Nerve Tissue Notes Markand 2024 PDF

NERVE TISSUE Learning objectives After attending lectures, doing the assigned reading, and doing the laboratory exercises, you should be able to: 1. Describe the features that characterize nerve tissue. You should be abl...

NERVE TISSUE Learning objectives After attending lectures, doing the assigned reading, and doing the laboratory exercises, you should be able to: 1. Describe the features that characterize nerve tissue. You should be able to use these features to identify nerve tissue in specimens on microscope slides, micrographs, and drawings. 2. Describe the structure of a neuron including its cellular processes, dendrite and axon. Describe functional properties of the neuron, including its processes. Characterize axonal transport. 3. Classify neurons by function and by morphology. 4. Describe the basic electrophysiology of the nerve tissue. 5. Describe the structure of a synapse. Classify synapses by morphology and type of impulse transmission. Describe the impulse transmission in a chemical synapse. Describe different morphotypes of synapses including the motor end-plate. 6. Describe the support cells of PNS (Schwann cells, satellite cells) and describe the myelin sheath. Describe myelinated and unmyelinated nerve fibers. 7. Describe the support cells of CNS (astrocytes, oligodendrocytes, microglial cells, and ependymal cells), and their functions. Describe the role of certain support cells in the pathologies of CNS. 8. Describe the histological structure of the major components of the peripheral nervous system, including peripheral nerves, ganglia, and special endings. 9. Describe the major types of encapsulated nerve endings including Meissner’s and Pacinian corpuscles. 10. Describe the structure of the muscle spindle. 11. Differentiate white and gray matter in the organs of the CNS. -1-  NERVE TISSUE I. Characteristics of nerve tissue II. Components of nerve tissue III. Neuron 1. Functional categories of neurons A. Sensory B. Interneurons C. Motor 2. Structure of neurons 3. Cell processes of neurons 4. Axon 5. Axonal transport A. Anterograde flow 1. Fast transport 2. Slow transport B. Retrograde flow 6. Dendrites 7. Major types of neurons A. Pseudounipolar B. Bipolar neurons C. Multipolar neurons 1. Golgi type I cells 2. Golgi type II cells 8. Electrophysiology of the nerve 9. Synapses A. Electrical B. Chemical 1. Excitatory 2. Inhibitory 10. Chemical synapses A. Presynaptic knob 1. Neurotransmitters B. Synaptic cleft C. Postsynaptic membrane D. Impulse transmission E. Impulse elimination 11. Morphotypes of synapses A. Axodendritic B. Axosomatic C. Axoaxonic D. Motor end-plate E. Neuroglandular junction 12. Clinical comments IV. Support cells 1. Peripheral nervous system support cells A. Schwann cells 1. Nodes of Ranvier -2-  2. Myelinated and unmyelinated nerve fibers B. Satellite cells 2. Central nervous system support cells A. Astrocytes 1. Protoplasmic astrocytes 2. Fibrous astrocytes 3. Clinical comments: astrocytoma, glial scar B. Oligodendrocytes 1. Clinical comment: oligodendrocytoma 2. Clinical comment: multiple sclerosis C. Microglial cells D. Ependymal cells V. Peripheral nervous system 1. Nerves A. Endoneurium B. Perineurium C. Epineurium 2. Ganglia A. Sensory craniospinal ganglia B. Autonomic ganglia 3. Special endings: sensory nerve endings A. Special-sense nerve endings B. Somesthetic receptors 1. Meissner’s corpuscle 2. Pacinian corpuscle C. Proprioceptors 1. Muscle spindle VI. Central nervous system 1. Gray matter 2. White matter -3-  NERVE TISSUE I. Characteristics of nerve tissue. Nerve tissue provides rapid and specific communication between organs in the body. II. The two major components of nerve tissue are highly specialized cells called neurons and support cells, called the neuroglia, or glial cells. Neurons, similar to muscle cells, are electrically excitable. III. Our body contains on average about 100,000,000 neurons. 1. Functional categories of neurons. There are three functional categories of neurons: A. Sensory neurons, which gather information from receptors. B. Interneurons that form a communicating network between neurons. C. Motor neurons convey impulses from the nervous system to the effector cells. 