Chapter 10

*Lecture Outline

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Chapter 10 Outline

• Properties of Muscle Tissue

• Characteristics of Skeletal Muscle Tissue

• Contraction of Skeletal Muscle Fibers

• Types of Skeletal Muscle Fibers

• Skeletal Muscle Fiber Organization

• Exercise and Skeletal Muscle

• Levers and Joint Biomechanics

(continued)

Chapter 10 Outline—continued

• Naming of Skeletal Muscles

• Characteristics of Cardiac and Smooth Muscle

• Aging and the Muscular System

• Development of the Muscular System

Introduction

• There are three types of muscles in the body:1. skeletal2. cardiac3. smooth

• Most of this chapter emphasizes skeletal muscle

• There are over 700 skeletal muscles and together they form the muscular system

Properties of Muscle Tissue

There are four unique characteristics of muscle tissue:

1. Excitability—outside stimuli can initiate electrical changes in the muscle fiber (cell) leading to contraction of that muscle fiber (cell)

2. Contractility—stimulation of muscle fiber can lead to contraction or shortening of the muscle fiber

Properties of Muscle Tissue—continued

3. Elasticity—a muscle fiber’s ability to return to its original length when the tension of contraction is released

4. Extensibility—the ability of a muscle fiber to be stretched beyond its relaxed length

Skeletal Muscle Tissue

• A skeletal muscle is considered an organ because it contains and is constructed of all four tissue types: epithelium, connective, muscle, and nervous tissues.

• Muscle fibers are striated (possessing stripes) when observed under a microscope.

Functions of Skeletal Muscle Tissue

1. Body movement

2. Maintenance of posture

3. Temperature regulation

4. Storage and movement of materials

5. Support

Gross Anatomy of Skeletal Muscle

Figure 10.1

Skeletal Muscle Composition

• Each muscle is comprised of muscle fibers.

• Muscle fibers are organized into bundles called fascicles.

• Muscle fibers contain myofibrils.

• Myofibrils are composed of myofilaments.

• Myofilaments are mainly composed of actin and myosin.

Order of Skeletal Muscle Structural Organization

• Muscle

• Fascicles

• Muscle fiber

• Myofibrils

• Myofilaments

• Actin and myosin

Order of Skeletal Muscle Structural Organization

Connective Tissue Components

• Each muscle has three layers of concentric connective tissue that is comprised mainly of collagen and elastic fibers.

• This connective tissue provides protection, sites for blood vessel and nerve distribution, and a means of attaching the muscle to the skeleton.

Layers of Connective Tissue

1. Endomysium—the innermost layer that surrounds and electrically insulates each muscle fiber

2. Perimysium—surrounds individual fascicles

3. Epimysium—surrounds the entire muscle

4. Deep and superficial fascia—surround each muscle and separate muscles from each other

Muscle Attachments

• At the ends of each muscle, all of the connective tissue merge to form a tendon, which attaches the muscle to a bone.

• Tendons usually are cordlike in appearance, but sometimes they are flat and are called an aponeurosis.

Muscle Attachments

• Most muscles extend over a joint and have attachments to both articulating bones of that joint.

• Upon contraction of the muscle, one of the articulating bones moves and the other one does not.

• The point of attachment to the bone that does not move is called the origin.

• The point of attachment to the bone that does move is called the insertion.

Muscle Attachments

Figure 10.2

Muscle Fiber Terminology

Skeletal muscle fibers have many of the same components of a typical cell, but some are named differently. The following list shows the typical cell name and the name provided to the muscle fiber, respectively:

1. Plasma membrane = sarcolemma2. Cytoplasm = sarcoplasm3. Smooth ER = sarcoplasmic reticulum

Muscle Fiber Terminology

There are two main structures that are unique to the muscle fiber:

1. Transverse tubules (T-tubules)—deep invaginations of the sarcolemma that extend into the sarcoplasm

2. Terminal cisternae—blind sacs at the end of the sarcoplasmic reticulum

Microscopic Anatomy of Skeletal Muscle

Figure 10.3

Embryonic Development of the Skeletal Muscle Fiber

• Embryonic skeletal muscle cells are called myoblasts. Each myoblast contains a single nucleus.

• Skeletal muscle fibers are multinucleated and arise as a result of fusion of many embryonic myoblasts.

• The nuclei from the myoblasts are preserved in the mature skeletal muscle fiber.

Embryonic Development of the Skeletal Muscle Fiber

Figure 10.4

Myofibrils

• The sarcoplasm of a muscle fiber contains 100–1000 cylindrical structures that extend the entire length of the cell.

• These structures are termed myofibrils.

