02 membrane & action potentials

Examples of extracellular fluid

Blood plasma, lymph, CSF, synovial fluid, interstitial fluid

Membrane protein: ion channel

Allows diffusion of ions (passive)

Membrane protein: transporter

Active transport (uses ATP)

Ligand-gated channel

Opens when a chemical (ligand) binds

Voltage-gated channel

Opens in response to a change in membrane potential

Ligand (definition)

Anything that binds a receptor to exert an effect

Membrane potential

Difference in electrical charge across a membrane, in mV

Typical neuron resting membrane potential

About -70 mV (range -40 to -70 mV)

Where is Na+ concentrated at rest?

Outside the cell

Where is K+ concentrated at rest?

Inside the cell

Why does Na+ "want" to enter the cell at rest?

Higher concentration outside AND negative interior

Na+ entering the cell causes

Depolarization (more positive)

K+ leaving the cell causes

Hyperpolarization / repolarization (more negative)

Main contributor to negative charge inside the cell

Large intracellular proteins (anions that can't leave)

3 things that maintain RMP

1) K+ leaks out faster than Na+ leaks in; 2) trapped intracellular anions; 3) Na+/K+ ATPase

Na+/K+ ATPase ratio

3 Na+ out for every 2 K+ in

Why is the Na+/K+ pump electrogenic?

It pumps more cations out than in

Effect on RMP of blocking Na+ leak channels

RMP becomes more negative

Pump moving 2 Na+ out and 4 K+ in would cause

Depolarization (more positive inside)

How is the Na+/K+ pump cycle terminated?

Dissociation of ADP and Pi

Graded potentials (key features)

Short-lived, local changes from ligand-gated channels; can depolarize or hyperpolarize; can be summed

Where do graded potentials mainly occur?

Dendrites (and cell body)

Depolarizing graded potential is caused by

Ligand-gated Na+ channels opening (Na+ in)

Hyperpolarizing graded potential is caused by

Ligand-gated K+ channels opening (K+ out)

Spatial summation

Graded potentials summed because they are close in location

Temporal summation

Graded potentials summed because they are close in time

Conditions for summation of graded potentials

Close in space OR time

Graded potentials return to RMP via ___ feedback; APs are propelled by ___ feedback

Negative; positive

Threshold for an action potential

About -55 mV

Where are action potentials initiated?

Axon hillock (trigger zone)

Which cells can have action potentials?

Excitable cells: neurons (axon) and muscle

"All-or-nothing" means

An AP either fires fully or not at all; amplitude is always the same

Can action potentials be summed?

No

Action potential feedback type

Positive feedback (Na+ entry opens more voltage-gated Na+ channels)

AP resting phase: channels

All voltage-gated channels closed

AP depolarizing phase

Voltage-gated Na+ channels open → Na+ influx

AP repolarizing phase

Voltage-gated K+ channels open and Na+ channels close → K+ efflux

AP hyperpolarizing phase

Voltage-gated K+ channels still open (delayed closing) → dips below RMP

What causes the hyperpolarization (undershoot) phase?

Delayed closing of K+ channels

Order of AP phases

Resting → depolarizing → repolarizing → hyperpolarizing → resting

Absolute refractory period

During depolarization + repolarization; NO new AP possible

Relative refractory period

During hyperpolarization; new AP possible but needs a bigger stimulus

Purpose of the refractory period

Prevents backward propagation (one-way conduction), keeps APs discrete, lets the cell recover

What determines the strength of a nerve signal?

Frequency of action potentials

Blocking voltage-gated K+ channels would NOT change

The depolarization phase

If voltage-gated K+ channels didn't open

APs would not be followed by a wave of repolarization

An action potential is terminated by

Efflux of K+

Is an AP propagated by ligand-gated channels?

No - by voltage-gated channels

Conduction (propagation)

Movement of an impulse down an axon

Transmission (neurotransmission)

Movement of a signal across a synapse to another cell

Electrical synapse

Gap junctions directly connect cells; very fast; synchronizes cells

Locations of electrical synapses

Cardiac muscle (myocardium), smooth muscle (GI, uterus/myometrium), parts of the brain

Where are electrical synapses NOT found?

Skeletal muscle

Chemical synapse

Neurotransmitter crosses a synaptic cleft to bind receptors on the postsynaptic cell

What triggers NT release at the axon terminal?

Influx of Ca2+ through voltage-gated Ca2+ channels

Steps of a chemical synapse

AP arrives → voltage-gated Ca2+ opens → Ca2+ in → vesicles release NT → NT binds postsynaptic receptors → ligand-gated channels open → graded potential → AP if threshold reached

Axon terminals are filled with

Vesicles (containing neurotransmitter)

EPSP

Excitatory postsynaptic potential: depolarizes, moves cell toward threshold, can be summed

IPSP

Inhibitory postsynaptic potential: hyperpolarizes, moves cell away from threshold

More excitatory than inhibitory input but below threshold results in

An EPSP (no AP)

Lambert-Eaton syndrome effect

Presynaptic voltage-gated Ca2+ channels inactivated → NT not released

Are all neurotransmitters excitatory?

No

Catecholamines

Dopamine, norepinephrine (noradrenaline), epinephrine (adrenaline)

Is serotonin a catecholamine?

No

Serotonin is associated with

Sleep, dreaming, mood, nausea/vomiting

Neurotransmitter associated with allergic reactions

Histamine

Excitatory amino acid neurotransmitters

Glutamate and aspartate

Inhibitory neurotransmitters

GABA and glycine

Peptide NT involved in pain

Substance P

Peptide that stimulates uterine contraction and milk expression

Oxytocin

Enkephalins / endorphins

Opioid-like peptides that inhibit pain

Is histamine a peptide neurotransmitter?

No (it's a biogenic amine)

Inactivation (NT removal)

Enzyme breaks down the NT (e.g. acetylcholinesterase breaks down ACh)

Reuptake (NT removal)

NT taken back into the presynaptic neuron

Main removal method for catecholamines

Reuptake

SSRIs (e.g. fluoxetine)

Block serotonin reuptake → more serotonin in the synapse

Atropine

Blocks ACh receptors → decreases neurotransmission

What increases ACh levels in a cholinergic synapse?

More presynaptic action potentials

Increase in synaptic ACh → ___ neurotransmission

Increase

MAO inhibitors would NOT affect which NT?

Acetylcholine

Electromyography (EMG)

Measures electrical activity in muscle

Latency (EMG)

Delay between stimulation and contraction; used to calculate conduction velocity

Tetany

Prolonged contraction of skeletal muscle due to electrical stimulation

Why does the "death grip" occur?

Forearm flexors are stronger than extensors

Membrane protein: receptor

Binds signals (interaction)

Membrane protein: enzyme

Catalyzes reactions

Membrane protein: anchor

Structural stability

Membrane protein: identity marker

Immune recognition

Three stimuli that open gated channels

Chemical (ligand), electrical (voltage), mechanical

Intracellular fluid (ICF)

Fluid inside cells (cytosol); depends on precise regulation of ECF

What do blood, urine and CSF tests really measure?

Homeostasis (composition of extracellular fluid)

Why is one-way conduction especially important in the heart and GI tract?

They need unidirectional, progressive contraction

Duration and spread of a graded potential

About 10 ms; only a few microns along the membrane