Chapter 10

MUSCULAR TISSUE

OBJECTIVES:

General

1. Be able to describe the morphological and functional properties of muscular tissue that distinguish it from the other basic tissues.

2. Be able to relate the morphological features of each type of muscle to its unique function in the body.

 

Smooth Muscle

1. Be able to describe the morphological features of smooth muscle cells.

2. How does smooth muscle exert its contractile force on tissues around it?

3. Be able to name the molecules in smooth muscle which provide the contractile force.

Skeletal Muscle

1. Be able to describe the molecular structure of the sarcomere and relate it to the morphology seen in the light microscope.

2. Be able to describe the modifications of the endoplasmic reticulum in skeletal muscle. Relate them to the control of muscular contractions.

3. Be able to give the location of different organelles in a skeletal muscle cell.

4. Be able to describe the connective tissue sheaths of skeletal muscle and how they contribute to contraction. How do tendons and connective tissue sheaths interconnect?

 

Cardiac Muscle

1. Be able to describe the morphological and functional features that distinguish cardiac muscle from skeletal muscle at the level of the light and electron microscope.

2. Be able to describe the origin, function and morphological features of the intrinsic conducting system of cardiac muscle.

3. Be able to describe the morphological features and functions of the intercalated disc.

 

General Characteristics of Muscle Tissue

1. Most muscle tissue is derived from mesoderm.

2. Structural and functional unit is the 'Muscle Fiber' which is itself a muscle cell.

3. The various structures of the muscle fiber are termed as follows:

a. Sarcolemma - Plasma membrane

b. Sarcoplasm - Cytoplasm

c. Sarcoplasmic reticulum - Endoplasmic Reticulum

d. Sarcosomes - Mitochondria

4. Based upon the morphological and functional differences, muscle fibers are classified into three types.

a. Smooth: Unstriped; involuntary; visceral; contraction is weaker and of longer duration.

b. Skeletal: Striated; voluntary; contraction is stronger and of shorter duration.

c . Cardiac: Striated; involuntary; autogenic, contraction is strong, continuous and rhythmic.

SMOOTH MUSCLE

- Fibers are fusiform (spindle shaped); 6 micrometer in diameter and 20 micrometer to as much as 500 micrometer long, the latter is the case in the pregnant uterus.

- Cytoplasm is eosinophilic and contains many filaments which are not discernible at the light microscope.

- Nucleus is one, cigar shaped and centrally located.

- Fibers are often packed to form sheets or bundles.

- Cells are attached to each other by gap junctions.

- The light microscope structure of the smooth muscle will vary depending upon the plane of section and the level of smooth muscle through which the section has passed. (Fig. 10-1)

- Ultrastructure of smooth muscle is shown in Fig. 10-2.

- EM shows actin and myosin filaments, however, the latter are not nicely preserved.

- Cytoplasm contains most cell organelles.

- The notable feature is the numerous vesicles below the cell membrane.

- The Junctional Complex includes:

1. Fascia Occludens

2. Gap Junctions

- Vascular smooth muscle cells can synthesize elastin and collagen.

 

Organization of Smooth Muscle Cells

1. Isolated: Found in the capsule of organs or in the connective tissue.

2. Small Bundles: Found in the skin (errector pili muscle associated with hair follicles), papillary duct, and scrotum.

3. Long Sheets: Found surrounding the luminal organs. In most organs, the inner sheet is circularly arranged and the outer sheet is longitudinal.

 

Mechanism of Contraction

Contraction of actin and myosin filaments.

 

SKELETAL MUSCLE

Organization of Skeletal Muscle (Fig. 10-3)

Muscle is made of muscle fasciculi which in turn are made of muscle fibers. The connective tissue sheath enclosing the whole muscle is Epimysium; the connective tissue sheath enclosing the muscle fasciculus is Perimysium; the connective tissue sheath enclosing one muscle fiber is Endomysium. The muscle fiber or muscle cell is the structural unit which in turn is made of many myofibrils.

 

Skeletal Muscle Fiber

- Fiber is cylindrical with a diameter between 10 and 100 - micrometer and a length between 1 and 40 mm.

-Multinucleated; nuclei, oval and peripherally located.

-Muscle fiber is made of numerous myofibrils which in turn are made of myofilaments.

- Myofilaments are of two types:

1. Thin or actin filaments

2. Thick or myosin filaments

- Cytoplasm of a muscle fiber shows regular dark and light striations due to varied thickness of myofilaments.

- Functional unit of a muscle fiber is Sarcomere which is defined as a segment between two Z lines.

- Sarcomere is about 0.5 to 1 micrometer in diameter and 2 micrometers long.

- Sarcomere consists of the following parts (See Fig. 10-4):

1. A Band - mainly thick filaments, but also contains a part of thin filaments.

2. I Band - only thin filaments which extend on either side of Z lines.

3. H Band - the central part of thick filaments.

4. M Line - bisects the H band.

 

Ultrastructure of Skeletal Muscle Fiber (Fig. 10-5)

- Sarcoplasm contains glycogen granules, lipid droplets, mitochondria and Golgi complex.

