Physiology ch 12 neurons

Resistance

(1) In electrical circuits, the property that hinders the flow of electric current (charge movement) through a material, measured in ohms (Ω). It is the inverse of electrical conductance, the ease of current flow, measured in siemens. See also membrane resistance. (2) In heat transfer, the property that hinders dry heat transfer either through a material or between an animal and its environment. Speaking of the latter case, resistance to heat transfer is defined to be the difference between body temperature and ambient temperature divided by the rate of dry heat transfer between the animal and the environment. Contrast with thermal conductance. (3) See vascular resistance.

The equilibrium potential of Na+ could possibly be

+58 mV

A neuron is stimulated at several voltages to explore its membrane potential. From our recording above we know that the action potential threshold is ________ and we were least likely to expect an action potential initiation at ______

-30 mV, 6 ms

Imagine a cell with only two permeable ions: K+ (EK = -58 mV) and Na+ (ENa = +58 mV). 10,000 K+ channels are open, and 1 Na+ channel is open. What should be the approximate membrane potential at equilibrium?

-58 mV

Calculate the Nernst equilibrium potential for Cl- , assuming it is 10X more concentrated outside the cell.

-58mv

Calculate the Nernst equilibrium potential for K+ , assuming it is 10X more concentrated inside the cell

-58mv

Three ions are permeable in a cell, A, B, and C. Ea = +50 mV, Eb = -50 mV, Ec = 0 mV. 500 ion A channels are open. 10,000 ion B channels are open, and 20,000 ion C channels are open. What is the approximate membrane potential of the cell?

0 to - 50 mV

The Basis of the Membrane Potential The membrane potential is primarily caused by two factors

1) Ion gradients and channels: K+ dominates the resting potential because it is much more permeable than the other ions. 2) Active transport: Na+ / K+ ATPase maintains the Na+ and K+ gradients in the long term. Passive diffusion and active transport play a role in the maintenance of the membrane potential.

The equilibrium potential for a given ion depends on three factors:

1. Valance: 2. Direction of gradient: 3. Size of gradient: Note that the permeability of the ion is not relevan

How long would it take to change the concentration of sodium ions by half in the sciatic nerve at your big toe if you drastically changed sodium concentration in the neuron cell bodies in the spinal cord, if only diffusion were acting?

30 years

Calculate the Nernst equilibrium potential for Na+ , assuming it is 10X more concentrated outside the cell.

58mv

If the concentration gradient across the membrane is the same in both A and B, which should display the higher rate of diffusion?

A : more charge faster diffusion. rem. valence

Action potential

A brief electrical signal of about 100 mV across the cell membrane of a neuron or other excitable cell. It is initiated by a depolarization above threshold and is propagated to the end of the axon or cell. Also called a nerve impulse.

Hormone

A chemical substance, released by nonneural endocrine cells or by neurons, that is carried in the blood to distant target cells, where it exerts regulatory influences on their function. There are three main chemical classes of hormones: steroids, peptides or proteins, and amines.

Depolarization

A decrease in amplitude of the inside-negative electrical potential of a cell membrane toward zero. More generally, any increase in the inside positivity of a cell membrane, even if it exceeds zero

Which is true as the membrane potential moves farther away from the equilibrium potential?

A electromotive force increases

The resting membrane potential of a cell is -65 mV. The voltage threshold of the cell is -40 mV. The cell is electrically stimulated to reach -39 mV. What should happen?

A full action potential

Conductance

A measure of how easily electrical current will flow through a conductive pathway. Contrast with electrical resistance. See also thermal conductance.

Voltage

A measure of the potential energy present because of charge separation. Separated electrical charges exert electrostatic force on each other. These forces can cause charges to move, and when charges move, work is done. Voltage, also called electrical potential or potential difference, provides a measure of the rate of charge movement that can be achieved.

Patch-clamp recording

A method of measuring single-channel currents by sealing a glass capillary microelectrode to a patch of cell membrane. Other conformations of patch clamping can measure whole-cell current or voltage. See also single-channel current recording.

