The heart utilizes electrochemical signals to control the rate at which it contracts. It accomplishes this using bundles of “pacemaker cells” whose signals maintain an internal rhythm independent of the body. However, at rest the rhythm it maintains is slightly faster than what is ordinarily required. The neuroendocrine system usually dampens this rate, but it can elevate it as well using electrical or chemical signals (during exercise or stress). The neuroendocrine ( neuro – nerve; endocrine – hormone) system is a compilation of inputs from the nervous system as well as glands throughout the body that secrete chemicals. Both usually work together to account for transient or even chronic changes in the body. Using nerve signals or hormones, the neuroendocrine system changes cell activities that alter the diameter of blood vessels, the heart’s rate and strength of contraction, and even the amount of fluid the body retains.
This system works as a complex series of checks and balances on the heart and circulatory system in a way similar to the action of a thermostat. A thermostat detects transient changes in temperature and has a maximum and minimum temperature range allowed. When the room temperature falls below the minimum, the thermostat signals to the furnace to turn on and raise the temperature in the room. When the addition of heat causes the maximum temperature to be reached, the thermostat signals to the furnace to turn off. Much the same way, the neuroendocrine system monitors certain variables in the body and makes adjustments when variables aren’t in the needed range. When using the diagram below, think of the brain as a “thermostat” detecting changes, while organs act to change what the brain dictates.
This diagram represents a “feedback loop.” Just as the brain sends signals to alter activity in the heart, blood vessels, and adrenal glands, it must likewise receive input when the condition has been properly changed. When it receives this input (demonstrated by the blue arrows triggered by the heart and artery), it will halt its own signalling and allow the system to work more independently. This particular example is a negative feedback loop, as it reacts to reverse a situation and bring it within a threshold range. Unfortunately, the changes made by the system are not necessarily beneficial in the long run. Imagine that the thermostat in the above example is reset to a higher than desirable level. Once the furnace turns on, it will not receive any input that the room is too hot and will continue to pump heat into the room until the new threshold temperature is reached. In a similar sense, if the brain doesn’t receive a proper “off” signal, it will continue to raise blood pressure unnecessarily. Going one step further, impaired heart function will lead to ineffective, sustained signalling activity by the nervous system, which will further create conditions that strain the heart, driving it towards failure. The system isn’t perfect, but under normal conditions it affords the body a great deal of flexibility. Whether resting or working, neuroendocrine feedback works in the background to address the changing needs of the body.