Meninges PDF

JABIR IBN HAYYAN MEDICAL Lecture By : Alaa AL Husainy UNIVERSITY COLLEGE OF MEDICINE Lecture:3 DEPARTMENT OF HUMAN ANATOMY Section of Histology Mening...

JABIR IBN HAYYAN MEDICAL Lecture By : Alaa AL Husainy UNIVERSITY COLLEGE OF MEDICINE Lecture:3 DEPARTMENT OF HUMAN ANATOMY Section of Histology Meninges The skull and the vertebral column protect the CNS, but between the bone and nervous tissue are membranes of connective tissue called the meninges. Three meningeal layers are distinguished: the dura, arachnoid, and pia maters. 1- Dura Mater The thick external dura mater (L., dura mater, tough mother) consists of dense, fibroelastic connective tissue that is continuous with the periosteum of the skull. 2- Arachnoid The arachnoid (Gr. arachnoeides, spider weblike) has two components: (1) Sheet of connective tissue in contact with the dura mater (2) System of loosely arranged trabeculae composed of collagen and fibroblasts, continuous with the underlying pia mater layer. Surrounding the trabeculae is a large, sponge-like cavity, the subarachnoid space, filled with CSF. This fluid-filled space helps cushion and protect the CNS from minor trauma. The subarachnoid space communicates with the ventricles of the brain where the CSF is produced. The arachnoid and the pia mater are intimately associated and are often considered a single membrane called the pia-arachnoid. 3- Pia Mater The innermost pia mater consists of flattened, mesenchymally derived cells closely applied to the entire surface of the CNS tissue. The pia does not directly contact nerve cells or fibers, being separated from the neural elements by the very thin superficial layer of astrocytic processes (the Glia limitans), which adheres firmly to the pia mater. Together, the pia mater and the layer of astrocytic end feet form a physical barrier separating CNS tissue from CSF in the subarachnoid space. Blood vessels penetrate the CNS through long perivascular spaces covered by pia mater, although the pia disappears when the blood vessels branch to form the small capillaries. However, these capillaries remain completely covered by the perivascular limiting layer of astrocytic processes. Blood-Brain Barrier The blood-brain barrier (BBB) is a functional barrier that allows much tighter control than that in most tissues over the passage of substances moving from blood into the CNS tissue. The main structural component of the BBB is 1-The capillary endothelium, in which the cells are tightly sealed together with well- developed occluding junctions and with little or no transcytosis activity. 2-The limiting layer of perivascular astrocytic feet. 3-The basal lamina of the capillaries that completely enveloped by astrocytic feet. The BBB protects neurons and glia from:  Bacterial toxins  Infectious agents  Exogenous substances  Helps maintain the stable composition and constant balance of ions in the interstitial fluid that is required for normal neuronal function. The components of the BBB are not found in the choroid plexus where CSF is produced, in the posterior pituitary which releases hormones, or in regions of the hypothalamus where plasma components are monitored. Choroid Plexus The choroid plexus consists of highly specialized tissue with folds and many villi projecting into the four large ventricles of the brain. It is found in the roofs of the third and fourth ventricles and in parts of the two lateral ventricular walls, all regions in which the ependymal lining directly contacts the pia mater. Each villus of the choroid plexus contains a thin layer of well-vascularized pia mater covered by cuboidal ependymal cells. The function of the choroid plexus is production of CSF. CSF is clear, colorless fluid, contains Na+, K+, and Cl– ions but very little protein, and its only cells are normally very sparse lymphocytes. It is produced continuously and it completely fills the ventricles, the central canal of the spinal cord, the subarachnoid and perivascular spaces. It provides the ions required for CNS neuronal activity and in the arachnoid serves to help absorb mechanical shocks. There are no lymphatic vessels in CNS tissue. PERIPHERAL NERVOUS SYSTEM The main components of the peripheral nervous system (PNS) are the nerves, ganglia, and nerve endings. Nerves are bundles of nerve fibers (axons) surrounded by Schwann cells and layers of connective tissue. Nerve Fibers Nerve fibersare analogous to tracts in the CNS, containing axons enclosed within sheaths of glial cells specialized to facilitate axonal function. In peripheral nerve fibers, axons are sheathed by Schwann cells,or neurolemmocytes. The sheath may or may not form myelin around the axons, depending on their diameter. Myelinated Fibers As axons of large diameter grow in the PNS, they are engulfed along their length by a series of differentiating neurolemmocytes and become myelinated nerve fibers. The plasma membrane of each covering Schwann cell fuses with itself around the axon, and the fused membrane (or mesaxon) becomes wrapped around the axon as the glial cell body moves circumferentially around the axon many times. The multiple layers of Schwann cell membrane unite as a thick myelin sheath. Composed mainly of lipid bilayers and membrane proteins, myelin is a large lipoprotein complex that, like cell membranes, is partly removed by standard histologic procedures. Unlike oligodendrocytes of the CNS, a Schwann cell forms myelin around only aportion of one axon. Membranes of Schwann cells have a higher proportion of lipids than do other cell membranes, and the myelin sheath serves to insulate axons and maintain a constant ionic microenvironment most suitable for action potentials. Between adjacent Schwann cells on an axon the myelin sheath shows small nodes of Ranvier(or nodal gaps), where the axon is only partially covered by interdigitating Schwann cell processes. At these nodes the axolemma is exposed to ions in the interstitial fluid and has a much higher concentration of voltage-gated Na+ channels, which renew the action potential and produce salutatory conduction(L. saltare, to jump) of nerve impulses, their rapid movement from node to node. Unmyelinated Fibers Unlike the CNS where many short axons are not myelinated at all but run free among the other neuronal and glial processes, the smallest-diameter axons of peripheral nerves are stillenveloped within simple folds of Schwann cells. In these unmyelinated fibers the glial cell does not form the multiple wrapping of a myelin sheath. In unmyelinated fibers, each Schwann cell can enclose portions of many axons with small diameters. Without the thick myelin sheath, nodes of Ranvier are not seen along unmyelinated nerve fibers. Moreover, these small-diameter axons have evenly distributed voltage-gated ion channels; their impulse conduction is not salutatory and is much slower than that of myelinated axons. Nerve Organization In the PNS nerve fibers are grouped into bundles to form nerves. Except for very thin nerves containing only unmyelinated fibers, nerves have a whitish, glistening appearance because of their myelin and collagen content. Axons and Schwann cells are enclosed within layers of connective tissue. Immediately around the external laminae of the Schwann cells is a thinlayer called the (Endoneurium), consisting of reticular fibers, scattered fibroblasts, and capillaries. Groups of axons withSchwann cells and endoneurium are bundled together as fasciclesby a sleeve of (Perineurium), containing flat fibrocyteswith their edges sealed together by tight junctions. From twoto six layers of these unique connective tissue cells regulatediffusion into the fascicle and make up the (blood-nerve barrier) that helps maintain the fibers’ microenvironment. Externally,peripheral nerves have a dense, irregular fibrous coatcalled the (Epineurium), which extends deeply to fill the spacebetween fascicles.