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Special Topics in Biomedical Science
Neurons
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The body has two systems that help maintain homeostasis: the nervous system and the endocrine system.
3http://adapaproject.org/bbk/tiki-index.php?page=Leaf%3A+What+are+the+structures+and+primary+functions+of+the+major+organ+systems%3F
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The nervous system is a complex network of nervous tissue that sends electrical and chemical signals.
The nervous system includes the central nervous system (CNS) and the peripheral nervous system (PNS) together.
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Brain
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The central nervous system is made up of the brain and spinal cord, and the peripheral nervous system is made up of the nervous tissue that lies outside the CNS, such as the nerves in the legs, arms, hands, feet and organs of the body.
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The electrical signals of the nervous system move very quickly along nervous tissue.
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Nerve Cells
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1: Unipolar neuron 2: Bipolar neuron 3: Multipolar neuron 4: Pseudounipolar neuron
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Although the nervous system is very complex, there are only two main types of nerve cells in nervous tissue.
All parts of the nervous system are made of nervous tissue.
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The neuron is the "conducting" cell that transmits electrical signals, and it is the structural unit of the nervous system.
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The other type of cell is a glial cell. Glial cells provide a support system for the neurons, and recent research has discovered they are involved in synapse formation.
Some different glial cells.14
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A type of glial cell in the brain, called astrocytes, are important for the maturation of neurons and may be involved in repairing damaged nervous tissue.
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Immunocytochemical staining of Astrocytes in culture using an antibody against glial fibrillary acidic protein.
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Isolated Astrocyte shown with confocal microscopy.
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human astrocyte
19Metabolic interactions between astrocytes and neurons.
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Description Diagram appeared in an article in a CC-by licensed journal. The figure description provided by the article: "Thick arrows show uptake and release reactions. Dashed arrows indicate shuttle of metabolites between two cell types. Glutamate and α-ketoglutarate in transamination reactions are abbreviated as GLU and AKG, respectively. All reactions considered in the modeling are given inadditional file 1 [see Reaction set below]. The reaction numbers in the figure refer to the numbering in the reaction list of additional file 1. Here we only depict major reactions for simplicity.“
Date17 December 2007SourceCakir, Tunahan; Selma Alsan, Hale Saybasili, Ata Akin, Kutlu
Ulgen (2007). "Reconstruction and flux analysis of coupling between metabolic pathways of astrocytes and neurons: application to cerebral hypoxia". Theoretical Biology and Medical Modelling 4 (1): 48. DOI:10.1186/1742-4682-4-48.ISSN 1742-4682. Retrieved on 2007-12-17.
AuthorPrepared by User:OldakQuill from diagram in journal article (see source). Authors of article: Tunahan Cakir, Selma Alsan, Hale Saybasili, Ata Akin, Kutlu Ulgen
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Neurons and glial cells make up most of the brain, the spinal cord and the nerves that branch out to every part of the body.
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Both neurons and glial cells are sometimes called nerve cells.
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Structure of a Neuron
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Every neuron has a membrane that surrounds its cytoplasm and a nucleus that contains its genes. Neurons also have small organelles that let them produce energy and manufacture proteins.
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The neurons’ main job is to transmit information, so they also have two types of highly specialized extensions that distinguish them from other cells.
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Dendrites have a tree-like branching structure.
They gather information and send it to each neuron’s cell body.
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Axons are generally very long, and many neurons have only one. The axon carries information away from the neuron’s cell body toward other neurons, with which it makes connections called synapses.
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Axons can also directly stimulate other types of cells, such as muscle and gland cells.
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The special shape of a neuron allows it to pass an electrical signal to another neuron, and to other cells.
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Electrical signals move quickly along neurons so that they can pass “messages” from one part of the body to another.
These electrical signals are called nerve impulses.
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Neurons are typically made up of a cell body (or soma), dendrites, and an axon.
not all neurons have a myelin sheath.
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The cell body contains the nucleus and other organelles similar to other body cells.
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The dendrites extend from the cell body and receive a nerve impulse from another cell.
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The cell body collects information from the dendrites and passes it along to the axon.
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The axon is a long, membrane covered extension of the cell body that passes the nerve impulse onto the next cell. The end of the axon is called the axon terminal.
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The axon terminal is where the neuron communicates with the next cell.
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The dendrites of the neuron receive the information, the cell body gathers it, and the axons pass the information onto another cell.
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The axons of many neurons are covered with an electrically insulating phospholipid layer called a myelin sheath.
The myelin speeds up the nerve impulse along the axon.
The myelin is an outgrowth of glial cells.
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Schwann cells which are sometimes wrapped around the neuron, are a type of glial cell. Schwann cells are flat and thin, and like other cells, contain a nucleus and other organelles.
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Schwann cells make myelin for neurons that are not part of the brain or spinal cord.
Another type of glial cell, called oligodendrocytes, supply myelin to neurons of the brain and spinal cord.
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Myelinated neurons look white.
They are the "white matter" of the brain.
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A cross section of a myelinated neuron.
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Myelin is not continuous along the axon.
The gaps between the myelin are called Nodes of Ranvier.
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The nodes are the only places where ions can move across the axon membrane, through ion channels.
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The nodes strengthen the nerve impulse by concentrating the flow of ions at the nodes of Ranvier along the axon.
