The Muscular System Biology 105 Lecture 12 Chapter 6
Outline I. Characteristics of muscles II. Three types of muscles III. Functions of muscles IV. Structure of skeletal muscles V. Mechanics of muscle contraction VI. Energy source for muscle contraction
Muscular System Recall there are different types of muscles: smooth, cardiac, and skeletal. All muscle cells are elongated, and therefore are called muscle fibers. All muscle tissues contract. Muscles contain muscle fibers, connective tissue, blood vessels, and nerves.
Smooth Muscle Smooth muscles are involuntary muscles found in the walls of many internal organs (digestive tract, respiratory system, blood vessels). They aid in the function of other organs.
Cardiac Muscle Cardiac muscles are involuntary muscles found only in the heart wall. They function by contracting, which forces blood from the heart into the arteries.
Skeletal Muscle Skeletal muscles are voluntary muscles attached to the skeleton. They usually work in pairs.
Skeletal Muscles Work in Pairs Most skeletal muscles work in antagonistic pairs: One muscle contracts, while the other relaxes. Muscles are attached to the bone by tendons. Skeletal muscles are usually attached to two bones on opposite sides of a joint.
Skeletal Muscles Work in Pairs The origin of the muscle is attached to the bone that remains stationary during movement. The insertion is attached to the bone that moves. Bones act as levers in working with skeletal muscles to produce movement.
Skeletal Muscles Work in Pairs Origin of muscle: attachment of muscle to less moveable bone The biceps contracts and pulls the forearm up, flexing the arm. The relaxed triceps is stretched. (a) Flexion Insertion of muscle: attachment of muscle to more moveable bone Figure 6.1a
Functions of Skeletal Muscles 1. Support the body maintain posture 2. Movement of bones and other tissues 3. Help maintain a constant body temperature generate heat 4. Help move blood through the veins and lymphatic fluid through the lymphatic vessels 5. Help to protect vital organs and stabilize joints
Structure of Skeletal Muscles Muscles are covered by connective tissue called fascia. A muscle contains bundles of skeletal muscle fibers (muscle cells): The bundles are called fascicles. These bundles are covered by connective tissue. Blood vessels and nerves are between the fascicles.
Structure of Skeletal Muscles Skeletal muscle consists of many bundles of muscle cells. A bundle of muscle cells is called a fascicle. A muscle cell consists of many myofibrils. (a) A section of a skeletal muscle The striped (striated) appearance of a skeletal muscle cell is due to the regular arrangement of myofilaments. (b) A light micrograph of a longitudinal view of skeletal muscle cells Figure 6.3a b
Muscle Cell Components Muscle cells (muscle fibers) have many of the same components as typical cells, but some of their components have different names Sarcolemma plasma membrane (cell membrane). Sarcoplasm similar to cytoplasm, and contains large amounts of stored glycogen and myoglobin. Myoglobin is an oxygen-binding protein similar to hemoglobin, but found only in muscles. Sarcoplasmic reticulum similar to endoplasmic reticulum, and functions as a Ca 2+ store.
Muscle Cell Components Muscle fibers also have unique features: Multiple nuclei Transverse tubules (T tubules) extensions of the sarcolemma that come into contact with the sarcoplasmic reticulum.
c. myofibril a. T tubule b. Sarcoplasmic reticulum d. Z line e. sarcomere f. sarcolemma
Muscle Cells (Fibers) The muscle fiber is composed of long, thin myofibrils. Myofibrils are bundles of myofilaments that contract: There are two types of myofilaments: actin and myosin.
Muscle Contraction A sarcomere is the name for the structural unit of these myofilaments. When you look at the myofibril, the sarcomere lies between two dark lines called Z lines: The Z lines are protein sheets where the actin filaments attach. When muscle fibers are stimulated to contract, myofilaments slide past one another, causing sarcomeres to shorten.
Sarcomeres The striped (striated) appearance of a skeletal muscle cell is due to the regular arrangement of myofilaments. (b) A light micrograph of a longitudinal view of skeletal muscle cells Z line One sarcomere (c) A diagram and electron micrograph of a myofibril Figure 6.3b c
Sarcomeres Z line One sarcomere (c) A diagram and electron micrograph of a myofibril Z line One sarcomere Z line Actin Myosin (d) A sarcomere, the contractile unit of a skeletal muscle, contains actin and myosin myofilaments. Figure 6.3c d
Myofilaments Actin and Myosin The two myofilaments are: Actin filaments: thin filaments that are formed by two intertwining strands of the protein actin. Myosin filaments: thick filaments of the protein myosin that are shaped like a golf club with a round head. The myosin heads can bind and detach from the thin actin filament. When bound, they create cross-bridges.
Muscle Contraction A neuron signals the muscle to contract. The myosin heads attach to the actin, and then pull the actin toward the center of the sarcomere. The myosin heads detach.
