adrenal glands. Small organs located just above the kidneys. One part of the glands produces and releases epinephrine (adrenaline) to initiate the stress response; another part produces and releases cortisol and cortisol-like hormones when directed by signals from the HPA axis to ramp up the stress response. See also HPA axis.

aerobic metabolism. The long-term mode of energy conversion defined by the sufficient availability of oxygen to burn fuel — first fat, and then fat and stored glucose — to feed active muscle cells. It occurs at low to moderately intense levels of physical activity and can be sustained over long periods of time.

anaerobic metabolism. A mode of energy conversion defined by lack of sufficient oxygen to convert fat and glucose into usable body fuel. When the body is pushed so fast and so hard that the working muscles’ demand for fresh oxygen outstrips the circulating blood’s ability to deliver it, the muscles begin to burn fuel inefficiently.

anandamide. A neurotransmitter in the body and the brain that binds to cannabinoid receptors. Tetrahydrocannabinol (THC), the active ingredient in marijuana, also activates these receptors. When the cannabinoid receptors are activated, they help the body and the brain manage pain, mood, and pleasure.

atrial natriuretic peptide (ANP). A naturally occurring hormone that is produced in the heart and the brain. When the heart rate increases, ANP production increases, and ANP is released into the bloodstream. It passes through the blood-brain barrier to counteract the action of certain elements of the stress response. It blunts stress and anxiety and helps to regulate mood. See also blood-brain barrier.

blood-brain barrier. A web of capillaries with tightly packed cells that prevents the easy transmission of some nutrients and substances from the bloodstream to the brain. It filters out toxins and infections.

brain-derived neurotrophic factor (BDNF). A protein produced inside nerve cells when they are active. It serves as Miracle-Gro for the brain, fertilizing brain cells to keep them functioning and growing, as well as spurring the growth of new neurons.

cerebellum. A small, densely packed part of the brain that contains half of the total nerve cells in the entire brain and is involved in integrating sensory and automatic motor function. It is constantly busy, updating and computing incoming and outgoing information. Within the past two decades, scientists have recognized that the cerebellum is involved in maintaining rhythm and continuity for many brain functions, such as emotions, memory, language, and social interactions, as well as allowing us to walk in a straight line. I call it the rhythm and blues center.

cortex. The brain’s thin outer layer of gray matter, just six cells thick. As the last portion of the human brain to have evolved, it is the seat of rapid computing and guides the rest of the brain. Neurons throughout the brain extend their axons to connect with the cortex and thus inform it about a wide range of mental activity.

cortisol. The primary long-acting stress hormone that helps to mobilize fuel, cue attention and memory, and prepare the body and brain to battle challenges to equilibrium. Cortisol oversees the stockpiling of fuel, in the form of fat, for future stresses. Its action is crucial to our survival. At high or unrelenting concentrations, cortisol has a toxic effect on neurons, eroding the connections between them and breaking down muscles and nerve cells to provide an immediate fuel source.

dopamine. A neurotransmitter that is vital to movement, attention, cognition, motivation and pleasure, and addiction.

endocannabinoids. A class of hormones known as the brain’s own marijuana. They are similar to endorphins in that they blunt pain, although they are metabolized much more quickly than tetrahydrocannabinol (THC) and thus have relatively fleeting effects.

endorphins. Hormones produced in the body and the brain that serve as natural morphine. They are released when the body and the brain are taxed, to block pain signals so we can push through physically uncomfortable situations. They affect many physiological functions, such as pleasure, satisfaction, and bliss.

epinephrine. Also called adrenaline, it is both a neurotransmitter in the brain and a hormone released from the adrenal glands. It is immediately released during the stress response, to prepare the nervous system to react to challenges to survival.

fibroblast growth factor (FGF-2). A protein that is produced and released in the body and the brain when tissues are stressed. Like vascular endothelial growth factor (VEGF), FGF-2 helps create more blood vessels and other tissues. FGF-2 is involved in initiating the process of stem-cell division that is necessary for neurogenesis, and it also encourages long-term potentiation (LTP) and the formation of memories. See also long-term potentiation, neurogenesis, and vascular endothelial growth factor.

gamma aminobutyric acid (GABA). The principal inhibitory neurotransmitter of the brain. It inhibits overactivity of all nerve cells, particularly those in the limbic system, which is home to the amygdala, the emotional center. Many antianxiety medications target GABA receptors. GABA is involved in anxiety, aggression, mood, and seizure control.

glutamate. The principal excitatory neurotransmitter of the brain. It is critical to cell binding and, thus, neuroplasticity.

hippocampus. The way station for many aspects of learning and memory. It gathers incoming stimuli from throughout the brain, cross-references the new information with stored information, and bundles it together as a memory that is then sent to the prefrontal cortex for processing. In recent years, it has been shown to be a critical component in the biology of stress and mood, since it contains a vast number of cortisol receptors and is the first step in regulating the feedback loop of the fight-or-flight response. Its close relationship with cortisol leaves the hippocampus particularly vulnerable to the ravages of stress and aging. Conversely, it is one of only two structures in the brain that clearly produces its own new nerve cells. See also neurogenesis.

