Muscles and Bones

Muscles consist of groups of bundles of muscle fibers Bundles of muscle fibers contain several muscle cells that span the length of the muscle

Muscle fibers consist of cylinders called myofibrils, which are surrounded by a sarcoplasmic reticulum, which stores high concentrations of calcium ions necessary for contraction.

Myofibrils consist of repeating subnits called sarcomeres, which give muscle its striped appearance. Sarcomeres have a precise arrangement of thick and thin filaments. The plasma membrane which surrounds each muscle fiber has indentations called T tubules which come into close contact with the sarcoplasmic reticulum

Thick filaments are made up of the protein myosin. Thin filaments are made up of the protein actin, along with the accessory proteins troponin and tropomyosin.

Muscle Contraction Projections from the thick filament called cross bridges link temporarily to the thin filament. Using ATP, the cross bridges bend, slightly shortening the sarcomere, and reattach to another binding site on the thin filament.

Muscle Contraction: Opening the Binding Sites Binding sites on actin are normally blocked by accessory proteins. Binding sites open up only when accessory proteins are occupied with calcium ions released from the sarcoplasmic reticulum.

Initiation of Contraction Muscle contraction is initiated by signals from the nervous system called action potentials. The length and strength of muscle contractions is determined by frequency of action potentials and the number of muscle fibers stimulated. Motor Unit = the group of muscle fibers affected by a single neuron’s action potential

The Neuromuscular Junction When an action potential arrives at a neuromuscular junction, it passes down the muscle fiber’s T tubules.

The Neuromuscular Junction The electrical signal conveyed by the T tubules is converted into a chemical (Ca 2+ ) signal when it reaches the sarcoplasmic reticulum. Calcium ions are then released into the cytoplasm surrounding thick and thin filaments.

Cross Bridge Binding Cross bridge can’t bind because accessory proteins blocking the binding site. Cross bridge can bind because accessory proteins are bound to Ca 2+.

3 Types of Muscle Tissue 1.Skeletal Muscle is under our voluntary control and is used for movement. 2.Cardiac Muscle is located in the heart and initiates its own contractions. 3.Smooth Muscle lines our hollow organs such as intestines, bladder, and blood vessels. It is normally not under our control. Both skeletal and cardiac muscle are striated, or “striped,” because of their regular arrangement of sarcomeres.

Skeletal Muscle Action potentials in skeletal muscles are additive. Ex: 2 action potentials arriving at a muscle fiber produce a contraction twice as strong as that produced by a single action potential. But there is a limit: Tetanus = the maximum sustained contraction of a skeletal muscle, cause by high frequency of action potentials

Cardiac Muscle Cardiac muscle is unique in that it can initiate its own contractions at the sinoatrial (SA) node. Action potentials from the SA node spread rapidly throughout the heart via gap junctions. Gap junctions allow for synchronized contraction.

Movement of the Body The muscular system is structured with pairs of antagonistic muscles. As one muscle actively contracts, the other passively extends. Ex: biceps/triceps, quadriceps/hamstrings

Skeletons Skeleton = a supporting framework for a body Hydrostatic skeletons are made of fluid Exoskeletons provide support from outside the body Endoskeletons are inside the body

Functions of the Skeleton 1.Protection 2.Support 3.Movement 4.Production of blood cells. 5.Storage for minerals.

Composition of the Skeleton 2 major types of tissue in our skeleton: cartilage and bone. Both are living tissue. Cartilage is flexible and exists near joints. Before birth, our skeleton is primarily cartilage and gradually is replaced with bone.

Cartilage Chondrocytes = the living cells of cartilage are called\ Chondrocytes secrete a nonliving matrix of collagen. The collagen matrix provides the firm but flexible support that exists between vertebral discs, the outer ear, and the knees.

Bone Bone has a dense, rigid outer layer called compact bone, which provides an ideal site for muscle attachment. Inner bone layers consist of porous spongy bone, where bone marrow is found. Bone marrow is where blood cells are formed.

Video Segment Video showing how bone distributes weight to withstand great impact forces. (1:52) (1:52)

Bone 3 types of cells found in bone tissue: 1.Osteoblasts = bone-forming cells 2.Osteocytes = mature bone cells 3.Osteoclasts = bone-dissolving cells Functional unit of bone is the osteon, with rings of osteocytes surrounding a central canal with a capillary.

Bone Remodeling Bone tissue is continually breaking down and rebuilding itself in a process called bone remodeling. 1.Osteoclasts on the bone surface secrete acids and enzymes to dissolve the hardened matrix. 2.Osteoblasts and capillaries move into the dissolved space. 3.As osteoblasts lay down collagen and calcium phosphate, they become trapped in it and turn into mature osteocytes.

All bones in this diagram are test-worthy.

Test-Worthy Muscle Groups Front Side View Deltoid Pectoralis major Biceps brachii Quadriceps femoris Back Side View Trapezius Latissimus dorsi Triceps brachii External oblique Gluteus maximus Hamstrings Gastrocnemius