The cerebellum means the “little brain” (when translated from Latin) and is nestled behind the “big brain” (i.e., the cerebrum) within the skull. The cerebellum traditionally was considered to be responsible primarily for the coordination of movement and motor learning. However, more recent anatomical and functional studies indicate that the cerebellum (especially in humans) plays a wider role in many cognitive functions, such as language, executive functions, and spatial cognition. In addition, a number of neurologic and psychiatric conditions have been associated with cerebellar dysfunction, including autism, attention deficit hyperactivity disorder, mood disorders, and schizophrenia. In humans, the cerebellum is a highly convoluted structure; in mice its gross structure is somewhat less complex (central part of
Figure). However, in both humans and mice, the cellular organization is relatively simple. The cortex of the cerebellum is composed of three layers of neurons: the molecular layer, the Purkinje cell layer, and the granule cell layer. The cell bodies of the Purkinje neurons (in the Purkinje cell layer) extend long, elaborate, graceful dendrites into the molecular layer, as shown in the Golgi-filled Purkinje cell on the right. The major inputs into the cerebellum come from the inferior olive neurons and from neurons in the spinal cord and brain stem that send axons called mossy fibers into the cerebellar cortex. The mossy fibers synapse on the granule cells and the climbing fibers from olivary neurons wrap around the Purkinje cell dendrites as they climb into the molecular layer. The Purkinje cells are the only direct output neurons from the cerebellar cortex, sending inhibitory projections primarily to the deep cerebellar nuclei and a few extracerebellar regions, including the vestibular nuclei. Through indirect pathways, the cerebellum receives information from all sensory modalities, including auditory, visual, somatosensory, and proprioceptive systems as well as input from the neocortex. In turn, the cerebellum sends information indirectly throughout the brain. One important projection pathway leads to the thalamus and from there to the motor cortex. Functional neuroimaging studies in humans and physiological studies in rats and mice are revealing new insights into cerebellar function. One thing that is becoming clear is that the cerebellum plays a more complex role in how the brain functions than previously thought.
