The three main lobes of the cerebellum receive afferent inputs from the vestibular apparatus, the proprioceptive system and the cerebral cortex.
Cortico-pontine (cortico-bulbar) fibres run in parallel with the corticospinal tract and carry copies of motor commands to the pontine nuclei, which relay them to the cerebellar cortex.
These axons also transmit information from association areas of cortex, e.g. the area of parietal cortex concerned with spatial awareness, or the limbic system, concerned with motor responses of an emotional nature, such as flight, fright or fight.
These pathways indicate that cerebellum receives information from all sensory systems - visual auditory, somatosensory - so all signals that can communicate information about movement and the position of the body converge on the cerebellum.
The cerebellum also receives inputs from the proprioceptive system that informs the cerebellum about the precise movements that result from the motor commands. This is carried by the spino-cerebellar systems.
The medullary olive are also involved in signalling information from the proprioceptive and other systems, but this pathway has chracteristic features (see 'climbing fibres') that distinguish it from the ponto-cerebellar pathway (see 'mossy fibres').
The cerebellum acts as a comparator and feeds error signals forward to the cortex. This is a function of the cerebellar cortex, and the error signals are carried by the Purkinje cells to the deep cerebellar nuclei, that communicate with the Red Nucleus of the midbrain, the thalamus, and the cerebral cortex.
The main output pathway sends error signals to the cortex and allows adjustments to be made and ensure that the desired outcome is achieved.
Disorders of the cerebellum lead to involuntary imprecise movements that become worse while performing willed movements.
The cerebellum is also involved in motor memory and learning motor skills.
Key Words: Cerebellum, error detector, comparator, feedback. Motor Learning
Cortical Inputs to the Cerebellum
A copy of the signal sent to motoneurone is also sent to the cerebellum using the cortico-pontine and ponto-cerebellar neurones.
The diagram shows the location of the deep cerebellar nuclei through which information processed in the cortex is passed to the brainstem and cortex.
Connections between the Cortex and Cerebellum
The main function of the cerebellum is to compare motor commands with actual events in the musculature, visual and auditory space, and provide the motor cortex with information of any deviation from the intended movement.
The cerebellum receives copies of the messages (motor commands) sent down the corticospinal tract, along with a plethora of sensory information relayed by the cerebral cortex.
Several areas of Association cortex send information to the cerebellum:
The Parietal cortex (concerned with spatial awareness dependent on inputs form the visual and somatosensory systems)
The Cingulate cortex (limbic system concerned with emotional responses, such as flight or fight)
The Frontal Cortex (concerned with motor control and planning)
These cortical inputs reach the cerebellum via nuclei in the brainstem, particularly the pontine nuclei, which give rise to mossy fibres, so called because of the tufted appearance of their synapses.
After processing of the sensori-motor information by the cerebellar cortex, error signals are relayed back to the motor areas of the cortex via the dentate nucleus, whose neurones project to the red nucleus (midbrain) and the thalamus.
Image source: Schmidt and Wiesendenger, 1990
Proprioceptive Inputs to the Cerebellum
Spinal Input to the Cerebellum
A second source of mossy fibres is from the spino-cerebellar tracts and from the cuneo-cerebellar tracts. These pathways arise from Clarke's Column (Nucleus) in the spinal cord and the cuneate nuclei. They carry proprioceptive information from the trunk and limbs to the medial parts of the anterior lobe and the vermis.
A different input to the cerebellum arises in the contralateral inferior olive (in the medulla) and these axons project to the contralateral side of the cerebellum; their terminals are wound around Purkinje cell dendrites and are known as 'climbing fibres'.
Climbing fibres. The name 'climbing fibers' is used because of the way in which the axon terminals wrap themselves around the Purkinje cell dendrites. Climbing fibres arise from the contralateral inferior olivary nucleus and excite Purkinje cells
Every Purkinje cell receives a very powerful excitatory synaptic input from each climbing fibre, each of which innervates ony a small number of Purkinje cells; this is in contrast to the very large number of weaker excitatory inputs Purkinje cells receive from parallel fibres.
Somatotopic maps in the cerebellar cortex
There are several complex somatotopic maps of the body in the medial cerebellum and vermis that show that the whole body surface and musculature, including the head, neck and eye.
Note that, in addition to the spinal pathways, the parietal lobe of the cortex, concerned with spatial awareness is another important input the the cerebellum, allowing the cerebellum to integrate information about visual space with the position of the body and limbs.
The major input and output pathways to and from the cerebellum.
Crossing the midline.
The cortico-bulbar axons cross the midline in the pons, so the left cerebral hemisphere sends its information to the right side of the cerebellum. Similarly the output of the cerebellum passes in the superior cerebellar peduncle to the left hemisphere, via the contralateral Red Nucleus and ventro-lateral Thalamus.
The spinocerebellar tracts are essentially ipsilateral to the cerebellum, i.e. information from the left side of the body projects to the left half of the cerebellum.
It can be seen that there are two main pathways into the cerebellum :
Ponto-cerebellar neurones. These link the right cerebral hemisphere with the left half of the cerebellum.
Olivo-cerebellar neurones that arise in the inferior olive of the medulla. These axons cross the midline in the medulla: the right olivary nucleus connects with the left half of the cerebellum; the right olivary nucleus has somatosensory and proprioceptive inputs from the right side of the body.
The output pathway from the Dentate nucleus on each side communicates with the contralateral Red Nucleus, Thalamus and Cerebral cortex.
In addition there are inputs to the cerebellum from the Red Nucleus, the Superior colliculus, the Vestibular apparatus and nuclei, and the Locus Coeruleus, which is the source of noradrenergic axons in the cerebellum.
It has been noted previously that the flocculonodular lobe developed along with the vestibular appaaratus, and that the two are concerned with the same functions, such as balance. Note that the Purkinje cells of the Flocculo-nodular lobe project to the vestibular nuclei of the brainstem. The relationship between the cerebellum and vestibular nuclei is unilateral.
The Cerebellum is also essential for motor learning, be it in infant development of walking skills, the honing of precise movements as required in catching a ball, or fine movements of the fingers as in playing a musical instrument. The development of hand-eye coordination in sport or hearing-hand skills in music require repetition of motor tasks; cerebellar modules are involved in refining these skills. It has been suggested that the climbing fibres may be concerned with defining the movements appropriate to each motor skill during the learning process.