Smooth Muscle Tissue

• Smooth Muscle Tissue – The name “smooth muscle” comes from the fact that this muscle tissue lacks the visible striations that are a noticeable characteristic of the other two muscle tissues, skeletal and cardiac muscle.

• Smooth muscle fibers contain numerous myofibrils that are oriented along the long axis of the fiber, and which extend from end to end within the fiber. These myofibrils are composed of the same thin myofilaments of actin and myosin contained in the other muscle tissues, except that they are arranged in a more random fashion.

• This means that the thicker myosin filaments do not line up to produce a “banded” or “striped” look, consequently, smooth muscle contains no visible striations. This is why it is called non-striated muscle tissue.

• Smooth muscle is located in the walls of many internal organs which is why it is often called “visceral” muscle. Organs such as the stomach, intestines, urinary bladder, uterus, arteries, veins, base of hair follicles, and in the iris of the eye are all examples of organs containing smooth muscles in their walls.

smooth muscle in an artery wall

• Smooth muscle is therefore responsible for the movement of food through the digestive tract (called “peristalsis”), the constriction of blood vessels, the emptying of the bladder, the “labor” pains of childbirth as well as for pushing the baby out of the mother’s body, and many more important bodily functions.

• Individual smooth muscle fibers are elongated with tapering ends.

Can you see the elongated, tapered fibers?

• Smooth Muscle fibers are arranged with their tapered ends overlapping so they can form different functional structures. These functional structures are bundles, sheets, or cords.

Bundles

Cord

Sheet

• As with cardiac muscle tissue, each smooth muscle fiber has a single, oval shaped nucleus located in the center of the cell.

• Smooth muscle, unlike skeletal and cardiac muscle, contracts very slowly, maintains the contracted state for an extended period of time, and relaxes very slowly, even though the contraction mechanism is identical to that of the other two. As with cardiac muscle, smooth muscle tends not to fatigue even when the contracted state is held for a long time. This is very different than skeletal muscle.

• One additional important feature of smooth muscle is that it can “stretch” without causing an increase in the internal pressure of the organ. This is an important fact when one considers the possibility of a “full” stomach, bladder, intestine, or when a woman is pregnant and the baby continues to grow inside the uterus for up to nine months.

General Characteristics

1. Location –

Commonly found in the walls of internal organs

2. Cell type or description –

Elongated, tapered fibers arranged as bundles, sheets, or cords

What functional unit is this a picture of?

3. Myofibril

arrangement-

least dense, yet still run end to end within the cell

random; consequently no visible striations Notice the same actin & myosin as in

skeletal & cardiac muscle arranged in the same sarcomere formation.

4. Location of nucleus

or nuclei –

One oval nucleus located in the center of the cell.

Note all the darkly stained nuclei

5. Vascular Supply

and relative rank-

- Good blood supply when used, reduced to minimal levels when inactive, highly variable with activity

- Ranks 3rd among muscle tissues Arrows pointing to capillaries

6. Description of contraction

and control factor –

- Rhythmic, slow to contract & relax, holds contraction without fatigue.

- Involuntary contraction

7. Alternative names –

Involuntary, non-striated or Visceral muscle

Longitudinal View (long Axis)

Note the elongated, tapered fibers, lack of striations, and single, centered nucleus

Another Longitudinal View

Note the elongated, tapered fibers, lack of striations, and single, centered nucleus

Cross-sectional View (short axis)

Dark areas are single centered nuclei that were cut through

Some slides include both views in one!!

Longitudinal section

Cross-section

A comparison of all three

Note the organs from which each type comes

A. C.

1. What tissue is “A”?2. What tissue is “B”?3. What tissue is “C”?

B.

• “A” was Skeletal Muscle Tissue.

• “B” was Smooth Muscle Tissue.

• “C” was Cardiac Muscle Tissue.

Did you get them right?

A.

B.

C.

Now, name the primary and a secondary identifying characteristics that are visible in each slide that helped you name the tissue.

B.

• A hint here:

– The primary identifying characteristic would be one that is visible and unique to that tissue only.

– A secondary identifying characteristic is one that is common to two, but when combined with other visible traits, assures the tissue’s identity.

A.

B.

C.

Here they are again.

B.

• Primary for “A” - multiple, peripheral nuclei

• Secondary for “A” – dark striations, or long, cylindrical fibers

• Primary for “B” – no striations at high magnification

• Secondary for “B” – single, centered nucleus, elongated, tapered fibers

• Primary for “C” – intercalated discs

• Secondary for “C” – branching, weaving network, single, centered nucleus, or light striations

Can you identify the two muscle tissues in this slide?

Use primary identifying characteristics to identify them.

smooth

cardiac

Identify each of the following:

A. B. C.