Lecture 15: Cerebellum The cerebellum consists of two hemispheres and a medial area called the vermis. The cerebellum is connected to other neural structures by three pairs of peduncles. The figure shows a dorsal view of the cerebellum, and the peduncles and cerebellar nuclei are obscured (shown by black areas). Vermis Hemisphere Peduncles Dentate nucleus Interposed nucleus (emboliform and globose nuclei) Fastigial nuclei

Cerebellar Lobes Primary fissure Anterior lobe Posterior lobe Posteriorlateral fissure Flocculonodular lobe Nodulus Flocculus The cerebellum is divided into three lobes: the anterior lobe, the posterior lobe, and the flocculonodular lobe.

Somatotopical Projections on the Cerebellum

Neurons of the Cerebellum Output (to cerebellar nuclei and then to thalamus, brain stem, and vestibular nuclei) White matter Molecular layer Purkinje cell layer Granular layer Basket cell Granule cell Climbing fiber Mossy fiber Golgi cell Stellate cell The cerebellar cortex consists of three layers and five types of neurons. Inputs to the cerebellum are carried by mossy fibers (from the inferior olive) and by climbing fibers (from pontine nuclei, the vestibular system, and the spinal cord). The only output system of the cerebellum is the axons of Purkinje cells.

Cerebellar Neurons

A Glomerulum Mossy fiber Golgi cell axon Granule cell dendrites Rosette A single glomerulum consists of an incoming mossy fiber, clusters of small dendrites (called rosettes) from a few dozen granule cells, and the axons of the Golgi cells.

Inputs Into the Cerebellum Mossy fibers (spinocerebellar tract and brain stem nuclei) Granule cells Climbing fibers (inferior olive) Purkinje cells Excitatory inputs to the cerebellum are provided by mossy fibers and climbing fibers. The mossy fibers originate in the spinocerebellar tract and in brain stem nuclei; they excite granule cells. The climbing fibers originate in the medulla (the inferior olive); they make synapses on Purkinje cells.

Simple and Complex Spikes Simple spike Complex spike In response to a single excitatory stimulus, a Purkinje cell may generate a single action potential (a simple spike, in response to mossy fiber input), or a larger action potential, followed by a few smaller action potentials (a complex spike, in response to a signal from climbing fibers).

Wiring of the Cerebellum Stellate cells make inhibitory synapses on the dendrites of Purkinje cells. Parallel fibers activate Purkinje cells, basket cells, stellate cells, and Golgi cells. Basket cells inhibit relatively distant Purkinje cells. Golgi cells inhibit granular cells, decreasing their response to mossy fibers. Glomeruli Parallel fibers Golgi cells Basket cells Purkinje cells Distant Purkinje cells Stellate cells Granular cells Mossy fibers Climbing fiber

Neuronal Population Vectors of the Purkinje Cells and Neurons in the Cerebellar Nuclei

Cerebellar Memory? If an action potential in a climbing fiber and another action potential in a parallel fiber arrive simultaneously at a Purkinje cell, the cell may “remember” this event with the help of a chemical mechanism, changing the synaptic efficacy. Climbing fibers Parallel fibers Purkinje cell