CEREBELLUM BRAINS BLOOD SUPPLY THE CEREBELLUM CEREBELLUM The - - PowerPoint PPT Presentation

CEREBELLUM BRAIN’S BLOOD SUPPLY

THE CEREBELLUM

The principal function of the cerebellum is to regulate and maintain balance, and to co-ordinate timing and precision of body movements.

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The cerebellum has multiple connections with the cerebral cortex, reticular formation in the brainstem, thalamus and vestibular nuclei. Through these intricate connections, the cerebellum constantly monitors proprioceptive sensory input from joints, muscles and tendons, and accordingly refines and co-ordinates the contractions of skeletal muscles. However, unlike the cerebral cortex of the primary motor area, the cerebellum is incapable of initiating movement, nor is the cerebellum involved in the conscious perception of somatic or visceral sensations.

The cerebellum is located dorsal to the pons and the medulla.

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The fourth ventricle is found between the cerebellum and the dorsal aspect of the pons. The cerebellum functions in the planning and finetuning of skeletal muscle contractions. The cerebellum performs these tasks by comparing an intended with an actual performance.

The cerebellum consists of a midline vermis and 2 lateral cerebellar hemispheres.

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The cerebellar cortex consists of multiple parallel folds that are referred to as folia. The grey and white substance of cerebellum on the sagittal section form a typical structure called “arbor vitae - the tree of life“.

The cerebellum is connected to the brain stem by three peduncles

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The cerebellar peduncles: Superior cerebellar peduncle - primary output of the cerebellum with mostly fibers carrying information to the midbrain. Middle cerebellar peduncle - carry input fibers from the contralateral cerebral cortex. Inferior cerebellar peduncle - receives ipsilateral proprioceptive information from the spinal cord.

The cerebellar cortex contains several maps of the skeletal muscles in the body.

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The topographic arrangement of maps of the skeletal muscles:

• the vermis controls the axial and proximal musculature of the

limbs,

• the intermediate part of the hemisphere controls distal

musculature,

• the lateral part of the hemisphere is involved in motor planning,
• the flocculonodular lobe is involved in control of balance and eye

movements

Major input to the cerebellum travels in the inferior cerebellar peduncle (ICP) and middle cerebellar peduncle (MCP).

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Major outflow from the cerebellum travels in the superior cerebellar peduncle (SCP).

The 3 cell layers of the cortex are:

• the molecular layer
• the Purkinje layer
• the granule cell layer

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The molecular layer is the outer layer and is made up of basket and stellate cells as well as parallel fibers, which are the axons of the granule cells. The extensive dendritic tree of the Purkinje cell extends into the molecular layer.

The Purkinje layer is the middle and most important layer of the cerebellar cortex.

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All of the inputs to the cerebellum are directed toward influencing the firing of Purkinje cells, and only axons of Purkinje cells leave the cerebellar cortex. A single axon exits from each Purkinje cell and projects to one of the deep cerebellar nuclei or to vestibular nuclei of the brain stem.

The granule cell layer is the innermost layer of cerebellar cortex

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The granule cell is the only excitatory neuron within the cerebellar

• cortex. All other neurons in the cerebellar cortex, including Purkinje,

Golgi, basket, and stellate cells, are inhibitory.

From medial to lateral, the deep cerebellar nuclei in the internal white matter are the fastigial nucleus, interposed nuclei, and dentate nucleus.

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Two kinds of excitatory input enter the cerebellum in the form of climbing fibers and mossy fibers. Both types influence the firing of deep cerebellar nuclei by axon collaterals.

These afferent fibers (mossy and climbing) reach the cerebellum via the inferior and middle cerebellar peduncles, which connect the cerebellum with the brain stem.

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These afferent fibers (mossy and climbing) are excitatory and project directly or indirectly via granule cells to the Purkinje cells of the cerebellar cortex.

The axons of the Purkinje cells are inhibitory. The axons of the Purkinje are the only outflow from the cerebellar cortex.

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The axons of the Purkinje cells project to and inhibit the deep cerebellar nuclei (dentate, interposed, and fastigial nuclei) in the medulla.

From the deep nuclei, efferents project mainly through the superior cerebellar peduncle and drive the upper motor neurons of the motor cortex.

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Right side of Cb controls muscles

• n right side of

the body

The efferents from the hemisphere project through the dentate nucleus, to the contralateral ventral lateral/ventral anterior nuclei of the thalamus, to reach the contralateral precentral gyrus.

These influence contralateral lower motor neurons via the corticospinal tract.

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Lesions of the cerebellum give rise to symptoms and signs on the same side of the body. Unilateral lesions of the cerebellum will result in a patient falling toward the side of the lesion.

Hallmarks of cerebellar dysfunction include ataxia, intention tremor, dysmetria, and dysdiadochokinesia.

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ATAXIA is a neurological sign consisting of lack of voluntary coordination of muscle movements that can include gait abnormality, speech changes and abnormalities in eye movements. DYSMETRIA is a type of ataxia - lack of coordination of movement typified by the undershoot or overshoot of intended position with the hand, arm, leg, or eye.

