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Chapter 3

Chapter 3 Textbook Notes - Cells of the Nervous System

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Ayesha Khan

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Notes From Reading CHAPTER 3:C ELLS OF THE N ERVOUS SYSTEM (P GS.60-93) Neurons and Glia LO1: Differentiate between the characteristics of neurons and glia - The nervous system is made up of two types of cells: o Neuron – A cell of the nervous system that is specialized for information processing and communication o Glia – Cells in the nervous system that support the activities of neurons - There are at least 10-50 glia for every neuron The Structure of Neurons - Organelle – A small structure within a cell that carries out a specific function - Cell Body/ Soma – the main mass of a neuron containing the nucleus and many organelles - Axon – The branch of a neuron usually responsible for carrying signals to other neurons - Dendrite – The branch of a neuron that generally receive information from other neurons LO2: Identify the functions and major structural features of the neural membrane and cytoskeleton Neuron Membranes - Must separate the intracellular fluid and the extracellular fluid surrounding the neuron o Intracellular Fluid – The fluid inside a cell o Extracellular Fluid – The fluid surrounding a cell - The neuron membrane is made up of a double layer of phospholipids (fatty molecules that contain phosphate) o Don’t dissolve in water because they are fat (ie. old and water don’t mix) o The neural membrane is able to restrain the water=based fluids on either side, maintaining the structural integrity of the cell - Permeability – A property of a substance that determines the extend to which other substances may pass through it o Found within phospholipid membrane - Ion Channel – A protein structure embedded in a cell membrane that allows ions to pass without the use of additional energy o Allows ions to move passively without the expenditure of energy - Ion Pump – A protein structure embedded in a cell membrane that uses energy to move ions across the membrane o Require energy for ions to move - These structures provide pores, or channels, through which specific ions can move into or out of the neuron o Ion – An electrically charged particle in solution o Both channels and pumps show ion selectivity, or the ability to let particular type of ion pass and no other o The amino acids, that make up the ion channel or pump, can determine which ions will be allowed to passed through the membrane - Voltage-Dependent Channel – An ion channel that opens or closes in response to the local electrical environment - Ligand-Gated Channels – An ion channel in the neural membrane that responds to chemical messengers o Are typically our naturally occurring chemical messengers, but can be drugs from artificial sources as well - Synapse – The junction between two neurons at which information is transferred from one to another - Sodium-Potassium Pumps – An ion pump that uses energy to transfer three sodium ions to the extracellular fluid for every two potassium ions retrieved from the extracellular fluid o Help maintain the differences in chemical composition between the intracellular and extracellular fluids Notes From Reading CHAPTER 3:C ELLS OF THE N ERVOUS SYSTEM (PGS .60-93) - Calcium Pump – A protein structure embedded in the neural membrane that uses energy to move calcium ions out of the cell - Neurotransmitter – A chemical messenger that transfers information across a synapse - It is essential to maintain low levels of calcium within the cell The Neural Cytoskeleton - Cytoskeleton – A network of filaments that provides the internal structure of a neuron - Structural fibers move elements within the cell and anchor the various channel and receptor proteins in their appropriate places on the neural membrane - Microtubule – The largest fiber in the cell cytoskeleton responsible for the transport of neurotransmitters and other products to and from the cell body o Formed in the shape of hollow tubes - Anterograde Transport – Movement of materials from the cell body of a neuron to the axon terminal along the microtubules - Retrograde Transport – Movement of material from the axon terminal back to the cell body via the cell’s system of microtubules - Microtubules have been implicated in the development of Alzheimer’s disease o Characterized initially by memory loss, followed by progressive decline in cognitive and physical functions, eventually leading to death o Presence of neurofibrillary tangles consisting of a protein called tau o In healthy brain, tau connects adjacent microtubules and holds them in place o In Alzheimer’s disease, the tau levels become elevated, and an affected neuron adds molecules of phosphate to the tau protein  causes it to disconnect from microtubules - Neurofilament – A neural fiber found in the cell cytoskeleton that is responsible for structural support o Run parallel to the length of axon and provide structural support - Microfilament – The