Muscle Tissue

MUSCLE TISSUE

Muscle tissue is a type of soft tissue in the body. It is by definition contractile tissue and is

formed by the process of myogenesis in the embryo. As mentioned, the three types of muscle

tissue include skeletal (striated) muscle, smooth (nonstriated) muscle, and cardiac (semi-

striated) muscle. Figure 22 shows what these different muscle types look like:

There is no conscious control over smooth muscle and cardiac muscle but there is activation of

both muscle types by the central nervous system and the endocrine system. The contraction of

skeletal muscle is voluntary with input from the central nervous system. Deep tendon reflexes

(like the knee-jerk reaction) will happen involuntarily but depend on the central nervous system

without actual thinking involved.

Muscle cells are contractile and are called myocytes. They can be very short muscle cells or as

long as ten centimeters in length. They can be connected by arrangements of myofilaments,

which are regularly repeated to allow the muscle tissue to have definition.

Skeletal muscle is anchored by tendons or aponeuroses to bone as is used to cause skeletal

movement and to affect a certain posture. About 40 percent of the body by mass is made from

skeletal muscle. It is known for striations. Smooth muscle is non-striated and is found within

the walls of internal structures, such as the stomach, esophagus, bronchi, intestines, bladder,

urethra, ureters, blood vessels, and the arrector pili in the skin (which makes goosebumps).

Cardiac muscle is also striated and is found in the heart.

Both cardiac and skeletal muscle contains sarcomeres, which are packed bundles into bundles

of muscle. Sarcomeres are made from long, fibrous proteins that act as filaments that slide past

one other when a muscle contracts or relaxes. Skeletal muscle has parallel bundles of cells,

while cardiac muscle has intercalated discs that connect the striations and non-parallel angles.

Smooth muscle can contract continuously, while skeletal muscle contracts in bursts.

Skeletal muscle is the only type of muscle that connects to nerves via a neuromuscular junction,

which will be discussed in a later chapter. The fibers are cylindrical and long in skeletal muscle,

branching in cardiac muscle, and very short and fusiform in smooth muscle. There are multiple

nuclei in skeletal muscle but not in other muscle types. Cardiac muscle is highly self-regulated

with a rapid response, while smooth muscle has a slow but spontaneous response. Only skeletal

muscle is not self-regulated (relying on the neuromuscular junction).

There are two types of skeletal muscle: Type 1 (slow twitch) muscle and Type 2 (fast twitch

muscle). There are several types of fast twitch muscle fibers. Slow twitch muscle has many

capillaries and myoglobin, which gives the muscle its red coloration. It carries more oxygen and

has sustained aerobic activity. Type IIa muscle is aerobic and looks red in color like Type 1

muscle. Type 2x muscle is less dense in mitochondria and myoglobin. It is the fastest muscle

type in humans and is more forceful than other skeletal muscle but cannot sustain itself. Type

2b muscle is not found in humans and is pale in color. It is the fastest type of skeletal muscle

(found in rodents).

Smooth muscle is divided into single and multi-unit types. Single unit types will contract as a

syncytium (which is a multinucleated mass that isn’t separated into separate cells). Multi-unit

smooth muscle will allow for finer control and gradual responses because each separate muscle

is innervated separately so that, ultimately, the tissue contracts as a whole but with a graded

response.

Smooth muscle is divided into the types of things it supplies. For example, the smooth muscle

that is found in the blood vessel walls is called vascular smooth muscle. One type is the tunica

media layer seen in the vessels, while another is called uterine smooth muscle (because it is the

myometrium or muscle layer of the uterine cells). The iris of the eye has smooth muscle (called

the ciliary muscle). There is similar smooth muscle in the respiratory tract, bladder, lymph

tissue, and in the skin (the arrector pili). The mesangial cells in the glomeruli of the kidneys are

very similar to smooth muscle cells.