In this second video on the endocrine system, I will examine three additional hormones that play a critical role in the adjustments made by the body in response to exercise. I will begin by discussing the many contributions of epinephrine and norepinephrine. These hormones are released from the adrenal glands and more specifically, the adrenal medulla. I will conclude the video examining the role of the growth hormone plays during both exercise as well as into the post-exercise recovery period. Based on their structure, epinephrine and norepinephrine belong to the chemical family of catecholamines which also includes the neurotransmitter dopamine. Since epinephrine and norepinephrine are synthesized in the adrenal glands they are also commonly referred to as adrenaline and noradrenaline. Norepinephrine also functions as the major neurotransmitter for the sympathetic nervous system, which as we discussed in the cardiovascular system videos, plays an important role in the regulation of cardiac output and blood flow during exercise. The adrenal glands are located on top of the kidneys. Shown here are the sympathetic nerve fibers directly enervating the adrenal medulla. Thus, whenever sympathetic nerve activity is increased, as during exercise, the adrenal medulla releases epinephrine and norepinephrine into the blood Where these hormones will have a tremendous impact on a number of biochemical and physiological adjustments necessary to sustain exercise. This sympathetic nervous system adrenal access is responsible for the fight or flight response which occurs when you're frightened or nervous. It originates from the animal kingdom, when a predator spots a prey. One animal gets excited for a chance at its next meal while the other animal is frightened, not wanting to become the next meal. Both reactions elicit a large sympathetic response. These stress hormones can actually prepare the body for exercise prior to taking the first step. Shown here are the many biochemical and physiological variables affected by these hormones. You may have felt your heart pounding in your chest when you become frightened or nervous. This is the fight or flight response in action. This table demonstrates the many physiological and metabolic effects of epinephrine required for exercise. As discussed in our cardiovascular videos, epinephrine plays a major role in many physiological adjustments, including an increase in heart rate and stroke volume, and thus cardiac output and an increase in local muscle blood flow. All of these adjustments contribute to the increase in the delivery of oxygen and fuel to the working muscles. From a metabolic standpoint, epinephrine regulates the breakdown of glycogen in both muscle and liver. As well as stimulates fatty acid mobilization from adipose tissue. Together, these metabolic functions ensure that the working muscles have adequate fuel for ATP production. During the course of a graded exercise test to exhaustion, blood epinephrine and norepinephrine levels will increase exponentially as maximal oxygen cup consumption is reached. This represents the progressive increase in sympathetic nerve activity, resulting in adrenal medullary release of epinephrine and norepinephrine. Not surprisingly, as shown here, the epinephrine response to submaximal exercise will be dependent upon the exercise intensity. The greater the exercise intensity, the greater is the sympathetic nerve activity and thus, epinephrine release from the adrenal medulla. Before moving on to the next hormone I cannot emphasize enough the critical role that both the sympathetic nervous system and the adrenal medullary hormones, epinephrine and norepinephrine play in regulating multiple physiologic and metabolic adjustments necessary to sustain physical activity. Next, I will discuss the contribution of the growth hormone, both during exercise, as well as into the post exercise recovery period. The growth hormone is synthesized and secreted by the anterior pituitary gland. While the growth hormone's major effect is to promote protein synthesis in all tissues, during exercise it also plays a role in the mobilization and utilization of free fatty acids. During exercise, there was a slow or delayed response of growth hormone released into the blood. As just mentioned this will assist with the mobilization and utilization of free fatty acids. However, notice that after relatively high-intensity exercise, growth hormone levels continue to rise and can remain elevated up to over one hour post exercise. As discussed in the video on protein metabolism, there is a very significant increase in the rate of protein synthesis during the period immediately following exercise. This is a crucial time for the initiation of training adaptations for both endurance and strength training. The elevation in growth hormone is primarily responsible for the regulation of this increase in protein synthesis during this post-exercise period. Many studies but not all suggest that as a result of training the growth hormone responds to endurance training is more robust. This has potential implications for not only future training adaptations but for healthy aging as well. The implications for aging will be discussed in module four. For now, understand that both the frequency and magnitude of growth hormone release from the anterior pituitary gland decreases with advancing age and sanitary men and women. This can directly impact an individual's ability to maintain muscle mass and strength as we get older and threaten one's independence and quality of life. In summary, shown here is the typical response of the hormones that I have discussed in this and the previous video. Other hormones, such as cortisol, contribute to the adjustments made by the body during the stress imposed by a single bout of excercise. However, I have covered the five major players when it comes to the regulation of the critical biochemical and physiological adjustments necessary to sustain exercise.
