BIOPHYSICS OF ACTION POTENTIAL & SYNAPSE

1. BIOPHYSICS OF ACTION POTENTIAL& SYNAPSE Ivan Poliaček

2. Excitable tissues - neuron (nerve tissue) - muscle fiber (muscle tissue)Neuron- primary structural and functional unit of nerve tissue (brain, spinal cord, nerves, sensory cells)- 4 – 130 μm dendrite axon terminal node of Ranvier soma Schwann cell axon hillock initial segment myelin sheath nucleus

3. Propagation of neuronal excitation from dendrites to the axon dendrites soma axon with an axon collateral

4. Membrane potential, membrane depolarization, hyperpolarization

5. Cell membrane - reminder double-layer of phospholipide + cholesterol + proteins

6. INTRA & EXTRA-CELLULAR ION CONCENTRATIONS ioninsideoutside (e.g. plasma) Na+ 12 mM145 mM K+ 140 mM 4 mM Cl- 4 mM 115 mM HCO3 - 12 mM 30 mM protein - 140 mM10 mM Ca++ 0,0001 mM 2 mM

7. Membrane potential - Goldman equation A magnitude is determined by concentrations & permeabilities of membrane for potasium, chlorine and sodium P K+ , P Na+ , P Cl- - permeabilities for K+, Na+, Cl- [K+], [Na+], [Cl-] -concentrations 100 : 4 : 45 Resting membrane potential for neuron - about -70 mV What does keep these concentrations uneven? Is interior of the cell negatively charged? How can be membrane potential altered? What is responsible for permeability changes? How will changes in K+, Na+, Cl- permeabilities change membrane potential? What is term for lower MP – more polarized cell membrane? What is the term for higher MP – less polarized cell membrane?

8. Depolarization – less polarization = reduced magnitude of membrane potential (e.g. from -70 mV to -60 mV or more) Hyperpolarization – - more polarization = increased magnitudeof membrane potential(e.g. from -70 mV to -80 mV) Graded (local) responses = graded depolarizations or hyperpolarizations : - electricity - chemicals - generator potential (sensory) - synaptic EPSP (depolarization) IPSP (hyperpolarization)

9. Neuronal recording Rest # 1

10. SYNAPSE neurons signal to each other or to muscles or glands Electrical synapses – electric signal goes through „gap junction“ (bidirectional) Chemical synapses – chemical transmission (one-way) one-directional from a presynaptic to a postsynaptic cell 1 mm3 of human cerebral cortex - about a billion of synapses

11. Axo-dendritic synapses Synapses: - axo-dendritic - axo-somatic - axo-axonal

13. neuron

14. Synaptic transmission Action potential comes via axon to the terminal at pre-synaptic membrane • depolarizationopensvoltage gated Ca channels - Ca++ diffuse into neuron • Ca++ inside - vesicles towardsthe membrane (proteinsstenine and neurine)- exocytosis – release of neurotransmitter (mediator) in the synaptic cleft

15. 3. diffusion of mediator molecules through the cleft (30-50 nm) • mediator molecules activates receptors on subsynaptic part of postsynaptic membrane Synaptic transmission IONOTROPIC receptors – ligand-gated channels producing EPSP or IPSP at post-synaptic cell METABOTROPIC receptors – mediator at extracellular domain activates intracellular G-proteine leading to the intracellular signaling(gene expression, chemical reactions, channels opening / closing – membrane permeability changes)

16. Summary: - action potential - voltage gated Ca channels - Ca++influx - vesicles exocytosis -neurotransmitter (mediator) release - its diffusion through the cleft - interaction with receptors(e.g. ligand gated channels) How does action stop? elimination of neurotransmitter - reabsorbed by the presynaptic cell (re-packaged into vesicles) - broken down metabolically - diffused away

17. Neurotransmitters chemically : aminoacids(glutamate, GABA, aspartate, glycine),peptides(vasopresin, somatostatine, neurotensine,...),monoamines(norepinephrine, serotonine,acetylcholine,...) • Excitatory: • - acetylcholine • (neuromuscular junction)- glutamateInhibitory : • - GABA • - glycine (spinal reflexes) • excitation – inhibition mostly • determined by receptor • In the brain are essential: • glutamate and GABA

18. Rest # 2 Comments and requests are welcomed. Why are chemical synapses called chemical? What ion initiates synaptic transmission? Where is neuromediator stored? Where is it released? Where does neurotransmitter act? What is main difference between metabotropic receptor and ligand gated channel? What is the difference between ligand gated and voltage gated channel? Where are amployedligand and where voltage gated channels in the synaptic transmission?

