A/P 1 Quiz 10

Chapter 10

Muscular Tissue

Three Types of Muscular Tissue

1. Skeletal muscle

2. Cardiac muscle

3. Smooth muscle

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Functions of Muscular Tissue

• Producing body movements
• Stabilizing body positions
• Storing and mobilizing substances within the body
• Generating heat

4

Properties of Muscular Tissue

• Electrical excitability
• Contractility
• Extensibility
• Elasticity

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Muscle Proteins

Contractile:

Myosin
Actin

Regulatory:

Troponin
Tropomyosin

Structural:

Titin
Nebulin
Alpha-actin
Myomesin
Dystrophin

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The Sliding Filament Mechanism

• Myosin pulls on actin, causing the thin filament to slide

inward

• Consequently, Z discs move toward each other and the

sarcomere shortens

• Thanks to the structural proteins, there is a

transmission of force throughout the entire muscle,
resulting in whole muscle contraction

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The Sliding Filament Mechanism

Note the changes in the I band and H zone as the muscle
contracts

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The Contraction Cycle

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Excitation-Contraction Coupling

This concept connects the events of a muscle action
potential with the sliding filament mechanism

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Length-Tension Relationship

The force of a muscle
contraction depends on
the length of the
sarcomeres in a muscle
prior to contraction

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The Neuromuscular Junction (NMJ)

• The events at the NMJ produce a muscle action potential:

❑ Voltage-gated calcium channels in a neuron’s synaptic

end bulb open, resulting in an influx of calcium. This
causes exocytosis of a neurotransmitter (NT) into the
synaptic cleft

❑ NT binds to ligand-gated Na+channels on the motor

endplate, which causes an influx of Na+into the muscle

❑ This depolarizes the muscle and results in Ca2+release

from the sarcoplasmic reticulum

❑ NT gets broken down by acetlycholinesterase

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Muscle Metabolism

How do muscles derive the ATP necessary to power the
contraction cycle?

❑ Creatine phosphate

❑ Anaerobic glycolysis

❑ Cellular respiration

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Creatine Phosphate (CP)

• Creatine kinase catalyzes the transfer of a phosphate

group from CP to ADP to rapidly yield ATP

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Anaerobic Glycolysis

• When CP stores are depleted, glucose is converted into

pyruvic acid to generate ATP

30

Cellular Respiration

• Under aerobic conditions, pyruvic acid can enter the

mitochondria and undergo a series of oxygen-requiring
reactions to generate large amounts of ATP

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Muscle Fatigue

Muscle fatigue is the inability to maintain force of
contraction after prolonged activity
The onset of fatigue is due to:

❑ Inadequate release of Ca2+from SR

❑ Depletion of CP, oxygen, and nutrients

❑ Build up of lactic acid and ADP

❑ Insufficient release of ACh at NMJ

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Central Fatigue

Central fatigue occurs due to changes in the central
nervous system and generally results in cessation of
exercise

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Oxygen Consumption After Exercise

Why do you continue to breathe heavily for a period of
time after stopping exercise?

❑ To “pay back” your oxygen debt!

The extra oxygen goes toward:

❑ Replenishing CP stores

❑ Converting lactate into pyruvate

❑ Reloading O2 onto myoglobin

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Control of Muscle Tension

The strength of a muscle contraction depends on how
many motor units are activated

❑ A motor unit consists of a somatic motor neuron and

the muscle fibers it innervates

❑ Activating only a few motor units will generally result

in a weak muscle contraction

❑ Activating many motor units will generally result in a

strong muscle contraction

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Motor Unit Recruitment

Motor unit recruitment is the process in which the
number of active motor units increases

❑ Weakest motor units are recruited first, followed by stronger

motor units

❑ Motor units contract alternately to sustain contractions for

longer periods of time

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Twitch Contraction

The brief contraction of all
muscle fibers in a motor
unit in response to a single
action potential

❑ Latent period

❑ Contraction period

❑ Relaxation period

❑ Refractory period

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Frequency of Stimulation

Wave summation occurs when a second action potential
triggers muscle contraction before the first contraction
has finished

❑ Results in a stronger contraction

Unfused tetanus
Fused tetanus

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Frequency of Stimulation: Myogram

Notice the difference in contraction time and force
production for each condition below

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Muscle Tone

Even when at rest, a skeletal muscle exhibits a small
amount of tension, called tone

❑ Tone is established by the alternating, involuntary

activation of small groups of motor units in a muscle

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Isotonic vs. Isometric Contractions

Isotonic – tension is constant while muscle length
changes

❑ Concentric

❑ Eccentric

Isometric – muscle contracts but does not change length

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Isotonic vs. Isometric Contractions

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Cardiac Muscle

Cardiac muscle has the same arrangement as skeletal
muscle, but also has intercalated discs

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Cardiac Muscle

• Intercalated discs contain desmosomes and gap

junctions that allow muscle action potentials to spread
from one muscle fiber to another

• Cardiac muscle cells have more mitochondria and their

contractions last 10 to 15 times longer than skeletal
muscle contractions

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Smooth Muscle

• Smooth muscle looks quite different than cardiac and

skeletal muscle. It is thick in the middle, tapered on the
ends, and is not striated

• It can be arranged as either single-unit or multi-unit

fibers

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Smooth Muscle

• Smooth muscle contractions

start more slowly and last
longer than skeletal and
cardiac muscle contractions

• Smooth muscle can shorten

and stretch to a greater extent
than skeletal and cardiac
muscle

• Smooth muscle fibers shorten

in response to stretch!

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Regeneration of Muscle Tissue

Mature skeletal muscle fibers cannot undergo mitosis

❑ Hypertrophy

❑ Hyperplasia

❑ Smooth muscle and pericytes

57

Development of Muscle

• Most muscles are

derived from mesoderm
which develops into
somites

❑ Myotome

❑ Dermatome

❑ Sclerotome

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Aging and Muscle Tissue

Between 30–50 years of age, about 10% of our muscle
tissue is replaced by fibrous connective tissue and
adipose tissue. Between 50–80 years of age another 40%
of our muscle tissue is replaced. Consequences are:

❑ Muscle strength and flexibility decreases

❑ Reflexes slow

❑ Slow oxidative fiber numbers increase

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