Anatomy of Muscles and Muscle Tissue

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Human Anatomy & Physiology, Sixth Edition

Elaine N. Marieb

PowerPoint® Lecture Slides prepared by Vince Austin, University of Kentucky

9Anatomy of Muscles and Muscle Tissue


Muscle Overview

The three types of muscle tissue are skeletal, cardiac, and smooth

These types differ in structure, location, function, and means of activation


Muscle Similarities

Skeletal and smooth muscle cells are elongated and are called muscle fibers

Muscle contraction depends on two kinds of myofilaments – actin and myosin

Muscle terminology is similar:

Sarcolemma – muscle plasma membrane

Sarcoplasm – cytoplasm of a muscle cell

Prefixes – myo, mys, and sarco all refer to muscle


Skeletal Muscle Tissue

Packaged in skeletal muscles that attach to and cover the bony skeleton

Has obvious stripes called striations

Is controlled voluntarily (i.e., by conscious control)

Contracts very rapidly, but tires easily

Is responsible for overall body motility


Cardiac Muscle Tissue

Occurs only in the heart

Is striated like skeletal muscle but is involuntary (controlled by the autonomic nervous system)

Contracts at a fairly steady rate set by the heart’s pacemaker cells

Neural controls allow the heart to respond to changes in bodily needs


Cardiac Muscle Cells

A. General Features

1. involuntary muscle

2. one, centrally located nucleus

3. mitochondria larger and more numerous

B. Structure of Tissue

1. muscle fibers branch and interconnect

2. intercalated disc - thickening of sarcolemma

3. cells connected by gap junctions

a. allow passage of ions like Calcium

b. makes adjacent cells electrically linked

c. allows for rhythmic, domino-like contraction


Smooth Muscle Tissue

Found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages

Forces food and other substances through internal body channels

It is not striated and is involuntary (controlled by the autonomic nervous system)


Smooth Muscle

Composed of spindle-shaped fibers with a diameter of 2-10 m and lengths of several hundred m

Lack the coarse connective tissue sheaths of skeletal muscle, but have fine endomysium

Organized into two layers (longitudinal and circular) of closely apposed fibers

Found in walls of hollow organs (except the heart)

Peristalsis – alternating contractions and relaxations of smooth muscles that mix and squeeze substances through the lumen of hollow organs


Smooth Muscle

Figure 9.24


Smooth Muscle Cells

A. General Features

1. non-striated

2. 5-10 um in diameter; 30-200 um long

3. thick in middle; thinner, tapering off to the end

4. single oval centrally located nucleus

5. actin and myosin fibers not arranged as sarcomere

6. lack of organization (no bands) --> smooth muscle

7. also contain intermediate filaments


Smooth Muscle Cells

B. Difference in Contraction

1. intermediate filaments attach to dense bodies

2. as muscle cell contracts, twists like corkscrew

3. caveolae - like T tubules of skeletal muscle


Smooth Muscle Cells

C. Two Kinds of Smooth Muscle

1. visceral (single unit) musclea. small arteries and veinsb. viscera - stomach, intestines, uterus, bladderc. continuous network with gap junctionsd. action spreads from one cell to another

2. multiunit muscle

a. each fiber (cell) has it own nerve endingb. no gap junctionsc. large arteries, large airways, arrector pili


Skeletal Muscle: Nerve and Blood Supply

Each muscle is served by one nerve, an artery, and one or more veins

Each skeletal muscle fiber is supplied with a nerve ending that controls contraction

Contracting fibers require continuous delivery of oxygen and nutrients via arteries

Wastes must be removed via veins


Skeletal Muscle

Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and connective tissue

The three connective tissue sheaths are:

Endomysium – fine sheath of connective tissue composed of reticular fibers surrounding each muscle fiber

Perimysium – fibrous connective tissue that surrounds groups of muscle fibers called fascicles

Epimysium – an overcoat of dense regular connective tissue that surrounds the entire muscle


Figure 9.2 (a)

One Skeletal Muscle in Cross Section


One Skeletal Muscle Cell (Fiber)


