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THE MUSCULAR SYSTEM: SKELETAL MUSCLE TISSUE AND MUSCLE ORGANIZATION. Muscle Tissue: Functions . 1. producing body movements integrated action of skeletal muscle, joints and bones 2. stabilizing body positions skeletal muscle contraction stabilizes joints and bones

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Muscle Tissue: Functions ORGANIZATION

  • 1. producing body movements

    • integrated action of skeletal muscle, joints and bones

  • 2. stabilizing body positions

    • skeletal muscle contraction stabilizes joints and bones

    • postural muscles contract continuously when awake

  • 3. storing and moving substances in the body

    • contraction of ring-like smooth muscle sphincters – storage of material in an organ

    • storage of glucose within skeletal muscle

    • movement of blood by cardiac muscle and by smooth muscle within the blood vessels

    • movement of food through the GI tract by smooth muscles within abdominal viscera


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Properties of Muscles ORGANIZATION

Electrical excitability

-ability to respond to stimuli by producing electrical signals

such as action potentials

-two types of stimuli: 1. autorhythmic electrical signals

2. chemical stimuli

Contractility

-ability to contract when stimulated by an AP

-isometric contraction: tension develops, length doesn’t change

-isotonic contraction: tension develops, muscle shortens

Extensibility

-ability to stretch without being damaged

-allows contraction even when stretched

Elasticity

-ability to return to its original length and shape


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Gross Anatomy ORGANIZATION

  • muscle is wrapped in a protective fascia

  • -fascia = sheet of fibrous connective tissue

  • that supports and surrounds muscle or organs

  • a superficial fascia separates muscle from the overlying skin

  • -also known as the subcutaneous layer

  • -made up of areolar tissue and adipose tissue

  • -provides support for blood vessel and nerves

  • -the adipose tissue stores most of the body’s triglycerides

  • and provides insulation

  • muscles with similar functions are grouped and held together by layers of deep fascia

  • -dense irregular connective tissue

  • -allow free movement of muscles, carries nerves, BVs


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Gross Anatomy ORGANIZATION

  • muscles are really groups of

  • fascicles

  • the fascicles are groups of muscle fibers = considered to be an individual muscle cell

  • three layers of connective tissue extend from the deep fascial layer

    • Epimysium

    • Perimysium

    • Endomysium

  • these layers further strengthen and protect muscle

  • outermost layer = epimysium

    • encircles the entire muscle

  • next layer = perimysium

    • surrounds groups of 10 to 100 individual muscle fibers

    • separates them into bundles = fascicles

    • give meat its “grain” because the fascicles are visible

    • both epimysium and perimysium are dense irregular connective tissue


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  • all three of these connective tissue layers extend beyond the muscle

  • and attaches it to other structures

  • -called a tendon = cord of regular dense CT that attaches

  • a muscle to the periosteum of bone

  • when the CT extends as a broad flat sheet = aponeurosis

  • the muscle fiber is made up

  • of fused muscle cells

  • -these muscle cells have a unique cytoskeleton

  • made up of myofibrils

  • each myofibril is comprised of

  • repeating units of protein filaments = sarcomeres


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  • at the NMJ the neuron ends at an axon terminal with synaptic bulbs for

  • the release of neurotransmitters -> contraction


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Microanatomy of Muscle Fibers

  • New terminology

    • Cell membrane = sarcolemma

    • Cytoplasm = sarcoplasm

    • Internal membrane system = sarcoplasmic reticulum


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Muscle Cell Anatomy

  • Transverse tubules

    • Invaginations of sarcolemma

    • Carry electrical impulses

  • Myofibrils within sarcoplasm

    • Protein filaments

    • Sarcomeres – organized arrangement

  • Myofilaments form myofibrils

    • Thin filaments (actin, troponin, tropomyosin)

    • Thick filaments (myosin)


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Microanatomy of Muscle Fibers

  • New terminology

    • Cell membrane = sarcolemma

    • Cytoplasm = sarcoplasm

    • Internal membrane system = sarcoplasmic reticulum

  • Large, multinucleated cells

    • embryonic development - muscle fibers arise from fusion of a hundred or more mesodermal cells called myoblasts – organized into muscle fibers composed of myofibrils

    • once fused, these muscle cells lose the ability of undergo mitosis - number of muscle cells predetermined before birth


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Microanatomy of Muscle Fibers

