BIOPHYSICS OF ACTION POTENTIAL

&

SYNAPSE

Ivan Polia ček Ján Jakuš

Excitable tissues – nerve tissue, muscle tissue Neuron

- primary structural and functional unit of nerve tissue (brain, spinal cord, nerves, sensory cells) - 4 – 130 μm (soma – proteosyntesis, dendrites – input, axon – output

)

dendrite axon terminal soma node of Ranvier axon hillock initial segment myelin sheath Schwann cell nucleus

Propagation of neuronal excitation from dendrites to the axon

dendrites soma axon with an axon collateral

Cell membrane

- reminder

• double-layer of phospholipide + cholesterol + proteins • isolating the cell from surroundings + regulation of permeability + “communication” (receptors and irritability)

INTRA- & EXTRA CELLULAR ION CONCENTRATIONS

ion Na + K +

Cl HCO 3 -

prote ín inside 12 mM 140 mM

4 mM 12 mM

140 mM outside

(e.g. plasma)

145 mM 4 mM

115 mM 30 mM

10 mM

Neuronal recording

Resting membrane potential – polarization of the cell membrane - interior of the cell is NEGATIVE (neuron typically about 70 mV) Depolarization – reduction of the magnitude of membrane potential (e.g. from -70 mV to -60 mV or more) Hyperpolarization – increase of the magnitude of membrane potential (e.g. from -70 mV to -80 mV or more) Efflux of K +

(through K channels)

, or influx of Cl –

(through Cl channels)

rising phase depolarization

ACTION POTENTIAL

falling phase repolarization stimulation hyperpolarization Action potential (nerve impulse) occures at excitable tissues (mostly neuron fibers or muscle cells) when graded potential reaches the threshold (gate threshold) – firing level.

It is all-or-none (it happens or do not happen).

threshold and rising phase – Na channels are opening the peak – Na + slowly shut off permeability maximal, Na channels – transpolarization - till +30 mV falling phase- Na channels inactivation, high voltage opens also voltage-sensitive K channels – potential towards resting level...

and even „overshooting“ it - (after)hyperpolarization

Only very small numbers of ions are involved in 1 action potential considering the cell (axon) size ratio of membrane permeability during rising phase of action potential – perm K + : perm Na + : perm Cl = 1 : 20 : 0.45

at quiet (resting membrane potential) = 1 : 0.04 : 0.45

closed open Bacterial voltage-gated potasium channel

Extracellular recording of respiratory neuron

exp airway pressure insp diaphragm EMG expiratory neuron

expiratory neuron burst extracellular spike waveform

• Each action potential is followed by a

refractory period

• Refractory periods are caused by changes in the state of Na and K channels • An

absolute refractory period

-

it is impossible to evoke another action potential - Na channels are "inactivated" at the end and immediately after the spike - they cannot be made to open regardless of the membrane potential • A

relative refractory period

-

later, a stronger than usual stimulus is required in order to evoke an action potential (part of Na channels recovered)

Scheme of Na voltage gated channel and K voltage gated channel involved in processing of action potential

Propagation of action potential

Local current spread (electrotonic conduction) – depolarization of nearby part of membrane can initiate the spike

Propagation of action potential

refractoriness - the duration around and below 1 ms - without the depression (an energy comes from the cell) - a wave (a spot) of electrical negativity on the surface (electrical positivity on the internal site of membrane) - openning and closing of voltage gated ion channels

Saltatory conduction

from one node of Ranvier to the next one antidromic conduction orthodromic conduction

(intensity of current [mA])

Electrical stimulation of nerve fibers

anode

- higher polarization - lower excitability

cathode

- depolarization - higher excitability (duration of electrical pulse [ms])

Rheobase

- minimal current amplitude of infinite duration (practically a few 100 ms) that results in an action potential (or muscle contraction)

Chronaxy (-ie)

- minimum time over which an electric current double the strength of the rheobase needs to be applied, in order to stimulate a nerve cell (muscle fiber)

