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The Muscular System Chapter 5 Muscles Essential function is contraction (shortening) Responsible for all body movements Muscle types 3 types Skeletal Cardiac smooth Similarities of the 3 types All muscle cells are elongated (called muscle fibers) Ability of the muscle to contract depends on two types of microfilaments Terminology Myo, mys, and sarco all refer to muscles Skeletal muscle Attached to the skeleton Cigar-shaped, multinucleate cells Largest of the muscles Striated muscles (fibers appear striped) Voluntary muscles (under conscious control) but can be activated by reflexes (subconscious) Can contract rapidly with great force, but tires easily Muscle fibers are bound together by connective tissue which gives strength and support Skeletal muscle Connective fibers Endomysium – delicate connective tissue that sheaths each muscle fiber Perimysium – coarse connective tissue that wraps several muscle fibers together to make a bundle or fascicle Epimysium – bounds several fascicles together Tendons – blending of epimysia to form cords Aponeuroses – sheetlike groups of epimysia that indirectly connect muscle to bone, cartilages, or connective tissue coverings together Tendons Anchor muscles to bone Provide durability and conserve space Tough collagenic fibers that can cross bony projections (without tearing) Smooth muscle No striations Under involuntary control Found in walls of hollow organs Used to propel substances through a specific tract Also known as visceral and involuntary muscle Cells are spindle shaped with one nucleus; arranged in sheets or layers Layers are in pairs (one running circularly, the other longitudinally) Contractions are slow and sustained Cardiac muscle Found only in the heart Used to propel blood into blood vessels then to tissues Striated, under involuntary control Cardiac fibers cushioned by soft connective tissue arranged in spiral or figure 8 Cells are branching joined by junctions or intercalated discs Contraction causes the internal chambers to become smaller pushing blood out of the heart into the arteries Contraction occurs at a steady rate unless stimulated by the nervous system Muscle functions 4 important roles Produces movement Maintains posture Stabilizes joints Generates heat Producing movement All movements and manipulations are the result of skeletal muscle activity Allow us to respond to changes in the environment Allow us to express emotions Smooth muscle in vessels and hollow organs helps to force fluids and other substances through internal body channels Maintaining posture Fights gravity to maintain an erect or seated position Stabilizing joints Muscles pull on bones to cause movement but stabilize joints at the same time Tendons reinforce the joints that have poor articulating surfaces (shoulder) Generating heat By-product of muscle activity As ATP is used, 75% of the energy escapes as heat Heat helps maintain body temperature Skeletal muscle is 40% of total body mass, so is most important for heat production Microscopic anatomy of skeletal muscle Muscle cells are multinucleate; nuclei lying just beneath the plasma membrane or SARCOLEMMA Myofibrils push the nuclei aside; alternating bands of LIGHT (I) and DARK (A) bands give the “stripes” or striations Myofibrils are chains of tiny contractile units or sarcomeres aligned end-to-end Myofilaments within the sarcomeres produce the banding pattern Two types of myofilaments Thick filaments (myosin) made of protein and ATPase which provides the energy for muscle contraction Thin filaments (actin) made regulatory proteins and the contractile protein that may allow or prevent myosin from binding to actin Microscopic anatomy Sarcoplasmic reticulum – a specialized form of smooth ER Surrounds all myofibrils like a sleeve Stores calcium and releases calcium “on demand” when the muscle is stimulated (Ca ++ is the “go” signal for contraction) Microscopic anatomy Microscopic anatomy Muscle activity Muscles have special functional abilities Irritability Ability to receive and respond to a stimulus Contractility Ability to shorten (contract) forcibly when an adequate stimulus is received Neuromuscular junction Skeletal muscle cells must be stimulated by nerves One motor neuron may stimulate a few cells or many cells (one neuron and the cells it stimulates is a motor unit) When the axon of the neuron reaches the muscle it branches into axonal terminals, each forms a junction with the sarcolemma of a muscle cell (neuromuscular junctions) Nerve cells and the sarcolemmas do not touch but are separated by a small area called a synaptic cleft that is filled with fluid Action potential Nerve impulses that reach the axonal terminals cause neurotransmitters (acetylcholine or ACh) to be released ACh move across the synaptic cleft and attaches to receptors on the sarcolemma If enough ACh is released, the sarcolemma will become permeable to Na ions which rush into the muscle cell giving the interior a + charge This + charge causes an “upset” in the sarcolemma or an action potential Once started the action potential travels over the whole sarcolemma producing