Muscular System “The function of muscle is to pull and not to push, except in the case of the genitals and the tongue.” - Leonardo da Vinci

Muscle Tissue There are three types of muscle tissue. Smooth muscle lines the walls of hollow organs. ▫Involuntary. ▫No striations, single nucleus.

Cardiac muscle forms the walls of the heart. ▫Cells are branching and striated. ▫Single nucleus per cell. ▫Intercalated discs divide each cell.

Skeletal muscle attached to bones. ▫Voluntary. ▫Long, striated, with multiple nuclei. The muscular system refers to skeletal muscles only.

Muscular System Functions Produce movement of the skeleton. Maintain body position. Support soft tissues. Guard body openings. Maintain body temperature.

Muscle Fibers (cells) Individual skeletal muscle cells are called muscle fibers due to their length. Skeletal muscle fibers are very long, stretching the entire length of a muscle. Each may contain hundreds of nuclei. ▫These nuclei are needed to maintain control across the entire cell. ▫This arrangement makes mitosis impossible.

Muscle Anatomy – Connective Tissue The epimysium is a layer of collagen fibers that covers the entire muscle, separating it from surrounding organs. Each muscle is divided into bundles of muscle fibers, called a fascicle. ▫Each fascicle is divided by the perimysium, which also contains blood vessels and nerves for the muscle. The endomysium surrounds and separates each individual muscle fiber (cell).

Tendon Muscle Fascicle Fiber Myofibril Epimysium Perimysium Endomysium

Structures of the Muscle Fiber The sarcolemma – The cell membrane of the muscle fiber. Sarcoplasm – The cytoplasm of a muscle fiber. T-Tubules – Tubes that travel through the sarcolemma to transmit signals to contract or relax. Myofibrils – Bundles of protein filaments within the muscle fiber. ▫Thin, light filaments, made of the protein actin ▫Thick, dark filaments, made of the protein myosin. ▫The alternation of these filaments creates the striated pattern of skeletal muscle tissue. Sarcoplasmic reticulum – A special type of smooth ER found in muscle cells. Contains calcium needed for contraction.

T-Tubules Sarcoplasmic Reticulum Sarcolemma Myosin Actin

Muscle Contraction Muscle contraction begins with a nerve signal that is transmitted through the motor neuron. ▫The neurotransmitter used as a signal is acetylcholine. ▫Contraction ends when the neurotransmitter is broken down by an enzyme (acetylcholinesterase). This signal is sent to every fiber in the muscle simultaneously through the t-tubules. The sarcoplasmic reticulum releases calcium ions (Ca 2+ ), initiating muscle contraction.

Muscle Contraction The calcium influx stimulates the myosin filaments to form connections to the actin filaments. The myosin filaments pull the actin filaments inward, causing the muscle to contract.

Muscle Fibers Before Contraction

Muscle Fibers After Contraction

Muscle Contraction When a muscle contracts, it pulls bones closer together, creating movement. ▫Example: The biceps, the scapula, radius, and ulna. Contracted muscles become more visible because all of the volume (cytoplasm) is forced outward.

Twitch A twitch is a single sequence of a muscle fiber that includes: ▫Stimulus by a motor neuron. ▫Contraction of the muscle. ▫Relaxation of the muscle.

Twitch A twitch begins with a stimulus, or signal by a motor neuron. During the latent period, the signal is spreading across the muscle. No actual tension occurs. Resting Phase Latent Period

Twitch In the contraction phase, the myosin filaments pull on the actin, creating increasing amounts of tension. Resting Phase Latent Period Contraction Phase

Twitch Calcium levels drop during the relaxation phase, and linkages between actin and myosin decline. Resting Phase Latent Period Contraction Phase Relaxation Phase

Summation A single twitch is short (milliseconds) and doesn’t really produce a useful movement. If a second stimulus arrives before the relaxation phase completes, a more powerful contraction occurs. This is summation.

Summation Eventually this builds into the maximum tension possible with the muscle. This is called incomplete tetanus. ▫Relaxation phases still occur, just not completely.

Summation Complete tetanus occurs if the stimulus frequency is so high that the relaxation phase is completely eliminated.

