1
Muscular Tissue
Chapter 10
CHAPTER SUMMARY
This chapter begins with a general introduction to muscular tissue and a description of the three types of muscular tissue
present in the human body. The functions and characteristics of muscular tissue are explained. The anatomy of skeletal
muscle tissue is described in detail; this includes descriptions of connective tissue components, microscopic anatomy
(including the sarcolemma, T tubules, sarcoplasm, myofibrils, sarcoplasmic reticulum, sarcomere, etc.), muscle proteins,
nerve and blood supply, the neuromuscular junction, and the motor unit. The sliding filament mechanism of muscle
contraction is portrayed. Muscle tone is explained. The characteristics of the three major types of skeletal muscle fibers
are concisely described. The effects of exercise on skeletal muscle tissue are explained. The principal features of cardiac
muscle tissue and smooth muscle tissue are described in detail. The developmental biology of muscles is provided. The
effects of aging on muscular tissue are explained. A glossary of key medical terms associated with muscular tissue is
provided. This chapter concludes with a thorough study outline, an excellent self-quiz, critical thinking questions, and
answers to questions that accompany chapter figures.
STUDENT OBJECTIVES
1.
2.
3.
4.
5.
6.
7.
8.
9.
Correlate the three types of muscular tissue with their functions and special properties.
Explain the relation of connective tissue components, blood vessels, and nerves to skeletal muscles.
Describe the microscopic anatomy of a skeletal muscle fiber.
Explain how a skeletal muscle fiber contracts and relaxes.
Why is muscle tone important?
Compare the structure and function of the three types of skeletal muscle fibers.
Describe the main structural and functional characteristics of cardiac muscle tissue.
Describe the main structural and functional characteristics of smooth muscle tissue.
Describe the development of the muscular system.
10. Explain how aging affects skeletal muscle.
LECTURE OUTLINE
A. Introduction (p. 265)
1. Bones provide leverage and form the framework of the body, but motion results from alternating contraction
(shortening) and relaxation of muscles.
2. Muscle tissue also stabilizes the body’s position, regulates organ volume, generates heat, and propels fluids and
food through various body systems.
3. The study of muscles is called myology.
B. Overview of Muscle Tissue (p. 266)
1. There are three types of muscle tissue (see Table 10.2):
i. Skeletal muscle tissue
a. moves bones (and, in some cases, skin and other soft tissues)
b. striated
c. voluntary
ii. Cardiac muscle tissue
a. forms most of the wall of the heart
b. striated
c. involuntary
d. some cells have autorhythmicity
iii. Smooth muscle tissue
a. located in the walls of hollow internal structures (and arrector pili muscles)
b. nonstriated, i.e., smooth
2
c. involuntary
d. some cells have autorhythmicity
2. Muscle tissue has four major functions:
i. producing body movements
ii. stabilizing body positions
iii. storing and moving substances within the body
iv. producing heat
3. Muscle tissue has four major characteristics that enable it to perform its functions:
i. Electrical excitability is the ability to respond to certain stimuli by producing electrical signals called
action potentials (impulses)
ii. Contractility is the ability to shorten, thus developing tension (force of contraction)
a. in an isometric contraction, a muscle develops tension but does not shorten
b. in an isotonic contraction, tension remains relatively constant while the muscle
shortens
iii. Extensibility is the ability of the muscle to be stretched without being damaged
iv. Elasticity is the ability to return to its original shape after contraction or extension
C. Skeletal Muscle Tissue (p. 267)
1. Connective Tissue Components:
i. Fascia is a sheet or broad band of fibrous connective tissue beneath the skin (superficial fascia) or
around muscles or organs (deep fascia).
