Nervous Tissues General Information

Nervous tissues are one of the four tissues of the human body (the others being epithelial, connective, and muscular).  Nervous tissue has the ability to conduct electricity. 

Nervous tissue histology can be very difficult to understand.  Students are usually introduced to a neuron when beginning study of the nervous systemThe organ system that controls body functions using electrical and chemical signals..  However, rarely can we find neuronsThe functional cells of the nervous system that transmit signals. stretched out in the orientation in which we might draw it or find it in diagrams.  Furthermore, multipolar neurons are the standard neuron for teaching about the nervous system.  Although they are the most common type of neuron, they aren’t the only game in town.  Unipolar neurons and other shapes are common in the brain.

Axons of neurons can be seen in longitudinal and cross-section.  This can be difficult for students to visualize.  When unmyelinated, axons look like a line in the longitudinal section A cut or slice of the body or an organ for study. and a circle in the cross-section.  When myelinated, axons still appear as a line (yellow in the picture below), but the myelin will form what looks like pillows around it (purple in the pictures below).  In a cross-section, axons appear with lamina (sheets) of myelin encircling the axon.

Compare the histological picture below with the diagrams above.  On the left side of the picture below are axons cut in a longitudinal section.  The axons, although difficult to see continuously, are the darker pink line sweeping up and to the left.  The dark purple ovals are the nucleiClusters of neurons in the CNS responsible for processing information. of the Schwann cellsPNS glial cells that form the myelin sheath around peripheral nerve fibers. creating the myelin.  Schwann cellsThe basic structural and functional units of life. are responsible for the myelinationThe process by which glial cells wrap axons in myelin. of axons in the peripheral nervous systemPNS All nervous tissue outside the CNS; includes nerves and ganglia..  In the central nervous systemComposed of the brain and spinal cord; integrates and processes information. , oligodendrocytesGlial cells in the CNS that form the myelin sheath around axons. perform myelination.

On the right side of the picture below are axons cut in a cross-section.  Each dark pink circle is the endoneuriumA thin layer of connective tissue surrounding each nerve fiber., surrounding a lightly stained area of myelin and then a small dark pink center circle represents the axons.

One of the most important structures to identify on the myelinated axons of neurons cut in longitudinal sections is the nodes of Ranvier.  These areas of the axolemmaThe cell membrane of an axon. are left exposed, as is the entire length of the axon in unmyelinated neurons.  As action potentials are conducted or propagated from the cell bodyThe central part of a neuron containing the nucleus and organelles. to axon terminals, the exposed areas conduct the electricity.  Myelin sheaths smother their portions of the axon, preventing a continuous flow of electricity.  Thus, the action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission. jumps from node to node, kind of like playing hopscotch along the axon.  This jumping speeds up the conductionThe transmission of nerve impulses along neurons. of the action potential in a process called saltatory conductionThe rapid transmission of impulses in myelinated axons by jumping between nodes of Ranvier..  The root word “saltare” means “to jump.”

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The picture below shows nodes of Ranvier between myelin sheaths.  The axons are sloping from the upper left corner to the bottom right corner and appear as a faint purple in this picture.  What looks like bubbles are the myelin sheaths.  Areas where the bubbles look like they are pinched are the nodes of Ranvier.

The peripheral nervous system consists of all nervous tissue located outside of the brain or spinal cordThe central nervous system structure that relays signals between the brain and body..  This means that nerves like the ulnar nervenerve from the brachial plexus that controls hand and forearm muscles. (the funny bone nerve) and the sciatic nerveThe largest nerve in the body, arising from the sacral plexus. are part of the peripheral nervous system.  Nerves such as these are collections of axons either bringing afferent sensory information into the spinal cord or carrying efferent motor information out of the spinal cord.  Think of gathering a collection of strings, straws, toothpicks, or any circular item you can collect into a bunch.  If you were to look top-down at your bunch of straws, this would be similar to what the cross-section of a nerve looks like.

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The bundling of the axons is not random.  There are individual axons, bundled into fasciclesBundles of nerve fibers within a nerve.
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Bundles of nerve fibers within a muscle., and fascicles bundled into the entire nerve.  Wrappings of connective tissue separate axons and fascicles and provide routes for vascularization, innervation, and lymphatics.  The picture below and to the left is a picture of a peripheral nerve with the epineuriumConnective tissue sheath surrounding an entire nerve. wrapping the entire nerve, the perineuriumA connective tissue sheath that surrounds a bundle (fascicle) of nerve fibers. wrapping the fascicles and the endoneurium wrapping the individual axons.  It’s important to note that the endoneurium is not the axolemma, but a separate structure of connective tissue.  The sequence of three wrappings is very similar to the wrappings of muscles.

Because neurons are so specialized for the conduction of action potentials, support cells called neuroglia surround neurons.  These cells are different in the peripheral nervous system compared to the central nervous system.  They may have very similar functions but have earned specific names.

Axolemma – cell membrane of the axon.

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Axon – long thin extension of the cell connecting the cell body to axon terminals.

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Axon hillock – area that connects the cell body to the axon, also an area of resistanceThe opposition to airflow in the respiratory tract, influenced by airway diameter. for electricity

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Axon terminals – the very tips of the ends of the axon that flare out (not the cell body end)

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Cell body – area that contains all the usual organellestructures within a cell that perform specialized functions. (ER, Golgi, nucleusThe control center of the cell that contains DNA and directs cellular activities., etc…_

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Continuous conduction/propagation – slow transmission of an action potential using every protein channel in the axolemma because myelin is not present.

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Dendrites – the antenna or receiving part of a neuron

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Myelin – a lipid substance that insulates axons, smothering protein channelsProtein passages in the cell membrane that allow specific molecules to pass through. in the axolemma

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Multipolar – most common neuron, used as a basicA solution with a pH above 7, having a lower concentration of H⁺ ions. example for learning about neuron

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Neuroglia – support cells for neurons

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Neuron – cell capable of generating action potentials

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Nodes of Ranvier – exposed areas of the axolemma where myelin is not present

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Saltatory conduction/propagation – fast transmission of an action potential using only exposed protein channels on the nodes of Ranvier on the axolemma because myelin is present.

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