Overview: Lines of Communication Chapter 8 Neurons, Synapses, and Signaling Fig. 8- The cone snail kills prey with venom that disables neurons Neurons are nerve s that transfer information within the body Neurons use two types of signals to communicate: electrical signals (long-distance) and chemical signals (short-distance) 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 The transmission of information depends on the path of neurons along which a signal travels Concept 8.: Neuron organization and structure reflect function in information transfer ntroduction to nformation Processing The squid possesses extremely large nerve s and is a good model for studying neuron function Nervous systems process information in three stages: sensory input, integration, and motor output Sensors detect external stimuli and internal conditions and transmit information along sensory neurons Many animals have a complex nervous system which consists of: Processing of information takes place in simple clusters of neurons called ganglia or a more complex organization of neurons called a brain Fig. 8- Nerves with giant axons Ganglia Brain Arm Eye Nerve Mantle 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 A central nervous system (CNS) where integration takes place; this includes the brain and a nerve cord A peripheral nervous system (PNS), which brings information into and out of the CNS
Fig. 8- Neuron Structure and Function Fig. 8- Dendrites Sensor Sensory input ntegration Most of a neuron s organelles are in the body Most neurons have dendrites, highly branched extensions that receive signals from other neurons Nucleus Stimulus Presynaptic hillock Cell body Motor output The axon is typically a much longer extension that transmits signals to other s at synapses Synapse Synaptic terminals Effector Peripheral nervous system (PNS) Central nervous system (CNS) An axon joins the body at the axon hillock Neurotransmitter Postsynaptic Fig. 8-a Synapse Neurotransmitter Synaptic terminals Postsynaptic A synapse is a junction between an axon and another The synaptic terminal of one axon passes information across the synapse in the form of chemical messengers called neurotransmitters nformation is transmitted from a presynaptic (a neuron) to a postsynaptic (a neuron, muscle, or gland ) Most neurons are nourished or insulated by s called glia Fig. 8- Fig. 8-a Fig. 8-b Dendrites Dendrites Cell body Cell body Sensory neuron nterneurons Portion of axon 8 µm Cell bodies of overlapping neurons Motor neuron Portion 8 µm of axon Cell bodies of overlapping neurons Sensory neuron nterneurons
Concept 8.: on pumps and ion s maintain the resting of a neuron Every has a voltage (difference in electrical charge) across its plasma called a Messages are transmitted as changes in The resting is the of a neuron not sending signals Formation of the Potential n a mammalian neuron at resting, the concentration of is greater inside the, while the concentration of is greater outside the -potassium pumps use the energy of ATP to maintain these and gradients across the plasma These concentration gradients represent chemical energy The opening of ion s in the plasma converts chemical to electrical A neuron at resting contains many open s and fewer open s; diffuses out of the Anions trapped inside the contribute to the negative charge within the neuron Animation: Potential Fig. 8-6 Modeling of the Potential Fig. 8-7 OUTSDE [ ] CELL mm NSDE [ ] CELL mm (a) [ ] mm [Cl ] mm [] mm [Cl ] mm [A ] mm Key OUTSDE CELL NSDE CELL (b) potassium pump can be modeled by an artificial that separates two s The concentration of KCl is higher in the inner and lower in the outer diffuses down its gradient to the outer Negative charge builds up in the inner At equilibrium, both the electrical and chemical gradients are balanced nner 9 mv mm mm KC KC Outer Cl (a) Membrane selectively permeable to Cl mm NaC E K = 6 mv(log mm mm) = 9 mv E Na = 6 mv(log +6 mv mm NaC (b) Membrane selectively permeable to Na+ mm mm ) = +6 mv Fig. 8-7a nner 9 mv Outer mm mm KC KC Cl (a) Membrane selectively permeable to The equilibrium (E ion ) is the voltage for a particular ion at equilibrium and can be calculated using the Nernst equation: E ion = 6 mv (log[ion] outside /[ion] inside ) The equilibrium of (E K ) is negative, while the equilibrium of (E Na ) is positive n a resting neuron, the currents of and are equal and opposite, and the resting across the remains steady E K = 6 mv(log mm mm) = 9 mv
