NEUROSCIENCE: RESTING MEMBRANE POTENTIAL

What is membrane potential (Vm)?

The voltage across a neuron's membrane at any moment, measured inside relative to outside.

What is an excitable membrane?

A membrane whose voltage can change rapidly to produce electrical signals.

Which two tissues have excitable membranes?

Nervous tissue and muscle tissue.

What is resting membrane potential?

The membrane voltage when a neuron is at rest.

What typical resting membrane potential is given in the notes?

Approximately -65 mV; the inside is more negative than the outside.

What did Hodgkin and Huxley record in 1939?

An early intracellular recording of the squid giant axon action potential.

What is an action potential?

A rapid change in membrane potential used for neuronal signaling.

Does an action potential diminish with distance?

No; it propagates along the axon without decreasing in amplitude.

Which action potential characteristics are relatively fixed?

Size and duration under given conditions.

How is information encoded in action potentials?

Through the frequency of action potentials rather than their individual size.

How does membrane polarity change during the positive phase of an action potential?

The inside becomes more positive than the outside.

What is another name for the knee-jerk reflex?

The patellar tendon reflex.

What is the basic knee-jerk reflex pathway?

Stimulus activates a sensory neuron; the signal enters the spinal cord; a motor neuron activates muscle.

Which neuronal types participate in the knee-jerk reflex circuit?

Sensory neurons, motor neurons, and spinal inhibitory interneurons.

What does the inhibitory interneuron help do in the knee-jerk reflex?

Inhibit the opposing muscle so the movement can occur.

Why is water a polar solvent?

It has an uneven distribution of electrical charge and dissolves many polar or charged substances.

Where is water found around a neuron?

In the cytosol and extracellular fluid.

Why is oxygen partially negative in water?

It attracts the shared electrons more strongly than hydrogen.

What charge do hydrogen atoms have in water?

A partial positive charge.

What holds the atoms of water together?

Covalent bonds.

What is an ion?

An atom or molecule with a net electrical charge.

What determines ionic charge or valence?

The difference between the numbers of protons and electrons.

What is a cation?

A positively charged ion.

What is an anion?

A negatively charged ion.

What does monovalent mean?

Having a charge magnitude of 1, such as Na+, K+, or Cl-.

What does divalent mean?

Having a charge magnitude of 2, such as Ca2+.

What is an ionic bond?

Electrical attraction between oppositely charged ions.

Why are ions important in neurons?

Their distribution and movement produce resting and action potentials.

What is a sphere of hydration?

Water molecules surrounding an ion.

What do hydration spheres do?

Help separate dissolved ions and reduce direct interactions between them.

What does hydrophilic mean?

Water-loving; polar or charged substances interact with and often dissolve in water.

What is a hydrophilic example?

Salt.

What does hydrophobic mean?

Water-fearing; nonpolar substances do not dissolve readily in water.

What is a hydrophobic example?

Oil.

What is the neuronal membrane made of?

An amphipathic phospholipid bilayer with embedded proteins.

What does amphipathic mean?

Having both hydrophilic and hydrophobic regions.

How are phospholipids arranged in the bilayer?

Hydrophilic heads face water; hydrophobic tails face inward toward each other.

How do membrane properties affect neuronal signaling?

Selective permeability contributes to resting membrane potential and action potentials.

What are proteins made of?

Amino acids joined into polypeptides.

What are the main parts of an amino acid?

An alpha carbon attached to an amino group, carboxyl group, hydrogen, and R group.

What bonds join amino acids?

Peptide bonds.

What is primary protein structure?

The amino acid sequence.

What is secondary protein structure?

Local structures such as alpha helices and beta sheets.

What is tertiary protein structure?

The overall three-dimensional shape of one polypeptide, including interactions among R groups.

What is quaternary protein structure?

Multiple polypeptide subunits assembled together.

What are ion channels?

Membrane-spanning proteins forming pathways for ions to cross the membrane.

Why do channel proteins contain polar and nonpolar R groups?

Different regions interact with water, ions, and the hydrophobic membrane interior.

What is ion selectivity?

A channel's ability to conduct particular ions preferentially.

What helps create ion-channel selectivity?

