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Lecture 18


11 Pages

Biological Sciences
Course Code

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HermoniseAuguste Section: W1 I. Neurons, Synapses and Signaling A. Overview 1. Neurons a. Nerves cells that transfer information within the body b. Two types of communication signals i. Electrical signals (long-distance) ii. Chemical signals (short-distance) (1) Transferring information from one cell to another 2. Transformation of information a. Depends on the path of neurons a signal travels 3. Processing of information a. Takes place in ganglia i. Clusters of neurons b. Brain i. Complex organization of neurons B. Neuron organization and structure 1. Introduction to Information Processing a. Sensory input; Sensors i. Detect external stimuli and internal conditions ii. Transmit information along sensory neurons b. Integration i. Sensory information is sent to the brain or ganglia ii. Interneurons integrate the information (1) Neurons take into account the immediate context and animals experience iii. Vast majority of neurons in the brain are interneurons c. Motor Output i. Motor neurons transmit signals to muscle cells d. Nervous system consists of i. Central Nervous System (CNS) (1) Where integration takes place HermoniseAuguste Section: W1 (2) Includes the brain and a nerve cord ii. Peripheral Nervous System (1) Brings information into and out of the CNS 2. Neuron Structure and Function a. Neuron i. Receiving and transmitting information (1) Based on highly specialized cellular organization ii. Neuron’s organelles are in the cell body iii. Dendrites (1) Highly branched extensions that receive signals from other neurons iv. Axons (1) Much longer extension that transmits signals to other cells at synapses (2) Joins the cell body at the axon hillock v. Glial cells or glia (1) Supporting cells (2) Nourish or insulate the axons of neurons (3) Regulate the extracellular fluid surrounding neurons b. Synapse i. Junction between an axon and another cell ii. Synaptic terminal (1) Part of each axon branch that forms a junction iii. Neurotransmitters (1) Chemical messengers (2) Pass information from the transmitting neuron to the receiving cell c. Two types of transmitting neurons i. Presynaptic (a neuron) ii. Postsynaptic (a neuron, muscle or gland cell) C. Ion pumps and Ion channels 1. Ion distribution a. Unequally distributed HermoniseAuguste Section: W1 i. Between interior of cells and the fluid around them ii. Inside of a cell is negatively charged relative to the outside b. Membrane potential i. Difference in electrical charge (voltage) ii. Messages are transmitted as changes in membrane potential c. Resting potential a i. Membrane potential of a neuron not sending signals 2. Formation of the Resting Potential a. Mammalian neuron i. The concentration of K+ is greater inside while concentration of Na+ is greater outside the cell (1) Essential role in the formation of the resting potential ii. Sodium-potassium pump (1) Uses the energy ofATP hydrolysis to maintain K+ and Na+ gradients across the plasma membrane ~ Na+ out K+ in (2) 3 Na+ for every 2 K+ (3) Net export of positive charge iii. Ion channels (1) Pores formed by clusters of specialized proteins that span the membrane (2) Opening of channels converts chemical potential to electrical potential ~ occurs because ions that do so are selective permeable (3) Ion diffusion carry units of electrical charge ~Any resulting net movement of positive or negative charge will generate a membrane potential (voltage across the membrane) iv. Neurons at resting potential (1) Contains many open K+ channels and fewer open Na+ channels ~ K+ diffuses out of the cell (2) Concentration of K+ is 140 mM, Na+ is mM ~ More K channels than Na channels ~ Concentration gradient favors net outflow of K+ HermoniseAuguste Section: W1 ~ K+ outflows leads to net negative charge inside the cell ~ Buildup of negative charge (ANIONS) within the neuron is the major source of the membrane potential (3) Stopping buildup of negative charge ~ Excess (-) charge exert attractive force that opposes the flow of additional positively charge Na+ ~ Counterbalance chemical concentration gradient of K+ 3. Modeling of the Resting Potential a. Artificial membrane model i. Concentration of KCl higher in inner chamber ii. K+ diffuse down its gradient to the outer chamber iii. Negative charge builds up in the inner chamber (1) Cl- lacks channel to travel down its gradient b. Equilibrium i. Electrical and chemical gradients are balanced ii. Equilibrium potential (1) Magnitude of the membrane voltage at equilibrium for a particular ion (2) Nernst equation ~ Eion62 mV (log [ion in]/ [ion out]) (3) E oion+ is negative, while E of Naions positive iii. Resting neuron (1) Currents of K+ and Na+ ~Are equal and opposite ~ The resting potential across the membrane remains steady D. Action potentials 1. Gated ion channels a. Ion channels that open or close in response to stimuli i. Alters the membrane’s permeability to particular ions ~ alters the membrane potential 2. Hyperpolarization and Depolarization a. Hyperpolarization HermoniseAuguste Section: W1 i. Increase in magnitude of the membrane potential ii. Ex. when gated K+ channels open, K+ diffuses out of the cell (1) Inside of cell more negative iii. In resting neuron (1) Results from any stimulus that increases the outflow of positive ions or inflow of negative ions b. Depolarization i. Reduction in the magnitude of the membrane potential ii. Ex. Gated Na+ channels open, Na+ diffuses into the cell iii. In resting neuron (1) If stimulus opens Na+ channel membranes permeability to Na+ increases ~ Na+ diffuses into the cell along concentration gradient ~ Membrane potential shifts toward E (Na) 3. Graded Potentials andAction Potentials a. Graded Potential i. Strong stimulus resulting in massive change in membrane voltage ii. Induce small electrical currents that leaks out of the neuron as it flows along the membrane (1) Decay with distance from their source iii. Major effect on the generation of nerve signals b. Action Potential i. Massive change in membrane voltage caused by strong stimulus ii. Have constant magnitude iii. Regenerate in adjacent regions of the membrane (1) Can spread along axons (2) Well suited for transmitting a signal over long distances iv. Brief all-or-none response to stimuli v. Once initiated action potential has magnitude that is independent of the strength of the triggering stimulus vi. Depolarization opens voltage-gated channels; opening of sodium channels cause further depolarization c. Voltage-gated ion Channels HermoniseAuguste Section: W1 i. Respond to change in membrane potential ii. Ex. Stimulus depolarizes the membrane (1) Na+ channels open, allowing Na+ to diffuse into the cell ~ Movement of Na+ into the cell increase the depolarization and causes even more Na+ channels to open (Positive Feedback) ~ Triggers very rapid opening of channels and marked change in membrane potential d. Threshold i. Particular value in membrane voltage (1) Prime indicator of whether action potential occurs if stimulus changes threshold 4. Generation ofAction Potentials a. Membrane depolarization opens both types of channel i. Response is independent and sequential (1) Sodium channels open first initiating the action potential (2) As action potential proceeds, Sodium channels become inactivated ~ Loop of channel protein moves, blocking ion flow through the opening (3) Sodium channels remain inactivated until membrane returns to resting potential and channels close (4) Potassium channels open more slowly but remain open and function un
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