Thrombosis of the posterior inferior cerebellar artery (PICA) gives rise to a characteristic syndrome (PICA syndrome) marked by ataxia and hypotonia of the ipsilateral limbs owing to involvement of the inferior cerebellar peduncle and cerebellar cortex, signs of cranial nerve involvement (V to X) and contralateral loss of pain and thermal sensibility (spinothalamic tract involvement).

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DYSDIADOCHOKINESIA often abbreviated as DDK, is the medical term for an impaired ability to perform rapid, alternating movements

Examination of cerebellar function: TAXIS - the finger-to-nose test and heel-to-shin test with and with out eye control. Damage of neocerebellum leads to ipsilateral problems with taxis.

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Examination of cerebellar function:

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Examination of cerebellar function: REBOUND PHENOMENA – inability to stop the movement after removing the contra-pressure which is generated by examiner.

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Examination of cerebellar function: TEST OF TRUNK RETROFLEXION (GROSS ASYNERGY) - standing patient, examiner performs a dorsal pull which is normally followed by flexion of lower limbs - in disease of cerebellum this synergy is impaired and patient usually falls in the direction of the pull.

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Examination of cerebellar function: SMALL ASYNERGY - patient is unable to sit up from a supine position without the help of his/her upper limbs and the elevation of the lower limb on the side of the lesion. DIADOCHOKINESIS - fast alternation of antagonist movements (pronation and supination, flexion and extension) of upper limbs

• simultaneously. We search for asymmetry, rhythm disruptions and

movement slowness.

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ARTERIAL SUPPLY OF THE BRAIN

The arterial supply of the brain is derived from the internal carotid and vertebral arteries, which lie, together with their proximal branches, within the subarachnoid space at the base of the brain.

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The INTERNAL CAROTID ARTERY arises from the bifurcation of the common carotid artery, ascends in the neck and enters the carotid canal of the temporal bone. The INTERNAL CAROTID ARTERY ascends in the neck and enters the carotid canal of the temporal bone.

The INTERNAL CAROTID ARTERY has petrous, cavernous and intracranial parts.

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PETROUS PART The petrous part of the internal carotid artery ascends in the carotid canal. The petrous part of the internal carotid artery curves anteromedially and then superomedially above the cartilage that fills the foramen lacerum, and enters the cranial cavity. The petrous part of the ICA lies at first anterior to the cochlea and tympanic cavity The petrous part of the ICA is separated from the latter and the pharyngotympanic tube by a thin, bony lamella that is cribriform in the young and partly absorbed in old age.

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PETROUS PART Further anteriorly, petrous part of the ICA is separated from the trigeminal ganglion by the thin roof of the carotid canal, although this is often deficient. The petrous part of the ICA is surrounded by a venous plexus and by the carotid autonomic plexus, derived from the internal carotid branch of the superior cervical ganglion.

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PETROUS PART The petrous part of the ICA gives rise to two branches:

• caroticotympanic artery
• pterygoid artery

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The caroticotympanic artery is a small, occasionally double, vessel which enters the tympanic cavity by a foramen in the carotid canal and anastomoses with the anterior tympanic branch of the maxillary artery and the stylomastoid artery. The pterygoid artery is inconsistent: when present, it enters the pterygoid canal with the nerve of the same name, and anastomoses with a (recurrent) branch of the greater palatine artery.

CAVERNOUS PART The cavernous part of the ICA ascends to the posterior clinoid process. The cavernous part of the ICA turns anteriorly to the side of the sphenoid within the cavernous sinus The cavernous part of the ICA curves up medial to the anterior clinoid process, to emerge through the dural roof of the sinus. The oculomotor, trochlear, ophthalmic and abducens nerves are lateral to it within the cavernous sinus.

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CAVERNOUS PART This part of the artery gives off a number of small vessels. Cavernous branches supply the trigeminal ganglion, the walls of the cavernous and inferior petrosal sinuses and the nerves contained therein. After piercing the dura mater, the ICA turns back below the optic nerve to run between it and the oculomotor nerve.

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INTRACRANIAL PART The ICA reaches the anterior perforated substance at the medial end

• f the lateral fissure and terminates by dividing into the anterior and

middle cerebral arteries. Just before splitting into the middle and anterior cerebral arteries, the internal carotid artery gives rise to the ophthalmic artery. The ophthalmic artery enters the orbit through the optic canal and supplies the eye, including the retina and optic nerve.

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The two terminal branches of the internal carotid:

• anterior cerebral artery
• middle cerebral artery

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The anterior cerebral artery is the smaller of the two terminal branches of the internal carotid The ANTERIOR CEREBRAL ARTERY passes anteromedially above the

• ptic nerve to the great longitudinal fissure.

The ANTERIOR CEREBRAL ARTERY connects with its fellow by a short transverse anterior communicating artery.

The anterior communicating artery is about 4 mm in length and may be double.

