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NEUROANATOMY ANS NERVOUS SYSTEM • • - Central Peripheral Somatic Autonomic SOMATIC NERVOUS SYSTEM - Somatic sensory system – transmits sensations of touch, pain, temperature and position from sensory receptors - Somatic motor system – innervates skeletal muscles – contraction Somatic = voluntary, concious actions AUTONOMIC NERVOUS SYSTEM - Sympathetic – „fight or flight” - Parasympathetic – „ rest and digest” Autonomic - regulates the body's unconscious actions. SYMPATHETIC? PARASYMPATHETIC? The name of this system can be traced to the concept of sympathy, in the sense of "connection between parts", first used medically by Galen. Sympathetic and parasympathetic divisions typically function in opposition to each other. This natural opposition is better understood as complementary in nature rather than antagonistic. For an analogy, one may think of the sympathetic division as the police responders and the parasympathetic division as the court system. The sympathetic division typically functions in actions requiring quick responses. AUTONOMIC (VISCERAL) NERVOUS (MOTOR) SYSTEM - Target/effector organ (efferent fibers): smooth (involuntary) muscle, modified cardiac muscle (the intrinsic stimuating and conducing tissue of the heart) and glandular cells. - Afferent fibers – autonomic reflexes and visceral pain impulses conduction. - Regulation of visceral function – efferent and afferent fibers. Somatic innervation – single neuron!! ANS – 2 multipolar neurons!! grey A) presynaptic (preganglionic) – matter of CNS B)postsynaptic (postganglionic) – autonomic ganglia. Ganglion - group of nerve cell bodies located in the ANS. Ganglia house the cell bodies of afferent nerves. GANGLIA Three major groups of ganglia: -Dorsal root ganglia (also known as the spinal ganglia) contain the cell bodies of sensory (afferent) neurons. - Cranial nerve ganglia, contain the cell bodies of cranial nerve neurons. -Autonomic ganglia contain the cell bodies of autonomic nerves. The efferent nerve fibers and ganglia of ANS are organized in two divisions: A) Sympathetic – Thoracolumbar B) Parasympathetic - Craniosacral SYMPATHETIC (THORACOLUMBAR) Cell bodies of presynaptic neurons are located in interomediolateral cell columns (IMLs) (R&L) IML’s are a part of grey matter of the thoracic (T1-T12) and the upper lumbar (L1- L2/3) segments of the spinal cord. They are organized somatotopically. Cell bodies of postsynaptic neurons of the sympathetic NS occur in 2 locations: A) Paravertebral ganglia (R&L) B) Prevertebral ganglia A) Paravertebral ganglia (R&L) The sympathetic trunks are two ganglioned nerve trunks that extend the whole lenght of vertebral column. They are arranged thus: • Cervical ganglia- 3 ganglia (superior, middle and inferior) • Thoracic ganglia - 11 or 12 ganglia • Lumbar ganglia - 4 or 5 ganglia • Sacral ganglia - 4 or 5 ganglia Superior paravertebral (cervical) ganglion (base of the cranium) Ganglion impar (level of the coccyx) B) Prevertebral ganglia Are in the plexuses that round the origins of the main branches of the abdominal aorta The nerves that synapse in the prevertebral ganglia innervate the pelvic viscera. These include: 1. The celiac ganglia (which can include the aorticorenal ganglion), 2. Superior mesenteric ganglia, and 3. Inferior mesenteric ganglia. EFFERENT NERVE FIBERS OF THORACOLUMBAR DIVISON The IML’s of the spinal cord (Th1-L3) possess the cell bodies of of the preganglionic neurons. The myelinated axons of these cells leave the cord in the anterior nerve roots and pass via the white rami communicantes (white- covered with myelin) to the paravertebral ganglia of the sympathetic trunk. Once these fibers (preganglionic) reach the ganglia they are distributed as follows: PREGANGLIONIC FIBERS 1) Ascend and then synapse when appropriate nerves are superior to the part of IML involved (inervation of neck and upper limb) 2) Descend and then synapse when spinal nerves involved are inferior to to the part of IML’s involved (innervation of lower limb) 3) Ented and synapse immediately at the same level – when appropriate nerves are at approximately same level as the part of the IML’s involved ( innervation of middle trunk) 4) Pass through the ST without synapsing. These myelinated