Histology-Nervous System chp1 PDF

CENTRAL NERVOUS SYSTEM The major structures comprising the CNS are the Cerebrum, Cerebellum, Spinal cord. Cereberral cortex and cerebellar medulla The CNS is completely covered by connective tissue layers, the meninges, but CNS tissue contains very little collagen or similar material, making it rela...

CENTRAL NERVOUS SYSTEM The major structures comprising the CNS are the Cerebrum, Cerebellum, Spinal cord. Cereberral cortex and cerebellar medulla The CNS is completely covered by connective tissue layers, the meninges, but CNS tissue contains very little collagen or similar material, making it relatively soft and easily damaged by injuries affecting the protective skull or vertebral bones. Many regions show organized areas of white matter and gray matter, differences caused by the differential distribution of lipid-rich myelin. WHITE MATTER - The main components of white matter are myelinated axons), often grouped together as tracts, and the myelin-producing oligodendrocytes. Astrocytes and microglia are also present, but very few neuronal cell bodies. Most white matter is found in deeper regions. GRAY MATTER - contains abundant neuronal cell bodies, dendrites, astrocytes, and microglial cells, and is where most synapses occur. Gray matter makes up the thick cortex or surface layer of both the cerebrum and the cerebellum; Deep within the brain are localized, variously shaped darker areas called the cerebral nuclei, each containing large numbers of aggregated neuronal cell bodies. In the folded cerebral cortex, neuroscientists recognize six layers of neurons with different sizes and shapes. The most conspicuous of these cells are the efferent pyramidal neurons. Neurons of the cerebral cortex function in the integration of sensory information and the initiation of voluntary motor responses. There are 6 classically recognized layers of the cortex: I.Outer plexiform (molecular) layer: sparse neurons and glia II.Outer granular layer: small pyramidal and stellate neurons III.Outer pyramidal layer: moderate sized pyramidal neurons IV.Inner granular layer: densely packed stellate neurons (usually the numerous processes aren’t visible, but there are lots of nuclei reflecting the cell density) V.Ganglionic or inner pyramidal layer: large pyramidal neurons VI.Multiform cell layer: mixture of small pyramidal and stellate neurons Pyramidal cells in layers III and V tend to be larger because their axons contribute to efferent projections that extend to other regions of the CNS –pyramidal neurons in layer V of motor cortices send projections all the way down to motor neurons in the spinal cord! Layers of Cortex 1 -Molecular layer: nerve fibers 2 -External granular layer Densely packed, small nonpyramidal cells (e.g., stellate cells) Small pyramidal cells that project axons to other cortical areas 3- External pyramidal layer: small to medium-sized pyramidal cells that project axons to other cortical areas Pyramidal cells in layers III and V tend to be larger because their axons contribute to efferent projections that extend to other regions of the CNS – pyramidal neurons in layer V of motor cortices send projections all the way down to motor neurons in the spinal cord! 4 - Internal granular layer Termination area of thalamocortical projections Filled with densely packed, medium-sized pyramidal and nonpyramidal cells 5- Internal pyramidal layer: large pyramidal cells give rise to the axons that form the corticospinal tracts and corticobulbar tracts (pyramidal tracts to the brainstem and spinal cord) 6- Multiform layer: composed of pyramidal and nonpyramidal cells of various size Cortical pyramidal neurons Photomicrograph of cerebral cortex (silver stain, 300x magnification) Multiple pyramidal neurons and neuronal processes are visible. Pyramidal cells consist of a pyramidal cell body, a dominant apical dendrite, several basal dendrites, and a thin axon that originates from the base of the cell body (indicated with a black arrow). Non pyramidal cells The stellate cells play the role of interneurons within the cerebral cortex; their axons do not leave the cortex. The axons of pyramidal cells leave the cortex and project to a variety of other structures within the central nervous system. layer III of cerebral cortex showing two types of neurons: pyramidal and stellate. Pyramidal neurons (P) have a prominent apical dendrite (D). The axons (A) of pyramidal neurons exit the cortex and travel through white matter to the spinal cord, to the opposite cortex and to various nuclei in the brainstem. In contrast, stellate neurons (S) are local circuit neurons