The Muscular System Anatomy and Physiology: Chapter 6

Functions of Muscles Produce movement Maintain posture Stabilize joints Generate heat

The Muscular System Cells are elongated (muscle cell = muscle fiber) Muscles are responsible for all types of body movement Contraction of muscles is due to the movement of microfilaments The Muscular System

The Muscular System Cardiac Muscle Smooth Muscle Skeletal Muscle

3 Basic Muscle Types Cardiac Muscle Smooth Muscle Skeletal Muscle Has striations Usually has a single nucleus Joined to another muscle cell at an intercalated disc Involuntary Found only in the heart Smooth Muscle Has no striations Spindle-shaped cells Single nucleus Involuntary – no conscious control Found mainly in the walls of hollow organs Skeletal Muscle Most are attached by tendons to bones Cells are multinucleate Striated – have visible banding Voluntary – subject to conscious control Cells are surrounded and bundled by connective tissue

Connective Tissue Wrappings of Skeletal Muscle Endomysium Around and between single muscle fibers Perimysium Around a fascicle (bundle) of fibers Epimysium Covers the entire skeletal muscle Blends into a tendon to attach the muscle to bone

Microscopic Anatomy of Skeletal Muscle Sarcolemma Specialized plasma membrane Several nuclei Myofibrils Long organelles within muscle Composed of Sarcomeres Tiny contractile units Myofilaments Actin Thin filaments Myosin Thick filaments

Microscopic Anatomy of Skeletal Muscle Myofibrils are aligned to give distinct bands I band = light band A band = dark band Myosin filaments have heads (extensions, or cross bridges) Myosin and actin overlap somewhat At rest, there is a bare zone that lacks actin filaments.

Sliding Filament Theory of Muscle Contraction Activation by nerves causes myosin heads (crossbridges) to attach to binding sites on the thin filament. Myosin heads then bind to the next site of the thin filament. This continued action causes a sliding of the myosin along the actin. The result is that the muscle is shortened (contracted).

The Sliding Filament Theory

Nerve Stimulus to Muscles Synaptic cleft: gap between nerve and muscle Nerve and muscle do not make contact Filled with interstitial fluid Neuromuscular junctions: association site of nerve and muscle Motor unit: one neuron and the muscle cells stimulated by that neuron Skeletal muscles must be stimulated by a nerve to contract.

Transmission of Nerve Impulse to Muscle Neurotransmitter Chemical released by nerve upon arrival of nerve impulse Acetylcholine Neurotransmitter for skeletal muscle Attaches to receptors on the sarcolemma stimulating muscle contraction. Once started, muscle contraction cannot be stopped.

Contraction of a Skeletal Muscle Muscle fiber contraction is “all or none” Within a skeletal muscle, not all fibers may be stimulated during the same interval Different combinations of muscle fiber contractions may give differing responses Graded responses – different degrees of skeletal muscle shortening, rapid stimulus = constant contraction or tetanus

Muscle Response to Strong Stimuli Muscle force depends upon the number of fibers stimulated More fibers contracting results in greater muscle tension Muscles can continue to contract unless they run out of energy

Energy for Muscle Contraction Stored ATP provides energy for the first 4-6 seconds worth of activity. Creatine phosphate (CP) can regenerate ATP for another 20 seconds worth of activity. Cellular respiration Aerobic respiration Breaks down glucose which provides hours of energy Requires constant oxygen so it can’t be used for intense activity Glycolysis Anaerobic respiration Also breaks down glucose but only provides enough energy for 30-60 seconds of activity Fast process so it can be used for intense activity

Muscle Fatigue and Oxygen Debt When a muscle is fatigued, it is unable to contract The common reason for muscle fatigue is oxygen debt Oxygen must be “repaid” to tissue to remove oxygen debt Oxygen is required to get rid of accumulated lactic acid Increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less

Types of Muscle Contractions Isotonic contractions Myofilaments are able to slide past each other during contractions The muscle shortens Isometric contractions Tension in the muscles increases The muscle is unable to shorten

Muscle Tone Some fibers are contracted even in a relaxed muscle. Different fibers contract at different times to provide muscle tone. The process of stimulating various fibers is under involuntary control.

Effects of Exercise on Muscle Results of increased muscle use Increase in muscle size More contractile filaments are produced Increase in muscle strength Increase in muscle efficiency How they burn fuel for energy Muscle becomes more fatigue resistant

Muscles and Body Movements Movement is attained due to a muscle moving an attached bone Muscles are attached to at least two points Origin: attached to an immoveable or less movable bone Insertion: attached to a movable bone When the muscle contracts, the insertion moves toward the origin.

Types of Ordinary Body Movements Flexion Decreases angle of joint and brings two bones closer together Extension Opposite of flexion Rotation Movement of a bone in longitudinal axis, shaking head “no” Abduction/Adduction (see slides) Circumduction (see slides)

Types of Ordinary Body Movements

Abduction -Moving the leg away from the midline Circumduction -Cone-shaped movement, proximal end doesn’t move, while distal end moves in a circle. Adduction -Moving toward the midline

Types of Muscles Prime mover Muscle with the 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 and helps prevent rotation Fixator Stabilizes the origin of a prime mover

Naming of Skeletal Muscles Direction of muscle fibers Example: rectus (straight) Relative size of the muscle Example: maximus (largest) Location of the muscle Example: many muscles are named for bones (e.g., temporalis) Number of origins Example: triceps (three heads) Location of the muscles origin and insertion Example: sterno (on the sternum) Shape of the muscle Example: deltoid (triangular) Action of the muscle Example: flexor and extensor (flexes or extends a bone)

Head and Neck Muscles

Trunk Muscles

Deep Trunk and Arm Muscles

Muscles of the Pelvis, Hip, and Thigh

Muscles of the Lower Leg

Superficial Muscles: Anterior

Superficial Muscles: Posterior

Disorders Relating to the Muscular System Muscular Dystrophy: inherited, muscle enlarge due to increased fat and connective tissue, but fibers degenerate and atrophy Duchenne MD: lacking a protein to maintain the sarcolemma Myasthemia Gravis: progressive weakness due to a shortage of acetylcholine receptors