• Myofibrils have the ability to shorten, resulting in contraction of the muscle and the production of motion.

Myofilaments

• Myofibrils are composed of short bundles of myofilaments.

• Myofilaments do not run the entire length of the muscle fiber but are organized into repetitive groupings.

• Myofilaments are of two types:1. Thin filaments—actin and associated

proteins2. Thick filaments—myosin

Microscopic Anatomy of Skeletal Muscle

Figure 10.3

Thin Filaments

• 5–6 nm in diameter• Comprised of two strands (F-actin or

filamentous actin) of bead-shaped (G-actin or globular actin) molecules twisted around each other

• Two regulatory proteins are also part of the thin filament:– Tropomyosin– Troponin

Thin Filaments

Figure 10.5

Thick Filaments

• 11 nm in diameter (twice as thick as thin filaments)

• Composed of bundled molecules of myosin

• Myosin molecule has a head and elongated tail

• The heads form crossbridges with the thin filaments during contraction

Thick Filaments

Figure 10.5

Comparison of Thick and Thin Myofilaments

Figure 10.5

Organization of Thin and Thick Filaments

• Organization of thin and thick filaments causes the basis of skeletal muscle being striated.

• The dark bands, the A bands, contain the entire myosin molecule and an overlapping portion of actin.

• The light bands, the I bands, contain thin filaments but no thick filaments.

• The I bands also contain the protein titin (see Table 10.2 for its function).

Muscle Fiber Components

Other Components of the A Bands and I Bands

Using an electron microscope, other components can be seen within both A bands and I bands:

1. H zone (H band)—light, central region of the A band where there are no thin filaments

2. M line—a protein meshwork in the H zone that keeps the thick filaments aligned

3. Z disc (Z band)—a protein structure in the middle of the I band that serves as the attachment site for one end of the thin filaments

Fig08.02

Myosin (thick)filaments

Actin (thin)filaments

Myofibril

Sarcomere

Sarcoplasmicreticulum

(a)

Skeletal muscle fiber

Z lineH zone M line

I band A band I band A band

(b)

Z line

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34

Bands and Zones Within a Myofibril

Figure 10.6

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I band

MyofibrilsA band

Z disc

(b)Z disc A bandH zoneM lineI band

Thinfilament

Thickfilament

Thickfilament

Thinfilament

Titinfilament

Connectinfilaments

M lineZ disc

Sarcomere

H zone

(a)

Organization of a Sarcomere

• The sarcomere is the functional contractile unit in a skeletal muscle fiber

• Defined by the area between two adjacent Z discs

• Myofibrils contain multiple and repeating sarcomeres

• Each sarcomere shortens as the muscle fiber contracts

Structure of a Sarcomere

Figure 10.6

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

I band

MyofibrilsA band

Z disc

(b)Z disc A bandH zoneM lineI band

Thinfilament

Thickfilament

Thickfilament

Thinfilament

Titinfilament

Connectinfilaments

M lineZ disc

Sarcomere

H zone

(a)

Structure of a Sarcomere

Figure 10.6

Components of a Sarcomere

Mechanisms of Skeletal Muscle Contraction

• Muscle fibers shorten by the interaction between thin and thick filaments within each sarcomere.

• The mechanism for contraction is explained by the sliding filament theory.

Sliding Filament Theory

During contraction, the thick and thin filaments interact and slide past each other. This causes the following changes within each sarcomere:

• width of A band remains constant, but H zone disappears

• Z discs in each sarcomere move closer together• sarcomere narrows in length• I bands narrow• length of the thick and thin filaments never changes

whether the muscle is contracted or relaxed

Sliding Filament Model of Contraction

Figure 10.7

Neuromuscular Junctions

• Muscle contraction begins when a motor neuron impulse stimulates an impulse in a muscle fiber.

• The neuromuscular junction is the region where the motor neuron comes into close proximity to the muscle fiber.

Neuromuscular Junction

Figure 10.8

Components of the Neuromuscular Junction

1. Synaptic knob—expanded end of the neuron2. Synaptic vesicles—membrane-bound sacs filled with

acetylcholine (ACh)3. Motor end plate—region of sarcolemma across from

the synaptic knob that has folds and indentations to increase the surface area in that region

4. Synaptic cleft—narrow space separating the synaptic knob from the motor end plate

5. ACh receptors—in the motor end plate that bind to ACh

6. Acetylcholinesterase (AChE)—an enzyme in the synaptic cleft that rapidly breaks down ACh

Neuromuscular Junction

Figure 10.8

Muscle Contraction:A Summary

1. A nerve impulse causes ACh to be released into the synaptic cleft.

2. ACh binds to receptors in the motor end plate initiating a muscle impulse along the sarcolemma and T-tubule membranes.