a contraction of the muscle cell Return to rest Returning the cell to a resting state includes two events Diffusion of K out of the cell Operation of the Na - K pump which moves the Na and K ions back to their initial locations (Na is outside the cell and K is inside the cell) Mechanism of contraction Fibers are activated by the nervous system Cross bridges (myosin heads) attach to thin filaments ATP provides energy Cross bridges attach and detach several times to create tension to pull the thin filaments Attachment of myosin to thin filaments requires Ca Action potential stimulates sarcoplasmic reticulum to release Ca to the sarcoplasm As action potential ends, Ca is reabsorbed and muscle cell relaxes ACh is broken down by enzymes Graded responses of contraction “all-or-none” law only applies to muscle cells not to whole muscles Graded responses occur since muscles have thousands of cells Graded contractions are produced by 1. changing the frequency of the stimulation 2. changing the number of cells being stimulated Response to increasingly rapid stimuli Nerve impulses delivered at rapid rate Cells can’t relax between stimuli Effects are “summed” so that contractions get stronger and smoother When contractions are sustained and smooth, the muscle is in fused or complete tetanus Response to stronger stimuli Strength of a contraction depends on the number of motor units stimulated Only a few motor units used, then slight to moderate contraction All motor units used, then very strong contraction Providing energy for muscle contraction Bonds in ATP are broken to release needed energy for contraction Muscles store a limited supply of ATP (4-6 seconds worth) ATP is the ONLY energy source that can be used ATP must be continuously regenerated for contraction to continue Three pathways to get ATP 1. Direct phosphorylation of ADP by creatine phosphate CP only found in muscle CP transfers phosphate to ADP to make ATP CP supplies run out in about 20 seconds 2. Aerobic respiration Produces 95% of ATP used for contraction Occurs in mitochondria, slow, needs lots of oxygen Breakdown of glucose to produce ATP by oxidative phosphorylation 3. Anaerobic glycolysis and lactic acid fermentation Occurs in cytoplasm when O2 and glucose are depleted Converts pyruvate to lactic acid, producing only 5% of ATP 2 ½ times faster than aerobic respiration Muscle fatigue and oxygen debt Muscle fatigue – failure of the muscle to contract even if it is still being stimulated Good blood supply can prevent muscle fatigue from occurring to quickly Oxygen debt – strenuous and prolonged muscle activity prevents the body from delivering adequate oxygen to cells Low oxygen leads to lactic acid build up and depletion of ATP Leads to fatigue Must be paid back (lactic acid is converted back to pyruvate and sent into aerobic respiration when O2 levels return to normal) Types of muscle contraction Isotonic contractions Myofilaments are successful in the sliding movements Muscle shortens, movement occurs Isometric contractions Muscles do NOT shorten Myosin myofilaments are trying to slide, but muscles are facing an immovable object (like lifting 400 lbs) Muscle tone Continuous partial contraction of the muscle at rest Direct result of different motor units being stimulated by the nervous system in a systematic way Keeps the muscle tissue firm, healthy and ready for action Effect of exercise on muscles Lack of use of muscles leads to muscle weakness and wasting (atrophy) Regular exercise increases muscle strength and endurance Two main types of exercise Aerobic activity or endurance resistance Endurance Examples: aerobic classes, biking, jogging Results in stronger, more flexible muscles Greater resistance to fatigue Increased blood supply to muscles More mitochondria (more ATP) and more oxygen in muscle cells Also Improves overall metabolism, digestion, coordination, and strengthens skeleton Improves heart function and circulation Resistance Exercises require little time and little equipment Forcing muscles to contract with as much force as possible Increases muscle size and strength by increasing size of muscle cells Increases connective tissue that surrounds the muscle tissue 5 golden rules of muscle activity 1. all muscles cross at least one joint 2. the bulk of the muscle lies proximal to the joint crossed 3. all muscles have at least two attachments: the origin and the insertion 4. muscles can only pull; they NEVER push 5. during contraction, the muscle insertion moves toward the origin Two attachments of skeletal muscle Origin Attached to the immovable or less movable bone Insertion Attached to the movable bone During contraction, the insertion moves toward the origin Most common movements Flexion Movement in saggital plane Decreases angle of the joint, brings bones close together Typical for hinge joints, common for ball-and-socket joints Extension Opposite of flexion Increases angle of joint, increases distance between bones Greater than 180 degrees is called hyperextension Rotation Movement around longitudinal axis Common for ball-and-socket joints and the atlas vertebrae Most common movements Abduction Moving