2. Structure of neurons. A. Neurons have a large, rounded usually euchromatic nucleus with prominent nucleolus(i). B. Perikaryon is the cell body which varies in size between 5-135 m. C. The rough endoplasmic reticulum is extremely well-developed and forms dense structures, visible in the light microscope, that are called Nissl bodies. Nissl bodies are formed by parallel arrays of RER cisternae. D. Golgi complex is usually well-developed. E. The well-developed RER and Golgi reflect the need for the neuron to produce membrane and neurotransmitter in large quantities. F. Neurons have many mitochondria. G. Lysosomes are usually present in the cytoplasm of neurons. H. Neurons usually lack the centrioles. It means that the mature cell is not capable of cellular division. Research has shown that some neurons retain the centrosome, which may play a role in the nucleation of microtubules. However even in the adult brain there are few neuroblasts that are able to divide and reproduce in small numbers. The only neurons that are replaced in an adult human body on a regular basis are the olfactory neurons. -4-  I. Cytoskeleton of neurons is very well developed and consists of neurofilaments (a type of intermediate filaments), microfilaments (composed of actin), and microtubules. 3. Cell processes of neurons. Neurons have two types of processes: axon and dendrite. Neurons form synapses, which are used for communication with other neurons, muscle cells, and glandular cells. 4. Each neuron has only one axon per cell. A. Axons are designed to convey signals from the perikaryon to the next neuron or to the effector cell. They end with an axon terminal. B. Axons vary in length but are usually fairly long (up to 1 meter), and have more or less constant diameter throughout their length. C. Axons begin from an elevated platform on the perikaryon called the axon hillock. Nissl bodies are absent from the axon hillock. D. Many axons are covered with an insulation sheath called the myelin sheath. Myelin sheath is extremely important because it allows the electric impulse to travel rapidly through the axons. Abnormalities in the formation of the myelin sheath result in severe disorders. E. The axon is enclosed by the continuation of the plasma membrane called axolemma. F. The cytoplasm of the axon, also called axoplasm, does not contain either Nissl bodies or ribosomes, but has well-developed smooth endoplasmic reticulum. G. The cytoskeleton of the axon is formed by numerous microtubules and neurofilaments. 5. Axonal transport. The presence of microtubules indicates intense transport of materials through the axon. The transport from the perikaryon to the periphery of the axon is called the anterograde flow and the transport from the distal part of the axon towards the perikaryon is called the retrograde flow. A. Anterograde flow allows transport of actin filaments, proteins, organelles, such as mitochondria, and vesicles from the perikaryon to the distal portion of the axon. The motor used in this type of transport is kinesin. There are two types of anterograde transport, slow and fast. Both types are mediated by kinesins. -5-  1. Slow axonal transport (~1-6 mm/day) is used to move such substances as tubulin molecules, actin molecules, and proteins that form neurofilaments, from the perikaryon to the end of the axon. 2. Fast axonal transport (~ 100-400 mm/day) is used to move membrane- bound organelles, such as SER compartments, synaptic vesicles, and mitochondria. B. The fast retrograde flow provides transport of materials taken up by endocytosis at axon terminal back to perikaryon. This is the pathway that some viruses (herpes simplex, rabies) use to travel through the nervous system. Toxins, such as the tetanus toxin, can be taken up to the perikaryon by the retrograde flow as well. The motor for this type of transport is dynein. 6. Most neurons have several dendrites per cell. Dendrites are designed to deliver the signal from the cell periphery to the perikaryon. A. Dendrites are typically numerous thick, short, and tapered processes of nerve cells. Dendrites branch profusely to form a dendritic tree, which increases the area for synaptic contacts. There can be up to 200,000 synapses in one dendritic tree. B. The surface of dendrites is covered with dendritic spines, where synapses with axonal processes of other neurons are formed. The dendritic spines have a “mushroom” shape and the “head” is where most postsynaptic receptors are located. C. Dendrites are not myelinated. D. The cytoplasmic composition of dendrites is similar to that of the perikaryon. They contain ribosomes and RER, but not Golgi apparatus. Here is a comparative