Introduction
The cerebellum, which in Latin means "little brain", makes up only 10% of the total brain yet has the same amount of cells as the rest of the brain combined. The cerebellum is a prominent structure that lies behind the spinal cord where it is almost completely covered by the cerebral cortex. The cerebellum receives input from the cerebral cortex, the basal ganglia, the reticular system, nuclei dispersed throughout the upper spinal cord, and from spinal pathways containing sensory inputs from muscles spindles, the tendons, and the joints. The cerebellum continually receives information about the activity of skeletal muscles as well as receiving information from all areas of the brain that regulates these muscles.
Some of the brain regions in which the cerebellum exerts an important influence over include: the reticular system and vestibular nuclei, thereby greatly modifying the activity of the motor neurons that control postural muscles and muscle tone and the entire motor system which is involved in the execution of rapid, skilled movements.
There are three fiber bundles called superior, middle, and inferior peduncles that connect the cerebellum to the brainstem. The superior peduncle originates in the deep cerebellar nuclei and carries most of the output from the cerebellum. It brings information to the red nucleus, the thalamus and the medulla. The middle pedluncle is responsible for input to the cerebellum and carries cerebellar afferents called the mossy fiber system coming from the pontine nuclei. The inferior peduncle carries mostly cerebellar input as well. It carries information from the inferior olive, cortex, spinal cord and the brainstem to the vestibular nuclei and to all of the cerebellum.
The vestibular nuclei along with the deep cerebellar nuclei are responsible for all the output of the cerebellum. The deep cerebellar nuclei consists of three nuclei: the dentate nucleus, the interposed nuclei (which is composed of the emboliform and globose nuclei), and the fastigial nucleus. These three act as a relay to pass information to other parts of the brain.
The cerebellum is divided functionally into three longitudinal zones. A thin segment in the midline, which looks like a worm, is known as the vermis (Latin for worm). The vermis projects to the fastigial nucleus and controls trunk movements. The paravermis, or the intermediate zone, is located on either side of the vermis. The paravermis projects to the interposed nuclei and controls ipsilateral limb movement. The lateral hemispheres, which are located on either side of the cerebellum, project to the dentate nucleus and control motor planning.
Two fissures, the primary fissure and the posteriorlateral fissure, create the three anatomical divisions of the cerebellum. There is the anterior lobe at the top of the cerebellum, the posterior lobe below that, and the flocculonodular lobe at the bottom. The flocculonodular lobe projects to vestibular nuclei outside the cerebellum and is concerned primarily with vestibular functions (posture and eye movements). The anterior and posterior lobes are further divided into lobules by shallower fissures. There appears to be branches in sections of the cerebellum with small offshoots called folia.
The three functional divisions of the cerebellum define how it works. The first division is the spinocerebellum, which consists of the vermis and the paravermis. The next division is the vestibulocerebellum, made up of the flocculonodular lobe. Finally, there is the cerebrocerebellum comprised of the lateral hemispheres. It is easier to study their distinct functions when looking at the consequences of damage to certain areas of the cerebellum and thus will be explored later in this section.
There are five types of cells in the cerebellum: Purkinje cells, granule cells, basket cells, stellate cells, and golgi cells. There are l5 million Purkinje cells found in the human cerebellum. Their cell bodies are about 50-80mm (micrometers) in size. They have extensive arborization in only one plane (two dimensional). They use the neurotransmitter GABA and therefore are inhibitory. Their axons are myelinated.
The granule cells amounting to 1011, exceeds the total number in the cerebral cortex. They are about 5-8mm in size and each have 4-5 dendrites. They are excitatory and use the neurotransmitter glutamate. The basket cells inhibit Purkinje cells and synapse on their cell bodies. Stellate cells also inhibit Purkinje cells but they synapse on dendrites instead. The golgi cells inhibit granule cells. The basket cells, stellate cells, and the golgi cells are all interneurons.
