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,