19. Summation of postsynaptic potentials (stimulation of several synapses with ligand gated ion channels)

20. EPSP – excitatory post-synaptic potential that depolarize IPSP – inhibitory post-synaptic potential that hyperpolarize Higher magnitude of PSP ? more neurotransmitter (and more receptors), the membrane already partially depolarized, for how long is neurotransmitter available (it must be quickly removed from the cleft or inactivated) Further from synapse (subsynaptic membrane) e.g. at axon hillock - less effect of PSP – it is GRADED and LOCAL electrical response that spreads with FALLOFF

21. SUMMATION of PSPs 1 EPSP - rare to the threshold , but temporal summation of 2 EPSP additive effect of many synaptic potentials at a neuron if : - the time span between the stimuli is short - temporal summation - they arrive at a given region of a neuron - spatial summation

22. SUMMATION of PSPs 1 EPSP - rare to the threshold , but spatial summation of 2 EPSP additive effect of many synaptic potentials at a neuron if : - the time span between the stimuli is short - temporal summation - stimuli arrive at several synapsesof neuron - spatial summation

23. Synaptic integration - The combining of EPSPs and IPSPs on a neuron. - In order for an action potential = ACTIVATION to occur, the threshold depolarization has to be reached at initial segment = axon hillock = trigger zone

24. action potential arises at trigger zone = initial segment if the depolarization there reaches the threshold dendrites axon with an axon collateral soma

26. Rest # 3 I recommend to search GOOGLE summation of postsynaptic potentials at the axon hillock Animation 5.2 - Summation of Postsynaptic Potentials and see the following http://sites.sinauer.com/neuroscience5e/animations05.02.html Suggestions what is important and how to remember that important are appreciated, aren’t they?

27. ACTION POTENTIAL rising phase depolarization falling phase repolarization stimulation hyperpolarization Action potential (nerve impulse) - at excitable conductivetissues = nerve fibers & muscle cells if depolarizationreaches the gate threshold = firing level. It is all-or-none (it happens or do not happen).

28. 500 times Na permeability At rest permeabilites for K+ : Na+ : Cl-100 : 4 : 45 At spike depolarization 100 : 2000 : 45 - Local (graded) depolarization to the threshold - firing level - Na channels open (voltage gated) - Na+ influx - rapid depolarization - SPIKE - eventranspolarization – positivecharge at internal side of membrane for a short moment (and negative outside)- Na channels close (voltage gated) and K channels open (voltage gated) – Na+ influx STOP + K+ efflux - rapid repolarization

29. 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

30. 1 action potential requires high, but limited number of ions - considering the whole cell it is capable of producing many action potentials What keeps the ion distribution appropriate? • Each spike is followed by a refractory period. • An absolute refractory period - it is impossible to evoke another action potential – during spike and right after it • (Na channels are open and after that inactivated) • A relative refractory period - a strongerthanusualstimulus is required to evokeanactionpotential (hyperpolarization; part of Na channelsrecovered)

32. exp insp Rest # 4 airway pressure diaphragm EMG expiratory neuron expiratory neuron burst extracellular spike waveform

33. Propagation of action potential Local current spread(electrotonic conduction) –depolarization of nearby part of membrane

34. Propagation of action potential – local currents refractoriness - without the depression (an energy comes from the cell) along nerve or muscle fibers - a wave (a spot) of electrical negativity on the surface (electrical positivity on the internal site of membrane) due toopenning and closing of voltage gated ion channels

37. Saltatory conduction from one node of Ranvier to the next one orthodromicconduction antidromicconduction

38. intensity of current [mA] Electrical stimulation of nerve (muscle) 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)

39. 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

40. I will be pleased to take your requests and comments. Please, let me know how to improve the lecture (and lecturer) Thank you for your attention.