These cells are found in different layers of the cerebellum along with three different fibers: climbing fibers, parallel fibers, and mossy fibers. The inner granule layer contains granule cells which receive the terminal arborizations from the mossy fibers. Mossy fibers originate external to the cerebellum and represent the major afferent pathway. The granule cells then project axons into the outer molecular layer of the cerebral cortex. Here, the axons bifurcate and form fiber bundles, which run parallel to the cortical surface, known as parallel fibers. Parallel fibers each pass through 500 Purkinje cells and each Purkinje cell receives information from 200,00 parallel fibers. They are unmyelinated. They activate Purkinje cells at a rate of 50-100 times per second (50-100 Hz) in the form of a single action potential. This is called a simple spike. The granular layer also contains golgi cells. These cells project short, profusely branching axons to the granule cells and extend their dendrites into the molecular layer of the cortex.
The external molecular layer of the cerebral cortex contains relatively few cells. Those present in this layer are mostly stellate cells but there are basket cells as well. The basket cells receive afferents from climbing fibers which may carry information from outside as well as inside the cerebellum. Climbing fibers are myelinated, excitatory, and they can innervate ten Purkinje cells. Although, each Purkinje cell only receives input from one climbing fiber. They also send collaterals to the deep cerebellar nuclei. When climbing fibers innervate a Purkinje cell it elicits a large action potential followed by a burst of smaller ones. This is known as a complex spike, which happens at a rate of one per second (1Hz). Stellate cells project ascending as well as descending dendritic processes in a transverse direction to the folium. Stellate cells in the inferior parts of the molecular layer have profusely branching axons which form complicated nests or baskets around Purkinje cells. The cell bodies of the Purkinje cells are located in a layer between the molecular and granule cell layers called the Purkinje cell layer.
The Purkinje cells are the major source of efferent projections from the cerebellar cortex to its deep nuclei. Their axons always emerge from the inferior end of the cell and project numerous collaterals to adjacent areas of the cortex. The dendritic processes of Purkinje cells project into the molecular layer of the cortex. There is a fourth layer consisting of white matter and made up of axons only.
Nystagmus, rapid oscillation of the eyes, occurs when there is damage to the vestibulocerebellum. This results because the vestibulocerebellum receives input from areas involved in the processing of visual information (LGN. superior colliculi and striate cortex), from the vestibular nuclei and from ganglion cells in the periphery. As a result of the vestibulocerebellum's projection of output to the vestibular nuclei, it also plays a role in equilibrium and balance. If there are damages to the vestibulocerebellum and the spinocerebellum, one notices an unsteadiness of gait. The spinocerebellum receives primarily somatosensory information but input from cortical motor areas as well. It controls the execution of movement and thus, is responsible for smoothing out movement and correcting deviations from the intended movement. Hypotonia, which is a decrease in muscle tone, can result if damage occurs here. Also, patients have shown disturbances in speech. The cerebrocerebellum, on the other hand, seems to be involved in the preparation of movement. If damage occurs to the cerebrocerebellum, five different deficits have been known to occur: ataxia, dysmetria, dysdiadochokinesia, asynergia, and terminal tremor. Ataxia involves abnormalities in voluntary movements. A person with dysmetria will make their movements too excessive or too small. dysdiadochokinesia is the inability to perform regular timing and force to a movement. Asynergia is marked by an action being performed as individual successive movements. Finally, terminal tremor is defined as instability at the end of movements. Sometimes this can occur at the beginning of movements as well.
The actions of climbing fibers have been found to modify parallel fiber activity . Studies have shown too, that the circuits of the cerebellum are modified by experience, which is important for motor learning. The mossy fibers and climbing fibers send collaterals to the deep cerebellar nuclei, sculpting motor output. This removes excess activation to make motions smooth. The cerebellum's general role in motor learning can be seen through classical conditioning. The unconditioned stimulus carried by the climbing fibers whereas the conditioned stimulus is carried by the mossy fibers.