Neuroanatomy lesson 4.pdf

NEUROANATOMY Lesson 4: Anatomy of the Cerebellum Cerebellum embryology  Cerebellum embryology Cerebellar Cerebellum rudiments enlarges and appear from ala...

NEUROANATOMY Lesson 4: Anatomy of the Cerebellum Cerebellum embryology  Cerebellum embryology Cerebellar Cerebellum rudiments enlarges and appear from alar bulges out of the lamina of fourth ventricle metencephalon Cerebellar Lateral rudiments grow hemispheres and into the roof vermis can be plate of the distinguished metencephalon to meet in the midline The cerebellum develops from the dorsolateral part of the alar lamina of the metencephalon Overview of the Cerebellum The cerebellum, which stands for “little brain”, is a structure of the central nervous system, and also is a vital component as it plays a role in motor movement regulation and balance control. The cerebellum coordinates gait and maintains posture, controls muscle tone and voluntary muscle activity but is unable to initiate muscle contraction. Damage to this area in humans results in a loss in the ability to control fine movements, maintain posture, and motor learning. Cerebellum: anatomical location The cerebellum is located at the back of the brain, immediately inferior to the occipital and temporal lobes, and within the posterior cranial fossa. It is separated from these lobes by the tentorium cerebelli, a tough layer of dura mater. It lies at the same level of and posterior to the pons, from which it is separated by the fourth ventricle.  Cerebellum: structure Petrous part of the temporal bone Posterior Cranial fossa  Cerebellum: structure The cerebellum consists of two hemispheres which are connected by the vermis, a narrow midline area. The cerebellum consists of an outer layer of highly convoluted gray matter (cerebellar cortex) surrounding a highly branched body of white matter known as the arbor vitae (Latin for “tree of life”), which in turn surrounds the pairs of deep cerebellar nuclei embedded in the central cerebellar white matter (corpus medullaris). Cerebellum: anatomy – Superior view Cerebellum: anatomy – Inferior view Cerebellum: anatomy – Anterior view Cerebellum: anatomy – Lateral view Cerebellum: anatomy – Lateral view Cerebellum: Cerebellar Cortex (Histology) The cerebellar cortex, which covers the surface of the cerebellum, is composed of three layers: The superficial molecular layer is relatively acellular. It is packed with axons of granule cells (parallel fibers) synapsing on dendrites of Purkinje neurons. Cell bodies of Purkinje cells distributed along the deep surface of the molecular layer constitute the middle Purkinje cell layer. The deep granule cell layer features densely packed nuclei of very small neurons. Cerebellum: Neurons of Cerebellar Cortex (Histology) Purkinje cells (neurons) are the largest neurons in the brain. They have extensive dendritic trees that are broad perpendicular and compressed parallel to the length of the folium. The dendrites fill the molecular layer. The axons run through the white matter and synapse in cerebellar nuclei (or in vestibular nuclei). Purkinje axons engage in inhibitory synapses; they release GABA as a neurotransmitter. Granule cells (neurons) are the smallest neurons in the brain. They are the only excitatory neurons in the cerebellar cortex; they release glutamate as a neurotransmitter. Axons of granule cells enter the molecular layer, bifurcate, and run along the length of a folium (parallel fibers). Each granule cell makes synaptic contact with numerous Purkinje neurons along a longitudinal band. Cerebellum: Neurons of Cerebellar Cortex (Histology) Basket cells (neurons) are located in the depth of the molecular layer. Their dendrites receive excitatory synaptic input from parallel fibers. Their axons run transversely in the folium and inhibit bands of Purkinje cells that border the excited zone. Axonal branches form basket synaptic contacts around initial segments of Purkinje axons and release GABA. Golgi cells in the granule cell layer and stellate cells in the molecular layer serve as inhibitory interneurons  Cerebellum: Cortical afferent fibers (Histology) Two types of afferent fibers enter the cerebellar cortex. Both afferent types are excitatory to the cortex and they send excitatory collateral branches to cerebellar nuclei before entering the cortext. The afferent fibers are: Mossy fibers: have terminations that resemble moss. The moss endings synapse in glomeruli, with dendrites of granule cells. Mossy afferents come from all sources (spinal cord, pontine nuclei, vestibular nuclei & nerve) except the reticular formation. Climbing fibers: terminate by climbing the dendritic tree of a Purkinje neuron, exciting it greatly via numerous synaptic contacts. All climbing fibers come from the olivary nucleus. Like mossy fibers, climbing fibers send excitatory collaterals to cerebellar nuclei before terminating in the cortex.  