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BIOL 273 (56)


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University of Waterloo
BIOL 273
Heidi Engelhardt

BIOL 273 Midterm #1 Notes Neurophysiology Introduction • The nervous system is a network of nerve cells linked together to form the rapid control system of the body • Nerve cells, also called neurons, are designed to carry electrical signals rapidly over long distances o They link together to transfer a signal by passing chemical signals called neurotransmitters across the small gap between each neuron, called a synapse o Sometimes (more rarely) they are instead linked by gap junctions • Pathways (paths along which these signals travel) are not necessarily linear – sometimes a signal influences multiple neurons, or many neurons affect one single neuron Organization of the Nervous System • Sensory receptors monitor conditions in the internal and external environments of the body, and send this information through afferent (or sensory) neurons to the central nervous system • The central nervous system (or CNS) is the integrating center for neural reflexes – it receives these afferent signals and then decides what to do with it • Then if an action is needed, the signal for this is sent out through efferent neurons o Efferent neurons are divided into the somatic motor division (which controls skeletal muscles) and the autonomic division (which controls smooth and cardiac muscles, exocrine and some endocrine glands, and some types of adipose tissue)  Autonomic neurons are further divided into sympathetic and parasympathetic branches o The afferent and efferent neurons form the peripheral nervous system Cells of the Nervous System • The nervous system is composed primarily of two cell types: glial cells (“support cells”) and nerve cells/neurons (the basic signaling units of the nervous system) Neurons are Excitable Cells that Generate and Carry Electrical Signals • The neuron is the functional unit of the nervous system (the smallest structure that can carry out the functions of a system) o SEE FIGURE 8.2, PAGE 242 • Neurons have long things (called appendages or processes) sticking out from their cell body, either dendrites (which receive incoming signals) or axons (which carry outgoing information) o The shape, number, and length of these things vary from neuron to neuron The Cell Body is the Control Center of the Neuron • The cell body of the nerve is just like a typical cell body • A cytoskeleton extends into the axon and dendrites • The position of the cell body (in relation to the dendrites, etc.) varies with different kinds of neurons, but almost always it is small (less than 1/10 of total neuron volume) Dendrites Receiving Incoming Signals • Dendrites are thin, branched processes that receive incoming information from neighboring cells • They increase a cell’s surface area • Basically, their job is to receive incoming information and send it to an integrating region within the neuron (e.g. the nucleus of the cell) Axons Carry Outgoing Signals to the Target • Most neurons only have one of these • It comes out of a part of the cell called the axon hillock • They can be as long as a meter long, or just a few micrometers long • But along the axon, they branch and form collaterals o Each collateral ends in an axon terminal • The functionality of an axon is to transmit outgoing electrical signals from the integrating center (i.e. nucleus) of the neuron to the end of the axon • The region where an axon terminal meets its target cell is called a synapse o The neuron which delivers the signal to the synapse is called the presynaptic cell o The cell that receives the signal is the postsynaptic cell o The narrow space between the two cells is the synaptic cleft Glial Cells are the Support Cells of the Nervous System • Glial cells provide important physical and biochemical support to neurons • They outnumber neurons in the nervous system by 10-50:1 • Neurons do not have much outward support, so glial cells provide structural stability to neurons by wrapping around them o They also provide metabolic support to neurons o And help maintain homeostasis of the brain’s extracellular fluid by taking + up excess metabolites and K • The peripheral nervous system has two types of glial cells: Schwann cells and satellite cells • The central nervous system has four types of glial support cells: oligodendrocytes, astrocytes, microglia, and ependymal cells o Astrocytes are highly branched cells which transfer nutrients between neurons and blood vessels o Microglia are specialized immune cells which reside permanently within the CNS, and remove damaged cells and foreign invaders o Ependymal cells are epithelial cells that create a selectively permeable barrier between compartments in the brain o Schwann cells in the peripheral nervous system and oligodendrocytes in the CNS support and insulate axons by creating myelin (multiple concentric