Parasympathetic preganglionic neurons in the sacral spinal cord

Two types of preganglionic neurons have been identified in the sacral parasympathetic nucleus (SPN) of the cat. These neurons could be differentiated by various characteristics including axonal conduction velocities, morphology, location in the nucleus, organ of innervation and central reflex mechanisms controlling their activity. Neurons having myelinated axons (B-PGNs) with conduction velocities between 3.3 and 13 m/s were located in the lateral band of the SPN and innervated the urinary bladder. Neurons with unmyelinated axons (C-PGNs) with conduction velocities of 0.5–1.4 m/s were located in the dorsal band of the nucleus and innervated the large intestine. B-PGNs were excited by distension of the bladder and inhibited by distension or mechanical stimulation of the intestine, whereas C-PGNs exhibited the opposite responses to these stimuli. C-PGNs often exhibited a low level of spontaneous discharge in absence of stimulation but exhibited marked firing (3.5–10 spikes/s) during a defecation reflex elicited by mechanical stimulation of the rectum-anal canal. The excitatory responses were elicited by C-fiber afferents via a spinal reflex pathway. B-PGNs were inactive when intravesical pressure was below the threshold for inducing micturition (5 cm H₂O) but raising the pressure above the threshold induced firing consisting of repetitive bursts of action potentials occurring at relatively high frequencies (15–60 spikes/s). These bursts coincided with rhythmic bladder contractions. The frequency of bladder contractions and associated bursts of PGN-firing and the mean PGN-firing rate (2–8 spikes/s) increased as intravesical pressure was increased in steps between 5 and 30 cm H₂O. However, as indicated by interspike interval histograms, the frequency of firing within a burst of action potentials was unchanged. It is concluded that the micturition reflex pathway is organized as a simple on-off switching circuit and that B-PGNs receive a maximal synaptic input when intravesical pressure exceeds the micturition threshold. This circuit was triggered by vesical Aδ afferents via a spinobulbospinal pathway. Transection of the spinal cord interrupted the reflex pathway and blocked micturition. However, in chronic spinal animals a spinal reflex mechanism emerged which contributed to the recovery of bladder function. This mechanism, which was weak or non-existent in animals with an intact neuraxis, exhibited a number of important differences from the normal micturiton reflex, most notably being activated by a C-fiber afferent rather than a Aδ afferent limb. The mechanism underlying the emergence of C-fiber evoked bladder reflexes in spinal animals is uncertain.