Physiology of Anxiety
To better understand what happens when someone experiences anxiety, let’s take a look at what happens in the body. The physiological response associated with anxiety primarily takes place in the area of the brain that controls our emotions, called the limbic system51.
The limbic system produces and regulates the biochemical reactions that control emotions52. Neurotransmitters are a result of these biochemical reactions that govern emotions such as anxiety5,53. Neurotransmitters essentially act as communicators by sending messages throughout the body via synapses, the connections between nerve cells53. The neurotransmitters serotonin and gamma aminobutyric acid (GABA) are produced by the pineal gland and have inhibitory effects on anxiety. This inhibitory effect triggers a part of the parasympathetic nervous system, or PNS, which is responsible for the relaxation response in the body and, in turn, calms the fight or flight response when danger is perceived5,51. Serotonin and GABA play an important role in emotions as low levels of either contributes to feelings of anxiety53. Epinephrine (adrenaline) and norepinephrine (noradrenaline) are neurotransmitters involved in emotions, and are produced by the adrenal gland53. These neurotransmitters have the opposite effect of serotonin and GABA as they put the body in a state of alertness by channeling blood to organs such as muscles and increasing the heart and breathing rates to prepare the body to fight or flee a threatening situation53,54; hence, their overproduction can lead to anxiety53. For more information on the fight-or-flight response see figure 3.
The amygdala may also play a role in MRI anxiety as it is responsible for receiving and interrupting visual and auditory stimuli, and determining if an environment is safe or unsafe55. The amygdala’s role in MRI anxiety is clear as it is one of the main limbic structures associated with anxiety disorders and responsible for interpreting the level of threat in an environment or a sound55. This correlation is supported by a study that determined the loud noise of MRI machines as a contributing factor in patient stress29,37,55. The amygdala has direct connections between other parts of the limbic system that regulate emotions such as the hippocampus, thalamus, periaqueductal grey matter, and prefrontal cortex5.
The role of the amygdala’s internal structures in emotion regulation are significant as they control two major branches of the autonomic nervous system: the parasympathetic and sympathetic5,51. The sympathetic nervous system (SNS) governs the response to perceived or real threats, referred to as the “fight or flight response,” whereas the parasympathetic nervous system (PNS) does the opposite and helps regulate or calm the nervous system4. These branches of the nervous system primarily govern involuntary bodily functions such as blood pressure and heart rate52. A structure located within the amygdala, called the “central nucleus,” controls emotions by regulating the heart and breathing rates and blood pressure, and are major players in mood regulation5,53. Furthermore, as a significant player in anxiety regulation, the central nucleus is the area of the brain that medications target in order to reduce panic attack symptoms51.
The prefrontal cortex is regulated by the sympathetic nervous system (SNS) of the autonomic nervous system (ANS) and regulates the brain’s reactions to dangerous situations56. The prefrontal cortex is responsible for turning off the fight or flight mechanism of the sympathetic nervous system, which is important when a patient perceives an MRI as a threat56,57. The olfactory region of the limbic system also plays a major role in anxiety regulation. You are probably wondering what scent has to do with anxiety. Scent is one of the major memory triggers. Think about people who say they hate the smell of hospitals. Well, that smell may trigger a negative response and, in some cases, causes anxiety. When a scent molecule enters the nasal cavity, the ANS is activated via the limbic system and releases neurotransmitters. The olfactory portion of the limbic system plays an important role in anxiety and will be discussed in greater detail in the MRI Anxiety Interventions chapter.
Figure 3. Fight-or-Flight Response