Nervous System Block 3 Module 1 Cases 1-2 PDF
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Dr. Cecille Espino
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This document provides an overview of the nervous system, focusing on the structure and function of the scalp's components, including nerves, arteries, and veins. The document details the different layers of the scalp and their neural and vascular supply .
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NERVOUS SYSTEM SYSTEM COORDINATOR: DR. CECILLE ESPINO BLOCK 3 MODULE 1 (CASES 1 AND 2) BASIC BIOMEDICAL SCIENCES I NOT FOR SALE | DO NOT UPL...
NERVOUS SYSTEM SYSTEM COORDINATOR: DR. CECILLE ESPINO BLOCK 3 MODULE 1 (CASES 1 AND 2) BASIC BIOMEDICAL SCIENCES I NOT FOR SALE | DO NOT UPLOAD IN ONLINE SITES ------------------------------------------------------------------------------------------------------------------------------------------------------- CASE 1 The zygomaticotemporal nerve, a branch of the maxillary SCALP division of the trigeminal nerve, supplies the scalp over the temple. The scalp extends from the superciliary arches anteriorly to the external The auriculotemporal nerve, a branch of the mandibular division occipital protuberance and superior nuchal lines posteriorly and to the of the trigeminal nerve, ascends over the side of the head from in temporal lines laterally. front of the auricle. Its terminal branches supply the skin over the temporal region. Cervical spinal nerve branches: The lesser occipital nerve (C2), a branch of the cervical plexus, ascends along the posterior edge of the sternocleidomastoid muscle and supplies the scalp over the lateral part of the occipital region and the skin over the medial surface of the auricle. The greater occipital nerve, a branch of the posterior ramus of the second cervical nerve, ascends over the back of the scalp and supplies the skin as far forward as the vertex of the skull. Arterial Supply o The scalp has a rich blood supply to nourish the hair follicles, and, for this reason, the smallest cut bleeds profusely. o As with the cutaneous nerves, the arteries run through the dense It consists of five layers. Conveniently, the first letters of each layer connective tissue layer (the "C" layer) of the scalp, typically follow the together spell SCALP, making recall easier. The first three layers are nerves, and form an extensive, freely anastomosing network. Moving intimately bound together and move as a unit. laterally from the anterior midline, the following arteries are present: o Skin- thick and hair bearing and contains numerous sebaceous glands. The supratrochlear and the supraorbital arteries, branches of the o Connective tissue beneath the skin ophthalmic artery (a branch of the internal carotid artery), ascend This is a dense fibrofatty layer containing fibrous septa that unite the over the forehead in company with the supratrochlear and skin to the underlying epicranial aponeurosis. supraorbital nerves. This layer contains numerous blood vessels. The arteries are The superficial temporal artery, the smaller terminal branch of the derived from both the external and internal carotid arteries, and free external carotid artery, ascends in front of the auricle in company anastomoses occur between them. with the auriculotemporal nerve. It divides into anterior and posterior o Aponeurosis (epicranial) branches, which supply the skin over the frontal and temporal This is a thin, tendinous sheet that unites the occipital and frontal regions. bellies of the occipitofrontalis muscle. The posterior auricular artery, a branch of the external carotid The lateral margins of the aponeurosis are attached to the temporal artery, ascends behind the auricle to supply the scalp above and fascia. The subaponeurotic space is the potential space deep to the behind the auricle. epicranial aponeurosis. The occipital artery, a branch of the external carotid artery, It is limited in front and behind by the origins of the occipitofrontalis ascends from the apex of the posterior triangle in company with the muscle, and it extends laterally as far as the attachment of the greater occipital nerve and pierces the trapezius muscle to reach the aponeurosis to the temporal fascia. scalp. It supplies the skin over the back of the scalp and reaches as o Loose areolar tissue high as the vertex of the skull. This occupies the subaponeurotic space and loosely connects the Venous Drainage epicranial aponeurosis to the periosteum of the skull (the o The supratrochlear and supraorbital veins unite at the medial pericranium). margin of the orbit to form the facial vein. This is the plane of movement of the scalp, that is, when the scalp o The superficial temporal vein unites with the maxillary vein in the moves, the first three layers (SCA) slide along this layer relative to substance of the parotid gland to form the retromandibular vein. the underlying periosteum. o The posterior auricular vein unites with the posterior division of The areolar tissue contains a few small arteries, but it also contains the retromandibular vein, just below the parotid gland, to form the some important emissary veins. The emissary veins are valveless external jugular vein. and connect the superficial veins of the scalp with the diploic veins o The occipital vein drains into the suboccipital venous plexus, of the skull bones and with the intracranial venous sinuses. which lies beneath the floor of the upper part of the posterior triangle; o Pericranium the plexus in turn drains into the vertebral veins or the internal jugular The pericranium is the periosteum covering the outer surface of the vein. skull bones. The pericranium is continuous with the periosteum on o The veins of the scalp freely anastomose with one another and are the inner surface of the skull bones (endosteum) at the sutures connected to the diploic veins of the skull bones and the intracranial between the individual skull bones. venous sinuses by valveless emissary veins. Sensory Nerve Supply o The main trunks of the sensory nerves lie in the dense connective tissue layer (the "C" layer) of the scalp. o The nerves are arranged in two main groups: (1) branches of the trigeminal nerve located anterior to the ear and (2) branches of cervical spinal nerves located posterior to the ear. o Moving laterally from the anterior midline, the following nerves are present. Trigeminal branches: The supratrochlear nerve, a branch of the ophthalmic division of the trigeminal nerve, winds around the superior orbital margin and supplies the scalp. It passes backward close to the median plane MENINGES OF THE BRAIN AND SPINAL CORD and reaches nearly as far as the vertex of the skull. Embryologic Origin The supraorbital nerve, a branch of the ophthalmic division of the o The meninges are derived from the cells of the neural crest and the trigeminal nerve, winds around the superior orbital margin and mesenchyme (mesoderm), which migrate to surround the developing ascends over the forehead. It supplies the scalp as far backward CNS between 25 and 35 days of gestation. as the vertex. o By the end of the first trimester, the meninges have generally reached This folding is responsible for separating the cranial cavity into the overall plan as seen around the adult brain and spinal cord. supratentorial and infratentorial compartments. Main Layers The tentorial notch is formed by the union of the edges of the right o The brain in the skull is surrounded by three protective membranes or and left tentorial with the straight sinus. meninges: the dura mater, the arachnoid mater, and the pia mater. The occipital lobe is above the tentorium, the cerebellum is