Chapter 7. The Cerebellum
The cerebellum is located in the posterior fossa of the skull, dorsal to the pons and medulla from which it is separated by the Aqueduct of Sylvius and the fourth ventricle.
Like the cerebrum, the cerebellum is covered by cortex and consists of two hemispheres, each of which is divided into lobes. The hemispheres are separated from one another by a thin structure called the vermis, or "worm."
The anterior lobe, or paleocerebellum, is the second oldest part of the cerebellum. It receives proprioceptive input from the spinal cord and controls the anti-gravity muscles of the body, thus regulating posture.
The posterior lobe, or neocerebellum, is the newest part of the cerebellum. It is involved in the coordination of muscle movement via the inhibition of involuntary movement. Inhibitory neurotransmitters, especially GABA, are found here. This lobe plays an important role in fine motor coordination.
The flocculonodular lobe consists of the flocculi, the most ancient part of the cerebellum, and the nodulus, the narrowest and most inferior part of the vermis. This lobe is involved in the maintenance of equilibrium.
Four different nuclei are located deep within each cerebellar hemisphere; the dentate nucleus, the emboliform nucleus, the globose nucleus, and the fastigal nucleus. These deep nuclei have axons that project to the brain stem, sending messages out to be conveyed to other parts of the central nervous system.
The deep nuclei are regulated by radish-shaped cells located in the cerebellar cortex called Purkinje cells. The Purkinje cells control the output of the cerebellum by inhibiting the firing of the deep nuclei. The Purkinje cells located in the lateral cortex of the cerebellum project to the dentate nuclei, while those in the intermediate cortex synapse with the emboliform and globose nuclei. The fastigial nuclei receive input from Purkinje cells found in the cerebellar cortical covering of the vermis.
The cerebellum is involved in a feedback loop for muscle movement. When the cortex sends a message for motor movement to the lower motor neurons in the brain stem and spinal cord it also sends a copy of this message to the cerebellum.This is conveyed from pyramidal fibers in the cortex on the corticopontinecerebeller tract to the cerebellum. In addition, information gets to the cerebellum from muscle spindles, joints and tendons. This information (proprioception and kinesthesia) lets the cerebellum know about the movements that have been executed, so that it can determine how well motor commands coming from the cortex are being carried out. This has been called its comparator function.
The cerebellum plays a major role in the coordination of muscle activity for the production of smooth movement through its connections with the pyramidal and extrapyramidal systems and the descending reticular formation .Due to its role in the coordination of fine motor movements the cerebellum makes important contributions to the control of rapid, alternating muscle movements necessary for speech.
Three fiber bundles called peduncles connect the cerebellum to the brain stem. On these tracts, information runs in both directions, with all messages sent and received by the cerebellum traveling on these fibers.
The superior cerebellar peduncle also called the superior brachium conjunctivum connects the cerebellum to the midbrain and contains efferent fibers from the dentate, emboliform, and globose nuclei. These are the axons that send feedback to the motor cortex in the frontal lobe via the red nucleus in the midbrain and the thalamus. Afferent fibers traveling in this peduncle take proprioceptive information to the cerebellum from the lower body.This information ascends along the spinal cord in the ventrospinocerebellar tract, before entering the cerebellum.
The middle cerebellar peduncle or the middle brachium pontis is the largest of the peduncles and links the cerebellum with the pons. Via this connection, the cerebellum receives a copy of the information for muscle movement that the pyramidal tract is carrying down to lower motor neurons.
The inferior cerebellar peduncle or the restiform bodies connects the cerebellum with the medulla, but also to the vestibular nuclei located in the lower pons and medulla and with cells of the reticular formation.
Proprioceptive information from the upper body, traveling along the dorsospinocerebellar tract enters the cerebellum on the inferior cerebellar peduncle.
Sensory and motor input to the cerebellum:
The cerebellum receives both proprioceptive and kinesthetic information from the periphery. It also gets information about the strength and type of muscle movements occurring.Proprioception, according to Love and Webb, 1992, refers to sensory information about equilibrium, pressure, movement, vibration, position, and pain received from muscles, joints and tendons.
Kinesthesia is a more specific term than proprioception. It refers to feedback that comes only from muscle spindles. Kinesthesia is the "ability to detect the range and direction of movements of the limbs" (Bhatnager & Andy, 1995, pg. 341).
Sensory information ascends to the cerebellum along the spinal cord. The two main tracts that bring information from the periphery to the cerebellum are the ventro-spino-cerebellar tract and the dorso-spino-cerebellar tract.
