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CHAPTER 6 The Muscular System Chapter 6: The Muscular System skeletal muscle fibers endomysium perimysium fascicle epimysium tendons ligaments aponeuroses smooth muscle cardiac muscle intercalated discs sarcolemma myofibrils light (I) bands dark (A) bands Z disc H zone sarcomeres unfused (incomplete) tetanus myofilaments aerobic respiration myosin (thick) filaments glycolysis myosin anaerobic glycolysis actin isotonic contractions actin (thin) filaments isometric contractions myosin heads (cross bridges) origin bare zone insertion sarcoplasmic reticulum prime mover motor unit antagonist axon synergist neuromuscular junctions fixators synaptic cleft neurotransmitter acetylcholine (ACh) action potential fused (complete) tetanus SECTION 1: OVERVIEW OF MUSCLE AND THE MUSCULAR SYSTEM Primary Functions of the Muscular System 1.) Producing movement 2.) Maintaining posture 3.) Stabilizing joints and providing structure and support for organs and tissues 4.) Generating heat Overview of Muscle Tissues The essential function of muscle is contraction or shortening – a unique characteristic that sets it apart from other body tissue. All muscles cells are elongated and called muscle fibers. The ability of muscles to shorten, or contract, depends on two types of myofilaments -- filaments composing the myofibrils, which are the contractile organelles in the cytoplasm of muscle cells. There are two types of myofilaments: actin and myosin. Myo or mys means “muscle,” and sarco means “flesh” Muscle Tissue: 3 Types 1. 2. 3. Muscle tissue enables the movement of body structures. There are three types of muscle tissue: smooth muscle tissue, cardiac muscle tissue, and skeletal muscle tissue. skeletal muscle tissue smooth muscle tissue cardiac muscle tissue intercalated discs Skeletal muscle is an example of voluntary muscle, and appears banded or striated. Skeletal muscle is sometimes called striated muscle, and tires after short periods of activity. Smooth muscles are involuntary muscles that have no striations. Smooth muscles propel substances along a definite tract, or pathway. Cardiac muscle is involuntary muscle whose cells are striated. Cardiac muscle is different from skeletal muscle in that it does not get tired. SKELETAL MUSCLE: BRIEF SUMMARY voluntary muscle banded or striated (sometimes called striated muscle) cigar-shaped (cylindrical), long, multinucleate cells, and the largest of the muscle fiber types found in skeletal muscles that attach to the body’s skeleton functions to contract and relax to move skeletal bones tires easily and must rest after short periods of activity skeletal muscle tissue SMOOTH MUSCLE: BRIEF SUMMARY involuntary muscle no striations spindle-shaped and have a single nucleus (uninucleate cells), arranged in sheets or layers found mainly in the walls of hollow visceral organs – stomach, urinary bladder, interior walls of blood vessels, and respiratory passages propels substances along a definite tract, or pathway Smooth muscle contraction is slow and sustained. smooth muscle tissue CARDIAC MUSCLE: BRIEF SUMMARY involuntary muscle found only in the heart striated cardiac fibers cushioned by soft connective tissue arranged in spiral or figure 8-shaped bundles branching chains of uninucleate cells joined by junctions called intercalated discs contracts at a fairly steady rate set by the heart’s pacemaker, but can be stimulated by the nervous system to shift to “high gear” for short periods does not get tired cardiac muscle tissue intercalated discs Muscle Types: Skeletal Muscle Skeletal muscle is striated, and associated with voluntary movement. It also provides structure and support for organs and tissues. Anatomy of Skeletal Muscles The endomysium is a delicate, thin connective tissue sheath that surrounds each muscle cell. The endomysium surrounds the individual skeletal muscle cells and loosely interconnects adjacent muscle fibers. The endomysium is flexible and contains capillary networks and nerve fibers. Figure 6.1 page 164 epimysium perimysium muscle fascicle tendon skeletal muscle endomysium skeletal muscle fiber (cell) Anatomy of Skeletal Muscles The perimysium is the coarser, fibrous connective tissue that envelops several sheathed bundles of muscle fibers, and divides the skeletal muscle into a series of compartments. A fascicle is a bundle of muscle fibers bound together by connective tissue. Figure 6.1 page 164 epimysium perimysium muscle fascicle tendon skeletal muscle endomysium skeletal muscle fiber (cell) Anatomy of Skeletal Muscles The perimysium is made of connective tissue fibers and divides the skeletal muscle into a series of compartments. The perimysium contains blood vessels and nerves that maintain blood flow and innervate the muscle fibers within the fascicle. Each compartment contains a bundle of muscle fibers called a fascicle. Many fascicles are bound together by an even tougher “overcoat” of connective tissue called an epimysium, which covers the entire muscle. Figure 6.1 page 164 epimysium perimysium muscle fascicle tendon skeletal muscle endomysium skeletal muscle fiber (cell) Your turn! Figure 6.1 page 164 epimysium 5. perimysium 3. muscle fascicle tendon 7. 4. 