L4 human phys

Resting membrane potential

The steady voltage difference across the membrane of an unstimulated cell, with the inside negative relative to the outside

What it means for a neuron to have a resting membrane potential

Electrical charges are separated across the plasma membrane so the intracellular side is negative relative to the extracellular side, creating stored electrical potential energy

Typical neuronal resting membrane potential

A typical neuron rests near negative 70 mV, although resting potentials vary among cells depending on their ion gradients and relative membrane permeabilities

Electrochemical gradient

The combined effect of an ion concentration gradient and the electrical gradient across the membrane that determines the direction an ion tends to move

Equilibrium potential

The membrane potential at which the electrical force on a particular ion exactly opposes its concentration gradient so there is no net movement of that ion

Nernst equation

Calculates the equilibrium potential for a single ion from its concentration gradient and charge

Ionic basis of the resting membrane potential

Resting membrane potential depends mainly on Na positive, K positive, and sometimes Cl negative concentration gradients together with the membrane permeability to each ion

Why potassium dominates resting membrane potential

Resting neurons have many more open K positive leak channels than Na positive leak channels, so the membrane is much more permeable to K positive and resting potential lies near the K positive equilibrium potential

Potassium movement at rest

K positive tends to leave the cell through leak channels down its concentration gradient, leaving the inside relatively negative

Sodium movement at rest

A small amount of Na positive enters through leak channels and pulls the membrane potential away from the K positive equilibrium potential toward the Na positive equilibrium potential

Role of the sodium potassium pump in resting potential

The Na positive K positive ATPase maintains Na positive and K positive concentration gradients by pumping 3 Na positive out and 2 K positive in. Its direct electrical effect is small but its indirect role is essential

Resting potential as a steady state

Na positive continuously leaks inward and K positive leaks outward, but the sodium potassium pump balances these movements so ion concentrations and membrane potential remain stable

How permeability affects membrane potential

Increasing permeability to an ion makes that ion exert a greater influence on membrane potential and moves the membrane potential toward that ion's equilibrium potential

Effect of increasing sodium permeability

More Na positive enters the cell and membrane potential moves toward the positive Na positive equilibrium potential, causing depolarization

Effect of increasing potassium permeability

More K positive leaves the cell and membrane potential moves toward the negative K positive equilibrium potential, generally causing hyperpolarization

Depolarization

A change in membrane potential that makes the inside of the cell less negative than at rest

Repolarization

Return of membrane potential toward its resting value after depolarization

Hyperpolarization

A change that makes membrane potential more negative than the resting level

Goldman Hodgkin Katz equation

Calculates membrane potential from the concentration gradients and relative membrane permeabilities of multiple ions rather than considering only one ion

GHK equation main principle

Each permeable ion pulls membrane potential toward its own equilibrium potential, and ions with greater permeability have a greater effect on the final membrane potential

Why GHK predicts different resting potentials in different cells

Different cell types have different relative permeabilities to Na positive, K positive, and Cl negative, so their weighted contributions to membrane potential differ

Major parts of a neuron

A neuron contains a cell body, dendrites, an axon hillock, an axon, axon branches, and axon terminals

Cell body of a neuron

Contains the nucleus and protein synthesis machinery and serves as the major metabolic and control center of the neuron

Dendrites

Highly branched processes that receive incoming signals from other neurons. Dendritic spines increase surface area for receiving synaptic input

Axon hillock and initial segment

Region where the axon begins and where graded potentials are integrated. Its high density of voltage gated Na positive channels makes it the usual trigger zone for action potentials

Axon

Long neuronal process that conducts outgoing electrical signals away from the cell body toward target cells

Axon terminals

End regions of axons that communicate with target cells and release neurotransmitters at chemical synapses

Synapse

Region where the axon terminal of a presynaptic neuron communicates with a postsynaptic target cell

Major parts of a chemical synapse

The presynaptic axon terminal contains neurotransmitter vesicles and voltage gated Ca2 positive channels, the synaptic cleft separates the cells, and the postsynaptic membrane contains neurotransmitter receptors

Presynaptic versus postsynaptic cell

The presynaptic cell releases the signal while the postsynaptic cell receives the signal through receptors

Major glial cells

Schwann cells, oligodendrocytes, satellite cells, astrocytes, microglia, and ependymal cells provide different forms of neuronal support

Schwann cells and oligodendrocytes

Schwann cells form myelin around axons in the PNS while oligodendrocytes form myelin in the CNS. One Schwann cell myelinates part of one axon while one oligodendrocyte can myelinate portions of several axons

Astrocytes

CNS glial cells that regulate extracellular ions and neurotransmitters, support neurons metabolically, contribute to the blood brain barrier, and help guide neuronal development

Microglia

Specialized immune and scavenger cells of the CNS

Ependymal cells

CNS glial cells that line fluid filled spaces and help regulate cerebrospinal fluid. The slides also identify them as a source of neural stem cells

Satellite cells

PNS glial cells associated with neuron cell bodies that provide support and are described in the slides as nonmyelinating Schwann type cells

Current flow

Movement of electrical charge. In neurons, ion movement through channels creates electrical current

Resistance

Opposition to electrical current. Higher resistance means less current can flow for a given voltage difference

Conductance

The ability of ions to flow through a membrane and is opposite to resistance. Opening more ion channels increases conductance and decreases resistance

Ohm's law

Voltage equals current times resistance. Therefore at a constant voltage, lowering resistance increases current and raising resistance decreases current

Membrane resistance and internal resistance

Membrane resistance opposes current leaking across the plasma membrane while internal resistance opposes current flow through the cytoplasm

Graded potentials

Local membrane potential changes whose amplitude varies with stimulus strength. They may depolarize or hyperpolarize, can summate, have no threshold or refractory period, and decrease in strength with distance

