Final answer:
The main difference between internal intercostals and innermost intercostals is their fiber direction. The internal intercostals aid in expiration, while innermost act as synergists. Both are vital for breathing, altering the ribcage dimensions and pleural cavity pressures.
Step-by-step explanation:
The difference between internal intercostals and innermost intercostals is the direction of muscle fibers. The internal intercostals are situated just below the external intercostals and have a fiber direction that runs at a right angle to that of the externals, aiding in expiration by drawing the ribs together and constricting the rib cage. On the other hand, the innermost intercostals, located even deeper, share a similar fiber direction with the internals and also act as synergists for the action of the internal intercostals during exhalation.
Muscles of the thorax, such as the internal and innermost intercostals, play a crucial role in breathing. They work with the diaphragm to change the dimensions of the rib cage and, in turn, affect the volume-pressure ratio inside the pleural cavities. Ultimately, these muscle actions orchestrate the rhythmic process of inspiration and expiration.