2Muscle Types: General Characteristics There are three types of muscle tissue: skeletal, smooth & cardiacMost 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
3Muscle Types: Skeletal Muscle Tissue Attached to bonesMakes up 40% of body weightResponsible for locomotion, facial expressions, posture, respiratory movements, other types of body movementUnder conscious (voluntary) control; controlled by somatic motor neuronsMicroscopically, tissue appears striated; Cells are long, cylindrical and multi-nucleate
4Muscle Types: Smooth Muscle Tissue Makes up walls of organs and blood vessels; Propel urine, mix food in digestive tract, dilate/constrict pupils, regulate blood flowInvoluntary control by endocrine and autonomic nervous systemTissue is non-striated; Cells are short, spindle-shaped and mononucleatedExtremely extensible but maintains ability to contract
5Muscle Types: Cardiac Muscle Tissue Makes up myocardium of heartAutorhythmic, generates movement of bloodUnconciously (involuntarily) controlled by endocrine and autonomic nervous systemsMicroscopically appears striated; cells are short, branching and mono-nucleatedCells connected to each other at intercalated disks
6Functional Characteristics of Muscle Excitability – can receive and respond to stimuliContractility – can shorten/thicken and generate a pulling forceExtensibility – can be stretched and lengthenElasticity – after contracting or lengthening, will recoil to original resting length
7Muscle 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 cavitiesMaintaining body temperature (muscle contractions generate heat, “thermogenesis”)Stabilizing Joints
8Gross Anatomy of a Skeletal Muscle Tendon: connects the muscle to boneEndomysium: connective tissue sheath that wraps each muscle fiberPerimysium: collagenic sheath surrounding bundles, or fascicles, of muscleEpimysium: Coarse sheath that wraps and strengthens the entire muscleNormal activity is dependent on rich supply of nerves and blood
9Basic Features of a Skeletal Muscle Most skeletal muscles span joints and are attached to bones in at least two placesWhen 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)
10Nerve & Blood SupplyEach muscle is usually served by one nerve, an artery and one or more veins that enter/exit near the center of the muscleMuscle capillaries are long & winding to accommodate changes in muscle length.
16Sliding 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 sarcomereSarcomere shortens, I bands get smaller, H zone gets smaller, & zone of overlap increases
20Motor Unit: Nerve/Muscle Functional Unit Muscles that control fine movement (fingers, eyes) have small motor unitsLarge weight bearing muscles (thighs, hips) have large motor unitsMuscle fibers from a motor unit are spread throughout muscle. Contraction of single motor unit causes a weak contraction of the entire muscleStronger and stronger contraction require more motor units being recruited
21Wave Summation and Tetanus Twitch: a single stimulus is delivered; the muscle contracts and relaxesWave summation: stimuli are delivered more frequently, so that the muscle does not have adequate time to relax completely and contraction force increasesUnfused (incomplete) tetanus: more complete twitch fusion occurs as stimuli are delivered more rapidlyFused (complete) tetanus: a smooth continuous contraction without any evidence of relaxation
25Types of Muscle FibersMuscle fibers can be classified based on speed of contraction & pathway of ATP formationSlow 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 oxidativeFast oxidativeFast glycolyticMost muscles have a mixture of fiber types, giving a range of contractile speeds and fatigue resistance
26Influence of Exercise Endurance Exercise Increases: Improves: Resistance ExerciseIncreases:# of capillaries# of mitochondriaAmount of myoglobinImproves:Overall metabolismEfficiency of NM coordinationGI mobilitySkeletal strengthStroke volume of heartFatty 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 pairCross-training yields the benefits of both leading to muscles with more mitochondria, more myofilaments, more glycogen reserves etc.
29Contraction of Smooth Muscle Still stimulated by release of Ach from nerve endings which generates an action potential; T-tubules are absent, but there are membrane infoldings called caveoli to allow rapid influx of calcium ins.SR is less developed, although it does release some calcium ions, most comes from intracellular space.Smooth muscle does not contain sarcomeres but it does contain alternating thick and thin filaments.The filaments are arranged on a diagonal, causing smooth muscle to contract in a cork-screw manner.Adjacent cells generally exhibit slow, synchronized contractions (entire sheet contracts together)
30Skeletal Muscle Interaction in the Body Body muscles work either together or in opposition to achieve wide variety of motionMuscles 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.
