The Muscular System Chapters 9 & 10. Muscle Types: General Characteristics There are three types of muscle tissue: skeletal, smooth & cardiac Most muscle.

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

1. Excitability – can receive and respond to stimuli

2. Contractility – can shorten/thicken and generate a pulling force

3. Extensibility – can be stretched and lengthen

4. Elasticity – after contracting or lengthening, will recoil to original resting length

Muscle Functions1. Producing Movement - (both voluntary and involuntary)

- Respiration (diaphragm contractions)- Constriction of organs & vessels (peristalsis, vasoconstriciton, pupils)- Heartbeat

- Communication (non-verbal & facial)

2. Maintaining Posture- Also support soft tissues within body cavities

3. Maintaining body temperature (muscle contractions generate heat, “thermogenesis”)

4. 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

(a)Twitch: a single stimulus is delivered; the muscle contracts and relaxes

(b)Wave summation: stimuli are delivered more frequently, so that the muscle does not have adequate time to relax completely and contraction force increases

(c)Unfused (incomplete) tetanus: more complete twitch fusion occurs as stimuli are delivered more rapidly

(d)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 ExerciseEndurance 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)

Resistance Exercise

• 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 MusclesMultiple 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

(a)

(b)

(e)

(d)

(g)

(f)

(c)

(a) Circular (orbicularis oris)

(b) Convergent (pectoralis major)

(c) Parallel (sartorius)

(d) Unipennate (extensor digitorum longus)

(f) Fusiform (biceps brachii)

(g) Multipennate (deltoid)

(e) Bipennate (rectus femoris)

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