The Muscular System

The Muscular System (The machines of the body) Muscles are responsible for all types of body movement Three basic muscle types are found in the body (Skeletal, Cardiac, and Smooth) Functions of Muscles Produce movement Maintain posture Stabilize joints Generate heat

Characteristics of Muscles Muscle cells are elongated (muscle cell = muscle fiber) Contraction of muscles is due to the movement of myofilaments All muscles share some terminology Prefix myo refers to muscle Prefix mys refers to muscle Prefix sarco refers to flesh Ex. Cytoplasm in cells = Sarcoplasm in muscle cells

Characteristics of Smooth Muscle Found in walls of hollow visceral organs No striations Involuntary, controlled by nervous and endocrine systems Very slow, contractions, often rhythmic (propel substances through tract) Characteristics of Cardiac Muscle Heart only Branching chains of striated cells Involuntary, nervous and endocrine controlled Slow, rhythmic contractions

Skeletal Muscle Characteristics Most are attached by tendons to bones Cells are multinucleate (largest is 1 foot long) Striated – have visible banding Voluntary – subject to conscious control Cells are surrounded and bundled by connective tissue

Connective Tissue Wrappings of Skeletal Muscle Endomysium – ____________________ Perimysium – ________________________________________ Epimysium – ________________________________________ Fascia – ________________________________________ Figure 6.1

Skeletal Muscle Attachments Epimysium blends into a connective tissue attachment Tendon – cord-like structure Aponeuroses – sheet-like structure Sites of muscle attachment Bones Cartilages Connective tissue coverings

Microscopic Anatomy of Skeletal Muscle Cells are multinucleate Nuclei are just beneath the sarcolemma Sarcolemma – plasma membrane of a muscle cell Sarcoplasmic reticulum – (Endoplasmic reticulum) stores calcium that is needed for muscle contraction Figure 6.3a

Microscopic Anatomy of Skeletal Muscle Myofibril - organelle that fills muscle cells, made of bundles of myofilaments I band = light band A band = dark band These give it a striped appearance. Figure 6.3b

Microscopic Anatomy of Skeletal Muscle Sarcomere – from z disc to z disc Contractile unit of a muscle fiber, when lined up form a myofibril Bundles of myofibrils = muscle cell Figure 6.3b

Microscopic Anatomy of Skeletal Muscle Organization of the sarcomere Thick filaments = myosin filaments Thin filaments = actin filaments Figure 6.3c

Microscopic Anatomy of Skeletal Muscle Myosin filaments have heads (extensions, or cross bridges) Myosin and actin overlap somewhat At rest, there is a bare zone that lacks actin filaments Figure 6.3d

Properties of Skeletal Muscle Activity Irritability – ability to receive and respond to a stimulus Contractility – ability to shorten when an adequate stimulus is received Graded Response- different degrees of skeletal muscle shortening Frequency of stimulation and number of cells stimulated Twitch – incomplete contraction, breaks in the impulse Complete Tetanus is sustained and smooth contraction

Nerve Stimulus to Muscles Skeletal muscles must be stimulated by a nerve to contract (show human machine video) Motor unit One neuron Muscle cells stimulated by that neuron Figure 6.4a

Nerve Stimulus to Muscles Neuromuscular junctions – where muscles and nerves interact Synaptic cleft – gap between nerve and muscle, they DO NOT contact each other Ach - Acetylcholine Figure 6.5b

Transmission of Nerve Impulse to Muscle Neurotransmitter – chemical released by nerve upon arrival of nerve impulse -The neurotransmitter for skeletal muscle is acetylcholine (Ach) Neurotransmitter attaches to receptors on the sarcolemma Sarcolemma becomes permeable to sodium (Na+) K+ (Potassium) leaves and inside of cell gains a + charge Sodium rushing into the cell generates an action potential Once started, muscle contraction cannot be stopped Ach is broken down so a single nerve cell produces only one contraction Cell returns to rest by diffusion of K+ in and Na+ out

The Sliding Filament Theory of Muscle Contraction Activation (Ach crosses the gap) by nerve causes myosin heads to attach to binding sites on the actin. Myosin heads then bind to the next site on the actin filament This continued action causes the actin filaments to “slide” closer together, towards the center of the sarcomere. This causes the muscle to shorten (contract). The contraction ends when the Ach is broken down. Figure 6.7

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 Initially, muscles used stored ATP for energy Bonds of ATP are broken to release energy Only 4-6 seconds worth of ATP is stored by muscles After this initial time, other pathways must be utilized to produce ATP

Energy for Muscle Contraction

Muscle Fatigue and Oxygen Debt When a muscle is fatigued, it is unable to contract The common reason for muscle fatigue is oxygen debt Increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less

Muscles and Body Movements Movement is attained due to a muscle moving an attached bone Muscles are attached to at least two points Origin – attaches to the less movable bone Insertion- attachment to the more movable bone Results of increased muscle use: increase in strength and smoothness Figure 6.12

Head and Neck Muscles Figure 6.15

Deep Trunk and Arm Muscles Trunk Muscles Deep Trunk and Arm Muscles

Muscles of the Pelvis, Hip, and Thigh Muscles of the Lower Leg

Superficial Muscles: Anterior Figure 6.21

Superficial Muscles: Posterior Figure 6.22