layers of phospholipids membrane) • Myelin forms when these glial cells wrap around an axon and they squeeze out the glial cytoplasm to form membrane layers • SEE FIGURE 8.6, PAGE 247 • So a single axon can have up to 500 Schwann cells wrapped around it, making the myelin sheath • Between these cells there is a very small part the axon membrane still in contact with the extracellular fluid, called a node of Ranvier o Satellite cells form supportive capsules around nerve cell bodies located in ganglia Electrical Signals in Neurons • Nerve and muscle cells are called excitable tissues because they can propagate electrical cells when ions move across the cell membrane The Nernst Equation Predicts Membrane Potential for a Single Ion • Membrane potential is the electrical disequilibrium which results from the uneven distribution of ions across the cell membrane o It is affected by two factors:  Concentration gradients of ions across the membrane • Normally, sodium, chloride, and calcium are more concentrated in the extracellular fluid than in the cytosol (inside), while potassium is more concentrated in the cell than in extracellular fluid  Membrane permeability to those ions • Like, since the resting cell membrane is much more permeable to potassium, it is the major ion contributing to the resting membrane potential • The Nernst equation gives us the membrane potential which a single ion would produce if the membrane were only permeable to that one ion o It is called the equilibrium potential of that ion o SEE COURSE NOTES FOR THE EQUATION • So for example, if we take the Nernst equation and check potassium, we will see a membrane potential of -90 mV, but we know that a neuron’s resting membrane potential is -70 mV, so other ions must be contributing to the potential The GHK Equation Predicts Membrane Potential Using Multiple Ions • Also known as simply the “Goldman equation”, it calculates the resting membrane potential which results from the contributions of all ions which can cross the membrane o SEE COURSE NOTES FOR THE EQUATION o You will notice that the permeability of the membrane to an ion is in the equation, because it has an effect on how much a single ion will contribute to the resting membrane potential Ion Movement Across the Cell Membrane Creates Electrical Signals • The membrane potential of a cell can be changed by either having the potassium concentration gradient changed (so there is an imbalance, and more potassium has to move across to correct this) or having ion permeabilities change (so that other ions can get in on the action, baby!) o So like, if a membrane becomes very permeable to Na , then more will end the cell and since it is positive, the cell will move down the electrochemical gradient – this is a depolarization of the cell membrane, and it creates an electrical signal o But hyperpolarization can also occur – like for example if the cell is more permeable to K , and it moves out, so we lose positive charge from the cell and it becomes more negative, or if negative ions from outside move in all of a sudden • Note that just because a change in membrane potential occurs, it doesn’t mean that the concentration gradient of the ions changes – because only very few ions have to move in order for the potential to change Gated Channels Control the Ion Permeability of the Neuron • The simplest way for a cell to change its ion permeability is by opening or closing existing channels in the membrane o The other, slower, more ghetto method would be to add or remove channels • Most channels only allow one kind of ion through, but sometimes there are monovalent channels which allow more than one kind of ion to pass • There are leak channels, which spend most of their time being open • On the other hand there are channels which have gates which open or close in response to particular stimuli o Mechanically gated ion channels are found in sensory neurons and open in response to physical forces such as pressure or stretch o Chemically gated ion channels in most neurons respond to a variety of ligands, such as extracellular neurotransmitters o Voltage gated ion channels play an important role in the initiation and conduction of electrical signals  Some leak channels are actually voltage gated channels which are just always open in the voltage range of the resting potential • When the ion channels open, ions move in/out of the cell, and this movement of electrical charge depolarizes or hyperpolarizes the cell, creating an electrical signal • The electrical signals can be one of two basic types: o Graded potentials are variable-strength signals which travel over short distances and lose strength as they travel through the cell o Action potentials are large, uniform depolarizations which can travel for long distances through the neuron without losing strength Graded Potentials Reflect the Strength of the Stimulus that Initiates Them • Graded potentials are depolarizations or