smallest fiber found in the cell cytoskeleton that may participate in the changing of the length and shape of axons and dendrites o Located in the branches of the neurons o Participate in changing the shape and length of these structures during development and in response to learning LO3: Describe the functions of the major organelles found in the cell body The Neural Cell Body - Specialized to participate in the communication function of the neuron - Nucleus – The substructure within a cell body that contains the cell’s DNA - Nucleolus – A substructure within a cell nucleus where ribosomes are produced - Ribosomes – An organelle in the cell body involved with protein synthesis - Endoplasmic Reticulum – An organelle in the cell body that participate in protein synthesis o Rough endoplasmic reticulum has many ribosomes bound to its surface, giving it the bump appearance o There are no ribosomes attached to the smooth endoplasmic reticulum - Golgi Apparatus – An organelle in the cell body that packages proteins in vesicles - Mitochondria – Organelles that provide energy to the cell by transforming pyruvic acid and oxygen into molecules of adenosine triphosphate (ATP) - ATP is the major energy source for the neuron LO4: Describe the major features of axons and dendrites Dendrites - Along with the cell body, the dendrites serve as locations at which information from other neurons is received Notes From Reading CHAPTER 3:C ELLS OF THE N ERVOUS SYSTEM (PGS .60-93) o The greater the surface area of dendritic membrane neuron has, the large the number of connections or synapses it can form with other neurons - Synaptic Gap – The tiny fluid-filled space between neurons forming a synapse - Dendritic Spine – A knob on the dendrite that provide additional membrane area for the formation of synapses with other neurons - Spines appear to be able to change their shape based on the amount of activity occurring at the synapse o Contributes to the process of learning and memory o Abnormal dendritic spines cause some type of human mental retardation Axon - Responsible for carrying neural messages to other neurons - Axon Hillock – The cone-shaped segment of axon located at the junction of the axon and cell body that is specialized for the generation of action potentials - Axon diameter is crucial to the speed of signaling o Larger diameter axons are much faster than smaller diameter axons - Myelin – The fatty insulating material covering some axons that boosts the speed and efficiency of electrical signaling - Axons also vary in length o Local Circuit Neuron – A neuron that communicates with neurons in its immediate vicinity o Projection Neuron – A neuron with a very long axon that communicates with neurons in distant areas of the nervous system - Collateral – One of he branches near the end of the axon closest to its target - Axon Terminal – The selling at the tip of an axon collateral specialized for the release of neurotransmitter substances - Synaptic Vesicles – A small structure in the axon terminal that contains neurotransmitters - In the adult human, the vast majority of CNS neurons and peripheral motor neurons are myelinated - Myelin is formed by certain types of glia that wrap themselves or their branches around segments of axons - Node of Ranvier – The uncovered section of axon membrane between two adjacent segments of myelin - Advantages of myelin o Allows human axons to be smaller in diameter without sacrificing transmission speed (the more neural tissue we can pack into our skull and more info we can process) o Reduces the energy requirements of neurons by decreasing the amount of work done by Na- K pumps o There is no need for ion channels under a myeline sheath - Unmyelinated axon has ion channels along its entire length o During signaling, fewer ions move through the ion channels of a myelinated axon membrane than through an nmyelinated axon membrane of the same length LO5: Differentiate between the structural and functional types of neurons Structural Variations in Neurons - Unipolar Neurons – A neuron with one branch that extends a short distance from the cell body then splits into two branches (involved with touch, temperature, and pain) o Typical of invertebrate nervous systems that may be found in sensory systems and ANS - Bipolar Neuron – A neuron with two branches: one axon and one dendrite o Important role in sensory systems, including in the retina of the eye - Multipolar Neuron – A neuron that has multiple branches, usually one axon and numerous dendrites o Have many branches extending form the cell body o Pyramidal cells in the cerebral cortex and the hippocampus have cell bodies that are shaped like pryamids Notes From Reading CHAPTER 3:C ELLS OF THE NERVOUS S YSTEM (P GS.60-93) o Purkinje cells of the cerebellum have dramatic dendritic trees that allow a single cell to form as many as 150,000 synapses Functional Variations in Neurons - Sensory Neurons – A specialized neuron that translates incoming sensory information into electrical signals - Motor Neurons – A specialized neuron