6. skeletal muscle endomysium 2. skeletal muscle fiber (cell) 1. The epimysium separates the muscle from surrounding tissues and organs. The epimysium is connected to the deep fascia, a dense connective tissue layer. The epimysia blend into the strong, cordlike tendons, or into fibrous membranous sheets called aponeuroses which attach muscles indirectly to bones, cartilages, or connective tissue coverings of each other. Tendons serve to anchor muscles to bones, but also provide durability, and conserve space. Tendons are mostly tough collagenic fibers, so they can cross rough bony projections without tearing. Because of their relatively small size, more tendons than fleshy muscles can pass over a joint. Muscles vary considerably in the way their fibers a arranged. Many are spindle-shaped, but in others the fibers are arranged in a fan shape or in a circle. Looking back at the anatomy of a muscle cell, and placing the structures in order from largest to smallest gives this arrangement: a fascicle – a bundle of muscle fibers made up of individual muscle fibers that are composed of myofibrils – organelles found in the cytoplasm of muscle cells that are made up of tiny units called sarcomeres -- composed of chains of tiny contractile units called myofilaments – threadlike protein filaments of actin or myosin Muscle Types: Smooth Muscle Smooth muscle is nonstriated, and acts in a number of involuntary processes in the body. Smooth muscles is found mainly in the walls of hollow visceral organs such as the stomach, urinary bladder, and respiratory passages. Smooth muscles helps to propel substances along a definite tract, or pathway, within the body. Muscle Types: Smooth Muscle Smooth muscle cells are spindle-shaped and have a single nucleus (uninucleate). Smooth muscle cells are arranged in sheets or layers. Most often, there are two layers, one running circularly and the other longitudinally. As the two layers alternately contract, and relax, they change the size and shape of the organ. Smooth muscle contraction is slow and sustained. Peristalsis is the wavelike contractions seen in tube-like organs that propels substances along a tract. Muscle Types: Smooth Muscle Some of the involuntary processes of smooth muscle include: allows the expansion and contraction of arteries and veins lines the bladder and reproductive tracts lines the entire gastrointestinal tract Did you know... Tiny smooth muscle fibers in the skin called arrector pili are responsible for “goose bumps.” Muscle Types: Cardiac Muscle Cardiac muscle (heart muscle) is striated but functions involuntarily. It is solely responsible for propelling blood throughout the body. Cardiac muscle fibers are branching cells joined by special junctions called intercalated discs. The heart’s pacemaker is tissue that sets and maintains the heart’s steady rate. Unlike other muscle types, cardiac muscle does not tire. Cardiac fibers are cushioned by small amounts of soft connective tissue and arranged in spiral or figure 8-shaped bundles. Cardiac muscle fibers are branching cells joined by special anchoring structures called intercalated discs that contain gap junctions. These two structural features and the spiral arrangement of the muscle bundles in the heart allow heart activity to be closely coordinated. smooth muscle cardiac muscle is involuntary muscle is involuntary muscle is involuntary muscle is voluntary muscle is voluntary muscle is voluntary muscle has striations has striations skeletal muscle has striations does not have striations does not have striations does not have striations TYPE OF MUSCLES voluntary muscles skeletal muscles involuntary muscles cardiac muscle smooth muscle Microscopic Anatomy of Skeletal Muscle Microscopic Anatomy of Skeletal Muscle The sarcolemma is the cell membrane of a muscle cell that is found in skeletal, cardiac, and smooth muscle. A sarcolemma consists of a true cell membrane, called the plasma membrane, and an outer coat made up of a thin layer of polysaccharide material that contains numerous thin collagen fibrils. At each end of the muscle fiber, this surface layer of the sarcolemma fuses with a tendon fiber, and the tendon fibers in turn collect into bundles to form the muscle tendons that then insert into bones. The membrane is configured to receive and conduct stimuli. Microscopic Anatomy of Skeletal Muscle Myofibrils are small cylindrical, contractile organelles found in the cytoplasm of muscle cells. Alternating light (I) bands and dark (A) bands along the length of the perfectly aligned myofibrils give the muscle cell as a whole its striped appearance. The light I band has a midline interruption, a darker area called the Z disc. The dark A band has a lighter central area called the H zone. A myofibril consists of approximately 10,000 sarcomeres end to end. A sarcomere is the functional component within the myofibril that is composed of chains of tiny contractile units called myofilaments. Sarcomeres are aligned end-to-end along the length of the myofibrils. A sarcomere lies in the area between the two Z lines. Within the sarcomere, there are two types of threadlike protein myofilaments within each of the sarcomeres: actin and myosin. The protein filaments actin and myosin allow the cell to expand and contract in a three-step process. Myosin Filaments Larger thick filaments, or myosin filaments, are the thick filaments made mostly of bundled molecules of the protein myosin. Myosin contains ATPase enzymes, which split ATP to generate the power for muscle contraction. The midpoints of the thick filaments are smooth, but their ends are studded with small projections called myosin heads, which are sometimes called cross bridges because they link the thick and thin filaments