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Cerebellum, Basal Ganglia and the Motor Cortex

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Cerebellum, Basal Ganglia and the Motor Cortex Voluntary movement: Cerebellum, basal ganglia and the motor cortex 1. Voluntary movement: Organization and major pathways 2. Extrapyramidal system of movement - Basal ganglia 3. Cerebellum - Functional organization of the cerebellum - Microanatomy of the cerebellar cortex - Principles of function of the cerebellar cortex - The cerebellum in motor learning Voluntary movement Control of subcortical motoric centers by the cortex Sulcus centralis 4 6 Voluntary movement Control of subcortical motoric centers by the cortex primary motor cortex (area 4) → lowest intensity of stimula- tion elicits movement 4 6 premotor cortex (area 6) Voluntary movement Somatotopic organization of the motor cortex Sensory homunculus Motor homunculus Voluntary movement Major pathway of voluntary motor information: Corticospinal tract (Tractus corticospinalis, “Pyramidenbahn”) Massive bundles of fibers (approx. 1 million of axons) Originates from primary motor cortex, premotor cortex and somatosensory cortex Somatotopic organization Main tract crosses at the medulla (pyramidal decussation) and descends in the lateral column (Tractus corticospinalis lateralis) Most end at interneurons between dorsal and ventral horn Voluntary movement Only 70-90% of the axonal fibres cross at the pyramidal decussation Remaining uncrossed fibers descend in the ventral column (Tractus corticospinalis anterior) and cross at the position of their endings Tractus corticospinalis lateralis Tractus corticospinalis Control more anterior distal limb muscles Control of posture by integrating →important for visual, vestibular and goal-directed somatosensory information movement Voluntary movement Corticospinal tract usually ends at Zona intermedia between ventral and dorsal horn at interneurons Minority directly contact motoneurons at the ventral horn (in most cases motor neurons that innervate flexors of the distal limb) → direct control by the corticospinal tract Extrapyramidal system of movement Corticospinal tract is phylogenetically young → older system: „extrapyramidal motor system“ → Involves group of neurons (basal ganglia) together with integrating centers, in particular the cerebellum Function: regulation of involuntary movements for keeping posture and trained movements Acts as a „servo-mechanism“ for voluntary movement and provide feed- back loops → required for „smooth“ movement Extrapyramidal system of movement Basal ganglia Group of nuclei which have a high content of iron: Striatum (putamen and caudatum) Pallidum Nucleus subthalamicus Nucleus ruber Substantia nigra Belong to different parts of the brain: telencephalon (Striatum), diencephalon (Pallidum, N. subthalamicus) brain stem (N. ruber, S. nigra) Extrapyramidal system of movement Basal ganglia Highly connected with each other and the cortex and cerebellum Pallidum Striatum (caudatum) Striatum (putamen) Nucleus subthalamicus Nucleus ruber Substantia nigra Striatum: integration unit of the extrapyramidal motor system Characterized by the presence of striasomes (patches) which stain for neuropeptides and opioid receptors and a matrix which stains for acetylcholinesterase Extrapyramidal system of movement Connections of the basal ganglia (schematic representation) Nigrostriatal pathway Reciprocal coupling between - Striatum and S. Nigra - Pallidum and N. subthalamicus Extrapyramidal system of movement Extrapyramidal syndromes Lesions in the basal ganglia lead to characteristic disturbances of movement: e.g., Lesion of the N. subthalamicus: Hemiballism – involuntary (often violent) movement of the limbs e.g., Degeneration of the Striatum: Chorea Huntington – involuntary movement, cognitive impairment (dementia) genetic disease Parkinson‘s disease Most frequent disease of the motoric system (incidence: 1-5%) paucity of spontaneous movement („Maskengesicht“), increased muscle tone (rigidity), characteristic tremor at rest („Schüttellähmung“) Extrapyramidal system of movement Parkinson‘s disease Selective loss of dopaminergic neurons in Substantia nigra Nigrostriatal pathway X Cerebellum Latin: „little brain“, constitutes 10% of the brain volume but 50% of its neurons Neurons are arranged in highly regular manner as repeating units → basic circuit modules Major output: premotor and motor cortex, basal ganglia of the brain stem Many parallel convolutions called folia („leaves“) Connected to the brain stem via pedunculi cerebellaris („Kleinhirnstiele“) Cerebellum Somatotopy of sensory input and motor