How long can muscles contract? The duration of muscle contraction depends on three factors: ▫How long the stimulus from the neuron lasts. ▫How many electrolytes needed for contraction (K +, Na +, Ca 2+ ) are available within the muscle. ▫How much ATP energy is available within the muscle. ATP is the direct unit of energy used by muscle fibers. If the supply of ATP is exhausted, the muscle becomes fatigued and will not contract. ▫ATP is converted to ADP when it is used. ▫An active skeletal muscle fiber may use 600 trillion molecules of ATP per second.

ATP while Resting While at rest, muscles build up their energy reserves. ▫Energy in the form of fatty acids or glucose is brought in by the blood. Mitochondria convert the fatty acids and glucose into ATP by cell respiration.

Cell Respiration There are two main parts of cell respiration: Glycolysis, which occurs in the cytoplasm. ▫Breaks down glucose (6-carbon sugar) into two molecules of pyruvate (3-carbon) ▫Produces 2 molecules of ATP. ▫Occurs quickly, does not require oxygen. The rest occurs in the mitochondria. ▫Metabolizes the pyruvate into 3 molecules of CO 2. ▫Produces 34 molecules of ATP. ▫Requires oxygen.

Energy Storage in Muscles Creatine (C 4 H 9 N 3 O 2 ) is an organic acid that can store energy in muscles as creatine-phosphate. Glycogen is a polysaccharide similar in structure to starch. ▫Made of multiple glucose molecules bonded together. Some atheletes take supplements to increase creatine levels in muscle.

Blood VesselFatty Acids Mitochondria Glucose Glycogen Creatine This diagram shows a muscle fiber at rest. ATP is generated in the mitochondria from fatty acids. Excess ATP is used to generate creatine-phosphate or glycogen.

ATP During Moderate Activity During moderate activity (e.g. jogging), the stored creatine phosphate and glycogen in the muscle is metabolized. ▫CP can be directly used to generate ATP. ▫Glycogen can be broken down into glucose, which the mitochondria can use to generate ATP. Moderate activity can continue as long as: ▫Oxygen levels are sufficient ( ↑ breathing rate) ▫Glycogen is not used up.  “Hitting the wall”

Blood Vessel Fatty Acids Mitochondria GlycogenGlucose Pyruvate ATP used as energy for contraction. This diagram shows a muscle fiber in moderate activity. ATP is being generated in the mitochondria and cytoplasm from glucose and fatty acids.

ATP During Intense Activity During intense activity (e.g. sprinting), oxygen is unable to diffuse into the muscle fast enough to provide the needed ATP by cell respiration. The muscle begins relying on glycolysis to make up for this deficiency. ▫This creates an excess of pyruvate, which is converted to lactic acid. ▫Lactic acid lowers the pH of the muscle and interferes with normal function.  Soreness.

Lactic Acid Glycogen Glucose Pyruvate ATP used as energy for contraction. Lactic Acid This diagram shows a muscle fiber in intense activity. Oxygen levels are insufficient; only glycolysis can occur. Excess pyruvate is converted to lactic acid. Creatine Phosphate is utilized as a source of ATP.

Fatigue Fatigue occurs when the muscle can no longer contract, despite stimulus from the brain. Caused by rapid and repeated contractions. ▫ATP levels are too low. ▫Lactic acid levels are too high. The muscles will not regain their normal ability to contract until pH, oxygen, and ATP levels have been restored.

Case Study: Rigor Mortis When an organism dies, skeletal muscle is deprived of nutrients and oxygen. Calcium ions will leak out of the sarcoplasmic reticulum, causing one last sustained muscle contraction. ▫Because no ATP is left in the muscle, the myosin and actin are unable to separate, causing rigor mortis.

Case Study: Rigor Mortis As the cells of the body die, enzymes are released from the lysosomes. These enzymes break down the actin and myosin filaments, and the muscles permanently go limp.

Endurance Two factors influence muscle endurance: ▫The type of fibers within the muscle. ▫Physical conditioning.

Types of Muscle Fibers Fast-twitch fibers are able to reach peak tension within 0.01 seconds or less of neural stimulation. ▫Large in diameter. ▫Densely packed with myofibrils (actin and myosin). ▫Large glycogen reserves. ▫Fewer mitochondria. Fast-twitch fibers produce the most tension, but get fatigued quickly.