Fig. 8-7b +6 mv Concept 8.: s are the signals conducted by axons Fig. 8-8 mm NaC mm NaC Neurons contain gated ion s that open or close in response to stimuli TECHNQUE Cl Microelectrode Voltage recorder (b) Membrane selectively permeable to E Na = 6 mv (log mm mm ) = +6 mv Reference electrode Fig. 8-9 Fig. 8-9a Stimuli + Membrane changes in response to opening or closing of these s When gated s open, diffuses out, making the inside of the more negative This is hyperpolarization, an increase in magnitude of the Membrane (mv) Stimuli + + Hyperpolarizations (a) Graded hyperpolarizations Membrane (mv) Stimuli (b) Graded depolarizations Depolarizations Membrane (mv) Strong depolarizing stimulus + 6 (c) Membrane (mv) Hyperpolarizations (a) Graded hyperpolarizations Fig. 8-9b Stimuli Production of Potentials + Other stimuli trigger a depolarization, a reduction in the magnitude of the For example, depolarization occurs if gated s open and diffuses into the Graded s are changes in polarization where the magnitude of the change varies with the strength of the stimulus Membrane (mv) Depolarizations (b) Graded depolarizations Voltage-gated and s respond to a change in When a stimulus depolarizes the, s open, allowing to diffuse into the The movement of into the increases the depolarization and causes even more s to open A strong stimulus results in a massive change in voltage called an action
Fig. 8-9c Strong depolarizing stimulus Generation of Potentials: A Closer Look Membrane (mv) + An action occurs if a stimulus causes the voltage to cross a particular threshold An action is a brief all-or-none depolarization of a neuron s plasma s are signals that carry information along axons A neuron can produce hundreds of action s per second The frequency of action s can reflect the strength of a stimulus An action can be broken down into a series of stages 6 (c) Fig. 8-- Fig. 8-- Fig. 8-- Key Key Key Rising phase of the action + + + Depolarization Membrane (mv) Time Depolarization Membrane (mv) Time Depolarization Membrane (mv) Time Extraular fluid Extraular fluid Extraular fluid nactivation loop nactivation loop nactivation loop state state state Fig. 8-- Fig. 8-- Key Key Rising phase of the action Falling phase of the action Depolarization Membrane (mv) + Time Rising phase of the action Falling phase of the action Depolarization Membrane (mv) + Time At resting. Most voltage-gated and s are closed, but some s (not voltagegated) are open Extraular fluid Extraular fluid nactivation loop state nactivation loop state Undershoot
When an action is generated. Voltage-gated s open first and flows into the. During the rising phase, the threshold is crossed, and the increases. During the falling phase, voltage-gated s become inactivated; voltage-gated s open, and flows out of the. During the undershoot, permeability to is at first higher than at rest, then voltagegated s close; resting is restored 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 BioFlix: How Neurons Work Animation: Potential Conduction of Potentials Fig. 8-- An action can travel long distances by regenerating itself along the axon At the site where the action is generated, usually the axon hillock, an electrical current depolarizes the neighboring region of the axon nactivated s behind the zone of depolarization prevent the action from traveling backwards s travel in only one direction: toward the synaptic terminals Fig. 8-- Fig. 8-- Conduction Speed The speed of an action increases with the axon s diameter n 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 s in the PNS 6
Fig. 8- Fig. 8-a Fig. 8-b Node of Ranvier Node of Ranvier Layers of myelin Layers of myelin Schwann Nodes of Myelin sheath Ranvier Schwann Nucleus of Schwann Schwann Schwann. µm Nodes of Myelin sheath Ranvier Nucleus of Schwann Myelinated axon (cross section). µm Fig. 8- Concept 8.: Neurons communicate with other s at synapses s are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage-gated Na+ s are found s in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction Cell body Schwann Depolarized region (node of Ranvier) Myelin sheath At electrical synapses, the electrical current flows from one neuron to another At chemical synapses, a chemical neurotransmitter carries information across the gap junction Most synapses are chemical synapses Fig. 8- Fig. 8- Postsynaptic neuron Synaptic terminals of presynaptic neurons µm 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 Voltage-gated Ca + Ca + Synaptic cleft Synaptic vesicles containing neurotransmitter Ligand-gated ion s Presynaptic Postsynaptic 6 Na+ Animation: Synapse 7
