48 Neurons, Synapses, and Signaling CAMPBELL BIOLOGY TENTH EDITION Reece Urry Cain Wasserman Minorsky Jackson Lecture Presentation by Nicole Tunbridge and Kathleen Fitzpatrick Concept 48.1: Neuron organization and structure reflect function in information transfer neurons clusters of neurons called ganglia or a more complex organization of neurons called a brain Neurons use two types of signals to communicate: electrical signals (long-distance) and chemical signals (short-distance) 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 The cone-shaped base of an axon is called the axon hillock The synaptic terminal of one axon passes information across the synapse in the form of chemical messengers called neurotransmitters A synapse is a junction between an axon and another cell 1
Figure 48.3 Most neurons are nourished or insulated by cells called glia or glial cells 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 the information Motor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity Figure 48.5 Cell body Axon Dendrites Sensory neuron Interneuron Motor neuron 2
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 Concept 48.2: Ion pumps and ion s establish the resting of a neuron Every cell has a voltage (difference in electrical charge) across its plasma membrane called a membrane The resting is the membrane of a neuron not sending signals Changes in membrane act as signals, transmitting and processing information Formation of the Resting Potential Sodium-potassium pumps use the energy of ATP to maintain these and gradients across the plasma membrane - is highest inside the cell, while the concentration of is highest outside the cell These concentration gradients represent chemical energy 3
Figure 48.6 A neuron at resting contains many open s and fewer open s; diffuses out of the cell Key Sodiumpotassium pump OUTSIDE OF CELL ++ + Potassium Sodium - - - INSIDE OF CELL - - - The opening of ion s in the plasma membrane converts chemical to electrical A neuron at resting contains many open s and fewer open s; diffuses out of the cell The resulting buildup of negative charge within the neuron is the major source of membrane Animation: Resting Potential 4
The equilibrium (E ion ) is the membrane voltage for a particular ion at equilibrium and can be calculated using the Nernst equation E ion = 62 mv (log[ion] outside /[ion] inside ) The equilibrium of (E K ) is negative, while the equilibrium of (E Na ) is positive Figure 48.7 Inner chamber 140 mm KCI 90 mv Outer chamber 5 mm KCI Inner +62 mv Outer chamber At equilibrium, chamber both the electrical 15 mm and chemical 150 mm NaCl NaCI gradients are Cl balanced Potassium Cl Artificial membrane (a) Membrane selectively permeable to In Sodium a resting neuron, the currents of and are equal and opposite, and the (b) Membrane resting selectively across permeable to Na the + membrane remains steady Modeling the Resting Potential Resting can be modeled by an artificial membrane that separates two chambers The concentration of KCl is higher in the inner chamber and lower in the outer chamber diffuses down its gradient to the outer chamber Negative charge (Cl ) builds up in the inner chamber At equilibrium, both the electrical and chemical gradients are balanced 5
Concept 48.3: Action s are the signals conducted by axons Changes in membrane occur because neurons contain gated ion s that open or close in response to stimuli Figure 48.8 Technique Microelectrode Voltage recorder Reference electrode Figure 48.6 A neuron at resting contains many open s and fewer open s; diffuses out of the cell Key Sodiumpotassium pump OUTSIDE OF CELL Potassium Sodium - - - - K - - - ++ + + INSIDE OF CELL 6
Membrane (mv) Membrane (mv) 11/10/2016 Figure 48.10a (a) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to +50 Stimulus When gated s open, diffuses out, making the inside of the cell more negative. This is hyperpolarization, an increase in magnitude of the membrane 0 50 100 Threshold Resting Hyperpolarizations 0 1 2 3 4 5 Time (msec) Figure 48.10b (b) Graded depolarizations produced by two stimuli that increase membrane permeability to +50 Stimulus depolarization occurs if gated s open and diffuses into the cell 0 50 Threshold 100 Resting Depolarizations 0 1 2 3 4 5 Time (msec) If a depolarization shifts the membrane sufficiently, it results in a massive change in membrane voltage called an action Action triggered by a depolarization that reaches the threshold Action s have a constant magnitude, are allor-none, and transmit signals over long distances They arise because some ion s are voltagegated, opening or closing when the membrane passes a certain level 7
Membrane (mv) Membrane (mv) Membrane (mv) 11/10/2016 Figure 48.UN03 Action +50 0 Rising phase Falling phase 50 100 70 Depolarization 0 1 2 3 4 5 6 Time (msec) Threshold ( 55) Undershoot Resting Figure 48.11f +50 Action 0 50 100 3 2 4 Threshold 1 5 Resting Time 1 Figure 48.11 Key 3 Rising phase of the 4 Falling phase of the action action +50 Action 0 3 2 Depolarization 2 4 Threshold 50 1 1 5 Resting 100 Time OUTSIDE OF CELL Sodium Potassium INSIDE OF CELL Inactivation loop 1 Resting state 5 Undershoot 8
