© 2012 Pearson Education, Inc. Lectures by Kathleen Fitzpatrick Simon Fraser University Signal Transduction Mechanisms: I. Electrical and Synaptic Signaling.

© 2012 Pearson Education, Inc. Signal Transduction Mechanisms: I. Electrical and Synaptic Signaling in Neurons Cell membranes can regulate the flow of ions between the interior and exterior of the cell The most dramatic example of regulation of the electrical properties of cells is the nerve cell or neuron Nerve cells have special mechanisms for using electrical potentials to transmit information over long distances

© 2012 Pearson Education, Inc. Neurons Almost all animals have a nervous system in which impulses are transmitted along the specialized plasma membranes of nerve cells Vertebrates have a central nervous system (CNS), consisting of the brain and spinal cord, and a peripheral nervous system (PNS), which comprises other sensory or motor components

© 2012 Pearson Education, Inc. Cells of the nervous system The nervous system has two main types of cells –Neurons send and receive electrical impulses (nerve impulses) –Glial cells encompass a variety of cell types

© 2012 Pearson Education, Inc. Neurons Sensory neurons are a diverse group of cells specialized for the detection of stimuli Motor neurons transmit signals from the CNS to the muscles and glands with which they make connections (innervate) Interneurons process signals and transmit information between parts of the nervous system

© 2012 Pearson Education, Inc. Glial cells Microglia fight infections and remove debris Oligodendrites and Schwann cells form the insulating myelin sheath around neurons of the CNS and peripheral nerves Astrocytes control access of blood-borne components into the extra-cellular fluid around the nerve cells forming the blood-brain barrier

© 2012 Pearson Education, Inc. Neurons Are Specially Adapted for the Transmission of Electrical Signals The cell body of a neuron is similar to that of other cells, and includes the nucleus and other endomembrane components Neurons also contain branches called processes Processes that receive signals are dendrites and those that conduct signals are axons

© 2012 Pearson Education, Inc. Axons The cytosol within an axon is called axoplasm Many vertebrate axons are surrounded by a discontinuous myelin sheath The sheath insulates the segments of axon separating the nodes of Ranvier A nerve is a tissue composed of bundles of axons

© 2012 Pearson Education, Inc. Motor neurons A motor neuron has multiple branched dendrites and a single axon, which is much longer than the dendrites The branches terminate in structures called synaptic boutons (terminal bulbs, or synaptic knobs) The boutons transmit the signal to the next cell, a neuron, muscle, or gland cell

© 2012 Pearson Education, Inc. Synapses The junction between a nerve cell, gland, or muscle cell is called a synapse For neuron-to-neuron junctions, synapses occur between an axon and a dendrite, but they can also occur between two dendrites Typically, neurons make synapses with other neurons, at the ends of axons and along their length as well

© 2012 Pearson Education, Inc. Understanding Membrane Potential Membrane potential is a fundamental property of all cells Cells at rest normally have excess negative charge on the outside and positive charge on the inside of the cell The resulting electrical potential is called the resting membrane potential

© 2012 Pearson Education, Inc. The squid giant axon The very large squid giant axon has been used for studies of nerve transmission since the 1930s It’s large size allows for easy insertion of microelectrodes to measure and control electrical potentials The resting membrane potential can be measured

© 2012 Pearson Education, Inc. Resting membrane potential Electrodes compare the ratio of negative to positive charge inside and outside the cell The resting membrane potential is about – 60mV for the squid giant axon Nerve, muscle, and certain other cell types exhibit electrical excitability

© 2012 Pearson Education, Inc. Electrically excitable cells In electrically excitable cells, certain stimuli trigger a rapid set of changes in membrane potential This is known as an action potential During the action potential the membrane potential changes from negative to positive and then back again in a very short time

© 2012 Pearson Education, Inc. Measuring changes in membrane potential Microelectrodes can be used to measure changes in the membrane potential The stimulating electrode is connected to a power source and inserted into the axon some distance from the recording electrode A brief impulse from the stimulating electrode depolarizes the membrane, measured at the recording electrode

