Vertebrate Physiology - Ch 4

Excitable cells are cable of

Undergoing transient, rapid changes in membrane potential

Types of excitable cells

Nerve cells and muscle cells

What do neurons do?

Recieve, process, intiate, and trasmit messages

Membrane potential definition

Separation o f charges across plasma membrane

What is membrane potential measured in?

Millivolts

How does membrane potential work?

Attractive forces cause separate pos/neg charges to accumulate along inner/outer surface of membranes

These charges each have potential to do work

All living cells have membrane potential with excess negative charges on ____

Inside of membrane

What allows a resting membrane potential?

Differences in distribution and permeance of certain ions

Is Na+ greater in the extracellular or intracellular fluid?

Extracellular fluid

Is K+ greater in the extracellular or intracellular fluid?

Intracellular fluid

How are concentration differences maintained in resting membrane potential?

Na+/K+ pump

Different solubilites and affinity for proteins

What is the equilibrium potential for K+?

-90 mV

What is the equilibrium potential for Na+?

+61 mV

What is the resting membrane potential (mV)?

-70 mV

Greater permeability of plasma membrane →

Greater tendency for ion to drive membrane potential towards its own equilibrium

Polarization definition

Charages separated across plasma membrane

Depolarization:

Decrease in potential → membrane less negative

Symbol for membrane potential

Vm

Repolarization

Return to resting potential after depolarization

Hyperpolarization

Increase in potential → membrane more negative

Changes in Vm result from…

Ion movement through ion channels in plasma membrane

When are leak channels open?

All the time

When are gated channels open?

They open/close in response to certain stimuli

Triggering events for gated channels

1) Voltage

2) Chemicals or ligands

3) Mechanical stimulation

4) Thermal stimulation

What do voltage-gated channels respond to?

Changes in membrane potential

*These are crucial to action potentials

What do chemically gated channels respond to?

Chemical binding messenger

Four steps for cellular response through a chemically gated channel

1) Extracellular messenger binds to receptor

2) Messenger binds → channel opens

3) Ions enter cell

4) Entry of ions causes desired result inside cell

What do mechanically gated channels respond to?

Stretching or mechanical deformation

Ex) Touch, hearing, sensory transduction

What do thermally gated channels respond to?

Local changes in temperature

What are graded potentials?

Local changes in membrane potential that occur in varying amounts

The stronger a triggering event of a graded potential →

Greater current entering/exiting cell

Examples of graded potentials

Depolarization or hyperpolarization

How are graded potentials spread?

By passive current flow

What’s important about passive current flow?

Its magnitufe diminished with distance from the origin

When a graded potential occurs locally in a nerve or muscle cell membrane, this means that the rest of the membrane is at..

Resting potential

Active area definition

Temporarily depolarized region

Current definition

Any flow of electrical charges

Direction of current flow designates …

The direction in which a positive charge is moving

Depolarization spreads by _____, away from _____

Local current flow; point of origin

Conductors have

Low resistance

Insulators have

High resistance

The lipid bilayer has

High resistance

How do currents cross membranes?

Through ion channels

Current loss across the plasma membrane leads to

Decremental spread of graded potential

What are the four physiologically relevant graded potentials?

1) Postsynaptic potentials

2) Receptor potentials

3) End-plate potentials

4) Pacemaker potentials

Action potential definition

Brief, rapid, large changes in membrane potential

Action potentials serve as

Long-distance signals

What are the events that occur during action potential?

1) Depolarization of threshold potential (-50 to -55 mV)

2) At threshold potential → explosive depolarization (Vm rises to 30 - 40 mV)

3) Membrane respolarizes back to resting potential (-70 mV)

4) Transient hyperpolarization (-80 mV)

What’s responsible for permeability changes during action potential?

Voltage-gated ion channels

What two gates do voltage-gated Na+ channels have?

Activation gate and inactivation gate

What form of gate is an activation gate and what does it do?

Hinge; guards channel

What form of gate is an inactivation gate and what does it do?

Ball and chain; blocks channel opening

What form of gate does the voltage-gated K+ channel have?

Hinge

At resting potential, which channels/gates are open/closed?

K+ leak channels and Na+ inactivation gates open

Voltage-gated Na+ and K+ channels and Na+ activation gates closed

Which gate opens when the membrane reaching its threshold potential?

Na+ activation gates

→ Allows Na+ to enter cell → depolarization

At the peak of action potential, which gates open/close?

Na+ inactivation gate close

K+ activation gate open

What does the input zone of a neuron do?

