Chapter 48 Neurons, Synapses, and Signaling

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Overview: Lines of Communication Neurons are nerve cells that transfer information within the body. Neurons use two types of signals to communicate: electrical signals (long-distance) and chemical signals (short-distance).

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Sensors detect external stimuli and internal conditions and transmit information along sensory neurons. Sensory information is sent to the brain or ganglia, where interneurons integrate / process the information. Motor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity = response.

Information Processing Sensor: Detects stimulus Sensory input Integration Processing Effector: Does response Motor output Peripheral nervous system ( PNS ) Central nervous system ( CNS )

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings I. Neurons Dendrites = receive signals from other neurons. Axon = longer extension that transmits signals from its terminal branches to other cells at synapses.

Neurons Dendrites Stimulus Nucleus Cell body Axon hillock Presynaptic cell Axon Synaptic terminals Synapse Postsynaptic cell Neurotransmitters

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Synaptic terminal of one axon passes information across the synapse in the form of chemical messengers (neurotransmitters) Presynaptic cell (a neuron) to a postsynaptic cell (a neuron, muscle, or gland cell). II. S ynapse

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings III. Resting Potential At resting potential, the [K] is greater inside the cell, while the [Na] is greater outside the cell. Sodium-potassium pumps use the energy of ATP to maintain these K + and Na + gradients across the plasma membrane. The concentration gradients represent chemical potential energy.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The opening of ion channels in the plasma membrane converts chemical potential to electrical potential. Resting neuron contains open K + channels and fewer open Na + channels; K + diffuses out of the cell, leading to negative charge inside cell

The Basis of the Membrane Potential OUTSIDE CELL [K + ] 5 mM Na mM [Cl – ] 120 mM INSIDE CELL K mM [Na + ] 15 mM [Cl – ] 10 mM [A – ] 100 mM (a)(b) OUTSIDE CELL Na + Key K+K+ Sodium- potassium pump Potassium channel Sodium channel INSIDE CELL

OUTSIDE CELL Na + Key K+K+ Sodium- potassium pump Potassium channel Sodium channel INSIDE CELL

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings IV. Production of Action Potentials Na + and K + channels respond to a change in membrane potential. Stimulus depolarizes the membrane, Na + channels open, allowing Na + to diffuse into the cell. This increases the depolarization and causes even more Na + channels to open. Strong stimulus = a massive change in membrane voltage = action potential (signal)

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings AP occurs if stimulus causes voltage to cross a threshold. AP is a all-or-none depolarization of the plasma membrane.

Strong depolarizing stimulus +50 Membrane potential (mV) –50 Threshold Resting potential – Time (msec) (c ) Action potential = change in membrane voltage Action potential 6

The role of voltage-gated ion channels in the generation of an action potential Key Na + K+K+ +50 Action potential Threshold –50 Resting potential Membrane potential (mV) –100 Time Extracellular fluid Plasma membrane Cytosol Inactivation loop Resting state Sodium channel Potassium channel Depolarization Rising phase of the action potential Falling phase of the action potential 5 Undershoot

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings V. Conduction of Action Potentials APs can travel long distances by regenerating along the axon. Action potentials travel toward the synaptic terminals.

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 +

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings A. Conduction Speed Speed of AP increases with the axon’s diameter. Axons are insulated by a myelin sheath (speed increases) – Myelin sheaths are made by glia— oligodendrocytes in the CNS and Schwann cells in the PNS.

Schwann cells and the myelin sheath Axon Myelin sheath Schwann cell Nodes of Ranvier Schwann cell Nucleus of Schwann cell Node of Ranvier Layers of myelin Axon

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings APs are formed at nodes of Ranvier (gaps in the myelin) APs in myelinated axons jump between the nodes of Ranvier = saltatory conduction

Saltatory conduction Cell body Schwann cell Depolarized region (node of Ranvier) Myelin sheath Axon

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings V. Neurons communicate at synapses Electrical synapses = electrical current flows from one neuron to another. Chemical synapses = neurotransmitter carries information across the synapse.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Presynaptic neuron puts the neurotransmitter in synaptic vesicles The action potential causes the release of the neurotransmitter. The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell.

Chemical synapse 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 +

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings VI. Generation of Postsynaptic Potentials Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels in the postsynaptic cell. Neurotransmitter binding causes ion channels to open, creating a postsynaptic potential.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Postsynaptic potentials fall into two categories: – Excitatory postsynaptic potentials (EPSPs) are depolarizations that bring the membrane potential toward threshold. – Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizations that move the membrane potential farther from threshold.

Summation of postsynaptic potentials 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 E2E2 E1E1 I Action potential E 1 + E 2 (c) Spatial summation I E1E1 E 1 + I (d ) Spatial summation of EPSP and IPSP E2E2 E1E1 I

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings You should now be able to: 1.Distinguish among the following sets of terms: sensory neurons, interneurons, and motor neurons; membrane potential and resting potential; ungated and gated ion channels; electrical synapse and chemical synapse; EPSP and IPSP; summation. 2.Explain the role of the sodium-potassium pump in maintaining the resting potential.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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 saltatory conduction. 6.Describe the events that lead to the release of neurotransmitters into the synaptic cleft.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 7.Explain the statement: “Unlike action potentials, which are all-or-none events, postsynaptic potentials are graded.” 8.Name and describe five categories of neurotransmitters.