Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings PowerPoint ® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Chapter 48 Neurons, Synapses, and Signaling

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Why do multicellular organism have nervous systems? Must have one centralized organ that can detect and integrate external and internal environment and coordinate function Relying on typical cell signaling (diffusion) of molecules over a long distance is too slow to respond to immediate danger. Electrical impulses allow signals to be transmitted rapidly from one part of the body to another. Electrical impulses on a can travel 120 m/s.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The neuron is the basic structure of the nervous system that reflects function 1.Dendrites: Detection 2.Cell body: Signal integration 3.Axon: Signal transmission 4.Myelin sheath: Electrical insulator – Formed by Schwann cells – Signal jumps from node to node.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Nervous system requires a large amount of energy to generate electrical activity The brain is ~3% of the body by mass but requires approximately 30% of the body’s glucose. Why? It must maintain a “resting potential” or difference in charge across the cell membrane of the axon. The Na+/ K+ ATP Pump keeps the neuron Polarized: inside is more negative than the outside by An electrical impulse is a rapid and transient. reversal of charge across the membrane (inside becomes more positive than the outside)

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Strong depolarizing stimulus +50 Membrane potential (mV) –50 Threshold Resting potential – Time (msec) Action potential 6 Neurons transmit electrical impulses along the axon called an Action Potential Action potential: a rapid and temporary change in membrane potential. Caused by opening and closing of voltage gated ion channels.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Neuron Cell Membrane Na+/ K+ Pump Na+ Channel K+ Channel Voltage gated Closed Opens at ~ -45 mV Closes at ~ +35 mV Opens at ~ +35 mV Closes at ~ -75mV After closing they have a refractory phase where they cannot open. This prevents the action potential from traveling in the wrong direction

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Neuron membranes are polarized by Na+/K+ pump. Membrane Polarization: The Na+/K+ pump establishes an electrical potential across the membranes called the “resting potential” Na+/K+ pump is powered by ATP Na+/ K+ Pump 2 K+ in 3 Na+ out Outside of cell is more positive than inside “Resting Potential” inside is~ -70mV Na+ Channel K+ Channel

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Depolarization: Stimulus triggers Na+ channels to open Ligand-gated ion channels allow Ca 2+ to enter the cell slowly (slightly depolarizing the membrane) When membrane depolarizes to -45mV, Na+ channels open Na+ rushes into the cell, causing local depolarization of the membrane Na+ channels close at +35 mV Na+/ K+ Pump 2 K+ in 3 Na+ out Na+ Channel K+ Channel Na+

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Repolarization occurs when Na+ channels close and K+ Channels open At +35 mV K+ gated channels open. K+ exits the cell. The cell membrane becomes hyperpolarized K+ channel closes at -75mV Na+/K+ pump reestablishes the resting potential at -70 mV Na+/ K+ Pump 2 K+ in 3 Na+ out Na+ Channel K+ Channel K+

Strong depolarizing stimulus +50 Membrane potential (mV) –50 Threshold Resting potential – Time (msec) (c ) Action potential = change in membrane voltage Action potential 6 K+ Channels Close Na+ Channels close K+ Channels open Na+ Channels open Ligand gated channels open, allowing some Ca 2+ to enter

Conduction of an Action Potential Signal Transmission Axon Plasma membrane Cytosol Action potential Na + Action potential Na + K+K+ K+K+ Action potential K+K+ K+K+ Na +

Saltatory conduction: the action potential “jumps” along the membrane, traveling faster than if myelin sheath were absent. Cell body Schwann cell Depolarized region (node of Ranvier) Myelin sheath Axon

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Voltage-gated Ca 2+ channel Ca Synaptic cleft Ligand-gated ion channels Postsynaptic membrane Presynaptic membrane Synaptic vesicles containing neurotransmitter 5 6 K+K+ Na + Transmission of information between neurons occurs across synapses using neurotransmitters.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Generation of Postsynaptic Potentials Neurotransmitter & Receptor binding can be excitatory or inhibitory The same neurotransmitter can be excitatory or inhibitory in different parts of the brain – Depends on the properties of the target receptor Hyperpolarization (more negative) = Inhibitory Depolarization (more positive)= excititory

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings After release, the neurotransmitter – May diffuse out of the synaptic cleft – May be taken up by surrounding cells – May be degraded by enzymes

Summation of postsynaptic potentials  All inputs (excitatory and inhibitory) are summed or added together in the cell body.  If the combined input is enough to reach threshold at the axon hillock an action potential will occur.  Action potentials is all or nothing. Terminal branch of presynaptic neuron E1E1 E2E2 I Postsynaptic neuron Threshold of axon of postsynaptic neuron Resting potential E1E1 E1E1 0 –70 Membrane potential (mV) (a) Subthreshold, no summation (b) Temporal summation E1E1 E1E1 Action potential I Axon hillock E1E1 E2E2

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Nervous System Review 1.Explain the roles of dendrites, cell body, axon, and synapse in neuron function. 2.Explain the role of the sodium-potassium pump in maintaining the resting potential. 3.Describe the stages of an action potential; explain the role of voltage-gated ion channels in this process. 4.Explain why the action potential cannot travel back toward the cell body. 5.Describe how the action potential travels along the axon. 6.Describe the events that lead to the release of neurotransmitters into the synaptic cleft. 7.How do neurotransmitters affect the postsynaptic neuron? 8.Why is it imprecise to define a neurotransmitter as excitatory or inhibitory? 9.Independent reading challenge: see chapter The brain can be divided up into different regions responsible for different functions. Choose one of the functions or regions below and describe how the brain processes that function or where the function occurs in the brain. – Vision – Hearing – Muscle movement – Abstract thought and emotions – Neuro-hormone production – Forebrain (cerebrum), midbrain (brainstem) and hindbrain (cerebellum) – Right and left cerebral hemispheres in humans