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Brain and Cranial Nerves.docx

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McMaster University
Krista Howarth

Brain and Cranial Nerves Development of the CNS - The CNS develops from a flat plate of ectodermal tissue, the neural plate on the dorsal surface of the embryo, influence in part by the underlying rod shaped notochord. - Three brain regions can be identified in the early embryo: a forebrain, or prosencephalon, a midbrain, or mesencephalon, and a hindbrain or rhombencephalon. - During development, the forebrain divides into the telencephalon, which becomes the cerebrum, and the diencephalon. - The midbrain remains as a single structure, but the hindbrain divides into the metencephalon, which becomes the pons and cerebellum, and the myencephalon, which become the medulla oblongata. Brainstem - The medulla oblongata, pons and midbrain constitute the brainstem. - The brainstem connects the spinal cord to the remainder of the brain and is responsible for many essential functions. - Damage to small brainstem areas often causes death because many reflexes essential for survival are integrated in the brainstem, whereas relatively large areas of the cerebrum or cerebellum may be damaged without life-threatening consequences. Medulla Oblongata - About 3 cm long, is the most inferior part of the brainstem, and is continuous inferiorly with the spinal cord. - Contains sensory and motor tracts; cranial nerve nuclei; other related nuclei and part of the reticular formation. - Discrete nuclei, clusters of gray matter composed of mostly neuron cell bodies and having specific functions, are found in the medulla oblongata, whereas the gray matter of the spinal cord extends as a continuous mass in the center of the cord. - The tracts within the medulla do not have the same organization as those of the spinal cord. - Several medullary nuclei function as centers for reflexes such as those involved in the regulation of heart rate, blood vessel diameter, respiration, swallowing, vomiting, hiccupping, coughing, and sneezing. - The two prominent enlargements on the anterior surface of the medulla oblongata are called pyramids because they are broader near the pons and taper toward the spinal cord. - The pyramids are descending tracts involved in the conscious control of skeletal muscles. - Near their inferior ends, most of the fibers of the descending tract cross to the opposite side, or decussate. - This decussation accounts for the fact that each half of the brain controls the opposite half of the body. - Two rounded, oval structures called olives, protrude from the anterior surface of the medulla oblongata just lateral to the superior ends of the pyramids. - The olives are nuclei involved in functions such as balance, coordination and modulation of sound from the inner ear. - Nuclei of cranial nerves V, IX, X, XI AND XII also are located within the medulla. Pons - Part of the brainstem just superior to the medulla oblongata. - Contains ascending and descending tracts and several nuclei. - The pontine nuclei, located in the anterior portion of the pons, relay information from the cerebrum to the cerebellum. - Nuclei for cranial nerves V, VI, VII, VIII AND IX are contained within the posterior pons. - Other important areas include the pontine sleep center and respiratory center, which work with the respiratory centers in the medulla to help control respiratory movements. Midbrain - The midbrain or mesencephalon is the smallest region of the brain. - It is located just superior to the pons and contains the nuclei of cranial nerve III, IV, AND V. - The tectum of the midbrain consists of four nuclei that form mounds on the dorsal surface, collectively called corpora quadrigemina. - Each mound is called a colliculus; the two superior mounds are called superior colliculi, and the two inferior mounds are called inferior colliculi. - The superior colliculi are involved in visual reflexes. - The reflexes control the movement of the head, eyes and body toward visual, auditory or tactile stimuli, such as loud noises, flashing lights, or startling pain. - The superior colliculi receive input from the eyes, the inferior colliculi, the skin and the cerebrum. - The inferior colliculi are involved in hearing and are an integral part of the auditory pathways in the CNS. - Neurons conducting action potenials from the structures of the inner ear to the brain synapse in the inferior colliculi. - Collateral fibers from the inferior colliculi to the superior colliculi provide auditory input that stimulates visual reflexes. - The tegmentum of the midbrain largely consists of ascending traits such as the spinothalamic tract and the medial lemniscus, which carry sensory information from the spinal cord to the brain. - The tegmentum also contain paired red nuclei, which are so-named because in fresh brain specimens they are pinkish in color as a result of an abundant blood supply. - The red nuclei aid in the unconscious regulation and coordination of motor activities. - Cerebral peduncles constitute the portion of the midbrain ventral to the tegmentum. - They consist primarily of descending tracts, which carry motor information from the cerebrum to the brainstem and spinal cord. - The substantia nigra is a nuclear mass between the tegmentum and cerebral peduncles, containing cytoplasmic melanin granules that five it a dark gray or black color. - The substantia nigra is interconnected with other basal nuclei of the cerebrum, and it is involved in maintaining muscle tone and in coordinating movements. Reticular Formation - A diffuse system consisting of several loosely packed nuclei scattered throughout the length of the brainstem. - The reticular formation receives axons from a large number of sources and especially from nerves that innervate the face. - The functions of the reticular formation involve “cycles” of activity such as the sleep-wake cycle. Cerebellum - Cerebellum means little brain - Attached to the brainstem posterior to the pons. - Communicates with other regions of the CNS through the three large tracts: the superior, middle and inferior cerebellar peduncles which connect the cerebellum to the midbrain, pons and medulla oblongata. - Has a gray cortex and nuclei, with white medulla in between. - Cerebellar cortex has ridges called folia. - White matter of medulla resembles a branching tree and is called the arbor vitae. - Nuclei of the cerebellum are located in the deep inferior center of the white matter. - The cerebellar cortex contains several cell types, including stellate cells, basket cells, granule cells, golgi cells, and purkinje cells. - It also contains mossy fibers, ,which are afferent axons that branch extensively within the cerebellum. - Purkinje cells are the largest cells in the CNS. - They receive 200000 synapses, are inhibitory neurons and are the only cerebellar cortex neurons that send axons to the cerebellar nuclei. - The cerebellar cortex contains more than 10^12 neurons, more than the entire cerebral cortex. - Cerebellum consists of three parts: - 1. Flocculonodular lobe: simplest part of cerebellum, helps control balance and eye movements. - 2. A narrow central vermis: involved in the control of posture, locomotion, and fine motor coordination, thereby producing smooth flowing movements. - 3. Two large lateral hemispheres: function in concert with the frontal lobes of the cerebral cortex in planning, practicing and learning complex movements. - Each lateral hemisphere is divided by a primary fissure into an anterior lobe and a posterior lobe. - Lobes are subdivided into lobules which contain the folia. Diencephalon - Located between the brainstem and the cerebrum. Thalamus - Largest part of diencephalon. - Consists of a cluster of nuclei shaped like a yoyo with 2 large lateral portions connected in the center by a small stalk called the interthalamic adhesion or intermediate mass. - The space surrounding the interthalamic adhesion and separating the two large portions of the thalamus is the third ventricle of the brain. - Except for olfactory neurons, all sensory neurons that project to the cerebrum first synapse in the thalamus. - Thalamic neurons then send projections to the appropriate areas of the cerebral cortex where sensory input is localized. - Thalamus is considered the sensory relay center of the brain. - Axons carrying auditory information synapse in the medial geniculate nucleus of the thalamus. - Axons carrying visual info synapse in the lateral geniculate nucleus. - Most other sensory impulses synapse in the ventral posterior nucleus. - Axons originating in the ventral posterior nucleus project to the dorsal tier of nuclei, which register pain. - Other axons project to the cerebral cortex where sensory input is localized. - The ventral anterior and ventral lateral nuclei are involved
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