a limb away from the midline Fanning movement of fingers and toes when spread apart Adduction Opposite of abduction Movement of limbs toward the midline Circumduction Combination of flexion, extension, abduction, and adduction Common in ball-and-socket Proximal end of the limb is stationary, distal end moves in a circle Special movements Dorsiflexion Lifting the foot at the ankle to approach the shin plantar flexion Depressing the foot at the ankle (pointing the toes) Inversion Turning the sole of the foot medially Eversion Turning the sole of the foot laterally Special movements Supination Forearm rotates laterally so the palm faces anteriorly; radius and ulna are parallel Pronation Forearm rotates medially and palm faces posteriorly; radius crosses the ulna Opposition Moving the thumb to touch the tips of other fingers on the same hand Types of muscles Prime mover Has the major responsibility for causing a movement Antagonists Muscles that reverse or oppose a movement Synergists Help prime movers Produce the same movement or reduce undesirable movements Fixators Hold a bone still or stabilize the origin of a prime mover so all tension moves the insertion bone Naming skeletal muscles Direction of muscle fibers Some named in reference to an imaginary line Names including rectus (straight) are fibers that run parallel to the imaginary line; ex: rectus femoris is the thigh muscle that is parallel to the femur Obliques are muscles that run in a slanted direction to the imaginary line Naming skeletal muscles Relative size of the muscle Maximus (largest), minimus (smallest), longus (long) Gluteus maximus is the largest muscle in the gluteus group Location of the muscle Some muscles named for the bone they are associated with Temporalis overlies the temporal bone of the skull Naming skeletal muscles Number of origins Biceps, triceps, quadriceps; prefixes indicate the number of origins Location of the origin and insertion Sometimes muscles are named for attachment sites Sternocleidomastoid has origins on the sternum and clavicle but inserts on the mastoid process of the temporal bone Naming skeletal muscles Shape of the muscle Distinctive shapes can identify muscles Deltoid means “triangular” Action of the muscle Some muscles named for their actions Flexor, extensor, adductor Head and neck muscles 2 main groups Facial Inserted into soft tissues like muscles or skin Pulling on skin allows us to show expression Chewing Begins the mechanical breakdown of food Facial muscles Frontalis Covers the frontal bone from cranial aponeurosis down to the eyebrows where it inserts (can raise eyebrows and wrinkle forehead) Orbicularis oculi Fibers run in circles around the eyes closing eyes, squinting, blinking and winking Orbicularis oris Circular muscle of the lips Closes mouth, protrudes lips (kissing muscle) Facial muscles Buccinator Fleshy muscle that runs horizontally across the cheek Inserts into the orbicularis oris Flattens cheek, holds food between teeth for chewing Zygomaticus Extends from corner of mouth to cheek bone Smiling muscle; raises corner of the mouth Chewing muscles Masseter Covers angle of lower jaw Runs from zygomatic process to the mandible Closes the jaw Temporalis Fan-shaped; overlies the temporal bone Inserts into the mandible Is a synergist of the masseter in closing the jaw Neck muscles Used to move head and shoulders Platysma Sheetlike; covering the anterolateral neck Originates from chest and inserts around the mouth Causes frowning Sternocleidomastoid Paired muscles on each side of the neck Two origins: sternum and clavicle Inserts into the mastoid process of the temporal bone Flex the neck (bowing); rotates the head Trunk muscles These include Those muscles that move the backbone Anterior thorax muscles (move head, ribs, arms) Muscles of the abdominal wall Anterior muscles Pectoralis major Large, fan-shaped, covers upper part of chest Origin is form shoulder girdle and first 6 ribs Inserts on proximal end of humerus Helps to adduct and flex arms Intercostal muscles Deep muscles between rib bones External intercostals aid in breathing (raise rib cage) Internal intercostals depress rib cage Anterior muscles Muscles of abdominal girdle Reinforce body trunk Fibers of each muscle run in a different direction Used to contain and protect organs in the abdomen Muscles of abdominal girdle (anterior muscles) Rectus abdominus Paired muscles; Most superficial in abdomen Run from pubis to rib cage; enclosed in an aponeurosis Flex the backbone; compress the abdomen during defecation and childbirth External oblique Paired, superficial muscles Make up the lateral walls Fibers run downward and medially from last 8 ribs Insert into ilium Flex backbone; rotate the trunk and bend the trunk laterally Muscles of abdominal girdle (anterior muscles) Internal oblique Paired muscles but deep to external obliques Fibers run perpendicular to external obliques Come from iliac crest and insert into last 3 ribs Same functions as external obliques Transversus abdominis Deepest muscle of the abdominal wall Fibers run horizontally across the abdomen Arises at lower ribs and iliac crest and inserts into pubis Compresses abdominal contents Posterior Muscles Trapezius Most superficial of posterior neck and upper trunk Runs from occipital bone, down the backbone, to the end of thoracic vertebrae Inserts on scapular spine and clavicle Extend the head (antagonists of sternocleidomastoid), elevate, depress, adduct, and stabilize the scapula Latissiumus Dorsi Paired muscles covering lower back Originates in lower spine and ilium; inserts into proximal end of humerus Extends, adducts the humerus Posterior Muscles Erector spinae Prime mover of back extension Paired muscles, deep, with 3 muscle columns (longissimus, iliocostalis, spinalis) Provide resistance to control action of bending at waist Deltoid Rounded portion of shoulders Origin crosses the shoulder girdle from spine of scapula to clavicle Inserts into proximal humerus Prime movers of arm abduction Upper limb muscles 3 groups Muscles from shoulder girdle that cross the shoulder and insert at the humerus Already described (pectoralis major, latissimus dorsi, and deltoid) Move the arm Muscles that enclose the humerus and insert on the forearm bones Muscles that insert on the hand bones and cause movement of the hand Anterior arm muscles Cause elbow flexion Biceps brachii Bulges when elbow is flexed Originates at the shoulder girdle; inserts on the radial tuberosity Prime mover for elbow flexion and supinates the arm Brachialis Deep to biceps Prime mover for elbow flexion Anterior arm muscles Brachioradialis Weak muscle Arises on the humerus and inserts on the distal forearm Triceps brachii Fleshes the posterior humerus Arise from shoulder girdle and proximal humerus; inserts into the olecranon process of the ulna Prime mover of elbow extension Antagonist of biceps brachii Muscles of the lower limb Largest, strongest muscles of the body Specialized for walking and balancing Many span 2 joints and cause movement at both Thigh muscles hold the body upright and fight gravity Thigh muscles can act on the lower leg as well as the hip Muscles at the hip Gluteus maximus Superficial; forms flesh of buttock Hip extensor (climbing, jumping) Originates from sacrum and iliac bones and inserts on gluteal tuberosity of the femur Gluteus medius From ileum to femur; beneath the maximus Hip abductor needed to steadying the pelvis during walking Important site for IM injections (not the maximus since it overlies the sciatic nerve) Muscles at the hip Iliopsoas 2 fused muscles; from iliac bone to lower vertebrae Deep into the pelvis and inserts on lesser trochanter of femur Prime mover of hip flexion Keeps body from falling backwards when standing Adductor muscles Muscle mass at the medial side of each thigh Press thighs together (adduction) Origin on pelvis and insert on proximal aspect of femur Muscles at the knee Hamstring Forms posterior thigh 3 muscles (biceps femoris, semimembranosus, semitendinosus) Originate on ischial tuberosity insert on both sides of the proximal tibia Sartorius Thin, straplike Most superficial of thigh muscles Runs obliquely across the thigh from anterior iliac crest to medial side of tibia Weak flexor muscle Muscles of the knee Quadriceps group Four muscles (rectus femoris, and 3 vastus muscles) Flesh the anterior thigh Vastus originate from femur; rectus femoris originates on pelvis All insert into tibial tuberosity by patellar ligament Extends the knee Rectus femoris can also flex the hip Lateral vastus and rectus femoris common IM injection sites, particularly in small children Muscles of ankle and foot Tibialis anterior Superficial on anterior leg Arises from upper tibia; parallels the anterior crest Inserts into tarsals with long tendon Dorsiflex and invert the foot Extensor digitorum longus Lateral to tibialis anterior Arises from lateral tibial condyle and proximal radius Inserts into phalanges of toes 2-5 Prime mover of toe externsion and dorsiflexor of foot Muscles of ankle and foot Fibularis muscles 3 (longus, brevis, tertius) Lateral part of leg Arise from fibula and insert into metatarsal bones Plantar flexes and everts the foot Gastrocnemium Two-bellied muscle forming the curved calf of posterior leg Arises from both sides of the distal femur Inserts through the calcaneal tendon (Achilles) into heel Prime mover for plantar flexion Muscles of ankle and foot Soleus Deep to gastrocnemius Arises on tibia Does NOT affect knee movement Strong plantar flexor of foot Common Injuries Strains Contusions Cramps Muscle soreness Tendinitis and tendinosis Rotational injuries Overuse Shin splints Whiplash Strains Muscle is stretched beyond normal limits Often occurs during running, accelerations, and/or changing directions Classified as Grades I, II, III Grade I – mild; tightness in the muscle the day after injury Grade II – moderate strain; partial tear in the muscle; associated weakness and temporary loss of function Grade III – severe strain; torn muscle; loss of function; internal bleeding; swelling Contusions Bruise or bleeding within the muscle Results from impact Can proceed to myositis ossificans Formation of calcium mass in the muscle over 3-4 weeks Should begin to dissolve in 6-7 weeks May leave a bony lesion