together during contraction. muscle fiber sarcomere Z-line myofibril myosin dark, thick filaments actin light, thin filaments Thin myofilament actin subunit Myosin molecule of thick myofilament myosin head Actin Filaments Actin filaments are the thin filaments composed of the contractile protein actin. Actin filaments are anchored to the Z disc. The light I band includes part of two adjacent sarcomeres and contains only the thin filaments. Although the thin filaments overlap the ends of the thick filaments, they do not extend into the middle of a relaxed sarcomere. Thus, the central region (the H zone, which lacks actin filaments and looks a bit lighter), is sometimes called the bare zone. muscle fiber sarcomere Z-line myofibril myosin dark, thick filaments actin light, thin filaments Thin myofilament actin subunit Myosin molecule of thick myofilament myosin head SECTION 2: SKELETAL MUSCLE ACTIVITY Skeletal muscles must be stimulated by nerve impulses to contract. A motor neuron, or nerve cell, may stimulate a few muscles cells or hundreds of them depending on the particular muscle and the work it does. A motor unit is composed of one neuron, or nerve cell, and all the skeletal muscle cells it stimulates. Nerve Stimulus and Action Potential When a long threadlike extension of the neuron, called the nerve fiber or axon, reaches the muscles, it branches into a number of axonal terminals, each of which forms a junction with the sarcolemma of a different muscles cell. These junctions are called neuromuscular junctions. A neuromuscular junction is the region where a motor neuron comes into close contact (but does not touch) a skeletal muscle cell. An axon is a neuron process that carries impulses away from the nerve cell body. Nerve Stimulus and Action Potential The gap between the motor neuron and a skeletal muscle cell is known as a synaptic cleft. The synaptic cleft is filled with interstitial tissue. A neurotransmitter is a chemical released by neurons that stimulates or inhibits cells. Neurotransmitters are stored within motor neuron endings. Nerve Stimulus and Action Potential Acetylcholine, or ACh, is the specific neurotransmitter that stimulates skeletal muscle cells. ACh diffuses across the synaptic cleft and attaches to receptors that are part of the sarcolemma. If enough acetylcholine is released, the sarcolemma becomes temporarily permeable to sodium ions (Na+), which rush into the muscle cell. Nerve Stimulus and Action Potential The sudden inward rush of sodium ions gives the cell interior and excess of positive ions, which upsets and changes the electrical conditions of the sarcolemma. This “upset” generates an electrical current called an action potential. An action potential is an electrical event occurring when a stimulus of sufficient intensity is applied to a neuron or muscle cell, allowing sodium ions to move into the cell and reverse the polarity. Nerve Stimulus and Action Potential Once begun, the action potential is unstoppable and travels over the entire surface of the sarcolemma, conducting the electrical impulse from one end of the cell to the other. The result is contraction of the muscle cell. The events that return the cell to its resting state include the diffusion of potassium ions (K+) out of the cell, and the operation of the sodium-potassium pump that moves sodium and potassium back to their original positions. Another very important muscle fiber organelle is the sarcoplasmic reticulum. The sarcoplasmic reticulum is a specialized and elaborate network of smooth endoplasmic reticulum made up of interconnecting tubules and sacs that surround each and every myofibril. The main function of the sarcoplasmic reticulum is to store calcium and to release it “on demand” when the muscle fiber is stimulated to contract. Calcium provides the final “go” signal for muscle contraction. The sodium-potassium pump carries sodium ions out of and potassium ions into the cell and is absolutely necessary for normal transmission of impulses by nerve cells. ATP provides the energy for a “pump” protein to move 3 sodium ions out of the cell and 2 potassium ions into the cell. Both ions are moved against their concentration gradient. 3 Steps of Muscle Contraction 1) Before the muscle is stimulated, actin and myosin filaments partially overlap one another. 2) A nerve cell releases a signal which causes the actin and myosin filaments to “slide” along one another and overlap even more. 3) This contracts the myofibril and subsequently the entire muscle cell. When the nervous signal changes, the filaments relax and return to their original state. The Sliding Filament Theory While the action potential is occurring, acetylcholine, which begins the process, is broken down by enzymes present on the sarcolemma. For this reason, a single nerve impulse produces only one contraction. This prevents continued contraction of the muscle cell in the absence of additional nerve impulses. The muscle cell relaxes until stimulated by the next round of acetylcholine release. Sarcomere Graded Responses In skeletal muscles, the “all-or-none” law of muscle physiology applies to the muscle cell, not to the whole muscle. The “all-or-none” law states that a muscle cell will contract to its fullest extent when it is stimulated adequately – it never partially contract. Skeletal muscles are organs that consist of thousands of muscle cells, and they react to stimuli with graded responses, or different degrees of shortening. Graded