• Body movement

• Maintenance of posture

• Respiration

• Production of body heat

• Communication

• Constriction of organs and vessels

• Heart beat

• Contractility: ability of a muscle to shorten with force

• Excitability: ability of muscle to receive & respond to stimuli

• Extensibility: muscle can be stretched beyond its normal resting length and is still able to contract

• Elasticity: ability of muscle to recoil to original resting length after stretched

• Muscles are responsible for all types of body movement

• Three basic muscle types are found in the body– Skeletal muscle– Cardiac muscle– Smooth muscle

• These types differ in structure, location, function, and means of activation

• Skeletal and smooth muscle cells are elongated and are called muscle fibers

• Muscle contraction depends on two kinds of myofilaments – actin and myosin

• Muscle terminology is similar– Prefixes – myo, mys, and sarco all refer to

muscle– Sarcolemma – muscle plasma membrane– Sarcoplasm – cytoplasm of a muscle cell

• Skeletal muscles that attach to and cover the bony skeleton

• Has obvious stripes called striations

• Is controlled voluntarily (i.e., by conscious control)

• Contracts rapidly but tires easily

• Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement

• Is extremely adaptable and can exert forces ranging from a fraction of an ounce to over 70 pounds

• Found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages

• Some functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow

• Not striated

• Controlled involuntarily by endocrine and ANS

• Occurs only in the heart

• Striated like skeletal muscle

• Involuntary

• Heart: major source of movement of blood

• Autorhythmic

• Controlled involuntarily by endocrine and ANS

• Composed of muscle cells (fibers), connective tissue, blood vessels, nerves

• Fibers are long, cylindrical, multinucleated

• Striated appearance due to light and dark banding

• The three connective tissue sheaths are:– Endomysium – fine sheath of

connective tissue composed of reticular fibers surrounding each muscle fiber

– Perimysium – fibrous connective tissue that surrounds groups of muscle fibers called fascicles

– Epimysium – Dense regular connective tissue that surrounds the entire muscle (many fascicles)

• Each muscle is served by one nerve, an artery, and one or more veins

• Each skeletal muscle fiber is supplied with a nerve ending that controls contraction

• Contracting fibers require continuous delivery of oxygen and nutrients via arteries

• Wastes must be removed via veins

• Most skeletal muscles span joints and are attached to bone in at least two places

• When muscles contract the movable bone, the muscle’s insertion moves toward the immovable bone, the muscle’s origin

• Muscles attach:– Directly – epimysium of the muscle is

fused to the periosteum of a bone

– Indirectly – connective tissue wrappings extend beyond the muscle as a tendon or aponeurosis

• Outer boundary of the cell is made of plasma membrane – sarcolemma

• Multiple nuclei present inside sarcolemma

• Cytoplasm of muscle cell - sarcoplasm

• Endoplasmic reticulum – Sarcoplasmic reticulum

• Most of muscle fiber is packed with myofibrils

• Other organelles, such as mitochondria, glycogen granules, sarcoplasmic reticulum, and T tubules are found between myofibrils

• Myofibrils: are thread like organelles

– Composed of protein threads called myofilaments:

– thin (actin) – thick (myosin)

– Sarcomeres: repeating units of myofilaments

• Interaction between actin and myosin filaments leads muscle to shorten or contract

• The tropomyosin/troponin complex regulates the interaction between actin and myosin

• Consists of two strands of fibrous (F) actin, troponin and tropomyosin molecules

• Strands of F actin coiled to form double helix

• Each F actin strand is composed of G actin monomers each of which has an active site

• Each F actin strand consists of 200 G actin monomers

• Active site binds with myosin during muscle contraction

• Tropomyosin: an elongated protein winds along the groove of the F actin double helix

• Troponin is composed of three subunits: one that binds to actin, a second that binds to tropomyosin, and a third that binds to calcium ions

• The tropomyosin/troponin complex regulates the interaction between active sites on G actin and myosin

• Thick filaments are composed of the protein myosin

• Shaped like golf clubs • Each myosin molecule has a rod-

like tail and two globular heads

– Tails – two interwoven, heavy polypeptide chains

– Heads – Each head contains two smaller, light polypeptide chains

• Each myosin filaments consists of about 300 myosin molecules

• Properties of Myosin heads

1. Can bind to active sites on the actin molecules to form cross-bridges

2. Attached to the rod portion by a hinge region, bend and straighten during contraction

3. Heads have ATPase activity, activity that breaks down ATP, releasing energy

• The smallest contractile unit of a muscle

• The region of a myofibril between two Z discs

• The point where actin originates is called Z disk

• Each sarcomere has alternating actin and myosin filaments

• The arrangement of myosin (dark in color and is called anisotropic band or A band)

• And actin (light in color and is called isotropic or I band) alternatively

• Gives the muscle a striated appearance

• H zone (bare zone) - lacks actin filament

• M line: middle of H zone; delicate filaments holding myosin in place

• Upon stimulation, myosin heads bind to actin and sliding begins

• Actin myofilaments slide over myosin to shorten sarcomeres

– Actin and myosin do not change length

– Shortening sarcomeres responsible for skeletal muscle contraction

• Nervous system controls muscle contractions through action potentials

• Electrical signals, called action potentials travel from brain or SC via axons of the nerve to muscle fibers and cause them to contract

