Action potentials

Neurons

  • Neurotransmitters (fast)

  • Neuromodulators (slow)

  • Neurohormones (very slow)


Anatomy of a neuron

  Neurons are excitable cells and can be pushed out of resting state into an “action potential” - a brief but large change in membrane potential


Neurons: information Flow

   Neurons are excitable (negative state)  cells and can be pushed out of resting state into an “action potential” - a brief but large change in membrane potential

  Information flow through neurons: Dendrites → cell body → axons

  • Dendrites: collect electrical signals

  • Cell body: integrates incoming signals and generates outgoing signal to axon

  • Axon: passes electrical signals to dendrites of another cell or to an effector cell


Electrical vs chemical information

    Electrical signaling within a neuron                          Chemical signaling between neurons


Neuron structure

  • Different shapes of neurons reflect their function

    • Motoneurons (motor neurons)

      • Large with long axons to stimulate muscles

      • The cell body of a motor neuron is approx. 100 microns (0.1 millimeter) in diameter and the axon is about 1 meter in length

      • So the axon of a motor neuron is 10,000 times as long as the cell body is wide. 

    • Sensory neurons

      • Have varied shapes to respond to environmental stimuli, such as light, odor or touch

    • Interneurons

      • Small axons

      • Receive input from and send input to other neurons within a region


Some terminology

  • An electrical signal or message between neurons. The electrochemical communication is the basis for all nerve functions

  • When the electrical signal travels down the presynaptic neuron to the postsynaptic neuron

  • When the electrical signal reaches the end of the axon (pre-synaptic neuron), synaptic vesicles open and release chemical  neurotransmitter into synapse

  • Action potential

    • Large, fast, long distance spread of membrane potential change that travels along the axon

    • Same magnitude and same pattern every time ; Travels one direction

    • Rapid reversal of the membrane potential that briefly makes the inside of the neuron positive with respect to the outside.

    • Applying larger depolarization does not lead to a larger action potential (amplitude is always the same)

    • “All or none” - fires at full amplitude or not at all

    • Stronger stimuli can lead to multiple action potentials


3 phases of action potential

  • Depolarization (rising phase)

    • Membrane becomes less polarized as positive ions move IN (inside the cell becomes more positive). Membrane potential goes from negative → positive

  • Repolarization (falling phase)

    • Membrane returns to resting value after signal is complete

  • Hyperpolarization (undershoot)

    • Membrane comes more polarized as positive ions move OUT (IC - more negative). Membrane potential goes from positive → negative


Electrical Signals

   What happens if we apply hyperpolarizing or depolarizing stimuli to a neuron via electrodes?

  • Hyperpolarizing (more negative)

  • Adding a hyperpolarizing stimulus to the membrane produces an immediate response that mirrors the stimulus pulse

  • The greater the stimulus, the greater the response

  • The farther away you are recording, the smaller the membrane response decays (graded potential)

  • Depolarizing 

    • Applying a depolarizing stimulus works in the same way to produce local graded potentials up to a point

    • At -40mV, you reach threshold and an action potential or ‘spike’ occurs


Ionic mechanisms of an Action Potential

  • The resting membrane potential is dependent on K+ ions

  • The action potential is dependent on Na+ ions

    • voltage - gated Na+ channels initiate the action potential

    • During the resting state,the conductance for K+ ions is 50-100 times greater than the conductance for Na+. This is due to leakage of K+ ions through the leak channels

    • At the onset of the action potential, Na+ sodium channels open and allow up to 5000-fold increase of Na+ conductance.

    • The inactivation process than closes the Na+ channels

    • The onset of the action potential also triggers the voltage-gating of the K+ channels, causing them to open at the time the Na+ channels close

    • This produces a 30-fold increase in K+ conductance

    • At the end of the action potential, the return of the membrane potential to the negative state causes the K+ channels to close slowly 


Steps of an Action Potential

  1. Leaky (non-gated) K+ channels are open and voltage-gated Na+ channels are closed at rest

  2. A depolarizing force brings the membrane potential closer to threshold, as voltage gated Na+ channels begin to open

  3. At threshold, ALL voltage-gated Na+ channels open causing Na+ ions to flood the cell, rapidly changing the membrane potential to ~+40mV - the action potential

  4. Na+ channels stay open for < 1 ms, then automatically close at +30mV. Voltage gated K+ channels open and K+ exits to help repolarize, and even hyperpolarize the cell

  5. All gated channels close, cell returns to resting potential with the help of leak K+ channels and the Na+/K+ pump


Action potential: voltage gated Na+ channels

  Voltage gated Na+ channels have two gates: an activation gate and an inactivation gate

  • Activation gate closed at rest ; opens at -50mV to +30mV

  • Inactivation gate closes at +30mV

  • The activation gate opens quickly when the membrane is depolarized, and allows Na+ to enter. 

