BIOPHYSICS OF ACTION POTENTIAL & SYNAPSE

Ivan Poliaček

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)

soma

dendrite

nucleus

axon terminal

myelin sheath

axon hillockinitial segment

node of Ranvier

Schwann cell

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 inside outside (e.g. plasma)

Na+ 12 mM 145 mMK+ 140 mM 4 mMCl- 4 mM 115 mMHCO3

- 12 mM 30 mM

proteín - 140 mM 10 mM

Neuronal recording

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)

Resting membrane potential – polarization of the cell membrane - interior of the cell is NEGATIVE (neuron typically about 70 mV)

ACTION POTENTIAL

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).

stimulation

rising phasedepolarization

falling phaserepolarization

hyperpolarization

threshold and rising phase – Na channels are opening

the peak – Na+ permeability maximal, Na channels slowly shut off – 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 : 0.45at quiet (resting membrane potential) = 1 : 0.04 : 0.45

closed open

Bacterial voltage-gated potasium channel

Extracellular recording of respiratory neuron

airway pressure

diaphragm EMG

expiratory neuron

exp

insp

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 impossibleto evoke another action potential - Na channelsare "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 requiredin order to evokean 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 nearbypart of membrane can initiatethe spike

- 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

Propagation of action potential

refractoriness

Saltatory conduction

from one node of Ranvier to the next one

orthodromicconduction

antidromicconduction

Electrical stimulation of nerve fibers

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)

anode - higher polarization - lower excitability

cathode - depolarization- higher excitability

(duration of electrical pulse [ms])

(intensity of current [mA])

SYNAPSEneurons signal to each other and to muscles or glands

• Electrical synapses – electric signalgoes 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 - 1014 to 5 × 1014 (100-500 trillion) synapses (1 mm3 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 – Ca2+ influx throughvoltage gated Ca channels

• Ca2+ activates proteins (stenine and neurine) attached to vesicles (containing a neurotransmitter) – pulling the vesiclesto 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, glycinePeptides : 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 NEUROTRANSMITTERdue 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 potentialthat depolarize

IPSP – inhibitory post-synaptic potentialthat 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 PSPs1 EPSP temporal

summationof 3 EPSP

The effect of more than onesynaptic potential arrivingat 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 PSPSynaptic 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