The Muscular System Structure & Function

Skeletal Muscle Functions Produce movement Maintain posture Stabilize joints Generate heat

The Muscular System Muscles are responsible for all types of body movement 3 muscle types are found in the body Skeletal muscle Attached to bone & skin, some facial muscles Produce movement, stabilize joints, provide posture & generate heat Cardiac muscle Walls of the heart Protect the heart & produce contractions of the heart walls Smooth muscle Walls of hollow visceral (i.e, stomach) organs (except heart) Protect organs & produce slow contractions

Comparison of Skeletal, Cardiac & Smooth Muscles

Characteristics of Muscles Skeletal & smooth muscle cells are elongated (muscle cell = muscle fiber) Contraction of muscles is due to the movement of microfilaments All muscles share some terminology myo & mys refer to “muscle” sarco refers to “flesh”

Smooth Muscle Characteristics Lacks striations Spindle-shaped cells Single nucleus Involuntary—no conscious control Contractions happen very slowly Found mainly in the walls of hollow organs Stomach, urinary bladder, respiratory passages

Cardiac Muscle Characteristics Striations Usually has a single nucleus Branching cells Joined to another muscle cell at an intercalated disc Involuntary Contractions happen slowly Found only in the heart

Skeletal Muscle Characteristics Most are attached by tendons to bones Cells are multinucleated Striated—have visible banding Voluntary—subject to conscious control Contractions happen slow to fast Muscles are either contracted or relaxed Contracted the muscle exerts pulling force = muscle shortens Since muscles can only pull (not push), they work in pairs (antagonistic muscles) Example: Biceps & Triceps Muscle that bends joint is called the flexor muscle Muscle that straightens joint is called the extensor muscle Which is the flexor muscle? Extensor muscle?

EXAMPLE

The Muscular System Skeletal Muscle Structure & Contractions

Connective Tissue Wrappings & Attachments of Skeletal Muscle Cells are surrounded & bundled by connective tissue Endomysium—encloses a single muscle fiber Perimysium—wraps around a fascicle (bundle) of muscle fibers Epimysium—covers the entire skeletal muscle Fascia—on the outside of the epimysium Tendons—cord-like structures Mostly collagen fibers Attach muscle to bone Figure 6.1

Microscopic Anatomy of Skeletal Muscle Sarcolemma—specialized plasma membrane Myofibrils—long organelles inside muscle cell

Microscopic Anatomy of Skeletal Muscle Sarcomere—contractile unit of a muscle fiber Organization of the sarcomere Myofilaments Thick filaments = myosin filaments Thin filaments = actin filaments Figure 6.3b

Microscopic Anatomy of Skeletal Muscle Myofibrils are aligned to give distinct bands I band = light band Contains only thin filaments A band = dark band Contains the entire length of the thick filaments Figure 6.3b

Microscopic Anatomy of Skeletal Muscle Thick filaments = myosin filaments Composed of the protein myosin Myosin filaments have heads (extensions, or cross bridges) Myosin & actin overlap somewhat (A-band) Thin filaments = actin filaments Composed of the protein actin Anchored to the Z disc Figure 6.3c

The Nerve Stimulus & Action Potential Skeletal muscles must be stimulated by a motor neuron (nerve cell) to contract Motor unit— one motor neuron & all skeletal muscle cells stimulated by that neuron Figure 6.4a

The Nerve Stimulus & Action Potential Neuromuscular junction Association site of axon terminal of motor neuron & muscle Neuromuscular Junction Animation

The Nerve Stimulus & Action Potential Figure 6.5a Action Potentials & Muscle Contraction

The Nerve Stimulus & Action Potential Synaptic cleft Gap between nerve & muscle Do not make contact Filled with interstitial fluid

Transmission of Nerve Impulse to Muscle Neurotransmitter—chemical released by nerve upon arrival of nerve impulse Skeletal muscle – acetylcholine (ACh) ACh attaches to receptors on sarcolemma Sarcolemma becomes permeable to sodium (Na+)

Transmission of Nerve Impulse to Muscle Na+ rushes into cell, generating an AP Once started, muscle contraction cannot be stopped

The Sliding Filament Theory of Muscle Contraction Activation by nerve causes myosin heads (cross bridges) to attach to binding sites on thin filament Myosin heads then bind to next site of thin filament & pull them toward center of sarcomere Continued action causes a sliding of the myosin along actin Result – muscle is shortened (contracted)

