When a hormone is triggered by the brain instantaneously, how is it possible for chemical reactors to traverse the distance between the brain and gland with sufficient speed to create an instantaneous response?
Good question! However, somewhat an outdated idea of how the brain relates and functions in accordance to the nervous and endocrine system. First of all, the brain, specifically the hypothalamus, receives afferent neuronal feedback from the internal body (homeostatic systems, which monitor hormone levels throughout the body) through the nervous system and specifically neurotransmitters (typically epinephrine, norepinephrine, and dopamine). These neurochemicals trigger the initial response from the hypothalamus (let's say, a stress response) to the pituitary gland first manufactures corticotrophicreleasing hormone (CRH). The pituitary gland is then stimulated into releasing adrenocorticotrophic hormone (ACTH). This hormone then makes the adrenal glands secret cortisol in the blood, which then affects various organs and glands (and specific brain structures, such as the hippocampus, which is why we typically struggle with memory recall during stressful periods in our life). This specific stress response is commonly referred to as the HPA-Axis (Hypothalamic-Pituitary-Adrenal axis). The important distinction to make is that physiological response is not simultaneous, per se. We have what is called a "dual pathway" in the nervous system which is based on our modern understanding of the brain and the neural pathways that intertwine in the spinal cord and up to the brain. When the body receives an external stimuli (or an internal imbalance is noticed by the hypothalamus), the HPA system responds (seemingly) instantaneously in what we typically call the 'fight-or-flight' (or 'freeze') response. When this occurs there is a two-fold process that is co-occuring in your central nervous system: First, there is the release of those Catecholamines (epinephrine, norepinephrine, and dopamine) in the Limbic system to the amygdala (emotion center), which happens unconsciously to us because the neural pathways those Catecholamines travel are in the mammalian brain (Limbic system), which is technically 'pre-conscious' as far as function is concerned. Then, literally milliseconds later, the parallel processing capacity of the brain sends a separate electrochemical message further out toward the cortex of the brain (specifically the frontal lobes) where we consciously are able (then) to cognitively process the stressful event in a 'rational' sense (which means we can properly assess the context of the situation and accurately evaluate the threat perceived by the autonomic nervous system). So, an example of this would be, let's say...you're walking in the woods behind your house and you subconsciously (or passively) think you perceived a snake in the leaves - you freeze, your heart starts racing, your breathing increases, salivation decreases, etc. - 'fight-or-flight' has kicked in almost instantaneously, as we can perceive it, but a split second later you look down and realize that it's just a tree branch in the fallen foliage (and thus your parasympathetic system kicks into gear to calm you back down by releasing acetylcholine, etc.). // It's facinating just how quickly and complexly our brain and CNS can respond and process our environment and be pretty accurate most of the time! Although it may 'feel' like those hormones (glucocorticoids & cortisol) are instantaneously, but actually they come at the end of an extensive and elaborate chain reaction of events!
