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The Cells That Make Us Who We Are How Neurons Communicate With Communication within the Nervous System The Cells that make us who we are How neurons communicate with one another Garrett: Brain & Behavior 4e 1 The Cells That Make Us Who We Are • How many are there? • Neurons: 100 billion • Make up 10% of brain volume • Glia: Many more! • Make up 90% of brain volume • Neurons: Jobs include • convey sensory information to the brain; • carry out operations involved in thought and feeling; • Send commands out to the body. • Dendrites • Cell body or soma Garrett: Brain & Behavior 4e • Axons insulated with myelin (secreted by glia), with end terminals that release neurotransmitters from vesicles into the synapse 2 The Cells That Make Us Who We Are Figure 2.3: Components of a Neuron Garrett: Brain & Behavior 4e 3 The Cells That Make Us Who We Are Figure 2.4 a,b : The Three Shapes of Neurons • Unipolar neurons (a) • Bipolar neurons (b) • Multipolar neurons • Figure 2 .3, previous slide Garrett: Brain & Behavior 4e 4 The Cells That Make Us Who We Are Table 2.1: The Three Types of Neurons Figure 2.4c: The Three Shapes of Neurons Type Shape Description Motor neuron Multipolar Output to muscles/organs Sensory neuron Unipolar or Bipolar Input from receptors Interneuron Multipolar Most within the CNS. Most common. Garrett: Brain & Behavior 4e 5 The Cells That Make Us Who We Are Figure 2.5: Composition of the Cell Membrane • Lipids • Heads attracted to water in and outside the cell, tails repelled by water • Creates a double-layer membrane • Proteins • Hold the cells together • Controls the environment in and around the cell Garrett: Brain & Behavior 4e 6 The Neural Membrane • The neuron has a selectively-permeable membrane. • Water and gases pass freely through • Other substances are barred from entry. • Others pass through protein channels in the membrane under certain circumstances. • Polarization results from selective permeability of the membrane. • Polarization: difference in electrical charge between the inside and outside of the cell. • This difference in electrical charge is referred to as a voltage. Garrett: Brain & Behavior 4e • A potential is any change in a membrane’s voltage. 7 The Neural Membrane • The resting potential is the difference in charge between the inside and outside of the membrane of a neuron at rest. • Between -40 and -80 millivolts (mV) in different neurons. • A typical neuron’s resting potential is around -70 mV. • Caused by unequal distribution of ions on either side of the membrane. • Outside contains mostly sodium (Na+) and chloride (Cl-) ions. Garrett: Brain & Behavior 4e • Inside contains mostly potassium (K+) ions and organic anions (A-). 8 The Neural Membrane Figure 2.7: The Sodium-Potassium Pump • Sodium-potassium pump • Moves 3 Na+ outside for every 2 K+ inside • Force of diffusion: • Ions flow from high to low concentration • Electrostatic pressure • Ions attracted to the opposite charge (+ to -), and repelled by the same charge (+ from +) Garrett: Brain & Behavior 4e 9 The Neural Membrane • Excitatory signals cause a partial hypopolarization (or depolarization), in a small area of the membrane. • The hypopolarization is caused by a change in ion balance, which also affects the adjacent membrane. • This spreading hypopolarization diminishes over distance, so it is often referred to as a local potential. • At the axon hillock, if hypopolarization reaches threshold (around -60 mV), an action potential Garrett: Brain & Behavior 4e will be triggered. 10 The Neural Membrane The Action Potential Depolarization is the change in the resting neuron’s polarity toward zero +30 mV 0 mV Hypopolarized threshold Resting Potential -70 mV -80 mV Garrett: Brain & Behavior 4e Hyperpolarized 11 The Neural Membrane Figure 2.8a: The Action Potential 1. Membrane depolarized past threshold through a series of graded potentials. 2. Voltage-gated Na ion channels open, Na enters 3. Voltage-gated K channels open, K exits. 4. K channels slowly close and membrane returns to resting potential. 5. The Action Potential lasts about 1 millisecond Garrett: Brain & Behavior 4e 12 The Neural Membrane • Movement of action potentials down the axon is not a flow of ions but a chain of events... depolarizing adjacent membrane areas which triggers another action potential. • When the action potential reaches the terminals it passes the message on to the next cell “in line”. • The local potential is a graded potential, but the action potential follows the all-or-none law. • Always occurs at full strength and doesn’t vary with stimulus intensity. • Nondecremental Garrett: Brain & Behavior 4e • Message travels over long distances at the same amplitude • Rate Law: Firing rate of neuron proportional to stimulus 13 intensity The Neural Membrane Absolute vs. Relative Refractory Period +60 mV +20 mV AP Threshold AP -20 mV Garrett: Brain & Behavior 4e -60 mV Absolute Refractory Period Relative Refractory Period 14 0 1 2 3 4 5 6 7 Time between Action Potentials (ms) The Cells That Make Us Who We Are Effects of Neurotoxins and Anesthetics • Neurotoxins affect ion channels involved in the action potential. • Tetrodotoxin blocks sodium channels. • Scorpion venom opens sodium channels, prolonging the action potential. • Beneficial drugs affect these ion channels as well. • Local anesthetics block sodium channels. • General anesthetics work by opening potassium Garrett: Brain & Behavior 4e channels. • Optogenetics. 