ii. Three layers of connective tissue extend from the deep fascia:
a. outer epimysium encircles the whole muscle
b. perimysium surrounds fascicles (bundles of muscle fibers)
c. endomysium surrounds individual muscle fibers
iii. Extensions of these three layers are continuous with:
a. tendon, a cord of dense connective tissue that attaches a muscle to the periosteum of a bone;
some tendons are enclosed in tendon (synovial) sheaths which permit tendons to slide back
and forth more easily
b. aponeurosis, a broad, flat sheet of connective tissue that attaches a muscle to the periosteum of
a bone, another muscle, or the skin
2. Microscopic Anatomy: (p. 267)
i. Growth of a skeletal muscle is achieved primarily by hypertrophy because skeletal
muscle fibers do not significantly increase in number after birth; skeletal muscle tissue does
have limited powers of regeneration due to the presence of satellite cells which can fuse
with or replace existing skeletal muscle fibers to a limited extent
- if significant skeletal muscle damage or degeneration occurs, skeletal muscle tissue
undergoes fibrosis, the replacement of muscle fiber by fibrous scar tissue
ii. Each muscle fiber has a sarcolemma (plasma membrane) that surrounds the sarcoplasm (cytoplasm);
the latter contains an oxygen-binding protein called myoglobin
iii. The sarcoplasm contains numerous myofibrils;
a. The myofibrils contain numerous parallel (protein) myofilaments called thin filaments and
thick filaments that are arranged in functional units called sarcomeres
b. Adjacent sarcomeres are separated by Z discs (lines) and have regions called the A band, I
band, H zone, and M line and a zone of overlap.
c. Thick filaments consist of myosin molecules with heads or crossbridges.
d. Thin filaments consist of actin molecules plus two regulatory proteins called troponin and
tropomyosin.
e. Sarcomeres also contain structural proteins (e.g., titin and dystrophin) which keep the thick and
thin filaments properly aligned, give the myofibril elasticity and extensibility and link the
myofibrils to the sarcolemma and extracellular matrix.
iv. The sarcoplasm contains the sarcoplasmic reticulum (SR) with dilated end sacs called terminal
cisterns which store calcium ions; release of calcium ions into the cytosol triggers muscle
contraction.
3
v.
The sarcolemma has tunnel-like infoldings called transverse tubules (T tubules) that penetrate into
the muscle fiber at right angles to the myofilaments; these T tubules conduct muscle action potentials
which cause the release of calcium ions from the SR.
3. Nerve and Blood Supply: (p. 273)
i. Motor neurons send impulses down their axons to stimulate muscle fibers to contract.
ii. The junction or synapse between a motor neuron and a skeletal muscle fiber is called a
neuromuscular junction (NMJ).
iii. When a nerve impulse (action potential) arrives at this junction, the synaptic vesicles in synaptic end
bulbs release acetylcholine (neurotransmitter molecules) which diffuse across the synaptic cleft
and bind to acetylcholine receptor molecules on the motor end plate; this triggers a muscle action
potential that travels along the sarcolemma and ultimately initiates contraction of the muscle fiber.
iv. A motor unit consists of a motor neuron plus all the muscle fibers that it innervates; all fibers in a
motor unit therefore contract and relax simultaneously
a. the average motor unit contains 150 muscle fibers but there is a wide range that extends from
2–3 muscle fibers per motor unit to 2000-3000 muscle fibers per motor unit
b. small motor units provide precise control of movement while large motor units provide greater
strength of contraction
v. Blood vessels called capillaries deliver nutrients and oxygen to muscle fibers and carry wastes and
heat away from muscle fibers.
4. Contraction and Relaxation of Skeletal Muscle Fibers: (p. 274)
i. Muscle contraction is triggered when a muscle action potential is propagated along the sarcolemma
through the T tubule system and to the sarcoplasmic reticulum, where it causes the release of calcium ions from
the SR into the cytosol
ii. Muscle contraction occurs via the sliding filament mechanism:
- in the presence of ATP and calcium ions, myosin cross bridges pull thin filaments toward the
center of the sarcomere, resulting in a shortening of the sarcomere
iii. The effect of acetylcholine is brief because it is rapidly broken down by
acetylcholinesterase located in the synaptic cleft.
5. Muscle tone is a sustained, partial state of contraction that gives firmness to a relaxed skeletal muscle; in a
constantly shifting pattern, a few motor units become active while others become inactive within a skeletal
muscle. This is essential for maintaining posture.
D. Types of Skeletal Muscle Fibers (p. 276)
1. Skeletal muscle fibers are not all identical in structure or function:
i. Muscle fibers that have a high content of myoglobin, a red oxygen-storing protein (as well as more
mitochondria and a rich blood supply), are called red muscle fibers.
ii. Muscle fibers that have a low content of myoglobin are called white muscle fibers.
iii. Based on structural and functional characteristics, skeletal muscle fibers are classified into three types
(see Table 10.1):
a. slow oxidative (SO) fibers
b. fast oxidative-glycolytic (FOG) fibers
c. fast glycolytic (FG) fibers
iv. Most skeletal muscles contain a mixture of all three types of skeletal muscle fibers, but their
proportion varies depending on the usual action of the muscle; however, the muscle fibers in any one
motor unit are all of the same type.
