MEMBRANE CURRENT AND POTENTIAL CHANGE DURING NEUROTRANSMISSION IN SMOOTH-MUSCLE

Smooth muscle cells are electrically coupled to one another in a syncy tium, and this renders their electrophysiology during neurotransmissio n strikingly different from that at other synapses. The postjunctional depolarizing responses of sympathetically innervated smooth muscle su ch as the vas deferens, particularly, the excitatory junction potentia ls (EJPs), possess intriguing properties which for several years have resisted explanation. A principal issue has been the temporal relation ship of transmitter-generated membrane current to the resulting potent ial change, which seems to differ depending upon whether transmitter r elease is spontaneous or is nerve-stimulation-evoked. Accordingly, smo oth muscle electrical properties appear to change with different patte rns of transmitter release. Until some years ago this relationship was an area of uncertainty, firstly because transmitter-activated membran e current could not be measured directly and secondly because intracel lular membrane potential measurements gave rise to conflicting results . Many of the uncertainties have now been resolved with refinements in techniques of measurement that have allowed membrane current time cou rse during neurotransmission to be estimated. As a result, our underst anding of smooth muscle electrical properties has been clarified and d eepened. These developments are outlined in this review, and it is sho wn how our comprehension of neurotransmission has at every stage been influenced strongly by the techniques adopted for investigation.