Relation of intrinsic heart rate and autonomic nervous tone to resting heart rate in the young and the adult of various domestic animals.

This study explores the connection between intrinsic heart rate (IHR) and autonomic nervous tone (ANT) to the resting heart rate (RHR) in a variety of domestic animals including horses, cows, pigs, goats, and chickens. It finds that both IHR and ANT vary based on species, weight, and age, with marked differences in the way each species adjust their autonomic nervous tone.

Measurement of IHR and ANT

• The researchers estimated the intrinsic heart rate and the autonomic nervous tone based on changes in the heart rate after the administration of specific drugs including atropine and propranolol.
• The study was conducted using both young and adult animals of different species – horses, cows, pigs, goats, and chickens – to provide comprehensive data across a range of domestic species.

IHR and ANT Changes with Age and Growth

• The study found that the IHR decreased progressively as the animals grew larger from young to adult.
• The ANT, on the other hand, shifted towards parasympathetic predominance, a state typically associated with relaxation and digestion.
• This alteration was brought about by a decrease in sympathetic tone (associated with the body’s “fight or flight” response) and/or an increase in parasympathetic tone.

Relationship between RHR, IHR and ANT

• As the IHR decreased as the animal grew, so too did the resting heart rate.
• However, it was also found that a shift towards parasympathetic predominance in the ANT also contributed to the decrease in resting heart rate.
• Therefore, both a decrease in IHR and an increase in parasympathetic predominance in the ANT can result in a lower resting heart rate.

Species Differences in ANT Alteration

• Notably, the study found a considerable difference between species in terms of how their ANT changed with growth or age.
• This suggests that the regulatory mechanisms of the autonomic nervous system may differ significantly between species.