Histology of Muscle Tissue

4 types of tissue in body

- epithelial tissue - CT - *muscle tissue* - nervous tissue

what are the 3 types of skeletal muscle fibers (cells)?

1. type I -- slow, red oxidative fibers 2. type IIa -- fast, intermediate oxidative-glycolytic fibers 3. type IIb -- fast, white glycolytic fibers

quiz

ID the tissue = skeletal muscle tissue ID the CT layer at arrow = endomysium which component of the intercalated disc allows the ionic continuity between adjacent cardiac muscle cells for the observed coordinated activity? gap junction

smooth muscle tissue ileum

Here is a histological section through the ileum. In the wall, you can see the inner layer of circularly arranged smooth muscle tissue, as well as the outer layer of longitudinally arranged smooth muscle tissue.

type IIa -- fast, intermediate oxidative-glycolytic fibers

intermediate in diameter. many mitochondria and lots of myoglobin, but also have lots of glycogen (polysaccharide that yields glucose when hydrolyzed). use *BOTH* oxidative metabolism and anaerobic glycolysis to produce energy. adapted for *rapid contractions and short bursts of energy* associated with activities like *walking/sprinting*

type IIb -- fast, white glycolytic fibers

largest in diameter, fewer mitochondria and myoglobin, lots of glycogen (pale color). derive energy primarily via *anaerobic glycolysis*. adapted for *rapid contractions, but fatigue quickly* *weight lifting and throwing a baseball*

what does muscle tissue develop from?

mesoderm

muscle cell other terms?

muscle fiber, myofiber

skeletal muscle longitudinal section

number of large, elongated, cylindrical, multinucleated skeletal muscle fibers in longitudinal section. cell continues in both directions. see striations!

prefixes associated with muscle

sarco-, myo-, mys-

plasma membrane (plasmalemma) of a muscle cell?

sarcolemma

cytoplasm of a muscle cell?

sarcoplasm

smooth ER of muscle cell?

sarcoplasmic reticulum

type I -- slow, red oxidative fibers

smallest in diameter. many mitochondria and lots of myoglobin (dark red color). myoglobin binds oxygen. type I fibers derive energy from *aerobic oxidative phosphorylation of fatty acids*. they're adapted for *slow, continuous contractions over long periods* found in *postural muscles of the back*

skeletal muscle tissue

under voluntary control, strong quick contractions, striated due to organization of actin/myosin filaments. skeletal muscle cells are large elongated cylindrical and multinucleated. each skeletal muscle cell is considered a *structural syncytium*, as each developed from fusion of many embryonic mesodermal cells called myoblasts. diameter = 100 micrometers and cell length varies from a few mm to almost a meter (sartorius). many oval nuclei at the periphery of the cell directly under sarcolemma (diff than cardiac and smooth!!) long section: see striations. cell is in the boxed area and the same cell continues on in both directions!

connective tissue sheaths associated with skeletal muscle (the organ):

1. *endomysium* -- a delicate layer of CT surrounding an individual muscle fiber/cell 2. *perimysium* -- a thicker CT layer surrounding each fascicle 3. *epimysium* -- an external sheath of dense CT surrounding the entire muscle such as the biceps brachii muscle

special characteristics of muscle tissue

1. *excitability* (irritability): the ability to respond to stimuli by producing electrical signals. plasma membrane of muscle cells can produce/conduct APs like the axons of neurons! muscle cells use APs to release Ca2+ ions from the sarcoplasmic reticulum, which are the final trigger/signal for contraction to occur 2. *contractility*: the ability to generate force (tension) when adequately stimulated. when tension generated by muscle > resistance of object to be moved, shortening occurs. contraction *requires ATP* to generate force through binding actin/myosin 3. *extensibility*: the ability to be stretched/extended 4. *elasticity*: ability of muscle to return to original length after being shortened/stretched

functions of muscle tissue?

1. *producing movement* 2. *maintaining posture* (skeletal muscle tissue) 3. *stabilizing joints* -- skeletal muscles and associated tendons cross joints and work to stabilize them. important for knee and shoulder especially. 4. *generating heat* (thermogenesis) -- during the process of gaining energy to produce muscle contraction from ATP, some energy is lost as heat

explain the different types of muscle contraction

1. isotonic contraction = the tension developed by the contracting muscle remains constant, while the muscle changes length - concentric isotonic contraction = when the tension generated by the contracting muscle is great enough to overcome the resistance of the object. muscle shorts as seen in a. - eccentric isotonic contraction = muscle generates tension while muscle is lengthened, as seen in b, when lowering the book through controlled, coordinated, purposeful movement due to tension is generated through biceps brachii. 2. isometric contraction = the tension developed by the contracting muscle is not great enough to exceed the resistance of the object to be moved, muscle doesn't change shape. hold book steady

what do the connective tissue sheaths of the skeletal muscle do?

1. provide a pathway for vessels traveling within the skeletal muscle 2. transmit mechanical forces generated by the contracting cells

3 types of muscle tissue

1. skeletal muscle tissue 2. cardiac muscle tissue 3. smooth muscle tissue

what does a skeletal muscle (i.e. the organ) contain?