3. Calcium is released from T-tubules into the sarcoplasm.

Muscle Contraction: A Summary—continued

4. Calcium ions bind to troponin, causing tropomyosin to uncover active binding sites.

5. Myosin heads then bind to actin and form crossbridges.

6. In the presence of ATP, myosin cycles through attachment, pivot, detach, and return events. ATP is also necessary for relaxation of the muscle fiber.

Muscle Contraction

Figure 10.9

Motor Units

• A motor unit consists of a single motor neuron and the muscle fibers it controls.

• A motor unit controls only a few muscle fibers in an entire muscle.

• Larger muscles have more motor units than do smaller muscles.

• Each muscle fiber obeys the all-or-none principle—a muscle fiber contracts completely or not at all.

• When a motor unit is stimulated, all muscle fibers under its control will contract.

Motor Unit

Figure 10.10

Two Types of Muscle Contraction

Figure 10.11

Types of Skeletal Muscle Fibers

Skeletal muscles are comprised of a mixture of three different type of muscle fibers. The ratio of these three fiber types within a muscle will determine the speed of the muscle’s contraction and the sustainability of the contraction.

1. Slow (Type I, slow oxidative)2. Intermediate (Type IIa, fast aerobic)3. Fast (Type IIb, fast anaerobic)

Structural and Functional Characteristics of Skeletal Muscle

Fiber Types

Microscopic View of Fiber Types in Skeletal Muscle

Figure 10.12

Distribution of the Three Muscle Fiber Types

• Usually, skeletal muscles are comprised of a combination of all three muscle fiber types, but a single motor unit controls only muscle fibers from one of the three types.

• Slow fibers dominate muscles such as back and calf muscles, which contract almost continuously.

• There are no slow muscle fibers in muscles that require swift but brief contractions, such as eye and hand muscles.

Skeletal Muscle Fiber Organization

Muscle fibers are organized into fascicles within a muscle but there are four different patterns of fascicle arrangements:

1. circular2. parallel3. convergent4. pennate

– unipennate– bipennate– multipennate

Fascicle Arrangements

Exercise and Skeletal Muscle

• Muscle atrophy—a wasting of tissue that results in reduction of muscle size, tone, and power. This can be caused by a lack of stimulation (exercise) to the muscle.

• Muscle hypertrophy—an increase in muscle fiber size (not an increase in number of muscle fibers). Hypertrophy results from repetitive stimulation of muscle fibers. Mitochondria increase in number, therefore the amount of ATP increases. Both myofibrils and myofilaments increase in number, all resulting in the muscle increasing in size.

Levers and Biomechanics

• A lever is an elongated, rigid object that rotates around a fixed point called a fulcrum.

• Rotation occurs when an effort applied to one point of the lever exceeds a resistance located at some other point of the lever.

• There are three classes of levers found in the human body.

Three Classes of Levers

Figure 10.13

Actions of Skeletal Muscles

1. Agonist (prime mover)—produces a specific movement when it contracts. For instance, the biceps brachii is an agonist that causes flexion of the elbow joint.

2. Antagonist—is a muscle whose action opposes that of an agonist. The triceps brachii would be an antagonist to the biceps brachii

3. Synergist—is a muscle that assists the agonist or prime mover.

Naming of Skeletal Muscles

Muscle names can be confusing, but their names incorporate the following qualities of the muscle:

1. Muscle action

2. Body region

3. Muscle attachments

4. Orientation of muscle fibers

5. Muscle shape and size

6. Muscle heads/tendons of origin

Naming of Skeletal Muscles

Figure 10.14

Characteristics of Cardiac and Smooth Muscle

• The three types of muscle in the human body are skeletal, cardiac, and smooth muscle.

• There are similarities and differences among the three types of muscles.

Cardiac Muscle Fibers

Cardiac muscle fibers are found almost exclusively within the heart wall. They have the following qualities:

1. striated2. one or two nuclei3. form Y-shaped branches4. join other adjacent cells to form junctions termed

intercalated discs comprised of gap junctions5. autorhythmic (able to generate a muscle impulse

without nervous stimulation)6. under involuntary control

Cardiac Muscle Fibers

Figure 10.15

Smooth Muscle Fibers

1. Found in the walls of viscera and blood vessels

2. Short fusiform cells (widest in the middle and tapered at each end)

3. One centrally located nucleus

4. No striations

5. Thin filaments attached to dense bodies

6. Under involuntary control

Smooth Muscle Fibers

Figure 10.16

General Comparison of Muscle Tissue Types

Development of Skeletal Muscle

Figure 10.17