  • muscle fibers are bound by a plasma membrane = sarcolemma

  • thousands of tiny invaginations in this sarcolemma called T or transverse

  • tubules - tunnel in toward the center of the cell

  • -T tubules are open to the outside of the fiber - continuous with

  • the sarcolemma

  • - filled with interstitial fluids

  • action potentials travel along the sarcolemma and the T tubules

  • - allows for the even and quick spread of an action potential

  • the cytoplasm is called a sarcoplasm

  • -substantial amounts of glycogen - can be broken into glucose

  • -contains myoglobin - binds oxygen needed for muscle ATP

  • production


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Microanatomy of Muscle Fibers

  • contractile elements of the muscle fibrils = myofilaments

  • -2 microns in diameter

  • -comprised of actin or myosin

  • -give the muscle its striated appearance

  • fibers have a system of fluid-filled membranes = sarcoplasmic

  • reticulum

  • -encircles each myofibril

  • -similar to the ER

  • -have dilated end sacs = terminal cisterns

  • -stores calcium when at rest - releases it during contraction

  • -release is triggered by an AP


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M line

Sarcomere Structure

  • sarcomere = regions of myosin (thick myofilament) and actin (thin myofilament)

  • bounded by the Z line (actinin)

  • actin filaments project out from Z line

  • myosin filaments lie in center of sarcomere - overlap with actin and connect

  • via cross-bridges

  • myosin only region = H zone

  • myosin filaments are held in place by the M line proteins.

  • actin region = I band

  • length of myosin filaments = A band

  • contraction = “sliding filament theory”

  • -actin and myosin myofilaments slide over each other and sarcomere shortens


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The Proteins of Muscle

  • Myofibrils are built of 3 kinds of proteins

    • contractile proteins

      • myosin and actin

    • regulatory proteins which turn contraction on & off

      • troponin and tropomyosin

    • structural proteins which provide proper alignment, elasticity and extensibility

      • titin, myomesin, nebulin and dystrophin


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Structural proteins of muscle

  • Nebulin, an inelastic protein helps align the thin filaments.

  • Dystrophin links thin filaments to sarcolemma and transmits the tension generated to the tendon.

  • Titin anchors thick filament to the M line and the Z line.

  • -the portion of the molecule between the Z line and the end of the thick filament can stretch to 4 times its resting length and spring back unharmed.

  • -has a role in recovery of the muscle from being stretched.


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Contractile proteins of muscle

  • actin filament is associated with troponin and tropomyosin

  • the myosin-binding site on each actin molecule is covered by tropomyosin in relaxed muscle

  • myosin thick myofilament is a bundle of myosin molecules

  • -each myosin protein has 2 globular “heads” each with a site to bind ATP and to bind actin


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Contraction: The Sliding Filament Theory

  • Contraction:

    • Active process

    • Elongation is passive

    • Amount of tension produced is proportional to degree of overlap of thick and thin filaments

  • SF Theory:

    • Explains how a muscle fiber exerts tension

    • Four step process

      • Active sites on actin

      • Crossbridge formation

      • Cycle of attach, pivot, detach, return

      • Troponin and tropomyosin control contraction


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-calcium binds to troponin and exposes sites that can interact with myosin

- Ca+2 binds to troponin & causes troponin-tropomyosin complex to move & reveal

myosin binding sites on actin

-ATP binding, hydrolysis and ADP release changes the conformation of the head (“power

stroke”) and causes actin to “slide” along the myosin myofilaments

-shortens the distance between the Z lines


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Sarcoplasmic Reticulum and Calcium release

  • the SR wraps around each A and I band

    • segmented with T-tubules between each SR segment

  • each segment forms saclike regions at the ends = lateral sacs

  • between the lateral sac and the T-tubule is an orderly arrangement of proteins = foot proteins (ryanodine receptors)

    • these foot proteins bridge the gap between SR and T-tubule

    • serve as Ca release channels

    • 50% of the foot proteins of the SR are “zipped” together with similar proteins found on the T-tubule (dihydropyridine receptors)

    • the T-tubule receptors respond to changes in voltage – voltage-gated sensors

    • when an AP travels down the T-tubule – the local depolarization activates these sensors which then open the foot proteins on the SR

    • the opening of these foot proteins triggers the SR to open the remaining foot proteins that are not connected to the T-tubule

