Class 11 Transport of Small Molecules Across Cell Membranes Channels – Passive Transport

Channels – Passive Transport

• Aquaporins

• Ion Channels

• Ion Channels and Membrane Potential

• Ion Channels and Nerve Cell Signaling

Ion Channels and Nerve Cell Signaling

• Action potentials – allow rapid long-distance communication along axons

• Action potentials - mediated by voltage-gated cation channels

• Voltage-gated $Ca^{2+}$ channels in nerve terminals à convert an electrical signal into a chemical signal

• Transmitter-gated Ion channels in postsynaptic membrane à convert chemical signal back into an electrical signal

• Neurotransmitters – can be excitatory or Inhibitory

• Most psychoactive drugs - affect synaptic signaling by binding to neurotransmitter receptors

• Ion channel proteins in plasma membranes of neurons = critical components of machinery that lets us think, act, feel, learn, remember, walk, talk, LIVE!

Action Potentials - Allow Rapid Long-Distance Communication Along Axons

• No matter what meaning of signal

  • Visual info from eye

  • Motor command to a muscle

  • One step in complex network of neural processing in brain

• Form of signal always same à changes in electrical potential across neuron’s plasma membrane

Measuring Membrane Potentials

• (a) Measurement of resting membrane potential requires two electrodes:

  • Inserted inside cell (recording electrode)

  • One placed in fluid surrounding cell (reference electrode)

  • Differences between two are amplified by voltage amplifier and displayed

• (b) Measurement of an action potential requires four electrodes:

  • One in axon for stimulation

  • Another in axon for recording

  • and two in fluid surrounding cell for reference

  • Stimulating electrode is connected to a pulse generator which delivers a pulse of current to axon when switch is momentarily closed

  • Nerve impulse this generates is propagated down axon and can be detected a few milliseconds later by recording electrode

Action Potential

• Establishment of resting membrane potential and its dependence on ion gradients and ion permeability are properties of almost all cells

• Unique feature of electrically excitable cells à their response to membrane depolarization

• A nonexcitable cell that has been temporarily and slightly depolarized à will return to its original resting membrane potential

• Electrically excitable cell depolarized to same degree à will respond with an action potential

Action Potentials – allow rapid long-distance communication along axons

1. Neuron - receives signal that initiates a change in membrane potential

2. Signal - relayed to next cells in pathway forming a neural circuit

3. Action potential aka nerve impulse - traveling wave of electrical excitation that carries message without weakening from one end of neuron to other at up to 100 m/s

Early experiments establishing membrane excitability done on giant axon of squid

• Very large diameter so can insert electrode and record its electrical activity

• Can measure axon’s membrane potential

Loligo pealeii (longfin inshore squid) has some of largest nerve cell axons in nature

• Squid giant axons

  • Length: can reach 10 cm (~4 in.) in length

  • Diameter: >100 X diam of mammalian axon ~pencil lead in width

• In general, the larger the diameter of axon, the faster signals can travel along its length

Scientists can study nerve cell excitability using an isolated axon from squid

• Measure resting membrane potential and monitor action potentials induced when axon is electrically stimulated

• An electrode can be inserted into cytoplasm (axoplasm) of a squid giant axon

• Reverses polarity – inside of cell becomes positive relative to outside of cell

  • Axoplasm - cytosol within an axon

  • A nerve - a tissue composed of bundles of axons

Measuring Membrane Potential

• Can measure membrane potential (voltage difference between inside and outside of axon) as an action potential sweeps past tip of electrode à Trigger action potential by applying brief electrical stimulus to one end of axon

• Stimulus must exceed a certain threshold

• Action potential is all or nothing response

Action Potential Shape

• Shape of action potential depends on concentration of $Na^+$ outside squid axon

• Action potentials - recorded when the external medium contains 100%, 50%, or 33% of the normal extracellular concentration of $Na^+$

Action Potentials – Mediated by Voltage-Gated Cation Channels

1. When neuron stimulated à membrane potential of plasma membrane shifts to less negative value (toward zero)

2. If depolarization is large enough, voltage-gated $Na^+$ channels in membrane will open transiently, allowing $Na^+$ to enter cell down electrochemical gradient