The Sliding Filament Theory Myofilament Contraction animation Figure 6.8a

The Sliding Filament Theory Figure 6.8b

The Sliding Filament Theory Sarcomere Contraction animation Figure 6.8c

5 Golden Rules of Skeletal Muscle Activity

Muscles & Body Movements Movement is attained due to a muscle moving an attached bone Muscles are attached to at least 2 points Origin Attachment to moveable bone Insertion Attachment to immovable bone

Types of Muscle Contractions Isotonic contractions Myofilaments are able to slide past each other during contractions Muscle shortens & movement occurs Ex: lifting a dumbbell Isometric contractions Tension in muscles increases Muscle is unable to shorten or produce movement Ex: pushing on a wall

Effect of Exercise on Muscles Exercise increases muscle size, strength & endurance Aerobic (endurance) exercise (biking, jogging) results in stronger, more flexible muscles with greater resistance to fatigue Makes body metabolism more efficient Improves digestion, coordination Resistance (isometric) exercise (weight lifting) increases muscle size & strength

The Muscular System Skeletal Muscles to Identify

Types of Muscles Prime mover—muscle with major responsibility for a certain movement Antagonist—muscle that opposes or reverses a prime mover Synergist—muscle that aids a prime mover in a movement & helps prevent rotation Fixator—stabilizes origin of a prime mover

Naming Skeletal Muscles By direction of muscle fibers Examples Rectus (straight) Transverse (horizontal) Oblique (angle) By relative size of the muscle Maximus (largest) Minimus (smallest) Longus (longest) By location of the muscle Examples Temporalis (temporal bone) Occipitalis (occipital bone) By number of origins Triceps (3 heads) Biceps (2 heads) Quadriceps (4 muscles in group)

Naming Skeletal Muscles By location of the muscle’s origin & insertion Example Sternocleidomastoid (on the sternum) By shape of the muscle Examples Deltoid (triangular) Trapezius (trapezoid) By action of the muscle Examples Flexor (flexes a bone) Extensor (extends a bone) Abductor (abducts a limb) Adductor (adducts a limb)

Head & Neck Muscles Facial muscles Chewing muscles Frontalis—raises eyebrows Orbicularis oculi— closes eyes, squints, blinks, winks Orbicularis oris— closes mouth & protrudes lips Buccinator—flattens cheek, chews Zygomaticus—raises corners of mouth Chewing muscles Masseter—closes jaw & elevates mandible Temporalis—synergist of masseter, closes jaw

Head & Neck Muscles Neck muscles Platysma—pulls corners of mouth inferiorly Sternocleidomastoid—flexes neck, rotates head

Muscles of Trunk, Shoulder & Arm Anterior muscles Pectoralis major— adducts & flexes humerus “Pecs” Intercostal muscles External intercostals— raise rib cage during inhalation Internal intercostals— depress rib cage to move air out of lungs when you exhale forcibly

Muscles of Trunk, Shoulder & Arm Posterior muscles Trapezius—elevates, depresses, adducts & stabilizes scapula Latissimus dorsi—extends & adducts humerus Deltoid—arm abduction

Muscles of Trunk, Shoulder & Arm Muscles of the abdominal girdle Rectus abdominis— flexes vertebral column & compresses abdominal contents (defecation, childbirth, forced breathing) External & internal obliques—flex vertebral column; rotate trunk & bend it laterally Transversus abdominis— compresses abdominal contents

Muscles of the Upper Limb Biceps brachii—supinates forearm, flexes elbow Brachialis—elbow flexion Brachioradialis—weak muscle Triceps brachii—elbow extension (antagonist to biceps brachii)

Muscles of the Lower Limb (Pelvis, Hip, Thigh) Gluteus maximus—hip extension Gluteus medius—hip abduction, steadies pelvis when walking Iliopsoas—hip flexion, keeps upper body from falling backward when standing erect Adductor muscles—adduct thighs

Muscles of the Lower Limb Muscles causing movement at the knee joint Hamstring group— thigh extension & knee flexion Biceps femoris Semimembranosus Semitendinosus

Muscles of the Lower Limb (Pelvis, Hip, Thigh) Muscles causing movement at the knee joint Sartorius—flexes thigh Quadriceps group—extends knee Rectus femoris Vastus muscles (3)

Muscles of the Lower Limb Muscles causing movement at ankle & foot Tibialis anterior— dorsiflexion & foot inversion Extensor digitorum longus—toe extension & dorsiflexion of foot Fibularis muscles— plantar flexion, everts foot Soleus—plantar flexion Gastrocnemius— plantar flexion

Superficial Muscles: Anterior You should be able to identify these superficial muscles for your Quiz NEXT WEEK!

Superficial Muscles: Posterior You should be able to identify these superficial muscles for your Quiz NEXT WEEK!