    • efflux of calcium into the sarcoplasm


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The Neuromuscular Junction

  • end of neuron (synaptic terminal or axon bulb) is in very close association

  • with a single muscle fiber (cell)

  • nerve impulse leads to release of neurotransmitter(acetylcholine) from the synaptic end terminal

  • AcH binds to receptors on myofibril surface (ligand-gated Na channels)

  • binding leads to influx of sodium ions and depolarization of the membrane potential of the sarcolemma

  • creation of an action potential that travels through the muscle cell – eventual contraction

  • Acetylcholinesterase breaks down ACh

  • Limits duration of contraction


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Muscle Contraction: A summary

  • called excitation-contraction coupling – describes the events linking generation of an AP (excitation) to the contraction of the muscle

  • ACh released from synaptic vesicles at each neuromuscular junction

  • Binding of ACh to motor end plate (muscle cell of the NMJ)

    • entrance of Na ions and depolarization

  • Generation of electrical impulse in sarcolemma

    • action potential

  • Conduction of impulse along T-tubules

    • AP flows along the outside of the muscle cell via the sarcolamma

    • also enters the inside of the muscle cell via T-tubules

    • close association of T-tubules with the sarcoplasmic reticulum (SR)

  • Release of Calcium ions by SR

    • AP results in release of Ca by the SR

    • SR is in close physical association with each A and I band

    • Ca binds to troponin and “pulls it away” from the actin filament

  • Exposure of active sites on actin

  • Cross-bridge formation with myosin

  • Formation of ATP by the muscle cell

    • sliding filaments & contraction



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Relaxation

  • Acetylcholinesterase (AChE) breaks down ACh within the synaptic cleft

  • Muscle action potential ceases

  • Ca+2 release channels (foot proteins) close

  • Active transport pumps Ca2+ back into storage in the sarcoplasmic reticulum

    • Ca ATPase pumps

    • the rate of pumping Ca back into the SR is slower than the rate of efflux

    • so as long as the muscle is being stimulated via the T-tubules – more Ca in the sarcoplasm

  • Calcium-binding protein (calsequestrin) helps hold Ca+2 in SR

    • enables more calcium to be stored in the SR

      • calcium concentration is 10,000 more concentrated in the SR than in the sarcoplasm

  • Tropomyosin-troponin complex recovers binding site on the actin


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Rigor Mortis

  • Rigor mortis is a state of muscular rigidity that begins 3-4 hours after death and lasts about 24 hours

  • After death, Ca+2 ions leak out of the SR and allow myosin heads to bind to actin

  • Since ATP synthesis has ceased, crossbridges cannot detach from actin until proteolytic enzymes begin to digest the decomposing cells.


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Length of Muscle Fibers: Length Tension relationship

  • Normally

    • resting muscle length remains between 70 to 130% of the optimum

  • Optimal overlap of thick & thin filaments

    • produces greatest number of crossbridges and the greatest amount of tension

    • optimal length = lo (muscle length at which maximum force is generated)

    • optimal length = point A

  • As stretch muscle (past optimal length)

    • length of the muscle fiber is greater than lo

    • fewer cross bridges exist & less force is produced = point B

    • when muscle is stretched to about 70% than lo of its (point C) the actin filaments are completely pulled out from between the myosin – no cross-bridges possible

  • If muscle is overly shortened (less than optimal)

    • length of the muscle fiber is less than lo

    • thick filaments crumpled by Z discs and the actin filaments overlap – poor cross-bridge formation

    • fewer cross bridges exist & less force is produced = point D

    • even less calcium released from the SR - ??

A

B

D

C



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Motor Units

  • Each skeletal fiber has only ONE NMJ

  • MU = Somatic neuron + all the skeletal muscle fibers it innervates

  • Number and size indicate precision of muscle control

  • Muscle twitch

    • Single momentary contraction in one muscle fiber

    • too small to generate any significant force

    • Response to a single stimulus

  • All-or-none theory

    • Either contracts completely or not at all

  • Motor units are grouped together to provide a greater force

    • in a whole muscle fire asynchronously

    • -some fibers are active others are relaxed

    • -delays muscle fatigue so contraction can be sustained

  • Muscle fibers of different motor units are intermingled so that net distribution of force applied to the tendon remains constant even when individual muscle groups cycle between contraction and relaxation.