chart of morphological differences between an axon and a dendrite: Axon Dendrite No Nissl bodies Nissl bodies Myelin sheath No myelin sheath Constant diameter Tapered Restricted branching Branches profusely Smooth surface Rough surface (spines) -6- 7. Major types of neurons. Based on the shape and number of cellular processes neurons can be classified into pseudounipolar, bipolar, and multipolar. A. Pseudounipolar neurons are primarily sensory neurons that have a single large process that begins from the perikaryon. This single cellular process branches into the peripheral and central processes. The peripheral process (PP) reaches the sensory area and collects the information, which is delivered to the central nervous system through the central process (CP). Both conduct like one axon. The two most typical locations for the pseudounipolar neurons are dorsal root ganglia and some cranial nerve ganglia. The name pseudounipolar is due to the fact that in the beginning of the development of the neuron two processes are formed, a dendrite and an axon, but they fuse later on to form one larger process that begins from the perikaryon. B. Bipolar neurons are sensory neurons that are rather limited in their distribution. They are found primarily within the major sense organs, such as eye retina, olfactory mucosa, and cochlea and semicircular canals of the inner ear. The bipolar neurons have two processes that extend from the cell body: an axon and a dendrite. Dendrite branches in the sensory area and acts as a receptor. Axon delivers the impulse to the central nervous system. C. Multipolar neurons represent the most common type of neurons. Both motor and interneurons belong to this type. These neurons have one axon and many dendrites. Depending on the relative length of the axon the multipolar neurons of the CNS can be either classified as: 1. Golgi type I cells, which have a long axon. These are the large motor neurons found in the motor nuclei of the CNS. 2. Golgi type II cells, which have a short axon. These are smaller interneurons found in the CNS. 8. Electrophysiology of the nerve. A. Nerve cells are similar to muscle cells in the fact that the plasma membrane of a nerve cell is an electric capacitor, like the sarcolemma of a muscle cell. The voltage on the inner side of the plasma membrane is negative (~ -70mV) relative to outer side, so there is a negative membrane potential in a resting cell. This is -7-  possible because the Na+ ions are actively pumped outside of the cell, so the concentration of Na+ is ten times greater outside of the cell than inside. B. Action potentials are brief positive going changes in the membrane potential that are propagated along the length of the membrane at speed up to 120 m/sec. As the action potentials (or waves of depolarization) travel along the membrane they open the voltage-sensitive channels and let the Na+ diffuse into the cell, which makes the membrane potential less negative. This is called depolarization of the membrane. C. The membrane is said to be hyperpolarized when the membrane potential becomes even more negative, which makes the membrane more difficult to depolarize. 9. Nerve cells communicate with each other and with other types of cells (e.g. muscle cells) via synapses. There are two major types of synapses: electrical and chemical. A. Electrical synapses in mammals are represented by gap junctions, which allow direct passage of ions from one cell to another to transmit the wave of depolarization. B. Chemical synapses are the principal type of synapses found in mammals. In a chemical synapse there is no protoplasmic continuity between the two cells and the signal is transmitted by release of a chemical (neurotransmitter) by one cell. Binding of the neurotransmitter to the receptors of the other cell results in either depolarization or hyperpolarization of the membrane. There are two principal types of chemical synapses, excitatory and inhibitory. Excitatory (or type I) synapses depolarize the membrane of the postsynaptic cell making the generation of an action potential more likely. These are mostly located on the dendrites and their spines. The inhibitory (or type II) synapses dive the membrane potential of the postsynaptic cell even more negative, which hyperpolarizes the membrane, thus making it less likely to generate the action potential. They are primarily located on the perikaryon. 