Responses In general, graded muscle contractions can be produced two ways: (1) by changing the frequency of muscle stimulation, and (2) by changing the number of muscle cells being stimulated. Muscle twitches are brief, jerky contractions that sometimes occur as a result of certain nervous system problems, and is not the way the nervous system normally operates. When a muscle is stimulated so rapidly that no evidence of relaxation is seen and the contractions are completely smooth and sustained, the muscle is said to be in fused or complete tetanus, or in tetanic contraction. Until this point is reached, the muscle is said to be exhibiting unfused or incomplete tetanus. Although tetanus produces stronger muscle contractions, its primary role is to produce smooth and prolonged muscle contractions. How forcefully a muscle contracts depends on how many cells are stimulated. When only a few muscle cells are stimulated, the contraction of the muscle as a whole will be slight. In the strongest contractions, when all the motor units are active and all the muscle cells are being stimulated, the muscle contraction is strong as it can get. As a muscle contacts, the bonds of ATP molecules are hydrolyzed to release the needed energy. Muscles store only 4 to 6 seconds’ worth of ATP – just enough to get you going. Since ATP is the only energy source that can be used directly to power muscle activity, ATP must be regenerated continuously if contraction is to continue. There are three pathways for ATP regeneration: (1) direct phosphorylation of ADP by creatine phosphate (2) aerobic respiration (oxidative phosphorylation) (3) anaerobic glycolysis and lactic acid formation (1) direct phosphorylation of ADP by creatine phosphate The unique high-energy molecule creatine phosphate is found in muscle fibers, but not other cell types. Although muscle cells store perhaps five times as much CP as ATP, the CP supplies are also soon exhausted in about 20 seconds. (2) aerobic respiration Aerobic respiration occurs in the mitochondria and involves a series of metabolic pathways that use oxygen. These pathways are collectively referred to as oxidative phosphorylation. Although aerobic respiration provides a rich ATP harvest – 36 ATP per 1 glucose molecule – it is fairly slow and requires continuous delivery of oxygen and nutrient fuels to the muscles to keep it going. (3) Anaerobic glycolysis and lactic acid formation The initial steps of glucose breakdown occur via a pathway called glycolysis, which does not use oxygen and is an anaerobic part off the metabolic pathway. During glycolysis, which occurs in the cytosol, glucose is broken down to pyruvic acid, and small amounts of energy are captured in ATP bonds – 2 ATP per 1 glucose molecule. If aerobic mechanisms cannot keep up with the demands for ATP, then the pyruvic acid generated during glycolysis is converted to lactic acid, and the overall process is referred to as anaerobic glycolysis. Anaerobic glycolysis produces only about 5 percent as much ATP from each glucose molecule as aerobic respiration. However, it is some 2 ½ times faster, and it can provide most of the ATP needed for 30 to 60 seconds of strenuous muscle activity. The main shortcomings of anaerobic glycolysis are that it uses huge amounts of glucose for a small ATP harvest, and accumulating lactic acid promotes muscle fatigue and muscle soreness, commonly called “muscle burn.” SECTION 3: Types of Body Movements The Muscular System: Tendons and Aponeuroses tendon Muscles move body parts because they are attached to bones by tendons or aponeuroses. Tendons are strips of dense, cordlike connective tissue attaching a muscle to a bone. aponeuroses An aponeuroses is a membranous sheet connecting a muscle and the part it moves. The Muscular System: Tendons Besides simply acting to anchor muscles, tendons also perform the important function of providing durability and conserving space. Tendons are mostly tough collagenic fibers, so they can cross rough bony projections which would tear the more delicate muscle tissues. Because of their relatively small size, more tendons than fleshy muscles can pass over a joint. All muscles are attached to bone, or to other connective tissue, at no less than two points: the point of origin and the point of insertion. The point of origin is the attachment of a muscle to the immovable or less movable bone that remains relatively fixed during muscular contraction. The point of insertion is the attachment to the movable bone. When the muscle contracts, the insertion moves toward the origin. Skeletal Muscle: Origin and Insertion Points The place where muscle attaches to a stationary bone is called the origin. The place where the same muscle attaches to the movable bone and “pulls” is called the insertion. Insertion Origin The Muscular System: Ligaments A ligament is a cord of tough fibrous tissue that connects bone to bone. Challenge Question How are tendons different from ligaments? Skeletal Muscle: Extensor and Flexor Skeletal muscle can be characterized as either an extensor or a flexor. An extensor causes a joint to straighten or extend. A flexor causes a joint to bend or flex. Skeletal Muscle: Prime Movers and Antagonists A muscle that has the major responsibility for causing a particular movement is called a prime mover. A muscle that oppose or reverse a movement is an antagonist. In flexing the forearm at the elbow, the biceps brachii acts as the prime mover, and the triceps brachii acts as