Histology of Nervous Tissue

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Nervous Tissue:

A specialized constellation of nerve cells (neurons) and support cells (glia) whose main function is:

• Reception of information from external and internal environment.

• Integration of incoming information Generation of new signals.

• Transformation and conduction of messages to special responding tissues (effectors).

Electrochemical Signaling via Synapses:

NOTE: Circuitry is very plastic especially in early development and there is incredible diversity of cell shape.

• Cell body = soma = perikaryon.

• Single Axon - conduct away from soma.

• Axon terminals (synaptic terminals).

• Multiple Dendrites - short, branching processes which form receptive fields.

• Conduct toward the soma.

Central Nervous System:

Covering:

Anything covered by dura. Integrates internal and external sensory input to produce motor or glandular output.

Parts of the Central Nervous System:

Cerebrum:

• Integrates sensory information, voluntary motor responses initiates and coordinated, language, learning and memory.

Brain stem:

• Basic functions like breathing.

Cerebellum:

• Modulates motor impulses to coordinate movements of muscle groups.

Peripheral Nervous System:

Extra-dural structures outside brain and spinal cord. Somatic or cerebrospinal system of 12 pairs of cranial nerves and 31 pairs of spinal nerves. Conveys information to skeletal muscle.

Autonomic Nervous System:

Supplies smooth and cardiac muscle and glands.

Sympathetic:

• Thoracolumbar division.

Parasympathetic:

• Craniosacral division.

Nervous System Terminology:

Ganglion

• A collection of neuron cell bodies outside the CNS, ex. DRG and sympathetic chain ganglia.

Nucleus

• A cluster of neuron cell bodies in the brain or spinal cord.

Tract

• A functional aggregation of axons interconnecting distant nuclei in the CNS.

Nerve

• A collection of axons in the PNS. A neuron is not a nerve.

Soma

• Is the trophic and signal integration portion of the neuron. Accounts for ~10% of volume of neuron. Nucleus is generally central and highly euchromatic. 1-4 nucleoli. RER as Nissl substance. Intermediate filaments (neurofilaments), microtubules and microtubules plentiful. Transcription and translation of proteins occurs in the soma. No RER or free ribosomes in axons.

Nervous System Proteins and their locations:

• Most NF proteins go to the axon.

• Nuclear lamins remain in the soma.

Nervous System Glossary:

Dendrite:

Arborization of dendrites makes it possible for one neuron to receive and integrate information from many other axon terminals, ex. 200,000 axon terminals interact with 1 Purkinje cell in the cerebellum. Dendritic spines are specialized areas to receive axon synapses.

Axon:

No Nissl in axon hillock. Axons from spinal cord to foot may be 40 inches long. There is only one axon per neuron but the axon may branch to form terminal arborizations and give off many collaterals to several targets. Each arborization ends in a dilation called the end bulb (synaptic boutons) that are part of the synapse. A variety of synapse types occur depending on what area the axon associates with.

Synaptic vesicles:

Synaptic vesicles contain neurotransmitters such as dopamine, acetylcholine, noradrenalin, etc. Presynaptic dense bars or dense projections containing the protein synapsin characterize the axon terminal. The axon terminal is devoid of microtubules and IF but rich in MF. Active membrane turnover occurs at the synapse. Endocytosis mediates membrane turnover.

Myelinated or Unmyelinated:

Peripheral nerves may be myelinated or unmyelinated. All axons are ensheathed by cytoplasm of glial cells. Myelinated axons are enveloped by segmental multilayered covering composed mostly of lipid derived from plasmalemmae of glial cells. Unmyelinated axons are ensheathed by a single layer of glial cytoplasm.

Unmyelinated axons are "pushed" into the side of support cells and lack nodes of Ranvier.

Mesaxon:

A narrow cleft formed by apposed plasmalemma of glial cells.

Schwann cell:

Schwann cell performs myelination in the PNS. One Schwann cell effects myelination of a part of a single axon. Intraperiod lines are formed by fusion of the outer leaflets of the two glial cells. Major dense lines are fusion of the inner cytoplasmic leaflets.

Incisures of Schmidt-Lanterman:

Incisures of Schmidt-Lanterman are helical cytoplasmic channels extending from the outside of the sheath to the inside to nourish the peripheral-most extension of the Schwann cell that is nearest the axon. Incisures appear as a herringbone pattern when viewed in longitudinal section.

Nodes of Ranvier:

Myelin is interrupted at nodes of Ranvier. The space is the internode. Saltatory conduction increases the speed of impulses in large myelinated axons. Neurokeratin is a fixation artifact of LM preparation that extracts the myelin and leaves a network of residual protein.

Oligodendrocytes:

Oligodendrocytes perform myelination in the CNS. One oligodendrocyte may participate in myelinating up to 50 axons.

Multiple sclerosis (MS):

Demyelinating diseases include multiple sclerosis (MS). The cause is unknown but viral and autoimmune causes are suspected. Systemic demyelinating diseases also occur in cases of carbon monoxide poisoning, chronic cerebral edema (due to meningitis, tumors, hydrocephalus), exposure to lead, tin, hexachlorophene.

Classification of Neurons:

According to function:

Motor

• Carry impulses that produce movement and include neurons of the ANS and anterior (ventral) horn of the spinal cord.

Sensory

• Neurons of the dorsal root ganglion (DRG) are of this type.

Internuncial, intercalated or association

• Connect neurons in the brain and spinal cord.

According to the length of the axon:

Golgi type I. neurons

• Have very long axons and leave their place of origin and extend a considerable distance. They form tracts, commissures and projection fibers.

Golgi type II. neurons

• Are very short and never leave the gray matter. Neurons of this type are numerous in the cerebral cortex, cerebellar cortex and spinal cord.

