Anxiety is the feeling of fear, nervousness or worry that you experience when anticipating something unpleasant. Thinking about an upcoming performance, misplacing something important or preparing for a confrontation are all situations that may cause anxiety. Some people have anxiety disorders that cause them to feel anxious more often or more intensely than normal. These feelings may be triggered by specific situations, by irrational thoughts or by nothing at all. Regardless of what external factors trigger anxiety, the resulting chemical reactions that occur in the body are complex.
Most people are familiar with the effects of anxiety, including rapid heart rate, sweating, faster breathing and a tight, somewhat nauseous feeling in the stomach. When you’re anxious, your blood pressure increases, your metabolism speeds up and your muscles get tense. These symptoms are part of the “fight or flight” response, which is how the body deals with fear--by preparing to act quickly by either facing the fear or fleeing from it. The difference between fear and anxiety is that fear relates to an immediate threat whereas anticipating something fearful causes anxiety. Nonetheless, the body responds in the same way to both emotions.
Anxiety symptoms are activated by a part of the brain stem called the locus ceruleus. When something stressful is sensed, neurons in the locus ceruleus start firing more intensely than usual. Norepinephrine, a neurotransmitter, transports neural messages from the locus ceruleus to the spinal cord and other parts of the brain. Norepinephrine is then released from the nerve endings to act on the heart, blood vessels and respiratory centers, causing the rapid heartbeat, higher blood pressure and quick breathing.
The amygdala and the hippocampus are two parts of the brain that play the most important role in anxiety. The amygdala lies deep within the brain and interprets incoming sensory signals. If there is a threat, it will alert the rest of the brain, including the hippocampus, which creates memories from the threatening event that are then stored back in the amygdala. The amygdala and hippocampus are both responsible for activating the hypothalamic-pituitary-adrenocortical (HPA) axis, the system that regulates the stress response.
In the HPA axis, the hypothalamus is the first part of the system to be activated by the amygdala. The hypothalamus then stimulates the sympathetic nervous system to release the stress hormone corticotrophin-releasing hormone, or CRH. CRH then acts on the adrenal cortex to release glucocorticoids, which are hormones that balance the stress response by facilitating its activation and also inhibiting it when the response has been adequate. The amygdala also connects to the periaqueductal gray matter in the brain, which sends signals to the spinal cord to initiate an analgesic response. This can suppress pain in an emergency and initiate defensive actions--for example, when a scared animal freezes up.
Five neurotransmitters--serotonin, norepinephrine, gamma-aminobutyric acid (GABA), corticotropin-releasing hormone (CRH) and cholecystokinin--are involved in anxiety. Serotonin and GABA are inhibitory as they quiet the stress response, while the others play a part in triggering it. In anxiety disorders, some of these neurotransmitters may be imbalanced, causing a change in the normal sequence of events. Medication given for anxiety disorders acts on one or more of these. For example, benzodiazepine medications such as Valium use GABA to suppress anxiety. Antidepressant selective serotonin re-uptake inhibitors (SSRI’s) are also used to treat anxiety. They work by increasing the availability of serotonin in the brain, which helps to calm anxiety, panic and obsession disorders.