The Human Brain : From Neurone to Nervous System

Cerebellum


  1. The cerebellum is a large solid structure involved in producing precise movements, and motor memory and learning. It compares motor commands generated in the motor cortex, with the actual movements generated, using somatosensory, visual and auditory feedback, and provides error signals that allow the motor cortex to correct any deviation from the intended movement.

  2. The cerebellum monitors the outcome of motor commands issued by the motor cortex as they are happening, i.e. it compares the feedback from the muscles, i.e. feedback about the actual movements, with the motor commands (a copy of which is fed to the cerebellum by cortico-bulbar-cerebellar pathways). It sends signals back to the cortex as to whether the movement achieved is what was desired, or whether there are errors or lack of precision in the actual movement. The feedback of an error signal allows adjustments to be made and ensure that the desired outcome is achieved.

  3. Disorders of the cerebellum lead to involuntary imprecise movements that become worse doing voluntary movements.

  4. The cerebellum is also involved in motor memory and learning motor skills.

  5. Key Words: Cerebellum, error detector, comparator, feedback. Motor Learning




Connections between the Cerebral Cortex and the Cerebellum
Connections between the Cortex and Cerebellum

The main function of the cerebellum is to compare motor commands with actual events in the musculature and 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 plethor a sensory information relayed by the cerebral cortex.

These cortical inputs reach the cerebellum via nuclei in the brainstem .

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.

Note that the Purkinje cells of the Flocculo-nodular lobe project to the vestibular nuclei of the brainstem.

There are two main pathways into the cerebellum :

  • Ponto-cerebellar neurones.
  • Olivo-cerebellar neurones that arise in the inferior olive of the medulla

The terminals of the ponto-cerebellar axons are known as 'mossy fibres' because of the ultrastructure of their terminals.

Mossy fibres synapse on granule cells, the origin the parallel fibres that distribute information from many areas of the cortex throughout the cerebellar cortex. These inputs are concerned with motor commands signals and somatosensory, visual and auditory information about the position of parts of the body. The terminals of the spino-cerebellar pathway also have mossy fibre morphology

The second input to the cerebellum arises in the inferior olive (in the medulla) and these fibres climb around the Purkinje cells and are known as 'climbing fibres'.

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.



The diagram shows mossy and climbing fibres in contact with Purkinje Cells. Note the way in which climbing fibres wind themselves around Purkinje cell dendrites.
The climbing fibres reach out to a limited number of Purkinje cells, which together form a functional unit called the Cerebellar module.
The Cerebellar Cortex

The cerebellar cortex has three layers -

  • a surface molecular layer of parallel fibres, consisting of the axons of granule cells and the dendrites of Purkinje cells
  • the innermost granular layer, consisting of the cell bodies of the granule cells are densely packed in the innermost layer
  • the Purkinje cell layer, one cell in thickness, between them, and is the main output pathway from the cerebellar cortex.

The mossy fibres of the ponto-cerebellar neurones make contact with the granule cells.

The axons of Granule cells are unmyelinated and run towards the surface of the molecular layer where they bifurcate, sending collaterals in opposite directions to make contact with Purkinje cells.

Climbing fibres from the inferior olive are the other input to the Purkinje cells

Purkinje cells have axons that terminate on neurones in the deep cerebellar nuclei; these neurone also receive axon collaterals of the mossy and climbing fibres that provide inputs to these Purkinje cells.

Most neurones in the deep cerebellar nuclei send their information to the red nucleus of the midbrain and onward to the thalamus and cortex

The sifting of feedback signals to extract information about errors is done by Purkinje cells, the principal cell of each cerebellar module.

The communication of error signals to the cortex depends on the output of Purkinje cells, through the deep cerebellar nuclei to the cortex (with synapses in the red nucleus and the thalamus).

The Cerebellar Cortex is where comparisons are made between the command signal and the feedback signals from proprioceptors, touch, vision and hearing.

A copy of the command signal is carried by ponto-cerebellar neurones whose axons are called 'mossy fibres', on account of the structure of their terminations on granule cells.



JM 2016

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