Grey and White Matter:

In the spinal cord, gray matter is central and white matter peripheral. In the cerebral cortex there is an additional outer cortex of gray matter.

Cover of a PNS nerve:

In the PNS, a nerve is covered by 3 connective tissue sheaths:

epineurium

• Around the entire nerve (Note: Small peripheral nerves lack epineurium).

perineurium

• Around bundles of axons.

endoneurium

• Around a single axon.

Function of Connective Tissue of CNS:

In the CNS, connective tissue forms the enveloping, protective and vascular coats (meninges). Most parts of the body are supported by intercellular substances produced by connective tissue derived from mesoderm. Gray and white matter of the CNS develop from neuroectoderm except endothelium and pericytes of capillaries and lack connective tissue and fibers. The substance of the brain and spinal cord is, therefore soft and mushy like pate and requires support provided by their neuroectodermally derived glia cells.

Non-neuronal Glial Cells:

• From neuroepithelium: astrocytes (both types), oligodendrocytes, and ependyma.

• From neural crest: Schwann cells (these are the only support cells in the PNS).

• From mesoderm: microglia.

Neuroglia:

Neuroglia or glia are non-neuronal support cells of the CNS. Outnumber neurons 10:1. Have only one type of cell process (not axons and dendrites). Do not form synapses. Can undergo mitosis in adults.

Protoplasmic Astrocytes:

Mainly in gray matter; processes cover non-synaptic and synaptic areas so that they primarily segregate and isolate receptive surfaces of neurons and prevent the flow of impulses in a haphazard manner.

Fibrous Astrocyte (long legged spiders):

Chiefly in white matter; many cytoplasmic filaments, thus, they are the chief supporting cells of the CNS; sometimes called scarring cells because they fill the gap after injury. If the void is too large for astrocytes alone to handle, fibroblasts invade to produce connective tissue fiber scars in a process called sclerosis.

Microglia:

• Arise from mesoderm late in development, no long before birth.

• ~10% of glia.

• In injury, they divide, enlarge, retract their processes, migrate to the site of injury and become phagocytic, thus their important function as scavengers of the CNS, comparable to macrophages elsewhere.

• In cases of severe injury, monocytes may enter and convert to macrophages and assist microglia.

• Act as antigen presenting cells.

Ependyma:

• Single row of cuboidal to low columnar cells that line the ventricular system of the brain and central canal of the spinal cord.

• (The ependyma that covers capillaries of the choroid plexus are called the lamina epithelialis).

• Cells are ciliated to move cerebrospinal fluid.

Staining of Glial Cells:

• Only glial cells stained by conventional methods.

Characteristics of Spinal Ganglia (dorsal root ganglia):

• Pseudounipolar.

• Cells bodies are separated from connective tissue framework by a layer of satellite or capsule cells.

• No synapses.

• Cell bodies are pushed to the periphery of the ganglion.

Characteristics of Autonomic Ganglia:

• Neurons are multipolar.

• Synapses present.

• Nuclei are eccentrically located.

• Satellite cell layer is less distinct.

• More than one nucleus in satellite cell covering neurons are evenly distributed in the ganglion.

Parasympathetic Ganglia:

• Parasympathetic ganglia are found in the walls of the alimentary tract.

Meissner's Submucosal Plexus:

• Found at base of submucosa Auerbach's myenteric plexus between circular and longitudinal smooth muscle layers.

Nerve Fibers in White Matter:

In white matter, nerve fibers either arise from cell bodies of neurons in the brain and descend to different levels of the cord or from cell bodies in the cord or sensory ganglia where they ascend to the brain. There is a progressive increase in white matter from lower to higher levels and for an increase in gray matter at sites of origin of nerve plexuses.

Function of Connective Tissue Meninges:

Connective tissue meninges envelope the cord and brain.

Leptomeninges:

Pia and arachnoid mater together are called the leptomeninges.

Pachymenix:

The dura is called the pachymenix.

Structure of Dura Mater:

Dura extends between the cerebral hemispheres as the falx cerebri and partitions the cerebrum from cerebellum as the tentorium cerebelli.

Cerebral Hemispheres Mediate:

• Consciousness.

• Premeditated thoughts.

• Sensory awareness (not reflex).

• Conscious initiation of motor activities.

• Emotions.

Cerebellum Functions:

• Coordination of movements.

• Maintain posture, impulses from tendons, muscles, joints, eyes, ears are coordinated in the cerebellum and resulting stimuli discharged for effective motor responses.

• Cerebellum with Purkinje cells.

Choroid Plexus:

Brain ventricles contain a choroid plexus that consists of numerous villi or folds that project into the ventricles. Each villus is covered with cuboidal ciliated ependymal epithelium. Choroid plexus produces cerebrospinal fluid (CSF).

Additional Reading:

Basic Histology

1. Introduction to Histology
2. Basic Cell Physiology
3. Actin, Microtubules, and Intermediate Filaments
4. Mitochondria, Nucleus, Endoplasmic Reticulum, Golgi
5. Epithelium (Epithelial Tissue)
6. Connective and Adipose Tissue
7. Types of Cartilage
8. Osteogenesis
9. Nervous Tissue
10. Muscle Tissue
11. Cardiovascular System
12. Blood and Hematopoiesis
13. Lymphoid Tissue
14. Digestive Tract I: Oral Cavity
15. Digestive Tract II: Esophagus through Intestines
16. Liver, Pancreas, and Gall Bladder
17. Respiratory System
18. Integument
19. Urinary System
20. Endocrine System
21. Male Reproductive System
22. Female Reproductive System
23. Eye and Ear

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