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Neural control of Motor Units

  • 1. input from afferent neurons

    • at the level of the SC

    • by interneurons within the SC = spinal reflex

    • afferent information is needed to control skeletal muscle activity

    • the CNS must know the position of your body prior to initiating movement and must know how the movement is progressing = prioprioceptive input

    • comes from information from your eyes, joints, inner ear and from the muscles themselves (prioprioceptors)

    • muscle spindles and tendon organs within the muscle monitor changes in muscle length and tension (see lecture 9)

  • 2. input from the motor cortex

    • fibers originating from neuronal cell bodies within the primary motor cortex = pyramidal cells

    • descend directly (as one continuous axon) to synapse with motor neurons in the SC

    • part of the corticospinal motor system (lecture 8)

  • 3. input from the brain stem

    • extrapyramidal motor system

    • involves many regions of the brain

    • final link is the brain stem


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  • 1. muscle spindles

    • monitor changes in muscle length

    • used by the brain to set an overall level of involuntary muscle contraction = motor tone

    • consists of several sensory nerve endings that wrap around specialized muscle fibers = intrafusal muscle fibers

      • very plentiful in muscles that produce very fine movements – fingers, eyes

      • stretching of the muscle stretches the intrafusal fibers, stimulating the sensory neurons – info to the CNS

      • IFMs also receive incoming information from gamma motor neurons – end near the IFMs and adjust the tension in a muscle spindle according to the CNS

    • also have extrafusal muscle fibers which are innervated by alpha motor neurons

      • response to a stretch reflex

  • 2. tendon organs

    • located at the junction of a tendon and a muscle

    • protect the tendon and muscles from damage due to excessive tension

    • consists of a thin capsule of connective tissue enclosing a few bundles of collagen

      • penetrated by sensory nerve endings that intertwine among the collagen fibers


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Motor Tone

  • Resting muscle contracts random motor units

    • Constant tension created on tendon

    • Resting tension – muscle tone

  • Stabilizes bones and joints


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Muscle Metabolism

  • Production of ATP:

  • -contraction requires huge amounts of ATP

  • -muscle fibers produce ATP three ways:

  • 1. Creatine phosphate

  • 2. Aerobic metabolism

  • 3. Anaerobic metabolism


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Creatine Phosphate

  • Muscle fibers at rest produce more ATP then they need for resting metabolism

  • Excess ATP within resting muscle used to form creatine phosphate or phosphocreatine

  • Creatine phosphate: 3-6 times more plentiful than ATP within muscle

  • the first storehouse of energy used upon the onset of contraction when additional ATP is needed

  • Its quick breakdown provides energy for creation of ATP

  • Sustains maximal contraction for 15 sec (used for 100 meter dash)

    • or about 8 muscle twitches

    • creatine phosphate breakdown is favored by muscles undergoing explosive movements

  • Athletes tried creatine supplementation

    • gain muscle mass but shut down bodies own synthesis (safety?)


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Aerobic Cellular Respiration

  • Muscles deplete creatine – make ATP in anaerobically or aerobically

  • aerobic respiration produces ATP for any activity lasting over 30 seconds

    • if sufficient oxygen is available, pyruvic acid enters the mitochondria to generate ATP, water and heat via the electron transport chain

    • fatty acids and amino acids can also be used by the mitochondria

  • Provides 90% of ATP energy if activity lasts more than 10 minutes

  • also can keep pace with moderate activities like walking

  • Each glucose = 36 ATP

  • Fatty acids = ~100 ATP

  • Sources of oxygen – diffusion from blood, released by myoglobin (hemoglobin-like molecule of muscle cells)


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Anaerobic Cellular Respiration

  • Muscles deplete creatine – make ATP in anaerobically via glycolysis only

  • Glycogen converted into glucose

  • normally ATP produced from the breakdown of glucose into pyruvic acid during glycolysis and this enters the citric acid cycle and electron transport chain to make ATP

  • if insufficient oxygen is present glycolysis creates the products for oxidative phosphorylation

    • glucose is broken down into two pyruvic acid molecules to yield 2 ATP

    • BUT in low oxygen this pyruvic acid is further processed to yield more ATP

    • by-product = lactic acid

  • Glycolysis can continue anaerobically to provide ATP for 30 to 40 seconds of maximal activity (200 meter race)

http://www.indstate.edu/thcme/mwking/oxidative-phosphorylation.html


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Muscle Fatigue

  • Inability to contract after prolonged activity

    • central and peripheral fatigue

    • central fatigue is feeling of tiredness and a desire to stop (protective mechanism)