below it o Dura Mater and the midbrain passes through the tentorial notch. Outermost layer; also called the pachymeninx o Falx Cerebelli It is thick, tough and fibrous. The 3rd dural infolding is the falx cerebelli which is located below the Has 2 layers, the endosteal layer and the meningeal layer which are tentorium cerebelli on the midline of the occipital bone. closely united except along certain lines where they separate to form This small infolding extends into the space found between the venous sinuses. cerebellar hemispheres and usually contains a small occipital sinus Endosteal layer- the periosteum covering the inner surface of the that communicates with the confluence of sinuses. skull bones o Diaphragma Sella Meningeal layer- is the dura mater proper which is a dense, The diaphragma sella is the smallest of the dural foldings. strong fibrous membrane covering the brain and is continuous It forms the roof of the hypophyseal fossa and encircles the pituitary through the foramen magnum with the dura mater of the spinal stalk. cord The cavernous sinuses are found beside the sella turcica while the These venous sinuses are found where the dura mater gives rise anterior and posterior intracavernous sinuses are found within their to indeed folding or reflections. respective edges of the diaphragma sella. The spinal dura mater lacks periosteal layer and ends at the level of the 2nd sacral vertebrae as a blind sac. o Arachnoid Mater Middle layer Has 2 parts, the arachnoid buried cell layer which is directly opposed to the dural border, and the spindly cells that traverse the subarachnoid space constituting the arachnoid trabeculae. A delicate, impermeable membrane covering the brain and lying between the pia mater internally and the dura mater externally. Separated from the dura by a potential space, the subdural space, filled by a film of fluid; it is separated from the pia by the subarachnoid space, which is filled with CSF. The outer and inner surfaces are covered with flattened mesothelial cells. Epidural space found between the cranium and cranial dura mater and vertebral column and spinal dura mater. The arachnoid projects into the venous sinuses to form arachnoid villi, which serve as sites where the CSF diffuses into the bloodstream. Aggregations of arachnoid villi are referred to as arachnoid granulations. o Pia Mater A vascular membrane covered by flattened mesothelial cells that closely invests the brain, covering the gyri and descending into the deepest sulci. It extends out over the cranial nerves and fuses with their epineurium. The cerebral arteries entering the substance of the brain carry a sheath of pia with them. The pia mater and the arachnoid layers form the leptomeninges. It is a double-layered membrane. Specializations of Spinal Pia Mater: Linea splendens- Ribbon like glistening structure lying over the anterior median fissure. Denticulate ligament- Tools like lateral projection found between the antero-lateral and postero-lateral sulci. Filum terminale- Thread like structure extending from the tip of the conus medullaris (cone shaped termination or end of the spinal cord) Dural Foldings o Falx Cerebri Largest Dural Nerve Supply It is attached to the crista rostrally to the midline of the inner surface o Branches of the trigeminal, vagus, and 1st three cervical spinal nerve of the skull, and to the surface of the tentorium cerebelli (another and branches from the sympathetic trunk pass to the dura folding) caudally. o The dura possesses numerous sensory ending that are sensitive to Separates the right hemisphere from the left. stretching, which produces the sensation of headache. The superior sinus found where the falx cerebri is attached to the Stimulation of the sensory endings of the trigeminal nerve above the straight sinus where it fuses with the tenrorium cerebelli, and the level of the tentorium cerebelli produces referred pain to an area of inferior sagittal sinus in its free edge. skin on the same side of the head. o Tentorium Cerebelli Stimulation of the dural endings below the level of the tentorium 2nd largest infolding. produces pain referred to the back of the neck and the back of the Rostrally it is attached to the clinoid processes where the superior scalp along the distribution of the greater occipital nerve. petrosal sinus is located, and caudolateral to the inner surface of the Dural Arterial Supply occipital bone and a small part of the parietal bone where the o Numerous arteries supply the dura mater from the internal carotid, transverse sinus is found. maxillary, ascending pharyngeal, occipital, and vertebral arteries. The most important is the middle meningeal artery, which can be o In the lateral recumbent position, the pressure, as measured by spinal damaged in head injuries. tap, is about 60 to 150 mm of water. o The middle meningeal artery arises from the maxillary artery in the o This pressure may be raised by straining, coughing, or compressing infratemporal fossa. It enters the cranial cavity through the foramen the internal jugular veins in the neck spinosum and then lies between the meningeal and endosteal layers o Functions of dura. The artery then runs forward and laterally in a groove on the Cushions and protects the central nervous system from trauma upper surface of the squamous part of the temporal bone. Provides mechanical buoyancy and support for the brain The anterior branch deeply grooves or tunnels the anterior-inferior Serves as a reservoir and assists in the regulation of the contents of angle of the parietal bone, and its course corresponds roughly to the the skull line of the underlying precentral gyrus of the brain. Nourishes the central nervous system The posterior branch curves backward and supplies the posterior Removes metabolites from the central nervous system part of the dura mater. Serves as pathway for pineal secretions to reach the pituitary gland o The meningeal veins lie in the endosteal layer of dura. The middle meningeal vein follows the branches of the middle meningeal artery and drains into the pterygoid venous plexus or the sphenoparietal sinus. The veins lie lateral to the arteries. Dural Venous Sinuses o The venous sinuses of the cranial cavity are situated between the layers of the dura mater. o Their main function is to receive blood from the brain through the cerebral veins and the CSF from the subarachnoid space through the arachnoid villi. o The blood in the dural sinuses ultimately drains into the internal jugular veins in the neck. The dural sinuses are lined by endothelium, and Circulation their walls are thick but devoid of muscular tissue. They have no valves. o Cerebrospinal fluid is formed at a rate of about 500 ml/day, which is Functions three to four times as much as the total volume of fluid in the entire o Protect the underlying brain and spinal cord cerebrospinal fluid system. o Serve as a support framework for important activities veins and o About two-thirds or more of this fluid originates as secretion from the sinuses choroid plexuses in the four ventricles, mainly in the two lateral o Enclose a fluid filled cavity, the subarachnoid space which is vital and ventricles. Additional small amounts of fluid are secreted by the normal function of the brain and spinal cord ependymal surfaces of all the ventricles and by the arachnoidal CEREBROSPINAL FLUID membranes. A small amount comes from the brain through the The cerebrospinal fluid is a colorless liquid, similar to plasma in its ionic perivascular spaces that surround the blood vessels passing through composition the brain. At present, cerebrospinal fluid is considered to be an actively secreted o The arrows (in