Final answer:
The iris contains specialized contractile epithelial cells, in addition to connective tissue and smooth muscle. Smooth muscle tissue in the iris functions to adjust pupil size, which is a reflexive involuntary movement due to the presence of contractile proteins actin and myosin in these non-striated muscle fibers.
Step-by-step explanation:
In addition to connective tissue and smooth muscle, the iris contains specialized contractile epithelial cells. These cells, including muscle and epithelial components, make up the complex tissue structure of the iris.
Smooth muscle tissue, characterized by spindle-shaped cells with a single nucleus and no visible striations, is responsible for the involuntary movements in the internal organs such as the digestive, respiratory, and reproductive systems, as well as the iris. In the iris, smooth muscle functions to adjust the pupil size in response to light intensity.
Muscle tissue's ability to contract is what allows smooth muscle to perform its function in the various systems of the body, including altering the shape of the iris in the eye.
Specifically, the fiber cells of smooth muscle tissue in the iris adjust the diameter of the pupil, controlling the amount of light that enters the eye. Unlike skeletal muscle, these smooth muscle fibers lack striations, but they do contain contractile proteins such as actin and myosin.
These thin filaments are anchored within the muscle's structure, enabling smooth muscle contraction without the need for voluntary control. The cellular arrangement of smooth muscle in the iris is crucial for the reflexive and precise adjustment of pupil size, a function integral to vision.