fibers leave the sympathetic trunk to reach prevertebral ganglia. (innervation of abdominopelvic viscera only) Term splanchnic is used to describe visceral organs. POSTGANGLIONIC FIBERS 1)Innervation of neck, body wall and limbs: from paravertebral ganglia of sympathetic trunks through grey rami communicantes to spinal nerves. Vasomotion – contraction of the blood vessels Pilomotion – „goose bumps” contraction of arrector muscle Sudomotion – sweating 2)Innervation of head (plus dilator muscle of iris). These nerves have cell bodies in superior cervical ganglion and they travel through cephalic arterial branch (peri-arterial plexus of nerves) and follow branches of carotid artery to reach their destination in head. SPLANCHNIC NERVES Include both efferent and afferent nerves to and from viscera. (through white rami comminicantes) A)Cardiopulmonary splanchnic nerve – postsynaptic nerves for viscera of the thoracic cavity (heart, lungs, esophageus etc). Begin in paravertebral ganglions of ST. B)Abdominopelvic splanchnic nerves (greater, lesser and least thoracic and lumbar) – presynaptic nerves for the viscera of abdominopelvic cavity (stomach, intestines etc.) Begin in IML’s and synapse in prevertebral ganglia. Then thy for peri-arterial plexuses, which follow branches of the abdominal aorta to their destination. C)Innervation of adrenal medulla - pass through prevertebral ganglia without synapsing and terminates directly on cells of adrenal gland. (EXCEPTION!) PARASYMPATHETIC (CRANIOSACRAL) Presynaptic parasympathetic nerve cell bodies are located in two sites within CNS and their fibers exit by two routes. Hence the craniosacral term. A)Cranial part - in the grey matter of brainstem, the fibers exit the CNS within cranial nerves III, VII, IX and X. Innervation of head (III,VII and IX) and thoracic and abdominal viscera (to the lext colic flexure) (X) B)Sacral part – in grey matter of S2-S4 segments (Intermediomedial cell column) of spinal cord, the fibers exit the CNS within sacral spinal nerves S2-S4 and the pelvic splanchnic nerves that arise from the anterior rami. Innervation of pelvic viscera and GI (descending and sigmoid colon and rectum) PARASYMPATHETIC - GANGLIA From the CNS to target organ – 2 neurons. Preganglionic nerve is long and synapses either in terminal ganglia which is located just by or on the target organ (n.X and S2-S4) or in parasympathetic ganglia of spinal nerves: 1.ciliary ganglion (sphincter pupillae, ciliary muscle) (n. III – occulomotor) 2.pterygopalatine ganglion (lacrimal gland, glands of nasal cavity) (n. VII – facial) 3. submandibular ganglion (submandibular and sublingual glands) (n. VII – facial ) 4.otic ganglion (parotid gland) (n. IX - glossopharyngeal) Postganglionic nerve is short. SYMPATHETIC VS PARASYMPATHETIC Parasympathetic distributes only to head, visceral cavities of the trunk and erectile tissues of the external genitalia. DOES NOT reach the body wall or limbs. Except from S2-S4 its fibers are not part of spinal nerves. Sympathetic: presynaptic fibers are short, postsynaptic long. Parasympathetic: presynaptic very long, postsynaptic short. Because postsynaptic cell bodies are located in or on the wall of the target organ. HOW DOES THE ANS WORK ON BLOOD VESSELS? Vasoconstriction – sympathetic excitation of smooth muscles of vessels. Vasodilation – decresaed sympathetic excitation of smooth muscles of vessels (exceptions: coronary, skeletal muscle and external genitalia – excitation results in vasodilation). IMPORTANT AUTONOMIC INNERVATIONS EYE Upper lid Superior cervical sympathetic ganglion sympathetic postganglionic fibers smooth muscle fibers of the levator palpabrae superioris. IRIS Dilator pupillae: Superior cervical sympathetic ganglion sympathetic postganglionic fibers in short and long ciliary nerves Sphincter pupillae: Edinger-Westphal nucleus n. III ciliary ganglion ->postganglionic fibers in short ciliary nerves LACRIMAL GLAND Superior salivary nucleus n. VII -> pterygopalatine ganglion ------- lacrimal nerve Superior cervical sympathetic ganglion sympathetic postganglionic fibers in plexus of the internal carotid artery ------- lacrimal nerve HORNER’S SYNDROME 1. constriction of the pupil 2. slight droping of the eyelid (ptosis) 3. enophthalmos 4. vasodilation of skin