that communicates with muscles and glands - Interneuron – A neuron that serves as a bridge between sensory and motor neurons LO6: Identify the major structures, types and functions of the macroglia and microglia Glia - Glia, the cells in the nervous system that support the activities of neurons, are generally categorized by size Macroglia – Large glial cells, including astrocytes, oligodendrocytes, and Schwaan cells - Astrocyte – A large, star shaped glial cell of the CNS, responsible for structural support, isolation of the synapse, control the extracellular chemical environment at the synapse, and possibly communication o Provide a structural matrix for the neurons o Form connections with the blood supply of the brain and contributes to blood-brain barrier  prevents most toxins circulating in the blood from entering the brain o Can form scar tissues when CNS neurons are damaged by digesting the dead neurons and prevents axons and dendrites from rebuilding their connections o Chemical signaling in brain; excite&suppress activity of neighboring neurons&astrocytes - Oligodendrocyte – A glial cell that forms the myelin on CNS axons o Provide the myelin in the CSN o A single oligodendrocyte may myelinate axons from an average of 15 different neurons, these macroglia contribute to the structural stability of the brain and spinal cord o Lacks the capacity to help guide regrowth of axons - Schwaan Cell – A glial cell that forms the myelin on axons in the PNS o Supply the myelin for the peripheral nerves exciting the brain and spinal cord o A single Schwaan cell provides a single myelin segment on one peripheral axon o Can help guide the regrowth of damaged axons Microglia – Tiny, mobile glial cells that migrate to areas of damage and digest debris - Microglia serve as the brain’s cleanup crew - Branches of microglia reach out and sample their immediate environments - Uncontrolled activation of microglia can damage the brain through their release of substances that produce inflammation - Inflammation caused by microglia activation is under investigation as a contributor to neurodegenerative diseases, incl. Alzheimer’s disease, Parkinson’s disease and multiple sclerosis LO7: Describe events responsible for the resting potential, the action potential, the refractory periods, and the propagation of action potentials down the length of the axon The Generation of the Action Potential - Action Potential – The nerve impulse arising in an axon - When the action potential arrives at the axon terminal, the process switches from electrical to chemical signaling - Presynaptic neuron releases molecules of neurotransmitter from its terminal - Neurotransmitter molecules float across the synaptic gap to the waiting postsynaptic neuron - Generation of an electrical signal within neurons requires a source of energy o Energy source for electrical signaling is provided by the characteristics of the intracellular and extracellular fluids The Ionic Composition of the Intracellular and Extracellular Fluids - Important responsibility of the membrane was to keep the intra and extracellular fluids apart Notes From Reading CHAPTER 3:C ELLS OF THE NERVOUS SYSTEM (P GS.60-93) - Also, the Na-K pumps also maintain the differences between these two fluid (it is the difference between these two types of fluid that provides the neuron with a source of energy for signaling) - Extracellular fluid is characterized by large concentrations of sodium and chloride ions and a relatively small concentration of K ions - Intracellular fluid f the resting neuron contains large numbers of K ions and relatively few Na and Cl ions o Large proteins in ion form within the intracellular fluid that are negatively charge - Can use a voltmeter to measure the difference between the microelectrode and a write placed in the extracellular fluid adjacent to our cell o The difference between the inside and outside of a neuron is approx. 70millivolts (mV) - Resting Potential – The measurement of the electrical charge across the neural membrane when the cell is not processing information The Movement of Ions - Neural signals, and action potentials results from movement of ions - Diffusion – The force that moves molecules from areas of high concentration to areas of low concentrations - Concentration Gradient – An unequal distribution in the concentration of molecules across a cell membrane - Electrical Force – The force that moves molecules with like electrical charges apart and molecules with opposite electrical charges together - K is found in larger concentrations on the inside of the cell than on the outside o Diffusion would move the K ions along their concentration gradient from the inside (high concentration) to outside (low concentration) o Diffusion pressure is balanced by electrical force - Cl is more concentrated outside the cell than in it, diffusion pressure works to push Cl inside cell o Diffusion is counteracted by electrical force o Negative interior of the cell repels the negatively charge
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