1. Intracellular recordings were made from the parasympathetic ganglio
n cells that lie in the epicardium of the left atrium of guinea-pig he
art near the interatrial septum. 2. Three distinct types of neurone we
re identified on the basis of their electrophysiological properties. I
n one group of neurones, S cells, somatic action potentials were follo
wed by brief after-hyperpolarizations. In the other two sets of neuron
es, somatic action potentials were followed by prolonged after-hyperpo
larizations. The neurones with prominent afterhyperpolarization were f
urther subdivided: one group of neurones, P cells, showed inward recti
fication at membrane potentials near the resting membrane potential wh
ilst neurones in the other group, SAH cells, did so only at more negat
ive potentials. 3. In the group of neurones that displayed inward rect
ification at potentials near rest, rectification resulted from the act
ivation of an inward current, which resembled the hyperpolarization-ac
tivated inward current present in cardiac muscle pacemaker cells. 4. T
he three different types of neurone received different patterns of syn
aptic input. Each SAH cell received a synaptic excitatory connection f
rom the vagus which in most cells released sufficient transmitter to i
nitiate an action potential in that cell; several SAH cells also recei
ved a separate connection, which could be activated by local stimulati
on. Although most S cells failed to receive a synaptic input from the
vagus, all of those tested received an excitatory synaptic input which
could be activated by local stimulation. Virtually all P cells failed
to receive a synaptic input from the vagus; in addition, local stimul
ation failed to initiate synaptic potentials in P cells. 5. When the s
tructure of cardiac ganglion cells was determined, by loading the cell
s with either biocytin or neurobiotin, it was found that most cells la
cked extensive dendritic processes. S cells were invariably monopolar,
most P cells were dipolar or pseudodipolar, whereas many SAH cells we
re multipolar. 6. In many neurones an on-going discharge of action pot
entials was detected in the absence of obvious stimulation. In S and S
AH cells, the action potentials resulted from an on-going discharge of
excitatory synaptic potentials. However, when a spontaneous discharge
of action potentials was detected in P cells a discharge of excitator
y synaptic potentials was not detected. 7. The results are discussed i
n relation to the idea that the three different types of cell may have
different functions and that some of the cells may be organized in su
ch a way as to permit the local handling of neuronal information withi
n the heart.