the antagonist. When extending the forearm back to a straight position, the triceps brachii becomes the prime mover, and the biceps brachii becomes the antagonist. When a prime mover is active, its antagonist is stretched and relaxed. Skeletal Muscle: Synergists Synergists help prime movers by producing the same movement or by reducing undesirable movements. When a muscle crosses two or more joints, its contraction will cause movement in all the joints crossed unless synergists are there to stabilize them. The finger-flexor muscles cross both the wrist and the finger joints. You can make a fist without bending your wrist because synergist muscles stabilize the wrist joints and allow the prime mover to act on the finger joints. Skeletal Muscle: Fixators Fixators are specialized synergists that hold a bone still or stabilize the origin of a prime mover so all the tension can be used to move the insertion bone. The postural muscles that stabilize the vertebral column are fixators, as are the muscles that anchor the scapula to the thorax. Types of Body Movements Pages 177-178 show examples of each type of body movement described below. Flexion is a movement, generally in the sagittal plane, that decreases the angle of the joint and brings two bones closer together. Flexion is typical of hinge joints, but is also common at ball-andsocket joints. Extension is the opposite of flexion and is a movement that increases the angle, or distance, between two bones of parts of the body. If extension is greater than 1800, it is known as hypertension. Types of Body Movements Rotation is movement of a bone around its longitudinal axis and is a common movement of ball-and-socket joints. An example of rotation is the movement of the atlas around the dens of the axis as in shaking your head “no.” Abduction is moving a limb away (generally on the frontal plane) from the midline, or median plane, of the body. The term also applies to the fanning movement of the fingers or toes when they are spread apart. Adduction is the opposite of abduction and is the movement of a limb toward the body midline. Types of Body Movements Circumduction is a combination of flexion, extension, abduction, and adduction commonly seen in ball-and-socket joints such as the shoulder. The proximal end of the limb is stationary, and its distal end moves in a circle. The limb as a whole outlines a cone. Dorsiflexion and plantar flexion is the up and down movements of the foot at the ankle. Dorsiflexion is lifting the foot so that its superior surface approaches the shin such as standing on your heels. Plantar flexion is depressing the foot as in pointing the toes. Inversion is the movement of the sole of the foot medially. Eversion is the movement of the sole of the foot laterally. Supination (turning backward) and pronation (turning forward) refers to movements of the radius around the ulna. Supination occurs when the forearm rotates laterally so that the palm faces anteriorly, and the radius and ulna are parallel. Pronation occurs when the forearm rotates medially so that the palm faces posteriorly, and brings the radius across the ulna so that the two bones form and X. Opposition is the action by which the thumb is moved to touch the tips of the other fingers on the same hand. Opposition is the unique action that makes the human hand such a fine tool for grasping and manipulating things. Supine refers to a body lying with the face upward. Prone refers to a body lying with the face down. Skeletal Muscles You Need to Know! Skeletal Muscles You Need to Know! Skeletal Muscles You Need to Know! Can you think of different ways in which the muscular system works with other body systems to maintain homeostasis? Aerobic vs. Anaerobic Exercise Aerobic respiration occurs in the mitochondria and involves a series of metabolic pathways that use oxygen. These pathways are collectively referred to as oxidation phosphorylation. During aerobic respiration, glucose is broken down completely to carbon dioxide and water, and some of the energy released as the bonds are broken is captured in the bonds of the ATP molecules. Aerobic respiration produces 36 ATP molecules per 1 glucose molecule, but requires a continuous delivery of oxygen to keep it going. Aerobic vs. Anaerobic Exercise Anaerobic glycolysis is the breakdown of glucose via a pathway called glycolysis which does not use oxygen. Anaerobic glycolysis occurs in the cytosol where glucose is broken down into pyruvic acid, and small amounts of energy are captured in ATP bonds (2 ATP per 1 glucose molecule). As long as enough oxygen is present, the pyruvic acid then enters the oxygen-requiring aerobic pathways that occur within the mitochondria to produce more ATP. When the muscle activity is intense, or oxygen and glucose delivery is temporarily inadequate to met the needs of the working muscles, the sluggish aerobic mechanism cannot keep up with the demands for ATP. Under these conditions, the pyruvic acid generated during glycolysis is converted to lactic acid, and the overall process is referred to as anaerobic glycolysis. Lactic acid is the product of anaerobic metabolism, especially in muscle. Anaerobic glycolysis produces only about 5 percent as much ATP from each glucose molecule as aerobic respiration. However, it is 2 ½ times faster, and it can provide most of the ATP needed for 30 to 60 seconds of strenuous muscles activity. The main shortcomings of aerobic glycolysis are that it uses huge amounts of glucose for