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Concept 48.1: Neuron structure and organization reflect function in information transfer Neurons are nerve cells that transfer information within the body Neurons use two types of signals to communicate: Electrical signals (long-distance) Chemical signals (short-distance) © 2017 Pearson Education, Inc.

Neuron Structure and Function Most of a neuron’s organelles are in the cell body Most neurons have dendrites, highly branched extensions that receive signals from other neurons The axon is typically a much longer extension that transmits signals to other cells at synapses\ © 2017 Pearson Education, Inc.

Neuron Structure and Function A synapse is a junction between an axon and another cell The synaptic terminal of one axon passes information across the synapse in the form of chemical messengers called neurotransmitters Information is transmitted from a presynaptic cell (a neuron) to a postsynaptic cell (a neuron, muscle, or gland cell) Most neurons are nourished or insulated by cells called glia, or glial cells © 2017 Pearson Education, Inc.

Dendrites Axon hillock Nucleus Cell body Axon Presynaptic cell Synapse terminals Synaptic terminal Figure 48.2 Neuron structure Synaptic terminals Postsynaptic cell Postsynaptic cell Neurotransmitter

Introduction to Information Processing Nervous systems process information in three stages: Sensory neurons transmit information about external stimuli such as light, touch, or smell Interneurons integrate (analyze and interpret) the information Motor neurons transmit signals to muscle cells, causing them to contract © 2017 Pearson Education, Inc.

Introduction to Information Processing Many animals have a complex nervous system that consists of: A central nervous system (CNS), where integration takes place; this includes the brain and a nerve cord A peripheral nervous system (PNS), which carries information into and out of the CNS The neurons of the PNS, when bundled together, form nerves © 2017 Pearson Education, Inc.

Cell body Dendrites Axon Sensory neuron Interneuron Motor neuron Figure 48.5 Structural diversity of neurons Motor neuron

Concept 48.2: Ion pumps and ion channels establish the resting potential of a neuron Every cell has a voltage (difference in electrical charge) across its plasma membrane called a membrane potential The resting potential is the membrane potential of a neuron not sending signals Changes in membrane potential are called action potentials © 2017 Pearson Education, Inc.

Formation of the Resting Potential In most neurons, the concentration of K+ is higher inside the cell, while the concentration of Na+ is higher outside the cell Sodium-potassium pumps use the energy of ATP to maintain these K+ and Na+ gradients across the plasma membrane These concentration gradients represent chemical potential energy © 2017 Pearson Education, Inc.

Formation of the Resting Potential The opening of ion channels in the plasma membrane converts chemical potential to electrical potential A neuron at resting potential contains many open K+ channels and fewer open Na+ channels; K+ diffuses out of the cell The resulting buildup of negative charge within the neuron is the major source of membrane potential © 2017 Pearson Education, Inc.

OUTSIDE OF CELL INSIDE OF CELL Na+ Sodium- potassium pump Potassium Figure 48.6 The basis of the membrane potential INSIDE OF CELL Na+ Sodium- potassium pump Potassium channel Sodium channel K+

Generation of Action Potentials: A Closer Look An action potential results from changes in membrane potential as ions move through voltage- gated channels © 2017 Pearson Education, Inc.

Axon Plasma Action membrane potential Cytosol Action potential Action Na+ Cytosol Action potential K+ Na+ Figure 48.12_3 Conduction of an action potential (step 3) K+ Action potential K+ Na+ K+

Evolutionary Adaptations of Axon Structure The speed of an action potential increases with the axon’s diameter In vertebrates, axons are insulated by a myelin sheath, which causes an action potential’s speed to increase Voltage-gated sodium channels are restricted to nodes of Ranvier, gaps in the myelin sheath © 2017 Pearson Education, Inc.

Node of Ranvier Layers of myelin Axon Schwann cell Schwann cell Axon Nucleus of Schwann cell Nodes of Ranvier Myelin sheath 0.1 µm Figure 48.13 Schwann cells and the myelin sheath

Schwann cell Depolarized region (node of Ranvier) Cell body Myelin sheath Axon Figure 48.14 Propagation of action potentials in myelinated axons

Concept 48.4: Neurons communicate with other cells at synapses At chemical synapses (which most are), a chemical neurotransmitter carries information between neurons The presynaptic neuron synthesizes and packages the neurotransmitter in synaptic vesicles located in the synaptic terminal The action potential causes the release of the neurotransmitter The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell © 2017 Pearson Education, Inc.

Presynaptic cell Postsynaptic cell Axon Synaptic vesicle containing neurotransmitter Synaptic cleft Postsynaptic membrane Presynaptic membrane Figure 48.15 A chemical synapse K+ Ca2+ Voltage-gated Ca2+ channel Ligand-gated ion channels Na+

Termination of Neurotransmitter Signaling After a response is triggered, the chemical synapse returns to its resting state The neurotransmitter molecules are cleared from the synaptic cleft Blocking this process can have severe effects The nerve gas sarin triggers paralysis and death due to inhibition of the enzyme that breaks down the neurotransmitter controlling skeletal muscles © 2017 Pearson Education, Inc.

Enzymatic breakdown of neurotransmitter in the synaptic cleft PRESYNAPTIC NEURON Neurotransmitter Fragments of inactivated neurotransmitter Neurotransmitter receptor POSTSYNAPTIC NEURON Inactivating enzyme Enzymatic breakdown of neurotransmitter in the synaptic cleft Figure 48.18 Two mechanisms of terminating neurotransmission Neurotransmitter Neurotransmitter transport channel Neurotransmitter receptor Reuptake of neurotransmitter by presynaptic neuron

Neurotransmitters A single neurotransmitter may bind specifically to more than a dozen different receptors Following is a sample of common neurotransmitters © 2017 Pearson Education, Inc.

1. Acetylcholine Acetylcholine is a common neurotransmitter in vertebrates and invertebrates involved in muscle stimulation, memory formation, and learning A number of toxins disrupt acetylcholine neurotransmission These include the nerve gas sarin and the botulism toxin produced by certain bacteria Insecticide poisoning © 2017 Pearson Education, Inc.

2. Amino Acids Glutamate is one of several amino acids that can act as a neurotransmitter, in vertebrates and invertebrates Gamma-aminobutyric acid (GABA) is the neurotransmitter at most inhibitory(releases) synapses in the brain © 2017 Pearson Education, Inc.

3. Biogenic Amines Have a central role in a number of nervous system disorders Parkinson’s disease is associated with a lack of dopamine in the brain Examples include: Serotonin: “Happy Molecule” Dopamine: Reward, motivation and attention Epinephrine: Stimulant Norepinephrine: Alertness in the brain © 2017 Pearson Education, Inc.

4. Neuropeptides Relatively short chains of amino acids, including: Substance P: perception of pain and inflammation response Endorphins: perception of pain and euphoria Opiates bind to the same receptors as endorphins and can be used as painkillers and are highly addictive Morphine Fentanyl Oxycodone © 2017 Pearson Education, Inc.

5. Gases Gases such as nitric oxide (NO) are local regulators in the PNS Unlike most neurotransmitters, NO is not stored in cytoplasmic vesicles, but is synthesized on demand Important in vasodilation of blood vessels It is broken down within a few seconds of production Although inhaling CO can be deadly, the vertebrate body synthesizes small amounts of it, some of which is used as a neurotransmitter © 2017 Pearson Education, Inc.