picture) show that the main channels of fluid flow from product whose composition is dictated by specific transport mechanisms the choroid plexuses and then through the cerebrospinal fluid system. The majority of cerebrospinal fluid is produced primarily by the choroid The fluid secreted in the lateral ventricles passes first into the third plexus. ventricle; then, after addition of minute amounts of fluid from the third The cerebrospinal fluid apparently is formed by filtration of blood through ventricle, it flows downward along the aqueduct of Sylvius into the the fenestrations of choroidal capillaries, followed by the active transport fourth ventricle, where still another minute amount of fluid is added. of substances (specifically sodium ions) across the choroids epithelium o Finally, the fluid passes out of the fourth ventricle through three small into the ventricle openings, two lateral foramina of Luschka and a midline foramen of Magendie, entering the cisterna magna, a fluid space that lies To maintain osmotic balance, water flows through the epithelium behind the medulla and beneath the cerebellum. CSF is not only formed in the choroids plexus. Extrachoroidal CSF o The cisterna magna is continuous with the subarachnoid space that comes primarily from the brain parenchyma, with fluid moving across the surrounds the entire brain and spinal cord. ependymal lining into the ventricle. The exact amount of CSF formed Almost all the cerebrospinal fluid then flows upward from the cisterna from this source is not exactly known but most authors would agree that magna through the subarachnoid spaces surrounding the cerebrum. well over half of CSF is produced by the choroid plexus. From here, the fluid flows into and through multiple arachnoidal villi The rate of formation of new CSF (an average of 0.35 ml/min. In Snell it that project into the large sagittal venous sinus and other venous is 0.5 ml/min) is relatively constant and is minimally affected by the sinuses of the cerebrum. Thus, any extra fluid empties into the systemic blood pressure or intraventricular pressure, implicating that the venous blood through pores of these villi. total volume of CSF is renewed more than 3x per day. o The rate of CSF formation is approximately 500 mL per day (300 mL The entire cerebral cavity enclosing the brain and spinal cord has a secreted by the choroid plexus and another 200 mL produced from capacity of about 1600 to 1700 ml. About 150 ml of this capacity is other sources). occupied by cerebrospinal fluid and the remainder by the brain and cord. The total CSF volume in an adult is about 150 mL (25 to 50 mL This fluid is present in the ventricles of the brain, in the cisterns within the ventricular system and 100 mL in the subarachnoid around the outside of the brain, and in the subarachnoid space around space); therefore, this total volume is replaced 2-3 times a day. both the brain and the spinal cord. All these chambers are connected o Subarachnoid CSF passes into the brain along paravascular spaces with one another, and the pressure of the fluid is maintained at a that encircle arterioles and, at this level or at the level of the capillary surprisingly constant level. endothelium, can pass into the closely opposed astroglial end-feet, Physical Properties and Composition which contribute to the formation of the blood-brain barrier. o The CSF is found in the ventricles of the brain and in the subarachnoid Abnormality in CSF Accumulations space around the brain and spinal cord. o A disturbance in CSF hydrodynamics will cause an accumulation of o Has a volume of about 150 mL. CSF within the ventricular system: a state of hydrocephalus. o Clear, colorless fluid and possesses, in solution, inorganic salts similar o Causes to those in the blood plasma. Hydrocephalus is an abnormal increase in CSF volume within the o Glucose content is about half that of blood, with only a trace of protein. skull. If the hydrocephalus is accompanied by raised CSF pressure, o Only a few cells are present, and these are lymphocytes. then it is due to one of the following: (1) abnormal increase in CSF o The normal lymphocyte count is 0 to 3 cells per cubic millimeter. formation, (2) blockage of CSF circulation, and (3) diminished CSF o CSF pressure is kept remarkably constant. absorption. Rarely, hydrocephalus occurs with normal CSF pressure; these patients exhibit compensatory hypoplasia or atrophy Ligamentum flavum, (6) Areolar tissue containing the internal of the brain substance. vertebral venous plexus, (7) Dura mater, and (8) Arachnoid mater Excessive Cerebrospinal Fluid Formation The cerebrospinal fluid pressure may be measured by attaching a Excessive CSF formation is a rare condition that may occur with a manometer to the needle. tumor of the choroid plexuses. The depth to which the needle will have to pass will vary from 1 inch Blockage of Cerebrospinal Fluid Formation (2.5 cm) or less in a child to as much as 4 inches (10cm) in an obese An obstruction of the interventricular foramen by a tumor will adult. Puncture above the L2-L3 interspace is inadvisable. block the drainage of the lateral ventricle on that side. Continued In children, puncture is performed in the L4-L5 or L5-S1 interspace. CSF production by the choroid plexus of that ventricle will cause Indications distention of that ventricle and atrophy of the surrounding neural o To withdraw a sample of CSF for microscopic or bacteriologic tissue. examination An obstruction in the cerebral aqueduct may be congenital or o To inject drugs to combat infection may result from inflammation or pressure from a tumor. This o To induce anesthesia causes a symmetrical distention of both lateral ventricles and Contraindications distention of the third ventricle. o Known or suspected increased intracranial pressure (especially those Obstruction of the median aperture (foramen of Magendie) in the of localized mass lesions) roof of the fourth ventricle and the two lateral apertures (foramina o Sudden alteration in CSF pressure dynamics can lead to herniation of of Luschka) in the lateral recesses of the fourth ventricle by the brain contents through the foramen magnum and clinical inflammatory exudate or by tumor growth will produce symmetrical decompensation or death. dilatation of both lateral ventricles and the third and fourth VENTRICLES OF THE BRAIN ventricles. The ventricles are 4 fluid-filled cavities located within the brain: Sometimes, inflammatory exudate secondary to meningitis will o The two lateral ventricles communicate through the interventricular block the subarachnoid space and obstruct CSF flow over the foramina (of Monro) with the third ventricle. outer surface of the cerebral hemispheres. Here again, the entire o The third ventricle is connected to the fourth ventricle by the narrow ventricular system of the brain will become distended. cerebral aqueduct (aqueduct of Sylvius). Diminished Cerebrospinal Fluid Absorption o The fourth ventricle, in turn, is continuous with the narrow central Interference with CSF at the arachnoid granulations may be canal of the spinal cord and, through the 3 foramina in its roof, with the caused by inflammatory exudate, venous thrombosis or pressure subarachnoid space. on the venous sinuses, or obstruction of the internal jugular vein. The central canal in the spinal cord has a small dilatation at its inferior o Varieties (Types) end, referred to as the terminal ventricle. The ventricles are lined Two varieties of hydrocephalus are described throughout with ependyma and are filled with CSF. The ventricles are Non-communicating hydrocephalus- the raised CSF pressure is developmentally derived from the