a small ATP harvest, and accumulating lactic acid promotes muscle fatigue and muscle soreness. Types of Muscle Contractions The event that is common to all muscle contractions is that tension develops in the muscle as the actin and myosin myofilaments interact and the myosin cross bridges attempt to slide the actin-containing filaments past them within the muscles fibers. Isotonic contractions occur when the myofilaments are successful in their sliding movements and the muscle shortens and movement occurs. Isotonic means “uniform tension” or “of the same tone.” Types of Muscle Contractions Isometric movements occur when the myosin myofilaments are trying to slide, but the muscle is pitted against some more or less immovable object, and the tension keeps increasing. Isometric means “of the same length.” Muscle tone is the state of continuous partial contraction that is the result of different motor units, which are scattered through the muscle, being stimulated by the nervous system in a systematic way. Naming Skeletal Muscles DIRECTION OF MUSCLE FIBERS: Some muscles are named in reference to some imaginary line, usually the midline of the body or the long axis of a limb bone. rectus – runs straight or parallel to imaginary line oblique – runs slanted to imaginary line RELATIVE SIZE OF THE MUSCLE: maximus – largest minimus – smallest longus – long Naming Skeletal Muscles LOCATION OF THE MUSCLE: Some muscles are named for the bone with which they are associated. temporalis – overlaps the temporal bones of the skull frontalis – overlaps the frontal bones of the skull NUMBER OF ORIGINS: biceps – two origins triceps – three origins quadriceps – four origins Naming Skeletal Muscles LOCATION OF THE MUSCLE ORIGIN AND INSERTION: Some muscles are named for their attachment sites. sternocleidomastoid – origin on the sternum (sterno) and clavicle (cleido) and inserts on the mastoid process of the temporal bone zygomaticus – extends from the corner of the mouth to the cheekbone (zygomatic bone) SHAPE OF THE MUSCLE: Some muscles have a distinct shape that helps to identify them. deltoid – triangular (deltoid means triangular) Naming Skeletal Muscles ACTION OF THE MUSCLE: Some muscles are named for their actions. flexor – decreases the angle of the joint to bring bones closer together as in a hinge joint extensor - extends by increasing the angle, or distance, between two bones adductor – moves the limb toward the body midline abductor – moves the limb away (generally on the frontal plane) from the midline, or median plane of the body Gross Anatomy of Skeletal Muscles MUSCLES OF THE HEAD AND NECK frontalis cranial aponeurosis orbicularis oculi temporalis SEE PAGE 182. Muscles of the Head and Neck zygomaticus occipitalis buccinator masseter sternocleidomastoid orbicularis oris platysma trapezius Muscles of the Head and Neck See locations of these muscles on pages 181182. FACIAL MUSCLES: frontalis: allows the eyebrows to be raised and the forehead to be wrinkled occipitalis: connects to the frontalis and moves the scalp orbicularis oculi: allows the eyes to close, squint, blink, and wink Muscles of the Head and Neck orbicularis oris: closes the mouth and protrudes the lips as in kissing buccinator: flattens the cheek as in whistling, and is a chewing or sucking muscle because it compresses the cheek to hold the food between the teeth during chewing zygomaticus: raises the corners of the mouth upward as in smiling Muscles of the Head and Neck See locations of these muscles on pages 181-182. NECK MUSCLES: masseter: closes the jaw by elevating the mandible temporalis: acts as a synergist of the masseter in closing the jaw platysma: pulls the corners of the mouth inferiorly, producing a downward sag of the mouth sternocleidomastoid: paired muscles found on each side of the neck-- if one of these muscles contracts, the head is rotated toward the opposite side frontalis cranial aponeurosis orbicularis oculi temporalis SEE PAGE 182. Muscles of the Head and Neck zygomaticus occipitalis buccinator masseter sternocleidomastoid orbicularis oris platysma trapezius MUSCLES OF THE ANTERIOR TRUNK ANTERIOR TRUNK MUSCLES See page 183. aponeurosis Anterior Trunk Muscles See locations of these muscles on pages 183-184. pectoralis major: forms the anterior wall of the axilla and acts to adduct and flex the arm external intercostal muscles: help to raise the rib cage for breathing air in See Figure 6.20 on page 190. Anterior Trunk Muscles internal intercostal muscles: lie deep to the external intercostal muscles and depress the rib cage, helping to move air out of the lungs when exhaling forcibly See Figure 6.20 on page 190 – these muscles lie deep to the external intercostal muscles. rectus abdominis: main function is to flex the vertebral column Anterior Trunk Muscles See locations of these muscles on pages 183-184. external oblique: flex the vertebral column, but also rotate the trunk and bend it laterally internal oblique: flex the vertebral column, but also rotate the trunk and bend it laterally transversus abdominis: compresses the abdominal contents Anterior Trunk Muscles linea alba: a fibrous structure that runs down the midline of the abdomen. The name means white line and the linea alba is composed mostly of collagen connective tissue. The linea alba is formed by the fusion of the aponeuroses of the abdominal muscles, and it separates the left and right rectus abdominis muscles. In muscular individuals, its presence can be seen on the skin, forming the depression between the left and right halves of a "six pack.