Neurotransmitter

A molecule that is used as a chemical signal in synaptic transmission.

Neuron

A nerve cell; the fundamental signaling unit of the nervous system, composed of a cell body and elongated processes—dendrites and axon—that carry electrical signals.

Motor neuron

A neuron that conveys motor signals from the central nervous system to the periphery to control an effector such as skeletal muscle.

Efferent neuron

A neuron that conveys signals from the central nervous system to the periphery, usually exerting motor control.

Interneuron

A neuron that is confined to the central nervous system and is therefore neither a sensory neuron nor a motor neuron.

Nonspiking neuron

A neuron that transmits information without generating action potentials.

Afferent neuron

A neuron, normally sensory, that conducts signals from the periphery into the central nervous system.

Axon

A process of a neuron specialized for conveying action potentials (usually) away from the cell body. An axon may be myelinated (ensheathed in myelin) or unmyelinated

Reflex

A simple, relatively stereotyped, but graded behavioral response to a specific stimulus.

Synapse

A specialized site of communication between two neurons, between a neuron and an effector, or between a non-neuronal sensory cell and a neuron.

Control system

A system that sets the level of a particular variable that is being controlled. To do so, it uses information from sensors to determine signals it sends to effectors that can modify the controlled variable. Control systems often (but not always) operate on negative feedback and are stabilizing

Oligodendrocyte

A type of ensheathing glial cell (non-neuron cell) in the vertebrate central nervous system

Schwann cell

A type of ensheathing non-neuronal glial cell found in the vertebrate peripheral nervous system. Schwann cells form, for example, the myelin sheath of myelinated axons.

Astrocyte

A type of glial cell (non-neuronal cell) of the vertebrate central nervous system that regulates extracellular ion concentrations and metabolically supports neurons. Important in the blood-brain barrier.

Graded potential

A voltage change that is variable in amplitude—that is, not all-or-none like an action potential. Examples include synaptic potentials and receptor potentials.

Hyperpolarization

A voltage change that makes a cell membrane potential more inside-negative (normally moves it further from zero).

An action potential is a momentary reversal of membrane potential from approximately -65 mV to +40 mV, lasting only 1-2 ms, followed by a restoration of the original membrane potential. Action potentials are all or none

Action potentials are triggered by any depolarization of the membrane beyond a critical value, called the voltage threshold.

The Action Potentia

Action potentials are voltage-dependent, all-or-none electrical signals Action potentials result from changes in membrane permeabilities to ions The molecular structure of the voltage-dependent ion channels reveals their functional properties BOX 12.1 Evolution and Molecular Function of Voltage-Gated Channels There are variations in the ionic mechanisms of excitable cells BOX 12.2 Optogenetics: Controlling Cells with Light, Matthew S. Kayser

The Propagation of Action Potentials

Action potentials propagate because the membrane's underlying permeabilities to ions are voltage-dependent. Local circuits of current flow spread the depolarization along the axon, depolarizing a new region to threshold. Behind an advancing action potential, Na+ channels remain inactivated long enough to prevent reexcitation by the local currents. The conduction velocity of an action potential depends on axon diameter, myelination, and temperature. Larger-diameter axons have higher conduction velocities because their length constants are longer, so local currents spread farther along the axon. Myelin greatly increases conduction velocity by increasing Rm (increasing the length constant) while decreasing Cm (preventing an increase in the time constant). Increasing temperature speeds the gating of channels so that the membrane responds faster to the local current

