Muscle Biology

Non-striated muscles are called as smooth muscles. Their activities are not under the voluntary control of the nervous system and are therefore known as involuntary muscles. They are called as visceral muscles because they line the blood vessels, stomach, digestive tract and other internal organs. So, the correct answer is option D. 

Multi unit smooth cells, however, are not electrically coupled and hence, the cells must be stimulated separately. These smooth muscles contain few gap junctions, and therefore contractions do not spread from cell-to-cell as in single-unit smooth muscle. Each cell is therefore situated close to an axon terminal where it can easily make contact with a neurotransmitter. As the cells are not electrically coupled the force of contraction can be controlled by the number of contractile muscle fibres. Multi-unit smooth cells can be found in the in the trachea, the large elastic arteries, the iris of the eye, in the lungs, erectile tissues of hair follicles ( Arrector pili of the dermis) and the vas deferens in the male genital tract.
The response of the whole muscle depends on upon the number of motor units active, the frequency of discharge in the fibers and the relative amount of excitatory and inhibitory input. Multi-unit smooth muscle activity can be initiated by hormones, but it is not much affected by rapidly stretching the muscle.

Most of the skeletal muscles are attached to the bone on their ends by tendons. As the muscles contract, a force is exerted on the bones, which helps to support and move our body along with its appendages. Usually, one end of the muscle is fixed in its position, while the other end moves during contraction.
The "origin" is the attachment site that doesn't move during contraction, while the "insertion" is the attachment site that does move when the muscle contracts. The insertion is usually distal, or further away, while the origin is proximal, or closer to the body, relative to the insertion. "Belly" of muscle is between origin and insertion.

Visceral muscles are located in the inner walls of hollow visceral organs of the body like the alimentary canal, reproductive tract, etc. Each spindle-shaped smooth muscle fiber contains a single, central nucleus. They do not exhibit any striation and are smooth in appearance. Hence, they are called smooth muscles (non-striated muscle). Their activities are not under the voluntary control of the nervous system and are therefore known as involuntary muscles. They assist, for example, in the transportation of food through the digestive tract and gametes through the genital tract.

Muscular strength usually refers to the "ability to exert a force on an external object". The strongest muscle in humans, based on its weight is the masseter muscle or the jaw muscle. It is the muscle that can generate the largest externally measurable force attributable to the action of a specific, single muscle. Its strength is the result of incredibly densely packed muscle fibers combined with being attached to a short-armed lever. With all muscles of the jaw working together it can close the teeth with a force as great as 55 pounds (25 kilograms) on the incisors or 200 pounds (90.7 kilograms) on the molars.

Striated and unstriated are the types of muscles. Skeletal muscles are closely associated with the skeletal components of the body like limbs. They have a striped appearance under the microscope and hence, are called striated muscles. As their activities are under the voluntary control of the nervous system, they are known as voluntary muscles too. They are primarily involved in locomotory actions and changes of body postures. 

Epimysium is the outermost white fibrous connective tissue that completely surrounds and encases skeletal muscle. It is composed of dense irregular connective tissue. The most important biological purposes of this layer are protection from strain and friction. The smoothness of the tissues promotes seamless gliding, and also protect joints and other high-impact areas from rips or tears and promotes smooth movement of the limbs and joints.

The gluteus maximus (also known as glutus maximus or, collectively with the gluteus medius and minimus, the glutes) is the largest and most superficial of the three gluteal muscles. It makes up a large portion of the shape and appearance of the hips. It permits a variety of motion and allows us to sit upright.

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. 

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'