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The most common aneurysm site in the circle of Willis is where the anterior communicating artery joins an anterior cerebral artery. The two anterior cerebral arteries travel together in the great longitudinal fissure. The anterior cerebral artery (ACA) supplies:

• 1. Medial surface of frontal and parietal lobes
• 2. Anterior four-fifths of corpus callosum
• 3. Anterior limb of internal capsule

The anterior cerebral artery supplies the medial surface of the frontal and parietal lobes, which include motor and sensory cortical areas for the pelvis and lower limbs.

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The anterior cerebral artery also supplies the anterior four-fifths of the corpus callosum and approximately 1 inch of the frontal and parietal cortex on the superior aspect of the lateral aspect of the hemisphere. Occlusion of the anterior cerebral artery results in spastic paresis of the contralateral lower limb and anesthesia of the contralateral lower limb. Urinary incontinence may be present, but this usually occurs only with bilateral damage. The anterior cerebral artery also supplies the anterior limb of the internal capsule.

The MIDDLE CEREBRAL ARTERY is the larger terminal branch of the internal carotid.

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The middle cerebral artery runs at first in the lateral fissure, then posterosuperiorly on the insula. The middle cerebral artery divides into branches distributed to the insula and the adjacent lateral cerebral surface. Like the anterior cerebral artery, it has cortical and central branches.

Occlusion of the middle cerebral artery results in spastic paresis of the contralateral lower face and upper limb and anesthesia of the contralateral face and upper limb.

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The middle cerebral artery (MCA) supplies:

• the lateral surface of the frontal, parietal, and upper temporal

lobes

• the posterior limb and genu of the internal capsule
• most of the basal ganglia

VERTEBRAL ARTERY The vertebral arteries are derived from the subclavian arteries. They ascend through the neck in the foramina transversaria of the upper six cervical vertebrae. The vertebral arteries enter the cranial cavity through the foramen magnum, close to the anterolateral aspect of the medulla. They converge medially as they ascend the medulla and unite to form the midline basilar artery at approximately the level of the junction between the medulla and pons. A small anterior spinal artery arises near the end of the vertebral artery, and descends anterior to the medulla oblongata to unite with its fellow from the opposite side at mid-medullary level.

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VERTEBRAL ARTERY The largest branch of the vertebral artery is the posterior inferior cerebellar artery. Minute medullary arteries arise from the vertebral artery and its branches and are distributed widely to the medulla oblongata. The basilar artery is a large median vessel formed by the union of the vertebral arteries at the mid-medullary level. The basilar artery lies in the pontine cistern, and follows a shallow median groove on the ventral pontine surface, extending to the upper border of the pons.

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BASILAR ARTERY The basilar artery ends by dividing into two posterior cerebral arteries at a variable level behind the dorsum sellae. Numerous small pontine branches arise from the front and sides of the basilar artery along its course and supply the pons. The anterior inferior cerebellar artery is given off from the lower part

• f the basilar artery and runs posterolaterally.

The superior cerebellar artery arises near the distal portion of the basilar artery.

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The POSTERIOR CEREBRAL ARTERY is a terminal branch of the basilar artery.

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The posterior cerebral artery (PCA) supplies:

• 1. Occipital lobe
• 2. Lower temporal lobe
• 3. Splenium
• 4. Midbrain

The CIRCULUS ARTERIOSUS (CIRCLE OF WILLIS) is a large arterial anastomosis which unites the internal carotid and vertebrobasilar systems

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The primary role of the circle of Willis is to allow for this eventuality by redirecting blood from other sources such as the contralateral internal carotid and vertebrobasilar system. The circle of Willis lies in the subarachnoid space within the interpeduncular cistern, and surrounds the optic chiasma and infundibulum

Anteriorly, the anterior cerebral arteries, derived from the internal carotid arteries, are linked by the small anterior communicating artery.

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Posteriorly, the two posterior cerebral arteries, formed by the division

• f the basilar artery, are joined to the ipsilateral internal carotid artery

by a posterior communicating artery.

CENTRAL OR PERFORATING ARTERIES Numerous small central (perforating or ganglionic) arteries arise from the circulus arteriosus, or from vessels near it. Many of these enter the brain through the anterior and posterior perforated substances. Central branches supply nearby structures on or near the base

• f the brain together with the interior of the cerebral hemisphere

including the internal capsule, basal ganglia and thalamus.

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ISCHAEMIC STROKE Stroke refers to the clinical syndrome of a rapidly developing focal neurological deficit that is not due to seizure activity. If the cause is lack of, or reduced, blood supply to a portion of the brain then the term ischaemic stroke is used, as opposed to haemorrhagic stroke e.g. subarachnoid haemorrhage. The lack of blood flow can be due to pathology in the vessel lumen, such as thrombosis or embolus (common), pathology outside the blood vessel, such as occlusion from mass effect of a tumour or haematoma (rare), or pathology of the vessel wall, such as inflammatory or infective arteritides (rare).

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ISCHAEMIC STROKE The symptoms and signs of ischaemic stroke depend on the location and extent of the arterial infarction. In certain locations, even a small volume stroke can have devastating effects.

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