When you’re startled, the two branches work together, regulating the body without any conscious thought needing to be involved. Thanks to these automatic responses, the brain’s cortex is allowed to remain free to do other things-process sensory information, register emotion, pursue rational thoughts, and initiate voluntary movements. This can happen because the parasympathetic nervous system briefly lowers the heart rate, breathing, and other functions. That gives the cortex time to do its job, assessing any possible threats from the external world. Within a flash, the sympathetic nervous system sends signals to release neurotransmitters that put the body on full alert to prepare for the next step.

Meanwhile, the cortex uses the data it has collected to make a decision on an appropriate response to the startling stimulus. If the cortex perceives a real threat-a tiger on the loose from the zoo, for example-the brain automatically sends signals straight to the hypothalamus. The hypothalamus then releases a stress hormone known as CRF. It increases anxiety, puts the senses on extreme alert, and orders the release of the stress hormones cortisol and epinephrine (adrenaline) from the adrenal glands.


Next, the hypothalamus also signals to the pituitary gland to release hormones into the bloodstream that energize all of the body’s organs. Thanks to all this interaction and coordination, a person is now primed to run from the tiger, climb a tree, or fight back if necessary.

The tiny hypothalamus, less than one percent of the brain, is rich in neural connections and receptors for hormones, and it strongly influences the pituitary gland. Damage to the hypothalamus weakens the immune system and its response to viruses and germs. Conversely, electrical stimulation boosts immunity.

Similar Posts

Leave a Reply