The Muscular System Chapters 9 & 10

Muscle Types: General Characteristics There are three types of muscle tissue: skeletal, smooth & cardiac Most muscle cells are elongated and called muscle fibers (not true of cardiac cells) Muscle contraction depends on two kinds of myofilaments, thin actin and thicker myosin containing microfilaments. Prefixes myo and mys (“muscle”) and sarco (“flesh”) always refer to muscles. Ex) Sarcolemma (“muscle husk”) is the plasma membrane of muscle fibers

Muscle Types: Skeletal Muscle Tissue Attached to bones Makes up 40% of body weight Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement Under conscious (voluntary) control; controlled by somatic motor neurons Microscopically, tissue appears striated; Cells are long, cylindrical and multi-nucleate

Muscle Types: Smooth Muscle Tissue Makes up walls of organs and blood vessels; Propel urine, mix food in digestive tract, dilate/constrict pupils, regulate blood flow Involuntary control by endocrine and autonomic nervous system Tissue is non-striated; Cells are short, spindle-shaped and mononucleated Extremely extensible but maintains ability to contract

Muscle Types: Cardiac Muscle Tissue Makes up myocardium of heart Autorhythmic, generates movement of blood Unconciously (involuntarily) controlled by endocrine and autonomic nervous systems Microscopically appears striated; cells are short, branching and mono-nucleated Cells connected to each other at intercalated disks

Functional Characteristics of Muscle Excitability – can receive and respond to stimuli Contractility – can shorten/thicken and generate a pulling force Extensibility – can be stretched and lengthen Elasticity – after contracting or lengthening, will recoil to original resting length

Muscle Functions Producing Movement - (both voluntary and involuntary) - Respiration (diaphragm contractions) - Constriction of organs & vessels (peristalsis, vasoconstriciton, pupils) - Heartbeat - Communication (non-verbal & facial) Maintaining Posture - Also support soft tissues within body cavities Maintaining body temperature (muscle contractions generate heat, “thermogenesis”) Stabilizing Joints

Gross Anatomy of a Skeletal Muscle Tendon: connects the muscle to bone Endomysium: connective tissue sheath that wraps each muscle fiber Perimysium: collagenic sheath surrounding bundles, or fascicles, of muscle Epimysium: Coarse sheath that wraps and strengthens the entire muscle Normal activity is dependent on rich supply of nerves and blood

Basic Features of a Skeletal Muscle Most skeletal muscles span joints and are attached to bones in at least two places When a muscle contracts, the movable bone (insertion) moves toward the immovable/less movable bone (origin) Attachments may be direct or indirect (anchored by tendons or an aponeurosis; more common)

Nerve & Blood Supply Each muscle is usually served by one nerve, an artery and one or more veins that enter/exit near the center of the muscle Muscle capillaries are long & winding to accommodate changes in muscle length.

Organizational Levels of Skeletal Muscles

Microanatomy of a Skeletal Muscle

Microanatomy of a Muscle Fiber

Structure of Myofilaments

Sarcoplasmic Reticulum & T tubules

Sliding Filament Mechanism of Contraction Myosin heads attach to actin molecules (at binding, active, site) Myosin “pulls” on actin, causing thin myofilaments to slide across thick myofilaments, towards the center of the sarcomere Sarcomere shortens, I bands get smaller, H zone gets smaller, & zone of overlap increases

Role of Ionic Calcium in Contraction

Sequence of Events in Sliding Filament

The Neuromuscular Junction

Motor Unit: Nerve/Muscle Functional Unit Muscles that control fine movement (fingers, eyes) have small motor units Large weight bearing muscles (thighs, hips) have large motor units Muscle fibers from a motor unit are spread throughout muscle. Contraction of single motor unit causes a weak contraction of the entire muscle Stronger and stronger contraction require more motor units being recruited

Wave Summation and Tetanus Twitch: a single stimulus is delivered; the muscle contracts and relaxes Wave summation: stimuli are delivered more frequently, so that the muscle does not have adequate time to relax completely and contraction force increases Unfused (incomplete) tetanus: more complete twitch fusion occurs as stimuli are delivered more rapidly Fused (complete) tetanus: a smooth continuous contraction without any evidence of relaxation

