Question

A particular muscle is under involuntary control, but appears striated. It is most likely to be: A. The myocardium, the muscular portion of the heart. B. The muscle(s) that surrounds the esophagus and causes peristalsis. C. The biceps, the muscle that moves the forearm. D. The diaphragm, the muscle that allows breathing to occur.

Short answer

The correct answer is A. The myocardium, the muscular portion of the heart.

Step-by-step solution

Identify Characteristics

Start by identifying the unique characteristics described in the exercise. It mentions that the muscle is under 'involuntary control' and appears 'striated'. Involuntary muscles are not consciously controlled by an individual, and striated muscles have a banded appearance under a microscope.

Eliminate the Voluntary Muscles

We can eliminate option C, the biceps, because it is a muscle that is consciously controlled by individuals, making it voluntary rather than involuntary.

Distinguish Between Striated and Non-Striated Muscles

Striated muscles have a distinctive banded look, whereas non-striated muscles do not. The muscles that cause peristalsis around the esophagus, mentioned in option B, are primarily smooth (non-striated) and involuntary.

Identify Muscles of the Heart

Recognize that the myocardium, the muscular portion of the heart, mentioned in option A, is indeed striated for contraction strength but operates under involuntary control to pump blood continuously without conscious effort.

Consider the Diaphragm

Option D, the diaphragm, is a muscle that assists in breathing. While it is under involuntary control for most of the time, it can also be voluntarily controlled to an extent, such as when taking a deep breath intentionally. Therefore, it does not entirely fit the description.

Choose the Correct Answer

By elimination and matching the provided characteristics, option A, the myocardium, is the correct answer. It is the muscle part of the heart, it is under involuntary control, and it is striated.

Key concepts

Involuntary vs Voluntary Muscles

Muscles in the human body can broadly be classified into two types based on control: voluntary and involuntary muscles. Voluntary muscles, such as the biceps, are those you can control consciously. These muscles allow you to perform actions like walking, grabbing, or making facial expressions.
Involuntary muscles, on the other hand, operate automatically without your conscious input. They are responsible for vital bodily functions such as heartbeats, digestion, and other internal processes. The muscles causing peristalsis around the esophagus is a good example of involuntary muscles, although they are non-striated, differing from the muscle group being asked about in the exercise.

Striated Muscle Tissue

When we examine muscle tissue under a microscope, we can identify striated muscle tissue by its characteristic dark and light bands, giving it a striped appearance. This striation is due to the orderly overlapping of protein fibers within the cells, which allows for controlled and forceful contractions. Skeletal muscles, which are voluntary, and cardiac muscles, which are involuntary, both exhibit this striated pattern. This striated design is not present in smooth muscle tissue, which is generally found in the walls of internal organs and is also involuntary. Thus, in the context of the exercise, the myocardium of the heart represents the striated and involuntary category.

Cardiac Muscle Function

The cardiac muscle, exclusive to the heart, represents a specialized form of striated muscle that operates under involuntary control. Its primary function is to contract and pump blood throughout the body, a critical and continuous process that does not rely on conscious thought. Cardiac muscle fibers are connected by intercalated discs which facilitate synchronized contraction, ensuring the heart functions as a cohesive unit. Due to its crucial role and unique properties, including its endurance and ability to operate without fatigue, the myocardium perfectly fits the description of the muscle in question from the exercise.

Biology MCAT Content

For students preparing for the Medical College Admission Test (MCAT), understanding the differences between muscle types is part of the essential biology MCAT content. It encompasses the study of muscle physiology, including the molecular basis of muscle contraction, types of muscle tissues, and their roles in the human body. This knowledge not only forms a foundational part of the MCAT but is also crucial for future medical professionals. Recognizing that the myocardium is an involuntarily controlled striated muscle is the kind of detail that could be tested on in the biological sciences section of the exam.