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2. Nerve Cells and Nerve Impulses.pdf

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York University
PSYC 2240

2. Nerve Cells and Nerve Impulses Sunday, May 26, 2013 4:52 PM Main Ideas 1) The nervous system is composed of two kinds of cells: neurons and glia. Only neurons transmit impulses from one location to another. 2) The larger number of neurons have branches, known as axons and dendrites, which can change their branching pattern as a function of experience, age, and chemical influences. 3) Many molecules in the bloodstream that can enter other body organs cannot enter the brain. 4) The action potential, an all-or-none change in the electrical potential across the membrane of a neuron, is caused by the sudden flow of sodium ions into the neuron and is followed by a flow of potassium ions out of the neuron. 5) Local neurons are small and do not have axons or action potentials. Instead, they convey information to nearby neurons by graded potentials. 2.1 The Cells of the Nervous System LearningObjectives 1) Know the main structures of neurons and the structural differences among neurons. 2) Know the main types of glia and their functions. 3) Be able to describe the advantages and disadvantages of the blood-brain barrier. Anatomyof NeuronsandGlia • Neurons receive information and transmit it to other cells • The brain, like the rest of the body, consists of individual cells ○ Santiago Ramon y Cajal (1852-1934)-- founder of neuroscience credited with this discovery by using newly discovered staining techniques to view neurons • Behavior does not follow from the properties of any one neuron • The Structuresof an AnimalCell ○ Membrane-- the surface of a cell that separates the inside of the cell from the outside environment  Most chemicals cannot cross the membrane, but specific protein channels in the membrane permit a controlled flow of water, oxygen, sodium, potassium, calcium, chloride, and other important chemicals ○ Nucleus-- the structure that contains the chromosomes  All cells have a nucleus except for mammalian red blood cells ○ Mitochondria-- the structure that performs metabolic activities, providing the energy that the cell requires for all other activities; require fuel and oxygen to function ○ Ribosomes-- the sites at which the cell synthesizes new protein molecules ○ Endoplasmic reticulum-- a network of thin tubes that transport newly synthesized proteins to other locations • The Structureof a Neuron ○ Neurons are distinguished from other cells by their shape ○ Larger neurons have these components: 1) Dendrites 1) Dendrites 2) Soma (cell body) 3) Axon 4) Presynaptic terminal ○ A motor neuron has its soma in the spinal cord ○ A sensory neuron is specialized at one end to be highly sensitive to a particular type of stimulation, such as light, sound, or touch ○ Dendrites are branching fibers that get narrower near their ends and receive information from other neurons via synaptic receptors ○ Dendritic spines-- short outgrowths that increase the surface area available for synapses ○ The soma contains the nucleus, ribosomes, mitochondria, and other structures found in most cells ○ Axon-- a thin fiber of constant diameter, in most cases longer than dendrites, which sends an information-conveying impulse towards other neurons or an organ or muscle  Covered by a myelin sheath with interruptions known as nodes of Ranvier ○ Presynaptic terminal-- the point from which the axon releases chemicals that cross through the junction between one neuron and the next ○ A neuron can have any number of dendrites, but only one axon (which can range to a meter or more in length; e.g., from the spinal cord to the foot) ○ An afferent axon brings information into a structure ○ An efferent axon carries information away from a structure ○ Interneuron or intrinsic neuron-- the neuron's dendrites and axon are entirely contained within a single structure (e.g., the thalamus) • VariationsAmongNeurons ○ Neurons vary enormously in size, shape, and function ○ The function of a neuron relates to its shape • Glia ○ Glia (or neuroglia)-- the other major components of the nervous system  Do not transmit information over long distances as neurons do  Exchange chemicals with adjacent neurons ○ Glia are smaller but more numerous than neurons ○ Several types of glia with different functions:  Astrocytes wrap around the presynaptic terminals of a group of functionally related axons, taking up chemicals released by the axons enabling the axons to send messages in waves □ Remove waste matter when neurons die □ Control the amount of blood flow to each brain area □ Release chemicals that modify the activity of neighboring neurons  Microglia-- very small cells which remove waste material as well as viruses, fungi, and other microorganisms  Oligodendrocytes in the brain and spinal cord and Schwann cells in the periphery are specialized types of glia that build the myelin sheaths that surround and insulate certain vertebrate axons  Radial glia-- guide the migration of neurons and their axons and dendrites during embryonic development TheBlood-BrainBarrier • Many chemicals cannot cross from the blood to the brain • The mechanism that keeps most chemicals out of the vertebrate brain is known as the blood-brain barrier barrier • Why We Needa Blood-BrainBarrier ○ The blood-brain barrier keeps out most viruses, bacteria, and harmful chemicals which, when attacked by the immune system, would lead to irreparable brain damage (the vertebrate brain does not replace most damaged neurons) ○ Advantage-- keeps out viruses ○ Disadvantage-- keeps out most nutrients • Howthe Blood-BrainBarrierWorks ○ The blood-brain barrier depends on the arrangement of endothelial cells that form the walls of the capillaries ○ Small uncharged molecules, including oxygen and carbon dioxide, cross freely ○ Molecules that dissolve in the fats of the membrane also cross passively (i.e., not actively), such as vitamins A and D ○ For certain other essential chemicals, the brain uses active transport-- a protein-mediated process that expends energy to pump chemicals from the blood into the brain (e.g., glucose, amino acids, purines, choline, iron, and certain hormones) ○ Disadvantage-- poses a difficulty in medicine because it keeps out nearly all medications TheNourishmentof VertebrateNeurons • Neurons depend almost entirely on glucose, a simple sugar • The metabolic pathway that uses glucose requires oxygen; consequently, the neurons consume an enormous amount of oxygen compared with cells of other organs • Glucose is practically the only nutrient that crosses the blood-brain barrier in adults (with the exception of ketones, a kind of fat) • Neurons need thiamine (vitamin B1) to use glucose 2.2 The Nerve Impulses LearningObjectives 1) Understand why the neuron uses considerable energy to produce a resting potential. 2) Understand the competing forces of the electrical and concentration gradients on potassium ions and how this competition produces the resting potential. 3) Be able to describe the function and the molecular basis of the action potential. 4) Understand how an action potential is conducted down an axon and how myelin sheaths contribute to this process. 5) Know how local interneurons transmit information without benefit of action potentials. Introduction • Instead of simply conducting an electrical impulse, the axon regenerates an impulse at each point • The brain cannot register small differences in the time of arrival of touch messages, though it can for visual stimuli • The properties of impulse conduction in an axon are well adapted to the exact needs for information transfer in the nervous system TheRestingPotentialof theNeuron • The membrane of a neuron maintains an electrical gradient-- a difference in electrical charge between the inside and outside of the cell • All parts of a neuron are covered by a neuron which provides the cell with a good combination of • All parts of a neuron
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