“ ANTERIOR TRUNK MUSCLES See page 183. aponeurosis MUSCLES OF POSTERIOR UPPER BACK POSTERIOR TRUNK MUSCLES See page 184. Posterior Trunk Muscles trapezius: diamond-shaped superficial muscles of the posterior neck and trunk that help to extend the head. The trapezius muscles are the antagonists of the sternocleidomastoids and can elevate, depress, adduct, and stabilize the scapula. latissimus dorsi: extends and adducts the humerus Posterior Trunk Muscles erector spinae: a group of muscles that is a prime mover of back extension. The erector spinae acts as powerful back extensors (erectors) and also provide resistance that helps control the action of bending over at the waist. The erector spinae consists of three muscle columns (longissimus, iliocostalis, and spinalis) that collectively span the entire length of the vertebral column. deltoid: prime movers of arm abduction Posterior Trunk Muscles rhomboideus minor: a small skeletal muscle on the back that connects the scapula with the vertebrae of the spinal column rhomboideus major: a skeletal muscle on the back that connects the scapula with the vertebrae of the spinal column. It acts together with the rhomboid minor to keep the scapula pressed against the thoracic wall and to retract the scapula toward the vertebral column. levator scapulae: situated at the back and side of the neck, its main function is to lift the scapula rhomboideus minor rhomboideus major levator scapulae POSTERIOR TRUNK MUSCLES See page 184. MUSCLES OF THE UPPER LIMB Deltoid Muscles of the Upper Limb See pages 183-186 Muscles of the Upper Limbs biceps brachii: the powerful prime mover for flexion of the forearm and acts to supinate the forearm (supinate: forearm rotates laterally so that palm faces anteriorly) brachialis: deep to the biceps muscle and is important as the biceps in elbow flexion brachioradialis: weak muscles that resides mainly in the forearm triceps brachii: powerful prime mover of elbow extension and is the antagonist of the bicep brachii Muscles of the Upper Limbs The anconeus muscle is a small muscle on the posterior aspect of the elbow joint. It assists in extension of the elbow, where the triceps brachii is the principal antagonist. The anconeus muscle also prevents the elbow joint capsule being pinched in the olecranon fossa during extension of the elbow. The anconeus muscle also abducts the ulna and stabilizes the elbow joint. Muscles of the Upper Limbs deltoid: fleshy, triangular-shaped muscles that stabilize the joint at the proximal end of the humerus. The deltoids are the prime movers of arm abduction. extensor carpi radialis longus: this muscle is an extensor at the wrist joint and travels along the radial side of the arm, so will also abduct (radial abduction) the hand at the wrist Deltoid Muscles of the Upper Limb See pages 183-186 MUSCLES OF THE LOWER LIMBS Muscles of the Lower Limbs See pages 187-189. Quadricep muscles The quadriceps are a group of four muscles that sit on the anterior or front aspect of the thigh. They are the vastus medialis, vastus intermedius, rectus femoris, and the vastus lateralis. The quadriceps attach to the front of the tibia and originate at the top of the femur. The exception to this rule is the rectus femoris which actually crosses the hip joint and originates on the pelvis. In this diagram, the vastus intermedius and rectus femoris overlap. Muscles of the Lower Limbs gluteus maximus: a powerful hip extensor that acts to bring the thigh in a straight line with the pelvis, and also helps to extend the hip when climbing stairs and when jumping gluteus medius: a hip abductor and is important in steadying the pelvis during walking iliopsoas: the prime mover of hip flexion and also acts to keep the upper body from falling backward when standing erect adductor muscles: a group of muscles that adduct or press the thighs together Muscles of the Lower Limbs sartorius: thin, strap-like muscle that runs obliquely across the thigh and acts as a synergist to help cross the legs quadriceps group: includes the rectus femoris, and three vastus muscles (vastus lateralis, vastus medialis, vastus intermedius) that as a whole act to extend the knee powerfully, as when kicking a ball tibialis anterior: acts to dorsiflex (standing on the heels) and invert the foot (turning the sole medially) extensor digitorum longus: it is a prime mover of toe extension and a dorsiflexor of the foot Muscles of the Lower Limbs See pages 187-189. MUSCLES OF THE ANTERIOR AND POSTERIOR BODY See Handout and pages 190193. Anterior Muscles See locations of these muscles on pages 183-184. pectoralis major: forms the anterior wall of the axilla and acts to adduct and flex the arm external intercostal muscles: help to raise the rib cage for breathing air in See Figure 6.20 on page 190. internal intercostal muscles: lie deep to the external intercostal muscles and depress the rib cage, helping to move air out of the lungs when exhaling forcibly See Figure 6.20 on page 190 – these muscles lie deep to the external intercostal muscles. rectus abdominis: main function is to flex the vertebral column Anterior Muscles See locations of these muscles on pages 183-184. external oblique: flex the vertebral column, but also rotate the trunk and bend it laterally internal oblique: flex the vertebral column, but also rotate the trunk and bend it laterally transversus abdominis: compresses the abdominal contents