  • However, the same change in membrane potential also causes the inactivation gate to close

  • The closure of the inactivation gate is slower than the opening of the activation gate

  • As a result, the channel is open for a very brief time (from the opening of the activation gate to the closure of the inactivation gate)


Steps on an action potential pt.2

  1. Membrane is at rest at -65mV to 70 mV

  2. The membrane is at rest at -70mV, but a depolarizing stimulus triggers an ion channel to open. Ions pass in and alter the membrane’s charge…

  3. At -40 to -55mV, voltage gated sodium channels open! A flood of positively charged sodium ions enter the cell and it becomes rapidly positively charged or depolarized

  4. At +30mV, sodium channels close and voltage-gated potassium channels open, allowing positively charged potassium ions to leave the cell, called repolarization. 

  5. Voltage-gated K+ channels also open, causing K+ to also leave the cell - the neuron briefly below its resting state of -70mV (hyperpolarization). The entire process lasts 1-2 ms (1/1000th of a second)


Mechanisms of an Action Potential

  • All-or-none property - size (amplitude) of the action potential is independent of the size (magnitude) of the stimulus

  • Membrane permeability changes to specific ions

    • First, permeability to Na+ (Pna) ramps up to depolarize

    • Second, permeability to K+ (Pk) ramps up to repolarize


Refractory period

     Absolute refractory period: brief period of time immediately after the action potential is produced in which the membrane is completely insensitive to further stimulation. This is due to the Na+ channel inactivation period, during which a new spike cannot be triggered by any stimuli. 

      Relative refractory period: reduced sensitivity of the membrane following the absolute refractory period; only strong stimulation, well beyond threshold, can produce another action potential. K+ channels are open, and only a very large depolarization will cause a signal, because as the Na+ flows in, in an attempt to create an action potential, the K+ will flow out, short-circuiting the attempt. 

  • Refractory periods limit action potential frequency


Propagation of action potentials

  • Propagation along the axon

  • 3 types of conduction

    • Electrotonic

    • Active

    • Saltatory

  • Action potentials are actively propagated along the neuron

  • The AP is a spike of depolarizing electrical activity, which regenerates along the length of the axon

Electrotonic conduction

  • Generates depolarization in a small area

  • Passive conduction, decays with distance from initiation site

  • Spread of a graded response

Active conduction

  • Action potentials propagate from axon hillock to axon terminals

  • Action potentials are constantly regenerated

    • They do NOT diminish over the length of the axon

  • Opening of voltage-gated Na+ channels depolarizes adjacent regions of the axon, eliciting action potential

  • Region behind advancing action potential is refractory and prevents back propagation

  • Myelin sheaths speed up the conduction

Saltatory conduction

  • Large myelinated axons - highest conduction velocities

    • Large diameter axon has less resistance

    • Myelin insulation reduces the ability of current to leak out of the axon

    • Na+ current travel faster and further, needing less regeneration

    • Action potentials boosted at nodes of Ranvier (between myelins) (packed with Na+ channels)

      • Latin saltare “to jump”

  • Saltatory conduction - combination of electrotonic & active conduction

  1. The ap is gelerated by voltage gate Na+ channels (active conduction_)

  2. The ap passively (electronic conduction) travels ot the next myelin gap (Node of Ranvier)

  3. Where its regenerated (active conduction)


Myelin

  • Produced by glial cells *oligodendrocytes - CNS, Schwann cells - PNS)

  • Made up consists of multiple layers of closely opposed glial membranes (lipids, proteins, cholesterol - basis of “white matter”)

  • Increases action potential speed and reduced energy required for repeated firing

  • Underlies many “leaps” in cognitive development

  • Myelination continues into teenage years

    • Unmyelinated axon conduction velocities - 0.5 to 10 m/s

    • Myelinated axons can conduct up to 150 m/s


Summary

  • Neurons are excitable cells

  • Basic features of the action potential (AP)

  • Mechanisms underlying the action potential

  • The refractory period

  • Propagation of action potentials

Outline

  • Review

  • Synapse and synaptic transmission

    • Terminology

    • Steps of neurotransmitter release

    • EPSPs and IPSPs

    • Spatial and temporal summation

    • Termination of synaptic transmission

  • Next Class: receptors


Review

  • Membrane potential: ions, ionic movement, permeability

  • Action potential: measurement

    • Reference electrode in ECF

    • Recording electrode in axon

    • Compare differences

    • Rmp: -70mV

  • Resting membrane potential

    • Initial phase, threshold

    • Depolarization

    • Repolarization

    • hyperpolarization/relative refractory period 

    • Return to rest


Synapse

  • Pre synaptic cell is said to “innervate” the post synaptic cell


Some terminology: chemical transmission

  • Neurotransmitter (NT)