Generation of Postsynaptic Potentials Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion s in the postsynaptic Neurotransmitter binding causes ion s to open, generating a postsynaptic Postsynaptic s fall into two categories: Excitatory postsynaptic s (EPSPs) are depolarizations that bring the toward threshold nhibitory postsynaptic s (PSPs) are hyperpolarizations that move the farther from threshold After release, the neurotransmitter May diffuse out of the synaptic cleft May be taken up by surrounding s May be degraded by enzymes Summation of Postsynaptic Potentials Fig. 8-6 Fig. 8-6ab Unlike action s, postsynaptic s are graded and do not regenerate Most neurons have many synapses on their dendrites and body A single EPSP is usually too small to trigger an action in a postsynaptic neuron f two EPSPs are produced in rapid succession, an effect called temporal summation occurs Terminal branch E E of presynaptic E neuron E E E Postsynaptic neuron hillock of axon of postsynaptic neuron 7 Membrane (mv) E E E E E + E (a) Subthreshold, no (b) Temporal summation (c) Spatial summation summation E E E E + (d) Spatial summation of EPSP and PSP Terminal branch E of presynaptic E neuron E E Postsynaptic neuron hillock Membrane (mv) of axon of postsynaptic neuron 7 E E E E (a) Subthreshold, no (b) Temporal summation summation Fig. 8-6cd E E n 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 Membrane (mv) E 7 E + E E E E + Through summation, an PSP can counter the effect of an EPSP The summed effect of EPSPs and PSPs determines whether an axon hillock will reach threshold and generate an action (c) Spatial summation (d) Spatial summation of EPSP and PSP 8
Modulated Synaptic Transmission Neurotransmitters Table 8- n indirect synaptic transmission, a neurotransmitter binds to a receptor that is not part of an ion This binding activates a signal transduction pathway involving a second messenger in the postsynaptic Effects of indirect synaptic transmission have a slower onset but last longer The same neurotransmitter can produce different effects in different types of s There are five major classes of neurotransmitters: acetylcholine, biogenic amines, amino acids, neuropeptides, and gases Acetylcholine Biogenic Amines Amino Acids Acetylcholine is a common neurotransmitter in vertebrates and invertebrates n vertebrates it is usually an excitatory transmitter Biogenic amines include epinephrine, norepinephrine, dopamine, and serotonin They are active in the CNS and PNS Two amino acids are known to function as major neurotransmitters in the CNS: gammaaminobutyric acid (GABA) and glutamate Neuropeptides Fig. 8-7 EXPERMENT Fig. 8-7a Radioactive naloxone EXPERMENT Several neuropeptides, relatively short chains of amino acids, also function as neurotransmitters Neuropeptides include substance P and endorphins, which both affect our perception of pain Protein mixture Proteins trapped on filter RESULTS Drug Measure naloxone bound to proteins on each filter Radioactive naloxone Protein mixture Drug Opiates bind to the same receptors as endorphins and can be used as painkillers Proteins trapped on filter Measure naloxone bound to proteins on each filter 9
Fig. 8-7b Gases Fig. 8-UN RESULTS Gases such as nitric oxide and carbon monoxide are local regulators in the PNS Membrane (mv) + 7 Rising phase Depolarization Falling phase ( ) Undershoot You should now be able to:. Distinguish among the following sets of terms: sensory neurons, interneurons, and motor neurons; and resting ; ungated and gated ion s; electrical synapse and chemical synapse; EPSP and PSP; temporal and spatial summation. Explain the role of the sodium-potassium pump in maintaining the resting. Describe the stages of an action ; explain the role of voltage-gated ion s in this process. Explain why the action cannot travel back toward the body. Describe saltatory conduction 6. Describe the events that lead to the release of neurotransmitters into the synaptic cleft 7. Explain the statement: Unlike action s, which are all-or-none events, postsynaptic s are graded 8. Name and describe five categories of neurotransmitters