Anatomy of the Muscular System

Interesting Facts Muscles are bundles of cells and fibers. Muscles are bundles of cells and fibers. Muscles work in a very simple way. All they do is tighten up--that is, contract--and relax. Muscles work in a very simple way. All they do is tighten up--that is, contract--and relax. Muscles work in pairs to allow bones to move. Muscles work in pairs to allow bones to move. There are 630 active muscles in your body. There are 630 active muscles in your body. Muscles can only pull. They never push. Muscles can only pull. They never push. Muscles make up 40-50% of our body weight. Muscles make up 40-50% of our body weight. The longest muscle has muscle cells that can be over a foot long. The longest muscle has muscle cells that can be over a foot long. The smallest muscles are in the middle of the ear; examples are the tensor tympani, and stapedius. The smallest muscles are in the middle of the ear; examples are the tensor tympani, and stapedius. The strongest, pound for pound, are the masseters, the chewing muscles. The strongest, pound for pound, are the masseters, the chewing muscles.

Skeletal Muscle Structure

Muscle cell - muscle fiber; one muscle can be made up of thousands of muscle fibers bundled together and surrounded by the epimysium Muscle cell - muscle fiber; one muscle can be made up of thousands of muscle fibers bundled together and surrounded by the epimysium Epimysium – coarse sheath covering the muscle as a whole Epimysium – coarse sheath covering the muscle as a whole Fascia - fibrous layers of connective tissue surrounding the epimysium and tendon; surrounds and separates muscles Fascia - fibrous layers of connective tissue surrounding the epimysium and tendon; surrounds and separates muscles Fasciculus – “fascicle”; a discrete bundle of muscle cells surrounded by the perimysium Fasciculus – “fascicle”; a discrete bundle of muscle cells surrounded by the perimysium Perimysium – tough connective tissue binding together fascicles Perimysium – tough connective tissue binding together fascicles

Endomysium – connective tissue membrane that covers each muscle fiber/cell Endomysium – connective tissue membrane that covers each muscle fiber/cell The three fibrous components (epimysium, perimysium, and endomyhsium may be continuous with fibrous tissue that extends from the muscle as a cordlike tendon, or the fibrous wrapping may extend as a broad, flat sheet of connective tissue called aponeurosis to help merge with the fibrous wrappings of another muscle. The three fibrous components (epimysium, perimysium, and endomyhsium may be continuous with fibrous tissue that extends from the muscle as a cordlike tendon, or the fibrous wrapping may extend as a broad, flat sheet of connective tissue called aponeurosis to help merge with the fibrous wrappings of another muscle. Tendons and aponeuroses are so tough and strong that they are not often torn Tendons and aponeuroses are so tough and strong that they are not often torn

Fascia Fascia is a series of fibrous layers found outside the epimysium and tendon. Fascia is a series of fibrous layers found outside the epimysium and tendon. Superficial fascia, also known as the hypodermis is very thin over the back of the hands and feet, but is extremely dense over the palms, soles of the feet and scalp where it has a cushioning effect. Superficial fascia, also known as the hypodermis is very thin over the back of the hands and feet, but is extremely dense over the palms, soles of the feet and scalp where it has a cushioning effect.fascia Deep fascia is dense and fibrous with no fat. It lies directly on the outer surface of muscle groups or around bone. It may form a thick septum between different muscle groups, as well as surround individual muscles. Deep fascia is dense and fibrous with no fat. It lies directly on the outer surface of muscle groups or around bone. It may form a thick septum between different muscle groups, as well as surround individual muscles.fascia

Superficial and Deep Fascia

Muscle Cell Muscle cell = muscle fiber; threadlike; each contains many nuclei, mitochondria, myofibrils, and is surrounded by a plasma membrane called the sarcolemma; cytoplasm is the sarcoplasm Muscle cell = muscle fiber; threadlike; each contains many nuclei, mitochondria, myofibrils, and is surrounded by a plasma membrane called the sarcolemma; cytoplasm is the sarcoplasm Myofibril – cylindrical organelles; fibers found within muscle cells that run parallel to the muscle length; made up of myofilaments called actin and myosin Myofibril – cylindrical organelles; fibers found within muscle cells that run parallel to the muscle length; made up of myofilaments called actin and myosin Actin – thin myofilament Actin – thin myofilament Myosin – thick myofilament Myosin – thick myofilament

Types of Muscles 1. Skeletal – striated and voluntary; attach to bones; usually multi-nucleate 2. Smooth – non-striated and involuntary; lines organs, blood vessels, lymphatic vessels 3. Cardiac – striated and involuntary; heart wall