arterioles 5. loss of sweating (anhydrosis) HIRSCHPRUNG’S DISEASE Also called congenital megacolon or congenital aganglionic megacolon, is a form of megacolon that occurs when part or all of the large intestine or antecedent parts of the gastrointestinal tract have no ganglion cells and therefore cannot function. During normal prenatal development, cells from the neural crest migrate into the large intestine (colon) to form the networks of nerves called the Auerbach plexus (between the smooth muscle layers of the gastrointestinal tract wall) and the submucosal plexus (Meissner plexus) (within the submucosa of the gastrointestinal tract wall). In Hirschsprung's disease, the migration is not complete and part of the colon lacks these nerve bodies that regulate the activity of the colon. The affected segment of the colon cannot relax and pass stool through the colon, creating an obstruction. In most affected people, the disorder affects the part of the colon that is nearest the anus. In rare cases, the lack of nerve bodies involves more of the colon. VISCERAL AFFERENT FIBERS Provides information about the condition of the body’s internal environment. This information is integrated in the CNS often triggering visceral or somatic reflexes or both. Visceral reflexes regulate bood pressure and chemistry by altering such functions as heart and respiratory rates and vascular resistance. Visceral sensation that reaches a concious level is generally perceived as pain that is either poorly localized or felt as crumps or that may conver a feeling of hunger, fullness or nausea. VISCERAL PAIN Visceral pain is pain that results from the activation of nociceptors of the thoracic, pelvic, or bdominal viscera (organs). Visceral structures are highly sensitive to distension , stretch), ischemia and inflammation, but relatively insensitive to other stimuli that normally evoke pain such as cutting or burning. Visceral pain is diffuse, difficult to localize and often referred to a distant, usually superficial, structure. The pain may be described as sickening, deep, squeezing, and dull. VAGUS NERVE STIMULATION Vagus nerve stimulation or vagal nerve stimulation (VNS) is a medical treatment that involves delivering electrical impulses to the vagus nerve. It is used as an adjunctive treatment for certain types of intractable epilepsy and treatment-resistant depression. VAGUS NERVE ACTION. Vagus, the tenth cranial nerve, arises from the medulla and carries both afferent and efferent fibers. The afferent vagal fibers connect to the nucleus of the solitary tract which in turn projects connections to other locations in the central nervous system. Little is understood about exactly how vagal nerve stimulation modulates mood and seizure control but proposed mechanisms include alteration of norepinephrine release by projections of solitary tract to the locus coeruleus, elevated levels of inhibitory GABA related to vagal stimulation and inhibition of aberrant cortical activity by reticular activation system. Because the vagus nerve is associated with many different functions and brain regions, research is being done to determine its usefulness in treating other illnesses, including various anxiety disorders, Alzheimer's disease, migraines, fibromyalgia, obesity, and tinnitus. DIRECT VAGUS NERVE STIMULATION This is currently the only widely used method of therapeutic VNS. It requires the surgical implantation of a stimulator device. The VNS devices consist of a titaniumencased generator about the size of a pocket watch with a lithium battery to fuel the generator, a lead wire system with electrodes, and an anchor tether to secure leads to the vagus nerve. The battery life for the pulse generator is "between 1 [and] 16 years, depending on the settings. Implantation of the VNS device is usually done as an outpatient procedure. The procedure goes as follows: an incision is made in the upper left chest and the generator is implanted into a little "pouch" on the left chest under the clavicle. A second incision is made in the neck, so that the surgeon can access the vagus nerve. The surgeon then wraps the leads around the left branch of the vagus nerve, and connects the electrodes to the generator. Once successfully implanted, the generator sends electric impulses to the