• Skeletal muscles are stimulated by motor neurons

• Axons of neurons branch (axon terminals) as they enter muscles

• Each axonal branch forms a neuromuscular junction with a single muscle fiber

• Neuromuscular junctions – association site of nerve and muscle

• The neuromuscular junction is formed from:

– Axonal endings, which have small membranous sacs (synaptic vesicles) that contain the neurotransmitter acetylcholine (ACh)

– Sarcolemma of the muscle fiber is highly folded that contains ACh receptors and helps form the neuromuscular junction

• Axonal ends and muscle fibers are always separated by a space called the synaptic cleft

• When a nerve impulse reaches the end of an axon at the neuromuscular junction:

– Voltage-regulated calcium channels open and allow Ca2+ to enter the axon

– Ca2+ inside the axon terminal causes synaptic vesicles to fuse with the axonal membrane

– This fusion releases ACh into the synaptic cleft via exocytosis

– ACh diffuses across the synaptic cleft to ACh receptors on the sarcolemma

– Binding of ACh to its receptors initiates an action potential in the muscle

• ACh bound to ACh receptors is quickly destroyed by the enzyme acetylcholinesterase

• This destruction prevents continued muscle fiber contraction in the absence of additional stimuli

• At rest, membranes are polarized

• There is voltage difference across

membranes

• Difference between charge inside and outside cell membrane = RESTING MEMBRANE POTENTIAL (RMP)

• Inside of cell membrane is more negative charge than outside

• Due to the presence of more positive ions (Na+) outside the cell

• Axonal terminal of a motor neuron releases ACh and binds to the receptors of sarcolemma

• Causes the opening of Sodium channels

• Sodium ions enter rapidly– RMP becomes more positive

• If the stimulus is strong enough, reaches threshold

• Depolarization occurs

• An action potential is initiated

• Thus, the action potential travels rapidly along the sarcolemma

• Once initiated, the action potential is unstoppable, and ultimately results in the contraction of a muscle

• Immediately after the depolarization wave passes, the sarcolemma permeability changes

• Na+ channels close and K+ channels open

• K+ diffuses from the cell

• Repolarization takes place

• The ionic concentration of the resting state

is restored by the Na+-K+ pump

• All-or-none principle: like camera flash system

• AP produced in sarcolemma of muscle lead to muscle fiber contraction by Excitation-Contraction Coupling

• Involves

– Sarcolemma– Transverse (T) tubules: invaginations

of sarcolemma– Sarcoplasmic reticulum: smooth ER– Terminal cisternae: Enlarged SR– Triad: T tubule, two adjacent terminal

cisternae– Ca2+

– Troponin

• AP produced at neuromuscular junction:– Is propagated along the

sarcolemma– Travels down the T tubules– Triggers Ca2+ release from

terminal cisternae

• Ca2+ released from SR binds to troponin and causes: – The blocking action of

tropomyosin to cease

• Active binding sites of actin are exposed

• Myosin heads attach with actin and forms cross bridges

• Thin filaments move toward the center of the sarcomere

• Hydrolysis of ATP powers this cycling process

• Ca2+ is removed into the SR, tropomyosin blockage is restored, and the muscle fiber relaxes

• A muscle twitch is contraction of muscle in response to a stimulus that causes action potential in one or more muscle fibers

• There are three phases to a muscle twitch– Lag (latent) phase– Contraction phase– Relaxation phase

• Lag phase – first few msec after stimulus; EC coupling taking place

• Contraction phase – cross bridges form; muscle shortens

• Relaxation phase – Reentry of Ca2+ in SR

• In response to each action potential muscle fiber produces contraction of equal force

• Follow All-or-none law - muscle fibers

• Sub-threshold stimulus: no action potential; no contraction

• Threshold stimulus: action potential; contraction

• Stronger than threshold : action potential; contraction equal to threshold stimulus

• Muscle fiber contraction is “all or none”

• But in whole muscle not all fibers may be stimulated during the same interval

• Different combinations of muscle fiber contractions may give differing responses – Graded Response

• Motor units (Nerve-Muscle Functional Unit) : a single motor neuron and all muscle fibers innervated by it

• Strength of contraction is graded: ranges from weak to strong depending on stimulus strength

• Multiple motor unit summation: strength of contraction depends upon no. of motor units

• A muscle has many motor units

– Submaximal stimuli– Maximal stimulus– Supramaximal stimuli

• As the frequency of action potentials increase, the frequency of contraction increases– Incomplete tetanus: muscle

fibers partially relax between contraction

– Complete tetanus: no relaxation between contractions

– Multiple-wave summation: muscle tension increases as contraction frequencies increase

• Graded response

• Occurs in muscle rested for prolonged period

• Each subsequent contraction is stronger than previous until all equal after few stimuli

• Possible explanation: more and more Ca2+ remains in sarcoplasm and is not all taken up into the sarcoplasmic reticulum

