Neurons, Synapses, and Signaling

Chapter 8 Neurons, Synapses, and Signaling PowerPoint Lectures for Biology, Eighth Edition Overview: Lines of Communication The cone snail kills prey with venom that disables neurons Neurons are nerve cells that transfer information within the body Neurons use two types of signals to communicate: electrical signals (longdistance) and chemical signals (shortdistance) Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp and Janette Lewis Copyright 8 Pearson Education, nc., publishing as Pearson Benjamin Cummings What makes this snail such a deadly predator? Nervous system organization The transmission of information depends on the path of neurons along which a signal travels Processing of information takes place in simple clusters of neurons called ganglia or a more complex organization of neurons called a brain Many animals have a complex nervous system which 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 brings information into and out of the CNS Arm Ganglia Brain Eye Nerve Mantle Nerves with giant axons

Concept 8.: Neuron organization and structure reflect function in information transfer Neuron organization and structure reflect function in information transfer Neurons are the functional unit of the nervous system Nervous systems process information in three stages:. sensory input. integration. motor output Neurons Transmit nformation stages of information processing. Sensory reception: Sensors detect external stimuli and internal conditions and transmit information along sensory neurons. ntegration: 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 Summary of information processing Sensor Effector Sensory input Motor output Peripheral nervous system (PNS) ntegration Central nervous system (CNS) Neurons Transmit nformation 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 An axon joins the cell body at the axon hillock Neuron Structure and Organization Dendrites Stimulus Neuron Structure and Organization Nucleus Cell body hillock Presynaptic cell Synapse Synaptic terminals Synapse Synaptic terminals Neurotransmitter Postsynaptic cell Neurotransmitter Postsynaptic cell

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 nformation is transmitted from a presynaptic cell (a neuron) to a postsynaptic cell (a neuron, muscle, or gland cell) Structural diversity of neurons Dendrites Cell body Sensory neuron nterneurons Portion 8 µm of axon Cell bodies of overlapping neurons Motor neuron Nervous System Organization Most neurons are nourished or insulated by cells called glia Astrocytes: star shaped- nourish Oligodendrocytes make myelin sheaths (insulation) for neurons in the CNS Schwann cells make myelin sheaths for neurons in the PNS Concept 8.: on pumps and ion s maintain the resting of a neuron on pumps and ion s maintain the resting of a neuron Every cell 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 Formation of the resting : The resting is typically -7 mv n a mammalian neuron at resting, the concentration of is greater inside the cell, while the concentration of is greater outside the cell Sodium-potassium pumps use the energy of ATP to maintain these and gradients across the plasma These concentration gradients represent chemical energy

Formation of the Potential The basis of the 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 cell Anions trapped inside the cell contribute to the negative charge within the neuron OUTSDE [ ] CELL 5 mm NSDE [ ] CELL mm [Na+ ] 5 mm [ ] [Cl ] 5 mm mm [Cl ] [A ] mm mm Key OUTSDE CELL NSDE CELL Sodiumpotassium pump Potassium Sodium (a) (b) Animation: Potential Modeling of the Potential can be modeled by an artificial 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 builds up in the inner chamber At equilibrium, both the electrical and chemical gradients are balanced Formation of the Potential 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 nner chamber 9 mv mm 5 mm KC KC Potassium Outer chamber Cl Formation of the Potential n a resting neuron, the currents of and are equal and opposite, and the resting across the remains steady Many ion s are open, most Na ion s are closed so the is around -7 mv (a) Membrane selectively permeable to E K = 6 mv(log 5 mm mm) = 9 mv

+6 mv 5 mm NaC 5 mm NaC nner chamber 9 mv Outer chamber mm 5 mm KC KC 5 mm NaC +6 mv 5 mm NaC Cl Cl Cl Sodium Potassium Sodium (b) Membrane selectively permeable to 5 mm E Na = 6 mv (log 5 mm ) = +6 mv (a) Membrane selectively permeable to (b) Membrane selectively permeable to E K = 6 mv(log 5 5 mm = 9 mv E mm) Na = 6 mv (log 5 mm ) = +6 mv Concept 8.: s are the signals conducted by axons Neurons contain gated ion s that open or close in response to stimuli TECHNQUE Microelectrode Reference electrode Voltage recorder Graded Potentials The changes in response to opening or closing of these s Hyperpolarization When gated s open, diffuses out, making the inside of the cell more negative This is hyperpolarization, an increase in (negative) magnitude of the Remember that resting is around -7 mv Membrane (mv) +5 5 Threshold Stimuli Hyperpolarizations 5 Time (msec) (a) Graded hyperpolarizations Graded Potentials Depolarizations Other stimuli trigger a depolarization, a reduction in the magnitude of the For example, depolarization occurs if gated s open and diffuses into the cell Graded s are changes in polarization where the magnitude of the change varies with the strength of the stimulus 5

Membrane (mv) +5 5 Threshold Stimuli (b) Graded depolarizations Depolarizations 5 Time (msec) Production of Potentials Production of action s Voltage-gated and s respond to a change in When a stimulus depolarizes the, s open, allowing to diffuse into the cell The movement of into the cell increases the depolarization and causes even more s to open A strong stimulus results in a massive change in voltage called an action Membrane (mv) +5 Strong depolarizing stimulus 5 Threshold Potentials 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 5 Time (msec) 6 (c) Graded s and an action in a neuron Membrane (mv) Stimuli +5 +5 Membrane (mv) 5 Threshold 5 Threshold Stimuli Hyperpolarizations Depolarizations 5 5 Time (msec) Time (msec) (a) Graded hyperpolarizations (b) Graded depolarizations Membrane (mv) +5 Strong depolarizing stimulus 5 Threshold (c) 5 6 Time (msec) Generation of Potentials: A Closer Look Generation of Potentials: A Closer Look 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