© 2012 Pearson Education, Inc. The Resting Membrane Potential Depends on Differing Concentrations of Ions Inside and Outside the Neuron and on the Selective Permeability of the Membrane The cytosol and extracellular fluid of a cell contain different compositions of anions and cations Extracellular fluid contains dissolved salts, mostly sodium chloride The cytosol contains potassium as its main cation due to the action of the Na + /K + pump

© 2012 Pearson Education, Inc. Potassium ions and the membrane potential The uneven distribution of potassium ions inside and outside the cell is the potassium ion gradient Because of this gradient, potassium ions will tend to diffuse out of the cell toward the region of lower concentration Ions in solution are present in pairs, one negative and one positive (electroneutrality)

© 2012 Pearson Education, Inc. Counterions For any given ion, there must be an oppositely charged ion in the solution The oppositely charged ion is called the counterion In the cytosol, potassium (K + ) ions serve as counterions for the trapped anions; outside the cell, Na + is the main cation with Cl – as its counterion

© 2012 Pearson Education, Inc. Electrical potential A solution must have an equal number of positive and negative charges overall, but they can be unevenly distributed, with one region more positive and another more negative Even when separated, they will tend to flow back toward each other (electric potential, or voltage) When the oppositely charged ions are moving toward each other, current is flowing, measured in amperes (A)

© 2012 Pearson Education, Inc. Resting potential forms as a result of ionic compositions inside and outside the cell Some types of potassium channels in the plasma membrane allow K + to diffuse out of the cell As K + leaves the cytosol, increasing numbers of anions are left behind without counterions Excess negative charge accumulates outside the cell and excess positive charge accumulates outside, resulting in the membrane potential

© 2012 Pearson Education, Inc. Equilibrium K + diffuses out of cell down its gradient, but eventually the gradient is balanced by the K + electrical potential and net movement of K + stops When a chemical gradient is balanced by an electrical potential, it is called electrochemical equilibrium The membrane potential at the point of equilibrium is known as an equilibrium (or reversal) potential

© 2012 Pearson Education, Inc. The Nernst Equation Describes the Relationship Between Membrane Potential and Ion Concentration The Nernst equation describes the mathematical relationship between an ion gradient and the equilibrium potential that will form when the membrane is permeable only to that ion.

© 2012 Pearson Education, Inc. The Na + /K + pump The Na + /K + pump continually pumps sodium ions out of the cell to compensate for the small amount of leakage of sodium into the cell At the same time, potassium is carried inward On average, three sodium are transported inward for every two potassium ions transported outward

© 2012 Pearson Education, Inc. Steady-State Concentrations of Common Ions Affect Resting Membrane Potential Equation 13-2 is not complete because it doesn’t account for the effects of anions Because of the unequal distributions of Na +, K +, and Cl – across the membrane, each has a different impact on the membrane potential Each ion diffuses down its electrochemical gradient and affects the membrane potential

© 2012 Pearson Education, Inc. Effect of ions on membrane potential K + tends to diffuse out of the cell, making the membrane potential more negative Na + tends to flow into the cell, driving the potential in the positive direction, causing depolarization Cl – tends to diffuse into the cell but is repelled by the negative membrane potential, so enters along with positive ions

© 2012 Pearson Education, Inc. Increased membrane permeability to Cl – decreases excitability Increasing the membrane permeability to chloride has two effects, and both decrease neuronal excitability –The net entry of chloride ions (without a matching cation) causes hyperpolarization (membrane potential is more negative) –When the membrane becomes permeable to sodium, some chloride will also enter

© 2012 Pearson Education, Inc. The Goldman Equation Describes the Combined Effects of Ions on Membrane Potential Even in the resting state the cell is a little permeable to sodium, chloride, and potassium ions The Nernst equation doesn’t account for leakage of sodium and chloride into the cell; it deals with just one ion at a time It is helpful to consider the steady-state ion movements across the membrane

© 2012 Pearson Education, Inc. Steady-state movement of ions across the plasma membrane A membrane permeable only to K + will have a membrane potential equal to the equilibrium potential for K + If the membrane is also slightly permeable to Na +, the membrane potential will be partially depolarized as Na + leaks into the cell There is now less restraint on K + leaving the cell, so K + diffuses outward, balancing the inward movement of Na +