    • Chemical released from the presynaptic axon terminal into the synapse

  • Presynaptic

    • Region of a synapse that releases neurotransmitter

  • Postsynaptic

    • Region of a synapse that receives and responds to neurotransmitter


Neuronal communication: synapses

  1. Axo-dendritic: axon to dendrite

  2. Axo-somatic: axon to cell body

  3. Axo-axonic: axon to axon

  4. Dendro-dendritic: dendrite to dendrite


Signaling steps

  1. Electrical signal(graded potentials) in the dendrite and cell body

  2. Chemical neurotransmitter (NT) travels across the synaptic gap to the dendrites/cell body of the next cell

  3. Electrical signal(graded potential) in the dendrite and cell body

  4. IF threshold is met at the axon hillock, an action potential is triggered

  5. Ap travels down the axon to the axon terminal

  6. This triggers the release of Nt

  7. NT travels across the synaptic gap to the dendrites/cell body of the next cell


Electrical signaling types

  • Action potential: a large, fast, long distance spread of potential change that is the same magnitude every time it occurs

  • Graded potential: a small, local change in membrane potential that can vary in size and does not travel far

    • Can build up to trigger a larger response


Signaling steps: NT Release

  1. Action potential arrives at axon terminal

  2. Voltage-gated Ca2+ channels open

  3. Ca2+ enters the presynaptic neuron

  4. Ca2+ signals to neurotransmitter vesicles

  5. Vesicles move to the membrane and dock

  6. Neurotransmitters released via exocytosis

  7. Neurotransmitters bind to receptors

  8. Signal initiated in postsynaptic cell


Neurotransmitters - basic criteria

  • Synthesized in presynaptic neuron and stored in axon terminals(vesicles)

  • Released when action potentials reach axon terminals

  • Recognized by receptors on postsynaptic membrane

  • Causes changes in the postsynaptic neuron

  • Blocking its release interfered with ability of presynaptic cell to affect postsynaptic cell


Calcium control of vesicles

  • Calcium mobilizes vesicles from reserve pool

  • Calcium leads to exocytosis of vesicles from ‘readily releasable pool’


Synapses - steps of the NT Release

  • How does the postsynaptic cell react to the action potential?

    • It depends on what type of ion channels are opened on the postsynaptic cell

    • With graded potentials - changes in membrane potential that passively reflect the stimulus

    • Effect on postsynaptic depends on receptors

      • Temporary receptors: they always change

  • Graded potentials: 

    • If + ions flow through, then the cell becomes depolarized and has small excitatory postsynaptic potentials (EPSPs) - more likely to action potential

    • If - ions, the cell becomes hyperpolarized and has inhibitory postsynaptic potentials (IPSPs)


EPSPs and IPSPs

  • EPSP - Excitatory Postsynaptic Potential

    • Na+ influx locally depolarizes dendrite

    • Increases probability of action potential firing

    • E.g. synapses using glutamate as neurotransmitter

  • IPSP - inhibitory Postsysnaptic Potential

    • Cl- influx locally hyperpolarizes dendrite

    • Decrease probability of action potential firing

    • E.g. synapses using GABA as neurotransmitter


  • EPSP and IPSP are graded and not “all-or-none” like action potentials

    • Graded potentials that lead to an action potential are called synaptic potentials

      • Can be larger or smaller depending on how much neurotransmitter is released

    • The amount of neurotransmitter released depends on amount of presynaptic Ca2+


Synaptic potentials to action potentials

  • EPSP and IPSPs constantly happening on dendrites and soma

  • An individual EPSP generally not enough to trigger an action potential

  • Net sum of many EPSP minus IPSP must reach threshold


Dendritic spines

  • Small ~1um protrusion of dendritic membrane

  • All different shapes

  • Highly modifyable : plasticity

  • Shape important from biophysical and biochemical standpoints


Spatial and temporal summation

  • EPSPs and IPSPs travel passively (electrotonically) so they decay over distance and time

    • EPSP or IPSP closer to the axon hillock will have a greater influence over action potentials (less time to decay)

    • EPSP or IPSP arriving near each other (spatially) will summate better (more likely to combine together)