The Action Potential

An action potential is a voltage change—a brief, transient reversal of membrane potential from inside-negative to inside-positive. Action potentials are all-or-none responses to any depolarization beyond a voltage threshold and are each followed by a brief refractory period. Action potentials result from voltage-dependent changes in membrane permeability to ions. Depolarization first opens voltage-gated Na+ channels, allowing Na+ ions to flow in and further depolarize the membrane toward ENa. The voltage-gated Na+ channels rapidly become inactivated to terminate the rising phase of the action potential; then voltage-gated K+ channels open to repolarize the membrane. The effects of depolarization on membrane permeability to ions can be studied at the level of single channels by patch clamp, and at the whole-cell level by voltage clamp. Ongoing investigations are clarifying the molecular structures of voltage-gated channels. The principal protein subunit of a K+ channel is a single chain with six transmembrane regions; a K+ channel consists of four of these protein subunits around a central pore. Na+ and Ca2+ channels consist of a single polypeptide chain with four similar domains; each domain corresponds to one of the four subunits of the K+ channel. Functional attributes of the channels can be localized to particular regions of the proteins. Nonspiking neurons do not generate action potentials, and the ionic mechanisms of action potentials in excitable cells can vary. Calcium ions can make substantial contributions to action potentials in cardiac muscle cells and in some neurons. Other varieties of voltage-gated channels modify the excitable properties of neurons.

Electroneutral pump

An active-transport process that pumps charges across a membrane such that no difference of charge is created across the membrane; it therefore is not a current source. Contrast with electrogenic pump.

Electrogenic pump

An active-transport process that pumps net charge across a membrane, acting to generate an electric current across the membrane and to produce a voltage difference across the membrane. Contrast with electroneutral pump.

The Goldman Equation Calculates the membrane potential when more than one ion is permeable The Goldman-Hodgkin-Katz equation calculates membrane potential by taking into account the all ions

An approximation for membrane potential can be reached by simply calculating the potentials of the three most permeable ions: K+ , Na+ , and Cl- , but the permeability of each ion must be taken into account.

Capacitance

An electrical term meaning the ability of a capacitor or a capacitor-like structure, such as a cell membrane, to store electrical charges. A cell membrane acts like a capacitor because of its electrically insulating properties. Capacitance (C, in farads) is a measure of the amount of charge stored per unit of voltage. See capacitor.

Goldman equation

An equation that describes membrane potential in terms of the concentrations of and membrane permeabilities to more than one ion species.

Nernst equation

An equation used to determine the equilibrium electrical potential for a particular ion, given the ion concentrations on both sides of a membrane

Voltage clamp

An experimental method to measure ionic current flow by imposing a selected membrane potential on a cell and monitoring the exogenously applied current necessary to maintain that potential by bucking the ionic current.

Myelin

An insulating sheath around an axon, composed of multiple wrappings of glial cell membranes, that increases the velocity of propagation of action potentials.

Pacemaker potential

An intrinsic depolarization of a neuron or other excitable cell, leading to an action potential and thus making the cell spontaneously active.

Membrane Potential

At rest, a standard membrane potential is -65 or -90 mV

At which site are action potentials formed?

Axon hillock

If the concentration gradient across the membrane is the same in both A and B, which should display the higher rate of diffusion?

B: more openings larger rate of diffusion

Imagine a cell with only two permeable ions: K+ (EK = -58 mV) and Na+ (ENa = +58 mV). 7,500 K+ channels are open and 5,000 Na+ channels are open. What should be the approximate membrane potential at equilibrium?

Between 0 and -58 mV.

The Ionic Basis of Membrane Potentials

Cell membranes have passive electrical properties: Resistance and capacitance Resting membrane potentials depend on selective permeability to ions: The Nernst equation Ion concentration differences result from active ion transport and from passive diffusion Membrane potentials depend on the permeabilities to and concentration gradients of several ion species: The Goldman equation Electrogenic pumps also have a small direct effect on Vm

The Ionic Basis of Membrane Potentials

Cell membranes have properties of electrical resistance and capacitance, which allow them to maintain a voltage (membrane potential) and regulate current flow across the membrane. Cells have inside-negative resting membrane potentials. The passive electrical properties of membranes determine how membrane potentials change with time (the time constant, τ) and with distance (the length constant, λ). Membrane potentials depend on selective permeability to ions. Any ion species to which the membrane is permeable will tend to drive the membrane potential toward the equilibrium potential for that ion. The Nernst equation calculates the equilibrium potential of a single ion species in terms of its concentrations on both sides of the membrane. All cells have higher concentrations of K+ inside than outside, higher concentrations of Na+ outside than inside, and higher concentrations of Cl- outside than inside. Ion concentrations inside and outside cells are maintained by active ion pumps, as well as by passive Donnan-equilibrium effects. Membrane potentials depend on the permeabilities to and concentration gradients of several ion species: The resting membrane is dominated by permeability to K+, so the resting membrane potential is near EK. The Goldman equation describes how changing the membrane permeability of an ion species changes the membrane potential. In addition to their major role of maintaining the nonequilibrium concentrations of ions, electrogenic ion pumps generate a current that makes a small, direct contribution to Vm. In addition, only those ions that are freely diffusible contribute to Vm, so corrections for bound ions may be necessary.