• Isometric Contraction: no change in muscle length but tension increases during contraction– Eg. Postural muscles of body, muscles that hold the spine erect while

a person is sitting or standing

• Isotonic Contraction: change in length but tension constant during contraction, eg. Movement of upper limbs, fingers such as waving, using a computer

– Concentric: Tension in muscle overcomes opposing resistance and muscle shortens, eg. Lifting a loaded backpack from the floor to table

– Eccentric: tension maintained, enough opposing resistance to cause the muscle to increase in length, eg. Person slowly lowers the heavy weight

• Decreased capacity to work and reduced efficiency of performance

• Types– Psychological: depends on emotional state of

individual– Muscular: results from ATP depletion – Synaptic: occurs in Neuromuscular junction due to lack

of acetylcholine – Physiological contracture: state of fatigue where due

to lack of ATP neither contraction nor relaxation can occur

– Rigor mortis: development of rigid muscles several hours after death. Ca2+ leaks into sarcoplasm and attaches to myosin heads and cross bridges form. Rigor ends as tissues start to deteriorate

• ATP provides immediate energy for muscle contractions

• Produced from three sources

– The interaction of ADP with creatine phosphate (CP) – Anaerobic respiration – Aerobic respiration

• Direct phosphorylation of ADP by creatine phosphate

– Muscle cells contain creatine phosphate (CP)

• CP is a high-energy molecule

– ADP is left, after ATP is depleted,

– CP transfers energy to ADP, to regenerate ATP

– CP supplies are exhausted in about 15 seconds

• Aerobic Respiration

– Series of metabolic pathways occur in the mitochondria, require oxygen

– Known as oxidative phosphorylation

– Glucose is broken down to carbon dioxide and water, & release energy in the form of ATP

– This is a slower reaction that requires continuous oxygen and nutrient fuel

– 36 ATP/ glucose

• Anaerobic glycolysis– Reaction that breaks

down glucose without oxygen

– Glucose is broken down to pyruvic acid to produce some ATP

– Pyruvic acid is converted to lactic acid

• Anaerobic glycolysis

• This reaction is not as efficient, but is fast

• 2ATP/glucose

• Huge amounts of glucose are needed

• Lactic acid produces muscle fatigue

• Vigorous exercise causes dramatic changes in muscle chemistry

• For a muscle to return to a resting state:

– Oxygen reserves must be replenished– Lactic acid must be converted to pyruvic acid– Glycogen stores must be replaced– ATP and CP reserves must be resynthesized

• Slow-twitch oxidative– Contract more slowly, smaller in diameter, better blood

supply, more mitochondria, more fatigue-resistant than fast-twitch, large amount of myoglobin.

– Postural muscles, more in lower than upper limbs. Dark meat of chicken.

• Fast-twitch – Respond rapidly to nervous stimulation, contain myosin

ATPase that can break down ATP more rapidly than that in Type I, less blood supply, fewer and smaller mitochondria than slow-twitch

– Lower limbs in sprinter, upper limbs of most people. White meat in chicken.

• Not striated, fibers smaller than those in skeletal muscle

• Spindle-shaped; single, central nucleus

• More actin than myosin

• Caveolae: indentations in sarcolemma; may act like T tubules

• Ca2+ required to initiate contractions; binds to calmodulin which regulates myosin kinase. Cross-bridging occurs

• Relaxation: caused by enzyme myosin phosphatase

Smooth Muscles• Arranged in two layers:• circular layer• longitudinal layer

• These two layers alternately contract and relax

• And move food through digestive tract, emptying the bowels & bladder

• Maintain housekeeping activities

• Slow and steady

• Visceral or unitary: cells in sheets; function as a unit– eg. Digestive, reproductive, urinary tracts– Numerous gap junctions– Allow Action potential to pass from cell to cell– Often autorhythmic

• Multiunit: cells or groups of cells act as independent units– Sheets (blood vessels); bundles (arrector pili and iris);

single cells (capsule of spleen)– Fewer gap junctions– Contracts when stimulated by nerves or hormones

• Slow waves of depolarization and repolarization transferred from cell to cell

• Depolarization caused by spontaneous diffusion of Na+ and Ca2+ into cell

• Does not follow all-or-none law

• Contraction regulated by nervous system and by hormones

• Some visceral muscle exhibits autorhythmic contractions

• Tends to contract in response to sudden stretch but not to slow increase in length

• Exhibits relatively constant tension: smooth muscle tone

• Amplitude of contraction remains constant although muscle length varies

• Innervated by autonomic nervous system

• Neurotransmitters are acetylcholine and norepinephrine

• Hormones important as epinephrine and oxytocin

• Receptors present on plasma membrane to which neurotransmitters or hormones bind determines the response of smooth muscle

• Found only in heart• Striated• Each cell usually has one

nucleus• Has intercalated disks and gap

junctions• Autorhythmic cells• Action potentials of longer

duration and longer refractory period

• Ca2+ regulates contraction

• Reduced muscle mass

• Increased time for muscle to contract in response to nervous stimuli

• Reduced stamina

• Increased recovery time

• Loss of muscle fibers

• Decreased density of capillaries in muscle