Resting Membrane Potential

Fig. 12.09a,b

Types of Ion Channels

Recording of Resting and action potentials It is recorded by cathode ray oscilloscope it is negative in polarized (resting, the membrane can be excited) state with the potential difference inside the cell membrane is negative relative to the outside. – -70 mV + + 0 mV Voltmeter + – Dentistry 07

Recording membrane potential + 60 - + 30 - 0 - - 30 - - 60 - - 90 - mV Electrotonic potential Localized non propagated Action potential

All or non law: Application of a threshold stimulus either produces a full response or not at all. Further increase in the intensity of a stimulus produces no increment or other changes in action potential. The action potential failed to occur if the stimulus is sub-threshold, it produces only local changes with no propagation. Latent period in a nerve: it is a period corresponding to the time taken from the site of simulation till the recording electrode.

Threshold and Action Potentials Threshold – membrane is depolarized by 15 to 20 mV Established by the total amount of current flowing through the membrane Weak (subthreshold) stimuli are not relayed into action potentials Strong (threshold) stimuli are relayed into action potentials All-or-none phenomenon – action potentials either happen completely, or not at all

The Action Potential Equilibrium potential of sodium (+60 mV) - 75 mV Resting potential (-75 mV) Equilibrium potential of potassium (-95 mV) Equilibrium potential of sodium (+60 mV) K Na - 75 mV Passive increase in positive charge Electrotonic potential

The Action Potential threshold Resting potential (-75 mV) Equilibrium potential of potassium (-95 mV) Equilibrium potential of sodium (+60 mV) - 55 mV Na Na K K K Opening of voltage-gated sodium channel threshold Electrotonic potential

The Action Potential Equilibrium potential of sodium (+60 mV) - 40 mV Resting potential (-75 mV) Equilibrium potential of potassium (-95 mV) Equilibrium potential of sodium (+60 mV) Depolarisation due to sodium influx - 40 mV Na Na K K K Opening of voltage-gated sodium channel Electrotonic potential

voltage-gated sodium channels turn to the inactivation phase The Action Potential voltage-gated sodium channels turn to the inactivation phase Resting potential (-75 mV) Equilibrium potential of potassium (-95 mV) Equilibrium potential of sodium (+60 mV) Depolarisation due to sodium influx + 50 mV Na Na K K K Inactivation of voltage-gated sodium channel Electrotonic potential

The Action Potential Equilibrium potential of sodium (+60 mV) + 50 mV Resting potential (-75 mV) Equilibrium potential of potassium (-95 mV) Equilibrium potential of sodium (+60 mV) Depolarisation due to sodium influx opening of voltage-gated potassium channel K + 50 mV Na Electrotonic potential

The Action Potential Equilibrium potential of sodium (+60 mV) - 85 mV Resting potential (-75 mV) Equilibrium potential of potassium (-95 mV) Equilibrium potential of sodium (+60 mV) Depolarisation due to sodium influx Repolarization due to potassium influx opening of voltage-gated potassium channel K - 85 mV Na Electrotonic potential

Resting potential (-75 mV) Equilibrium potential of sodium (+60 mV) The Action Potential Membrane potential approaches the ENa and voltage-gated sodium channels turn to the inactivation phase Resting potential (-75 mV) Equilibrium potential of sodium (+60 mV) Depolarisation due to sodium influx K - 75 mV Na repolarization due to potassium influx closing of voltage-gated potassium channel Electrotonic potential Repolarisation due to potassium influx Hyperpolarising afterpotential

The Action Potential threshold Inactivation of voltage-controlled sodium channel Equilibrium potential of sodium (+60 mV) Opening of voltage-controlled sodium channel Opening of voltage-controlled potassium channel threshold Electrotonic potential Resting potential (-75 mV) Hyperpolarization due to more outflux of potassium ions Dentistry 07

The Action Potential (excitability changes) Absolute refractory period Relative refractory period ENa (+60 mV) Depolarisation (due to sodium influx) Resting potential (-75 mV) Polarized state (resting) Hyperpolarising afterpotential EK (-95 mV)

Action Potential Propagation Dentistry 07

Saltatory Conduction: Action Potential Propagation in a Myelinated Axon Dentistry 07

Properties of action potentials are all-or-none events threshold voltage (usually 15 mV positive to resting potential) threshold -70 +60 mV Stimulus are initiated by depolarization action potentials can be induced in nerve and muscle by extrinsic (percutaneous) stimulation – APs do not summate - information is coded by frequency not amplitude.

Neuron F8-2 Axons carry information from the cell body to the axon terminals. Axon terminals communicate with their target cells at synapses.

Communication Between Neurons Electrical synapse Chemical synapse one-way conduction, always transmits signals in one direction. this allows signals to be directed toward specific goals.

Terminology Associated with Changes in Membrane Potential F8-7, F8-8 Depolarization- a decrease in the potential difference between the inside and outside of the cell. Hyperpolarization- an increase in the potential difference between the inside and outside of the cell. Repolarization- returning to the RMP from either direction. Overshoot- when the inside of the cell becomes +ve due to the reversal of the membrane potential polarity.

Graded Potentials F8-9 Graded potentials are depolarizations or hyperpolarizations whose strength is proportional to the strength of the triggering event. Graded potentials lose their strength as they move through the cell due to the leakage of charge across the membrane (eg. leaky water hose).

Graded Potentials Above Threshold Voltage Trigger Action Potentials Graded potentials travel through the neuron until they reach the trigger zone. If they depolarize the membrane above threshold voltage (about -55 mV in mammals), an action potential is triggered and it travels down the axon. F8-10

Spatial Summation F8-12 A neuron may receive greater than 10, 000 inputs from presynaptic neurons. The initiation of an action potential from several simultaneous subthreshold graded potentials, originating from different locations, is known as spatial summation.

Temporal Summation F8-13 When summation occurs from graded potentials overlapping in time, it is called temporal summation. Summation of graded potentials demonstrates a key property of neurons: postsynaptic integration.

Synaptic potentials EPSP a IPSP

Synaptic potentials

Action Potential Propagation Dentistry 07

Saltatory Conduction: Action Potential Propagation in a Myelinated Axon Dentistry 07

Synaptic potentials EPSP a IPSP