10. Chemical synapses. A. Presynaptic knob contains synaptic vesicles that are 40-60 nm in diameter and contain the neurotransmitter. 1. Neurotransmitters are a diverse group of chemicals (over 100 known) that are capable of binding to receptors to generate the wave -8-  of depolarization or hyperpolarization in the postsynaptic cell. Thus a given neurotransmitter acts in only one of two ways: excitatory or inhibitory. B. Synaptic cleft is a narrow space (~20 nm) between the plasma membranes of the presynaptic and postsynaptic cells. C. Postsynaptic membrane contains receptor sites for the neurotransmitter. D. Action potential is usually propagated along the membrane of the presynaptic cell from the perikaryon towards the axon terminal. As it reaches the presynaptic terminal it opens the Ca++ channels briefly. The influx of Ca++ into the cytoplasm causes the synaptic vesicles to migrate to the membrane and fuse with it. The neurotransmitter diffuses across the cleft. When the neurotransmitter is bound by the receptors on the membrane, it starts the local depolarization of the membrane of the J&C: fig. 9-7 postsynaptic cell. The extra plasma membrane that was formed as a result of fusion of synaptic vesicles with the plasma membrane is removed by endocytosis using clathrin-coated vesicles. E. The neurotransmitter that has been released into the synaptic cleft is deactivated through two main mechanisms: recapture or degradation. 1. Up to 80 % of the neurotransmitters, such as catecholamines (i.e., dopamine, norepinephrine) that has been released into the cleft can be recaptured through the mechanism known as high-affinity reuptake. The neurotransmitter is reincorporated by endocytosis into vesicles that are ready for repackaging. 2. Enzymes, associated with the synaptic membrane, break down the remaining neurotransmitter that is left in the synaptic cleft. Such neurotransmitters as acetylcholine are broken down into acetate and choline in the cleft. 3. Clinical comment. It has been shown that inhibition of the enzyme that breaks down the neurotransmitter norepinephrine, or inhibition of high-affinity reuptake, has beneficial effect in the treatment of depression. -9- 11. Morphotypes of synapses. We can differentiate several morphotypes of synapses based on the connections they make. A. If the connection is between an axon and a dendrite, the synapse is called axodendritic. B. If the connection is between an axon and a perikaryon (body of a neuron), the synapse is called axosomatic. C. If the connection is between an axon and another axon, it is called axoaxonic. D. Motor end-plate represents the neuromuscular junction and is a specialized type of synapse. Motor end-plate consists of: 1. Axon terminal that contains presynaptic vesicles with the neurotransmitter acetylcholine. 2. Synaptic cleft is the space between the plasma membranes of the nerve cell and the muscle cell. 3. Sarcolemma of a muscle cell forms multiple junctional folds in the area of the motor end-plate. The receptor sites for acetylcholine are located within the junctional folds. 12. Clinical comments. A. Several toxins disable the chemical synapses including motor end-plates and do not allow the depolarization of the sarcolemma. The two best known toxins are the curare toxin and botulinum toxin. 1. Curare toxin was originally used by the South American Indians to hunt prey. It binds to the acetylcholine receptors and acts a muscle relaxant. 2. Botulinum toxin is a neurotoxin protein produced by the bacterium Clostridium botulinum. The toxin prevents release of acetylcholine from the synaptic vesicles. a. Botox (brand name for the botulinum neurotoxin) is used in cosmetic surgery to relax the facial musculature. B. Autoimmune diseases. Certain diseases can affect the neuromuscular junctions. One of them is the myasthenia gravis, which is an autoimmune disease characterized by extreme muscle weakness. 1. Auto-antibodies to the acetylcholine receptor protein are produced. 2. Auto-antibodies bind to the receptor sites, which weakens the muscle response to the nerve stimuli. - 10 -  C. Rabies virus is carried by wild mammals, such as skunks or raccoons. If an infected animal bites a person a series of unfortunate events takes place. 1. When the muscle fibers are broken during the bite, the virus gets into the muscle and starts replicating. Replication takes place for about one or two weeks and this is the time when vaccine can still help. 2. After replication the virus finds a motor-end plate and gets into the cleft. 