cavity of the neural tube. due to blockage at some point between its formation at the choroid Lateral Ventricles plexuses and its exit through the foramina in the roof of the fourth o One of each of the two large lateral ventricles is present in each ventricle. cerebral hemisphere Communicating hydrocephalus- no obstruction exists within or to o A roughly C-shaped cavity and may be divided into a body which the outflow from the ventricular system; the CSF freely reaches the occupies the parietal lobe and from which anterior, posterior, and subarachnoid space and is found to be under increased pressure inferior horns extend into the frontal, occipital, and temporal lobes, Lumbar Puncture respectively. o Lumbar puncture is a procedure of obtaining CSF from the o The lateral ventricle communicates with the cavity of the third ventricle subarachnoid space of the lumbar vertebrae for routine diagnostic through the interventricular foramen. This opening, which lies in the tests. anterior part of the medial wall of the ventricle, is bounded anteriorly o Procedure by the anterior column of the fornix and posteriorly by the anterior end of the thalamus. o The body of the lateral ventricle extends from the interventricular foramen posteriorly as far as the posterior end of the thalamus. Here, it becomes continuous with the posterior and the inferior horns. The body of the lateral ventricle has a roof, a floor, and a medial wall. The roof is formed by the undersurface of the corpus callosum. The floor is formed by the body of the caudate nucleus and the lateral margin of the thalamus. The superior surface of the thalamus is obscured in its medial part by the body of the fornix. The choroid plexus of the ventricle projects into the body of the ventricle through the slit-like gap between the body of the fornix and the superior surface of the thalamus. This slit-like gap is known as the choroidal fissure; through it, the blood vessels of the plexus invaginate the pia mater of the tela choroidea and the ependyma of the lateral ventricle. The medial wall is formed by the septum pellucidum anteriorly; With the patient lying on his or her side or in the upright position, posteriorly, the roof and the floor come together on the medial wall. sitting with the vertebral column well flexed, the space between The anterior horn of the lateral ventricle extends forward into the adjoining laminae in the lumbar region is opened to maximum. frontal lobe. It is continuous posteriorly with the body of the ventricle An imaginary line joining the highest points on the iliac crests passes at the interventricular foramen. over the fourth lumbar spine. The anterior horn has a roof, a floor, and a medial wall. Using a careful aseptic technique and local anesthesia, the The roof is formed by the undersurface of the anterior part of the physician passes the lumbar puncture needle fitted with stylet, into corpus callosum; the genu of the corpus callosum limits the the vertebral canal above or below the fourth lumbar spine. anterior horn anteriorly. The needle will pass through the following anatomical structures The floor is formed by the rounded head of the caudate nucleus; before it enters the subarachnoid space: (1) Skin, (2) Superficial medially, a small portion is formed by the superior surface of the fascia, (3) Supraspinous ligament, (4) Interspinous ligament, (5) rostrum of the corpus callosum. The medial wall is formed by the septum pellucidum and the cells of the hippocampus. These axons converge on the medial anterior column of the fornix. border of the hippocampus to form a bundle known as the fimbria. The posterior horn of the lateral ventricle extends posteriorly into The fimbria of the hippocampus becomes continuous posteriorly the occipital lobe. with the posterior column of the fornix. The roof and lateral wall are formed by the fibers of the tapetum In the interval between the stria terminalis and the fimbria is the of the corpus callosum. Lateral to the tapetum are the fibers of temporal part of the choroidal fissure. Here, the lower part of the the optic radiation. choroid plexus of the lateral ventricle invaginates the ependyma The medial wall of the posterior horn has two elevations. The from the medial side and closes the fissure. superior swelling is caused by the splenial fibers of the corpus callosum, called the forceps major, passing posteriorly into the occipital lobe; this superior swelling is referred to as the bulb of the posterior horn. The inferior swelling is produced by the calcarine sulcus and is called the calcar avis. Third Ventricle o A slit-like cleft between the two thalami. o It communicates anteriorly with the lateral ventricles through the interventricular foramina (of Monro) and posteriorly with the fourth ventricle through the cerebral aqueduct (of Sylvius). Cerebral Aqueduct (aqueduct of Sylvius) o A narrow channel about 3/4 of an inch (1.8 cm) long, connects the third ventricle with the fourth ventricle. o It is lined with ependyma and is surrounded by a layer of gray matter called the central gray. o The cerebral aqueduct does not have a choroid plexus. Fourth Ventricle o A tent-shaped cavity filled with CSF. o It is situated anterior to the cerebellum and posterior to the pons and the superior half of the medulla oblongata. o It is lined with ependyma and is continuous above with the cerebral aqueduct of the midbrain and below with the central canal of the medulla oblongata and the spinal cord. o It possesses lateral boundaries, a roof, and a rhomboid-shaped floor. BLOOD-BRAIN BARRIER AND CSF-BRAIN BARRIERS The CNS requires a very stable environment in order to function normally. This stability is provided by isolating the nervous system from the blood by the existence of the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier. Blood-Brain Barrier Structure o Examination of a CNS electron micrograph shows that the lumen of a blood capillary is separated from the extracellular spaces around the neurons and neuroglia by the following structures: (1) the endothelial cells in the wall of the capillary, (2) a continuous basement membrane surrounding the capillary outside the endothelial cells, and (3) the foot processes of the astrocytes that adhere to the outer surface of the capillary wall. o The tight junctions between the endothelial cells of the blood capillaries are responsible for the BBB. The inferior horn of the lateral ventricle extends anteriorly into the Peripheral nerves are isolated from the blood in the same manner temporal lobe. The inferior horn has a roof and a floor. as those of the CNS. The endothelial cells of the blood capillaries in The roof is formed by the inferior surface of the tapetum of the the endoneurium have tight junctions; thus, there is a blood-nerve corpus callosum and by the tail of the caudate nucleus. The barrier. latter passes anteriorly to end in the amygdaloid nucleus. Medial o The BBB is thus a continuous lipid bilayer that encircles the endothelial to the tail of the caudate nucleus is the stria terminalis, which cells and isolates the brain tissue from the blood. This explains how also ends anteriorly in the amygdaloid nucleus. lipophilic molecules can readily diffuse through the barrier, whereas The floor is formed laterally by the collateral eminence, hydrophilic molecules are excluded. produced by the collateral fissure, and medially by the o Although a BBB exists in the newborn, it is likely more permeable to hippocampus. The anterior end of the hippocampus is expanded certain substances than it is in the adult. and slightly furrowed to form the pes hippocampus. The o BBB structure is not identical in all CNS regions: In those areas where hippocampus is composed of gray matter; however, the ventricular it appears to be absent, the capillary endothelium contains surface of the hippocampus is covered by a thin layer of white fenestrations across which proteins and small organic molecules may matter called the alveus, which is formed from the axons of the pass from the blood to the nervous tissue. The area postrema of the floor of the fourth ventricle and the The main sites for CSF absorption are the arachnoid villi that project hypothalamus may serve as sites at which neuronal receptors may into the dural venous sinuses, especially the superior sagittal sinus. sample the chemical content of the plasma directly. The arachnoid villi tend to be grouped together to form elevations known The hypothalamus, which is involved in the regulation of the as arachnoid granulations or pacchonian bodies. metabolic activity of the body, might bring about appropriate Structurally, each arachnoid villus is a diverticulum of the subarachnoid modifications of activity, thereby protecting the nervous tissue space that pierces the dura mater. The arachnoid diverticulum is capped Blood-Cerebrospinal Fluid Barrier Structure by a thin cellular layer, which, in turn, is covered by the endothelium of o Water, gases, and lipid-soluble substances pass freely from the blood the venous sinus. to the CSF. Macromolecules such as proteins and most hexoses other The arachnoid granulations increase in number and size with age and than glucose are unable to enter the CSF. A barrier similar to the BBB tend to become calcified with advanced age. may exist in the choroid plexuses. CRANIAL BONES o The lumen of a blood capillary is separated from the lumen of the Frontal bone ventricle by the following structures: (1) the endothelial cells, which o Consists of two portions: a vertical portion, the squama, are fenestrated and have very thin walls (the fenestrations are not true corresponding with the forehead, and an orbital or horizontal portion perforations but are filled by a thin diaphragm); (2) a continuous which enters into the formation of the roof of the orbital and nasal basement membrane surrounding the capillary outside the cavities. endothelial cells; (3) scattered pale cells with flattened processes; and o In the midline of the external surface of the squama remains the frontal (4) a continuous basement membrane, on which rest (5) the suture. The internal surface of the squama is concave and presents in choroidal epithelial cells. the middle a vertical groove, the sagittal sulcus, which ends in the o Tight junctions between the choroidal epithelial cells probably serve frontal crest; the sulcus lodges the superior sagittal sinus. as the barrier. Parietal bones Cerebrospinal Fluid-Brain Interface o Two portions which form the sides and roof of the cranium. o A comparable physiologic barrier between CSF and the CNS o Each has 2 surfaces: External, Internal extracellular compartment does not exist. However, three structures o Has 4 borders: Sagittal, Squamous, Frontal, Occipital separate CSF from nervous tissue: (1) the pia-covered surface of the o 4 angles: brain and spinal cord, (2) the perivascular extensions of the Frontal angle- a right angle and corresponds with the point of subarachnoid space into the nervous tissue, and (3) the ependymal meeting of the sagittal and coronal sutures surface of the ventricles. Sphenoidal angle The pia-covered surface of the brain consists of a loosely arranged Occipital angle- is rounded and corresponds with the point of layer of pial cells resting on a basement membrane. Beneath the meeting of the sagittal and lambdoidal sutures basement membrane are the astrocyte foot processes. Mastoid angle- is truncated; it articulates with the occipital bone No intercellular junctions exist between adjacent pial cells or o The internal surface of the parietal bone along the superior margin the between adjacent astrocytes; therefore, the extracellular spaces of edges afford attachment to the falx cerebri, a shallow groove still along the nervous tissue are in almost direct continuity with the the superior margin are several depressions for arachnoid subarachnoid space. granulations. The prolongation of the subarachnoid space into the central nervous Temporal bone tissue quickly ends below the surface of the brain, where the fusion o Consists of 3 parts, the squama, the petrous, the tympanic parts, and of the outer covering of the blood vessel with the pial covering of the the mastoid part nervous tissue occurs. Occipital bone The ventricular surface of the brain is covered with columnar o Has a trapezoid outline and is rather cup-like in shape ependymal cells with localized tight junctions. o It is pierced by a large aperture, the foramen magnum through which lntercellular channels permit free communication between the the cranial cavity communicates with the vertebral canal ventricular cavity and the extracellular neuronal space. Sphenoid bone The ependyma does not have a basement membrane, and o Found anterior to the occipital bone, between the two temporal bones, specialized astrocytic foot processes are absent because the at the base of the skull. neuroglia cells are loosely arranged. o Its different parts form part of the boundaries of the cranial, orbital, and Functional Significance of the Barriers nasal cavities. The following parts should be distinguished: o In normal conditions, the BBB and blood-cerebrospinal fluid barrier are Body two important semipermeable barriers that protect the brain and spinal Central mass of bone, lying anterior to the basilar part of the cord from potentially harmful substances while permitting gases and occipital bone nutriments to enter the nervous tissue. Characterized by the presence of a depression that resembles a ARACHNOID VILLI AND PACCHONIAN BODIES (ARACHNOID saddle GRANULATIONS) Although massive in appearance, it is lightened by the presence of an air sinus, the sphenoidal sinus. Great wings Two lateral processes spreading out from the body of the sphenoid and forming the floor of the middle cranial fossa, as well as part of the side of the skull. Small wings Two smaller flattened, triangular plates of bone projecting laterally from the anterior upper part of the body. The two roots by which each half is attached to the body, surrounds a small round hole - the optic foramen. Pterygoid processes Two projections springing from the lower part of the side of the body and great wings of the sphenoid Each process bears a lateral and medial bony lamina, enclosing a deep fossa between them. Ethmoid bone Central Nervous System o An irregularly shaped bone, inserted into the notch found between the o The brain and spinal cord are the main centers where correlation and orbital plates of the frontal bone, immediately in front of the sphenoid. integration of nervous information occur. o It forms part of the boundaries of the nasal and orbital cavities. o Both the brain and spinal cord are covered with a system of o The following parts are recognized membranes (meninges) and are suspended in cerebrospinal fluid Cribriform plate (CSF). Meninges are further protected by the bones of the skull and Thin, horizontal plate, separating the nasal cavity from the cranial the vertebral column. cavity. It is perforated by small holes. o Composed of large numbers of neurons, which are excitable nerve Perpendicular plate cells, and their processes, known as axons or nerve fibers. Neurons An irregular, pentagonal plate of bone that forms the upper are supported by specialized tissue called neuroglia. anterior part of the nasal septum o CNS interior is organized into