Chronic block of the cervical trunk increases synaptic efficacy in the superior and stellate ganglia of the guinea‐pig.

1. The effects of chronic conduction block with tetrodotoxin (TTX) of the cervical sympathetic trunk on synapses in the superior and stellate ganglia were examined in vitro with intracellular recording techniques. 2. The mean maximum amplitude of excitatory post-synaptic potentials (e.p.s.p.s) evoked in superior cervical ganglion neurones by stimulation of the cervical sympathetic trunk was increased significantly after 2 or 4 days of block. The electrical properties of the ganglion cells were not appreciably changed by the period of inactivity. 3. Chronic conduction block for 4 days also resulted in a significant increase in the amplitude of the e.p.s.p.s. evoked in stellate ganglion cells by active collaterals of the blocked fibres. 4. The number of steps in the synaptic response elicited in individual stellate ganglion neurones by graded stimulation of the ansa subclavia and the subclavian trunk (which join to form the cervical sympathetic trunk) was increased after 4 days of conduction block. 5. These results show that elimination of impulse activity in the cervical sympathetic trunk increased synaptic efficacy at the disused synapses in the superior cervical ganglion and at the active synapses in the stellate ganglion. In the latter case, collaterals of the blocked fibres apparently formed new synaptic connexions on stellate ganglion cells.