SYNAPSE

neurons signal to each other and to muscles or glands

•

Electrical synapses

– electric signal goes through „gap junction“ (bidirectional) •

Chemical synapses

– chemical transmission (one-way) directionally from a presynaptic to a postsynaptic cell (and are therefore asymmetric in structure and function) human brain - 10 14 (1 mm 3 to 5 × 10 14 (100-500 trillion) synapses of cerebral cortex - about a billion of synapses)

Axo-dendritic synaptic terminals – chemical synapses

•

Synaptic transmission

Action potential depolarizes pre-synaptic membrane of synaptic terminal – Ca 2+ influx through voltage gated Ca channels

•

Ca 2+ activates proteins (stenine and neurine) attached to vesicles (containing a neurotransmitter) – pulling the vesicles to the membrane, making them to fuse with the membrane, thereby opening the vesicles and dumping their neurotransmitter contents (each vesicle contains thousands molecules) into the synaptic cleft – exocytosis (active transport)

•

Neurotransmitter molecules diffuse across the synaptic cleft (30-50 nm between pre- and post-synaptic membrane) and bind to receptors on the subsynaptic membrane ( it is a part of post-synaptic membrane ) thus initiating the response (either via G-protein coupled effector enzymes or via ligand gated ion channels)

Types of neurotransmitter

Aminoacids

: glutamate, GABA, aspartate, glycine

Peptides :

vasopresin, somatostatine, neurotensine...

Monoamines :

norepinephrine, dopamine, serotonione, and acetylcholine

Crucial neuromediatiors in the brain are : glutamate and GABA RECEPTOR is mostly responsible for the effect not the neurotransmitter itself Excitatory - acetylcholine - ACh (neuromuscular junction - e.g. voluntary movement) - glutamate Inhibitory - GABA - glycine (spinal reflexes)

Ionotropic receptors

(ligand-gated ion channels) – permeability changes e.g. efflux of K and/or influx of Ca and Na on the subsynaptic membrane of the post synaptic cell –

graded

(post-synaptic)

potential

occurs - fast postsynaptic actions (synaptic delay usually 1-5 ms)

Metabotropic receptors

(G-protein-coupled receptors) - an extracellular domain binds to a neurotransmitter, an intracellular domain binds to G-protein – the second messenger (or intracellular messenger) – activated and released from the receptor interacts with other proteins e.g. with ion channels to open or close them (slow postsynaptic response - ms to minutes)

ELIMINATION OF NEUROTRANSMITTER

due to

thermal shaking

, neurotransmitter molecules eventually break loose from the receptors and drift away - reabsorbed by the presynaptic cell (re-packaged in vesicles for future release) - broken down metabolically - difused away

EPSP – excitatory post-synaptic potential that depolarize IPSP – inhibitory post-synaptic potential that hyperpolarize The magnitude of a PSP depends on:

• the amount of neurotransmitter (and receptors) • the electrical state of the postsynaptic cell (less neurotransmitter is necessary if already partially depolarized) • how long is neurotransmitter present in the synaptic cleft (it must be quickly removed or inactivated)

SUMMATION of PSPs

1 EPSP temporal summation of 3 EPSP

The effect of

more than one synaptic potential

arriving at a neuron is additive if : - the time span between the stimuli is short -

temporal summation

- they arrive at a given region of a neuron -

spatial summation

Spatial summation of PSP

Synaptic integration

- The combining of excitatory and inhibitory signals acting on adjacent membrane regions of a neuron.

In order for an action potential to occur, the sum of excitatory and inhibitory postsynaptic potentials (local responses) must be greater than a threshold value.

Summary

• • • • • • • • • •

depolarization, repolarization, hyperpolarization action potential – the shape, mechanisms refractory periods propagation of action potential (continual spreading, saltatory conduction) electrical stimulation – rheobase, chronaxy graded potential synapse, neurotransmitter, mechanisms of transmission receptors (ionotropic vs. metabotropic) EPSP, IPSP, summation (temporal, spatial) convergence, divergence