gray and white matter. It is continued above the perpendicular plate as a short process Gray matter- consists of nerve cells embedded in neuroglia (gray) called the crista galli. White matter- consists of nerve fibers embedded in neuroglia and Ethmoid labyrinth is white in color because of the presence of lipid material in nerve Found each side of the perpendicular plate, hanging vertically fiber myelin sheaths downwards Peripheral Nervous System The air spaces found inside it constitute the ethmoidal air sinuses o Cranial and spinal nerves, which consist of bundles of nerve fibers (or VERTEBRAL COLUMN axons), conduct information to and from the CNS. Components o The nerves are surrounded by fibrous sheaths as they run to different o The vertebral column or backbone is formed of a series of bones called parts of the body, they are relatively unprotected and are commonly vertebrae. damaged by trauma. o The 33 vertebrae are grouped under the names cervical, thoracic Autonomic Nervous System lumbar, sacral, and coccygeal according to the regions they occupy. o Part of the nervous system that innervates the body’s involuntary o There are 7 in the cervical region, 12 in the thoracic, 5 in the lumbar, structures, such as the heart, smooth muscle, and glands. 5 in the sacral, and 4 in the coccygeal. o Distributed throughout the CNS and PNS and is divided into 2 parts, Parts both containing afferent and efferent nerve fibers: o A typical vertebrae consists of two essential parts: Sympathetic- prepare the body for an emergency Body- ventral segment; largest part Parasympathetic- aimed at conserving and restoring energy Vertebral arch- which encloses the vertebral foramen, consists of a NERVOUS TISSUE pair of pedicles and a pair of laminae which together support 7 Protoplasmic Properties processes - 4 articular, 2 transverse, and 1 spinous. o Irritability- ability to respond to a stimuli o The average length of the vertebral column in the male is about 71 cm o Conductivity- ability to conduct a nerve impulse over long distances while the female column is about 61 cm in length. at great speed Classification Components o True or Movable Vertebrae- the bones composing the cervical, o Nerve cells or Neuron thoracic, and lumbar regions which remain separate throughout life. Structural and functional unit of the nervous system o False or Fixed Vertebrae- the bones composing the sacral and They do NOT undergo mitotic cell division and replication normally coccygeal regions which in the adult fuse into the sacrum and coccyx in the mature individual. respectively. o Neuroglia or Supporting Cells Characteristics Greater in number than neurons o These are the common characteristics present in all vertebrae, with Functions few exceptions. o The function of the nervous tissue: Body or centrum- the large anterior part Receive information from the environment or from other nerve cells. Vertebral or neural arch- the posterior curved part, made up of two Process information symmetrical halves. Sends information to other neurons or effector tissues Pedicles or roots- two short, thick processes by which the arch is NEURON/NERVE CELL attached to the upper part of the posterior surface concavity is Parts, Inclusions, Organelles, and Processes shallower. o Nerve cell body or Soma- nucleated part, bounded externally by a Vertebral notches- the concavities on the superior and inferior plasma membrane surfaces of the pedicles. The superior concavity is shallower. Shape: polygonal to spherical or angular Intervertebral foramen- the rounded hole procuded by the notches Nucleus: large, spherical to ovoid, centrally located, pale with a when two contiguous vertebrae are articulated. single prominent nucleolus (fish eye nucleus) and the fine chromatin Laminae- the broad plates that make up the greater part of the granules vertebral arch. From the pedicle, they project backwards and Cytoplasm (Perikaryon): rich in granular and agranular medially to unite in the median line posteriorly. endoplasmic reticulum (rough surfaced- abundant) and contains the Vertebral foramen- the large opening bounded by the body following organelles and inclusions: anteriorly, and by the arch laterally and posteriorly. Nissl Bodies- granular substance distributed throughout the Spinous process- the median projection that is directed backwards cytoplasm of the cell body, except for the region close to axon and downwards from the union of the laminae. called the axon hillock. Articular processes- two superior and two inferior projections from Broad cisternae in appearance the junctions of the pedicles and laminae. Extends into the proximal part of the dendrites Transverse processes- two projections directed laterally from the Responsible for the basophilia of the cytoplasm junctions of the pedicles and laminae, between the articular Involved in the synthesis of neurotransmitter substance processes. Rough ER Smooth ER- less obvious, abundant; Function is not clear but it CASE 2 is known to sequester calcium and contains proteins and provide MAIN PARTS OF THE NERVOUS SYSTEM pathway for their distribution. The nervous system is divided into 2 main parts: the CNS (brain and Golgi Apparatus spinal cord), and the PNS (cranial and spinal nerves and their associated Light Microscope: appear as a network of wavy threads around ganglia) the nucleus Electron Microscope: appears as clusters of flattened Golgi Type II cisternae and small vesicles made up of smooth ER Short axon that terminates in the neighborhood of the cell body or Functions: is entirely absent. They greatly outnumber the Golgi type I neurons. Stores temporarily the proteins by the Nissl Bodies The short dendrites that arise from these neurons give them a star- Active in lysosome production shaped appearance. Synthesis of the components of cell membrane- important in Examples: Numerous in the cerebral and cerebellar cortex and are the formation of synaptic vesicles at the axon terminals often inhibitory in function. Adds carbohydrate to protein molecule o Number of processes Mitochondria Unipolar neurons Scattered throughout the cell body, dendrites, and axon Possesses a single process seen during early embryonic Spherical or rod-shaped development E/M: walls show a characteristic double membrane Have a single neurite that divides a short distance from the cell Possess many enzymes body into two branches, one proceeding to some peripheral Function: Production of chemical energy structure and the other entering the central nervous system (CNS). Inclusions The branches of this single neurite have the structural and Lysosomes- membrane bound vesicles, contain enzymes functional characteristics of an axon. Melanin Granules- found in the cytoplasm of cells in certain The fine terminal branches found at the peripheral end of the axon parts of the brain (substantia nigra of the midbrain, locos at the receptor site are often referred to as the dendrites. coeruleus of pons) Examples: Posterior root ganglion Lipofuscin (pigment granule)- occurs as a yellowish-brown Bipolar neurons granules within the cytoplasm Possesses an elongated body, has a single dendrite and an axon Accumulates with age Have an elongated cell body, with a single neurite emerging from Believed to be formed as the result of lysosomal activity and it each end (has a single dendrite and an axon). represents a harmless metabolic byproduct Examples: Retinal bipolar cells, olfactory epithelium of the nasal Lipid Droplets- as a result of faulty metabolism cavity, and the cells of the sensory cochlear and vestibular ganglia Cytoskeletal components Pseudo-unipolar neuron Neurofibrils One process that divides into a short distance