Providing Energy for Contraction

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The Cori Cycle

Types of Muscle Fibers Muscle fibers can be classified based on speed of contraction & pathway of ATP formation Slow v. Fast (based on efficiency of ATPase) Oxidative (rely mainly on aerobic path) v. Glycolytic (rely mainly on anaerobic pathway) Based on these characteristics muscle fibers can categorized as: Slow oxidative Fast oxidative Fast glycolytic Most muscles have a mixture of fiber types, giving a range of contractile speeds and fatigue resistance

Influence of Exercise Endurance Exercise Increases: Improves: Resistance Exercise Increases: # of capillaries # of mitochondria Amount of myoglobin Improves: Overall metabolism Efficiency of NM coordination GI mobility Skeletal strength Stroke volume of heart Fatty acid deposits in blood vessels (removes them) Increases size of muscle fibers (not number of fibers) amount of connective tissue between cells (more protection from injury) Glycogen stores are increased. Important to focus on both parts of an antagonistic pair Cross-training yields the benefits of both leading to muscles with more mitochondria, more myofilaments, more glycogen reserves etc.

Smooth Muscle Tissue Cells are not striated and are narrower and much shorter than muscle cells Lack coarse connective tissue sheaths, but there is a thin layer of connective tissue between the smooth muscle cells Fibers smaller than those in skeletal muscle Spindle-shaped; single, central nucleus Lack highly structured neuromuscular junction; varicosities at the end of autonomic nerve fibers release NTs into wide synaptic cleft, “diffuse junction” Sarcoplasmic reticulum is less developed & T-tubules are absent

Smooth Muscle Structure Grouped into sheets (2 layers) in walls of hollow organs. Longitudinal layer – muscle fibers run parallel to organ’s long axis; contraction causes organ to shorten Circular layer – muscle fibers run around circumference of the organ; contraction causes organ to elongate Both layers involved in peristalsis by alternating contraction and relaxation

Smooth Muscle – Layers & Innervation

Peristalsis & Diffuse Junctions

Additional Differences Between Smooth & Skeletal Muscle Tissue Sarcolemma does have small infoldings called caveoli that hold extracellular fluid & Ca2+ ions Slow synchronized contractions Lack sarcomeres: 13x more actin than mysoin (v. 2x more in skeletal) The myofilaments are arranged diagonally resulting in a corkscrew-like contraction Tropomysosin is present but no troponin Non-contractile intermediate filaments resist tension by attaching to dense bodies at regular intervals which act as anchors (correspond to z-discs)

Contraction of Smooth Muscle Slow & synchronous contraction of entire sheet Some similarities with skeletal muscular contraction Sliding filament Ca2+ trigger ATP for energy Contraction is slow, sustained & resistant to fatigue Contraction/relaxation cycle is ~30x longer Tension can be maintained at 1% the energy costx

Cardiac Muscle Found only in heart; forms a thick layer called the myocardium Striated fibers that branch Each cell usually has one centrally located nucleus Fibers joined by intercalated disks Under ANS (involuntary) and Endocrine (hormones) control Some cells are autorhythmic (pacemaker cells)

Cardiac Muscle Tissue

Developmental Aspects of Muscle Tissue Cardiac and smooth muscle becomes amitotic but can lengthen and thicken Myoblast-like satellite cells show very little regenerative ability Cardiac cells lack satellite cells Smooth muscle has good regenerative ability

Developmental Aspects: Male v. Female Skeletal muscle makes up app. 35% of a woman’s body mass and 42% of a male’s. Difference is primarily due to males hormone testosterone With more muscle mass, men are generally stronger, however body strength per unit muscle mass is the same for both sexes

Developmental Aspects: Age Connective tissue increases and a muscle fibers decrease with age This results in muscles that become stringier and more sinewy 50% of muscle mass is lost by age 80 (sarcopenia) Density of muscle capillaries also decreases, which reduced stamina and increases recovery time Regular exercise reverses sarcopenia

Skeletal Muscle Interaction in the Body Body muscles work either together or in opposition to achieve wide variety of motion Muscles can only pull, never push. Therefore muscles or muscle groups usually work in pairs. As a muscle shortens, the insertion usually moves toward the origin; There is one muscles or muscle groups to pull the insertion toward the origin and a second muscle or muscle group to undo the action and pull the insertion away.

