12 microscopic anatomy

Most of the muscle tissue in the body is skeletal muscle, which attaches to the skeleton or associated connective tissue. Skeletal muscle shapes the body and gives you the ability to move—to walk, run, jump, and dance; to draw,

paint, and play a musical instrument; and to smile and frown. The remaining muscle tissue of the body consists of smooth muscle that forms the walls of hollow organs and cardiac muscle that forms the walls of the heart. Smooth and cardiac muscle move materials within the body. For example, smooth muscle moves digesting food through the digestive system, and urine from the kidneys to the exterior of the body. Cardiac muscle moves blood through the blood vessels.

Materials

• ▶▶  Three-dimensional model of skeletal muscle fibers (if available)

• ▶▶  Forceps

• ▶▶  Dissecting needles

• ▶▶  Clean microscope slides and coverslips

• ▶▶  0.9% saline solution in dropper bottles

• ▶▶  Chicken breast or thigh muscle (fresh from the market)

• ▶▶  Compound microscope

• ▶▶  Prepared slides of skeletal muscle (l.s. and x.s. views) and skeletal muscle showing neuromuscular junctions

• ▶▶  Three-dimensional model of skeletal muscle showing neuromuscular junction (if available)

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184

Exercise 12

Nuclei

(a)

Dark A band Light I band

Muscle fiber

Sarcolemma Mitochondrion

Figure 12.1 Microscopic anatomy of skeletal muscle. (a) Photomicrograph
of portions of two isolated muscle fibers (7253). (b) Part of a muscle fiber. One myofibril has been extended. (c) Enlarged view of a myofibril showing its banding pattern. (d) Enlarged view of one sarcomere (contractile unit) of a myofibril. (e) Cross-sectional view of a sarcomere cut through in different areas.

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(b)

Dark
A band

Light
I band

Nucleus

Myofibril

H zone

A band Sarcomere

M line

Thin (actin) filament Thick (myosin) filament

(c)

Thin (actin) filament

Elastic (titin) filaments Thick (myosin) filament

Z disc

I band

Z disc

I band

M line
Z disc

Z disc

(d)

(e)

I band

thin filaments only

H zone

thick filaments only

M line

Outer edge of A band

thick and thin filaments overlap

thick filaments linked by accessory proteins

Part of a skeletal muscle fiber (cell)

I band Z disc

A band

H zone

M line

I band Z disc

Microscopic Anatomy and Organization of Skeletal Muscle 185

Myofibril

Sarcolemma

Triad:

• T tubule

• Terminal cisterns
of the SR (2)

Tubules of the SR

Myofibrils Mitochondria

Sarcolemma

Figure 12.2 Relationship of the sarcoplasmic reticulum and T tubules to the myofibrils of skeletal muscle.

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Each of the three muscle types has a structure and function uniquely suited to its function in the body. Our focus here is to investigate the structure of skeletal muscle.

The Cells of Skeletal Muscle

Skeletal muscle is made up of relatively large, long cylindrical cells, called muscle fibers. These cells range from 10 to 100 μm in diameter. Some are up to 30 cm long.

Since hundreds of embryonic cells fuse to produce each muscle fiber, the cells (Figure 12.1a and b) are multinucleate; multiple oval nuclei can be seen just beneath the plasma mem- brane (called the sarcolemma in these cells). The nuclei are pushed peripherally by the longitudinally arranged myofibrils, long, rod-shaped organelles that nearly fill the sarcoplasm, the cytoplasm of the muscle cell. Alternating light (I) and dark (A) bands along the length of the perfectly aligned myofibrils give the muscle fiber its striped appearance.

Electron microscope studies have revealed that the myofi- brils are made up of even smaller threadlike structures called myofilaments (Figure 12.1d). The myofilaments are composed largely of two varieties of contractile proteins—actin and

Skeletal muscle is also known as voluntary muscle because it can be consciously controlled, and as striated muscle because it appears to be striped.

myosin—which slide past each other during muscle activity to bring about shortening or contraction of the muscle cells. The actual contractile units of muscle, called sarcomeres, extend from the middle of one I band (its Z disc) to the middle of the next along the length of the myofibrils (Figure 12.1c and d.) Cross sections of the sarcomere in areas where thick and thin filaments overlap show that each thick filament is surrounded by six thin filaments; each thin filament is surrounded by three thick filaments (Figure 12.1e).

