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Chapter 3

Chapter 3-Psyb64

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University of Toronto Scarborough
Janelle Leboutillier

PSYB64-CH3 C ells of the Nervous System • TTX blocks the formation and transmission of electrical signals, or action potentials, within nerve cells. Action Potentials: The nerve impulse arising in an axon. Without signals from the nervous system, lungs and other essential organs, don’t work. • The nervous system is made of two cells: neuron: A cell of the nervous system that is specialized for information processing and communication. Glia: Cells in the nervous system that support the activities of the neurons. There are atleast 10-50 glia for every neuron. The Structure of Neuron • All animal cells including neurons, have membranes, nuclei, and small internal structures as organelles. Many of these structures are found within the main mass of the neuron known as the cell body or soma. • Neurons differ structurally from other cells in that they have specialized branches extending from the cell body, known as the axons and dendrites. Axon: the branch of the nervous system usually responsible for carrying signals to other neurons. Dendrite: The branch of a neuron that generally receives information from other neurons. • The primary task of any cell membrane is to form a boundary between the cell and its external environment. • The neural membrane must separate the intracellular fluid of the cells interior from the extracellular fluid surrounding the neuron. Intracellular fluid: the fluid inside a cell. Extracellular fluid: the fluid surrounding a cell. • The chemical composition of extracellular fluid and intracellular fluid are quite different. Maintaining this difference is essential to the process of generating and sending neural messages. • The neural membrane is made up of a double layers of pholipids do not dissolve in water. Water and oil do not mix. As a result of lipid structure, the neural membrane is able to restrain the water-based fluids on either side, maintaining the structural integrity of the cell. Membranes are able to do this in spite of being only two phospholipid molecules wide, with resulting thickness of only five nanometers. (1 nanometer= 1 billion of a meter, or 10^-9 meters) • Permeability: a property of a substance that determines the extent to which other substances may pass through it. There are two primary types of protein structures of neural function, ion channels and ion pumps. Ion channels: a protein structure embedded in a cell membrane that allows ions to pass without the use of additional energy. Ion pump: a protein structure embedded in a cell membrane that uses energy to move ions across the membrane. These structures provide pores, or channels, through which specific ions, or electrically charged particles, can move into or out of neuron. Ion: an electrically charged particle in solution. Both channels and pumps show ion selectivity, or the ability to let a particular type of ion pass and no others. The amino acids, or protein building blocks, that make up the ion channel or pump determine which ions will be allowed to pass through the membrane. • Ion channels have the ability to open and close in response to stimuli in their immediate vicinity. Some ion channels known as voltage-dependent channels, open and close in response to the electrical status of adjacent areas of membrane. These changes form an important part of our discussion of electrical signalling within the neuron. Voltage-dependent channels: An ion channel that opens or closes in response to the local electrical environment. • Ligand-gated channels: open when they come in contact with specific chemicals. These chemicals are typically our naturally occurring chemical messengers but can be drugs from artificial sources as well. Ligand-gated channels become very important as we discuss events taking place at the synapse, or the junction between two neurons. Synapse: the junction between two neurons at which information is transferred from one to another. • The two most important pumps in neurons are the sodium-potassium pumps and the calcium pumps. Sodium-potassium pumps: an ion pumps that helps maintain the difference in chemical composition between the intracellular and extracellular fluids. Sodium-Potassium pumps do a “prisoner exchange” across the neural membrane by sending three sodium ions out of the cell while collecting two potassium ions from the extracellular environment. This process comes at a high cost to the neuron. 20-40 percent of the energy required by the brain is used to run the sodium-potassium pumps. Calcium pumps perform a similar function, although they do not collect another type of ion in exchange for the calcium they pump out of the cell. The release of chemical messenger or neurotransmitters by a neuron, it is essential to maintain low levels of calcium within the cell. Neurotransmitters: a chemical messenger that transfers information across a synapse. The Neural Cytoskeleton • Structural support that maintains the shape of a neuron is provided by the cytoskeleton. Cytoskeleton: a network of filaments that provides the internal structure of a neuron. • Three types of filament, or fiber, make up the neural cytoskeleton. These structural fibers also move elements within the cell and anchor the various channel and receptor proteins in their approximate places on the neural membrane. • Microtubules: largest of the three types of cytoskeleton, formed in the shape of hollow tubes with a diameter of about 25nm, responsible for the movement of materials within the cell, movement along the microtubules from the cell body to the axon terminal is known as anterograde transport, and movement back to the cell body from the periphery of the neuron is known as retrograde transport, implicated (connected) in the development of Alzheimer’s disease. This condition is characterized initially by memory loss, followed by progressive decline in cognitive and physical functions, eventually leading to death. One symptom of Alzheimer’s disease is the presence of neurofibrillary tangles. These tangles are made from a protein known as tau, which normally plays a big role in holding together the microtubules that form an important part of the neural cytoskeleton, In Alzheimers the tau protein detach and the microtubules separate. • Neurofilament: a neural fiber found in a cell cytoskeleton that is responsible for structural support. (middle sized fibers) • Microfilaments: the smallest fiber found in the cell cytoskeleton that may participate in the changing of the length and shape of axons and dendrites. (smallest fibers) • The neural cell body: specialized participate in the communication function of the neuron. • Nucleus: a substructure within a cell body that contains the cells DNA. Contains DNA information, contains substructure called nucleolus. Nucleolus builds organelles known as ribosomes, which engage in protein synthesis. The ribosomes produce proteins either on their own or in association with the endoplasmic reticulum, another small structure, or organelle located in the cell body. Mitochondria: organelles that provide energy to the cell by transforming pyruvic acid and oxygen into the molecules of adenosine triphosphate. • Dendrites: branches, tree like • Dendrite spines: de
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