Four Functional Groups Prime movers (agonist): Provides the major force for producing a specific movement Antagonists: Muscles that oppose or reverse a particular movement; Usually stretched or relaxed when prime mover is active, can provide resistance to prevent overshoot or help slow or stop the movement Synergists: help prime movers by adding extra force and reducing undesirable or unnecessary movements (stabilize joints) Fixators: category of synergists that help immobilize a bone or muscles origin (contribute to maintaining upright posture)

Naming Skeletal Muscles Multiple descriptive criteria can be used to name muscles: Location of the muscle: ex) temporalis, intercostal Shape of the muscle: ex) deltoid (triangle), trapezius Relative size of the muscle: ex) maximus, minimus, longus, brevis Direction of muscle fibers: ex) rectus, transversus, oblique Number of origins: ex) biccep, tricep, quadricep Location of attachments: ex) sternocleoidalmastoid Action: ex) flexor, extensor, adductor

Importance of Fascicle Arrangement Fascicle arrangement determines the range of motion and power of a muscle Skeletal muscles shorten up to 70% of resting length when they contract, the longer and more parallel the fibers are to the long axis of the muscle, the more the muscle can shorten (this does not equate to power) Power depends on the total number of muscle cells; Bipennate & multipennate muscles pack in a lot of cells and are very powerful despite relatively minimal shortening.

Types of Fascicle Arrangement Circular: fascicles arranged in concentric rings; “sphincters”, close openings when contracting Convergent: muscle has a broad origin and converges toward a single tendon or insertion Parallel: long axes of fascicle run parallel to long axis of muscle; strap-like Fusiform: parallel, but spindle shaped rather than strap-like Pennate: fasicles are short and attach obliquely to a central tendon that runs length of the muscle Unipennate: fascicles insert into only one side of the tendon Bipennate: fascicles insert from opposite sides (feather-like) Multipennate: multiple bipennate fused into central tendon

Muscle Mechanics: Fascicle Arrangement (b) (f) Circular (orbicularis oris) (b) Convergent (pectoralis major) (c) (e) (d) Unipennate (extensor digitorum longus) (c) Parallel (sartorius) (d) (e) Bipennate (rectus femoris) (f) Fusiform (biceps brachii) (g) Multipennate (deltoid)

Lever Systems A lever is a rigid bar that moves on a fixed point (fulcrum) when force is applied to it. The applied force (effort) is used to move a resistance (load) In the human body, joints are fulcrums, bones act as levers and muscle contraction provides the effort which is applied at it’s insertion point. The load that is moved is the insertion bone and overlying tissues and anything else associated.

Power Levers Power levers operate at a mechanical advantage. In this type of system, the lever allows the given effort to move a heavier load or move a load farther and faster than otherwise possible. A mechanical advantage exists if the load is close to the fulcrum and the effort is applied far from the fulcrum. A small effort over a large distance is used to move a large load over a small distance

Speed Levers Speed levers operate at a mechanical disadvantage because the force exerted by the muscle must be greater than the load moved or supported. A mechanical disadvantage exists if the load is far from the fulcrum and the effort is applied near the fulcrum These levers allow the load to move rapidly through a large distance

Classes of Lever Systems Levers are classified based on the relative position of the three elements: effort, fulcrum & load First-Class Levers: effort applied at one end of the lever and the load is at the other end with the fulcrum somewhere in the middle (E –F – L ); comparable to see-saws and scissors; can be power or speed Second-Class Levers: effort applied at one end, with fulcrum at the other and load in the middle (E – L – F); comparable to a wheelbarrow, uncommon in the body Third-Class Levers: Effort is applied between the load and the fulcrum (F – E – L); comparable to tweezers and forceps; always speed levers; most skeletal muscles operate this way

1st Class Levers

2nd Class Levers

3rd Class Levers