from the cell body Filamentous structures running parallel to each other through into a peripheral and central branches the cell body and into the neuritis The central branch enters the CNS and the peripheral branch Appear as linear fibrils proceed to its designation in the body Types Examples: Dorsal root ganglia Microtubules- spherical or rod-shaped linear tubes Multipolar neurons E/M- 3 microtubules associated Most common type of neuron MAP-1- regulates stability of microtubules and promote Have a number of neurites arising from the cell body, with the their assembly exception of the long process, the axon, the remainder of the MAP-2- abundant in perikaryon and dendrites and absent neurites are dendrites. in the axon Examples: Most neurons of the brain and spinal cord MAP-3- present only in axon; transport substance from cell body to the distal end of cell process Neurofilaments (intermediate filaments) Microfilaments (actin filament) Nucleus- group of structurally and functionally related neurons in the central nervous system. Ganglion- group of structurally and functionally related neurons in the peripheral nervous system. o Nerve Processes (Neurites) Axon Dendrites Classification based on o Length of axons AXON VS DENDRITES Golgi Type I Axons Dendrites Long axon that can stretch 1 m or more in length in extreme cases Number One Absent-Many Forms the long fiber tracts of the brain and spinal cord, and the Length Vary from microns to meters Microns; seldom more nerve fibers of peripheral nerves (longer). than a millimeter Examples: pyramidal cells of cerebral cortex, Purkinje cells of the (shorter). cerebellar cortex, and the motor cells of the spinal cord Size and Long and big in size Tree-like appearance Large numbers of astrocytic processes are interwoven at the outer Shape and inner surfaces of the CNS, where they form the outer and inner Branching Limited to collaterals, pre- Vary from simple to glial limiting membranes. Pattern terminals, and terminals complex arborizations Outer glial limiting membrane- found beneath the pia mater Nissl body Absent Present Inner glial limiting membrane- lies beneath the ependyma lining Surface/ Smooth. Vary from smooth to the ventricles of the brain and the central canal of the spinal cord Contour spiny. Astrocytic processes are also found in large numbers around the Coverings Supporting cells and Always naked initial segment of most axons and in the bare segments of axons at frequently myelin. the nodes of Ranvier. Axon terminals at many sites are separated Sheaths CNS- interfascicular glia and None from other nerve cells and their processes by an envelope of myelin sheath. astrocytic processes PNS- sheath of Schwann Functions of astrocytes: and myelin sheath. Bundles CNS- Fiber Tract None Forms a supporting framework for the nerve cells and nerve fibers. PNS- Nerve In the embryo, they serve as a scaffolding for the migration of Function Transport impulses from the Receive impulses and immature neurons. (Direction of cell body. transports them toward Serve as electrical insulators preventing axon terminals from impulse the cell body (cellulipetal influencing neighboring and unrelated neurons. conduction) conduction). Form barriers for the spread of neurotransmitter substances SUPPORTING ELEMENTS OF THE NERVOUS SYSTEM released at synapses. Central Nervous System- Neuroglial Cells Affected by GABA and glutamic acid secreted by the nerve o Ependyma terminals, thereby limiting the influence of these neurotransmitters. Line the cavities of the brain and the central canal of the spinal cord. Take up excess K+ ions from the extracellular space so that they They form a single layer of cells that are cuboidal or columnar in may have an important function during repetitive firing of a neuron. shape and possess microvilli and cilia. Stores glycogen within their cytoplasm. The glycogen can be The cilia are often motile, and their movements contribute to the broken down into glucose and even further into lactate, both of flow of the cerebrospinal fluid (CSF). which are released to surrounding neurons in response to The bases of the ependymal cells lie on the internal glial limiting norepinephrine. membrane. Phagocytes by taking up degenerating synaptic axon terminals. Ependymocytes Following the death of neurons due to disease, astrocytes Line the ventricles of the brain and the central canal of the spinal proliferate and fill in the spaces previously occupied by the cord and are in contact with the CSF. neurons, a process called replacement gliosis. Their adjacent surfaces have gap junctions, but the CSF is in free Serves as a conduit for the passage of metabolites or raw communication with CNS intercellular spaces. materials from blood capillaries to the neurons through their Function: Assist in CSF circulation within the cavities of the brain perivascular feet. and the central canal of the spinal cord by the movements of the Because astrocytes are linked together by gap junctions, they cilia. The microvilli on the free surfaces indicate that they also have enable ions to pass from one cell to another without entering the an absorptive function. extracellular space. Tanycytes Produce substances that have a trophic influence on neighboring Line the floor of the third ventricle overlying the median eminence neurons. of the hypothalamus. Secrete cytokines that regulate the activity of immune cells Have long basal processes that pass between the cells of the entering the nervous system in disease. median eminence and place endfeet on blood capillaries. Plays an important role in the structure of the blood-brain barrier. Function: Transport chemical substances from the CSF to the The astrocyte processes terminate as expanded feet at the hypophyseal portal system. They may play a part in the control of basement membrane of blood vessels. the hormone production by the anterior lobe of the pituitary. o Oligodendrocytes or Oligodendroglia or Oligoglia Choroidal epithelial cells Small cell bodies and a few delicate processes; their cytoplasm does Cover the surfaces of the choroid plexuses. not contain filaments. The sides and bases of these cells are thrown into folds; near their Found in rows along myelinated nerve fibers and surround nerve cell luminal surfaces, the cells are held together by tight junctions bodies that encircle the cells. The presence of tight junctions prevents the However, only one process joins the myelin between two adjacent leakage of CSF into the underlying tissues. nodes of Ranvier. Function: Involved in the production and secretion of CSF from Interfascicular glia or interfascicular oligodendrocyte the choroid plexuses. Found beside bundles of axons. o Astrocytes Responsible for manufacturing and maintaining myelin about the Small cell bodies with branching processes that extend in all axons of CNS, serving to insulate them. directions. Both astrocyte and oligodendrocyte are derived from ectoderm. Protoplasmic astrocytes Functions of oligodendrocytes: Found mainly in the gray matter, where their processes pass Responsible for the formation of the myelin sheath of nerve fibers between the nerve cell bodies in the CNS, much as the myelin of peripheral nerves is formed The processes are shorter, thicker, and more branched than those from Schwann cells. of the fibrous astrocyte This formation and maintenance of myelin around many CNS Their cytoplasm contains fewer filaments. axons provides them with an insulating coat and greatly Fibrous astrocyte (fibrocyte) increases the speed of nerve conduction along them. Found mainly in the white matter, where their processes pass Because Oligodendrocytes have several processes, unlike