Neuroscience: Exploring the Brain: Ch2 Neurons and Glia

• THE NEURON DOCTRINE
• There are two main types of brain cells; neurons and glia. The ratio of these cells are about 1:10.
• Neurons sense changes in the environment, communicate these changes to other neurons, and command the body's responses to these changes.
• Glial cells contribute to the brain function mainly by insulating, supporting, and nourishing neighboring neurons.
• The ability to examine the cells of the brain depended on the use of formaldehyde to fix the cells in place without distorting the structure, and the development of the microtome to slice the brain thin enough for it to be studied under a microscope. Stains, like the Nissl (crystal violet) and the Golgi stain, allowed us to study cells.
• The Nissl stain stains the nuclei of the cells as well as the clumps of material surrounding the nuclei (Nissl bodies). This stain allows us to distinguish between glial cells and neurons, as only the nuclei of the neurons will be stained. It also gives us the ability to study the arrangement of the cells (cytoarchitecture) in different parts of the brain.
• The study of cytoarchitecture led to the discovery that the brain is made up of many specialized regions.
• The Golgi stain is done by soaking the brain tissue is a silver chromate solution, which will stain a small amount of the neurons in their entirety showing that the neuron is made up a central cell area (soma) and a neurites (axons, and dendrites)
• Cajal used the Golgi stain to work out the circuitry of many regions of the brain and proposed that the neurites of different cells are fused together to form a continuous network, and that the brain is a continuation of cell theory ( the individual cell is the elementary functional unit of all animal tissue).
• Neuron doctrine is the idea that neurons adhere to the cell theory.

 

 

 

• THE PROTOTYPICAL NEURON
• THE SOMA  
• This is the body of the neuron, measuring about 20 nanometers in diameter, and filled with a watery fluid called cytosol, which is a salty potassium rich solution that is separated from the outside by the neuronal membrane. Within the cytosol, there are several organelles.
• Contains all the organelles that a normal cell would have.
• THE NEURONAL MEMBRANE
• this serves as a  5nm barrier to enclose the cytoplasm inside the neuron and to exclude certain substances that float in the fluid. It is studded with proteins and protein pumps that have the ability to regulate what can enter and leave the cell. The composition of the proteins depends on whether it is a soma, dendrite, or the axon.
• THE CYTOSKELETON
• this is the internal structuring of the cell and is composed of microtubules, microfilaments, and neurofilaments.  
• Microtubules- about 20 nm in diameter and run longitudinally down neurites. It consists of a hollow tube that is composed of smaller strands made of tubulin that are braided like a rope around the center. These can be regulated by the cell using proteins like Microtubule-associated proteins (MAPs) which help anchor the microtubules to one another and to other parts of the neuron. Changes in the axonal MAPs (tau) are associated with the dementia in Alzheimer's.
• Microfilaments- only 5nm in diameter and are particularly numerous in the neurites and are composed of two thin braided strands of the polymers of the protein actin. These are closely associated with the membrane and form a spider web like mesh of support for the cell.
• Neurofilaments- about 10 nm in diameter and exist in all cells of the body as intermediate filaments, and are organized like sausage links.

 

 

 

• THE AXON
• these are found only in neurons and are specialized for the transfer of information over distances in the nervous system.
• No rough ER extends into the axon, and there are few , if any, free ribosomes. And the protein composition of the axon membrane is different from the soma membrane.
• These different proteins mean that there is no protein synthesis and allows it to send information over long distances.
• The axon begins with the axon hillock, which tapers to form the initial segment of the axon.
• Axon collaterals- this is where the axon branches out, and will occasionally return to communicate with the same cell (recurrent collaterals).
• Axon proper- the middle part of the axon
• Axon terminal- also called the terminal bouton, and is where the axon comes into contact with other neurons or cells to pass information on to them through a point called a synapse.
• The cytoplasm of the axon terminal differs from the axon in several ways
• The microtubules do no extend into the terminal.
• The terminal contains synaptic vesicles.
• The inside surface of the membrane that faces the synapse has a particularly dense covering of proteins.
• It has mitochondria, indicating a high energy demand.

 

• SYNAPSE
• The synapse is composed of two sides, the presynaptic and postsynaptic. These indicate the direction of information flow. The presynaptic side tends to consist of an axon terminal, and the postsynaptic side may be a dendrite or a soma, with a space referred to as the synaptic cleft in between.
• At most of the synapses the electrical signal is changed into a chemical signals, called neurotransmitters, that can cross the synaptic cleft. This transmitter is stored in and released from the synaptic vesicles in the terminal.
• Axoplasmic transport- this is the movement of materials from the soma down the axon to the terminal by being connected to microtubule "tracks" by Kinesin. This transport is powered by ATP.
• The transport in the direction of soma to bouton is referred to as anterograde transport, and any transportation in the opposite direction would be referred to as retrograde transport (this uses dynein instead of kinesin).
• DENDRITES
• Dendrites function as antennas for the neuron, collectively forming the dendritic tree, and are covered in specialized protein molecules called receptors that can detect the neurotransmitters in the synaptic clef.
• Some dendrites are covered in dendritic spines, a specialized structure that receive some types of synaptic inputs. They are believed to isolate various chemical reactions that are triggered in some synaptic activation, and is sensitive to the type and amount of synaptic activity.

 

• CLASSIFYING NEURONS
• NUMBER OF NEURITES
• Unipolar- has only a single neurite.
• Bipolar- if there are two neurites.
• Multipolar- if there are three or more neurites. Most cells in the brain fall under this category. 
• NUMBER OF DENDRITES
• There are many different types of neurons based on dendrites, many specific to certain areas of the brain, but we will focus on only the main classes.
• Stellate cells- more star shaped dendritic trees.
• Pyramidal cells - pyramid shaped trees.
• These can then further be classified as either spiny or aspinous.
• TYPE OF CONECTIONS
• Primary sensory neurons- are neurons that have neurites in the sensory surfaces of the body, like the skin or other sensory organs.
• Motor neurons - are neurons that have axons that form synapses with the muscles and command movements.
• Interneurons- only form connections with other neurons. This is the most common of this category.
• AXON LENGTH
• Golgi type I - neurons that have axons that extend from one side of the brain to the other.
• Golgi type II - these only have axons that are short and tend to not extend past the vicinity of the soma.
• NEUROTRANSMITTER
• A way to chemically classify neurons by the neurotransmitter that they use. These collections of cells that use the same neurotransmitter makeup the brain's neurotransmitter systems.

 

• GLIA
• ASTROCYTES
• The purpose of these glial cells is to fill the space in between neurons, they may influence whether a neurite can grow or retract, and help to regulate the chemical content of the extracellular space.  Not only do these have the ability to uptake extra neurotransmitters from the synaptic clef, but they also have the ability to respond to these chemicals and can trigger electrical and biochemical events in the glial cells.
• MYELINATING GLIA
• Oligodendroglia and Schwann cells, are another type of glial cell that are responsible for forming the myelin sheath that helps promote the electrical charge that is passing though the axon of the neurons.
• The Oligodendroglia cells form the myelin sheath in only the central nervous system.
• Schwann cells form the sheath for the peripheral nervous system. (outside the skull and vertebral column) 
• The openings in between the segments of the myelin sheath are called the nodes of Ranvier.
• OTHER NON-NEURONAL CELLS
• Ependymal cells- these provide the lining of the ventricles with in the brain, as well as helping to direct cell migration during brain development.
• Microglia - these function as phagocytes in the brain, removing debris left by dead or degenerating neurons and glial cells.