Generation of Action Potentials: A Closer Look An action can be considered as a series of stages At resting 1. Most voltage-gated sodium ( ) and potassium ( ) s are closed When an action is generated 2. Voltage-gated s open first and flows into the cell 3. During the rising phase, the threshold is crossed, and the membrane increases 4. During the falling phase, voltage-gated s become inactivated; voltage-gated s open, and flows out of the cell 5. During the undershoot, membrane permeability to is at first higher than at rest, then voltage-gated s close and resting is restored Figure 48.11a Key OUTSIDE OF CELL Sodium Potassium INSIDE OF CELL Inactivation loop 1 Resting state Figure 48.11b Key 2 Depolarization 9
Figure 48.11c Key 3 Rising phase of the action Figure 48.11d Key 4 Falling phase of the action Figure 48.11e Key 5 Undershoot 10
Animation: Action Potential Conduction of Action Potentials At the site where the action is generated (usually the axon hillock) an electrical current depolarizes the neighboring region of the axon membrane Action s travel in only one direction: toward the synaptic terminals Inactivated s behind the zone of depolarization prevent the action from traveling backwards During the refractory period after an action, a second action cannot be initiated The refractory period is a result of a temporary inactivation of the s Figure 48.12-3 Action s travel in only one direction: toward the synaptic terminals Axon Inactivated s behind the zone of depolarization prevent the action from traveling backwards Action Action Action Plasma membrane Cytosol 11
Evolutionary Adaptations of Axon Structure The speed of an action increases with the axon s diameter In vertebrates, axons are insulated by a myelin sheath, which causes an action s speed to increase Myelin sheaths are made by glia oligodendrocytes in the CNS and Schwann cells in the PNS Figure 48.13 Node of Ranvier Layers of myelin Axon Axon Myelin sheath Schwann cell Nodes of Ranvier Schwann cell Nucleus of Schwann cell 0.1 µm Action s are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage-gated s are found Action s in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction 12
Concept 48.4: Neurons communicate with other cells at synapses At electrical synapses, the electrical current flows from one neuron to another through gap junctions At chemical synapses, a chemical neurotransmitter carries information between neurons Most synapses are chemical synapses Figure 48.16 Presynaptic cell Postsynaptic cell A chemical synapse 1 Axon Synaptic vesicle containing neurotransmitter Synaptic cleft Postsynaptic membrane Presynaptic membrane 3 4 Ca 2+ 2 Voltage-gated Ca 2+ Ligand-gated ion s The presynaptic neuron synthesizes and packages the neurotransmitter in synaptic vesicles located in the synaptic terminal The action causes the release of the neurotransmitter The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell Generation of Postsynaptic Potentials Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion s in the postsynaptic cell Neurotransmitter binding causes ion s to open, generating a postsynaptic 13
Membrane (mv) 11/10/2016 Animation: Synapse Postsynaptic s fall into two categories Excitatory postsynaptic s (EPSPs) are depolarizations that bring the membrane toward threshold Inhibitory postsynaptic s (IPSPs) are hyperpolarizations that move the membrane farther from threshold The summed effect of EPSPs and IPSPs determines whether an axon hillock will reach threshold and generate an action Figure 48.17 E 1 Terminal branch of presynaptic neuron E 1 E 1 E 1 E 2 E 2 E 2 E 2 I Postsynaptic neuron I Axon hillock I I 0 Threshold of axon of postsynaptic neuron Resting Action Action 70 E 1 E 1 E 1 E 1 E 1 + E 2 E 1 I E 1 + I (a) Subthreshold, no (b)temporal summation (c) Spatial summation (d) Spatial summation summation of EPSP and IPSP The summed effect of EPSPs and IPSPs determines whether an axon hillock will reach threshold and generate an action 14
Summation of Postsynaptic Potentials Most neurons have many synapses on their dendrites and cell body A single EPSP is usually too small to trigger an action in a postsynaptic neuron If two EPSPs are produced in rapid succession, an effect called temporal summation occurs In spatial summation, EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together The combination of EPSPs through spatial and temporal summation can trigger an action Neurotransmitters A single neurotransmitter may bind specifically to more than a dozen different receptors Receptor activation and postsynaptic response cease when neurotransmitters are cleared from the synaptic cleft Neurotransmitters are removed by simple diffusion, inactivation by enzymes, or recapture into the presynaptic neuron Figure 48.18 PRESYNAPTIC NEURON Neurotransmitter Neurotransmitter receptor POSTSYNAPTIC NEURON Inactivating enzyme (a) Enzymatic breakdown of neurotransmitter in the synaptic cleft Neurotransmitter receptor Neurotransmitter Neurotransmitter transport (b) Reuptake of neurotransmitter by presynaptic neuron 15
Acetylcholine Acetylcholine is a common neurotransmitter in vertebrates and invertebrates It is 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 Figure 48.1 The cone snail kills prey with venom that disables neurons Table 48.2 CNS and PNS CNS and PNS Opiates bind to the same receptors as endorphins and can be used as painkillers 16