© 2012 Pearson Education, Inc. Goldman, Lloyd, and Katz These were the first researchers to describe how gradients of several different ions each contribute to a membrane potential Their equation, known as the Goldman equation, is

© 2012 Pearson Education, Inc. Goldman equation The Goldman equation, unlike the Nernst equation, includes terms for permeability of the ions involved In this case P K, P na, and P Cl are the relative permeabilities for each ion Except under special circumstances, the contribution of other ions to membrane potential is negligible

© 2012 Pearson Education, Inc. An example of the Goldman equation To estimate resting membrane potential in a squid axon, we use known steady-state concentrations and relative permeabilities of the three ions K + can be assigned a permeability value of 1.0 and the others are determined relative to that The relative permeability of Na + is 4% (0.04) and Cl – is 45% (0.45)

© 2012 Pearson Education, Inc. An example of the Goldman equation Using the relative permeabilities and the concentrations of the ions from Table 13-1, one can estimate resting potential of the squid axon This comes to –60.3 mV

© 2012 Pearson Education, Inc. Nernst and Goldman equations When the relative permeability of one of the ions is very high, the Goldman equation reduced to the Nernst equation for that ion For instance, if we ignore the effect of Cl –, as we can when P na → P K

© 2012 Pearson Education, Inc. Electrical Excitability The unique feature of electrically excitable cells is their response to depolarization Excitable cells respond with an action potential Excitable cells have voltage-gated channels in their plasma membranes

© 2012 Pearson Education, Inc. Ion Channels Act Like Gates for the Movement of Ions Through the Membrane Ion channels: integral membrane proteins that form ion-conducting pores in the lipid bilayer Voltage-gated ion channels respond to changes in the voltage across a membrane Voltage-gated Na + and K + channels are responsible for the action potential

© 2012 Pearson Education, Inc. Other ion channels Ligand-gated ion channels open when a ligand binds to the channel Other channels contribute to the steady-state ionic permeability of membranes These leak channels allow cells to be somewhat permeable to cations

© 2012 Pearson Education, Inc. Patch Clamping and Molecular Biological Techniques Allow the Activity of Single Ion Channels to Be Monitored Patch clamping, or single-channel recording, records currents passing through individual channels

© 2012 Pearson Education, Inc. Patch Clamping An amplifier keeps the membrane at a fixed membrane potential despite changes in its electrical properties Then the voltage clamp measures tiny changes in current flow through individual channels

© 2012 Pearson Education, Inc. Specific Domains of Voltage-Gated Channels Act as Sensors and Inactivators Voltage-gated potassium channels are multimeric proteins, composed of four protein subunits Voltage-gated sodium channels are large monomeric proteins with four separate domains In both types of channels each domain or subunit contains six transmembrane  -helices

© 2012 Pearson Education, Inc. Channel specificity The size of the central pore and the way it interacts with an ion gives the channel its specificity Oxygen atoms in the amino acids at the center of the channel are positioned to interact with ions as they move through the selectivity filter

© 2012 Pearson Education, Inc. Channel gating Voltage-gated sodium channels can open rapidly in response to a stimulus and then close again; channel-gating The open or closed state is all-or-none; the channels are not partially open The fourth subunit, S4, acts as a voltage sensor, responding to changes in potential

© 2012 Pearson Education, Inc. Channel inactivation Most voltage-gated channels adopt a second type of closed state, channel inactivation When a channel is inactivated it cannot reopen immediately, even if stimulated to do so Inactivation is caused by part of the channel called the inactivation particle that inserted in the opening of the channel

© 2012 Pearson Education, Inc. The Action Potential The coordinated opening and closing of ion channels leads to an action potential The giant axon of the squid has been important in the study of action potential

© 2012 Pearson Education, Inc. Action Potentials Propagate Electrical Signals Along an Axon Depolarization that brings the membrane to the threshold potential initiates an action potential An action potential is a brief but large electrical depolarization and repolarization of the neuronal plasma membrane It is caused by inward movement of sodium and subsequent outward movement of potassium

© 2012 Pearson Education, Inc. Action Potentials Movement of sodium and potassium ions during the action potential are controlled by the opening and closing of voltage-gated channels Once the action potential is initiated it will travel along the membrane away from the origin by a process called propagation

