© John Messmer 1998
[Re: Involuntary Diaphragm???]
There are three types of muscle in the body: skeletal, smooth and cardiac. Skeletal contracts in response to a nerve impulse at the individual muscle cell's neural plate. It is not propogated to other muscles. Smooth muscle also uses a stimulus to contract, though its type of contraction is less forceful than skeletal muscle and can propogate to other smooth muscle cells. Smooth muscle is in intestines, for example. Cardiac muscle contracts spontaneously, but that's a separate topic.
The diaphragm has ONLY skeletal muscle, not smooth muscle - none. It will not contract spontaneously as cardiac muscle. It MUST have an impulse delivered to contract. That impulse can originate in the higher brain centers as when we voluntarily inhale and exhale or in the lower brain as when low oxygen levels or high levels of acid or carbon dioxide are present in the cerebrospinal fluid or blood. There are MANY sites of modulation of breathing in the brain too complicated for this forum.
I will repeat what I have said before: the diaphragm muscle is skeletal and *not considered voluntary or involuntary* - it is the NERVOUS SYSTEM which is either voluntary or involuntary depending on whether we are consciously or unconsciously breathing. The phrenic nerve carries motor fibers that originate in the upper brain - the cortex which serves voluntary actions, and lower brain - brainstem which serves involuntary actions. There are NO SYMPATHETIC OR PARASYMPATHETIC nerves to the diaphragm. (Lungs, yes; diaphragm, no.)
We determine whether our diaphragms are at rest, contracted or forced up into our thoraces by the proprioceptive (position) sensation in our abdominal and chest walls and the nociceptive (bad feeling) fibers in our intestines and lungs. When it is contracted, our abdominal organs are pushed down and our abdominal walls are pushed out. When relaxed, this reverses. To exhale fully, it requires us to contract the abdominal wall muscles and the intercostal muscles (between the ribs) since our diaphragms can not move any higher than fully relaxed.
I hope this clears it up.
John Messmer, MD, Medical Director
Penn State Geisinger Health Group, Palmyra, PA