hyperpolarizations that occur in the dendrites, cell body, or less frequently, near the axon terminals o SEE FIGURE 8.7, PAGE 251 • They occur when ion channels open or close, causing ions to enter or leave the neuron • They are called “graded” because their size or amplitude is directly proportional to the strength of the triggering event • So for example, a chemical or mechanical stimulus could open up sodium channels, and then sodium ions move into the neuron and electrical energy comes in o Then a wave of depolarization spreads through the cytoplasm (just like a stone thrown in water creates ripples), and this wave is called local current flow • Graded potentials lose strength as they move through the cytoplasm for two reasons: o Current leak – so like, if the initial wave is caused by lots of positive ions coming in, sometimes as the wave moves, positive ions move back outside, and the electrical difference becomes lesser o Cytoplasmic resistance – the cytoplasm itself resists the flow of electricity • If a graded potential is strong enough, eventually it will reach the area of the neuron called the trigger zone o For efferent neurons and interneurons, the trigger zone is the axon hillock and the very first part of the axon (called the initial segment) o For afferent neurons, it is immediately adjacent to the receptor, where the dendrites join the axon • In the trigger zone, there is a high concentration of voltage-gated Na channels o So if the current gets here and the membrane here is depolarized to a level called the threshold voltage, then these channels open and we start an action potential o If the depolarization (current) is not strong enough though, then nothing happens • So, a graded potential that depolarizes the cell and causes it to fire an action potential is called an excitatory postsynaptic potential (ESP) • But there are also graded potentials that are hyperpolarizing, and it moves the membrane farther away from the threshold value, and these are called inhibitory postsynaptic potentials Action Potentials Travel Long Distances Without Losing Strength • Action potentials (also called spikes), differ from graded potentials in several ways: o All action potentials are identical (i.e. not varied strengths) o They do not diminish in strength as they travel through the neuron • The action potential does not even depend on the graded potential which triggered it! Action Potentials Represent Movement of Na and K Across the Membrane • Action potentials are changes in membrane potential that occur when voltage- gated ion channels open, altering membrane permeability to sodium and potassium o SEE FIGURE 8.9, PAGE 253 – UNDERSTAND THIS FIGURE, IT WILL SAVE YOUR LIFE! + Na Channels in the Axon Have Two Gates • The voltage gated Na channel has two gates to regulate ion movement, rather than a single gate • These gates are called activation and inactivation gates, and they flip-flop back and forth to open and shut the Na channel o SEE FIGURE 8.10, PAGE 254 • When the neuron is at its resting membrane potential, the activation gate is closed and no Na moves through the channel, and the inactivation gate is still open (it is like a ball-and-chain amino acid sequence) • But when the cell membrane in the area depolarizes, the activation gate opens, and now Na can move into the cell because the inactivation gate is open from before • And the cell now gets even more depolarized! And as long as it still is depolarized, the activation gates remain open – this is called a positive feedback loop • But then, the second gate breaks this loop by closing o This happens 0.5 milliseconds after the activation gate opens – so basically, the activation gate opens then the inactivation gate closes 0.5 milliseconds afterwards, so during that little time, the Na can move into the cell o Enough Na moves in to make the rising phase of the action potential • The neuron repolarizes as K+ moves out… Action Potentials will not Fire During the Absolute Refractory Period • The fact that the Na channel is double gated plays a large role in the thing we call the refractory period, which is the span of time for 1 millisecond after an action potential has begun that no other action potential may occur, regardless of how large the stimulus is • This period is called the absolute refractory period, and it represents the time it takes for the Na gates to get to their original “starting positions”, from where they can open again and let more Na into the cell • This means that: o Action potentials cannot move backward! o And the rate at which signals can be transmitted down a neuron is limited • We also have a relative refractory period after the absolute refractory period, which is when a stronger-than-normal depolarizing graded potential is needed to bring the cell to the threshold for an action potential o This is partially because during this time, the K+ channels are still open (for repolarization of the cell