3. Influx of positive charge depolarizes membrane further, (makes it even less negative) thereby opening additional voltage-gated $Na^+$ channels and causing further depolarization

4. Within ~millisecond, membrane potential in local region of neuron’s plasma membrane has shifted from resting value of ~ -60 mV to ~40 mV

Action Potential Trigger

• An action potential is triggered by a depolarization of a neuron’s plasma membrane

  • Resting membrane potential in this neuron is –60 mV

  • stimulus that depolarizes plasma membrane to ~ –40 mV (threshold potential) is applied

  • This depolarizing stimulus is sufficient to open voltage-gated $Na^+$ channels in membrane and thereby trigger an action potential

  • As membrane rapidly depolarizes further, membrane potential (red curve) swings past zero, reaching +40 mV before it returns to its resting negative value as action potential terminates

  • green curve shows how membrane potential would simply have relaxed back to resting value after initial depolarizing stimulus if there had been no amplification by voltage-gated ion channels in plasma membrane

Voltage-Gated Channels Inactivation

• If voltage-gated channels continued to respond to depolarized membrane potential, cell would get stuck with most of its $Na^+$ channels open

• BUT What happens is that voltage-gated channels have an automatic inactivating mechanism that within a millisecond or so causes them to randomly adopt a special inactivated conformation in which channel is closed even though membrane is still depolarized

• The $Na^+$ channels remain in this inactivated state until membrane potential has returned to its resting negative value

Three Distinct States of Voltage-Gated $Na^+$ Channel: Closed, Open, Inactivated

• A voltage-gated $Na^+$ channel can flip from one conformation to another, depending on membrane potential

Voltage-Gated $Na^+$ Channel: Closed

• CLOSED

  • When the membrane is at rest and highly polarized, positively charged amino acids in the voltage sensors of the channel (red bars) are oriented by the membrane potential in a way that keeps the channel in its closed conformation.

  • When the membrane is depolarized, the voltage sensors shift, changing the channel’s conformation so the channel has a high probability of opening.

  • But in the depolarized membrane, the inactivated conformation is even more stable than the open conformation, and so, after a brief period spent in the open conformation, the channel becomes temporarily inactivated and cannot open.

  • When membrane is at rest and highly polarized: positively charged amino acids in voltage sensors of channel (red bars) are oriented by membrane potential in a way that keeps channel in its closed conformation

Voltage-Gated $Na^+$ Channel: Open

• OPEN

  • When membrane is depolarized (red arrow): voltage sensors shift, changing channel’s conformation so channel has a high probability of opening

Voltage-Gated $Na^+$ Channel: Inactivated

• INACTIVATED

  • But in depolarized membrane, inactivated conformation is even more stable than open conformation: so, after a brief period spent in open conformation, channel becomes temporarily inactivated and cannot open

Voltage-Gated $Na^+$ Channel Cycle Status

• A voltage-gated
  $Na^+$ channel returns to original conformation (closed) after membrane has repolarized

Action Potential - Rise and Fall

• How three distinct states of voltage-gated $Na^+$ channel: closed, open, inactivated contribute to rise and fall of an Action Potential

• Action Potential - triggered here by a brief pulse of electric current (arrow), which partially depolarizes membrane

Action Potential - Repolarization

• During action potential, voltage-gated $Na^+$ channels are helped by opening of voltage-gated $K^+$ channels to return depolarized axonal membrane to its resting negative potential

• Rapid outflow of $K^+$ through voltage-gated $K^+$ channels brings membrane back to its negative resting state much more quickly than outflow of $K^+$ out of $K^+$ leak channels could achieve alone

• Action potential spreads outward as a traveling wave from initial site of depolarization to axon terminals

• Once action potential has passed, $Na^+$ pumps in axon plasma membrane work to restore $Na^+$ and $K^+$ ion gradients to resting cell levels