"T" System and Sarcoplasmic Reticulum

The sarcolemma at the junction between A and I bands dips into the fiber as central tubules of the T-system. On either side of the central tubule lie cisternae of smooth endoplasmic reticulum. Thus, a central tubule and two lots of cisternae on either side of the central tubule constitutes a 'Triad'. Note that the position of a triad is at the Z line in the amphibian skeletal muscle (Fig. 10-5) and at the junction between A and I bands in the mammalian skeletal muscle.

 

Functions of a Muscle Triad

Cisternae of smooth endoplasmic reticulum store calcium which is released during contraction.

Note: Contraction of a muscle fiber depends upon the release of calcium ions; if there is no calcium release, there will be no muscle contraction.

Huxley's Sliding Filament Theory of Muscle Contraction

Thick and thin filaments maintain the same length but slide past each other.

Results of Muscle Contraction (Fig. 10-6)

1. I band reduces in length.

2. Sarcomere reduces in length.

3. H band disappears.

4. No change in the length of actin and myosin filaments.

Events and Chemistry of Muscle Contraction

1. Depolarization of sarcolemma i.e. propagation of action potential.

2. Depolarization extends down the central tubule and excites the adjacent cisternae of the smooth endoplasmic reticulum.

3. Smooth endoplasmic cisternae when stimulated release calcium ions.

4. The binding of calcium to troponin stimulates contractile links between actin and myosin.

5. Relaxation occurs when calcium is actively taken up by smooth endoplasmic reticulum.

 

Types of Skeletal Muscle Fibers

1. Red Fiber (Fig. 10-7)

2. White Fiber (Fig. 10-8)

3. Intermediate

Compared to white fibers, red fibers are richer in myoglobin and mitochondria. Red fibers maintain activity for longer periods because they are less prone to fatigue. Postural muscles are an example of red fibers. Most mammalian muscles are of intermediate type.

Motor End Plate

Point of attachment of a nerve with a skeletal muscle fiber is referred to as motor end plate.

Light Microscope View

Seen only after gold-chloride impregnation. The point of contact between a muscle fiber and an axon shows a structure resembling a "bunch of grapes".

 

EM Morphology (Fig. 10-9)

1. Axon branches distally, each distal end of the axon dilates to form a large vesicular structure.

2. At and/or near the dilation, the axon loses its myelin sheath.

3. The dilation is filled with acetylcholine (neurotransmitter) containing vesicles.

4. Sarcolemma adjacent to the dilation forms subneural clefts.

5. Cytoplasm below the sarcolemma shows aggregation of mitochondria and lacks striations.

 

Events of Impulse Conduction from the Axon to the Muscle Fiber

1. Depolarization of the distal end of the axon stimulates the rupture of acetylcholine vesicles.

2. Acetylcholine initiates depolarization of sarcolemma, followed by depolarization of T tubule, release of Ca++ions and contraction of myofilaments.

3. Acetylcholine is immediately inactivated by cholinesterase enzyme present in the subneural cleft. This inactivation results in the relaxation of muscle.

 

Diseases of Neuro-Muscular Junction (Motor End Plate)

1. Organophosphorus Toxicity: Cholinesterase enzyme is phosphorylated.

Result: Initial stimulation followed by depression.

2 . Botulism: This disease occurs due to Botulinus toxin secreted by 'Clostridium botulinum'. This toxin blocks the release of acetylcholine.

Result: Flacid Tetraparesis.

3. Myasthenia Gravis: Nerve terminals are deficient in acetylcholine. Usually reported in dogs and cats.

Results: Weakness in muscles.

CARDIAC MUSCLE

- Average muscle fiber is 100 micrometer long and 15 micrometer wide. - Each muscle fiber has only one or two nuclei, centrally located.

- Fibers branch and anastomose. The point of anastomosis (junction between two adjacent fibers) shows intercalated disc.

- Intercalated discs at the EM are made of gap junctions, desmosomes and fascia adherens.

- Similar to the skeletal muscle fiber, the cytoplasm of a cardiac fiber contains actin and myosin filaments which are responsible for its regular striations.

- The ultrastructure of a cardiac muscle, and a sarcomere, and the

physical and/or chemical events of cardiac muscle contraction are the same as described for the skeletal muscle with one exception that 'T' tubules in the mammalian cardiac muscle are located at the 'Z' line. (Fig. 10-10)

Purkinje Fibers (Fig. 10-11)

- Modified cardiac fibers with impulse conducting function.

- Located in the S.A. node, A.V. node, interventricular septum, septomarginal band, and beneath the endocardium.

- Purkinje fibers differ from the cardiac fibers in that they:

1. occur in groups.

2. are larger in diameter (50 micrometer or more).

3. contain pale staining cytoplasm.

4. contain fewer myofibrils which are peripherally located and do not reveal regular striations.

5. contain large amounts of glycogen.

6. contain less developed striations.