Multiple motor pathways to single smooth muscle cells in the ferret trachea

1. We investigated the distribution and characteristics of motor pathways to individual smooth muscle cells activated by electrical stimulation of either, single nerves which enter the tracheal plexus (inlet nerves), or a longitudinal nerve trunk (LNT) located near the entrance of an inlet nerve into the plexus. Excitatory junction potentials (EJPs) were recorded using intracellular microelectrodes as an index of smooth muscle cell activation. In all experiments EJPs were completely blocked by tetrodotoxin and by atropine. 2. In smooth muscle fields located in the caudal direction from the point of inlet or LNT nerve stimulation, neural input decreased as a function of distance. There was evidence of a demarcated area innervated by neurons entering the plexus in one inlet nerve. In smooth muscle fields located in the rostral or transverse direction from the site of nerve stimulation, no such demarcated area could be identified. 3. Of the smooth muscle cells located within the innervated fields studied, 83-95% were activated following stimulation of a single inlet nerve or LNT. Evoked EJPs were similar in different innervated cells or units of electrically coupled cells located within the same 1 mm2 'field'. 4. There was overlapping cholinergic motor input to single smooth muscle cells originating from neurons present in different inlet nerves or different neurons present in the same inlet nerve or region of the LNT. Multiple small step increases in the voltage used to stimulate a LNT resulted in three or four step increases in EJP amplitudes. This gives a minimal value for the number of motor pathways that can be activated by neurons in a region of LNT leading to a single smooth muscle cell. 5. Motor pathways to smooth muscle cells located in caudal and rostral fields ran initially in the LNT and exited in proximity to the smooth muscle cell studied. 6. Motor pathways used in transmitting signals to smooth muscle cells to different areas of trachealis muscle varied in their sensitivity to hexamethonium or curare. EJPs evoked in fields located in the caudal direction from the stimulating electrode were abolished by these drugs. Muscle cells located in different rostral fields showed EJPs that were either sensitive or resistant to these drugs. 7. The rostral hexamethonium-resistant pathway ran initially in the LNT but it exited from the LNT several millimetres before reaching the level of the smooth muscle field innervated. This pathway followed stimulation frequencies up to 25 Hz. The final neuron in this pathway released acetylcholine and evoked EJPs were entirely inhibited by atropine.(ABSTRACT TRUNCATED AT 400 WORDS)