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Neurons are specialized for the passing of cell signals.
Neurons have many functions in different parts of the nervous system, so there are many different shapes and sizes of neurons.
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E.g., the cell body of a neuron can vary from 4 to 100 micrometers in diameter.
Some neurons can have over 1000 dendrite branches, which connect with tens of thousands of other cells.
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Other neurons have only one or two dendrites, each of which has thousands of synapses.
A synapse is a specialized meeting place where neurons communicate with each other.
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A neuron may have one or many axons. The longest axon of a human motor neuron can be over a meter long, reaching from the base of the spine to the toes. Sensory neurons have axons that run from the toes to the spinal cord, over 1.5 meters in adults.
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Neurons form networks through which nerve impulses travel.
From each neuron’s dendrites to the tip of its axon, these impulses move through the neural membrane in the form of electricity.
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The neurons communicate without touching.
They use special molecules, called neurotransmitters, to pass nerve impulses from one neuron to the next.
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Chemical transmission of nerve impulses causes the axon and the dendrites to develop specialized structures that help it.
So, dendrites have thousands of “spines” sticking up out of their surface.
Spines on the dendrite of a medium spiny striatal neuron. Enhanced Green Fluorescent Protein (EGFP) was expressed in the neurons and seen using a laser microscope
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Main types of shapes for dendritic spines60
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The bulb-like terminal buttons of the axons, which secrete the neurotransmitters, are opposite these spines.
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The form of these structures of the synapse, and the overall form of the neurons varies greatly.
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64there are over 200 different kinds of neurons
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NEURONS have the basic characteristics of other cells, but have made them more important.
This includes transmembrane potential, the ability to form extensions of its cytoplasm, and so on.
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The extensions of neurons have become specialized, so that the ion channels and receptors in dendrite membranes are different from those in axon membranes.
The number, types and location of receptors and ion channels can be different in different parts of the membrane and different types of cell.
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Every neuron has its own unique shape, its own unique position in the nervous system, and its own unique connections to other neurons or to receptor (sensory) cells or effector (muscle or gland) cells.
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There are over 200 different kinds of neurons.
So some neurons do not have the standard basic morphology.
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For example, some axons may form synapses directly with another neuron’s cell body, or even with its axon.
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Neuronal cell bodies also vary widely both in size (small, medium, large, and giant) and in shape (star-shaped, fusiform, conical, polyhedral, spherical, pyramidal).
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The geometry of a neuron’s dendrites and axon also vary tremendously with its role in the neural circuit.
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Neurons can also be classified into various categories.
For example:
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Functional classification:
eg. sensory neurons that receive sensory signals from sensory organs and send them by short axons to the central nervous system
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Morphological classification based on the number of extensions from the cell body:
eg. pseudo-unipolar neurons with a short extension that quickly divides into two branches, one of which functions as a dendrite, the other as an axon
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Functional classification:
Eg. motor neurons that conduct motor commands from the cortex to the spinal cord or from the spinal cord to the muscles
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Morphological classification based on the number of extensions from the cell body:eg. multipolar neurons that have short dendrites coming from the cell body and one long axon
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Functional classification:
Eg. interneurons that interconnect various neurons within the brain or the spinal cord
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Morphological classification based on the number of extensions from the cell body:
Eg. bipolar neurons that have two main extensions of similar lengths
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GLIAL CELLS
The glial cells give nourishment, physical support, and protection to neurons.
Glial cells also dispose of the waste materials generated when neurons die, and accelerate neural conduction by acting as an insulating sheath around certain axons.
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Astrocytes, like most glial cells, are essential for supporting and maintaining nerve tissue.
But astrocytes may actually play a far more important role in neural communication.
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Astrocytes supply glucose needed for nerve activity. Glucose can enter the astrocytes which are next to the walls of the capillaries in the brain. The astrocytes partially metabolize it, then send it on to the neurons.
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More intense synaptic activity seem to promote a better supply of glucose by activating this astrocytic metabolisis.
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Astrocytes are connected with each other by “gap junctions” through which they can pass various metabolites. It is through these junctions that astrocytes send to the capillaries the excess extracellular potassium released during intense neuronal activity.
A gap junction or nexus is a specialized intercellular connection between a many animal cell-types. It directly connects the cytoplasm of two cells, which allows various molecules and ions to move freely between cells.
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A–D: Dye diffusion patterns after PI was injected into a single cell in various locations in the cochlea. The type of the cells that was injected is given at lower right corner of each panel. E–F: Diffusion patterns of four different fluorescent dyes after injecting into a single Claudius cell. Name of the dye is given in the lower right corner of each panel. Panels B), C), D), F) & H) were photographed with unfixed fresh samples. Panels A), E), G) were results obtained from fixed samples. They were labeled with fluorescent phalloidin (red in E, green in A&G) to outline the cell border. Scale bar on the top left of each panel represents approximately 100 µm.
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The network of intercommunicating astrocytes forms a syncytium, it behaves like a single thing. For example, through this network, the regulatory effects of waves of calcium ions might be sent to large numbers of synapses at the same time.
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The astrocytic extensions surrounding the synapses might have some control over the concentration of ions and the volume of water in the synaptic gaps.
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The network of astrocytes could act as a non-synaptic transmission system added to the neuronal system to play a major role in modulating neuronal activities.