15 • Modified ion channels that are triggered by light. Glial Cells Myelination, Axon diameter, and conduction speed • Myelin, secreted by glial cells, is a fatty tissue that surrounds axons, providing electrical insulation and support. • CNS: oligodendrocytes • PNS: Schwann cells • Increases the conduction speed from 1 m/s to over 120 m/s. • Myelin gaps called nodes of Ranvier are where action potentials occur... i.e. where sodium ions enter the axon. • Transmission between nodes (under the myelin) is by local potential. • Saltatory conduction: the action potential “jumps” from node to node. • Multiple sclerosis is a disease in which myelin is destroyed, reducing conduction speed. Garrett: Brain & Behavior 4e • Axons with a larger diameter will conduct signals faster than axons with a smaller diameter. 16 Glial Cells Figure 2.9: Glial Cells Produce Myelin for Axons Garrett: Brain & Behavior 4e 17 Glial cells Figure 2.11: Astrocyte Density Correlates With Behavioral Complexity • Scaffolds for migrating neurons... guides new neurons in fetal development • Respond to injury and disease by removing debris. • Provide energy to neurons. • 7X more neural connections when glia are present Garrett: Brain & Behavior 4e 18 Glial Cells Figure 2.10: Glial Cells Increase the Number of Connections Between Neurons. Garrett: Brain & Behavior 4e 19 SOURCE: From “Synaptic Efficacy Enhanced by Glial Cells in vitro,” by F. W. Pfrieger and B. A. Barres, Science, 277, p. 1684. © 1997. Used by permission of the author. How Neurons Communicate With One Another Figure 2.12: The Synapse Between a Presynaptic Neuron and a Postsynaptic Neuron • Synapse: connection between a neuron and another cell • Presynaptic neuron transmits the signal • Postsynaptic cell receives the signal • Synaptic cleft (gap) Garrett: Brain & Behavior 4e between the two 20 How Neurons Communicate With One Another Figure 2.13: Loewi’s Experiment Demonstrating Chemical Transmission in Neurons • One of two methods: • Stimulated vagus nerve, slowed heart A • Stimulated accelerator nerve, sped up heart A. • Injected solution from heart A into heart B • Heart B rate changed to match in similar ways • Loewi’s conclusion • Heart uses chemical Garrett: Brain & Behavior 4e messengers, not action potentials, to change 21 heart rate. How Neurons Communicate With One Another Steps of the Synaptic Event 1. Action potential depolarizes pre- synaptic membrane, Ca2+ channels open and Ca2+ enters cell 2. Neurotransmitter released into cleft 3. NT binds to post-synaptic receptors 4. Ionotropic receptor opens post- synaptic ion channels, changing the potential Ca2+ Garrett: Brain & Behavior 4e EPSP Receptor Na+ 22 “Ionotropic effect” http://www.youtube.com/watch?v=HXx9qlJetSU How Neurons Communicate With One Another Postsynaptic Receptor Types • Ionotropic receptors • cause ion channels to open, which • has a direct and rapid effect on the neuron. • Metabotropic receptors • open channels indirectly, • producing slower but longer-acting effects. • Synaptic transmission is much slower than axonal (electrical) transmission. Garrett: Brain & Behavior 4e 23 How Neurons Communicate With One Another Excitatory & Inhibitory Postsynaptic Potentials • Activation of receptors on the postsynaptic cell has two possible effects on the membrane potential. • Hypopolarization creates an excitatory postsynaptic potential (EPSP). • An EPSP opens sodium channels. • This makes the postsynaptic neuron more likely to fire. • Hyperpolarization creates an inhibitory postsynaptic potential (IPSP). • An IPSP opens potassium or chloride channels or both. Garrett: Brain & Behavior 4e • This makes it less likely an action potential will occur. 24 How Neurons Communicate With One Another Postsynaptic Potentials are Graded • EPSPs and IPSPs are graded potentials. • Accumulate over a short time (temporal summation) • Combine inputs from different locations on dendrites and cell body (spatial summation) • The neuron acts as a(n) • Information integrator (summation) • Decision maker (excitatory and inhibitory inputs combine algebraically, fires when above threshold) • The Decision Point is the Axon Hillock,
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