14.4: Central Control of Autonomic Function

By the end of this section, you will be able to:

• Describe the role of higher centers of the brain in autonomic regulation
• Explain how the hypothalamus exercises autonomic control
• Explain how the amygdala influences the autonomic function
• Describe the function of the medulla and the pathways important to descending control of the autonomic system

Autonomic control is based on the visceral reflexes, composed of the afferent and efferent branches. These homeostatic mechanisms are based on the balance between the two divisions of the autonomic system, which results in tone for various organs that is based on the predominant input from the sympathetic or parasympathetic systems. Coordinating that balance requires integration that begins with brain structures like the hypothalamus and continues into the brainstem and spinal cord.

Central Control of Autonomic Reflexes

The autonomic nervous system is controlled and influenced by the cerebral cortex, hypothalamus, limbic system, medulla oblongata and spinal cord. The hypothalamus is the control center for many homeostatic mechanisms, as it regulates both autonomic function and endocrine function.  The hypothalamus receives visceral and somatic sensory input as well as information related to emotions from the limbic system.  Stimulation of specific regions within the hypothalamus result in parasympathetic effects such as slowing of the heart, constriction of the pupil, and salivary secretion. On the other hand, stimulation other hypothalamic regions results in sympathetic effects such as increased in heart rate and blood pressure, pupillary dilation, and the fight-or-flight response. Axons from both regions project to autonomic nuclei within the brainstem and the spinal cord to maintain homeostasis.

Pupillary Reflexes

The optic nerve projects primarily to the thalamus, which is the necessary relay to the occipital cortex for conscious visual perception. Another projection of the optic nerve, however, goes to the hypothalamus. The hypothalamus then uses this visual system input to drive the pupillary reflexes. If the retina is activated by high levels of light, the hypothalamus stimulates the parasympathetic response which stimulates the circular fibers of the iris to contract and constrict the pupil. If the optic nerve message shows that low levels of light are falling on the retina, the hypothalamus activates the sympathetic response which dilates the pupil (Figure \(\PageIndex{1}\)). When light hits the retina in one eye, both pupils contract. When that light is removed, both pupils dilate again back to the resting position. When the stimulus is unilateral (presented to only one eye), the response is bilateral (both eyes). The same is not true for somatic reflexes. If you touch a hot radiator, you only pull that arm back, not both. Central control of autonomic reflexes is different than for somatic reflexes.

Figure \(\PageIndex{1}\): Pupillary Light Reflex. Three-quarter views of an Asian male human eye, taken consecutively in well lit (left) and dim (right) environments to illustrate pupil size changes in dark adaptation. The pupil measured 3mm on the left, and 9mm on the right. (Image credit: "Pupillary light reflex" by RapidReflex is licensed under CC BY-SA 4.0.)

Fight-or-Flight

The amygdala is a group of nuclei in the medial region of the temporal lobe that is part of the limbic system.  The amygdala sends signals to the hypothalamus indicating  various forms of stress (psychological, physical, or physiologic) and the hypothalamus will stimulate the sympathetic fight-or-flight response. The hypothalamus will also stimulate the release of stress hormones through its control of the endocrine system in response to amygdala input.

Cardiovascular Center

Has your heart ever "skipped a beat" at the surprise appearance of someone you love?  Your heart does not literally miss a contraction, but its rate does increase and decrease in response to stimuli that trigger feelings of surprise, excitement, and fear.  The medulla oblongata contains nuclei referred to as the cardiovascular center, which controls the smooth and cardiac muscle of the cardiovascular system through autonomic connections. When descending inputs from the hypothalamus stimulate this area, the sympathetic system can increase activity in the cardiovascular system by making your heart beat faster and stronger and by increasing blood pressure in the arteries.  When all is calm and stress is low, the hypothalamus signals the motor nucleus for the parasympathetic functions ascribed to the vagus nerve to decrease the heart rate and decrease blood pressure. 

EVERYDAY CONNECTIONS

Exercise and the Autonomic System

In addition to its association with the fight-or-flight response and rest-and-digest functions, the autonomic system is responsible for certain everyday functions. For example, it comes into play when homeostatic mechanisms dynamically change, such as the physiological changes that accompany exercise. Getting on the treadmill and putting in a good workout will cause the heart rate to increase, breathing to be stronger and deeper, sweat glands to activate, and the digestive system to suspend activity. These are the same physiological changes associated with the fight-or-flight response, but there is nothing chasing you on that treadmill.

This is not a simple homeostatic mechanism at work because “maintaining the internal environment” would mean getting all those changes back to their set points. Instead, the sympathetic system has become active during exercise so that your body can cope with what is happening. A homeostatic mechanism is dealing with the conscious decision to push the body away from a resting state. The heart, actually, is moving away from its homeostatic set point. Without any input from the autonomic system, the heart would beat at approximately 100 bpm, and the parasympathetic system slows that down to the resting rate of approximately 70 bpm. But in the middle of a good workout, you should see your heart rate at 120–140 bpm. You could say that the body is stressed because of what you are doing to it. Homeostatic mechanisms are trying to keep blood pH in the normal range, or to keep body temperature under control, but those are in response to the choice to exercise.

Concept Review

The autonomic system integrates sensory information and higher cognitive processes to generate output, which balances homeostatic mechanisms. The central autonomic structure is the hypothalamus, which coordinates sympathetic and parasympathetic efferent pathways to regulate activities of the organ systems of the body. The majority of hypothalamic output travels through the medial forebrain bundle and the dorsal longitudinal fasciculus to influence brainstem and spinal components of the autonomic nervous system. The medial forebrain bundle also connects the hypothalamus with higher centers of the limbic system where emotion can influence visceral responses. The amygdala is a structure within the limbic system that influences the hypothalamus in the regulation of the autonomic system, as well as the endocrine system.

These higher centers have descending control of the autonomic system through brainstem centers, primarily in the medulla, such as the cardiovascular center. This collection of medullary nuclei regulates cardiac function, as well as blood pressure. Sensory input from the heart, aorta, and carotid sinuses project to these regions of the medulla. The solitary nucleus increases sympathetic tone of the cardiovascular system through the cardiac accelerator and vasomotor nerves. The nucleus ambiguus and the dorsal motor nucleus both contribute fibers to the vagus nerve, which exerts parasympathetic control of the heart by decreasing heart rate.

Review Questions

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Critical Thinking Questions

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Glossary

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Contributors and Attributions

OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology