A hypothesis for
generation of central command signal during exercise
Kanji
Matsukawa
Department
of Physiology, Graduate School of Health Sciences, Hiroshima University
1-2-3
Kasumi, Minami-ku, Hiroshima 734-8551, Japan
We
can feel that a heartbeat is influenced by central command from higher brain
centers in association with emotions and voluntary activities including
exercise. In daily life, a positive
relationship between arterial pressure (AP) and heart rate (HR) is usually
obtained, suggesting that central command, but not the arterial baroreflexes,
determines the level of HR.
However, cardiac autonomic outflow in a conscious condition has been
little understood due to technical difficulty of direct recording, especially
cardiac vagal nerve activity (CVNA).
Furthermore, the neural networks for generating central command remain
little known, because it is difficult to accomplish a comprehensive study with
voluntary exercise. First, I would
like to summarize the response in cardiac sympathetic nerve activity (CSNA)
during voluntary exercise in conscious cats and the responses in CVNA and CSNA
during spontaneous fictive exercise in paralyzed decerebrate cats. CSNA increased prior to or at the
onset of voluntary exercise in conscious cats. Such abrupt increase in CSNA was also
observed during spontaneous fictive exercise, whereas a decrease in CVNA was
never obtained. Central command,
therefore, contributes to exercise tachycardia via sympathetic activation but
not vagal withdrawal as opposed to the traditional concept. In addition, the fact that the rapid
sympathetic response occurred during fictive exercise in decerebrate cats
suggests that the brainstem, but not the cerebral cortex, may play an essential
role in generating central command.
In the later part of my talk, I would like to argue a hypothesis for
generation of central command in the brainstem. Since the positive AP-HR relationship in
daily life was lost or weakened in idiopathetic Parkinson’s patients, we
hypothesized that the midbrain dopaminergic system [substantia nigra (SN) or
ventral tegmentum area (VTA)] may play a role in generating central
command. Indeed, the cardiovascular
response at the onset of voluntary exercise in Parkinson’s patients was blunted
as compared to normal subjects.
Chemical or electrical stimulation of VTA but not SN increased renal
sympathetic outflow, HR, and AP in anesthetized cats and rats. Taken together, we propose the
hypothesis that VTA may be involved in generation of central command during
exercise.
Key words: Central
command, Cardiac sympathetic and parasympathetic outflows, Voluntary exercise