  • Factors that contribute to muscle fatigue

    • depletion of creatine phosphate

    • decline of Ca+2 within the sarcoplasm

    • insufficient oxygen or glycogen

    • accumulation of extracellular K ions

    • drop in pH within muscle cell

    • buildup of lactic acid

    • buildup of ADP and inorganic phosphate from ATP hydrolysis

    • insufficient release of acetylcholine from motor neurons


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Isotonic and Isometric Contraction

  • Isotonic contractions = a load is moved

    • concentric contraction = a muscle shortens to produce force and movement

    • eccentric contractions = a muscle lengthens while maintaining force and movement

  • Isometric contraction = no movement occurs

    • tension is generated without muscle shortening

    • maintaining posture & supports objects in a fixed position


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  • Atrophy

    • wasting away of muscles

    • caused by disuse (disuse atrophy) or severing of the nerve supply (denervation atrophy)

    • the transition to connective tissue can not be reversed

  • Hypertrophy

    • increase in the diameter of muscle fibers

    • resulting from very forceful, repetitive muscular activity and an increase in myofibrils, SR & mitochondria


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Exercise-Induced Muscle Damage

  • Intense exercise can cause muscle damage

    • electron micrographs reveal torn sarcolemmas, damaged myofibrils an disrupted Z discs

    • increased blood levels of myoglobin & creatine phosphate found only inside muscle cells

  • Delayed onset muscle soreness

    • 12 to 48 Hours after strenuous exercise

    • stiffness, tenderness and swelling due to microscopic cell damage


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Three Types of Muscle Fibers

  • Fast fibers = fast twitchglycolytic

  • Slow fibers = slow twitchoxidative

    • 10 times slower than fast fibers

  • Intermediate fibers = fast twitch oxidative glycolytic

  • Fibers of one motor unit all the same type

  • Percentage of fast versus slow fibers is genetically determined

  • Proportions vary with the usual action of the muscle

    • - neck, back and leg muscles have a higher proportion of postural, slow oxidative fibers

    • - shoulder and arm muscles have a higher proportion of fast glycolytic fibers


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Fast Fibers

  • Large in diameter

  • Contain densely packed myofibrils

  • Large glycogen reserves

  • high ATPase activity – faster cross-bridge potential

  • Fast oxidative-glycolytic (fast-twitch A) (intermediate fibers)

    • red in color (lots of mitochondria, myoglobin & blood vessels)

    • higher ability to produce ATP via aerobic metabolism

    • highly vascularized

    • split ATP at very fast rate; used for walking and sprinting

  • Fast glycolytic (fast-twitch B)

    • white in color (few mitochondria & BV, low myoglobin)

    • higher concentration of enzymes for glycolysis

    • need less oxygen to function

    • anaerobic movements for short duration; used for weight-lifting

    • fatigue faster than fast-twitch A fibers


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  • Slow fibers

    • Half the diameter of fast fibers

    • Three times longer to contract

    • low ATPase activity, low glycogen content

    • high resistance to fatigue

    • higher ability to produce ATP via aerobic metabolism

    • many mitochondria

    • highly vascularized

    • Continue to contract for long periods of time

      • e.g. marathon runners


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Muscle Adaptation

  • long-term adaptive changes can occur with exercise depending on the pattern of neuronal discharge

  • 1. improvement of oxidative capacity

    • regular aerobic activity

    • induces metabolic changes in the oxidative fibers

    • increases number of mitochondria and capillaries to the fast and slow oxidative fibers

    • more efficient use of oxygen – prolonged activity without fatigue

  • 2. muscle hypertrophy

    • increased by regular bursts of short, anaerobic, high-intensity exercise

    • increases the diameter of the muscle fiber – increase synthesis of myosin and actin

    • exercise triggers the activation of specific genes that direct the synthesis of actin and myosin

    • also a role for muscle stem cells?