vagus nerve at regular intervals The left vagus nerve is stimulated rather than the right because the right plays a role in cardiac function such that stimulating it could have negative cardiac effects. NEUROANATOMY: BRAIN, VENTRICLES, MENINGES CENTRAL NERVOUS SYSTEM The CNS consists of the brain and spinal cord. The pricipal roles of the CNS are to integrate and coordinate incoming and outgoing neural signals and to carry out higher mental functions, such as thinking and learning. BRAIN - STRUCTURE 1. Telencephalon(Cerebrum) 2. Diencephalon 3. Brainstem A)Midbrain B)Pons C) Medulla oblongata 4. Cerebellum ANATOMICAL PLANES An anatomical plane is a hypothetical plane used to transect the human body, in order to describe the location of structures or the direction of movements. In human, three basic planes are used: 1. Saggital (lateral) 2. Coronal (frontal) 3. Horizontal (transverse, axial) WHITE AND GRAY MATTER A nucleus is a collection of nerve cell bodies in the CNS. A boundle of nerve fibers (axons) within the CNS connecting neighboring or distant nuclei of the cerebral cortex is a tract. The brain and spinal cord are composed of gray matter and white matter. The nerve cell bodies lie within and constitute the gray metter; the interconnecting fiber tract systems form the white matter. CEREBRUM The cerebrum is a large part of the brain containing the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb. The cerebrum is also divided into approximately symmetric left and right hemispheres. CEREBRAL CORTEX The cerebral cortex is the cerebrum's outer layer of neural tissue. It is divided into two cortices, along the sagittal plane: the left and right cerebral hemispheres divided by the medial longitudinal fissure. The cerebral cortex is composed of gray matter, consisting mainly of cell bodies and capillaries. It contrasts with the underlying white matter, consisting mainly of the white myelinated sheaths of neuronal axons. Each cortical ridge is called a gyrus, and each groove or fissure separating one gyrus from another is called a sulcus. BRODMANN AREA A Brodmann area is a region of the cerebral cortex defined by its cytoarchitecture, or histological structure and organization of cells It is correlated closely to diverse cortical functions. FRONTAL LOBE - Pimary motor cortex - Secondary motor cortex - Broca’s motor speech area PARIETAL LOBE - Primary somesthetic area - Secondary somesthetic area TEMPORAL LOBE - Primary auditory area - Secondary auditory area - Wernicke’s sensory speach area OCCIPITAL LOBE - Primary visual area - Secondary visual area IMPORTANT WHITE MATTER STRUCTURES - Semioval centre Corona radiata Corpus callosum Internal capsule Cerebral peduncles (midbrain) SEMIOVAL CENTER The semioval center is the white matter found underneath the grey matter on the surface of the cerebrum. The white matter, located in each hemisphere between the cerebral cortex and nuclei, as a whole has a semioval shape. It consists of cortical projection fibers, association fibers and cortical fibers. It continues ventrally as the corona radiata. CORONA RADIATA The corona radiata is a white matter sheet that continues ventrally as the internal capsule and dorsally as the semioval center. This sheet of both ascending and descending axons carries most of the neural traffic from and to the cerebral cortex. The corona radiata is associated with the corticospinal tract, the corticopontine tract, and the corticobulbar tract. CORPUS CALLOSUM The corpus callosum also known as the callosal commissure, is a wide, flat bundle of neural fibers about 10 cm long beneath the cortex in the brain at the longitudinal fissure. It connects the left and right cerebral hemispheres and facilitates interhemispheric communication. It is the largest fibre pathway in the brain. Rostrum, genu, trunk and splenium – parts. CEREBRUM The cerebrum is a large part of the brain containing the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb. The cerebrum is also divided into approximately symmetric left and right hemispheres. OLFACTORY BULB The olfactory bulb (bulbus olfactorius) is a neural structure of the cerebrum involved in olfaction, or the sense of smell. Olfactory bulb is on the inferior (bottom) side of the brain. The olfactory bulb is supported and