• Human brain - consumes ~20% of its total energy to power these $Na^+$ pumps

Action Potential Propagation

• An action potential propagates along length of an axon

• Red arrows: Changes in $Na^+$ channels and consequent flow of $Na^+$ across membrane alters membrane potential and gives rise to traveling action potential

• Blue: Region of axon with a depolarized membrane

• Action potential can only travel forward, away from site of depolarization, because $Na^+$ channel inactivation in aftermath of an action potential prevents advancing front of depolarization from spreading backward

Voltage-Gated $Ca^{2+}$ Channels

• Voltage-gated $Ca^{2+}$ channels in nerve terminals convert an electrical signal into a chemical signal

  • When action potential reaches terminals, signal needs to be relayed to target cells, usually neurons or muscle cells

  • Done at specialized junctions called synapses

  • Usually presynaptic and postsynaptic cells separated by narrow synaptic cleft usually ~20 nm across

  • Electrical signal can’t cross synaptic cleft

  • To get across gap, electrical signal converted into chemical signal in form of small, secreted signal molecule – a neurotransmitter

  • Neurotransmitters stored in nerve terminals within membrane- enclosed synaptic vesicles

Neurotransmitters and Synapses

• Electron micrograph of two nerve terminals forming synapses on a single nerve cell dendrite (center) in mammalian brain

• Neurotransmitters carry signal across synaptic cleft that separates presynaptic and postsynaptic cells

• Neurotransmitter in presynaptic terminal contained within synaptic vesicles, which release neurotransmitter into synaptic cleft

• Neurons connect to their target cells at synapses

• Both presynaptic and postsynaptic membranes thickened and highly specialized at synapses

Neurotransmitter Release

• When action potential reaches nerve terminal, some of synaptic vesicles fuse with plasma membrane, releasing neurotransmitter in synaptic cleft

• This involves activation of another type of voltage-gated channel: voltage-gated $Ca^{2+}$ channels in plasma membrane of presynaptic nerve cell terminal

  • $[Ca^{2+}]$ much higher outside cell, and channel opens and $Ca^{2+}$ rushes into cell

  • This causes increases in cytosolic $[Ca^{2+}]$ and triggers fusion of synaptic vesicles with plasma membrane which releases neurotransmitter into synaptic cleft

• Thanks to voltage-gated $Ca^{2+}$ channels electrical signal converted into chemical signal

Electrical Signal Conversion

• Electrical signal- converted into secreted chemical signal at nerve terminal

• When an action potential reaches a nerve terminal, it opens voltage-gated $Ca^{2+}$ channels in plasma membrane, allowing $Ca^{2+}$ to flow into terminal

• Increased $Ca^{2+}$ in nerve terminal stimulates synaptic vesicles to fuse with plasma membrane, releasing their neurotransmitter into synaptic cleft by exocytosis

Transmitter-Gated Ion Channels

• Transmitter-gated ion channels in postsynaptic membrane - convert chemical signal back into an electrical signal

• Released neurotransmitter rapidly diffuses across synaptic cleft and binds to neurotransmitter receptors concentrated in plasma membrane of postsynaptic target cell

• Neurotransmitters then have to be removed either by

  • Enzymes that destroy them

  • Pumps that return them to nerve terminal or to non-neuronal cells

• Rapid removal of neurotransmitter limits duration and spread of signal and makes certain that when presynaptic cell is quiet, postsynaptic cell will also fall quiet

Neurotransmitter Receptors

• Neurotransmitter Receptors Can be various types:

  • some mediate slow effects in target cell

  • others trigger rapid responses (milliseconds)

• The rapid responses depend on receptors that are transmitter-gated ion channels (aka ion-channel-coupled receptors)

  • subclass of ligand-gated ion channels

  • convert chemical signal carried by neurotransmitter back into an electrical signal

  • Channels open transiently in response to binding of neurotransmitter, changing ion permeability of postsynaptic membrane

  • This causes change in membrane potential

Chemical Signal Conversion

• Chemical signal converted into an electrical signal by postsynaptic transmitter- gated ion channels at synapse

• Released neurotransmitter binds to and opens transmitter-gated ion channels in plasma membrane of postsynaptic cell