Glial cells

Cells in an animal's neural tissue (e.g., brain) other than neurons. Glial cells are considered support cells, ensheathing neuronal processes or regulating the metabolism of neurons. They may play secondary roles in signaling and integration. Also called neuroglia.

Excitable cells

Cells that can generate action potentials because their cell membranes contain voltage-gated channels, notably neurons and muscle cells.

If the equilibrium potential for Cl is -58 mV, which is true of Cl- at a membrane potential of 0 mV? (assume that the membrane is permeable to Cl- )

Cl should diffuse into the cell

Imagine two identical cells. The equilibrium potential for Cl is -58 mV and the membrane potential is 0 mV in both cells. Only a few Cl- channels open in cell A. All Cl- channels are open in cell B. Which should be true of the two cells?

Cl- will diffuse faster in cell B, but the membrane potential of the cells will be equal when Cl reaches equilibrium.

Imagine that the equilibrium potential for Clis -58 mV, the membrane potential is 0 mV, and all Cl- are channels open. What should be true when Clreaches electrochemical equilibrium? (assume that this cell has no other ion gradients)

Concentration does not change: The membrane potential should be -58 mV

The Physiology of Control: Neurons and Endocrine Cells Compared

Control by a nervous system involves neurons that send axons to discrete postsynaptic cells. Neurons generate rapidly conducting action potentials to control the specific targets on which they end. They exert fast, specific control by releasing neurotransmitters at synapses. Endocrine cells release hormones into the bloodstream to mediate endocrine control. All body cells are potential targets of a hormone, but only those with specific receptors for the hormone actually respond. Hormonal control is slower, longer lasting, and less specific than neural control.

Ionic Current EMF and conductance determine current

EMF increases as Vm gets farther from Ex (i.e. more ions flow through the channels when the membrane potential is far from the equilibrium potential)

falling phase

Falling: Positive charge inside cell opens voltage gated K+ channels, increasing K+ permeability and leading to outflux of K+ ions (due to high EMF). Na+ channels are automatically inactivated.

A stimulus causes membrane potential to exceed the voltage threshold, opening voltage-gated Na+ channels. Rising:

High Na+ EMF results in influx of Na+ , and cell depolarizes (becomes more positively charged).

undershoot

Hyperpolarization: K+ channels remain open for a few milliseconds, drawing the membrane potential toward Ek , resulting in 'hyperpolarization' or membrane potential that is more negative than resting potential. Membrane potential nears Ek , voltage-gated K+ channels close, and the membrane potential approaches resting level.

Relative refractory period

In a neuron or other excitable cell, the brief period following an action potential during the generation of another action potential is relatively difficult because voltage conditions are further than usual from threshold.

The Nernst Equation The electrochemical equilibrium for an ion species

In electrochemistry, the Nernst equation is an equation that relates the reduction potential of an electrochemical reaction (half-cell or full cell reaction) to the standard electrode potential, temperature, and activities (often approximated by concentrations) of the chemical species undergoing reduction and oxidation. It is the most important equation in the field of electrochemistry

The Goldman Equation

In resting state, K+ is the most permeable ion, and thus the resting membrane potential is close to Ek

Node of Ranvier

In the myelin sheath surrounding an axon, spaces between adjacent glial cells. These interruptions in the sheath allow propagation of action potentials by saltatory conduction.

Here, Na+ is located on the outside of the cell, and K+ on the inside. Anions are located on both sides. The membrane is only permeable to K+ . The inside of the membrane should become relatively _________ compared to the outside.