protected by the cribriform plate of the ethmoid bone, which separates it from the olfactory epithelium, and which is perforated by olfactory nerve axons. The bulb is divided into two distinct structures: the main olfactory bulb and the accessory olfactory bulb. BASAL NUCLEI The basal ganglia (or basal nuclei) comprise multiple subcortical nuclei. Basal ganglia nuclei are strongly interconnected with the cerebral cortex, thalamus, and brainstem, as well as several other brain areas. The basal ganglia are associated with a variety of functions including: control of voluntary motor movements, procedural learning, routine behaviors or "habits" such as bruxism, eye movements, cognition and emotion. COMPONENTS OF BASAL GANGLIA TELENCEPHALON - Caudate nucleus - Putamen - Globus pallidus DIENCEPHALON AND MIDBRAIN - substantia nigra, - subthalamic nucleus. - red nucleus TERMINOLOGY • Striatum (nucleus caudatus and putamen) • Nucleus lentiformis (globus pallidus and putamen) BASAL GANGLIA - FUNCTION Currently, popular theories implicate the basal ganglia primarily in action selection; that is, it helps determine the decision of which of several possible behaviors to execute at any given time. In more specific terms, the basal ganglia's primary function is likely to control and regulate activities of the motor and premotor cortical areas so that voluntary movements can be performed smoothly. BASAL GANGLIA IMPAIRMENT The importance of these subcortical nuclei for normal brain function and behavior is emphasized by the numerous and diverse neurological conditions associated with basal ganglia dysfunction, which include: - disorders of behavior control such as Tourette syndrome, hemiballismus, and obsessive–compulsive disorder - dystonia; - addiction; - movement disorders a) Parkinson's disease, which involves degeneration of the dopamineproducing cells in the SUBSTANTIA NIGRA b) Huntington's disease, which primarily involves damage to the STRIATUM. LIMBIC SYSTEM The limbic system is a complex set of brain structures located on both sides of the thalamus, right under the cerebrum. It is not a separate system but a collection of structures from the cerebrum, diencephalon, and midbrain. It includes: - olfactory bulbs, - hippocampus, - hypothalamus, - amygdala, - fornix, - mammillary body, - limbic cortex, The limbic system supports a variety of functions including emotion, behavior, motivation, long-term memory, and olfaction. Emotional life is largely housed in the limbic system, and it has a great deal to do with the formation of memories. HIPPOCAMPUS The hippocampus (named after its resemblance to the seahorse), is a major component of the brain. Human brain has 2 hippocampi: one in each side of the brain. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. The hippocampus is located under the cerebral cortex; it is located in the medial temporal lobe, underneath the cortical surface. It contains two main interlocking parts: the hippocampus proper (also called Ammon's horn) and the dentate gyrus. In Alzheimer's disease, the hippocampus is one of the first regions of the brain to suffer damage; memory loss and disorientation are included among the early symptoms. DIENCEPHALON The diencephalon consists of structures that are lateral to the third ventricle, and include the thalamus, the hypothalamus, the epithalamus and the subthalamus. Diencephalon connects the midbrain to the cerebral hemispheres. THALAMUS The thalamus is a vital structure lying deep within the brain that has several important functions, such as sensory and motor function, attention, memory, speech, and emotion. Various thalamic nuclei with extensive nerve networks send signals all around the structures of the brain including the cerebral cortex. Thalamic lesions cause a wide variety of clinical symptoms; therefore, anatomical knowledge of the thalamus is important in clinical situations. HYPOTHALAMUS The hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions of the hypothalamus is to link the nervous system to the endocrine system via the pituitary gland (hypophysis). The hypothalamus is located below the thalamus and is part of the limbic system. It forms the ventral part of the diencephalon. The hypothalamus is responsible for certain metabolic processes and other