• Resulting ion flows alter membrane potential of postsynaptic cell, thereby converting chemical signal back into an electrical one

Synaptic Signaling

• If change in membrane potential is large enough, postsynaptic membrane will depolarize and à trigger action potential in postsynaptic cell

Neuromuscular Junction

• Neuromuscular junction – specialized synapse formed between a motor neuron and a skeletal muscle cell

• Well-studied and in vertebrate organisms: neurotransmitter acetylcholine stimulates muscle contraction by binding to acetylcholine receptor, a transmitter-gated ion channel in skeletal muscle cell’s membrane

Acetylcholine Receptor

• Acetylcholine receptor in plasma membrane of vertebrate skeletal muscle cells opens when it binds the neurotransmitter acetylcholine

• This transmitter-gated ion channel composed of five transmembrane protein subunits

• Two acetylcholine-binding sites

• Acetylcholine binds, protein conformation changes, and $Na^+$ can flow across membrane down electrochemical gradient and depolarize membrane

Neurotransmitters - Excitatory or Inhibitory

• Neurotransmitters can

  • excite

  • inhibit a postsynaptic cell

• Receptor that recognizes neurotransmitter determines how postsynaptic cell will respond

Neurotransmitters - Excitatory

• Chief receptors for excitatory neurotransmitters such as:

  • Acetylcholine

  • Glutamate

    • are ligand-gated cation channels

• Neurotransmitter binds and channel opens to allow influx of $Na^+$

• This depolarizes plasma membrane and tends to activate postsynaptic cell, encouraging it to drive an action potential

Neurotransmitters - Inhibitory

• Chief receptors for inhibitory neurotransmitters such as

  • Gamma-aminobutyric acid (GABA)

  • Glycine

    • are ligand-gated $Cl^-$ channels

• Neurotransmitter binds and channel opens allowing $Cl^-$ to enter cell

• Influx of $Cl^-$ inhibits postsynaptic cell by making its plasma membrane harder to depolarize

Excitatory and Inhibitory Signals

• Neurons receive excitatory (green) and inhibitory (red) signals

• An action potential - may be generated at axon hillock if combined effects of membrane potentials induced by these synapses result in depolarization above threshold potential

• Both temporal and spatial summation contribute to likelihood that an action potential will be initiated

  • Temporal summation – if two action potentials fire in rapid succession at presynaptic neuron, postsynaptic neuron will not have time to recover from first action potential and postsynaptic neuron will be more depolarized

  • Spatial summation – a neuron, with connections to many different neurons, integrates numerous small depolarizations that occur over its surface into one large depolarization

Ion Channel Review

Synapses

• (BioFlix™ How Synapses Work)

Neurotransmitter Receptor Toxins

• Toxins - that bind to neurotransmitter receptors have dramatic effects on animals

  1. Curare – causes muscle paralysis by blocking excitatory acetylcholine receptors at neuromuscular junction

     • Used by surgeons to relax muscles during surgery and used historically to poison arrows

  2. Snake venom – contains neurotoxins (e.g. cobratoxin) that bind to excitatory acetylcholine receptors at neuromuscular junction– paralyzes prey

  3. Strychnine – common ingredient in rat poisons causes muscle spasms, convulsions, death by blocking inhibitory glycine receptors on neurons in brain and spinal cord

Psychoactive Drugs

• Most psychoactive drugs – affect synaptic signaling by binding to neurotransmitter receptors

• Many drugs used to treat insomnia, anxiety, depression, schizophrenia act by binding to transmitter-gated ion channels in brain

  1. Examples:

     • Sedatives, tranquilizers such as barbituates, Valium, Ambien, Restoril bind to GABA-gated $Cl^-$ channels

     • Their binding makes it easier for GABA to open channel and neuron is more sensitive to GABA’s inhibitory action

Psychoactive Drugs - Antidepressants

• Antidepressant Prozac blocks $Na^+$-driven symport responsible for reuptake of excitatory neurotransmitter serotonin, increasing serotonin in synapses that use it

• Not known why boosting serotonin elevates mood