In this case this cell would be negative. Notice the k+ leaving. Which means more A- is present. in general More negative inside= negative. More positive inside = positive

Absolute refractory period

In, e.g., a neuron, the time during and immediately after an action potential in which the voltage threshold is infinite. Thus, no depolarization can exceed threshold and no new action potentials can be initiated.

Which ion would have the fastest flux rate in a resting neuron (Vm = -65 mV)?

Ion 3: E3 = +50 mV; 100 channels open

If the equilibrium potential for K+ is -58 mV, which is true of K+ at a membrane potential of 0 mV? (assume that the membrane is permeable to K+ )

K+ should diffuse out of the cell

Which is analogous to an electrical capacitor in a cell membrane?

Lipid Bilayer

he Propagation of Action Potentials

Local circuits of current propagate an action potential Membrane refractory periods prevent bidirectional propagation The conduction velocity of an action potential depends on axon diameter, myelination, and temperature BOX 12.3 Giant Axons

Membrane Potential

Membrane potential is generated from selective permeability (resistance and capacitance). • Membrane potential is the sum of many singular ion potentials (Nernst potentials), mostly K+ , Na+ , and Cl- , multiplied by their permeabilities. • The Nernst potential of K+ dominates the membrane potential because K+ has the greatest permeability. • Permeability is a function of the number of open ion channels

overshoot

Membrane potential nears Na+ Nernst potential and Na+ influx slows down. The inside of the cell is positively charged.

A negatively charged ion has a negative equilibrium potential. What is its concentration?

More concentrated on the outside of the cell

The velocity of conduction increases with nerve fiber diameter

Myelination in vertebrates allows small diameter nerves to conduct as fast as giant axons in squid. Propagation is unidirectional because of inactivation of Na+ channels

To conduct a signal at relatively comparable speeds, a smaller diameter axon needs more ____________, to compensate for the _____________ as axon size decreases.

Myelination, Increased resistance

If the equilibrium potential for Na+ is +58 mV, which is true of Na+ at a membrane potential of 0 mV? (assume that the membrane is permeable to Na+ )

Na+ should diffuse into the cell

Absolute and relative refractory periods are caused, respectively, by:

Na+ channels inactivated, K+ channels still open

If the equilibrium potential for K+ is -58 mV and +58 mV for Na+, which is true the ions at a membrane potential of -58mV? (assume that the membrane is permeable to both ions)

Na+ should diffuse into the cell

Neurons Are Organized into Functional Circuits in Nervous Systems The Cellular Organization of Neural Tissue

Neurons are structurally adapted to transmit action potentials Glial cells support neurons physically and metabolically

The Cellular Organization of Neural Tissue

Neurons are the principal cells of nervous systems. They have long processes (dendrites and axons) that are specialized to receive signals from other neurons (via dendrites) and to generate and propagate action potentials (via axons). Glial cells are the support cells of the nervous system. Schwann cells (in the PNS) and oligodendrocytes (in the CNS) form sheaths around neuronal axons, including insulating myelin sheaths around myelinated axons. Astrocytes surround capillaries and act as metabolic intermediaries between neurons and their circulatory supply. Microglial cells serve immune and scavenging functions

The Physiology of Control: Neurons and Endocrine Cells Compared

Neurons transmit electrical signals to target cells Endocrine cells broadcast hormones Nervous systems and endocrine systems tend to control different processes

Myelin provides electrical insulation, speeding the AP and minimizing "cross talk" between neighboring neurons

Nodes of Ranvier allow Na+ to enter the cell at spatial intervals. The velocity of conduction increases with nerve fiber diameter

The ____________ enable an action potential to be regenerated in vertebrate neurons, while _____________ minimizes leak of the signal

Nodes of Ranvier, Myelination

Saltatory conduction

Propagation of action potentials in a spatially discontinuous manner along a myelinated axon by jumping from one node of Ranvier to another.

What is the major determinant of the potential of a membrane in a standard electrolytic medium?