Action potentials

Rapid all or none depolarizations used for long distance signaling. They require threshold, use voltage gated channels, have refractory periods, and regenerate without decreasing in amplitude

Graded potentials versus action potentials

Graded potentials vary in size, can summate, decrease with distance, and have no threshold while action potentials are all or none, cannot summate, require threshold, have refractory periods, and propagate without decrement

Stimulus intensity in graded versus action potentials

A stronger stimulus creates a larger graded potential, but once threshold is reached action potential amplitude stays the same. Stronger stimuli are represented by greater action potential frequency rather than larger action potentials

Threshold potential

Critical membrane potential at which enough voltage gated Na positive channels open to initiate the positive feedback phase of an action potential

Action potential depolarization

Threshold opens many voltage gated Na positive channels. Na positive rapidly enters the cell and further depolarization opens additional Na positive channels through positive feedback

Action potential peak

As membrane potential becomes positive, voltage gated Na positive channels begin to inactivate and slower voltage gated K positive channels open

Action potential repolarization

Na positive permeability falls as Na positive channels inactivate while K positive permeability rises. K positive leaves the cell and drives membrane potential back toward negative values

Action potential afterhyperpolarization

Voltage gated K positive channels close slowly, so K positive continues leaving after resting potential is reached and temporarily makes the membrane more negative than resting

Return to resting membrane potential after an action potential

Voltage gated K positive channels close, voltage gated Na positive channels reset, and normal leak channel permeabilities again dominate membrane potential

Why Na positive and K positive channels have different roles in an action potential

Voltage gated Na positive channels open rapidly and then inactivate while voltage gated K positive channels open more slowly and close slowly, creating depolarization followed by repolarization and hyperpolarization

Absolute refractory period

Period when another action potential cannot occur regardless of stimulus strength because voltage gated Na positive channels are already open or inactivated and have not yet reset

Relative refractory period

Period after the absolute refractory period when another action potential is possible but requires a stronger stimulus because some Na positive channels remain unavailable and K positive channels are still open

Functions of refractory periods

They limit maximum action potential frequency, keep individual action potentials separated, and help prevent backward propagation along the axon

Action potential propagation

Local current from an active region depolarizes the adjacent membrane to threshold, opening voltage gated Na positive channels and regenerating the action potential farther along the axon

Myelin

Multiple layers of glial cell membrane that electrically insulate axons and increase membrane resistance, reducing current leakage across the membrane

How myelin speeds action potential conduction

Less charge leaks through myelinated membrane so local current spreads farther and brings the next node of Ranvier to threshold more rapidly

Nodes of Ranvier

Gaps between myelin segments with a high density of voltage gated Na positive channels where action potentials are regenerated

Saltatory conduction

Propagation in a myelinated axon in which action potentials are regenerated only at nodes of Ranvier, making the signal appear to jump from node to node

Why myelin improves efficiency

Because ions cross primarily at nodes, fewer ions must later be restored by membrane pumps, making myelinated conduction faster and less metabolically expensive

Electrical versus chemical synapses

Electrical synapses directly connect neighboring cells through gap junction channels while chemical synapses release neurotransmitter across a synaptic cleft to receptors on a postsynaptic cell

Chemical synaptic transmission sequence

An action potential reaches the terminal, depolarization opens voltage gated Ca2 positive channels, Ca2 positive enters, vesicles fuse with the presynaptic membrane, neurotransmitter is released, neurotransmitter crosses the cleft and binds postsynaptic receptors

Direct trigger for neurotransmitter release

Ca2 positive entry through voltage gated Ca2 positive channels activates regulatory proteins and triggers synaptic vesicle exocytosis

Neurotransmitter

Chemical messenger released from a presynaptic neuron that binds receptors on a target cell and alters its activity

Termination of neurotransmitter signaling

Neurotransmitter can be removed by reuptake into the presynaptic terminal, uptake into glial cells, diffusion away, or enzymatic degradation

Ionotropic receptor

Postsynaptic receptor that contains an ion channel in its structure so neurotransmitter binding directly changes ion flow and produces a relatively rapid response

Metabotropic receptor

Postsynaptic receptor that acts through G proteins or second messenger pathways to indirectly alter ion channels and cellular activity, generally producing slower and more complex effects

Fast versus slow synaptic responses

Fast responses commonly involve direct ionotropic receptor effects on ion channels while slower responses involve metabotropic pathways and intracellular signaling that can last longer

Excitatory postsynaptic potential or EPSP

A depolarizing graded potential that moves the postsynaptic membrane closer to threshold, commonly because opening cation channels produces greater Na positive influx than K positive efflux

Inhibitory postsynaptic potential or IPSP

A graded response that hyperpolarizes or stabilizes the postsynaptic membrane and reduces the chance of reaching threshold, commonly through opening Cl negative or K positive channels

Why opening potassium channels is inhibitory

Increased K positive permeability allows more K positive to leave the cell, moving membrane potential toward the negative K positive equilibrium potential and away from threshold

Why opening chloride channels can be inhibitory

Cl negative entry can hyperpolarize the cell or stabilize membrane potential near rest so incoming positive charge from excitatory synapses has less effect

Postsynaptic inhibition

An inhibitory neurotransmitter acts directly on the postsynaptic neuron and produces an IPSP or stabilizes its membrane potential, making an action potential less likely

Presynaptic inhibition

A modulatory neuron acts on the presynaptic terminal and decreases neurotransmitter release before the signal reaches the postsynaptic cell

Presynaptic versus postsynaptic inhibition

Presynaptic inhibition reduces how much neurotransmitter is released from the sending neuron while postsynaptic inhibition directly changes the receiving neuron's membrane potential or excitability