    • EPSP or IPSP arriving closer together in time (temporally) will summate better

  • Spatial - potentials sum if close enough in space

  • Temporal - potentials sum if close enough in time


Stronger stimuli, more APs, more neurotransmission

  • Weak stimulus releases little neurotransmitter

  • Strong stimulus causes more action potentials and releases more neurotransmitters


Terminating synaptic transmission

  • Reuptake

    • Transport into axon terminals for reuse or into glial cells

  • Enzymatic degradation

    • Enzymes inactivate neurotransmitters

  • Diffusion

    • Diffuse out of the synaptic cleft

  • Antidepressants: SSRIs (serotonin transporter)

  • Cocaine: DAT inhibitor (dopamine transporter)

  • Alzheimers, Parkinson, schizophrenia: AChEl (Ech esterase)


Recap: from action potential to synaptic transmission

  1. Membrane is at rest at -65mV to 70mV

    1. Leaky K+ channels open, all voltage gated K+ and Na+ channels closed

  2. Multiple EPSP’s come along and summate at the axon hillock

    1. Voltage-gated Na+ channels begin to slowly open due to EPSP and opening of the activation gate

  3. Threshold is reached

    1. Voltage reaches ~-55mV at the axon hillock, voltage gated Na+ open due to opening of the activation gate

  4. The cell becomes depolarized due to the MASSIVE influx of Na+ until the membrane potential reaches +30mV

    1. Voltage gated Na+ channels inactivate and close

    2. Voltage gated potassium channels open and K+ leaves

      1. Absolute refractory period

  5. Membrane charge drops, repolarization occurs

    1. Sodium potassium pump is pumping Na+ ions out

  6. Slow closing of the voltage gated K+ channels leads to hyperpolarization and return to resting membrane potential

    1. Relative refractory period


Postsynaptic responses can be fast or slow

   Ionotropic vs metabotropic receptors

  • Ionotropic receptors: ion receptors

    • Fast on/off

    • All or none on/off

    • Ligand gated ion channels made up of protein subunits that together form an ion-conducting pore in the center of the receptor

  • Metabotropic receptors: signaling molecules

    • Slower on/off

    • Can amplify or dampen signals

    • Indirectly linked with ion channels through signal transduction mechanisms such as G proteins


Diversity of signaling: GPCRs

   GPCR: G protein coupled receptor

  • Always metabotropic

  • Dont have own ionic channel

  • Signaling molecules that communicate with ion channels

  • Gi: less cAMP

  • Gs: more cAMP

  • Gq: more IP3, DAG production

  1. Signal molecule binds to G protein-coupled receptor (GPCR) which activated the G protein

  2. G protein turns on adenylyl cyclase, an amplifier enzyme

  3. Andelyl cyclase converts ATP to cyclic AMP

  4. cAMP activated protein kinase A

  5. Protein kinase A phosphorylates other proteins, leading ultimately to a cellular response

Gi: inhibits AC, decreasing cAMP (i=inhibitory)

Gs: activated AC, more cAMP (s=stimulates)

Gq: activate PLC, more IP3, DAG production


Neurotransmitters

  • Neuropeptides

  • Gaseous neurotransmitters

  • Small-molecule neurotransmitters

    • Amino acids

      • Excitatory

        • Glutamate

        • aspartate

      • Inhibitory

        • GABA

        • Glycine

  • Glutamate

    • The major excitatory neurotransmitter in vertebrate brain

      • Found throughout the brain, especially in cortical, limbic and basal ganglia circuits

      • Involved in learning and memory and, at some level, in all behaviors

      • Excitotoxicity from too much glutamate release/impaired reuptake

        • Alzheimer’s disease

        • Mild cognitive impairment

        • ALS

    • Subject to reuptake by excitatory amino acid transports 

      • Rapid process, astrocytes contribute

      • Recycled into vesicles

    • 5 main classes of receptors, including ionotropic and metabotropic

      • NMDA - plays central role 

      • AMPA

      • Kainate

      • Group I mCluRs

      • Group 2

  • GABA

    • The major inhibitory neurotransmitter in vertebrate brain

      • Found throughout the brain, predominantly interneurons

      • Involved at some level, in all behavior

      • Drugs that increase effects of GABA used to treat

        • Anxiety

        • Epilepsy

        • Stress

    • Subject to reuptake by GABA transports

      • Rapid process, astrocytes contribute

      • Recycled into vesicles

    • 2 main classes of receptors, including ionotropic and metabotropic

      • GABAa -  ionotropic transmission of Cl-

      • GABAb - metabotropic (G couple protein), open hyperpolarizing K+ channels