activities of the autonomic nervous system. It synthesizes and secretes certain neurohormones, called releasing hormones and these in turn stimulate or inhibit the secretion of pituitary hormones. HYPOTHALAMUS ANATOMY The hypothalamus occupies the ventral diencephalon and is composed of numerous fiber tracts and nuclei situated symmetrically about the third ventricle. In sagittal section, the hypothalamus is roughly diamond shaped; although its boundaries are not sharply demarcated, its perimeters can be correlated using neuroanatomic landmarks. Rostrally, the hypothalamus extends from the anterior commissure, lamina terminalis, and optic chiasm. Caudally, the hypothalamus extends to the periaqueductal gray matter of the midbrain, approximated by (from ventral to dorsal) the mammillary bodies, interpeduncular fossa, and cerebral peduncles. In the coronal plane, the boundaries of the hypothalamus are more distinct. Superiorly, the hypothalamus is divided from the thalamus by a groove in the lateral wall of the third ventricle, the hypothalamic sulcus. The lateral surface is contiguous with the thalamus and subthalamus and is bordered by the internal capsule and optic tracts. Medially, the hypothalamus is bound by the ependyma of the third ventricle. Finally, the inferior surface is continuous with the floor of the third ventricle. The external surface of the hypothalamic floor projects into the interpeduncular cistern. A median protuberance, the tuber cinereum lies between the optic chiasm (rostrally) and mammillary bodies (caudally) and is continuous anteriorly with the lamina terminalis. This projection continues as the infundibulum, terminating inferiorly on the pituitary gland. EPITHALAMUS The epithalamus comprises the habenular trigone, the pineal gland, and the habenular commissure. It is wired with the limbic system and basal ganglia. The function of the epithalamus is to connect the limbic system to other parts of the brain. Some functions of its components include the secretion of melatonin by the pineal gland (involved in circadian rhythms), and regulation of motor pathways and emotions. SUBTHALAMUS The subthalamus is a part of the diencephalon. Its most prominent structure is the subthalamic nucleus. The subthalamus connects to the globus pallidus, a basal nucleus. The subthalamus is located ventral to the thalamus, medial to the internal capsule and lateral to the hypothalamus. It is a region formed by several grey matter nuclei and their associated white matter structures, namely: - The subthalamic nucleus - Zona incerta - Subthalamic fasciculus, - Fields of Forel - Ansa lenticularis BRAINSTEM 1. Midbrain (continuous with the cerebral hemisphere above) 2. Pons 3. Medulla oblongata (continuous with the spinal cord below) The brainstem is located in posterior cranial fossa. CRANIAL NERVES The III and IV nerves emerge from the surface of the midbrain. The V from the pons. The VI , VII and VIII nerves emerge at the junction of the pons and medulla. The IX, X, XI and XII nerves emerge from the surface of the medulla. MIDBRAIN Parts of the midbrain The midbrain comprises two lateral halves, called the cerebral peduncles; which is again divided into an anterior part, the crus cerebri, and a posterior part, tegmentum, by a pigmented band of gray matter, substantia nigra. The narrow cavity is the cerebral aqueduct, which connects the 3rd and 4th ventricles. The tectum is the part of the midbrain posterior to the cerebral aqueduct; it has four small surface swellings two superior and two inferior colliculi. PONS Pons has a convex anterior surface marked by transversely running fibers which laterally forms a bundle called middle cerebellar peduncle. Main Features The trigeminal nerve emerges from the anterior surface at its junction with middle cerebellar peduncle. Presents a basilar sulcus in the midline which lodges basilar artery. In the groove between Pons and the medulla oblongata, there emerge, from medial to lateral, abducent, facial and vestibulocochlear nerves. Posterior surface of the pons is limited laterally by superior cerebellar peduncle and forms the upper part of the floor of the 4th ventricle. Main Features: The floor is divided into symmetrical halves by a median sulcus. Lateral to this sulcus is an elongated