Muscle Biology

Muscles cannot expand. They can only contract. Hence, muscles responsible for body movements frequently occur in pairs called agonist-antagonist pairs, for example, biceps and triceps. The biceps cause movement towards the body or central axis and triceps cause movement away from the body or away from the central axis.

There are two types of muscles, voluntary and involuntary. Voluntary muscles are under our own control while involuntary muscles are not. The arrector pili muscles are small muscles attached to hair follicles in mammals. Contraction of these muscles causes the hairs to stand on end. Each arrector pili is composed of a bundle of smooth muscle fibres, which attach to several follicles (a follicular unit) and is innervated by the sympathetic branch of the autonomic nervous system. The contraction of the muscle is, therefore, involuntary stresses such as cold, fear etc., may stimulate the sympathetic nervous system and thus, cause contraction, but the muscle is not under conscious control.

Shivering is an involuntary action of striated muscles ( skeletal muscles). Shivering is also called as rigors or shuddering. It is a bodily function in response to early hypothermia or just feeling cold in warm-blooded animals. When the core body temperature drops, the shivering reflex is triggered to maintain homeostasis. Skeletal muscles around begin to shake in small movements, creating warmth by expending energy. 

Autorhythmicity is a property of electrically active cells, that show rhythmic activity without being driven by the rhythmic external stimulation, e.g., heart muscle cells or neurons.
Thus, the correct answer is option (C), 'Heart'.

The shape of the skeletal muscle is cylindrical, while that of visceral muscle is pear-shaped or spindle-shaped. Actin filaments which are thin filaments and myosin which are thick filaments are contractile proteins in the skeletal muscle as well as in the visceral muscle.
Thus, the correct answer is the option (D), 'Presence of actin and myosin filaments'.

• Cardiac muscles have the property of both the striated muscles (or skeletal muscle) and unstriated muscle or smooth muscle.
• Cardiac muscle is involuntary, which is the property of smooth muscle.
• Cardiac muscles also have features of skeletal muscles. Like skeletal muscle, cardiac muscle cross striations formed by rotating segments of thick and thin protein filaments and the primary structural proteins of cardiac muscle are myosin and actin. 
• However, skeletal muscles are voluntary as they are supplied with the somatic nerve.

The muscles in the upper arm and shoulder are striated muscles. So, option B is incorrect.
Heart possesses unique muscles, called cardiac muscles, which are involuntary striated muscles. So, option C is incorrect.
The intestinal muscles are involuntary and are smooth muscle. So, option D is incorrect.

A syncytium is a multinucleated mass of cytoplasm, that is not separated into individual cells.
Example: Formation of skeletal muscle. Large skeletal muscle fibers formed by the fusion of thousands of individual muscle cells.
Cardiac muscle's syncytium is important because it allows rapid coordinated contraction of muscles along their entire length.
Smooth muscles, however, are not syncytial
So, the correct answer is 'Smooth muscle'

There are three types of muscle tissue such as cardiac, smooth or unstriated and skeletal or striated.
Cardiac muscle cells are located in the walls of the heart, appear striated and are under involuntary control.
Smooth or unstriated muscle fibers are located in the walls of hollow visceral organs, except the heart, appear spindle-shaped and are under involuntary control.
Skeletal or striated muscles fibers occur in muscles, which are attached to the skeleton. They are striated in appearance and are under voluntary control.
Thus, the correct answer is option D.

Myoglobin is a protein found in the muscle cells of animals. It functions as an oxygen-storage unit, providing oxygen to the working muscles.
Thus, the correct answer is option (A), 'Muscles'.

Smooth muscles are the muscle tissues that contracts without conscious control (involuntary), having the form of thin layers or sheets. It is made up of spindle-shaped, unstriated cells with single nuclei and found in the walls of the internal organs such as stomach, uterus, bladder, intestine and blood vessels (veins, arteries) except for heart.
Thus, the correct answer is option D.

Three major types of skeletal muscle fibers have been identified in vertebrates: slow-oxidative, fast-oxidative, and fast-glycolytic. Slow-oxidative fibers are specialized for endurance activities such as swimming, running long distances like the marathon, or maintaining posture. They contract slowly and fatigue slowly. These fibers require a steady supply of oxygen. They derive most of their energy from aerobic respiration and are rich in mitochondria and capillaries. Slow-oxidative fibers are rich in myoglobin, a red pigment similar to hemoglobin in red blood cells, which stores oxygen. Myoglobin enhances rapid diffusion of oxygen from blood into muscles during strenuous muscle exertion. Because of the myoglobin, slow-oxidative fibers are red. Birds that fly have red breast muscles specialized to support sustained activity. In humans, the proportions of slow-oxidative and fast-oxidative fibers vary among individuals and from muscle to muscle in the same person.

Six actin filaments surround each myosin filament and the active sites of an actin filament are available to three different myosin filaments in the myofibril of skeletal muscle fibre.

Thigh muscles are skeletal muscles, which are striated and under voluntary control.

Cardiac muscles are striated involuntary, Contract quickly and do not get fatigued.