as potassium moves out), and so any depolarization has to be strong enough to overcome that Stimulus Intensity is Coded by the Frequency of Action Potentials • So, remember how every action potential is identical to every other action potential for a given neuron? How do we transmit information through action potentials, then? o We communicate information through the frequency with which the action potentials occur o So, the stronger the graded potential, the higher the frequency of the many action potentials fired as a result of this graded potential Action Potentials are Conducted from the Trigger Zone to the Axon Terminal • The movement of an action potential through the axon at high speed is called conduction • Conduction is the flow of electrical energy from one part of the cell to another in a process that makes sure that any energy lost as a result of friction or leakage out of the cell is immediately replenished • So how does this flow process work? Well… o When a graded potential reaches threshold in the trigger zone, we have Na channels opening…And Na+ enters… o Now the cell depolarizes, and all this positive stuff comes in o But the positive charge in this area is repelled by the Na+ which has just entered, and the charge travels farther down the axon, and depolarizes the region down there! o Then the exact same thing happens – the Na+ channels open because of the depolarization, more Na+ enters, and so on • SEE FIGURE 8.14, PAGE 258 Larger Neurons Conduct Action Potentials Faster • Two key physical parameters influence the speed of action potential conduction: o The diameter of the neurons  Current flowing in an axon meets resistance from the membrane, so the larger the neuron is, the less percentage of the current which is being slowed down by the membrane o Resistance of the neuron membrane to current leak out of the cell  See next section Conduction is Faster in Myelinated Axons • An unmyelinated axon has low resistance to current leak because the entire axon membrane is in contact with the extracellular fluid, and so there are lots of ion channels which the current can leak through o SEE FIGURE 8.17, PAGE 260 • But myelinated axons limit the amount of membrane in contact with the extracellular fluid • As an action potential moves down an axon, the signal is “refreshed” every so often to compensate for current loss from the cell o Na channel open, and sodium entry reinforces the depolarization • So, in myelinated axons, this only happens at the nodes of Ranvier, where the axon membrane is “directly accessing” extracellular fluid – and so it seems like the action potential is jumping from node to node – this is called saltatory conduction • So, to explain the title of this section, conduction is more rapid in myelinated axons because when you’re having channels open all the time (as is the case with unmyelinated axons), the signal travels slower because opening the channels takes time o But myelinated axons only have channels open at the nodes of Ranvier, and so it goes much faster Cell-to-Cell Communcation in the Nervous System • The specificity of neural communication depends on several factors: the signal molecules secreted by neurons, the target cell receptors for these chemicals, and the anatomical connections between neurons and their targets, which occur in regions known as synapses Information Passes from Cell to Cell at the Synapse • A synapse is the point at which a neuron meets its target cell • There are three parts of the synapse: o The axon terminal of the presynaptic cell o The synaptic cleft o The membrane of the postsynaptic cell • Synapses are classified as either electrical or chemical, depending on the type of signal that passes between cells o Electrical synapses pass an electrical signal or current directly from the cytoplasm of one cell to another through gap junctions  Information can flow either way  Electrical synapses are mostly in the CNS  The primary advantage of these things are the rapid conduction of signals from cell to cell o Chemical synapses are the vast majority of synapses, and they use neurotransmitters to carry information from one cell to the next  Neurotransmitter synthesis can take place either in the nerve cell body or in the axon terminal  Neurotransmitters are “stored” in synaptic vesicles usually located near the synaptic cleft, just waiting to open and release their contents Calcium is the Signal for Neurotransmitter Release at the Synapse • The release of neurotransmitters into the synapse takes place by exocytosis o SEE FIGURE 8.20, PAGE 264, IT WILL SAVE YOUR LIFE! Integration of Neural Information Transfer Neural Pathways May Involve Many Neurons Simultaneously • Sometimes a single neuron branches, and its collaterals synapse on multiple target neurons – this is called divergence • Or we can have a single postsynaptic neuron having synapses with over 10,000 presynaptic neurons – this n-to-1 input is called convergence