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Brain Involvement
Brain control of exhalation can be broken down into voluntary control and involuntary control. During voluntary exhalation, air is held in the lungs and released at a fixed rate. Examples of voluntary expiration include: singing, speaking, exercising, playing an instrument, and voluntary hyperpnea. Involuntary breathing includes metabolic and behavioral breathing.

Voluntary Expiration
The neurological pathway of voluntary exhalation is complex and not fully understood. However, a few basics are known. The motor cortex within the cerebral cortex of the brain is known to control voluntary respiration because the motor cortex controls voluntary muscle movement. This is referred to as the corticospinal pathway or ascending respiratory pathway. The pathway of the electrical signal starts in the motor cortex, goes to the spinal cord, and then to the respiratory muscles. The spinal neurons connect directly to the respiratory muscles. Initiation of voluntary contraction and relaxation of the internal and external internal costals has been shown to take place in the superior portion of the primary motor cortex. Posterior to the location of thoracic control (within the superior portion of the primary motor cortex) is the center for diaphragm control. Studies indicate that there are numerous other sites within the brain that may be associated with voluntary expiration. The inferior portion of the primary motor cortex may be involved, specifically, in controlled exhalation. Activity has also been seen within the supplementary motor area and the premotor cortex during voluntary respiration. This is most likely due to the focus and mental preparation of the voluntary muscular movement.

Voluntary expiration is essential for many types of activities. Phonic respiration (speech generation) is a type of controlled expiration that is used everyday. Speech generation is completely dependent on expiration, this can be seen by trying to talk while inhaling. Using airflow from the lungs, one can control the duration, amplitude, and pitch.

Involuntary Expiration
Involuntary respiration is controlled by respiratory centers within the medulla oblongata and pons. The medullary respiratory center can be subdivided into anterior and posterior portions. They are called the ventral and dorsal respiratory groups respectively. The pontine respiratory group consists of two parts: the pneumotaxic center and the apneustic center. All four of these centers are located in the brainstem and work together to control involuntary respiration. In our case, the ventral respiratory group (VRG) controls involuntary exhalation. The neurological pathway for involuntary respiration is called the bulbospinal pathway. It is also referred to as the descending respiratory pathway. “The pathway descends along the spinal ventralateral column. The descending tract for autonomic inspiration is located laterally, and the tract for autonomic expiration is located ventrally.” Autonomic Inspiration is controlled by the pontine respiratory center and both medullary respiratory centers. In our case, the VRG controls autonomic exhalation. Signals from the VRG are sent along the spinal cord to several nerves. These nerves include the intercostals, phrenic, and abdominals. These nerves lead to the specific muscles they control. The bulbospinal pathway descending from the VRG allows the respiratory centers to control muscle relaxation, which leads to exhalation.

Receptors
Several receptor groups in the body regulate metabolic breathing. These receptors signal the respiratory centers to initiate inhalation or exhalation. Peripheral chemoreceptors are located in the aorta and carotid arteries. They respond to changing blood levels of oxygen, carbon dioxide, and H+ by signaling the pons and medulla. Irritant and stretch receptors in the lungs can directly cause exhalation. Both sense foreign particles and promote spontaneous coughing. They are also known as mechanoreceptors because they recognize physical changes not chemical changes. Central chemoreceptors in the medulla also recognize chemical variations in H+. Specifically, they monitor pH change within the medullary interstitual fluid and cerebral spinal fluid.