Receives and integrates incoming signals

What do the dendrites of neurons do?

Increase surface area for receiving signals from other nerve cells

What is the trigger zone?

Site on neurons where action potential is initiated

What’s importnat about the axon hillock?

It has abundant voltage-gated Na+ channels

What does the conducting zone of a neuron do?

It conducts action potentials away from cell body

Axon definition:

Single extension from cell body

What does the output zone do?

Releases chemical messengers to affect nearby cells

Axon terminal definition

Highly branched ending of axons

What are the steps of contiguous conduction of action potentials?

1) Depolarizing current spreas from site of Vm to inactive area around it

2) Threshold reached in inactive area → generates new action potential

3) Action potentials move along length of axon without loss of amplitude

What’s imporant about contiguious conduction of action potentials?

It provides long-distance signals without distortion or attenuation

What is the purpose of the refractory period?

Ensures unidirectional propagation of action potential

Absolute refractory period definition

Time when membrane’s unresponsive to further stimulation

What causes the absolute refractory period?

Na+ inactivation gates close

Relative refractory period definition

Time when second action potential can be produces, but only by a stronger than normal triggering event

What leads to the releative refractory period?

Slow closing of K+ activation gates and slow opening of Na+ inactivation gates

Myelinated axon definition

Axons of myelinated fibers wrapped by myelin-forming cells along length of axon

Nodes of ranvier

Exposed regio nbetween myelin-forming cells

What do the nodes of ranvier contain?

High concentrations of voltage-gated Na+ channels

Do myelinated or unmyelinated fibers propagate action potentials faster?

Myelinated fibers

How do action potentials move along myelinated fibers?

Action potential jumps from node to node → saltatory conduction

→ Conserves energy

How do large diamter fibers propagate action potentials more rapidly?

Lower resistance to current flow

Synapse definition

Specialized junctions between neuron and target cells

What is the presynaptic neuron?

A signaling cell

What is the postsynaptic cell?

Target cell

Ex) Neurons, gland cells, muscle cells

How do electrical synapses signal?

Gap junctions

How do chemical synapses signal?

Neurotransmitters released into synaptic cleft

What’s special about neurotransmitters?

Slower than electical synapses → synaptic delay

Operate in one direction only

Allow for various signaling events

When is the neurotransmitter stored?

In synaptic vesicles in synaptic knob

Five steps of synapse function:

1) Action potential reaches axon terminal of presynaptic cell

2) Ca2+ enters synaptic knob (presynaptic axon terminal)

3) Neurotransmitter released by exocytosis into synaptic cleft

4) Neurotransmitter binds to receptors of postsynaptic neuron

5) Binding of neurotransmitter to receptor opens that specific channel

Types of neuron-to-neuron synapses

Excitatory synapses and inhibitory synapses

How do excitatory synapses work?

Binding of neurotransmitter opnes nonspecific cation channels (Na+ and K+)

In excitatory synapses, the electrochemical gradient favors the ___ movement of ___ which leads to depolarization.

Inward; Na+

What does EPSP stand for?

Excitatory postsynaptic potential

How do inhibitory synapses work?

Binding of neurotransmitter opens specific K+ or Cl- channels

→ K+ efflux leads to hyperpolarization

→ Cl- movement buffers membrane potential

Three ways neurotransmitters removed from synaptic cleft

1) Diffuse away from synaptic cleft or

2) Inactivated by specific enzyme or

3) Transporte back into axon terminal by reuptake carriers

What does IPSP stand for?

Inhibitory postsynaptic potentials

Temporal summation

Summation of PSPs produced by repeated firing of presynaptic neuron

Spatial summation:

Sum of PSPs originating simultaneously from several different presynaptic inputs

Do EPSPs and IPSPs work with or against each other?

Work against each other

Grand postsynaptic potential (GPSP)

Composite of all EPSPs and IPSPs occurring at about same time

Neuromuscular junction

Synapse between motor neuron and skeleton muscle fiber

Where can you find the terminal bouton of the axon terminal

Shallow depression in muscle cell membrane (motor end plate)

Where is acetylcholine (ACh) stored?

In synaptic vesicles in terminal bouton

Process of neuromuscular transmission (5 steps)

1) ACh released by motor neuron

2) Chemically-gated ion channels in motor end plate serve as ACh receptor

3) Binding leads to depolarization (EPP)

4) Action potential initiated → spreads via contiguous conduction

5) Action potential leads to contraction of muscle fiber