31Four Functional Groups Prime movers (agonist): Provides the major force for producing a specific movementAntagonists: 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 movementSynergists: 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)
32Naming Skeletal Muscles Multiple descriptive criteria can be used to name muscles:Location of the muscle: ex) temporalis, intercostalShape of the muscle: ex) deltoid (triangle), trapeziusRelative size of the muscle: ex) maximus, minimus, longus, brevisDirection of muscle fibers: ex) rectus, transversus, obliqueNumber of origins: ex) biccep, tricep, quadricepLocation of attachments: ex) sternocleoidalmastoidAction: ex) flexor, extensor, adductor
33Importance of Fascicle Arrangement Fascicle arrangement determines the range of motion and power of a muscleSkeletal 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.
34Types of Fascicle Arrangement Circular: fascicles arranged in concentric rings; “sphincters”, close openings when contractingConvergent: muscle has a broad origin and converges toward a single tendon or insertionParallel: long axes of fascicle run parallel to long axis of muscle; strap-likeFusiform: parallel, but spindle shaped rather than strap-likePennate: fasicles are short and attach obliquely to a central tendon that runs length of the muscleUnipennate: fascicles insert into only one side of the tendonBipennate: fascicles insert from opposite sides (feather-like)Multipennate: multiple bipennate fused into central tendon
36Lever SystemsA 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.
37Power LeversSpeed 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
38Speed LeversSpeed 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 fulcrumThese levers allow the load to move rapidly through a large distance
39Classes of Lever Systems Levers are classified based on the relative position of the three elements: effort, fulcrum & loadFirst-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 speedSecond-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 bodyThird-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
43Smooth Muscle TissueCells are not striated and are narrower and much shorter than muscle cellsLack coarse connective tissue sheaths, but there is a thin layer of connective tissue between the smooth muscle cellsFibers smaller than those in skeletal muscleSpindle-shaped; single, central nucleusLack 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
44Smooth 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 shortenCircular layer – muscle fibers run around circumference of the organ; contraction causes organ to elongateBoth layers involved in peristalsis by alternating contraction and relaxation
45Additional Differences Between Smooth & Skeletal Muscle Tissue Sarcolemma does have small infoldings called caveoli that hold extracellular fluid & Ca2+ ionsLack sarcomeres:13x more actin than mysoin (v. 2x more in skeletal)The myofilaments are arranged diagnollyTropomysosin is present but no troponinNon-contractile intermediate filaments resist tension by attaching to dense bodies at regular intervals which act as anchors (correspond to z-discs)
46Contraction of Smooth Muscle Slow & synchronous contraction of entire sheetSome similarities with skeletal muscular contractionSliding filamentCa2+ triggerATP for energyContraction is slow, sustained & resistant to fatigueContraction/relaxation cycle is ~30x longerTension can be maintained at 1% the energy costx
47Cardiac MuscleFound only in heart; forms a thick layer called the myocardiumStriated fibers that branchEach cell usually has one centrally located nucleusFibers joined by intercalated disksUnder ANS (involuntary) and Endocrine (hormones) controlSome cells are autorhythmic (pacemaker cells)
49Developmental Aspects of Muscle Tissue Cardiac and smooth muscle becomes amitotic but can lengthen and thickenMyoblast-like satellite cells show very little regenerative abilityCardiac cells lack satellite cellsSmooth muscle has good regenerative ability
50Developmental 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 testosteroneWith more muscle mass, men are generally stronger, however body strength per unit muscle mass is the same for both sexes
51Developmental Aspects: Age Connective tissue increases and a muscle fibers decrease with ageThis results in muscles that become stringier and more sinewy50% of muscle mass is lost by age 80 (sarcopenia)Density of muscle capillaries also decreases, which reduced stamina and increases recovery timeRegular exercise reverses sarcopenia