Hypothalamic paraventricular nucleus and cardiovascular regulation

1. The hypothalamic paraventricular nucleus (PVN) is an important integrati ve site within the brain composed of magnocellular and parvocellular neuron s. It is known to influence sympathetic nerve activity. 2, The parvocellular PVN contains neurons that project to the intermediolat eral cell column of the thoraco-lumbar spinal cord (IML). This defines the PVN as an autonomic 'premotor nucleus', one of only five present within the brain. 3, Another projection arising from the PVN is a prominent innervation of th e presser region of the rostral ventrolateral medulla (RVLM), also a premot or nucleus. The distribution of the PVN neurons projecting to the RVLM is s imilar to that of the PVN neurons that project to the IML. 4. It has been found that up to 30% of spinally projecting neurons in the P VN also send collaterals to the RVLM. Thus, there are neurons in the PVN th at can: (i) directly influence sympathetic nerve activity (via PVN-IML conn ections); (ii) indirectly influence sympathetic nerve activity (via PVN-RVL M connections); and (iii) both directly and indirectly influence sympatheti c nerve activity (via neurons with collaterals to the IML and RVLM). 5, In the rat, results of studies using the protein Fos to identify activat ed neurons in the brain suggest that neurons in the PVN with projections to the IML or RVLM mag be activated by decreases in blood volume. 6, In conclusion, the PVN can influence sympathetic nerve activity Within t he PVN are neurons with anatomical connections that enable them to affect s ympathetic nerve activity either directly indirectly or via both mechanisms (via collaterals). Studies that hare examined the role of specific subgrou ps within the PVN suggest that PVN neurons with connections to the IML or t o the RVLM may play a role in the reflex changes in sympathetic nerve activ ity that are involved in blood volume regulation.