neural activity sent to the brain fromthe cardiac mechanosensory neurites. However, at the level of the macro-scale heartbraininteractions investigated here, the dynamics of the generation and transmission of cardiovascularafferent input involve many types of neurons and a multiplicity of pathways operating overdifferent time scales.INTEGRAL REVIEWDecember 2009Vol. 5, No. 2There are approximately 40,000 sensory neurites in the human heart involved in relayingafferent information to the brain. Of these, just 20% are mechanosensory neurons. Of this 20%,only a small proportion actually fire in unison with each heartbeat. Moreover, there are at leastfive different types of mechanosensory neurons. Almost all mechanosensory neurons aresensitive to rate of change, in that their activity levels increase in a nonlinear manner in responseto change in the system. Some increase their firing rate only when blood pressure decreases,while others increase only during pressure increases. Still others are only sensitive to largemovements in the rate of change of heart rate or blood pressure (Armour & Kember, 2004).Thus, there is only a minority of sensory neurites whose output activity exhibits a one-to-onerelationship to the heartbeat and regional changes in ventricular blood pressure.To add to the complexity, the hearts intrinsic nervous system has both short-term and long-term memory that affects cardiac function (and thus afferent signals) over two different timescales: (1) variations in activity patterns that occur in response to rapidly occurring alterations inlocal mechanical status over milliseconds; and (2) variations in activity patterns of a globalnature that operate over time scales of seconds to minutes (Armour, 2003; Armour & Kember,2004). Thus, in addition to the information related to a single cardiac cycle, there is alsorhythmic information occurring over longer time scales that may modulate brain activity. Thefact that many of the neurons respond primarily to rate of change, and that changes in activitypatterns can last for minutes, are important factors in understanding how heartbrain interactionsare affected during coherence and can have an extended carry-over effect. This is because in thecoherence mode there is an increased rate of change in beat-to-beat variability of both heart rateand blood pressure, in addition to the increased order in the temporal patterns of activity of thecardiovascular system. While it is likely, under normal pressure variations and heart rates, thatthe overall amount of afferent neural activity reaching the brain is the same or nearly the samefrom one heartbeat to the next, it is our contention that the macro-scale patterns of neuralactivity can be quite different.In this regard, Wolk and Velden made an important observation in noting that the frequencyand stability of the afferent input were important factors affecting sensory-motor performance(Wolk and Velden, 1989). In this context, however, we suggest that the concept of activitypattern is more appropriate than the concept of frequency. This is because it is in the interspikeinterval (the temporal space between consecutive spikes of the neural activity) that information isencoded. Thus, it is the overall pattern of activity and not merely its frequency that contains themeaning of the information enfolded in the signals. Furthermore, we consider the stability of thepattern over longer time scales, those of seconds to minutes. Therefore, to understand the effectsof cardiovascular