6 The Muscular System 1

Introduction to Muscles Muscle tissue is found in every organ Muscles participate in every activity that requires movement Large proportion of body weight is muscle 40% of body weight in males 32% of body weight in females Skeletal muscle: attaches to skeleton and provides strength and mobility Cardiac muscle: exclusively in the heart Smooth muscle: walls of digestive tract, blood vessels, uterus, ureters

Muscles Produce Movement or Generate Tension Muscles may produce movement Voluntary: conscious control over movement (picking up a pen) Involuntary: unconscious control over movement (beating of heart) Many muscles resist movement Maintenance of posture Maintenance of blood pressure Muscles generate heat

The Fundamental Activity of Muscle Is Contraction Excitable: contract in response to electrical or chemical stimuli All muscle cells have one mechanism of action: They contract (shorten), then relax (lengthen)

Pectoralis major Trapezius Serratus anterior Deltoid Biceps brachii Figure 6.1 Pectoralis major Draws arm forward and toward the body Trapezius Lifts shoulder blade Braces shoulder Draws head back Serratus anterior Helps raise arm Contributes to pushes Draws shoulder blade forward Deltoid Raises arm Biceps brachii Bends forearm at elbow Triceps brachi Straightens forearm at elbow Rectus abdominus Compresses abdomen Bends backbone Compresses chest cavity Latissimus dorsi Rotates and draws arm backward and toward body External oblique Lateral rotation of trunk Compresses abdomen Gluteus maximus Extends thigh Rotates thigh laterally Adductor longus Flexes thigh Rotates thigh laterally Draws thigh toward body Hamstring group Draws thigh backward Bends knee Figure 6.1 Major skeletal muscle groups and their functions. Sartorius Bends thigh at hip Bends lower leg at knee Rotates thigh outward Gastrocnemius Bends lower leg at knee Bends foot away from Quadriceps group Flexes thigh at hips Extends leg at knee Achilles tendon Connects gastrocnemius muscle to heel Tibialis anterior Flexes foot toward knee 5

Skeletal Muscles Cause Bones to Move Synergistic muscles: work together to created the same movement Antagonistic muscles: muscles that oppose each other Many muscles attach to bones via tendons Origin: end of muscle that attaches to relatively stationary bone Insertion: end of muscle attached to another bone across a joint, action pulls insertion toward origin

Movement. Antagonistic muscles Figure 6.2 Origins from scapula Scapula Tendons Biceps contracts, pulling forearm up Shoulder joint Triceps relaxes Humerus Origins from scapula and humerus Biceps muscle Triceps muscle Triceps contracts, pulling forearm down Tendon Biceps relaxes Figure 6.2 Movement of bones. Insertion on ulna Tendon Elbow joint Insertion on radius Ulna Radius Movement. Antagonistic muscles produce opposite movements. The forearm bends when the biceps contracts and the triceps relaxes. The forearm straightens when the biceps relaxes and the triceps contracts. Origin and insertion. The point of attachment of a muscle to the stationary bone is its origin; the point of attachment to the movable bone is its insertion. 7

A Muscle Is Composed of Many Muscle Cells Muscles Group of muscle cells with same origin, insertion, and function Fasicles Bundles of muscle fibers (cells) wrapped with connective tissue (fascia) Muscle fibers (muscle cells) Long, tube shaped Vary in length from few mm to 30 cm Multinucleate Packed with myofibrils, which are long cylindrical structures which contain proteins actin and myosin

Muscle bundle (fascicle) surrounded by connective tissue (fascia) Figure 6.3 Muscle bundle (fascicle) surrounded by connective tissue (fascia) Whole muscle Single muscle cell (fiber) Tendon Bone Figure 6.3 Muscle structure. 9

A single muscle cell contains many individual myofibrils Figure 6.4 Muscle cell Myofibril A single muscle cell contains many individual myofibrils and has more than one nucleus. Nuclei Muscle cell Figure 6.4 Muscle cells. A photograph of portions of several skeletal muscle cells. 10

Animation: Muscle Structure and Function Right-click and select Play

The Muscle Contractile Unit Is the Sarcomere Sarcomere: contractile unit Myosin: forms thick filaments Actin: forms thin filaments Z Lines: attachment points for sarcomeres A sarcomere is a segment of myofibril extending from one Z line to the next Arrangement of filaments gives rise to striated appearance of skeletal muscle