3. Virus enters the synaptic terminal and via retrograde axonal transport it reaches the body of the motor neuron in the CNS and is ready to spread to other neurons. 4. Very soon most of the CNS is affected, which causes severe inflammation. Any change in the light intensity or any sounds, like the running water, cause seizures. That’s why the old name for rabies: hydrophobia. 5. The virus spreads into the salivary glands and that is how it is transmitted from animal to animal with a bite. 6. After the symptoms have shown up, generally there is no cure. IV. Support cells of the nervous system. Neurons and their processes are nourished and protected by the support cells of the nervous system that far outnumber the neurons. 1. The peripheral nervous system (PNS) contains two major types of support cells: Schwann cells and satellite cells. A. Schwann cells form a lipid layer called myelin sheath that surrounds axons in the peripheral nerves. Schwann cells also “envelope” the unmyelinated axons. Myelin sheath isolates the axon from the surrounding tissue and provides electrical insulation for the nerve fibers. Myelin sheath is necessary for rapid conduction of electrical impulses. 1. The myelin sheath forms open gaps, where the myelin sheath interrupts. These gaps in the myelin sheath are called nodes of Ranvier. Nodes of Ranvier represent spaces between adjacent Schwann cells. The axolemma of myelinated nerve fibers in the areas at the nodes of Ranvier has high concentration of Na+ channels. Action potential in myelinated nerve fibers travels via saltatory conduction, which means that the membrane is only depolarized at the nodes of Ranvier. Because of the presence of electrical insulation in the form of the myelin sheath there is no charge leakage through the membrane, and, as a result, depolarization of the membrane at - 11 -  one node is sufficient to elevate the voltage at the next node to the level necessary to generate an action potential. Nodes of Ranvier also allow axons to form synapses with each other and to form branches. Branching is usually best expressed in the vicinity of the target group of cells. 2. Schwann cells support both myelinated and unmyelinated nerve fibers. a. In myelinated nerve fibers (A) a single axon, located in the middle is sheathed by a Schwann cell that wraps around it several times. The plasma membrane layers fuse together to form the myelin sheath, a lipoprotein complex. Action potential travels through myelinated nerve fibers using saltatory conduction (fast). b. In unmyelinated fibers of the PNS (B) several axons are enveloped into simple clefts in the Schwann cell. As a result, the Schwann cell is located in the middle of the nerve bundle. Action potential in unmyelinated fibers is wave-like. B. Satellite cells are support cells found primarily in the ganglia of the peripheral nervous system, where they surround bodies of individual neurons. They create a microenvironment around individual neurons and provide electrical insulation for the bodies of neurons. So, they act similar to Schwann cells, but they do not have myelin. Satellite cells also provide a pathway for metabolic exchange necessary for the neurons. 2. The central nervous system (CNS) contains several types of support cells, or neuroglia: astrocytes, oligodendrocytes, microglial cells, and ependymal cells. A. Astrocytes are among the largest neuroglial cells (8-10 m) and provide support for neurons and vascular structures of the CNS. Astrocytes have granular cytoplasm and large nuclei; the mitochondria are numerous in the cytoplasm. Astrocyte processes extend between neurons and blood vessels. Astrocytes play an important role in moving metabolic substances between blood and nerve cells. Together with the endothelial cells of blood capillaries astrocytes form the blood- brain barrier. There are two major types of astrocytes, protoplasmic and fibrous. Astrocytes stain positive for glial fibrillary acidic protein (GFAP), which forms the intermediate filament cytoskeleton of these cells. - 12 -  1. Protoplasmic astrocytes are found in the gray matter of the brain. They have numerous short, branching processes that form structures called perivascular feet along blood capillaries. 2. Fibrous astrocytes have more prominent cytoskeleton, than protoplasmic astrocytes, and are primarily found in the white matter of the brain. These cells have fewer processes with less expressed branching. 