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Nervous Tissue

via GIPHY

,    The primary function of nervous tissue is communication. It is composed of neurons and glial cells. Some neurons specialize in relaying sensory information to the brain, while others relay commands from the brain to our various muscles and organs. These signals can be conscious or they can be completely involuntary and occur without our knowledge or our conscious control.  

    Neurons are highly specialized cells that receive and transmit information in the form of electrochemical signals.  The electrical charge inside of the neuron differs from the outside of the neuron (by about -70 mV, depending on the type of neuron). This difference of electrical charge is called the membrane potential. The membrane potential allows the neuron to sense and respond to electrochemical changes. Neurons receive information (in the form of chemicals) from the synaptic cleft through  their dendrites. The dendrites have membranes embedded with specialized pores, channels, and receptors. These structures of the dendritic membrane will respond to different chemicals found in the synaptic cleft by altering the permeability of the membrane.  The result is a change in the membrane potential of the neuron. When the change in membrane potential results in a depolarization to threshold, the neuron responds by inducing an action potential which travels to the axon terminals where the release of neurotransmitters results. 

The nervous tissue of the central nervous system (the brain and spinal cord) also contains supportive cells, called glial cells or neuroglia, that act to provide nourishment, insulation, and protection neurons.

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Labelled Neuron Model

Structure	Description	Function
Cell Body (Soma)	The neuron has 3 main parts; the cell body, the dendrites and the axon. The cell body houses the nucleus, organelles and cytoplasm of the neuron.	Cell metabolism, carries out day-to-day functions of the cell.
Axon	The neuron has 3 main parts; the cell body, the dendrites and the axon.	Transmits (sends) signals from the cell body to the axon terminal in the form an action potential.
Dendrites	Processes that protrude from the cell body like branches. (Dendrite means 'branch').	Receives neurotransmitters (chemical messenger molecules).
Schwann Cells (Neurolemmocytes)	Schwann cells are the main type of glial cell in the peripheral nervous system.	Schwann cells wrap around axons to form the myelin sheath.
Neurolemma	Neurolemma refers to the outer portion of the myelin sheath that contains the cell bodies of the Schwann cells.	The neurolemma is the outer portion of the myelin sheathe. The myelin sheath provides support for the axon.
Myelin Sheath	The myelin sheath is made up of the plasma membrane of the Schwann cell   (in the peripheral nervous system (PNS)).	The myelin sheath provides support for the axon.
Node of Ranvier	Gaps in the myelin sheath that allows the axon membrane contact to the ions of the extracellular fluid.	Propagation of the action potential by allowing an influx of sodium ions (positively charged) into the axon to continue the to transmit the signal down to the axon terminal.
Synapse (Synaptic Cleft /Synaptic Space)	The synapse is the space between the neurons (between the presynaptic neuron and the postsynaptic neuron.	During neurotransmission, the presynaptic neuron releases neurotransmitters (chemicals) into the synaptic cleft, These chemicals passively diffuse to the dendrites of the postsynaptic neuron. This triggers a change in the membrane permeability of the postsynaptic cell.

Glial Cells
 Nervous tissue is composed of glial cells, as well as neurons. The glial cells function like a connective tissue (even though they are not connective tissue). They provide support. There are 4 types of glial cells:

1. microglia - macrophages of the CNS;
2. oligodendrocytes - myelinate the axons within the CNS
3. Schwann  cells - myelinate the axons within the PNS
4. astrocytes - supportive cells 

Histological Examination of Nervous Tissue

ASTROCYTES

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A thin axon arises from an axon hillock.  Both are recognizable by their pale axoplasm, which is pale because basophilic ribosomes are absent from axoplasm.

Cross-Section of Nervous Tissue

• The nuclei of the Schwann cells take up a lot of the nuclear stain and appear round and dark purple. 
• The myelin sheath has a lot of lipids, so it does not stain well. In a cross-section of the nerve, the myelin sheath will appear as a lighter ring around the darker axon.

Longitudinal Sections of Nervous Tissue

• ​The myelin sheath is the lighter portion surrounding the axon. The outer portion of myelin stains more darkly and is the neurolemma which has the cell bodies of the Schwann cells.  
• The gaps in the sheath are the Nodes of Ranvier. 

• ​Node of Ranvier (NoR) = gap in the myelin sheath that exposes the axon membrane to the extracellular fluid.
• Axon (a) = the entire length of where it is labeled ‘a’ is the axon.
• Schwann Cell (SCn) = this is a cell body of a Schwann cell. Schwann cells make up the myelin sheath that surrounds the axon. The outermost portion of the myelin sheath that contains the cell bodies of the Schwann cells, is called the neurolemma.
• M = myelin sheath
• N = neurolemma

Cross-Section of Spinal Cord

The Meninges

You have 3 membranes covering your brain and spinal cord. The word 'meninges' comes from the Greek word for 'membrane. The innermost layer is the pia mater, the intermediate layer is the arachnoid mater and the most superficial layer is the dura mater. . Cerebrospinal fluid lies in the subarachnoid space which lies between the arachnoid mater and the pia mater. The primary function of the meninges is to protect the central nervous system.

Diagram of Meninges of Brain

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