More About Cardiac and Smooth Muscle Tissue

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Much of the text material is from, “Principles of Anatomy and Physiology, 14th edition” by Gerald J. Tortora and Bryan

Derrickson (2014). I don’t claim authorship. Other sources are noted when they are used.

Mappings of the lecture slides to the 12th and 13th editions are provided in the supplements.

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Outline

• Cardiac muscle• Smooth muscle

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Cardiac Muscle

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Cardiac Muscle Fibers

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Structure

• The walls of the heart are mostly composed of cardiac muscle.

• Between the layers of cardiac muscle fibers are sheets of connective tissue that also contain blood vessels, nerves, and a conduction sys-tem.

• Cardiac muscle fibers have the same arrangement of thin and thick elements, and the same bands, zones, and Z discs as found in skel-etal muscle.

Chapter 10, page 317 Figure 20.9

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Structure (continued)

Chapter 10, page 318 Figure 4.2e

• Cardiac muscle fibers are attached end-to-end by transverse thicken-ings of the plasma membrane known as intercalated discs.

• The discs have desmosomes to strengthen the muscle tissue and hold the muscle fibers together during contractions.

• The fibers have gap junctions to enable the rapid conduction of action potentials in each heart chamber.

Desmosome = a cell structure specialized for cell-to-cell adhesion.

Gap junction = directly connects the cytoplasm of two cells, which allows various molecules and ions to pass freely

between cells.(Both definitions from http://en.wikipedia.org)

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Cardiac Muscle Response

• Cardiac muscle contractions are much longer in duration than skel-etal muscle contractions.

• The longer time results from the prolonged delivery of Ca2+ from the sarcoplasmic reticulum to the sarcoplasm.

Chapter 10, page 318

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Cardiac Muscle Stimulation

• As we discussed, skeletal muscle contracts when stimulated by ACh triggered by neuronal action potentials from somatic motor neurons.

• In comparison, cardiac muscle tissue contracts when stimulated by the autorhythmic muscle fibers in the heart—typically about 75 times per minute at rest.

• The autonomic nervous system regulates changes in heart rate and other aspects of cardiac activity in response to stress, physical exer-cise, and other conditions. �

Chapter 10, page 318

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ATP Production

• The mitochondria in cardiac muscle fibers are larger and more numer-ous than in skeletal muscle.

• Aerobic cellular respiration is the principal source of ATP in cardiac muscle—heart muscle requires more oxygen than many other tissues.

• Cardiac muscle can also use the lactic acid from contractions of skeletal muscle to produce ATP.

Chapter 10, page 318

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Smooth Muscle

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Smooth Muscle Fibers

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Single-Unit Tissue

• One type of smooth muscle is single-unit or visceral smooth muscle tis-sue.

• Their fibers form tubular arrangements in the walls of small arteries and veins.

• They also form the walls of hollow organs including in the digestive tract, urinary tract, and uterus.

• Gap junctions enable muscle action potentials to spread rapidly to allow smooth muscle fibers to contract in unison.

Chapter 10, page 318 Figure 10.16

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Multi-Unit Tissue

• Multi-unit tissue consists of individual fibers, each with its own motor neuron terminals and few (if any) gap junctions.

• The stimulation of a multi-unit fiber causes contraction of only that one fiber.

• Multi-unit smooth muscle tissues are found in:

- Walls of the large arteries- Airways (respiratory bronchioles) to the lungs - Muscles that attach to hair follicles- Muscles of the iris of the eye for constriction and dilation of the

pupils

- Ciliary muscles of the lens of the eye for focusing visual images on the retina

Chapter 10, page 318 Figure 10.16

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• Smooth muscle contracts in response to the release of neurotrans-mitters (acetylcholine and norepinephrine) from motor neurons of the autonomic nervous system.

• Some smooth muscles can also contract in response to:

- Stretching- Hormones- pH changes- Oxygen and carbon dioxide levels in the blood- Body temperature- Ion concentrations

Smooth Muscle Stimulation

Chapter 10, page 318

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Smooth Muscle Response

• The contractions of smooth muscle start more slowly and last longer than in skeletal muscle.

• Because there are fewer transverse tubules in smooth muscle, Ca2+ takes longer to reach the central region of the muscle fiber to trigger a contraction.

• Ca2+ also exits the sarcoplasm,more slowly, which delays relaxation.

Chapter 10, page 318

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Smooth Muscle Response (continued)

• Smooth muscle can shorten and stretch to a greater extent than skele-tal muscle and cardiac muscle.

• Smooth muscle fibers, unlike skeletal muscle, can be stretched consid-erably and still maintain their contractility.

• An example is a full stomach after a large meal—the stomach wall has substantial smooth muscle.

Chapter 10, page 318