One Skeletal Muscle Cell Myofibril


Each Myofibril composed of many Sarcomeres


Myofibrils

Myofibrils are densely packed, rodlike contractile elements

They make up most of the muscle volume

The arrangement of myofibrils within a fiber is such that a perfectly aligned repeating series of dark A bands and light I bands is evident


Many Myofibrils in One Muscle Cell

Figure 9.3 (b)


Sarcomeres

The smallest contractile unit of a muscle

The region of a myofibril between two successive Z discs

Myofilaments are of two types – thick and thin


Many Sarcomeres make up a single Myofibril

Figure 9.3 (c)


Myofilaments: Banding Pattern

Thick filaments (myosin filaments) – extend the entire length of an A band

Thin filaments (actin filaments) – extend across the I band and partway into the A band


Myofilaments: Banding Pattern

Figure 9.3 (c, d)



Ultrastructure of Myofilaments: Thick Filaments

Thick filaments are composed of the protein myosin

Each myosin molecule has a rodlike tail and two globular heads

Tails – two interwoven, heavy polypeptide chains

Heads – two smaller, light polypeptide chains called cross bridges


Ultrastructure of Myofilaments: Thick Filaments

Figure 9.4 (a)(b)


Ultrastructure of Myofilaments: Thin Filaments

Thin filaments are composed of the protein actin

Each actin molecule is a helical polymer of globular subunits called G actin

The subunits contain the active sites to which myosin heads attach during contraction

Tropomyosin and troponin are regulatory subunits bound to actin


Ultrastructure of Myofilaments: Thin Filaments

Figure 9.4 (c)


Arrangement of the Filaments in a Sarcomere

Longitudinal section within one sarcomere

Figure 9.4 (d)


Sarcoplasmic Reticulum (SR)

SR is an elaborate, smooth endoplasmic reticulum that mostly runs longitudinally and surrounds each myofibril

Functions in the regulation of intracellular calcium levels

T tubules associate with the paired terminal cisternae to form triads


Sarcoplasmic Reticulum (SR)

Figure 9.5


Neuromuscular Junction (NMJ)

1. motor neuron - nerve cell that innervates muscle

2. motor end plate - where axon meets muscle cell

3. neuromuscular junction - entire muscle/nerve site

4. synapse – name for where nerve terminal meets target cell

5. synaptic vesicles - inclusions in the axon terminal

a. neurotransmitter - chemical messenger

i. acetylcholine ACh (for muscular synapse)

6. synaptic cleft - space between axon and cell

7. motor unit - a neuron and all muscle cells it stimulates



Muscle Hypertrophy and Atrophy

1. muscular hypertrophy

a. increase in size of myofibers (muscle cells)

b. allows for increased strength

c. adult muscle cells are amitotic (do not divide)

2. muscular atrophy

a. progressive loss of myofibrils

i. disuse atrophy

ii. denervation atrophy (neuron lost)


Interactions of Skeletal Muscles

Skeletal muscles work together or in opposition

Muscles only pull (never push)

As muscles shorten, the insertion generally moves toward the origin

Whatever a muscle (or group of muscles) does, another muscle (or group) “undoes”


Muscle Classification: Functional Groups

Prime movers – provide the major force for producing a specific movement

Antagonists – oppose or reverse a particular movement

Synergists – work cooperatively to achieve a certain action

Add force to a movement

Reduce undesirable or unnecessary movement

Fixators – synergists that immobilize a bone or muscle’s origin


Naming Skeletal Muscles

1. Location of muscle – bone or body region associated with the muscle

2. Shape of muscle – e.g., the deltoid muscle (deltoid = triangle)

3. Relative size – e.g., maximus (largest), minimus (smallest), longus (long)

4. Direction of fibers – e.g., rectus (fibers run straight), transversus, and oblique (fibers run at angles to an imaginary defined axis)


Naming Skeletal Muscles

5. Number of origins – e.g., biceps (two origins) and triceps (three origins)

6. Location of attachments – named according to point of origin or insertion

7. Action – e.g., flexor or extensor, as in the names of muscles that flex or extend, respectively

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Anatomy of Muscles and Muscle Tissue