HPA axis. A signaling route from the hypothalamus to the pituitary gland to the adrenal gland, which controls the stress response. It is important to such vital functions as fuel regulation and the immune system. See also adrenal glands, hypothalamus, and pituitary gland.

human growth hormone (HGH). A hormone known as the master of all hormones, it is vital to the growth and development of all cells in the brain and the body into adulthood and is intimately involved with building the body. It controls fuel allocation and counteracts the natural cellular atrophy of aging.

hypothalamus. A small gland situated above the pituitary gland that makes and secretes hormones to signal the pituitary to release hormones and other factors. It is a switching station that translates instructions from the brain, carried by neurochemical signals, into hormonal signals that travel through the bloodstream and direct biological imperatives such as sex, hunger, sleep, and aggression. See also pituitary gland.

insulin-like growth factor 1 (IGF-1). A hormone that is produced mainly in the liver and works closely with human growth hormone (HGH) and insulin to stimulate cell growth and counteract natural cell deterioration.

long-term potentiation (LTP). The cellular mechanism for learning and memory that requires the strengthening of brain cells’ ability, or potential, to send a signal across the synaptic gap. It is crucial for the process of cell binding and, thus, communication. See also synapse.

Maximum heart rate. The physiological limit of the number of times an individual’s heart can beat in one minute. This number is useful in correctly calculating the intensity of physical exertion. It can be determined in a physiology lab, by working to exhaustion. More commonly, recreational athletes use a theoretical maximum derived by subtracting their age from 220.

mitochondria. Tiny structures within every cell nucleus that function as the cell’s furnace, using oxygen to convert glucose into a usable fuel during aerobic metabolism. When oxygen levels are insufficient, fuel conversion is shifted outside the mitochondria for anaerobic metabolism, a far less efficient process than aerobic metabolism. See also aerobic metabolism and anaerobic metabolism.

neurogenesis. The process of stem cells dividing and developing into functional new brain cells, or neurons, in the brain. That this happens in the adult human was firmly established in 1998, and it is believed to be confined to part of the hippocampus and another brain area called the subventricular zone, which is associated with the sense of smell. See also stem cells.

norepinephrine. A neurotransmitter that affects arousal, alertness, attention, and mood. Norepinephrine signals activate the sympathetic nervous system and sharpen the senses. See also sympathetic nervous system.

pituitary gland. A pea-size endocrine gland located just beneath the hypothalamus. It secretes hormones and factors that control other hormones throughout the body. See also hypothalamus.

prefrontal cortex. The region of the cortex located at the very front of the brain. As the last portion of gray matter to have evolved, the prefrontal cortex oversees the qualities that make us most human. It is the CEO of most brain functions, including, but not limited to, planning, sequencing, rehearsing, evaluating, and understanding. It is also the home of working memory, the brain’s RAM, which is crucial to decision making. See also cortex.

serotonin. A neurotransmitter vital to mood, anxiousness, impulsivity, learning, and self-esteem. Often called the policeman of the brain, serotonin helps quell an overactive or out-of-control response in a wide range of brain systems.

stem cells. Undifferentiated cells that can develop into fully functioning new cells. In the adult human brain, they are located in a part of the hippocampus called the dentate gyrus, and in another area called the subventricular zone. Stem cells are encouraged to divide and develop into new neurons by fibroblast growth factor (FGF-2) and vascular endothelial growth factor (VEGF). See also fibroblast growth factor, hippocampus, and vascular endothelial growth factor.

sympathetic nervous system. A vast network of nerve cells that connect the brain to the body and are activated by norepinephrine. It is a component of the always-on autonomic nervous system, but its activity is dramatically increased during the stress response.

synapse. The junction of the axon and dendrite of two adjacent neurons. In the axon, electrical impulses are translated into chemical messengers — neurotransmitters — to carry instructions across the synaptic gap. At the dendrite, the neurotransmitter’s signal is converted back into an electrical impulse, which prompts the receiving neuron to carry out a task.

vascular endothelial growth factor (VEGF). An important signaling protein produced and released in the body when tissues are taxed and there is not enough blood flow to fuel the demand. Like fibroblast growth factor (FGF-2), VEGF acts as a mitogen, signaling other cells to start dividing to make more blood vessels. Recently, scientists have learned that VEGF is also produced in the brain and is involved in cementing memories. See also fibroblast growth factor.

VO2 max. Maximal rate of oxygen consumption, a measure of the lungs’ capacity to process oxygen; also known as aerobic capacity. It is the primary indicator of cardiovascular fitness.