2. The total number of skeletal muscle fibers and the relative percentages of fast and slow fibers in each muscle
do not change significantly after birth, but the characteristics of the muscle fibers present may be somewhat
altered in various ways by various types of exercise (see p. 277).
E. Cardiac Muscle Tissue (p. 278)
1. Cardiac muscle tissue is located in the wall of the heart.
2. The cells have the following important characteristics:
i. are striated
ii. are involuntary
iii. some cells have autorhythmicity
4
iv. are branching cylinders that usually contain a single nucleus
v. compared to skeletal muscle fibers, the sarcoplasm:
a. is more abundant
b. has larger and more numerous mitochondria
c. has a less elaborate sarcoplasmic reticulum
d. is penetrated by fewer but wider transverse tubules
vi. neighboring fibers are connected by intercalated discs that contain:
a. desmosomes, which strongly hold the fibers together
b. gap junctions, which allow muscle action potentials to spread from one fiber to another fiber
3. Under normal resting conditions, cardiac muscle tissue rhythmically contracts and relaxes about 75 times per
minute; this rhythm may be increased or decreased by nerve or hormonal stimulation.
4. Cardiac muscle tissue has characteristics that permit significant increases in heart rate but prevent the heart
from undergoing tetanus.
5. Damaged cardiac muscle fibers are not repaired or replaced, and healing occurs by fibrosis; but they can
undergo hypertrophy in response to increased workload.
F. Smooth Muscle Tissue (p. 278)
1. Smooth muscle tissue is usually activated involuntarily.
2. There are two types of smooth muscle tissue:
i. visceral (single-unit) smooth muscle tissue is the more common type
a. located in the walls of small blood vessels and hollow viscera
b. it is autorhythmic; due to presence of gap junctions, entire muscle contracts as a single unit
when one fiber is stimulated by neurotransmitter, hormone or autorhythmic signal
ii. multiunit smooth muscle tissue
a. located in the walls of large arteries, bronchioles, arrector pili muscles, irises of the eyes, and
ciliary muscles of the eyes
b. each fiber contracts individually upon stimulation
3. The cells have the following important characteristics:
i. are narrow fibers with two tapering ends
ii. have a single nucleus
iii. the sarcoplasm:
a. contains both thick and thin filaments but they are not arranged into orderly sarcomeres, i.e.,
nonstriated and therefore called smooth
b. contains intermediate filaments attached to dense bodies
iv. when a smooth muscle fiber contracts, it turns like a corkscrew
v. compared to skeletal muscle, contraction starts more slowly and lasts much longer
vi. the prolonged presence of calcium ions in the cytosol provides for smooth muscle tone
4. Most smooth muscle fibers contract or relax in response to neurotransmitters, hormones, autorhythmic signals,
stretching or local factors.
5. Smooth muscle tissue exhibits the stress-relaxation response which allows it to be stretched considerably
while still retaining the ability to contract effectively.
6. Smooth muscle fibers can undergo hypertrophy; some smooth muscle fibers can divide and new fibers can be
formed from pericytes and, therefore, regeneration can occur to a limited extent.
G. Development of Muscles (p. 282)
1. Almost all muscles develop from mesoderm.
2. Skeletal muscles of the head and limbs develop from the general mesoderm and the remaining skeletal
muscles develop from the mesoderm of somites.
3. Cardiac muscle and smooth muscle develop from mesodermal cells.
H. Aging and Muscular Tissue (p. 283)
1. After 30 years of age, there is a progressive decrease in skeletal muscle mass that is replaced primarily by
fibrous connective tissue and adipose tissue.
2. This is accompanied by a decrease in maximal strength and a slowing of muscle reflexes.
3. In some muscles, there is a change in the proportions of the specific types of skeletal muscle fibers.
5
I. Key Medical Terms Associated with Muscular Tissue (p. 284)
1. Students should familiarize themselves with the glossary of key medical terms.