© 2012 Pearson Education, Inc. Action Potentials Involve Rapid Changes in the Membrane Potential of the Axon Development and propagation of an action potential (all within a few milliseconds) –Membrane potential rises dramatically to about +40 mV –It then falls slowly to about –75 mV (undershoot, or hyperpolarization) –It stabilizes again at the resting potential of about –60 mV

© 2012 Pearson Education, Inc. Action Potentials Result from the Rapid Movement of Ions Through Axonal Membrane Channels In a resting neuron the voltage-dependent channels are usually closed Because of leakiness to K +, the cell is about 100 times more permeable to K + than to Na + When a region of the nerve cell is slightly depolarized, some of the Na + channels open

© 2012 Pearson Education, Inc. Rapid movement of ions through axonal membrane channels The increased flow of Na+ through the channels increases membrane depolarization Increasing depolarization opens more channels, causing more Na+ to flow, etc. This positive feedback loop is called the Hodgkin cycle

© 2012 Pearson Education, Inc. Subthreshold Depolarization When the membrane is depolarized by a small amount, the membrane potential recovers because of K + movement through leak channels In this case no action potential occurs Levels of depolarization too small to initiate an action potential are called subthreshold depolarizations

© 2012 Pearson Education, Inc. The Depolarizing Phase When the membrane is depolarized past the threshold potential a significant number of Na + channels begin activating The membrane potential shoots upward rapidly It peaks at about +40 mV

© 2012 Pearson Education, Inc. The Repolarizing Phase Once the membrane potential has risen to its peak the membrane quickly repolarizes This is due to inactivation of sodium channels and opening of voltage-gated potassium channels The inactivated sodium channels remain closed until the membrane potential is negative again The cell repolarizes as K + leaves the cell

© 2012 Pearson Education, Inc. The Hyperpolarizing Phase (Undershoot) At the end of an action potential most neurons show a transient hyperpolarization or undershoot The membrane potential temporarily drops below the resting potential. This occurs because of increased potassium permeability. As the voltage-gated potassium channels close, the membrane potential returns to normal

© 2012 Pearson Education, Inc. The Refractory Periods For a few milliseconds after an action potential, it is impossible to trigger a second one This is the absolute refractory period, when sodium channels are inactivated and cannot open by depolarization During undershoot, sodium channels can open again but potassium channels are open, too

© 2012 Pearson Education, Inc. Refractory Periods Potassium leak channels and voltage-gated channels are open, driving the membrane potential down This is well below the threshold for triggering another action potential This time is called the relative refractory period

© 2012 Pearson Education, Inc. Changes in Ion Concentrations Due to an Action Potential During an action potential, cellular concentrations of Na+ and K+ hardly change at all The ions involved are a small fraction of the total ions in the cell Intense neuronal activity can lead to significant changes in ion concentration

© 2012 Pearson Education, Inc. Action Potentials Are Propagated Along the Axon Without Losing Strength Depolarization that occurs in one place along a membrane spreads to adjacent regions through passive spread of depolarization As it spreads away from the origin it decreases in magnitude, so signals cannot travel far by this means To go farther, the action potential must be propagated, actively generated without fading as it moves

© 2012 Pearson Education, Inc. Transmission of a signal Incoming signals are transmitted to a neuron at points of contact called synapses Incoming signals depolarize the dendrites and the depolarization spreads passively over the membrane to the base of the axon, the axon hillock This is where action potentials are most easily generated

© 2012 Pearson Education, Inc. Propagation of an action potential in a nonmyelinated nerve cell Stimulation of a resting membrane results in depolarization and an inward rush of Na + (1) Membrane polarity is temporarily reversed and this spreads (2) Nearby depolarization is above a threshold, and results in an inward movement of Na + (3)

© 2012 Pearson Education, Inc. Propagation of an action potential (continued) The original region on the membrane becomes permeable to K + ions, which rush out of the cell, and return the membrane to its resting state (4) Meanwhile, depolarization has spread farther, initiating the same sequence of events there (5) The propagation of these events is a propagated action potential or nerve impulse

© 2012 Pearson Education, Inc. The Myelin Sheath Acts Like an Electrical Insulator Surrounding the Axon Most vertebrate axons are surrounded by many concentric layers of membrane, forming a discontinuous myelin sheath It is formed by oligodendrites in the CNS and Schwann cells in the PNS