Types of Muscle Fibers Slow-twitch fibers can take three times as long to reach peak tension. ▫Half the diameter of fast-twitch fibers. ▫Increased network of capillaries, allowing for a greater and more reliable oxygen supply. ▫Contain a special protein called myoglobin that reserves additional oxygen within the muscle. ▫Higher numbers of mitochondria.

Chicken meat is a good example of the differences between muscles with lots of slow-twitch or fast-twitch fibers. The breasts are “white meat” because they have mostly fast-twitch fibers and less myoglobin. ▫Wings are only used for short intervals to escape predators. The legs and thighs are “dark meat” and have mostly slow-twitch fibers and more myoglobin. ▫Used much more frequently during the day, but not at the same intensity.

Physical Conditioning The percentage of fast vs. slow-twitch fibers in a muscle is genetically determined. The ability of fast-twitch muscles to resist fatigue can be increased through physical conditioning. Aerobic exercise focuses on improving endurance. ▫Improved oxygen intake and delivery. ▫Higher glycogen stores within muscle.

Physical Conditioning Anaerobic exercise focuses on improving strength. ▫The number of muscle fibers does not change, but their size can. ▫An increase in size is hypertrophy. ▫A decrease in size is atrophy.

Anabolic Steroids Anabolic steroids are chemical compounds that mimic the effects of testosterone. ▫Increases protein synthesis, causing muscle buildup. The change in hormone levels has a lot of other side effects: ▫Increase in blood cholesterol. ▫Acne ▫High blood pressure ▫Testicular atrophy ▫Increase in male characteristics in women.

Testicular Atrophy? One of the primary functions of the testes is to produce the hormone testosterone. If anabolic steroids are taken externally, the testes will shrink in size, as the body attempts to compensate. ▫This is called negative- feedback inhibition.

Polio Polio is a viral infection that can enter the nervous system, specifically infecting and destroying motor neurons. ▫This causes paralysis. Polio is now considered eradicated due to the use of a vaccine.

ALS (Lou Gehrig’s Disease) Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative disease that affects motor neurons in the brain and spinal cord. ▫Muscle begins to atrophy due to disuse.

Tetanus Tetanus is a disease caused by a bacteria called Clostridium tetanii. The bacteria releases a toxin that stops the normal motor neuron inhibition process. ▫Muscles will uncontrollably contract and maintain the contraction. ▫The toxin spreads, causing symptoms such as lockjaw.

Botulism Botulism occurs when food contaminated with a bacterial toxin are ingested. The toxin prevents the release of acetylcholine by motor neurons, causing paralysis.

Muscular Dystrophy Muscular dystrophy is a group of degenerative disorders that affect muscle tissue. ▫Multiple causes, the most common one is the lack of a protein needed to maintain muscle integrity.

Curare This is a toxin produced by poison dart frogs. Blocks acetylcholine receptors in muscles, causing paralysis. Neostigmine is a drug that prevents acetylcholine from breaking down. This makes it an effective treatment for curare. Discovered when it was observed that animals called peccaries were immune to the poison.

Muscle Anatomy

Muscle Attachment All muscles are attached to at least two points: ▫The origin is an attachment to a immoveable bone. ▫The insertion is an attachment to an movable bone.

Types of Muscle Movement Flexion ▫Most often found in hinge joints. ▫Brings two bones closer together. Extension ▫Opposite movement of flexion. ▫Pulls two bones farther apart. Hyperflexion or hyperextension ▫Flexion beyond the normal position.

Types of Muscle Movement Abduction ▫Pulls a limb away from the midline of the body. Adduction ▫Pulls a limb toward the midline of the body. Circumduction ▫Rotation of a limb.

Types of Muscle Movement Rotation is the movement of a bone in a circular direction around a central axis. Medial rotation is toward the midline. Lateral rotation is away from the midline.

Some muscles are named based on the direction of their fibers. Rectus means straight. ▫Rectus abdominis. Oblique means diagonally arranged. ▫External abdominal oblique. How Are Muscles Named?