At each junction of the A and I bands, the sarcolemma indents into the muscle cell, forming a transverse tubule (T tubule).These tubules run deep into the muscle fiber between cross channels, or terminal cisterns, of the elaborate smooth endoplasmic reticulum called the sarcoplasmic reticulum (SR) (Figure 12.2). Regions where the SR terminal cisterns border a T tubule on each side are called triads.

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186 Exercise 12 Activity 1

Examining Skeletal Muscle Cell Anatomy

1. Look at the three-dimensional model of skeletal muscle fibers, noting the relative shape and size of the cells. Identify the nuclei, myofibrils, and light and dark bands.

2. Obtain forceps, two dissecting needles, slide and coverslip, and a dropper bottle of saline solution. With forceps, remove a very small piece of muscle (about 1 mm diameter) from a fresh chicken breast or thigh. Place the tissue on a clean microscope slide, and add a drop of the saline solution.

3. Pull the muscle fibers apart (tease them) with the dissect- ing needles or forceps until you have a fluffy-looking mass of tissue. Cover the teased tissue with a coverslip, and observe under the high-power lens of a compound microscope. Look for the banding pattern by examining muscle fibers isolated at the edge of the tissue mass. Regulate the light carefully to obtain the highest possible contrast.

4. Now compare your observations with the photomicrograph (Figure 12.3) and with what can be seen in professionally pre- pared muscle tissue. Obtain a slide of skeletal muscle (longitudinal section), and view it under high power. From your observations, draw a small section of a muscle fiber in the space provided below. Label the nuclei, sarcolemma, and A and I bands.

What structural details become apparent with the prepared slide? ______________________________________________________

______________________________________________________

Nuclei of muscle fibers

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Muscle fibers, longitudinal view

Muscle fibers, cross-sectional view

Figure 12.3 Photomicrograph of muscle fibers, longitudinal and cross sections (8003).

Organization of Skeletal Muscle Cells into Muscles

Muscle fibers are soft and surprisingly fragile. Thousands of muscle fibers are bundled together with connective tissue to form the organs we refer to as skeletal muscles (Figure 12.4). Each muscle fiber is enclosed in a delicate, areolar connective tissue sheath called the endomysium. Several sheathed muscle fibers are wrapped by a collagenic membrane called the peri- mysium, forming a bundle of muscle fibers called a fascicle. A large number of fascicles are bound together by a much coarser “overcoat” of dense irregular connective tissue called the epimysium, which sheathes the entire muscle. All three sheaths converge to form strong cordlike tendons or sheetlike aponeuroses, which attach muscles to each other or indirectly to bones. A muscle’s more movable attachment is called its insertion, whereas its fixed (or immovable) attachment is the origin (Exercise 11).

Tendons perform several functions; two of the most impor- tant are to provide durability and to conserve space. Because tendons are tough dense regular connective tissue, they can span rough bony projections that would destroy the more deli- cate muscle tissues. Because of their relatively small size, more tendons than fleshy muscles can pass over a joint.

In addition to supporting and binding the muscle fibers, and providing strength to the muscle as a whole, the connective tis- sue wrappings provide a route for the entry and exit of nerves and blood vessels that serve the muscle fibers. The larger, more powerful muscles have relatively more connective tissue than muscles involved in fine or delicate movements.

As we age, the mass of the muscle fibers decreases,

and the amount of connective tissue increases; thus the skeletal muscles gradually become more sinewy, or “stringier.” +

Activity 2

Observing the Histological Structure of a Skeletal Muscle

Identify the muscle fibers, their peripherally located nuclei, and their connective tissue wrappings—the endomysium, perimysium, and epimysium, if visible (use Figure 12.4 as a reference).

Microscopic Anatomy and Organization of Skeletal Muscle 187

Bone Tendon

Epimysium

Epimysium

Perimysium Endomysium

Muscle fiber within a fascicle

(b)

Blood vessel

Perimysium wrapping a fascicle

Endomysium
(between individual muscle fibers)

Muscle fiber Myofibril

Fascicle

Perimysium

(a)

Figure 12.4 Connective tissue sheaths of skeletal muscle. (a) Diagram. (b) Photomicrograph of a cross section of skeletal muscle (403).

The Neuromuscular Junction

The voluntary skeletal muscle cells must be stimulated by motor neurons via nerve impulses. The junction between an axon of a motor neuron and a muscle fiber is called a neuromuscular junction (Figure 12.5, p. 188).