© 2012 Pearson Education, Inc. Consequences of myelination Myelination decreases the ability of the neuronal membrane to retain electric charge (i.e., capacitance) Nerve impulses can spread farther and faster than in the absence of myelination The action potential must still be renewed; this happens at nodes of Ranvier

© 2012 Pearson Education, Inc. Action potentials are renewed at nodes of Ranvier Nodes of Ranvier are spaced closely enough to ensure the action potential at one node can trigger one in the next node Action potentials jump from one node to the next, called saltatory propagation, more rapid than continuous propagation Nodes of Ranvier are highly organized structures

© 2012 Pearson Education, Inc. Synaptic Transmission Nerve cells communicate with one another and other cell types at synapses Electrical synapse: one neuron (presynaptic) is connected to a second neuron (postsynaptic) via gap junctions Ions move through the junctions between the cells and there is no delay in transmission

© 2012 Pearson Education, Inc. Chemical synapses Chemical synapse: presynaptic and postsynaptic neurons are not connected by gap junctions Instead the two cell membranes are separated by a small space, the synaptic cleft A signal at the terminus of the presynaptic neuron must be sent to the postsynaptic neuron chemically

© 2012 Pearson Education, Inc. Neurotransmitters Neurotransmitters are stored in synaptic boutons in the presynaptic neuron, released by the arrival of an action potential They diffuse across the cleft and bind to receptors in the plasma membrane of the postsynaptic cell They are converted into electric signals, to stimulate or inhibit an action potential in the receiving cell

© 2012 Pearson Education, Inc. Types of neurotransmitter receptors Neurotransmitter receptors fall into two classes –Ligand-gated ion channels (ionotropic receptors) –Receptors that exert their effects indirectly via a system of messengers (metabotropic receptors)

© 2012 Pearson Education, Inc. Neurotransmitters Relay Signals Across Nerve Synapses Neurotransmitter: any signaling molecule released by a neuron, detected by the postsynaptic cell through a receptor Excitatory receptors: cause depolarization of the postsynaptic neuron Inhibitory receptors: cause the postsynaptic cell to hyperpolarize

© 2012 Pearson Education, Inc. Criteria for neurotransmitters To qualify as a neurotransmitter a molecule must -1. Elicit the appropriate response when introduced to the synaptic cleft -2. Occur naturally in the presynaptic neuron -3. Be released at the right time when the presynaptic neuron is stimulated

© 2012 Pearson Education, Inc. Acetylcholine Acetylcholine is the most common neurotransmitter in vertebrates in synapses outside the CNS and for neuromuscular junctions It is an excitatory neurotransmitter Synapses using acetylcholine as their neurotransmitter are called cholinergic synapses

© 2012 Pearson Education, Inc. Catecholamines Catecholamines include dopamine and the hormones norepinephrine and epinephrine All are derivatives of tyrosine, and are also synthesized in the adrenal gland Synapses (in the brain and between nerves and smooth muscles in internal organs) that use catecholamines as neurotransmitters are called adrenergic

© 2012 Pearson Education, Inc. Amino Acids and Derivatives Some other neurotransmitters consist of amino acids and their derivatives: histamine, serotonin,  -amino butyric acid (GABA) as well as glycine and glutamate Serotonin is excitatory (causes potassium channels to close) and functions in the CNS GABA and glycine are inhibitory; glutamate is excitatory

© 2012 Pearson Education, Inc. Neuropeptides Neuropeptides are short amino acid chains formed by proteolysis of precursor proteins Hundreds are known; they act on groups of neurons and have long-lasting effects E.g., enkephalins inhibit the activity of neurons in the brain that are involved in pain perception

© 2012 Pearson Education, Inc. Elevated Calcium Levels Stimulate Secretion of Neurotransmitters from Presynaptic Neurons Neurotransmitter secretion is directly controlled by calcium ion concentration in synaptic boutons When an action potential arrives, depolarization causes a temporary increase in Ca 2+ due to opening of voltage-gated calcium channels The neurotransmitters are stored in small neurosecretory vesicles in the bouton

