Blood pressure control by neurotransmitters in the medulla oblongata and spinal cord

The roles of putative central neurotransmitters in the control of blood pressure have been reviewed with respect to the cardiovascular functions of individual nerve pathways in the medulla oblongata and spinal cord. Vasomotor activity of sympathetic preganglionic neurones originates from spinally-projecting neurones in the ventrolateral medulla which may include adrenaline neurones of the C1 group and serotonin neurones in the lateral B1 and B3 groups. Other bulbospinal monoamine nerves may modulate vasomotor activity at the spinal level, but the mechanism of this modulation is controversial. Evidence for two descending sympatho-inhibitory pathways has emerged: a noradrenergic projection from the A5 cell group and a serotonergic projection from the medullary raphe (medial B1 and B2 groups). The vasomotor influence of other bulbospinal pathways is unclear. Baroreflex control of blood pressure is mediated through the solitary tract nucleus (NTS). l-Glutamate and substance P are considered as candidates for transmitters in baroreceptor afferents to the NTS. Transmitters in efferent nerves relaying baroreflex activity from the NTS to cardiovagal motoneurones, medullary vasomotor neurones or sympathetic preganglionic neurones have not been identified but the monoamine transmitters present in the NTS appear to modulate baroreflexes. Noradrenaline and serotonin nerve endings may facilitate the vasodepressor component of the baroreflex while adrenaline nerves possibly inhibit the cardiovagal mechanism. Enkephalins and vasopressin act in the NTS to raise blood pressure and nerves containing these neuropeptides may constitute important links in reciprocal cardiovascular pathways between the lower brainstem and hypothalamus.