Selective permeability of different ions across the membrane

The skin of frogs is quite permeable to water, and because frogs that live in freshwater are hypertonic to the surrounding water, water will tend to enter the animal across the skin by osmosis. To avoid overhydration, frogs produce large amounts of very dilute urine, but this causes loss of Na+ in the urine. Frogs do not drink, so how do they replace the lost Na+? Can they obtain it from pond water across their skin? To test this hypothesis, you place a piece of skin from the abdomen of a frog in diffusion chamber, such that the apical surface of the skin is facing half-chamber A and the basolateral surface is facing half-chamber B. You then fill both half-chambers with frog Ringer's solution (containing 85 mM NaCl, 25 mM NaHCO3, 2 mM KCl and 1 mM CaCl2). Under these conditions, you measure a net Na+ flux from A to B of 1.5 nanomole/sec. Based on the results of your experiment, which of the following statements is most likely FALSE? Replacing the Ringer's solution with "potassium-free Ringer's solution" (87 mM NaCl, 25 mM NaHCO3 and 1 mM CaCl2) would reduce the Na+ flux rate The Na+ flux rate could also be calculated from the Nernst equilibrium potential for Na+, provided Na+ conductance (gNa) is known Reversing the orientation of the skin (such that the apical surface faces half-chamber B) would reverse the direction of net Na+ flux. The K+ flux rate in this system is -1.0 nanomole/sec

The Na+ flux rate could also be calculated from the Nernst equilibrium potential for Na+, provided Na+ conductance (gNa) is known

Single-channel current recording

The ability to record the ionic currents flowing though a single membrane channel by attaching a microelectrode to the membrane surrounding the channel. See also patch-clamp recording.

Inactivation (in ion transport)

The closing of an ion channel in response to a stimulus such as membrane depolarization. This occurs in a time-dependent manner.

Rule of membrane potential club:

The concentration gradient of ions does not change

Integration

The coordination of input signals, as by summing, to provide a harmonious control of output. Cellular integration refers to the integration of signals within a cell, and physiological integration refers to the integration of sensory, central nervous system, and endocrine signals for harmonious control of effectors in the bod

Voltage threshold

The critical value of membrane depolarization that is just enough to trigger an action potential

Hodgkin cycle

The cycle that explains the rising phase of an action potential: Depolarization opens voltage-gated Na+ channels, increasing membrane permeability to Na+. The resulting inflow of Na+ further depolarizes the membrane, opening more Na+ channels.

Passive electrical properties

The electrical properties of a cell that do not involve a change in membrane ion permeability, and thus involve no change in electrical resistance.

Membrane resistance (Rm)

The electrical resistance of a membrane per unit of area. Many cell membranes have an Rm of about 1000 ohm × cm2 (1000 Ω × cm2). (The unit is ohm × cm2 rather than ohm/ cm2 because membrane resistance decreases as area increases, as more resistances are added in parallel.)

Presynaptic terminal

The ending of a neuronal axon at the presynaptic side of a synapse.

Electric current

The flow of electric charges. In animals, electric current is carried by ions, unlike the flow of electrons in manmade circuits.

Which description of how the action potential is propagated through a neuron is most accurate?

The influx of Na+ at one end of the neuron causes more Na+ to influx in neighboring channels, which repeats until Na+ influx occurs toward the other end of the neuron.

What is an equilibrium potential?

The membrane potential at which a solute will be at equilibrium

Equilibrium potential

The membrane potential at which an ion species is at electrochemical equilibrium, with concentration-diffusion forces offset by electrical forces so that there is no net flux of that ion species across the membrane.

A cell at resting membrane potential is triggered by a neurotransmitter to open up Na+ ion channels. Which of the following is then least accurate

The membrane will hyperpolarize

Resting membrane potential (Vm)

The normal electrical potential across the cell membrane of a cell at rest.

Ionic hypothesis

The organizing principle that the distributions of ions across the membrane of a cell, and the permeability of the membrane to ions, determine membrane potentials and cellular electrical events.

Action potentials propagate from the axon hillock to the terminals

The passive spread of action potentials through a cell decreases exponentially with distance.