© 2012 Pearson Education, Inc. Neurotransmitter release The release of calcium into the bouton has two effects -1. Vesicles in storage are mobilized for rapid release -2. Vesicles near the membrane that are poised for release fuse with the plasma membrane and expel the contents into the cleft

© 2012 Pearson Education, Inc. Secretion of Neurotransmitters Involves the Docking and Fusion of Vesicles with the Plasma Membrane Vesicle fusion is thought to involve vesicles that are already “docked” at the plasma membrane Docking and fusion are mediated by t- and v-SNARE proteins Ca 2+ in the boutons is bound by synaptogamin, which undergoes conformational change and promotes t- and v-SNARES to interact efficiently

© 2012 Pearson Education, Inc. The active zone Docking takes place at the active zone where the synaptic vesicles and calcium channels are very close together Neurotoxins such as tetanus and botulism interfere with docking and release Compensatory endocytosis maintains the size of the nerve terminal by recycling membranes

© 2012 Pearson Education, Inc. Kiss-and-run exocytosis When neurons need to fire very rapidly they use a more transient method for neurotransmitter release A vesicle will temporarily fuse with the plasma membrane, release some neurotransmitter, and then reseal This is called kiss-and-run exocytosis

© 2012 Pearson Education, Inc. Neurotransmitters Are Detected by Specific Receptors on Postsynaptic Neurons Each neurotransmitter has a particular receptor that detects and binds it A few of these receptors are quite well understood

© 2012 Pearson Education, Inc. The Nicotinic Acetylcholine Receptor Acetylcholine binds a ligand-gated Na + channel called the nicotinic acetylcholine receptor (nAchR) When two molecules of acetylcholine bind, the receptor opens and lets Na + rush into the cell, causing depolarization nAchR has been studied using the electric organ of the electric ray (Torpedo californica)

© 2012 Pearson Education, Inc. NAchR structure The receptor forms rosette-like particles about 8 nm across It consists of four kinds of subunits (  ) The receptors play an important role in transmitting nerve impulses to muscle; in humans, autoimmune reactions against these receptors cause serious illnesses

© 2012 Pearson Education, Inc. The GABA Receptor The GABA receptor is also a ligand-gated channel, but when opened, it allows Cl– ions into the cell This causes hyperpolarization of the receiving cell and decreased likelihood that an action potential will be generated

© 2012 Pearson Education, Inc. Neurotransmitters Must Be Inactivated Shortly After Their Release Once the neurotransmitter has been secreted, it must be rapidly removed from the synaptic cleft Acetylcholinesterase hydrolyzes acetylcholine Neurotransmitter reuptake involves pumping neurotransmitters back into the presynaptic cell or nearby support cells

© 2012 Pearson Education, Inc. Integration and Processing of Nerve Signals A single action potential is usually not enough to cause firing of a postsynaptic cell Incremental changes in potential due to neurotransmitter binding are called postsynaptic potentials (PSPs) These can cause excitatory or inhibitory postsynaptic potentials depending on the neurotransmitter

© 2012 Pearson Education, Inc. Neurons Can Integrate Signals from Other Neurons Through Both Temporal and Spatial Summation Individual action potentials will produce only a temporary EPSP Two action potentials in rapid succession will result in a more depolarized receiving cell A rapid series of action potentials sums EPSPs over time, and pushes the postsynaptic cell past its threshold (temporal summation)

© 2012 Pearson Education, Inc. Spatial summation Action potentials received at a single synapse are usually not sufficient to induce an action potential When many action potentials cause neurotransmitter release simultaneously, it is more likely that an action potential will be induced This is called spatial summation

© 2012 Pearson Education, Inc. Neurons Can Integrate Both Excitatory and Inhibitory Signals from Other Neurons Postsynaptic neurons can receive both inhibitory and excitatory signals Neurons can receive thousands of inputs from other neurons, and physically sum EPSPs and IPSPs

© 2012 Pearson Education, Inc. Lectures by Kathleen Fitzpatrick Simon Fraser University Signal Transduction Mechanisms: I. Electrical and Synaptic Signaling.

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© 2012 Pearson Education, Inc. Lectures by Kathleen Fitzpatrick Simon Fraser University Signal Transduction Mechanisms: I. Electrical and Synaptic Signaling.

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