A closer view of a section of a myofibril Figure 6.5 Myofibril Z-line Z-line Sarcomere A closer view of a section of a myofibril showing that it is composed of sarcomeres joined end to end at the Z-line. Myosin Actin An electron micrograph cross section of a sarcomere in a region that contains both actin and myosin. Thin filament (actin) Thick filament (myosin) Sarcomeres contain thin filaments of actin that attach to the Z-lines and thicker filaments of myosin that span the gap between actin molecules. Figure 6.5 Structure of a myofibril. A transmission electron micrograph ( 11,300) of a longitudinal section of a sarcomere. The rounded red objects are mitochondria. 13

Individual Muscle Cells Contract and Relax Muscle contraction: each sarcomere shortens a little Basic process of contraction: Skeletal muscle must be activated by a nerve Nerve activation increases the concentration of calcium ions in the vicinity of the contractile proteins Presence of calcium permits contractions When nerve stimulation stops, contraction stops

Nerves Activate Skeletal Muscles Acetylcholine is released from motor neuron at neuromuscular junction Electrical impulse transmitted along T tubules Calcium (Ca) is released from sarcoplasmic reticulum (modified smooth endoplasmic reticulum) Ca initiates chain of events that cause contraction when it contacts the myofibrils

The release of acetylcholine at the neuromuscular junction causes an Figure 6.6 1 The release of acetylcholine at the neuromuscular junction causes an electrical impulse to be generated in the muscle cell plasma membrane Motor neuron Acetylcholine 2 Electrical impulse The electrical impulse ( ) is carried to the cell’s interior by the T tubules Ca2 T tubule Sarcoplasmic reticulum 3 The electrical impulse triggers the release of Ca2 from the sarcoplasmic reticulum Figure 6.6 How nerve activation leads to calcium release within a muscle cell. Muscle cell plasma membrane Myofibrils Z-line 16

Calcium Initiates the Sliding Filament Mechanism Thick filaments: myosin Thin filaments: actin Contraction: formation of cross-bridges between thin and thick filaments Ca must be present for cross-bridges to form In absence of Ca, protein complex of troponin-tropomyosin covers myosin binding sites on actin molecules Presence of Ca, binding sites available

Relaxed state. The myosin heads do Figure 6.7 Myofibril Myosin molecule head Myosin molecule Thick filament Thin filament Actin molecule Relaxed state. The myosin heads do not make contact with actin. Figure 6.7 Sliding filament mechanism of contraction. Contraction. The myosin heads form cross-bridges with actin and then bend, pulling the actin filaments toward the center of the sarcomere. 18

When Nerve Activation Ends, Contraction Ends Calcium is released from sarcoplasmic reticulum Calcium binds to troponin Troponin–tropomyosin complex shifts position Myosin binding site is exposed Myosin heads form cross-bridges with actin Actin filaments are pulled toward center of sarcomere Sarcomere shortens

Resting sarcomere. In the absence of calcium the Figure 6.8 Sarcoplasmic reticulum Myofibril Sarcoplasmic reticulum Electrical impulse Thick and thin filaments Tropomyosin Calcium release Ca2 Actin filament Ca2 Myosin binding sites Troponin Myosin head Figure 6.8 Role of calcium in contraction. Cross- bridge Myosin filament Resting sarcomere. In the absence of calcium the muscle is relaxed because the myosin heads cannot form cross-bridges with actin. Cross-bridge attachment. The binding of calcium to troponin causes a shift in the troponin-tropomyosin complex, allowing cross-bridges to form. 20

Muscles Require Energy to Contract and to Relax Nerve activation ends, contraction ends Ca pumped back into sarcoplasmic reticulum (requires ATP) Ca no longer bound to troponin Myosin binding site covered ATP must bind to myosin before myosin heads can detach from actin No calcium  no cross-bridges Muscle relaxes

Muscles Require Energy to Contract and to Relax Principle source of energy: ATP ATP required for contraction ATP required for relaxation ATP is replenished by a variety of means Creatine phosphate Stored glycogen Aerobic metabolism of glucose, fatty acids, and other high-energy molecules

Table 6.1 Table 6.1 Energy sources for muscle. 23

The Activity of Muscles Can Vary Isotonic contractions: muscle shortens, while maintaining a constant force, movement occurs Isometric contractions: force generated, muscle doesn’t shorten, no movement Degree of nerve activation influences force Terms to know: Motor unit Muscle tension All-or-none principle Muscle tone Recruitment