Sarcomeres Shorten During Muscle Contraction Figure 6.4
Steps of Muscle Contraction 1. Action potentials are transmitted through the neurons. 2. At the end of the neurons, neurotransmitters are released into the synaptic cleft. 3. Neurotransmitters bind to receptors on the sarcolemma.
Neuromuscular Junction Figure 6.7 (1 of 2)
Steps of Muscle Contraction 4. These receptors are ion channels that open. 5. An action potential travels through the T tubules of the muscle fiber. 6. The action potential goes to the sarcoplasmic reticulum. 7. The sarcoplasmic reticulum releases Ca 2+.
Steps of Muscle Contraction 8. The calcium binds to the troponin on the actin filament. 9. This uncovers the binding site for the myosin to attach. 10.Now the myosin binds to the actin. 11.ATP is needed for the myosin to slide past the actin.
Sarcomeres Figure 6.6 (1 of 2)
Sarcomeres Figure 6.6 (2 of 2)
Tropomyosin-Troponin Complex The tropomyosin-troponin complex is attached to the actin filament. Calcium binds to the troponin, causing a shift in the complex, which opens the sites for myosin to attach.
A bundle of muscle cells is called a: 1. Fascicle 2. Fascia 3. Muscle fiber 33% 33% 33%
What is the oxygen-binding protein found only in muscles? 1. Myosin 2. Actin 3. Hemoglobin 4. Myoglobin 25% 25% 25% 25% Myosin Actin Hemoglobin Myoglobin
Which ion is required for the myofilaments to bind to each other? 1. Potassium 2. Calcium 3. Chloride 4. Sodium 25% 25% 25% 25% Potassium Calcium Chloride Sodium
Where is the calcium stored? 1. Nucleus 2. Sarcolemma 3. Sarcoplasmic reticulum 33% 33% 33% Nucleus Sarcolemma Sarcoplasmic reticulum
ATP is needed for the myofilaments to slide past each other
ATP ATP is the energy currency like money in the bank! The bonds between the phosphate groups are high energy bonds.
The Energy Sources Muscle contractions take a lot of energy in the form of ATP. Muscles get their ATP from three sources: 1. Breakdown of creatine phosphate 2. Cellular respiration 3. Fermentation
1. Creatine Phosphate Creatine phosphate regenerates ADP to make ATP. This gives quick energy for a few seconds (up to 30 seconds). Only 1 ATP is produced per creatine phosphate. Oxygen is not needed. When a muscle is resting, the ATP in turn regenerates creatine phosphate.
2. Cellular Respiration In the mitochondria, glucose is broken down to produce ATP. Remember that oxygen is needed for the electron transport chain to produce ATP. Carbon dioxide is produced as a waste product during the Krebs Cycle of cellular respiration. Can provide energy for hours. Produces 36 ATP per glucose molecule. Can use glucose, as well as fatty acids and amino acids, for the energy source.
3. Fermentation This is when the cell only uses glycolysis, and glucose is broken down to lactic acid. Since the Krebs Cycle and the electron transport chain are skipped, no oxygen is required. No CO 2 is produced as a waste product, but lactic acid is produced. Can provide energy for 30 60 seconds. Only 2 ATP produced per glucose molecule.
ATP Comes from Many Sources Figure 6.10
CP Breakdown Cellular Respiration Fermentation Requires O 2 No Yes No Produces No Yes No CO 2 # ATP produced 1 36 2 Duration 30 sec Hours 30-60 sec
Which energy source would a long-distance runner mainly use on a run that lasted for hours? 1. Fermentation 2. Cellular respiration 3. Creatine phosphate 25% 25% 25% 25% Fermentation Cellular respiration Creatine Phosphate
Which energy source would a sprinter use in the first 5 seconds of the race? 1. Fermentation 2. Cellular respiration 3. Creatine phosphate 25% 25% 25% 25% Fermentation Cellular respiration Creatine Phosphate
Important Concepts Read Chapter 6 What are the three types of muscles? Where are they found, and are they under voluntary or involuntary control? What are the functions of skeletal, cardiac, and smooth muscles? How do skeletal muscles work in pairs? What is the function of tendons?
Important Concepts What is the overall structure of a muscle? What are the components of a muscle, and of a muscle cell (muscle fiber)? What are the functions of the muscle fiber components? You should be able to identify the muscle fiber components in an illustration, including: myofibrils, sarcomeres, Z lines, myofilaments (actin and myosin filaments), cross-bridges, sarcolemma, sarcoplasm, sarcoplasmic reticulum, T tubules
Important Concepts What stimulates a muscle to contract? You should be able to describe the steps of how the message is transmitted from the neuron to the myofilaments What is the role of Ca 2+? What happens when the message is received by the myofilaments? What are the components and the function of the tropomyosin-troponin complex?
Important Concepts What are the three energy sources used for muscle contraction? Which of these require oxygen and which produce carbon dioxide? How many ATP are produced, and how long can each energy source provide energy?
Definitions Muscle fiber, myoglobin, fascia, fascicle, myofibril, sarcomere, involuntary, voluntary, origin, insertion