with synaptic connections to neighboring neurons The cerebellum is composed of the outer gray matter (cerebellar cortex) and inner white matter (cerebellar medulla). Cerebellar cortex Receives afferent inputs from the cerebrum, spinal cord, and vestibular nuclei Sends neural impulses to the cerebellar nuclei Composed of 5 types of neuronal cells, densely packed and arranged in 3 layers The cortex is primarily an inhibitory structure; all cerebellar cells except granule cells are inhibitory. Circuitry of the cerebellum The cerebellar cortex is composed of three layers: - Molecular layer: contains basket cells, astrocytes (stellate cells), and parallel fibers (axons of granule cells). The astrocytes and basket cells receive excitatory input from parallel fibers and send inhibitory impulses to the Purkinje cells. - Ganglion cell layer: contains Purkinje cells, which receive excitatory input from climbing fibers and parallel fibers and send inhibitory impulses to the deep cerebellar nuclei - Granular cell layer: contains golgi cells and granule cells. The golgi cells receive excitatory impulses from the molecular layer and send inhibitory impulses to the granule cells. The granule cells receive excitatory input from mossy fibers and send excitatory efferents to all other cells of the cerebellar cortex. Granule cells are the only excitatory neurons in the cerebellar cortex and use the neurotransmitter glutamate (GABA is the neurotransmitter of the inhibitory cerebellar cells). Granule cells = parallel fibers Cerebellar medulla Composed of climbing fibers, mossy fibers, Purkinje cell axons, and the deep cerebellar nuclei Mossy fibers : afferent axons from the cerebral cortex, pons, spinal cord, and vestibular nuclei to the cerebellum. Terminate on granule cells → send excitatory stimuli to the Purkinje cells. Climbing fibers: afferent axons from the inferior olivary nuclei of the medulla → terminate on Purkinje cells Four deep cerebellar nuclei (from lateral to medial): dentate, emboliform, globose, and fastigial nucleus o In cross sections of the spinal cord, the white matter is peripheral and the gray matter forms a deeper, H-shaped mass The two anterior projections of this gray matter anterior horns - contain cell bodies of very large motor neurons whose axons make up the ventral roots of spinal nerves. o The two posterior horns contain interneurons, which receive sensory fibers from neurons in the spinal (dorsal root) ganglia. o Near the middle of the cord, the gray matter surrounds a small central canal, which develops from the lumen of the neural tube, is continuous with the ventricles of the brain, is lined by ependymal cells, and contains CSF. The spinal cord varies slightly in diameter along its length but in cross section always shows bilateral symmetry around the small, CSF-filled central canal (C). Unlike the cerebrum and cerebellum, in the spinal cord, the gray matter is internal, forming a roughly H-shaped structure that consists of two posterior (P) horns (sensory) and two anterior (A) (motor) horns, all joined by the gray commissure around the central canal. (a) The gray matter contains abundant astrocytes and large neuronal cell bodies, especially those of motor neurons in the ventral horns. (b) The white matter surrounds the gray matter and contains primarily oligodendrocytes and tracts of myelinated axon running along the length of the cord. (c) With H&E staining, the large motor neurons (N) of the ventral horns show large nuclei, prominent nucleoli, and cytoplasm rich in Nissl substance, all of which indicate extensive protein synthesis to maintain the axons of these cells that extend great distances. (d) In the white commissure ventral to the central canal, tracts (T) run lengthwise along the cord, seen here in cross section with empty myelin sheaths surrounding axons, as well as small tracts running from one side of the cord to the other. 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. Dura Mater The thick external dura mater (L. dura mater, tough mother) consists of dense irregular connective tissue organized as an outer periosteal layer continuous with the periosteum of the skull and an inner meningeal layer. These two layers are usually fused, but along the superior sagittal surface and other specific areas around the brain, they separate to form the blood-filled dural venous sinuses. Around the spinal cord, the dura mater is separated from the periosteum of the vertebrae by the epidural space, which contains a plexus of thin- walled veins and loose connective tissue. The dura mater may be separated from the arachnoid by formation of a thin