Each axon of the motor neuron usually divides into many branches called terminal branches as it approaches the muscle. Each of these branches ends in an axon terminal that par- ticipates in forming a neuromuscular junction with a single muscle fiber. Thus a single neuron may stimulate many muscle fibers. Together, a neuron and all the muscle fibers it stimu- lates make up the functional structure called the motor unit. Part of a motor unit, showing two neuromuscular junctions, is shown in Figure 12.6, p. 188. The neuron and muscle fiber

membranes, close as they are, do not actually touch. They are separated by a small fluid-filled gap called the synaptic cleft (see Figure 12.5).

Within the axon terminals are many mitochondria and ves- icles containing a neurotransmitter called acetylcholine (ACh). When an action potential reaches the axon terminal, voltage- gated Ca2+ channels open. Ca2+ enters the axon terminal and causes ACh to be released by exocytosis. The ACh rapidly diffuses across the synaptic cleft and combines with the recep- tors on the sarcolemma. When receptors bind ACh, a change in the permeability of the sarcolemma occurs. Ion channels open briefly, depolarizing the sarcolemma, and subsequent contrac- tion of the muscle fiber occurs.

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Studying the Structure of a Neuromuscular Junction

1. If possible, examine a three-dimensional model of skeletal muscle fibers that illustrates the neuromuscular junction. Iden- tify the structures just described.

2. Obtain a slide of skeletal muscle stained to show a portion of a motor unit. Examine the slide under high power to identify the axon branches that extend like a leash to the muscle fibers.

Follow one of the axons to its terminal branch to identify the oval-shaped axon terminal at the end of the branch. Com- pare your observations to the photomicrograph (Figure 12.6). Sketch a small section in the space provided below. Label the axon of the motor neuron, its terminal branches, axon termi- nals, and muscle fibers.

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188 Exercise 12

Ca2+ Sarcolemma

Sarcoplasm
of muscle fiber

Axon terminal of motor neuron

Fusing synaptic vesicles

ACh

Synaptic vesicle Mitochondrion

Synaptic cleft

Junctional folds of sarcolemma

Figure 12.5 The neuromuscular junction. Pink arrows indicate arrival of the action potential, which ultimately causes vesicles to release ACh. The ACh receptor is part of the ion channel that opens briefly,
causing depolarization of
the sarcolemma.

ACh receptors

Ca2+
containing ACh

Activity 3

Motor neuron axon branches

Terminal branch of an axon

Axon terminals at neuromuscular junctions

Muscle fibers

Figure 12.6 Photomicrograph of neuromuscular junctions (7503).

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REVIEW SHEET

Microscopic Anatomy and Organization of Skeletal Muscle

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EXERCISE

Name ______________________________________________________ Lab Time/Date ____________________________________

Skeletal Muscle Cells and Their Organization into Muscles

1. Use the items in the key to correctly identify the structures described below.

Key:

a. endomysium b. epimysium c. fascicle
d. myofibril

e. myofilament f. perimysium g. sarcolemma h. sarcomere

i. tendon

____________________ 1. ____________________ 2. ____________________ 3. ____________________ 4. ____________________ 5. ____________________ 6. ____________________ 7.

____________________ 8. ____________________ 9.

2. List three reasons why the

connective tissue covering a bundle of muscle fibers bundle of muscle fibers
contractile unit of muscle
superficial sheath that covers the entire muscle

thin areolar connective tissue surrounding each muscle fiber plasma membrane of the muscle fiber

a long organelle with a banded appearance found within muscle fibers

actin- or myosin-containing structure

cord of collagen fibers that attaches a muscle to a bone connective tissue sheaths of skeletal muscle are important.

   ____________________________________________________________________________________________________________

   ____________________________________________________________________________________________________________

   ____________________________________________________________________________________________________________

3. Why are there more indirect—that is, tendinous—muscle attachments to bone than there are direct attachments?

      ____________________________________________________________________________________________________________
      ____________________________________________________________________________________________________________
      ____________________________________________________________________________________________________________
   

4. How does an aponeurosis differ from a tendon structurally? _________________________________________________________ ____________________________________________________________________________________________________________ How is an aponeurosis functionally similar to a tendon? ____________________________________________________________ ____________________________________________________________________________________________________________

189

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