The association between membrane excitation and alterations in membrane electrical impedance, its dependence on extracellular Na+ and the accompanying transmembrane Na+ fluxes measurable by isotope tracer methods, gave rise to the Na+ hypothesis for the action potential. Here, suprathreshold depolarising stimulation increases the voltage-dependent Na+ membrane conductance. The latter in turn initiates a regenerative cycle of membrane depolarisation and further channel opening, culminating in the action potential upstroke phase. Subsequent action-potential recovery to the resting potential then follows a voltage-dependent Na+ channel inactivation and more gradual K+ channel opening. This hypothesis was tested by voltage-clamp experiments determining the ionic currents required to drive depolarising membrane-potential steps in cephalopod giant axons from the resting to varying test levels. These revealed Na+ and K+ conductances whose voltage-dependences and kinetic properties could be incorporated into a successful mathematical reconstruction of the timecourse and properties of experimentally observed propagating action potentials.

Action-potential propagation along the length of an axon beyond the regions of initial excitation requires current flow driven by Na+-channel activation to access remote, initially quiescent, regions of nerve. This current, and its effect on membrane potential, varies with membrane resistance and capacitance, and the electrical resistances of the adjacent extracellular and intracellular fluids. These variables quantify the spread of the consequent voltage change with time and distance through the cable equation. This in turn determines action-potential conduction velocity which, in combination with its refractory period, determines the wavelength of this advancing excitation. Conduction velocity in unmyelinated fibres increases with fibre diameter. That in myelinated fibres increases with the reduced electrical capacitance and increased resistance of their surrounding myelin sheath, resulting in a saltatory action potential conduction. Conduction is further modified by the threshold for the initial excitation, in turn dependent on the membrane Na+ channel density.

Smooth muscle cells are adapted for slow, sustained contractions reducing the lumina of the tubular structures in which they occur. These are often paced by networks of interstitial cells of Cajal. The latter are recurrently depolarised by Ano1-Cl- channel opening, triggered by inositol trisphosphate induced store Ca2+ release. This triggers T-type Ca2+ current causing propagating slow waves then transmitted, via gap junctions, to smooth muscle cells. The resulting smooth muscle cell activation by L-type Ca2+ channels elicits Ca2+-induced sarcoplasmic reticular Ca2+ release. Additional autonomic and local transmitter driven pharmacomechanical coupling mechanisms mediated by a range of G-proteins also promote second-messenger-mediated muscle contraction or relaxation. Cross-bridge activity is activated by a combined Ca2+-calmodulin-mediated caldesmon dissociation from thin filament actin and myosin light chain kinase activation. Termination of cross bridge cycling leads to either muscle relaxation or latch-bridge formation, permitting sustained shortening in an absence of ATP-dependent energy expenditure.

Vertebrate skeletal muscle has a negative, -90 mV, resting potential arising from Na+-K+-ATPase-generated transmembrane ionic gradients and inwardly rectifying K+, and Cl- membrane conductances. Three-electrode and loose-patch voltage-clamp experiments demonstrated that, as in nerve, muscle action potentials involve voltage-dependent Na+ followed by K+ channel activation. An additional transverse tubular action potential contributes a discrete and separable delayed component to the recorded voltage change. It is triggered by low-frequency components of the surface-membrane action-potential leaving high-frequency components to ensure rapid propagation of the surface wave. Tubular Cl- conductances decrease in both fast and slow twitch muscle and ATP-dependent K+ channel conductances increase in fast twitch muscle, in early and prolonged exercise. Mathematical modelling demonstrates that these respectively enhance and reduce tubular excitability and its triggering of contractile activity. They potentially furnish enhancing and fatiguing mechanisms for muscle activation and for clinical myotonia congenita.

The propagation of action potentials through successive regions of the heart can be clinically monitored by electrocardiographic recording. The cardiac action potential typically comprises Phase 0 upstroke, early Phase 1 recovery, Phase 2 plateau and Phase 3 repolarisation to Phase 4 electrical diastole. These are driven respectively by inward Na+, early K+ transient outward, inward Ca2+, rapid and slow outward K+ currents, and inward rectifying K+ currents. The inward Ca2+ currents trigger Ca2+-induced release of sarcoplasmic reticular Ca2+ by cardiac ryanodine receptors, thereby initiating excitation-contraction coupling leading to contractile activation. With repolarisation, this is reversed through Ca2+-ATPase-mediated Ca2+ reuptake into the sarcoplasmic reticulum, and electrogenic Na+-Ca2+ exchange expelling Ca2+ into the extracellular space. These processes are modulated by feedforward autonomic, sympathetic and parasympathetic triggering of specific G-protein signaling pathways, involving their respective transmitters norepinephrine and acetylcholine. Conversely, cytosolic Ca2+ exerts feedback effects on the initiating Na+ current generation.

Biochemical studies have cloned, isolated and sequenced the Na+ and other ion channels in their related protein family; cryo-electronmicroscopic structural determinations have characterised details of their structure. Biophysical measurements of intramembrane charge movement provided electrical signatures clarifying the dynamics and mechanisms of the channel conformational responses to membrane voltage change. Such charge movements were demonstrated, studied and quantified in a wide range of ion-channel species and cell types. Finally, radioactive tracer flux experiments examined the basis for their ion selectivity and permeation. Together these detailed characterisations separated and clarified the mechanisms for ion channel gating and channel permeability to specific ions. They identified voltage-sensing modules and how each domain contributed to the ion-specific pore module within each domain of the four-domain structure making up the ion-channel protein. These studies thus together provide a continuing clarification of the molecular basis through which ion channels mediate excitability in biological membranes.

