Excitatory and inhibitory inputs from medullary nuclei projecting to spinal cardioacceleratory neurons in the cat

In chloralosed vagotomized cats medullary structures projecting to spinal (T2) sympathetic neurons were identified by activating fibre terminals in the intermediolateral nucleus (ILN) at functionally and histologically identified cardioacceleratory sites and exploring the ipsilateral medulla for evoked field and single unit potentials.In 23 cats evoked field potentials with a mean latency of 1.5 msec were recorded at 264 sites in 216 penetrations in the right medulla and at 101 sites in 81 penetrations in the left medulla. These potentials followed stimulation at 300 Hz and persisted after asphyxia and administration of sodium pentobarbital.In 7 cats single unit activity was recorded from 39 units which followed the stimulus at a constant short latency of activation (mean 1.7 msec), exhibited cancellation of antidromic with orthodromic spikes and fractionation at high frequencies, and responded to paired stimuli delivered at short intervals (mean minimum interval 1.27 msec, range 0.58–2.3 msec).Stimulation of 79 sites at which evoked field potentials were recorded in the right medulla elicited cardioacceleration mainly from N. lateralis reticularis and N. parvocellularis, and cardiac slowing mainly from N. paramedium reticularis, raphe NN. and N. medullae oblongatae centralis subnucleus ventralis; stimulation of 101 sites in the left medulla changed heart rate at only 3 sites.It is concluded that: a) medullary inputs to spinal sympathetic neurons arise from discrete nuclei; b) structures from which cardioacceleration and cardiac slowing were elicited provide excitatory and inhibitory inputs, respectively, to spinal cardioacceleratory neurons; c) the efferent limb of reflexes controlling heart rate appears to be localized mainly on the right side of the medulla; d) the vasomotor centre provides multiple excitatory and inhibitory inputs from specific reticular nuclei to the ILN.