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Chapter 3

Neurons Chapter 3.docx

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University of Toronto Scarborough
Malcolm Mac Kinnon

Neurons Chapter 3:  Neurons: Cells in the nervous system that communicate with one another to perform information-processing tasks Discovery of How Neurons Function:  Santiago Ramon y Cajal learned about a new technique for straining neurons in the brain. This technique highlighted the appearance of the entire cell, revealing that they came in different shapes and sizes.  Each neuron was composed of a body with many threads extending towards another neuron, without physically touching other neurons  Neurons are the information-processing units of the brain Components of the Neuron:  Cell body: cells in the nervous system that communicate with one another to perform information-processing tasks. Surrounded by a porous layer: allows molecules to flow in and out  Cell body contains a nucleus: structures houses chromosomes that contain your DNA  Two types of specialized extensions of the cell membrane that allow them to communicate: dendrites and axons.  Dendrites: receive information from other neurons and relay it to the other cell  Axon: transmits information to other neurons ,muscles or glans  Myelin sheath: insulating layer of fatty material composed of glial cells (support cells found in the nervous system)  Glial cells digest dead neurons and provide physical and nutritional support for neurons  Myelin is used to help the axon transmit information more efficiently  An axon insulated with myelin can more efficiently transmit signals to other neurons  There is a small gap between the axon of one neutron and the dendrites of another cell body-this is referred to as the synapse  Synapse: is the junction or the region between the axon of one neuron and dendrites or cell body of another  transmission of information across the synapse is fundamental to communication between neurons-allows us to feel and behave Major Types of Neurons:  Sensory neurons: neurons that receive information from the external world and convert this information to the brain via the spinal cord  Motor neurons: neurons that carry signals from the spinal cord to muscles to produce movement  Interneurons: neurons that connect sensory neurons, motor neurons or other interneurons Neurons Specialized by Location:  Neurons are also specialized depending on the location  For example: purkinje cells are interneuron that carries info from cerebellum to the spinal cord and brain-they have dense and elaborate dendrites The Electrochemical Actions of Neurons:  First stage: conduction of an electric signal over long distances within neurons  Second stage: transmission of chemical signals between neurons and over synapse  These two stages combine are referred to as electrochemical action of neurons Electric signaling: Conducting Information  the porous membrane allows small electrically charged molecules called ions to flow in and out of the cell The resting potential:  Resting potential: difference in electric charge between the inside and outside of neuron’s cell membrane. This potential is created by the difference in concentrations of ions inside and outside of the cell’s membrane. The K+ molecules are able to move freely, while the Na+ pump is closed.  Resting potential: the concentration of positive ions is more outside compared to inside. Negative inside and positive outside  Action potential: K+ channels close and Na+ rush in and increase the charge inside to become positive.  Then the Na+ pump closes and the K+ ions move inside the axon The Action Potential:  Action potential: electric signal that is conducted along a neuron’s axon to a synapse.  if the electric stimulation is below the threshold fails to produce an electric potential-only produced if it is higher than the threshold  Action potential occurs when there is a change in the state of the axons membrane channels  When threshold is reached, K+ channels close down and the Na+ PUMP OPES UP  When the action potential reaches the maximum point it returns to the original state. However during the imbalance on ions the neurons cannot intimate another action potential-refractory period  The myelin covers only clumps of the axon with little break points –nodes of Ranvier Chemical Signaling:  Axons usually end in terminal buttons: Knoblike structures that branch out from an axon  The terminal button is filled with vesicles that contain neurotransmitter: chemicals that transmit info across the synapse to a receiving neurons dendrites  The dendrites have receptors: parts of the cell membrane that receive neurotransmitters and initiate/prevent a new electrical signal  Presynaptic neuron: the sending neuron that moves from a resting potential to an action potential  Once the neurotransmitters from the vesicles are released into synapse and the receptor site near the dendrites receive the neurotransmitters, this is referred to as the postsynaptic neuron  Neurons tend to form pathways in the brain based on the type of neurotransmitter  Neurotransmitters leave the synapse through three process: 1. Reuptake: occurs when the neurotransmitter is absorbed by terminal buttons of presynaptic neuron  2 neurotransmitter can be destroyed by enzymes  3rdneurotransmitters can bind to receptors sites on the presynaptic neuron Types and Function of Neurotransmitters:  Acetylcholine: neurotransmitter involved in a number of functions including voluntary motor control. Activates muscles to imitate motor behaviour, attention, learning, sleeping and memory  Dopamine: regulates motor behaviour, motivation, and emotional arousal  Glutamate: a major excitatory neurotransmitter involved in information transmission throughout the brain  GABA: primary inhibitory neurotransmitter in the brain-stops firing of the neurons  Norepinephrine: a neurotransmitter that influences mood and arousal  Serotonin: A neurotransmitter that is involved in the regulation of sleep and wakefulness, eating and aggressive behaviour  Endorphins: chemicals that act within the pain pathways and emotion centers of the brain How Drugs Mimic Neurotransmitters:  Many drugs affect the nervous system by mincing the function of neurotransmitters  Agonist: drugs that increase the action of a neurotransmitter  Antagonist: drugs that block the function of a neurotransmitter  Some chemical structures are similar to neurotransmitters and are able to bind
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