between the nerve fibers. Schwann cells, they can each form several internodal segments Each process is long, slender, smooth, and not much branched. of myelin on the same or different axons. The cell bodies and processes contain many filaments in their A single oligodendrocyte can form as many as 60 internodal cytoplasm. segments. Unlike Schwann cells in the PNS, Oligodendrocytes Many of the processes of astrocytes end in expansions on blood and their associated axons are not surrounded by a basement vessels (perivascular feet), where they form an almost complete membrane. covering on the external surface of capillaries. Myelination begins at about the 16th week of intrauterine life and Axis cylinder or axon continues postnatally until practically all the major nerve fibers Cells of Schwann- from both myelinated and unmyelinated coverings are myelinated by the time the child is walking. over axons of the CNS Surround nerve cell bodies (satellite Oligodendrocytes) and have Incisures of Schmidt-Lanterman- cone-shaped oblique clefts in the a similar function to the satellite or capsular cells of peripheral myelin sheath sensory ganglia. Myelin sheath- lipoid sheath covering the axon in the PNS and CNS Thought to influence the biochemical environment of neurons. Peripheral nerve is a bundle of nerve fibers (axons) surrounded by o Microglia several investments of connective tissue sheaths. Embryologically unrelated to the other neuroglial cells and are Bundles (fascicles) are surrounded by: Epineurium (centermost layer), derived from macrophages outside the nervous system. Perineurium (middle layer), and Endoneurium (innermost layer) Smallest of the neuroglial cells and are found scattered throughout the CNS Wavy branching processes arise from their small cell bodies that give off numerous spinelike projections. Closely resemble connective tissue macrophages. Migrate into the nervous system during fetal life. Increase in number in the presence of damaged nervous tissue resulting from trauma and ischemic injury and in the presence of diseases including Alzheimer disease, Parkinson disease, multiple sclerosis, and AIDS. Many of these new cells are monocytes that have migrated from the blood. Functions of microglia: Microglial cells in the normal brain and spinal cord appear to be inactive and are sometimes called resting microglial cells. Becomes immune effector cells in inflammatory disease of CNS. They retract their processes and migrate to the site of the lesion. They proliferate and become antigen-presenting cells, which, together with the invading T lymphocytes, confront invading RESTING MEMBRANE POTENTIAL (RMP) organisms. Genesis of the RMP They are also actively phagocytic; their cytoplasm becomes filled with lipids and cell remnants. The microglial cells are joined by monocytes from neighboring blood vessels. o Electrical potentials exist across the membranes of virtually all cells of the body. Some cells, such as nerve and muscle cells, generate rapidly changing electrochemical impulses at their membranes, and these impulses are used to transmit signals along the nerve or muscle membranes. In other types of cells, such as glandular cells, macrophages, and ciliated cells, local changes in membrane potentials also activate many of the cell’s functions. o When the potassium (K+) concentration is great inside a nerve fiber membrane but very low outside the membrane Peripheral Nervous System The membrane in this case is permeable to the potassium ions but o Capsule cells (Amphicyte or Satellite cells) not to any other ions Flattened cells that form a capsule around the neurons Because of the large potassium concentration gradient from the Example: Spinal Ganglion inside toward the outside, there is a strong tendency for potassium Types: ions to diffuse outward through the membrane. As they do so, they Cerebrospinal ganglia- example: Dorsal root ganglion carry positive electrical charges to the outside, thus creating Autonomic ganglia- example: Terminal, sympathetic and collateral electropositivity outside the membrane and electronegativity inside Differences the membrane because of negative anions that remain behind and BASIS DRG TERMINAL do not diffuse outward with the potassium. CELL SIZE Relatively larger Smaller Within about 1 millisecond, the potential difference between the SHAPE Globular or pyriform Stellate inside and outside, called the diffusion potential, becomes great PROCESSES Pseudo-unipolar Multipolar enough to block further net potassium diffusion to the exterior, NUCLEUS Centrally located Eccentrically located despite the high potassium ion concentration gradient. CAPSULE Distinct Less distinct In the normal mammalian nerve fiber, the potential difference is o Cell of Schwann about 94 millivolts, with negativity inside the fiber membrane. Forms a long thin tube called the sheath of Schwann or o In the same phenomenon, but this time with a high concentration of Neurilemmal sheath containing the axis cylinder with the Myelin sodium (Na2+) ions outside the membrane and a low concentration of sheath in between sodium ions inside Myelin sheath is a glistening lipoid sheath These ions are also positively charged and this time the membrane MYELINATED NERVE FIBER is highly permeable to the sodium ions but is impermeable to all Neurilemma other ions Node of Ranvier- sites of interruption at regular interval of the myelin sheath Diffusion of the positively charged sodium ions to the inside creates The ratio of sodium ions from inside to outside the membrane is a membrane potential of opposite polarity, with negativity outside 0.1, which gives a calculated Nernst potential for the inside of the and positivity inside. membrane of +61 millivolts. Again, the membrane potential rises high enough within In the normal nerve fiber, the permeability of the membrane to milliseconds to block further net diffusion of sodium ions to the potassium is about 100 times as great as its permeability to inside; however, this time, in the mammalian nerve fiber, the sodium. potential is about 61 millivolts positive inside the fiber. Using this value in the Goldman equation, and considering only o In both parts, a concentration difference of ions across a selectively sodium and potassium, gives a potential inside the membrane of permeable membrane can, under appropriate conditions, create a -86 millivolts, which is near the potassium potential. membrane potential. Many of the rapid changes in membrane The Goldman equation gives the calculated membrane potential potentials observed during nerve and muscle impulse transmission on the inside of the membrane when two univalent positive ions, result from such rapidly changing diffusion potentials. sodium (Na+) and potassium (K+), and one univalent negative ion, o Nernst Equation- describes the relationship of diffusion potential to chloride (Cl ), are involved, where P is the permeability of the the ion concentration difference across a membrane membrane to each ion, C is the concentration of the respective The diffusion potential across a membrane that exactly opposes the ions on the inside (i) and outside (o) of the membrane. net diffusion of a particular ion through the membrane is called the Nernst potential for that ion. The magnitude of the Nernst potential is determined by the ratio of Contribution of the Na+-K+ Pump the concentrations of that specific ion on the two sides of the The Na+-K+ pump is shown to provide an additional contribution to membrane. The greater this ratio, the greater the tendency for the the resting potential. There is continuous pumping of 3 sodium ion to diffuse in one direction and therefore, the greater t