Sarcomere – the contractile unit of the muscle; give skeletal and cardiac muscles their striated appearance Sarcomere – the contractile unit of the muscle; give skeletal and cardiac muscles their striated appearance

Size, Shape, and Fiber Arrangement

Muscle Attachment Origin – the point of attachment that does not move when the muscle contracts; origin bone is the most stationary bone at a joint when contraction occurs Origin – the point of attachment that does not move when the muscle contracts; origin bone is the most stationary bone at a joint when contraction occurs Insertion – point of attachment that moves when the muscle contracts; insertion bone moves toward the origin bone when the muscle shortens Insertion – point of attachment that moves when the muscle contracts; insertion bone moves toward the origin bone when the muscle shortens

Insertion moves toward the origin when contraction occurs.

Anatomical Terms of Motion In human anatomy, all descriptions of position and movement are based on the assumption that the body is in the complete anatomical position. In human anatomy, all descriptions of position and movement are based on the assumption that the body is in the complete anatomical position.

Anatomical Terms of Motion Flexion - Bending movement that decreases the angle between two parts. Bending the elbow or clenching a hand into a fist are examples of flexion. Knees are flexed when sitting down. Flexion - Bending movement that decreases the angle between two parts. Bending the elbow or clenching a hand into a fist are examples of flexion. Knees are flexed when sitting down. Extension - The opposite of flexion; a straightening movement that increases the angle between body parts. In a conventional handshake, the fingers are fully extended. When standing up, the knees are extended. Extension of the hip or shoulder moves the limb toward the posterior side of the body. Extension - The opposite of flexion; a straightening movement that increases the angle between body parts. In a conventional handshake, the fingers are fully extended. When standing up, the knees are extended. Extension of the hip or shoulder moves the limb toward the posterior side of the body.

Abduction - A motion that pulls a structure or part away from the midline of the body (or, in the case of fingers and toes, spreading the digits apart, away from the centerline of the hand or foot). Abduction - A motion that pulls a structure or part away from the midline of the body (or, in the case of fingers and toes, spreading the digits apart, away from the centerline of the hand or foot). Adduction - A motion that pulls a structure or part towards the midline of the body, or towards the midline of a limb. Dropping the arms to the sides, or bringing the knees together, are examples of adduction. Adduction - A motion that pulls a structure or part towards the midline of the body, or towards the midline of a limb. Dropping the arms to the sides, or bringing the knees together, are examples of adduction.

Pronation - a rotation of the forearm that moves the palm from an anterior-facing position to a posterior-facing position; radius crosses over ulna Pronation - a rotation of the forearm that moves the palm from an anterior-facing position to a posterior-facing position; radius crosses over ulna Supination - the opposite of pronation, the rotation of the forearm so that the palm faces anteriorly; radius is uncrossed Supination - the opposite of pronation, the rotation of the forearm so that the palm faces anteriorly; radius is uncrossed Eversion - refers to the foot (plantar surface) being turned outward so that the sole is pointing laterally. Eversion - refers to the foot (plantar surface) being turned outward so that the sole is pointing laterally. Inversion - the movement of the sole towards the median plane Inversion - the movement of the sole towards the median plane

pronation supination

Inversion

Muscle Actions Muscles usually work in groups or pairs. Muscles usually work in groups or pairs. Muscles help the body to maintain POSTURE (position of the body for a special purpose) Muscles help the body to maintain POSTURE (position of the body for a special purpose) Agonists – prime mover; a muscle or group of muscles that directly performs a specific movement (the action or function of that muscle) Agonists – prime mover; a muscle or group of muscles that directly performs a specific movement (the action or function of that muscle) Antagonists – when contracting, these muscles directly oppose the agonists and are relaxed when the agonist is contracting; responsible for moving the body part back to its original position Antagonists – when contracting, these muscles directly oppose the agonists and are relaxed when the agonist is contracting; responsible for moving the body part back to its original position Synergists – muscles that contract at the same time as the prime mover to compliment their actions; move joint in the same direction as another muscle Synergists – muscles that contract at the same time as the prime mover to compliment their actions; move joint in the same direction as another muscle Fixator muscles – joint stabilizers; help to maintain posture or balance; fix a joint in a given position during the movement of other joints Fixator muscles – joint stabilizers; help to maintain posture or balance; fix a joint in a given position during the movement of other joints Most muscles function as more than one of these depending on the particular movement pattern. Most muscles function as more than one of these depending on the particular movement pattern.

The biceps and triceps work together as an agonist and antagonist.

