UNIT 8 - Cerebellum
The cerebellum is a great reservoir for proprioceptive information from the body as well as from supraspinal cranial nerves and, in addition it receives important input from vestibular, visual, auditory, and exteroceptive receptors. It serves as:
1. A coordinator of motor activity
2. A regulator of muscle tone
3. Functions to maintain balance and equilibrium
4. Participates in motor planning
5. Plays a role in controlling visceral activity
A. Internal Structure
Unlike the cerebrum, the cerebellum is comprised of three layers that are uniform in size throughout the entire cortex. Like the cerebral cortex, the cortex of the cerebellum is located outside a centrally located mass of white matter called the corpus medullaris. Grooves (sulci and fissures) and gyri (called folia because of their appearance) mark the surface of the cerebellum and greatly increase its surface area. The white matter of the cerebellum is composed of afferent and efferent fibers. Located within the white matter are four pair of deep nuclei. They are: fastigial, dentate, emboliform, and globus nuclei which act as relay stations for all information entering and leaving the cerebellum.
Fig. 1a –
Fig. 1b –
The cerebellum and motor control, http://isis.uwimona.edu.jm/fpas/courses/physiology/
neurophysiology/Cerebellum.htm
1. Molecular Layer
The outer or molecular layer is mainly a synaptic layer that deals with the organizing of sensory input for integration by the middle purkinje layer. It is comprised of basket and stellate cells, the axons of granule cells, and the dendrites of both the purkinje and golgi cells.
2. Purkinje Layer
The middle or purkinje layer is responsible for integrating all incoming sensory information and formulating an appropriate response. This layer is comprised of a single layer of purkinje cells giving rise to an elaborate dendritic tree, which extends into the molecular layer. Climbing fibers from the inferior olivary nucleus are also received by the purkinje cells.
3. Granule Layer
The inner or granule layer is the layer of the cerebellum which receives incoming information. It contains the neurons of the golgi and granule cells, as well as the axons of the purkinje fibers. Afferent fibers that bring information to the granule cells are called mossy fibers. Golgi cells form a reverberating circuit with the molecular layer. Basket and stellate cells in the molecular layer receive their dendrites, with axonal attachments being made with granule cells in this layer. The granule cells in turn, have axonal connections with the basket and stellate cells of the molecular layer.
return to top of page
Fig. 2 –
Fig. 3 –
Basal ganglia and Cerebellum, An illustrated guide to the essential basics of clinical neuroscience, Neuroscience Tutorial, The Washington University School of Medicine
B. Cerebellar Peduncles
The cerebellum is connected to the brainstem by three cerebellar peduncles, which carry specific types of afferent and efferent information.
1. Superior Cerebellar Peduncle - Brachium Conjunctivum
The brachium conjunctivum carries efferent information to the red nucleus, superior colliculi and the VA and VL nuclei of the thalamus. It also carries afferent information from the ventral spinocerebellar tract, the fifth cranial nerve, the superior colliculi, the red nucleus, Area 19, areas 37, 39, 40 of the non-dominant hemisphere and areas 5-7.
2. Middle Cerebellar Peduncle - Brachium Pontis
The Brachium Pontis is comprised of afferent fibers from the pontine nuclei, which receive their information from the cerebral cortex (primarily areas 4-6-8).
3. Inferior Cerebellar Peduncle - Restiform Body
The restiform body carries afferent fibers from the extremities and axial musculature via the cuneo cerebellar, dorsal spinocerebellar, the medial leminiscal, and the spinothalamic tracts. The restiform body also carries afferent information from the fifth cranial nerve, the eighth cranial nerve (both vestibular and auditory), the vestibular nuclei, the inferior olivary nucleus, the cuneate nuclei, and the reticular formation. It also sends information from the cerebellum to the vestibular nuclei and reticular formation (after first synapsing with the fastigial nuclei).
Fig. 4 –
Cerebellar peduncle, The animated brain, http://www.brainviews.com/abFiles/DrwCerbped.htm
return to top of page
C. Cerebellar Lobes (Anatomical)
Anatomically the cerebellum is divided into three lobes which include the superior and inferior vermis, which in turn, form the central portion of the cerebellum.
1. Flocculonodular Lobe
The flocculonodular lobe consists of the nodulus of the inferior vermis and the two flocculi on either side. This is the smallest of the three cerebellar lobes and is found on the rostral edge of the anterior surface of the cerebellum.
2. Anterior Lobe
The anterior lobe is that part of the superior surface of the cerebellum rostral to the primary fissure, including the rostral portions of the superior vermis (lingula, central, lobule and the culmen).
3. Posterior Lobe
The posterior lobe is that area of the cerebellum posterior to the primary fissure, including the caudal portion of the superior vermis (declive and folium) and the entire inferior vermis except the nodulus (uvula, pyramis, tuber and two tonsils). It is by far the largest of the cerebellar lobes.
NOTE: The inferior vermis consists of the nodulus, uvula, pyramis, tuber, and two tonsils. The superior vermis consists of the lingula, central lobule, culmen, declive, and folium.
Fig. 5 –
Upper surface of the cerebellum. (Schäfer.) Henry Gray (1821–1865). Anatomy of the Human Body. 1918.
