CHAPTER SUMMARY
37.1 MUSCLES ARE BIOLOGICAL MOTORS COMPOSED OF ACTIN AND MYOSIN THAT GENERATE FORCE AND PRODUCE MOVEMENT FOR BODY SUPPORT, LOCOMOTION, AND CONTROL OF THE BODY’S INTERNAL PHYSIOLOGICAL FUNCTIONS.
- There are two main types of muscle: striated (skeletal and cardiac) and smooth.
- Skeletal and cardiac muscle fibers appear striated when viewed under the microscope because of the regular spacing of sarcomeres along their length.
- Smooth muscle fibers, which regulate airflow for breathing, blood flow through arteries, and the passage of food through the gut, lack regular sarcomere organization and appear smooth when viewed under the microscope.
- Skeletal muscle fibers are long thin cells composed of parallel sets of myofibrils built up from smaller parallel arrays of actin and myosin filaments.
- The sarcomere is the basic contractile unit of a skeletal muscle. Sarcomeres are arranged in series along the length of a myofibril.
- Muscles change length and produce force by the formation of actin–myosin cross-bridges, causing the sliding of myosin and actin filaments relative to each other.
- Muscles are stimulated by motor neurons at the fiber’s motor endplate, leading to depolarization of the muscle cell that triggers the release of Ca2+ ions from the sarcoplasmic reticulum. Ca2+ binds troponin, which moves tropomyosin off the myosin-binding sites on actin, allowing myosin heads to form cross-bridges with actin.
- Excitation–contraction coupling is the process in which depolarization of the muscle cell leads to its shortening.
37.2 MUSCLE FORCE DEPENDS ON MUSCLE SIZE, DEGREE OF ACTIN–MYOSIN OVERLAP, CONTRACTILE VELOCITY, AND STIMULATION RATE.
- Muscles exert their highest force when actin–myosin filaments have maximal overlap, allowing the most cross-bridges to form.
- Muscles exert more force when they contract at slow velocities compared to when they contract at high velocities.
- Because muscles can transmit force only by pulling on the skeleton, they are arranged as antagonist pairs to produce reciprocal motions of a joint or limb.
- In vertebrates, a motor unit consists of a single motor neuron and the muscle fibers (cells) it innervates.
- Muscle force is increased by increasing the motor neuron firing rate and, in vertebrates, by the number of motor units that are activated.
- Vertebrate muscles have two types of fiber: red slow-twitch fibers that contract slowly over longer time periods and white fast-twitch fibers that contract rapidly but fatigue quickly.
37.3 HYDROSTATIC SKELETONS, EXOSKELETONS, OR ENDOSKELETONS PROVIDE ANIMALS WITH MECHANICAL SUPPORT AND PROTECTION.
- The rigid element of a hydrostatic skeleton is an incompressible fluid within a body cavity, used to support the body and change shape.
- Invertebrate exoskeletons form an external rigid support system, which protects the animal and limits water loss, but also limits growth and repair.
- Vertebrates have a bony endoskeleton that provides rigid support and protection of body organs, and that can grow and be repaired.
- The vertebrate endoskeleton is organized into axial (central) and appendicular (limb) components.
- Vertebrate endoskeletons consist of bone and cartilage.
- Bone is a composite tissue that consists of calcium phosphate mineral and type I collagen and that is rigid and hard to break.
- Cartilage is a fluid-based gel reinforced by type II collagen and proteoglycans, providing cushioning support at joint surfaces.
37.4 VERTEBRATE ENDOSKELETONS ALLOW FOR GROWTH AND REPAIR AND TRANSMIT MUSCLE FORCES ACROSS JOINTS.
- Vertebrate bones develop by one of two processes: either directly or by way of a cartilage model (endochondral ossification).
- Cartilage forms much of the embryonic skeleton and remains as growth plates within the bone to provide rapid growth during adolescence, as well as forming the bone’s joint (articular) surfaces.
- Bone has two basic structures: Compact bone forms the solid walls of a bone’s shaft, and spongy bone forms a mesh that supports the cartilage at the bone’s ends.