Muscles within a group may have different names based on their size. Maximus and longus indicates a larger muscle. ▫Fibularis longus Minimus and brevis indicate a smaller muscle. ▫Fibularis brevis

Prefixes like bi- and tri- may be used to indicate multiple heads or attachment sites. ▫Triceps brachii

Muscles may also be named based on their origin and insertion bones. ▫Sternocleidomastoid: sternum, clavicle, mastoid process of the temporal bone

If a muscle resembles a shape, it can be named after that shape. ▫The deltoid is named after the Greek letter Delta, which is a triangle.

Finally, muscle names may indicate a specific action they perform. ▫Flexor carpi ulnaris

Orbicularis Oris Orbicularis Oculi Sternocleidomastoid Temporalis Frontalis Zygomatic Bone Buccinator Masseter Trapezius Zygomaticus

Head and Neck Muscles The frontalis raises the eyebrows. The masseter and temporalis both elevate the mandible. ▫Chewing muscles The buccinator flattens the cheeks during chewing, holding them against the teeth. The orbicularis oculi performs all eyelid movements, including opening, closing, blinking, etc. The orbicularis oris closes the mouth with the lips. The zygomaticus raises the corners of the mouth when smiling. The sternocleidomastoid rotates the head and flexes the neck.

Sternocleidomastoid Internal Abdominal Oblique Sternum Deltoid Serratus Anterior Clavicle External Abdominal Oblique Rectus Abdominis Pectoralis Major Trapezius Latissimus Dorsi

Muscles of the Trunk The pectoralis major adducts the humerus. The rectus abdominis flexes the vertebral column and compresses the contents of the abdomen. ▫The “pushing” muscle of defecation, childbirth, and forced breathing. ▫The transversus abdominis also performs this action. The external and internal obliques rotate the trunk.

Sternocleidomastoid Trapezius Deltoid Teres Major Infraspinatus External Oblique Latissimus Dorsi

Muscles of the Dorsal Trunk The trapezius elevates and depresses the scapula. The latissimus dorsi adducts the humerus. The deltoid abducts the arm.

Deltoid Brachioradialis Biceps Extensor Digitorum Triceps Extensor Carpi Ulnaris Extensor Carpi Radialis

Anterior Muscles of the Arm The biceps brachii and brachioradialis flex the arm. The triceps brachii extends the arm. The extensor carpi radialis and ulnaris extend the wrist. The extensor digitorum extends the four non- thumb digits.

Posterior Muscles of the Leg Gluteus medius Gluteus maximus Semitendinosus Biceps femoris Semimembranosus Gastrocnemius Soleus

Anterior Muscles of the Leg Gluteus medius Tensor Fascia Latae Sartorius Rectus Femoris Vastus Medialis Fibularis Tibialis Anterior Vastus Lateralis Soleus Gracilis

Muscles of the Hip, Thigh, and Leg The gluteus maximus adducts and extends the leg. The gluteus medius abducts the leg. The hamstring group flexes the knee. ▫Biceps femoris ▫Semitendinosus ▫Semimembranosus The gastrocnemius and soleus extend the foot.

Muscles of the Hip, Thigh, and Leg The sartorius flexes, abducts, and laterally rotates the thigh. ▫Look at the bottom of your foot while standing to demonstrate these actions. The quadriceps group extends the knee. ▫Rectus femoris ▫Vastus medialis ▫Vastus lateralis ▫Vastus intermedialis (a deep muscle)

Muscles of the Hip, Thigh, and Leg The tibialis anterior and fibularis muscles flex the foot.

Frontalis Masseter Sternocleidomastoid External Oblique Gluteus Medius Tensor Fascia Latae Rectus Femoris Vastus Lateralis Vastus Medialis Fibularis Longus Soleus Tibialis Anterior Temporalis Orbicularis Oculi Zygomaticus Orbicularis Oris Pectoralis Major Biceps Brachii Deltoid Rectus Abdominis Internal Oblique Adductor (Groin) Gracilis Sartorius

Trapezius Deltoid Triceps Brachii Latissimus Dorsi External Oblique Gluteus Medius Soleus Gastrocnemius Semitendinosus Biceps Femoris Adductor Gluteus Maximus