Anterior Muscles serratus anterior: The serratus anterior muscle is occasionally called the "big swing muscle" or "boxer's muscle" because it is largely responsible for the protraction of the scapula — that is, the pulling of the scapula forward and around the rib cage that occurs when someone throws a punch. The serratus anterior also plays an important role in the upward rotation of the scapula, such as when lifting a weight overhead. It performs this in sync with the upper and lower fibers of the trapezius. Anterior Muscles biceps brachii: the powerful prime mover for flexion of the forearm and acts to supinate the forearm (supinate: forearm rotates laterally so that palm faces anteriorly) brachialis: deep to the biceps muscle and is important as the biceps in elbow flexion brachioradialis: weak muscles that resides mainly in the forearm triceps brachii: powerful prime mover of elbow extension and is the antagonist of the bicep brachii Deltoid Muscles of the Upper Limb See pages 183-186 Anterior Muscles The anconeus muscle is a small muscle on the posterior aspect of the elbow joint. It assists in extension of the elbow, where the triceps brachii is the principal antagonist. The anconeus muscle also prevents the elbow joint capsule being pinched in the olecranon fossa during extension of the elbow. The anconeus muscle also abducts the ulna and stabilizes the elbow joint. Anterior Muscles deltoid: fleshy, triangular-shaped muscles that stabilize the joint at the proximal end of the humerus. The deltoids are the prime movers of arm abduction. extensor carpi radialis longus: this muscle is an extensor at the wrist joint and travels along the radial side of the arm, so will also abduct (radial abduction) the hand at the wrist Anterior Muscles iliopsoas: the prime mover of hip flexion and also acts to keep the upper body from falling backward when standing erect gastrocnemius: two-bellied muscle that forms the curved calf of the posterior leg and is a prime mover for plantar flexion of the foot soleus: arises on the tibia and is a strong plantar flexor of the foot QUADRICEP MUSCLES The quadriceps are a group of four muscles that sit on the anterior or front aspect of the thigh. They are the vastus medialis, vastus intermedius, rectus femoris, and the vastus lateralis. The quadriceps attach to the front of the tibia and originate at the top of the femur. The exception to this rule is the rectus femoris which actually crosses the hip joint and originates on the pelvis. In this diagram, the vastus intermedius and rectus femoris overlap. MUSCLES OF THE POSTERIOR BODY Posterior Trunk Muscles trapezius: diamond-shaped superficial muscles of the posterior neck and trunk that help to extend the head. The trapezius muscles are the antagonists of the sternocleidomastoids and can elevate, depress, adduct, and stabilize the scapula. latissimus dorsi: extends and adducts the humerus Posterior Trunk Muscles erector spinae: a group of muscles that is a prime mover of back extension. The erector spinae acts as powerful back extensors (erectors) and also provide resistance that helps control the action of bending over at the waist. The erector spinae consists of three muscle columns (longissimus, iliocostalis, and spinalis) that collectively span the entire length of the vertebral column. deltoid: prime movers of arm abduction rhomboideus minor rhomboideus major levator scapulae Posterior Trunk Muscles rhomboideus minor: a small skeletal muscle on the back that connects the scapula with the vertebrae of the spinal column rhomboideus major: a skeletal muscle on the back that connects the scapula with the vertebrae of the spinal column. It acts together with the rhomboid minor to keep the scapula pressed against the thoracic wall and to retract the scapula toward the vertebral column. levator scapulae: situated at the back and side of the neck, its main function is to lift the scapula Posterior Muscles of the Lower Limbs gluteus maximus: a powerful hip extensor that acts to bring the thigh in a straight line with the pelvis, and also helps to extend the hip when climbing stairs and when jumping gluteus medius: a hip abductor and is important in steadying the pelvis during walking iliopsoas: the prime mover of hip flexion and also acts to keep the upper body from falling backward when standing erect adductor muscles: a group of muscles that adduct or press the thighs together Posterior Muscles hamstring group: located on the posterior side of the upper leg and includes the biceps femoris, semitendinosus, and semimembranosus which function to flex the knee and extend the hip fibularis muscles: three fibularis muscles as a group aids in plantar flexes (pointing the toes) and everts the foot (turning the sole laterally) gastrocnemius: two-bellied muscle that forms the curved calf of the posterior leg and is a prime mover for plantar flexion of the foot soleus: arises on the tibia and is a strong plantar flexor of the foot Muscles of the Lower Limbs See pages 187-189. Pop Quiz! Identify skeletal muscle groups 1-14. 1. internal oblique 2. biceps femoris 3. deltoid 4. biceps brachii 5. external intercostals 6. rectus abdominus 7. adductor muscle 8. rectus femoris 9. pectoralis major 10. trapezius 11. triceps bachii 12. latissimus dorsi 13. gluteus maximus 14. gastrocnemius Figure 6.1 page 164 epimysium 5. perimysium 3. muscle fascicle tendon 7. 4. 6. skeletal muscle endomysium 2. skeletal muscle fiber (cell) 1. That’s all…for now!
