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histology summary notes, Essays (university) of Histology

Samarkand State UniversityHistology

summary notes of histology from lectures

Typology: Essays (university)

2019/2020

Uploaded on 01/19/2020

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Nerve system
The study of nervous system histology could easily comprise the entire subject matter of this
course and a couple more. Thus, we will just be considering only a broad overview of the
subject.
Nervous tissue is the subject of intense interest, not only because of its complexity, but
because it is the seat of thought, emotion, and behavior. All the technological, philosophical,
and cultural achievements of human beings are the result of the structure and metabolism of
this tissue.
Two major components of nervous system,
1. central nervous system (CNS) - brain, spinal cord
2. peripheral nervous system (PNS) - nerves extending toward or away from
CNS and ganglia (aggregations of nerve cell bodies outside CNS).
Nervous tissue consists of two major types of cells,
1. neurons - responsible for conduction, propagation, and reception of nervous
impulses. Processes called axons or dendrites extend from these cells.
2. glial cells - (neuroglia) cells associated with neurons. No axons or dendrites.
These cells are involved in nutrition, support, insulation, protection of neurons.
If brain or spinal cord are sectioned, we find that two major areas of brain tissue may be
defined on the basis of their color in fixed, unstained tissue. In living tissue gray matter is
actually pink due to blood in the many capillaries coursing through this tissue.
1. gray matter - neuron perikarya (cell bodies), glial cells, axons, dendrites,
synapses
2. white matter - axons + myelin sheaths and glial cells. No neuron perikarya,
no synapses
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Nerve system The study of nervous system histology could easily comprise the entire subject matter of this course and a couple more. Thus, we will just be considering only a broad overview of the subject. Nervous tissue is the subject of intense interest, not only because of its complexity, but because it is the seat of thought, emotion, and behavior. All the technological, philosophical, and cultural achievements of human beings are the result of the structure and metabolism of this tissue. Two major components of nervous system,

  1. central nervous system (CNS) - brain, spinal cord
  2. peripheral nervous system (PNS) - nerves extending toward or away from CNS and ganglia (aggregations of nerve cell bodies outside CNS). Nervous tissue consists of two major types of cells,
  3. neurons - responsible for conduction, propagation, and reception of nervous impulses. Processes called axons or dendrites extend from these cells.
  4. glial cells - (neuroglia) cells associated with neurons. No axons or dendrites. These cells are involved in nutrition, support, insulation, protection of neurons. If brain or spinal cord are sectioned, we find that two major areas of brain tissue may be defined on the basis of their color in fixed, unstained tissue. In living tissue gray matter is actually pink due to blood in the many capillaries coursing through this tissue.
  5. gray matter - neuron perikarya (cell bodies), glial cells, axons, dendrites, synapses
  6. white matter - axons + myelin sheaths and glial cells. No neuron perikarya, no synapses

BASIC NEURON STRUCTURE

  1. Basic neuron a. perikayon - nerve cell body, contains nucleus and typical cell organelles * Nucleus - large, central in most, large amount of euchromatin (intense synthetic activity), Barr body (Dormant X chromosome of females). * rough endoplasmic reticulum (RER) - lots for synthesis of structural and transport proteins, Nissl bodies seen with light microscope are condensations of ths RER and free ribosomes. * Golgi apparatus - only found near nucleus in perikaryon. Expected, since intense synthetic activity of neurotransmitters and/or neurohormones. * Mitochondria - abundant for high energy requirements * Neurofilaments, microtubules - neurofilaments are intermediate filaments (10 nm) Microtubules - important in transport of materials (e.g. neurotransmitters)

*Synapse can transmit action potential, or can polarize or depolarize the postsynaptic cell. *Synapses at end of an axon or axon branches are swollen into a club shape, called boutons terminaux. *Those along length of axon result in varicosities (swellings) in the axon, called boutons en passage. e. General structure of synapse

  • terminal or presynaptic membrane - this is part of the neuron plasmalemma
  • synaptic gap is present - this is a space between the presynaptic membrane of the axon and the plasmalemma of the cell that receives the synaptic input
  • postsynaptic membrane - part of plasmalemma of a cell that receives input *high concentrations of small vesicles in bouton that contain neurotransmitter. *when action potential reaches synapse, these vesicles are exocytosed at the presynaptic membrane and their contents (neurotransmitter) are released into the synaptic gap. *neurotransmitter binds to receptors on postsynaptic membrane and propagates electrical impulse (action potential) or membrane charge change (polarization or depolarization) in post-synaptic cell. TYPES OF NEURONS Three major catagories serve to describe most neurons
  1. multipolar - more than two processes (one axon plus multiple dendrites), most of neurons in brain and spinal cord are of this type.
  2. bipolar - two major processes (axon and dendrite), but may be branched at ends, sensory neurons in retina, cochlea, and olfactory epithelium are of this type.
  1. pseudounipolar - two major processes that are fused along portions closet to perikaryon - found in spinal ganglia and some cranial ganglia. Regardless of the type of neuron, the general structure dendrite(s)-perikaryon- axon-synapse is the same. Appearance may differ due to branching or fusion of processes. NEURON CLASSIFICATION BASED ON FUNCTION
  2. Motor neurons - efferent, action potential moves from CNS to effector organ (e.g. muscle)
  3. Sensory neurons - afferent, action potential moves from sensory organ to CNS (e.g. neuron processes (effectively dentrites, but often called axons) associated with pacinian corpuscles, touch, pressure)
  4. Interneurons - form connections between neurons GLIAL CELLS There are many more glial cells in the nervous system than there are neurons. These cells are situated among the neurons and are generally smaller. Special staining techniques are necessary if their cell bodies are to be easily differentiated from surrounding cells. In sections stained with hematoxylin - eosin, only the glial cell nuclei show up. Major glial types found in the CNS
  5. Astrocytes - two types a. protoplasmic astrocytes *granular cytoplasm, many branches on short processes *some of processes are closely applied to neurons, while others form intimate contacts with blood vessels. *thought to form a conduit for nutrients from blood vessels to neurons. *found in gray matter. b. fibrous astrocytes

STRUCTURE OF NERVES

  1. Nerves are surrounded by a thick connective tissue sheath composed of collagenous fibers and fibroblasts and containing small blood vessels that is called the epineurium. The epineruium extends into the nerve and between the fascicles.
  2. Within the epineurium are bundles of nerve fibers called facicles. a. Nerve fibers - groups of axons, each one of which is ensheathed by single or multiple folds of a sheath cell. b. Sheath cells are Schwann cells in PNS and oligodendrocytes in CNS.
  3. Each bundle (or facicle) of axons is surrounded by a layer of connective tissue called the perineurium
  4. Each axon within each bundle is individually surrounded by a layer of reticular connective tissue called the endoneurium a. Small diameter axons are usually sheathed by single fold of sheath cell and are not myelinated.

b. Larger axons are sheathed by multiple layers of specialized sheath cell plasma membrane with a high myelin content. Such axons are said to be myelinated. c. Myelin acts to insulate axon, speeds up conduction of electrical impulses due to the formation of nodes of Ranvier where adjacent sheath cells meet along the length of an axon.