Generation of Potentials: A Closer Look Key At resting. Most voltage-gated and s are closed, but some s (not voltagegated) are open Membrane (mv) +5 Threshold 5 5 Depolarization Time Extracellular fluid Sodium Potassium nactivation loop state Generation of Potentials: A Closer Look Key When an action is generated. Voltage-gated s open first and flows into the cell. 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 cell Depolarization Extracellular fluid nactivation loop Membrane (mv) Time Sodium Potassium +5 Threshold 5 5 state Key Key Rising phase of the action Rising phase of the action Falling phase of the action Membrane (mv) +5 Threshold 5 5 Membrane (mv) +5 Threshold 5 5 Depolarization Time Depolarization Time Extracellular fluid Sodium Potassium Extracellular fluid Sodium Potassium nactivation loop nactivation loop state state 7

Generation of Potentials: A Closer Look 5. During the undershoot, permeability to is at first higher than at rest, then voltage-gated s close; resting is restored as Na-K pumps pump sodium out and potassium in. Key Rising phase of the action Falling phase of the action +5 Threshold 5 5 Depolarization Time Membrane (mv) Extracellular fluid Sodium Potassium nactivation loop state 5 Undershoot Animation: Potential After an Potential 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 while sodium-potassium pumps return ions to the opposite side of the using ATP and restoring resting How Neurons Work Conduction of Potentials Conduction of action s 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 Conduction of Potentials nactivated s behind the zone of depolarization prevent the action from traveling backwards s travel in only one direction: toward the synaptic terminals 8

+5 Membrane (mv) 5 7 Rising phase Falling phase Threshold ( 55) Depolarization Undershoot Time (msec) Conduction Speed 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 cells in the PNS Schwann cells and the myelin sheath Myelin sheath Schwann cell Nodes of Ranvier Node of Ranvier Layers of myelin Schwann cell Nucleus of Schwann cell 9

Schwann cells and the myelin sheath Conduction Speed and the Myelin Sheath 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 Myelinated axon (cross section). µm Saltatory conduction Concept 8.: Neurons communicate with other cells at synapses Cell body Schwann cell Depolarized region (node of Ranvier) Myelin sheath Neurons communicate with other cells at synapses 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 Synaptic terminals on the cell body of a postsynaptic neuron (colorized SEM) Postsynaptic neuron Synaptic terminals of presynaptic neurons 5 µm Chemical Synapses Neurotransmitters 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 Animation: Synapse

A chemical synapse How Synapses Work Synaptic vesicles containing neurotransmitter Presynaptic 5 Na+ Voltage-gated Ca + Ca + Postsynaptic 6 Synaptic cleft Ligand-gated ion s Generation of Postsynaptic Potentials Generation of postsynaptic s 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 Postsynaptic s Postsynaptic s fall into two categories: Excitatory postsynaptic s (EPSPs) are depolarizations that bring the toward threshold For example: if Na+ s open in response to neurotransmitter nhibitory postsynaptic s (PSPs) are hyperpolarizations that move the farther from threshold May occur if K+ ion s open Postsynaptic Potentials After release, the neurotransmitter May diffuse out of the synaptic cleft May be taken up by surrounding cells May be degraded by enzymes Postsynaptic Potentials Summation of postsynaptic s Unlike action s, postsynaptic s are graded and do not regenerate 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 f two EPSPs are produced in rapid succession, an effect called temporal summation occurs

E Postsynaptic neuron Membrane (mv) E Terminal branch of presynaptic neuron E Threshold of axon of postsynaptic neuron 7 E E (a) Subthreshold, no summation E E E (b) Temporal summation hillock Summation of Postsynaptic Potentials 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 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 Summation of postsynaptic s E E E E E Terminal branch of presynaptic neuron E E E Membrane (mv) 7 E + E (c) Spatial summation E E + (d) Spatial summation of EPSP and PSP E Postsynaptic neuron Threshold of axon of postsynaptic neuron 7 Membrane (mv) E E (a) Subthreshold, no summation E E hillock E E E + E (b) Temporal summation (c) Spatial summation E E E + (d) Spatial summation of EPSP and PSP Modulated Synaptic Transmission 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 cell Effects of indirect synaptic transmission have a slower onset but last longer Neurotransmitters Neurotransmitters The same neurotransmitter can produce different effects in different types of cells There are five major classes of neurotransmitters: acetylcholine, biogenic amines, amino acids, neuropeptides, and gases

Neurotransmitters Acetylcholine 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 Neurotransmitters Amino acids Two amino acids are known to function as major neurotransmitters in the CNS: gamma-aminobutyric acid (GABA) and glutamate GABA is an inhibitory neurotransmitter Neurotransmitters Neuropeptides: relatively short chains of amino acids, Neuropeptides include substance P and endorphins, which both affect our perception of pain Opiates bind to the same receptors as endorphins and can be used as painkillers Gases such as nitric oxide and carbon monoxide are local regulators in the PNS Does the brain have a specific receptor for opiates? Does the brain have a specific receptor for opiates? EXPERMENT Radioactive naloxone RESULTS Drug Protein mixture Proteins trapped on filter Measure naloxone bound to proteins on each filter

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. Describe the conduction of an action down an axon 5. 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