Each skeletal muscle fibre comprises a repeating sarcomere structure, each containing interdigitated thin and thick filaments made up respectively of the proteins actin and myosin. Actin is globular protein occurring in a double chain. Myosin consists of globular heads with ATPase activity capable of interacting with actin, connected to fibrous protein components. Activation of the regulatory protein troponin, intercalated within the actin chains, by Ca2+ binding, causes configurational changes displacing the structural protein tropomyosin. This frees binding sites on actin, permitting cycles of cross-bridge formation and dissociation through their interaction with myosin heads, with expenditure of ATP, generating tension. This sliding filament hypothesis was confirmed by examining how peak isometric tension varied with sarcomere length and the consequent actin-myosin overlap determining the potential number of tension-generating crossbridges. The protein titin, amongst others, ensures structural integrity of thick and thin filament arrangement through the resulting cross bridge sliding underlying muscle contraction.

The cell surface membrane comprises an insulating lipid bilayer in which specialised proteins are embedded. This supports the membrane potential difference between intracellular and extracellular fluids brought about by their differing Na+, K+ and Cl- concentrations, their respective Nernst potentials for electrochemical equilibrium, and their relative membrane permeabilities. The concentration distributions arise from metabolically dependent active ion transport through membrane Na+-K+ ATPase activity, first demonstrated using radioactive tracers in cephalopod giant axons. In intact cells, these factors are combined with the Donnan distribution properties for intracellular and extracellular ions, reflecting the presence of impermeant intracellular charged protein, to determine the resting potential. The intact cell thereby forms an osmotically and electrically balanced system with relatively increased intracellular K+ and extracellular Na+ concentrations separated by a membrane across which there is a stable negative resting potential. The latter provides the electrophysiological background upon which cell excitation events are superimposed.

Nervous systems are a characteristic feature of higher animals. Their sensory components convey incoming information from the internal and external environment; their motor components convey instructions for reactions to such stimuli to their effector organs. Vertebrates possess both central and peripheral nervous systems, including an autonomic division concerned with homeostasis of the internal environment. The nerve cell is the anatomical, functional and trophic unit of nervous system function. Its cell body radiates dendritic and axonal nerve fibres that respectively transmit incoming information and the departing results of its processing. In contrast to non-myelinated nerve fibres, myelinated nerve fibres are ensheathed by glial cells in the central and Schwann cells in the peripheral nervous systems. Peripheral but not central nerves show a capacity for regeneration along their basement membranes thereby regaining their peripheral attachments. This property has attracted significant interest in connection with clinical repair following nerve injury.

Skeletal muscle contraction can be characterised under either isometric or isotonic conditions of constant length or load. These demonstrate an inverse, Hill, relationship between initial shortening velocity and load. The muscle contraction timecourse exceeds that of its initiating electrical and intracellular Ca2+ changes. Repetitive stimulation consequently produces summation and tetanic fusion of successive muscle twitches. All these variants of contractile activity incur energy expenditure immediately supplied by ATP breakdown, replenished successively from creatine phosphate, carbohydrate and lipid energy supplies. Continued activity leads to energetic depletion, and osmotic and electrolyte imbalances all contributing to fatigue. However, cellular H+ buffering mechanisms mitigate the osmotic and pH effects of the associated lactate production. Na+-K+-ATPase activity buffers the inward Na+ and outward K+ fluxes accompanying electrical activity, and their osmotic effects. Long-term increases in muscle activity exert positive trophic effects. In contrast, ageing is associated with sarcopaenia which contributes importantly to clinical frailty.

Excitation-contraction coupling refers to the events connecting surface membrane excitation and initiation of mechanical activity. It involves a steeply voltage-dependent Ca2+ release from its intracellular sarcoplasmic reticular store. The resulting cytosolic Ca2+ elevation, leading to troponin activation, is detectable through absorbance or fluorescence properties of intracellularly introduced optically sensitive dyes. The initiating transverse tubular depolarisation is sensed by intramembrane dihydropyridine receptors at triad junctions with terminal cisternal sarcoplasmic reticulum. Their underlying configurational changes were demonstrated and characterised through their associated intramembrane charge movements employing pharmacological agents known to modify excitation-contraction coupling. This separated a steeply voltage-dependent qγ transition allosterically and co-operatively coupled to opening of sarcoplasmic reticular ryanodine receptor Ca2+ release channels. These events and the associated Ca2+ release reverse with membrane repolarisation. Sarcoplasmic reticular Ca2+-ATPase activity then returns the released Ca2+ from cytosol to its sarcoplasmic reticular store. Clinical ryanodine receptor disorders cause malignant hyperthermia, important in anaesthetic practice.

Studies demonstrating, characterising and thereby clarifying our understanding of nerve function began from the experimental availability of electophysiological methods for recording and stimulation of bio-electric signals. The classical recording methods were developed to measure intracellular potentials directly from cephalopod giant axons, skeletal muscle fibres and other excitable cell types. These consistently demonstrated strongly negative resting potentials and monophasic action potentials in response to stimulation, whose detailed waveforms varied with different excitable tissue types through a wide range of species. Measurement of extracellular potential differences between different recording sites in the nervous system permitted study both of electrical events occurring at a point, and their propagation along lengths of nerve. This demonstrated and characterised the observed compound action potentials. It separated their components by conduction velocity attributing this to their different fibre diameters and degrees of myelination. It also demonstrated their threshold excitation, all-or-none and refractoriness properties.