The pore structure and amino acid residues that interact with ions.

What is channel gating?

Opening and closing of a channel.

What activates voltage-gated channels?

Changes in membrane voltage.

What activates ligand-gated channels?

Binding of a chemical ligand.

What are ligand-gated channels also called in these notes?

Receptor channels.

What activates mechanically gated channels?

Mechanical forces such as pressure or stretch.

How do channels affect membrane permeability?

They determine which ions can cross and how readily they cross.

What are ion pumps?

Membrane-spanning proteins using energy to transport ions against their electrochemical gradients.

What energy source do the pumps in these notes use?

ATP breakdown.

Why are ion pumps important?

They maintain ion concentration gradients needed for neuronal signaling.

What does the sodium-potassium pump move per cycle?

3 Na+ out and 2 K+ in, using ATP.

What gradients does the sodium-potassium pump maintain?

High Na+ outside and high K+ inside.

Does the sodium-potassium pump move ions with or against their concentration gradients?

Against their concentration gradients.

Where does the plasma membrane calcium pump transport Ca2+?

Out of the cytosol into extracellular fluid.

What is diffusion?

Net movement from higher concentration to lower concentration.

What two conditions allow concentration-driven ion diffusion across a membrane?

A concentration gradient and membrane permeability to that ion.

Is a concentration gradient alone enough for ion movement across a membrane?

No; a permeable pathway must also exist.

What is electrical current?

Movement of electrical charge.

How do electrical forces influence ions?

Opposite charges attract and like charges repel.

What is voltage?

An electrical potential difference that can drive charge movement.

What is Ohm's law?

V = IR; equivalently, I = gV because g = 1/R.

What do V, I, R, and g mean?

Voltage, current, resistance, and conductance, respectively.

How are resistance and conductance related?

They are reciprocals: R = 1/g and g = 1/R.

What happens when conductance increases?

Resistance decreases.

What happens when conductance decreases?

Resistance increases.

What units commonly describe neuronal voltage?

Millivolts (mV).

What units commonly describe neuronal resistance?

Megaohms (MΩ).

What units commonly describe neuronal current?

Nanoamperes (nA) or picoamperes (pA).

What units commonly describe neuronal conductance?

Nanosiemens (nS) or picosiemens (pS).

What is a chemical gradient?

A difference in an ion's concentration across the membrane.

What is an electrical gradient?

An electrical force on ions caused by the voltage difference across the membrane.

What is an electrochemical gradient?

The combined chemical and electrical forces acting on an ion.

Which major ion is more concentrated inside a neuron?

K+.

Which major ions are more concentrated outside a typical resting neuron?

Na+, Ca2+, and Cl-.

Which channels strongly influence resting membrane potential?

Potassium channels that permit K+ movement at rest.

How does K+ leaving help make the inside negative?

Positive charge leaves, making the inside more negative until electrical forces oppose further net outward movement.

How does the sodium-potassium pump support resting potential?

It maintains the K+ and Na+ concentration gradients.

Why is the membrane compared to a battery?

Charge separation across it creates an electrical potential difference.

What is equilibrium potential (Eion)?

The voltage at which an ion's electrical force exactly balances its concentration-driven force.

What happens at an ion's equilibrium potential?

No net movement or net current of that ion.

Does no net movement mean ions stop moving entirely?

No; movement in opposite directions balances.

Which way does the K+ chemical gradient drive K+?

Outward, because K+ concentration is higher inside.

Which way does a negative interior electrically attract K+?

Inward.

When is EK reached?

When the inward electrical force on K+ balances its outward concentration-driven force.

What does EK mean?

Potassium equilibrium potential.

What does ENa mean?

Sodium equilibrium potential.

What does the Nernst equation calculate?

The equilibrium potential for one ion.

What does the Nernst equation consider?

Ion charge, temperature, gas constant, Faraday's constant, and the outside-to-inside concentration ratio.

What is the general Nernst equation?

Eion = (RT/zF) ln([ion]outside/[ion]inside).

What is z in the Nernst equation?

The ion's valence, including its sign.

What is T in the Nernst equation?

Absolute temperature in kelvin.

What are R and F in the Nernst equation?

The gas constant and Faraday's constant.