell body Located in granule cell layer Inhibitory synapse (GABA) from Golgi cells Axon Projects from granule cell layer, through Purkinje cell layer, up to molecular layer, where it forms parallel fibers (which are oriented in a horizontal plane, parallel to the cerebellar folia) Forms excitatory synapses (glutamate) on Golgi, basket and stellate cell Golgi Cells Interneuron Cell body located in granule cell layer Dendrites in all three layers, receiving excitatory stimuli Granule cells parallel fibers form excitatory synapses (glutamate) in molecular layer Climbing fibers (from inferior olivary nucleus) form excitatory synapses (aspartate) in molecular layer Mossy fibers o form excitatory synapses (acetylcholine) in granule cell layer o arrive from spinocerebellar pathway, vestibular afferents, brainstem reticular formation, cerebropontocerebellar systems pontocerebellar projections o form the single largest input to cerebellum o indirectly excite Purkinje cells (in contrast to climbing fibers, which directly excite Purkinje cells) o Axons projects within the granule cell layer onto granule cells, forming inhibitory synapses (GABA), thereby acting as a “brake” to the excitation of granule cells; it curtails the duration of excitation Basket Cells Interneuron o Cell body located in molecular layer o Name derives from the fact that its axons form “basket-like” terminal arbors around soma of Purkinje cells Dendrites o Oriented in rostrocaudal plane (perpendicular to folia) o Receive excitatory stimuli (glutamate) from granule cells parallel fibers Axons o Oriented in rostrocaudal plane (perpendicular to folia) o Project from molecular cell layer to Purkinje cell layer o Form inhibitory synapses (GABA) with Purkinje cell body Stellate Cells Interneuron o Cell body located in molecular layer Dendrites o Oriented in rostrocaudal plane (perpendicular to folia) o Receive excitatory stimuli (glutamate) from granule cells parallel fibers Axons o Project within the molecular layer o Form inhibitory synapses (GABA and taurine) on Purkinje cell dendrites Cerebellar CircuitsMossy Fiber System Largest input to cerebellum (99%), most of which are from cerebropontocerebellar system (mostly from 1 & 2 motor areas and 1 somatosensory area) Form excitatory synapses (acetylcholine) Several afferents (spinocerebellar pathways, 1 & 2 vestibular afferents, brainstem reticular formation, cerebropontocerebellar systems pontocerebellar fibers) enter through all three cerebral peduncles Mossy fibers have various origins & projections: Vestibular afferents vestibulocerebellum & fastigial nucleus Spinal afferents spinocerebellum Globose & emboliform nuclei and pontocerebellar fibers cerebrocerebellum & dentate nucleus When a mossy fiber projects to the granular layer, it forms profuse synapses with dendrites of 500-600 granule cells Mossy fiber projections to spinocerebellum form ipsilateral somatotopic maps Multiple Homunculi /epob/epob3730rlynch/image/figure5-12.jpg/fpas/courses/physiology/neurophysiology/Cerebellum.htmClimbing Fiber System Climbing fibers (also called “olivocerebellar climbing fibers”) originate in the contralateral inferior olivary nucleus When a climbing fiber enters the cerebellum, it typically gives off a collateral branch to the cerebellar nuclei before projecting to the cerebellar cortex Axons project through granule cell layer into Purkinje cell layer, where each climbing fiber forms an excitatory synapse (aspartate) on soma of a single Purkinje cell The climbing fiber directly excites the Purkinje cell (in contrast with the mossy fibers, which indirectly excite the Purkinje cell) Stimulation of climbing fiber alters the responsiveness of the Purkinje cell to subsequent parallel fiber stimulation for long periods. This leads to various long-term responses, including protein phosphorylation reactions, calcium-calmodulin dependent protein kinases, and second messenger cascades. The long-term depression of Purkinje cells disinhibits neurons in the cerebellar nuclei, which in turn increase their basal firing rate and become more receptive to other excitatory signals (e.g., collateral from mossy fibers & climbing fibers). Due to these characteristics, the climbing fiber system is crucial in motor learning. Monoaminergic Fiber SystemThe monoaminergic projections to cerebellum send fibers to all three layers of cerebellar cortex. The number of monoaminergic projections is small. Function is relatively poorly understood. Projections include: Raphe nuclei send serotoninergic projections. Appears to inhibit Purkinje cells. Locus ceruleus sends noradrenergic projections. Appears to inhibit Purkinje cells. Dorsomedial nucleus of the hypothalamus sends histaminergic projections. May be involved with coordination of somatomotor & visceromotor functions. Cerebellar Arterial VasculatureSCA (superior cerebellar artery) Branches off distal basilar artery Supplies the bulk of the cerebellum and peduncles Follows pontomesencephalic border Gives off branches to tectum of the lower midbrain and superior cerebellar peduncles These vessels travel with peduncular fibers to deep nuclei (primarily the dentate) Supplies ventral vermis and paravermis Continues on to supply rostral vermis and rostroventral portions of both hemispheres Crosses tentorial margin en route to dorsal and lateral parts of rostral hemispheres These vessels are therefore vulnerable to compression (resulting in infarction of rostral cortex) AICA (anterior inferior cerebellar artery) Branches off proximal basilar artery Supplies flocculus and adjacent convolutions Internal auditory arteries are usually a branch of these Supplies pyramis, tuber, flocculus and portions of inferior surface of the cerebellar hemisphere PICA (posterior inferior cerebellar artery) Branches off distal vertebral artery Some branches go to dorsolateral medulla Supplies inferior cerebellar peduncle, inferior part of middle cerebral peduncle, and inferior part of cerebellum Medial branch supplies the inferior vermis (especially nodule and uvula), part of the dentate, the tonsil, and the choroid plexus of 4th ventricle Lateral branch supplies inferolateral surface of cerebellar hemisphere Clinical Correlates of Cerebellar diseasePostural instability Static (e.g., during standing). Patient cannot maintain steady position (of limbs, trunk, head, eyes) Broad stance Dynamic (e.g., during ambulation) Difficulty walking in a straight line Staggers “as if drunk” Usually reflects dysfunction of vestibulocerebellum Maneuvers: Walking (particularly tandem gait) Romberg Other Clinical Correlates/neuro/cerebell.htm Delayed initiation and termination of movement Slow to initiate a movement on command Tendency to overshoot a target Usually reflects dysfunction of cerebrocerebellum Maneuvers: o Checking o Rebound o Inability to perform continuous, repetitive movements o Poorly executed alternating movements o Irregular rhythm o Usually reflects dysfunction of the cerebellar hemisphere Maneuvers: o Seek dysdiadochokinesis (e.g., thigh patting) o Seek dysrhythmokinesis (e.g., finger or toe tap) Errors in smoothness and direction of movement Overreaching a target Underreaching a target Usually reflects dysfunction of the cerebellar hemisphere Maneuvers: o Finger to nose & heel to shin (seeking dysmetria