Cerebellum: Cortical Circuitry (Histology) Afferent fibers, both climbing and mossy, are excitatory. The fibers send collaterals to cerebellar nuclei before terminating in cerebellar cortex. Climbing fibers excite Purkinje cells directly. Mossy fibers excite granule cells. Granule cells excite bands of Purkinje cells and basket cells along the length of a folium. Basket cells inhibit Purkinje cells along the bilateral margins of the excited band. Purkinje cells axons terminate in cerebellar nuclei (or vestibular nuclei) where they selectively inhibit output neurons. Efferent axons from cerebellar nuclei constitute cerebellar output. Note: Granule cells are the only excitatory cells in the cerebellar cortex, all other cells are inhibitory. Purkinje cell axons are the only axons that leave the cerebellar cortex. Cerebellum: Gray and White Matter Grey matter – located on the surface of the cerebellum. It is tightly folded, forming the cerebellar cortex. White matter – located underneath the cerebellar cortex. Embedded in the white matter are the four cerebellar nuclei: – the Dentate nuclei – the Emboliform nuclei – the Globose nuclei – the Fastigial nuclei Cerebellum: nuclei conections Fibers from the dentate, emboliform, and globose nuclei leave the cerebellum through the superior cerebellar peduncle. Fibers from the fastigial nucleus exit through the inferior cerebellar peduncle. Cerebellum: function The cortex of the vermis coordinates the movements of the trunk, including the neck, shoulders, thorax, abdomen, and hips. Control of the distal extremity muscles is by the intermediate zone of the cerebellar hemispheres, located adjacent to the vermis. The remaining lateral area of each cerebellar hemisphere provides the planning of sequential movements of the entire body along with involvement in the conscious assessment of movement errors. Cerebellum: divisions - Anatomical lobes (There are three ways that the cerebellum can be subdivided – anatomical lobes, zones and functional divisions) There are three anatomical lobes that can be distinguished in the cerebellum: the anterior lobe the posterior lobe the flocculonodular lobe These lobes are divided by two fissures: the primary fissure the posterolateral fissure Cerebellum: divisions - Zones (There are three ways that the cerebellum can be subdivided – anatomical lobes, zones and functional divisions) There are three cerebellar zones: In the midline of the cerebellum is the vermis. Either side of the vermis is the intermediate zone. Lateral to the intermediate zone are the lateral hemispheres. There is no difference in gross structure between the lateral hemispheres and intermediate zones  Cerebellum: divisions - Functional divisions (There are three ways that the cerebellum can be subdivided – anatomical lobes, zones and functional divisions) There are three functional areas of the cerebellum: The Cerebrocerebellum o The largest division, formed by the lateral hemispheres. o It is involved in planning movements and motor learning. o It receives inputs from the cerebral cortex and pontine nuclei, and sends outputs to the thalamus and red nucleus. o This area also regulates coordination of muscle activation and is important in visually guided movements. Cerebellum: divisions - Functional divisions (There are three ways that the cerebellum can be subdivided – anatomical lobes, zones and functional divisions) There are three functional areas of the cerebellum: The Spinocerebellum o Comprised of the vermis and intermediate zone of the cerebellar hemispheres. o It is involved in regulating body movements by allowing for error correction. o It also receives proprioceptive information. Cerebellum: divisions - Functional divisions (There are three ways that the cerebellum can be subdivided – anatomical lobes, zones and functional divisions) There are three functional areas of the cerebellum: The Vestibulocerebellum o The functional equivalent to the flocculonodular lobe. o It is involved in controlling balance and ocular reflexes, mainly fixation on a target. o It receives inputs from the vestibular system, and sends outputs back to the vestibular nuclei. Cerebellum: blood supply The cerebellum receives its blood supply from three paired arteries: Superior cerebellar artery (SCA) Anterior inferior cerebellar artery (AICA) Posterior inferior cerebellar artery (PICA) The SCA and AICA are branches of the basilar artery, which wraps around the anterior aspect of the pons before reaching the cerebellum. The PICA is a branch of the vertebral artery. ) Cerebellum: blood supply The Venous drainage of the cerebellum is by the superior and inferior cerebellar veins. They drain into the superior petrosal, transverse and straight dural venous sinuses. Cerebellum: Pathways (neuron fibres) The cerebellum attaches to the brainstem by three groups of nerve fibers, through which efferent and afferent fibers pass to connect with the rest of the nervous system. : Superior cerebellar peduncle Middle cerebellar peduncle Inferior cerebellar peduncles Cerebellar Pathways: Connection of the Cerebellum with the Cerebrum Cerebellar Pathways: Connection of the Cerebellum with the Brainstem Cerebellar Pathways: Connection of the Cerebellum with the Spinal Cord