elevation, the medial eminence, which is bounded laterally by a sulcus limitans. Inferior end of medial eminence is slightly expanded to form facial colliculus, which is produced by facial nerve. The upper end of sulcus limitans presents a bluish-gray coloration and the area is called substantia ferruginosa. Area vestibule lies lateral to sulcus limitans. Parts of the Pons a posterior part, the tegmentum, and an anterior basilar part MEDULLA OBLONGATA The medulla oblongata is conical in shape. Its broad part joins the pons above and narrow part becomes continuous with the spinal cord. The junction between medulla and spinal cord coincides with the level of the upper border of Atlas (first cervical vertebra). Its length is about 3 cm and its width is about 2cm at its upper end. It is divided into 1. A lower closed part with central canal and 2. An upper open part posteriorly which is related to the lower part of the 4th ventricle Features on the anterior surface of Medulla Oblongata Anterior median fissure, is an upward continuation of similar fissure present on the spinal cord Anterolateral sulcus, on each side, is in line with the ventral roots of spinal cord -Gives attachment to the rootlets of the hypoglossal nerve Pyramid is an elevation on each side of the midline between anterior median fissure and anterolateral sulcus. Composed of bundles of nerve fibers of corticospinal tract that descends from the cerebral cortex to the spinal cord -Tapers inferiorly where the majority of fibers cross over to the opposite side, obliterating the medulla. These crossing fibers constitute the decussation of the pyramid. Olive is a prominent, elongated oval swelling that lies in the upper part of medulla posterolateral to the pyramid separated by anterolateral sulcus. The elevation is produced by the underlying inferior olivary nucleus. Features on posterior surface of the medulla oblongata Posterior median sulcus is upward continuation of the similar fissure on the spinal cord. Posterolateral sulcus lies in line with the dorsal roots of spinal nerves. Gives attachment to the rootlets of IX, X and XI cranial nerves. Between the posterior median sulcus and posterolateral sulcus, the medulla contains tracts (asccending) that enter it from the posterior funiculus of the spinal cord. Fasciculus gracilis lies medially and fasciculus cuneatus lies laterally Both fasciculi end in rounded elevations called gracile tubercle (nucleus gracilis) and cuneate tubercle (nucleus cuneatus) respectively. Just above these tubercles, medulla is occupied by a triangular fossa which forms the lower part of the 4th ventricle. This fossa is bounded on each side by inferior cerebellar peduncle which connect the medulla to cerebellum. Features on the posterior part of the medulla that forms the floor of the 4th ventricle: Presents median sulcus, on each side of which there is a longitudinal elevation called the median eminence (continuous above in the pontine part of the floor of 4th ventricle). The eminence is bounded laterally by sulcus limitans. The sulcus limitans is marked by a depression called inferior fovea. The part of the medulla below fovea presents hypoglossal triangle medially and vagal triangle laterally. Between the vagal triangle, above and gracile tubercle, below lies a small area called area postrema. The lowest part of the floor is called the calamus scriptorius (for its resemblance to a nib). The inferior angle where the lateral margins of the floor meet is called obex. The ventricular system is a set of communicating cavities within the brain. These structures are responsible for the production, transport and removal of cerebrospinal fluid, which bathes the central nervous system. FUNCTIONS OF CEREBROSPINAL FLUID Cerebrospinal fluid is an ultrafiltrate of plasma that surrounds the brain and spinal cord. It serves three main functions: Protection – It acts as a cushion for the brain, limiting neural damage in cranial injuries. Buoyancy – By being immersed in CSF, the net weight of the brain is reduced to approximately 25 grams. This prevents excessive pressure on the base of the brain. Chemical stability – The CSF creates an environment to allow for proper functioning of the brain. E.g. Maintaining low extracellular K+ for synaptic transmission.