  • 3. influence of testosterone

    • makes muscle fibers thicker

    • promotes the synthesis of myosin and actin


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Smooth muscle

  • slowest of contraction and relaxation of all three types of muscle

  • lowest O2 consumption rates

  • require less energy to contract

  • generates force over longer periods of time

    • maximum tension with only 25-30% of cross-bridges “active”

  • can still generate tension even when over-stretched

    • the nonstretched length of smooth muscle is shorter than skeletal

    • therefore it can be stretched quite a distance before the optimal length is reached

    • important for the contractile ability of hollow organs and blood vessels


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Characteristics of Smooth Muscle

  • have more variety

    • with differing properties

    • vascular, gastrointestinal, urinary, respiratory, reproductive, ocular

    • single model of smooth muscle function is impossible

  • anatomy is distinct

    • fibers are arranged in oblique bundles (“lattice-like) so that contractile forces are generated in multiple directions

    • also several organs have multiple layers of SM

  • contraction is controlled by hormones, paracrines and NTs

    • ACh, NE, Epi etc…

  • variable electrical properties

    • do NOT always respond to an AP with a twitch

    • some can hyperpolarize in response

    • others can contract without reaching threshold!!

  • multiple pathways can influence C and R

    • some paths illicit contraction, others inhibit C

    • SM can act as an integrating center – integrating multiple messages and deciding what to do


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Smooth muscle

  • three filaments: myosin, actin and intermediate sized filaments (don’t participate in contraction)

    • forms of actin, myosin are specific to smooth muscle

    • more actin vs. skeletal muscle

    • express tropomyosin only

  • have no sarcomeric structure

    • do not have Z lines

    • the actin and myosin filaments are organized in a lattice-like pattern rather than parallel to each other

    • have dense bodies

      • same proteins as found in Z lines

      • positioned throughout the cell and attach to the internal surface of the PM

      • held in place by the cytoskeleton of the cell

      • act as an anchor for actin filaments


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  • no T-tubule structure and poorly developed sarcoplasmic reticulum

    • neural excitation differs from skeletal muscle cells

    • AP travels along the PM of the smooth muscle cell – opens calcium channels in the PM – flows in from the ECF

    • this increased calcium from the ECF triggers the opening of ryanodine Ca receptors on the SR – these act as calcium channels also

  • can still generate tension even when over-stretched

    • the nonstretched length of smooth muscle is shorter than skeletal

    • therefore it can be stretched quite a distance before the optimal length is reached

    • important for the contractile ability of hollow organs and blood vessels


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  • turned on by calcium-dependent phosphorylation of myosin reticulum

    • since there is no troponin – how does the cell prevent cross-bridge formation at rest?

    • lightweight chains of proteins – myosin light chain proteins – attach to the head of myosin

    • increasing cytosolic calcium initiates a series of biochemical events that phosphorylates the myosin light chain – this allows an interaction between actin and myosin

    • so there is two forms of myosin in muscle cells (skeletal too!) – myosin heavy and myosin light

    • myosin light chains have no function in skeletal muscle contraction


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Smooth muscle reticulum

  • organized as multi-unit or single-unit smooth muscles

    • multi-unit – exhibits neural-like properties

      • muscle fibers within the muscle contract as a unit

      • multiple units per muscle

      • each unit is stimulated by nerves to contract – similar to skeletal muscle motor units

      • so multi-unit smooth muscle is neurogenic – “nerve produced”

      • supplied by the ANS

    • single unit – muscle fibers within the muscle contract as a separate, single unit

      • found in cell walls of organs and blood vessels

      • cells are linked by gap junctions for spread of AP

      • interconnected cells form a functional syncytium

      • myogenic – self excitable

        • clusters of cells exhibit spontaneous electrical activity without neural stimulation

          • clusters are specialized to initiate an AP BUT arenot specialized to contract

        • their membrane potential fluctuates automatically without any external influence

        • two type of spontaneous depolarizations: pacemaker and slow-wave potentials


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Smooth muscle reticulum

  • smooth muscle tone:

    • single-unit smooth muscle interconnection ensures that the entire muscle contracts upon initiation of an AP by a unit

    • can’t vary the number of muscle fibers contracting

    • BUT can vary the tension

      • varying the cytosolic calcium can alter the number of eventual cross-bridges that form – alters strength of contraction

      • many single units have sufficient calcium within their cytosol to ensure a low level of constant contraction = tone

  • smooth muscle activity:

    • innervated by the ANS

    • does not initiate the contraction

    • but modifies the rate and strength of contraction of single-unit smooth muscle

    • also can be modified by: hormones, muscle stretch, drugs

      • all act by modifying the permeability of the PM to calcium in the ECF