Agonists and Antagonists Pectoral and latissimus dorsi (chest and back) Pectoral and latissimus dorsi (chest and back) Anterior and posterior deltoids (front and back of shoulders) Anterior and posterior deltoids (front and back of shoulders) Trapezius and deltoids (upper back and shoulders) Trapezius and deltoids (upper back and shoulders) Left and right external obliques ( left and right side of abdomen) Left and right external obliques ( left and right side of abdomen) Quadriceps and hamstrings (front and back of thigh) Quadriceps and hamstrings (front and back of thigh) Tibialis anterior and gastrocnemius (shin and calf) Tibialis anterior and gastrocnemius (shin and calf)

Parts of a Lever System Lever – any rigid bar free to turn about a fixed point Lever – any rigid bar free to turn about a fixed point Fulcrum – fixed point about which the lever is free to move (F) Fulcrum – fixed point about which the lever is free to move (F) Load – resistance that is moved (R) Load – resistance that is moved (R) Effort – force or pull which produces movement (muscle contraction) (E) or (M) Effort – force or pull which produces movement (muscle contraction) (E) or (M)

Three Classes of Levers

Muscle-Bone Interaction Lever Systems Bones serve as levers, and joints serve as fulcrums (pivot points)of these levers. Bones serve as levers, and joints serve as fulcrums (pivot points)of these levers.

First Class Lever - resembles a see-saw with a fulcrum in the middle of the force and effort First Class Lever - resembles a see-saw with a fulcrum in the middle of the force and effort

Second Class Lever - resembles a wheel barrow - the gastrocnemius muscle pulls the heel (calcaneus) upward [like you are lifting the handles of the wheel barrow], this causes the metatarsal/phalange [the wheel] joint to press against the floor; when you stand up on your toes, the resistance is the weight of the body pressing down on the ankles Second Class Lever - resembles a wheel barrow - the gastrocnemius muscle pulls the heel (calcaneus) upward [like you are lifting the handles of the wheel barrow], this causes the metatarsal/phalange [the wheel] joint to press against the floor; when you stand up on your toes, the resistance is the weight of the body pressing down on the ankles

Third Class Lever - the fulcrum and resistance are at opposite ends, the effort in between; examples are the adductors of the thigh, tweezers, and lifting a shovel of dirt; the fulcrum is the hip joint, the effort is the adductors' contractions and the resistance is the weight of the leg Third Class Lever - the fulcrum and resistance are at opposite ends, the effort in between; examples are the adductors of the thigh, tweezers, and lifting a shovel of dirt; the fulcrum is the hip joint, the effort is the adductors' contractions and the resistance is the weight of the leg

Lever Systems (continued)

Naming Muscles Muscles are named according to… Muscles are named according to… Size: vastus (huge); maximus (large); longus (long); minimus (small); brevis (short) Size: vastus (huge); maximus (large); longus (long); minimus (small); brevis (short) Shape: deltoid (triangular); rhomboid (like a rhombus with equal and parallel sides); latissimus (wide); teres (round); trapezius (like a trapezoid, a four-sided figure with two sides parallel) Shape: deltoid (triangular); rhomboid (like a rhombus with equal and parallel sides); latissimus (wide); teres (round); trapezius (like a trapezoid, a four-sided figure with two sides parallel)

Direction of fibers: rectus (straight); transverse (across); oblique (diagonally); orbicularis (circular) Direction of fibers: rectus (straight); transverse (across); oblique (diagonally); orbicularis (circular) Location: pectoralis (chest); gluteus (buttock or rump); brachii (arm); supra- (above); infra- (below); sub- (under or beneath); lateralis (lateral) Location: pectoralis (chest); gluteus (buttock or rump); brachii (arm); supra- (above); infra- (below); sub- (under or beneath); lateralis (lateral) Number of origins: biceps (two heads); triceps (three heads); quadriceps (four heads) Number of origins: biceps (two heads); triceps (three heads); quadriceps (four heads)

Origin and insertion: sternocleidomastoideus (origin on the sternum and clavicle, insertion on the mastoid process); brachioradialis (origin on the brachium or arm, insertion on the radius) Origin and insertion: sternocleidomastoideus (origin on the sternum and clavicle, insertion on the mastoid process); brachioradialis (origin on the brachium or arm, insertion on the radius) Action: abductor (to abduct a structure); adductor (to adduct a structure); flexor (to flex a structure); extensor (to extend a structure); levator (to lift or elevate a structure); masseter (a chewer) Action: abductor (to abduct a structure); adductor (to adduct a structure); flexor (to flex a structure); extensor (to extend a structure); levator (to lift or elevate a structure); masseter (a chewer)