Fig. 6 –
Under surface of the cerebellum. (Schäfer.), Henry Gray (1821–1865). Anatomy of the Human Body. 1918.
return to top of page
D. Functional Divisions
Functionally, the cerebellum is divided into three functional divisions: 1.) according to its phylogenetic development or 2.) according to the destination of the major sources of afferent fibers. These divisions are so closely related anatomically that their names are used interchangeably and will so be used in this unit when considering function.
1. Archiocerebellum - Vestibulocerebellum
The archiocerebellum, or vestibulocerebellum, is the oldest division of the cerebellum and consists of the nodulus, paired flocculi, and the uvula. Afferently this functional division as well as the paleo-spino and neo-ponto divisions depend primarily on visual, vestibular, proprioceptive, extroceptive and perceptual input. Additionally, input from the red nucleus, reticular formation, vestibular nuclei, inferior olivary nuclei and areas 4, 6 and 8 is needed by these three functional lobes. Information from the motor corticles (4, 6, 8) is received via the cortical pontine and pontocerebellar fibers.
Efferent information is sent to the vestibular nuclei and the central group nuclei of the reticular formation. The vesstibular nuclei sends information to the apha and gamma motor neurons of lamina IX to effect control of our antigravity musculature through the M.V.S. and the L.V.S.
tracts, as well as to the ascending M.L.F. tract for occular control. The central group nuclei of the reticular formation send the information to the alpha and gamma motor neurons of Lamina IX
to affect control of our non-antigravity musculature through the medial and lateral reticulospinal tracts.
Functionally, this division of the cerebellum is responsible for controlling our postural control mechanism, which allows us to maintain or regain our balance and equilibrium (reactive and proactive control). Also this division is responsible for regulating or adjusting our muscle tone.
2. Paleocerebellum - Spinocerebellum
The paleocerebellum, or spinocerebellum, is the second division of the cerebellum to have evolved. This division consists of the superior vermis in the anterior lobe (lingula, central lobule and the culmen) and the adjacent medial half of the anterior lobe along with the tonsils, pyramis, and tuber of the inferior vermis. Afferently this functional division is dependent on information listed
under the archio-vestibular cerebellum.
Efferent responses from this lobe are sent to the red nucleus, the contralateral VA and VL nuclei of the thalamus, which relay this information to the ipsilateral to the motor cortices (Areas 4,6,8) and the central and lateral nuclei of the reticular formation. This division of the cerebellum exerts its influence ipsilaterally on our body musculature through the medial and lateral reticulospinal tracts, which originate from the central and lateral nuclei of the reticular formation. The final destination of this information is the alpha and gamma motor neurons in lamina IX of the spinal cord.
Functionally, this division monitors movement facilitated by Areas 4 & 6 so that the motor cortices can be prepared to instantaneously change the movement we are performing in order to meet our needs. Also, this monitoring allows us to adjust or alter the movement we are performing without stopping that movement, or having to initiate a new movement. The paleocerebellum adjusts our movements either by increasing the strength of contraction of the working muscles or by substantially increasing the activity of the synergistic muscles. This division alters movement by making slight modifications of the movement we are performing. These alterations when seen in a stress situation are referred to as associated movements and are basically stabilizing in nature being similar to equilibrium reactions. In addition, this division works with the archiocerebellum to regulate and adjust our muscle tone.
Fig. 6b –
3. Neocerebellum - Pontocerebellum
The neocerebellum, or pontocerebellum, is the last of the three divisions to have evolved and is only found in mammals. It is the largest in humans. This division consists of the lateral aspect of the anterior lobe and the entire posterior lobe (which includes the portion of the superior vermis found in this area, the declive and folium).
Afferently this functional division is dependent on information listed under the archio-vestibulocerebellum.
Efferent information is sent to the red nucleus, contralateral VA and VL nuclei of the thalamus, which relays this information to the ipsilateral motor cortices (Areas 4,6,8).
Functionally, this division is part of a system involved with the planning and programming of functional movement strategies. Other components of this system include the supplemental motor cortex and the corpus striatum. In general, this system helps decide the strategy of movement to be used, as well as the force, rate, direction, and range of movement. The neocerecellum also decides when the movement pattern is to be initiated and when it is to be terminated and type of contraction to be used.
Fig. 6c –
4. Miscellaneous Functions
Visual and auditory information is received in the superior vermis of the posterior lobe (Neocerebellum/Pontocerebellum).
In addition to the cerebellum's role in motor control, it has a role in controlling visceral activity. The paleocerebellum/spinocerebellum has been shown to produce respiratory, cardiovascular, pupillary, and urinary bladder responses. Sympathetic control of these functions occurs through that part of the paleocerebellum/ spinocerebellum found in the anterior lobe. Parasympathetic responses occur through tonsil activity. These functions are thought to be accomplished by information being relayed to the hypothalamus via the reticular formations, central group nuclei and the superior colliculi
Fig. 7 –
Phylogenetic regions (Left) and functional regions (Right) of the cerebellum. A. superior surface. B,inferior surface. The Cerebellum, Chapter-8. www.hallym.ac.kr/~de1610/nana/chp-8n.htm - 26k,
return to top of page
