Answer
The hypothalamus is the management center which coordinates and adjusts the activities of the autonomic nervous system(ANS). Heart activity, blood pressure, body temperature and water balance are vital life sustaining physiological parameters which must be kept under proper homeostatic control if health and life are to be maintained. It is the important work of the ANS to maintain the proper levels or tone of these physiological processes. Because these processes are so important the body has a double assurance mechanism -- an integrator or an operational manager that supervises and issues orders to the ANS if and when values get out of safe ranges. That integrator is the hypothalamus.
The control of the hypothalamus also is important in alerting us to danger or threat. When the limbic system senses danger or perceives a threat, it signals the hypothalamus to do some thing to preserve life and limb. The hypothalamus through sympathetic innervation directs the ANS to get the body ready to fight or flee. The ANS responds by releasing increased amounts of adrenergic neurotransmitters which produce some well-known effects: increases in heart rate and respiratory rate, dilation of pupils, shunting of blood to internal organs and increased muscle tension.
Work Step by Step
The hypothalamus is the integrator or operations manager of the autonomic nervous system (ANS). The anterior hypothalamus supervises the parasympathetic system and the posterior hypothalamus supervises the sympathetic system. Direct inputs from the hypothalamus can adjust ANS activity, but so can indirect control routed through the reticular formation (ARF/DRF) of the brain stem.
When the ANS allows important physiological parameters , for example, heart rate, blood pressure or body temperature to fall out of the proper homeostatic range the hypothalamus responds by sending messages to the ANS to get things in order. These hypothalamic messages are sent to the RF nuclei which transmit the messages to the appropriate level of the spinal cord. Preganglionic neurons carry the messages to the ANS chain from where a post-ganglionic fiber carries the ANS message to the relevant effector that carries out the adjustment.
The hypothalamus also has a role in protecting the body against physical injury; this is still important, but less so than it was in our evolutionary past. However, when one senses danger or perceives stress or anxiety through the limbic system-- amygdala (emotions), peri-aqueductal gray ( pain and anxiety), these emotions are communicated to the hypothalmus. If the flight/fight responses of the ANS are inappropriate or inadequate, the hypothalamus can modulate ( ramp up or dampen) the responses of the ANS to the emotion of fear or anxiety.