subdural space. ) ) Arachnoid The arachnoid (Gr. arachnoeides, spider web-like) has two components: a sheet of connective tissue in contact with the dura mater a system of loosely arranged trabeculae composed of collagen and fibroblasts, continuous with the underlying pia mater layer. Surrounding these 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 connective tissue of the arachnoid is said to be avascular because it lacks nutritive capillaries, but larger blood vessels run through it. Because the arachnoid has fewer trabeculae in the spinal cord, it can be more clearly distinguished from the pia mater in that area. The arachnoid and the pia mater are intimately associated and are often considered a single membrane called the piaarachnoid. In some areas, the arachnoid penetrates the dura mater and protrudes into blood-filled dural venous sinuses located there. These CSF-filled protrusions, which are covered by the vascular endothelial cells lining the sinuses, are called arachnoid villi and function as sites for absorption of CSF into the blood of the venous sinuses. Pia Mater The innermost pia mater (L. pia mater, tender mother) con- sists 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 glial limiting membrane, or 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 CNS tissue through long peri- vascular 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 layer of astrocytic processes Section of an area near the anterior median fissure showing the tough dura mater (D). Surrounding the dura, the epidural space (not shown) contains cushioning adipose tissue and vascular plexuses. The subdural space (SD). The middle meningeal layer is the thicker weblike arachnoid mater (A) containing the large subarachnoid space (SA) and connective tissue trabeculae (T). The subarachnoid space is filled with CSF and the arachnoid acts as a shock-absorbing pad between the CNS and bone. Fairly large blood vessels (BV) course through the arachnoid. The innermost pia mater (P) is thin and is not clearly separate from the arachnoid; together, they are sometimes referred to as the pia-arachnoid or the leptomeninges. The space between the pia and the white matter (WM) of the spinal cord here is an artifact created during dissection; normally the pia is very closely applied to a layer of astrocytic processes at the surface of the CNS tissue. (X100; H&E) 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 the capillary endothelium, in which the cells are tightly sealed together with well-developed occluding junctions, with little or no transcytosis activity, and surrounded by the basement membrane. The limiting layer of perivascular astrocytic feet, which closely envelops the basement membrane of the continuous capillaries in most CNS region, also contributes to the BBB and further regulates passage of molecules and ions from blood to brain. The BBB protects neurons and glia from bacterial toxins, infectious agents, and other exogenous substances, and helps maintain the stable composition and constant balance of ions in the interstitial fluid required for normal neuronal function. The BBB is not present in regions of the hypothalamus where plasma components are monitored, in the posterior pituitary that releases hormones, or in the choroid plexus where CSF is produced. Blood-brain barrier Description: a barrier of CNS blood vessels separating the brain from circulating blood Function: protects the central nervous system from microorganisms, cells, proteins, and drugs that can cause damage to the brain and other structures Components Capillary endothelial cells → have tight junctions The basal lamina Astrocytes Pericytes Transport mechanisms across the blood-brain barrier Ion channels (e.g., sodium, potassium) Selective transport (slow): amino acids, glucose, vitamin K, vitamin D Diffusion (fast): nonpolar and lipid-soluble substances (e.g., oxygen and carbon dioxide) Structures with no blood-brain barrier Structures located around the brain ventricles (circumventricular location), including: Area postrema: vomiting center (contains the chemoreceptor trigger zone) Organum vasculosum lamina terminalis (OVLT): osmoreceptors Neurohypophysis: oxytocin and ADH release directly into the blood Allow certain molecules to affect brain function (e.g., blood-borne drugs and hormones) Damage mechanisms Brain infarction and tumors break endothelial cell tight junctions, thereby causing vasogenic edema. Hyperosmolar solutions (e.g., mannitol) cause vasodilatation and shrinkage of endothelial cells, thereby disrupting the blood-brain barrier and increasing its permeability to other drugs.