1. skeletal muscle tissue 2. nerve fibers 3. connective tissue 4. vessels (arteries, capillaries, veins, lymphatic vessels)

cardiac muscle tissue

Cardiac muscle tissue is under *involuntary control*, has strong, quick, rhythmic contractions, and is striated due the organization of its actin and myosin filaments. However, the striations in cardiac muscle tissue are not as apparent as those in skeletal muscle tissue. Cardiac muscle cells are typically uninucleated cells. The single nucleus is found centrally within the cardiac muscle cell. Cardiac muscle cells are branched cells that are roughly 15µm in diameter and 85 to 100 µm in length. These branched cells are interwoven with one another in the myocardial layer of the heart wall and joined to one another through *intercalated discs*. This cartoon representation shows cardiac muscle tissue in longitudinal and in cross section. Notice that in the longitudinal section, striations are apparent. Also, note these striations in an actual histological section. Notice that these *branched cells are joined by intercalated discs*. Finally, notice how the *nucleus of a cardiac muscle cell is found centrally* in the cross section image.

cardiac muscle tissue -- intercalated discs

FA = fascia adherens - anchoring junction - links actin (thin) filaments D = desmosome (macula adherens) - anchoring junction - links intermediate filaments N = gap junction - communicating junction - intracellular communication Again, associated with cardiac muscle tissue are intercalated discs. Intercalated discs are dark staining transverse lines that indicate the interface between adjacent cardiac muscle cells. In this electron micrograph, we see portions of two cardiac muscle cells and the site of their interface at the intercalated disc. (curly wavy part in middle) The intercalated disc has a number of cellular junctions associated with it. First, we see fascia adherents (pl.) and desmosomes, which are anchoring junctions that bind cells together and keep the cells from pulling apart from one another during contractions. Specifically, a fascia adherens binds the actin (thin) filaments of one cell to the actin (thin) filaments of another cell, while a desmosome (macula adherens) binds the intermediate filaments of one cell to the intermediate filaments of another cell. Also associated with intercalated discs are *gap junctions, which are communicating junctions that connect the cytoplasm of one cell to the cytoplasm of another cell. This allows ions and messenger molecules to flow from one cell directly to another and causes the cells, although separate, to act as a functional syncytium. If one cell gets the signal to contract, they all get the signal to contract*.

cardiac muscle tissue longitudinal and cross sections

In the histological image on the left, we again see a longitudinal section of cardiac muscle tissue. In this image, red arrows indicated just a few of the *intercalated discs --- which are only associated with cardiac muscle tissue*. These intercalated discs are where one branching cardiac muscle cell connects to another branching cardiac muscle cell. This longitudinal section also allows you to see the striations present due to the organization of the actin and myosin filaments. In the histological image of the right, we again see a cross section of cardiac muscle tissue. Indicated by the green arrows, are a number of centrally located nuclei of cardiac muscle cells.

smooth muscle tissue cross section and longitudinal section

In the upper portion of the image, we see smooth muscle tissue in cross section, while in the bottom portion of the image, we see smooth muscle tissue in longitudinal section. In both views, a few of the centrally located nuclei are indicated by the arrows. The longitudinally sectioned smooth muscle tissue reminds you that smooth muscle tissue is a nonstriated tissue that is composed of fusiform-shaped cells.

smooth muscle tissue in intestine

Smooth muscle tissue is often found in the walls of organs organized into sheets - where all the cells in one particular sheet are arranged in the same direction. Here, for example, they are showing the two layers of the muscularis externa from the wall of the small intestine. There is an inner circularly arranged layer and an outer longitudinally arranged layer.

summary

There are three types of muscle tissue (i.e., skeletal muscle tissue, cardiac muscle tissue, and smooth muscle tissue), each with distinct morphological characteristics and specific distribution within the body.

smooth muscle tissue

Smooth muscle tissue is under involuntary control, has weak, slow contractions, and appears nonstriated due the organization of its actin and myosin filaments. Smooth muscle cells are uninucleated, fusiform-shaped cells. By fusiform, I mean wide in the middle and tapering at both ends. The *single nucleus is found centrally within the smooth muscle cell*. Smooth muscle cells vary in length based on location - with smooth muscle cells being as short as 20 μm in blood vessels or upwards of 500 μm in the wall of a pregnant uterus. Smooth muscle is often found in the *wall of hollow organs to propel substances along, such as in the wall of the digestive tract or urinary tract*. It can also be found in locations where the size or shape of something needs to be altered --- such as in the wall of blood vessels in order to change their diameter or in the iris in order to change the size of the pupil. This cartoon representation shows smooth muscle tissue in longitudinal and in cross section. Notice that in the *longitudinal section, striations are not seen*. Instead, the fusiform shape of the cells with the centrally located nuclei are seen. Finally, in the cross section image, notice how the nuclei are found centrally within the cells.

muscle tissue is composed of _____ that ______

contractile cells; produce movement

skeletal muscle cross section

one fiber outlined in green. number of nuclei of different skeletal muscle cells with arrows. nuclei right under sarcolemma. 1 cell labeled with asterick on right. 3 nuclei just under sarcolemma!

creating a skeletal muscle (image), like the brachioradialis muscle

take individual muscle fibers (cells) and wrap *endomysium* around each. then take a bunch of endomysium wrapped muscle fibers, group them, and wrap *perimysium* around them. this perimysium wrapped group = fascicle. then take these fascicles, group them together, put more CT and called the *epimysium* and we have a muscle. handed you a muscle on the outside it's the epimysium. epimysium is continuous with the tendon, which connects to the periosteum of the bone, and the periosteum of the bone is anchored to the bone tissue by *perforating (Sharpey's)* fibers. when a muscle contracts, it pulls on its surrounding endomysium, which pulls on perimysium, which pulls on epimysium, which pulls on tendon, which pulls on periosteum, which pulls on perforating fibers, then pulls on bone. this is how the force of the contracting cell is transferred to the bone.