The ventro-spino-cerebellar tract contains proprioceptive fibers from the lower body. (The axons of this tract decussate and travel upward on the contralateral side of the spinal cord for some distance before crossing again and continuing upward ipsilaterally.) The axons of this tract enter the cerebellum on the superior cerebellar peduncle.
The dorso-spino-cerebellar tract takes proprioceptive information from the upper body that reaches the cerebullum on the inferior cerebellar peduncle (restiform body).
The reticulo-cerebellar tract carries messages received by the reticular nuclei in various parts of the brain stem from the cortex, spinal cord, vestibular system and red nucleus.
The vestibulo-cerebellar tract brings information from the semi-circular canals of the inner ear to the cerebellum via the vestibular nucleus located in the lower pons and medulla. These fibers travel to the flocculi on the inferior cerebellar peduncle.
The cortico-pontine-cerebellar tract brings motor information to the cerebellum from the frontal lobe. It leaves the precentral gyrus (motor strip) and descends in the internal capsule along with pyramidal tract fibers. Its axons synapse with cells in the pons. These pontine nuclei then send second order neurons to the cerebellum on the middle cerebellar peduncle. The axons of the corticopontine tract bring the cerebellum a copy of the information that the corticobulbar and corticospinal tracts are conveying to the cranial and spinal nerves. Thus, the cerebellum "knows" the details of the messages being sent to lower motor neurons by the upper motor neurons. The messages sent by the corticopontinecerebellar tract includes information about the nature of the motor impulse being sent by the precentral gyrus: its destination, its strength and its speed. This connection is contralateral, meaning that information from the frontal lobe of one hemisphere is sent to the posterior cerebellar lobe on the opposite side of the brain.
The cerebellum acts on the motor messages carried by the corticopontinecerebellar tract by integrating the information and exerting control over the message through the firing of Purkinje cells.
I consider the cortico-pontine-cerebellar tract to be part of the extrapyramidal system, although it originates in the premotor, precentral (motor strip) and postcentral (sensory strip) gyri it synapses with cells in the pons rather than with spinal or cranial nerves. The portion of the tract from the cortex to the pons consist of first order neurons, and from the pons to the cerebellum, second order. The first order neurons may be considered by some to be part of the pyramidal system
The tracts discussed below may be considered part of the extrapyramidal system since they carry involuntary signals and are indirect and multisynaptic. As mentioned previously some sources (not current) consider the extrapyramidal tract to consist of the basal ganglia only.
Output to the red nucleus
The dentate nuclei, which receive inhibitory messages from the Purkinje cells in the lateral cerebellar cortex, and the emboliform and globose nuclei, which are controlled by the Purkinje cells in the intermediate cortical areas all project to the contralateral red nucleus in the midbrain via the superior cerebellar peduncle.
Some of the information sent on the superior cerebellar peduncle travels from the red nucleus to the ventrolateral nucleus of the thalamus. From there it passes on to the precentral gyrus of the frontal lobe. Other ascending projections travel to the cortex directly bypassing the thalamus. These connections allow the cerebellum to perform its comparator function by providing feedback to the motor cortex regarding the messages that it has received from cortico-pontine-cerebellar fibers as well as feedback from the muscles innervated by lower motor neurons that receive impulses from the pyramidal and extra pyramidal tracts.
The Structure of the Feedback Loop for Motor Movement
Precentral gyrus --> pontine nuclei -- > cerebellum -- > (after input from muscles innervated by lower motor neurons, and comparisons made by the cerebellum between output from the cortex and input from muscles) --> red nuclei --> thalamus --> precentral gyrus where adjustments are made.
Fibres from the red nucleus also descend to synapse with cranial and spinal nerves, forming the rubrospinal tract.
The vestibulospinal tract
The fastigal nucleus, which receives inhibitory control from the Purkinje cells of the vermis, sends messages to the vestibular nuclei in the lower pons and medulla. From the vestibular nuclei, information is sent to lower motor neurons in the brain stem and spinal cord. This tract brings information about the body's position in space to the antigravity muscles.
The reticulospinal tract
This tract carries information related to the functioning of the autonomic nervous system including circulation of the blood, dilation of blood vessels, respiration and visceral activity. (Note that while this tract influences autonomic activity, it is not part of the autonomic nervous system.)
Ataxia, is an incoordination of motor movement, resulting from cerebellar lesions. The term ataxia is also used to describe the unsteady walk and usual postures seen in patients who have suffered injury to the cerebellum. People sometimes compensate for these problems by walking with their feet far apart in a broad-based gait seen on the tandem walking test (Webb, Adler and Love, 2008).