output in the cerebellar cortex cerebellar vermis Cerebellum Cells of the cerebellar cortex Neurons are highly ordered and organized in repeat units → „modules“ Only five types of neurons: - Stellate neurons (“Sternzellen”) - Basket neurons (“Korbzellen”) Inhibitory neurons - Purkinje neurons - Golgi neurons - Granule cells (“Körnerzellen”) Excitatory neurons Dendritic spines Cerebellum Organization of the cerebellar cortex Organized in 3 layers: Molecular layer: cell bodies of stellate and basket cells, axons of granule cells (oriented as parallel fibers along the “folia”), dendrites of Purkinje cells (oriented perpendicular to the parallel fibers) Purkinje cell layer: single layer of Purkinje cell bodies, axons project to white matter Granule layer: many granule cells and some Golgi neurons Cerebellum Input connections of the cerebellar cortex Two main types of inputs: - input from mossy fibers - input from climbing fibers Cerebellum Input connections of the cerebellar cortex Two main types of inputs: - input from mossy fibers - input from climbing fibers Input from mossy fibers (“Moosfasern”): - excitatory input - originate from nuclei in the spinal cord and brain stem - terminate on dendrites of granule and Golgi cells in the granular layer Input from climbing fibers (“Kletterfaser”): - excitatory input - originate from a nucleus in the medulla (inferior olivary nucleus) - wrap around cell bodies and proximal dendrites of a single Purkinje neuron making numerous contacts and, via axon collaterals, to some stellate and basket cells Cerebellum Connections within the cerebellar cortex 1. Axons of granule cells travel to molecular layer and excite as “parallel fibers” many Purkinje neurons in the same transverse plane 2. Basket cells and stellate cells make inhibitory contacts to Purkinje neurons, thus producing an inhibitory side- loop Cerebellum Output connections of the cerebellar cortex Axons of the purkinje neurons project into the white matter (deep nuclei of the cerebellum) and provide the (entirely inhibitory) output of the cerebellar cortex (mediated by GABA) Cerebellum Principles of function of the cerebellar cortex I Incoming signals from mossy fibers (from spinal cord and brain stem) synapse at dendrites of granule and golgi cells Parallel fibers of granule cells excite only one row of purkinje cells Golgi cells Golgi cells are much larger than granule cells and have dendrites filling a large volume in all directions → are excited in a larger volume and locally inhibit (via short axons) neighboring purkinje cells → Enhancement of contrast Cerebellum Principles of function of the cerebellar cortex II Inhibitory interneurons Neurons of the subcortical nuclei are excited by axon collaterals of climbing and mossy fibers but strongly inhibited by Purkinje neurons. Thus, the transmission of signals can only occur, if Purkinje neurons are inhibited by inhibitory interneurons. Neurons of subcortical nuclei Climbing fiber Mossy fiber Cerebellum Principles of function of the cerebellar cortex II Cerebellum Principles of function of the cerebellar cortex III Neighboring purkinje cells receive signal of the same granule cell via the parallel fiber with increasing delay (speed of conductance of a parallel fiber: 0.2 m/s; corresponds to about 0.1 ms for every neighboring Purkinje cell) → Temporal correlation of signals can be determined and movement (activation of muscle fibers) can be segmented (there are often difficulties with cerebellar lesions) Cerebellum The cerebellum in motor learning e.g. vestibulo-ocular reflex: ensures that the eyes can fix a target when turning the head. When wearing prismatic glasses, the reflex turns after a learning phase. Destruction of the vestibulocerebellum prevents this adaptation Cerebellum The cerebellum in motor learning Simple model system: Rabbit eyelid conditioning Conditioned Mossy fiber tone Stimulus (CS) Cerebellar cortex Unconditioned Climbing fiber puff Stimulus (US) Learning depends on the interstimulus interval (ISI): Deep nuclei of the cerebellum (Interpositus nucleus) (from: Ohyama et al. (2003) What the cerebellum computes. Trends Neurosci. 26: 222-227) Cerebellum The cerebellum in motor learning Rabbit eyelid conditioning Cerebellar cortex Mossy Climbing fiber fiber + + Puff (US) - (CS) tone + + Deep nuclei of the cerebellum (Interpositus nucleus) Cerebellum The cerebellum in motor learning Rabbit eyelid conditioning Parallels between cerebellum- and amygdala-dependant conditioning Javier F. Medina, J. Christopher Repa, Michael D. Mauk & Joseph E. LeDoux. Nature Reviews Neuroscience 3, 122- 131(2002).
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