- Bone formation by osteoblasts and removal by osteoclasts is a continuing process of growth, shape change, and repair.
- The shape of a bone’s joint surfaces largely determines the range of motion and stability of a joint.
- Muscles produce joint movements by transmitting forces by means of rigid skeletal levers.
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Self-Assessment Question 1
Diagram a sarcomere, showing the basic organization of thin (actin) and thick (myosin) filaments. Indicate on your diagram the regions where myosin cross-bridges form with actin.
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Self-Assessment Question 2
Describe the sequence of events that occurs when an action potential arrives at a muscle fiber’s motor endplate, causing the muscle to be depolarized.
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Depolarization at the motor endplate of a muscle fiber is propagated from the cell surface to the sarcoplasmic reticulum (SR) via the T-tubule system. Depolarization of the SR causes release of Ca2+ ions from the SR into the cytoplasm. The Ca2+ ions bind to troponin and cause a conformational change in troponin that causes tropomyosin to move. The movement of tropomyosin exposes the myosin binding sites on actin, allowing for the formation of myosin-actin cross-bridges and muscle contraction.
Self-Assessment Question 3
Draw a graph of the isometric force–length relationship of striated muscle, indicating where maximal overlap between actin and myosin filaments occurs.
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Self-Assessment Question 4
Draw a graph of the force–shortening velocity relationship of striated muscle.
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Self-Assessment Question 5
Compare the force produced by a muscle over time when it is stimulated by a single twitch stimulus with the force produced by multiple stimuli at low frequency, and then with the force produced when the stimulation frequency is increased.
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A single twitch stimulus will result in a certain amount of force based on the number of muscle fibers in the motor unit. If the number of stimuli increase but the frequency is low, the muscle fully relaxes between stimuli. In this case, the force produced by each stimulus will be equal to that of a single twitch stimulus. When the frequency of the stimuli is increased further, so that the muscle does not have time to relax between stimuli, the forces produced by the individual stimuli are summed together to generate a greater force than a single twitch stimulus can produce. At a sufficiently high frequency, a sustained contraction of the muscle, known as tetanus, occurs.
Self-Assessment Question 6
Name the two basic structural elements common to all animal skeletons.
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All animal skeletons consist of rigid elements (such as cartilage, bones, or fluid-filled cavities) and groups of muscles that work as antagonists.
Self-Assessment Question 7
Compare and contrast three features of an exoskeleton and an endoskeleton.
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(1) While both endoskeletons and exoskeletons provide support and protection for an animal, exoskeletons cover the outside of an animal and provide external protection from harm and dessication, whereas endoskeletons are internal and provide protection to vital organs but not the animal’s exterior. (2) Both types of skeletons are made from composite materials, which makes them strong and flexible. However, endoskeletons, like those found in vertebrates, are first formed as cartilage and then mineralized to form hardened bone, whereas exoskeletons are formed by the deposition of mineralized proteins at the outer edge of the growing skeleton. (3) Endoskeletons allow for greater range of motion because they have joints between adjacent bones, while organisms with exoskeletons are more restricted in their movements since they usually only have joints between different body segments. (4) Exoskeletons limit growth because organisms must undergo molting and regeneration of their skeleton as they increase in size, and very large exoskeletons are especially prone to damage because they are so thin. In contrast, bone length in endoskeletons can increase over time to support growth of the organism during development and throughout its lifetime. (5) Damage to both types of skeletons can be repaired, but animals with exoskeletons must form an entirely new skeleton, whereas animals with endoskeletons can repair specific regions of damaged bone through the work of osteoblasts.
Self-Assessment Question 8
Identify the two primary components of bone tissue and explain how each of these contributes to a bone’s strength, stiffness, and resistance to fracture.
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Bone tissue is composed of hydroxyapitite mineral and collagen protein. Hydroxyapitite makes the bone strong and rigid, and collagen protein allows the bone to absorb force and resist breaking.
Self-Assessment Question 9
Diagram a limb bone, such as the tibia, showing the regions of articular cartilage, spongy trabecular bone, compact bone tissue, and the marrow cavity.
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