o So here we have input from multiple sources influencing the output of a single postsynaptic cell – the response of the postsynaptic cell is determined by the summed input from the presynaptic neurons • There is spatial summation, when several stimuli arrive at different dendrites of a single neuron, and they all meet together at the trigger zone to make an action potential o SEE FIGURE 8.26, PAGE 271 • Postsynaptic inhibition happens, though, when one of the graded potentials which enters a neuron is from an inhibitory neurotransmitter (creating an inhibitory graded potential), and in this case we can have it canceling out the effects of the other excitatory graded potentials • There is also temporal summation, when we have two graded potentials (could have arrived from the same presynaptic neuron) but they are so close together in time that the second contributes to the depolarization of the first, and it is enough to cause an action potential Efferent Division: Autonomic and Somatic Motor Control Introduction • The efferent side of the peripheral nervous system has two divisions: o The somatic motor neurons which control skeletal muscles  Generally, these things control voluntary movement o The autonomic neurons which control smooth muscle, cardiac muscle, many glands, and some adipose tissue  Generally, these things control involuntary movement The Autonomic Division • The reason for the prefix “auto” in this word is because the functions in the autonomic division are not under voluntary control • The autonomic division is subdivided into sympathetic and parasympathetic branches o The parasympathetic branches are more for “at rest” kinds of processes, like resting and digesting o The sympathetic branch is dominant in stressful situations, like automatically removing your hand from a fire, for example Antagonistic Control is a Hallmark of the Autonomic Division • The sympathetic and parasympathetic branches of the autonomic nervous system together display properties of homeostasis: o Maintenance of the internal environment o “Up-down” regulation by varying tonic control o Antagonistic control o Variable tissues responses that result from differences in membrane receptors • Most internal organs are under antagonistic control, where one autonomic branch is excitatory and the other branch is inhibitory o For example, sympathetic innervation increases heart rate and parasympathetic stimulation decreases it Autonomic Pathways Have Two Efferent Neurons in Series • The sympathetic and parasympathetic branches in the autonomic division share the same general structure • All autonomic pathways consist of two neurons in series o SEE FIGURE 11.4, PAGE 372 o The first neuron is called the preganglionic neuron, and it originates in the CNS and projects to an autonomic ganglion outside the CNS o Then the preganglionic neuron synapses with the second neuron in the pathway, the postganglionic neuron o Then the postganglionic neuron projects its axon to the target tissue • The autonomic ganglia is where the two efferent neurons synapse, and it is like an integrating center for preganglionic and postganglionic neurons o Divergence even occurs here – one preganglionic neuron can synapse here with 8 or 9 postganglionic neurons Sympathetic and Parasympathetic Branches Exit the Spinal Cord in Different Regions • There are two anatomical differences between sympathetic and parasympathetic branches o Firstly, the pathways originate from different places in the CNS o The locations of the autonomic ganglia are different as well o SEE FIGURE 11.5, PAGE 373 • Most sympathetic pathways originate in the thoracic and lumbar regions of the spinal cord o Sympathetic ganglia are found in a chain which runs close to the spinal column, and then there are long nerves which go to the spinal tissues o So that’s why sympathetic pathways have short preganglionic neurons and long postganglionic neurons • Many parasympathetic pathways originate in the brain stem originate in the brain stem o And parasympathetic ganglia are located on or near their target organs o And so we have long preganglionic neurons and short postganglionic neurons The Autonomic Nervous System Uses a Variety of Neurotransmitters and Modulators • All autonomic preganglionic neurons release acetylcholine (ACh) onto cholinergic nicotinic receptors • Most postganglionic sympathetic neurons secrete norepinephrine onto adrenergic receptors • Most postganglionic parasympathetic neurons secrete acetylcholine onto cholinergic muscarinic receptors • SEE FIGURE 11.7, PAGE 375 Autonomic Pathways Control Smooth Muscle, Cardiac Muscle, and Glands • The synapse between a postganglionic autonomic neuron and its target cell is called the neuroeffector junction • Autonomic axons are different from “normal” axons because they have a series of swollen areas at their distal ends, like beads spaced out along a string
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