Cell body

The portion of a neuron that contains the cell nucleus; also called the soma or perikaryon.

Dendrite

The receptive element of most neurons, which receives synaptic input from other neurons. Most neurons have many, multiply branching dendrites, in contrast to one sparsely branching axon

Internode

The region of a myelinated axon that lies between two nodes of Ranvier and is covered by a myelin sheath.

Axon hillock

The region of a neuronal cell body from which an axon originates; often the site of impulse initiation.

Axon initial segment

The region of an axon close to the cell body: often the site if impulse initiation.

The Basis of the Membrane Potential

The sodium potassium pump generates and maintains the ionic gradients of Na+ and K+ into the cell. The pump uses ATP to actively transport Na+ out of cell and K+ into cell. Although there is a substantial concentration gradient of sodium across the membrane very little net diffusion of Na+ occurs because there are few open sodium channels. Large number of open potassium channels allow net outflow of K+. Membrane is only weakly permeable to chloride and large anions, this outflow of K+ results in a net negative charge inside the cell.

Two identical cars, 1 and 2, are parked in the USF parking lot during physiology class. It is 85 ºF outside and the cars were in the sun during the entire class. Afterwards, the owners of the cars return to the parking lot. The owner of car 1 opens a window to let the car cool down, and the owner of car 2 opens all of the four doors. What will be true of the two cars as they cool down?

The temperature will drop faster in car 2, but the cars will ultimately drop to the same temperature

Neuron doctrine

The theory that the nervous system, like other organ systems, is composed of discrete cellular elements (neurons) that are its fundamental signaling elements.

Time constant (τ)

The time required for an exponential process to reach 63% of completion. In neurophysiology, it is a measure of the time needed to change membrane potential and is proportional to the product of resistance and capacitance.

Reticular theory

Theory in the nineteenth century, now discarded, that cells in the CNS were cytoplasmically connected to each other in a syncytial reticulum. Contrast neuron doctrine

Why is ion permeability important for calculating membrane potential, but not electrochemical equilibrium?

Time : equilibrium is timed state

Innervate

To provide neural input.

Given the ion concentrations shown in the figure below, how should Na+ be transported from inside to outside the cell? (assume no electrical gradient)

active transport

ions

are atoms or molecules that have a net charge (i.e. they have unequal numbers of protons and electrons)

Neurons

are cells transmit information quickly and accurately to specific locations in the body, via action of electrically excitable membranes. Neurons receive electrical signals, which act to depolarize the membrane, which is propagated down the axon toward the terminals.

When is a neuron membrane potential closest to its Nernst equilibrium for Na+?

at the peak of the action potential

Action Potentials

can't go past equilibrium

The neuron structure labelled B is a ___________ and performs the function of _________________.

dendrite, recieving information from receptors or neurons

Which of the following would most likely increase the likelihood of generating an action potential in a cell?

inhibition of K+ leak channels

Which of the following most ensures a one way movement of signal along a neuron?

inhibition of ion channels

The absolute refractory period

is the moment immediately following an action potential where a neuron cannot stimulate another AP because of Na channel inactivation.

Resistance (R)

is a measure of the impedance or difficulty of electrical flow (current). The inverse is conductance (G)

Capacitance (C)

is the ability to store a voltage (i.e. insulate between two conducting bodies)

Voltage (V)

is the difference in electrical charge across space, which represents an electrical potential.

Current (I)

is the net movement of charges (e.g. ions) through space

MS is a disease which damages the myelin sheath of neurons. What problem would this cause with signal propagation?

lower resistance to leakage

The Nernst Equation tells us that if a positively charged ion is ten times more concentrated inside the cell, its Nernst equilibrium potential is

negative

During the falling phase:

only voltage-gated K+ channels are open

You observe a cell with a membrane potential of approximately -20 mV. Which phase(s) of an action potential are potentially happening?

repolarization or depolarization

Action potential propagation rate is not influenced by:

stimulus strength

At rest:

the membrane is most permeable to K+ , although the permeability to all ions is relatively low