3. Clinical comments. a. Tumors derived from astrocytes are called astrocytomas. These are some of the most common tumors in the brain and represent 20% of all brain tumors (including the ones that were formed elsewhere and metastasized into the brain). Astrocytes give rise to 80 % of all tumors that originate in the brain. b. In case of the local damage to the brain astrocytes are responsible for the process called gliosis, which results in the formation of a glial scar. J&C: fig. 9-15 B. Oligodendrocytes are the most common neuroglial cells of the CNS. These are smaller cells, than astrocytes (6-8 m). They have small nuclei, abundant SER, and prominent Golgi apparatus. There are few tongue-like cell processes that extend from the oligodendrocyte cell body to wrap around the axons of the neurons of the CNS forming segments of myelin sheath. Gaps between individual segments of oligodendrocytes in the CNS represent the nodes of Ranvier. Thus oligodendrocytes of the CNS are similar to the Schwann cells of the PNS, but different in the way they form the myelin sheath. 1. Clinical comment: multiple sclerosis is a disease that is caused by damage to the myelin sheath of the axons in the CNS done by cells of the immune system. It results in the partial loss of the myelin sheath. Symptoms may include loss of sensitivity, partial paralysis, etc. depending on the area that is damaged. C. Microglial cells have distinctive phagocytic properties. Microglial cells are derived from blood monocytes and are part of the mononuclear phagocytic system. These are the smallest cells of the neuroglia (5-7 m). They have dark indented nuclei and limited cytoplasm. Cells have few short-twisted processes that are covered with spikes, which may be equivalent to the ruffled border seen - 13 -  in other phagocytic cells. The cytoplasm of microglial cells contains many lysosomes. The numbers of microglial cells in the brain increase with injury, so microglial cells are believed to remove the debris from the CNS. 1. Clinical comment. It has been noticed that microglial cells are abundant in patients with Alzheimer’s and Parkinson’s diseases. It is possible that microglial cells are partially responsible for the plaque formation, demyelination and destruction of nerve fibers in the CNS of patients with these disorders. D. The ventricles of the brain and cavities of the spinal cord are lined with ependymal cells. They are responsible for production and absorption of cerebrospinal fluid (CSF). These cells are arranged in a form of simple cuboidal epithelium, but unlike the true epithelia there is no basal lamina. The cells are tightly bound by junctional complexes and often possess microvilli, which are responsible for absorbing CSF. There are also few cilia attached to the luminal surface of some ependymal cells. The basal processes of ependymal cells interdigitate with astrocyte processes allowing exchange of metabolites between these cells. Glial fibrillary acidic protein is also present in ependymal cells. V. Peripheral nervous system (PNS) consists of cranial, spinal, and peripheral nerves, ganglia, and special nerve endings. 1. The nerves of the PNS are made of many nerve fibers that carry sensory and motor information between the organs and tissues of the body. They are composed of myelinated and non-myelinated axons. The nerve fibers are held together by sheets of connective tissue, the endoneurium, the perineurium, and the epineurium. A. Endoneurium surrounds individual nerve fibers. B. Perineurium surrounds nerve fascicles. C. Epineurium surrounds individual nerves and extends into the spaces between the fascicles. 2. Ganglia are clusters of neuron cell bodies outside the central nervous system. Ganglia are covered by a connective tissue capsule and usually have satellite cells associated with them. The two major types of ganglia are the sensory craniospinal ganglia and the motor ganglia of the autonomic nervous system. - 14 -  A. Sensory craniospinal ganglia contain pseudounipolar neurons. The pseudounipolar neurons have a single process, which T-branches into peripheral and central processes. The peripheral process is long and goes to the receptor organ. The central process is rather short and goes to the spinal cord (dorsal root ganglia) or to the brain (cranial ganglia) to form synapses with neurons of the CNS. Satellite cells surround the pseudounipolar neurons of craniospinal ganglia. B. The motor ganglia of the autonomic nervous system contain multipolar neurons and satellite cells. 