The Degree of Nerve Activation Influences Force Motor unit Motor neuron and all the muscle cells it controls Smallest functional unit of muscle contraction Muscle tension Mechanical force that muscles generate when they contract Determined by Motor unit size Number of active motor units Frequency of stimulation of motor units

The Degree of Nerve Activation Influences Force All-or-none principle Individual muscle cells are completely contracting or are relaxed Muscle tone Whole muscles—maintain intermediate level of force known as muscle tone Recruitment Activation of additional motor units increases muscle tone

all of the muscle cells it controls. Any one muscle cell Figure 6.9 Two motor neurons Muscle Muscle cells Neuromuscular junctions A motor unit consists of a motor neuron and all of the muscle cells it controls. Any one muscle cell is controlled by only one motor neuron, but a motor neuron controls more than one muscle cell. Figure 6.9 Motor units. Photograph of the muscle cells in a motor unit, showing branches of the motor neuron and neuromuscular junctions. 27

The Degree of Nerve Activation Influences Force Complete cycle of contraction-relaxation in response to stimulus Can be observed using a myogram (laboratory recording of muscle activity) Latent period Contraction Relaxation Summation Tetanic contraction

Muscle force Time (msec) Tetanus Latent period Summation Contraction Figure 6.10 Tetanus Latent period Summation Contraction Relaxation Muscle force Figure 6.10 How frequency of stimulation affects muscle cell contractile force. Stimulus Time (msec) 500 29

Slow Twitch versus Fast Twitch Fibers Contract slowly Make ATP as needed by aerobic metabolism Many mitochondria Well-supplied with blood vessels Store very little glycogen “Red” muscle Used for endurance activities Fast Twitch Contract quickly Rapidly break down ATP Fewer mitochondria Little or no mitochondria Store a lot of glycogen “White” muscle Capable of anaerobic metabolism Used for brief high-intensity activities

Exercise Training Improves Muscle Mass, Strength, and Endurance Strength training Resistance training Short, intense Builds more myofibrils, particularly in fast-twitch fibers Aerobic training Builds endurance Increases blood supply to muscle cells Increase in mitochondria and myoglobin Reach target heart rate for at least 20 minutes, three times a week

Cardiac and Smooth Muscles Have Special Features Involuntary Able to contract entirely on their own in absence of nerve stimulation Cardiac muscle cells are joined by intercalated disks Have gap junctions allowing cells to electrically stimulate the next one Smooth muscle cells joined by gap junctions allowing cells to activate each other Cardiac and smooth muscle cells respond to stimulation from autonomic nervous system, which can modify the degree of contraction

A view of several adjacent cardiac muscle cells showing Figure 6.12 Cardiac muscle cell Intercalated disc A view of several adjacent cardiac muscle cells showing their blunt shape and the intercalated discs that join them together. Figure 6.12 Cardiac muscle cells. Adhesion junction Protein channel Gap junction Cell membranes of adjacent cells A closer view showing that intercalated discs are bridged by gap junctions that permit direct electrical connections between cells. 33

Speed and Sustainability of Contraction Skeletal muscle: fastest Cardiac muscle: moderate Smooth muscle: Very slow Partially contracted all of the time Almost never fatigues

Arrangement of Myosin and Actin Filaments Cardiac muscle Sarcomere arrangement of thick and thin filaments Striated appearance Smooth muscle Filaments arranged in criss-crossed bundles, not sarcomeres No striations

Relaxed state. Contracted state. The A closer view showing how actin Figure 6.13 Relaxed state. Filament bundles Contracted state. The crisscross arrangement of bundles of contractile filaments causes the cell to become shorter and fatter during contraction. Thin filament Thick filament Figure 6.13 Smooth muscle. Cell membrane protein A closer view showing how actin filaments are attached to cell membrane proteins. 36

Diseases and Disorders of the Muscular System Muscular Dystrophy Genetic disease: Duchenne Muscular Dystrophy Modified dystrophin protein enables leakage of Ca into cells Extra Ca activates enzymes that destroy muscle proteins Muscle weakening and wasting Muscle mass is replaced with fibrous connective tissue Life expectance: approx. 30 years

Diseases and Disorders of the Muscular System Tetanus Infection of deep wound by bacteria, Clostridium tetani Bacteria produce tetanus toxin which causes muscles to contract forcefully Death due to respiratory failure Preventable by tetanus vaccine

Diseases and Disorders of the Muscular System Muscle cramps: often caused by dehydration and ion imbalances Pulled muscles: result from overstretching of a muscle, fibers tear apart Fasciitis: inflammation of fascia Plantar fasciitis: sole of foot