Their movement is controlled by conscious will.
Explanation
Muscular tissue is classified into three types based on structure and function: skeletal, smooth, and cardiac. The distinction between voluntary and involuntary muscles rests on whether their movements are under the conscious control of the nervous system.
Detailed Analysis:
- They contract rhythmically without external stimulation. is Incorrect: Muscles that contract rhythmically without external nervous stimulation are termed myogenic. This is a characteristic of cardiac muscles found in the heart, which are involuntary.
- Their movement is controlled by conscious will. is Correct: Voluntary muscles are those whose activity can be controlled by conscious will. These are primarily skeletal muscles attached to bones, facilitating locomotion and body movements. They are regulated by the somatic nervous system.
- They are primarily found in the walls of internal organs. is Incorrect: Muscles found in the walls of internal organs (such as the stomach, intestine, and blood vessels) are smooth muscles (visceral muscles). These are involuntary in nature, as their movement cannot be directly controlled by conscious will.
- They lack the striation bands seen in other muscles. is Incorrect: Voluntary (skeletal) muscles are also known as striated muscles because they exhibit alternating light and dark bands (striations) when viewed under a microscope. Smooth muscles lack these striations.
Key Takeaway:
Voluntary muscles are synonymous with skeletal muscles. They are characterized by the presence of striations and are under conscious control.