3. Special endings can be either motor or sensory. We already discussed the specialized motor nerve endings in the skeletal muscle tissue, called motor end-plates. Sensory nerve endings can be classified into two major types: special-sense nerve endings and somesthetic receptors. A. Special senses nerve endings are sensory endings specialized for smell, sight, hearing, and equilibrium. B. Somesthetic receptors are found throughout the body in epithelial tissues, connective tissues, muscles, and joints. 1. Free nerve endings are branched sensory endings that mediate pain. 2. Encapsulated nerve endings include Meissner’s corpuscle, Pacinian corpuscle, and several others. a. Meissner’s corpuscle is a cylindrical structure formed by the stacks of lamellae that surround one or two sensory nerve endings. These receptors provide the sense of touch and are most common in the skin of fingers and toes. b. Pacinian corpuscle is the largest of encapsulated nerve endings (up to 2 mm) and is the most complex type. It is spherical in shape and consists of up to 30 concentric sheets of connective tissue with fluid between the layers that surround a single nerve fiber. These receptors respond to vibrations and deep pressure and are found in the dermis of the skin, mesenteries, and inside internal organs (e.g. pancreas). - 15 -  C. Proprioceptors are designed to collect information about the angulations of joints and muscle tension. 1. Muscle spindle is a specialized receptor unit located in the skeletal muscle. It is a specialized stretch receptor. It is covered with two capsules, internal and external, with fluid-filled space separating them. Inside the spindle there are intrafusal fibers, which are thin skeletal muscle fibers that are surrounded by the nerve fibers of two types (sensory and motor). The sensory nerve fibers wrap around the intrafusal fibers and transmit information about the degree of stretching of the muscle. The motor nerve fibers are thought to regulate the sensitivity of the stretch receptor. VI. Central nervous system (CNS) consists of the spinal cord and the brain. Clusters of neurons in the CNS are called nuclei. The neurons are supported by the neuroglial cells, discussed earlier (V.2). Nerve fibers are organized into tracts. Organs of the CNS are supported by accessory structures, such as meninges, choroid plexuses, ventricles, etc. The nervous tissue of the central nervous system can be divided into the white matter and gray matter, which are organized in a different way in the spinal cord and in the brain. 1. Gray matter consists of neuron bodies and unmyelinated fibers that form dense fibrous network. The tissue has extensive vascular supply through a system of capillaries. A. In the spinal cord the gray matter is internal to the white matter (opposite to what we see in the brain). The gray matter is organized into two pairs of horns: anterior and posterior (dorsal and ventral). Ventral horns contain large motor neurons, while the dorsal horns receive information from the dorsal root ganglia. The gray matter on the left and right sides of the spinal cord is connected via the gray commissure. B. In the brain the gray matter is external to the white matter and is often thrown into deep folds called gyri. In the cerebellum these folds are called folia. - 16 - P. Kondrashov, Ph.D. NERVE TISSUE 1. The gray matter in the cerebrum is organized into 6 layers. The three main types of neurons found in the cerebrum are pyramidal cells, fusiform cells, and granule cells. 2. In the cerebellum the gray matter is organized into three layers. a. The molecular layer is the most external layer. It contains relatively few cell bodies of neurons called basket cells and numerous cell processes. b. The Purkinje cell layer is a thin layer composed of very large neurons called Purkinje cells. c. The most internal layer, adjacent to the white matter, is called the granular layer. It is highly cellular and is mostly composed of small neurons called granule cells. 2. White matter consists of myelinated axons and glial cells. It has limited blood supply compared to the gray matter. The tissue is rather dense with very limited extracellular space. Generally, there are no synaptic contacts within the white matter. In the spinal cord the white matter is external to the gray matter, while it is the opposite in the brain. - 17 -