Chapter 9 Muscle Tissue

Lab exam next Thurs. 2/12 CPR practice, essay entry complete this morning, remainder due next Tuesday Knee dissection Tuesday2/5

Objectives

          Discuss organization and functions of muscle as an organ including all tissue types Describe the structural modifications of muscle cells and their functional significance Describe the neuromuscular junction and events Discuss the sliding filament theory Describe a motor unit and neural control of muscle Explain the link between anatomy & physiology exemplified in the length/tension curve Compare and contrast the three types of skeletal muscle cells and relate these to muscular performance Compare and contrast the three types of muscle tissue Discuss developmental changes of muscle Human Anatomy Spring 2012 2 Apply knowledge of levers to human skeletal muscles(if time)

Muscle Function

1.

2.

3.

4.

• Movement – including moving substances within body    Contract against resistance Skeletal move against bone Cardiac move against fluid – blood Smooth move against other contents Muscles also maintain posture & stabilize joints Regulate organ volume Generate heat Human Anatomy Spring 2012 3

Functional Characteristics of Muscle Tissue

 Contractility – the ability to shorten forcibly is the unique feature of muscle  Excitability, or irritability – the ability to receive and respond to stimuli, have action potentials like neurons  Extensibility – the ability to be stretched or extended  Elasticity – the ability to recoil and resume the original resting length Human Anatomy Spring 2012 4

Tendon Epimysium Muscle fascicle Endomysium Perimysium Nerve Muscle fibers Blood vessels SKELETAL MUSCLE (organ) Perimysium Muscle fiber Endomysium Epimysium Blood vessels and nerves Perimysium MUSCLE FASCICLE (bundle of cells) Mitochondria Endomysium Sarcolemma Myofibril Axon Sarcoplasm MUSCLE FIBER (cell) Capillary Endomysium Myosatellite cell Nucleus

Human Anatomy Spring 2012 5

Human Anatomy 6

Skeletal Muscle Organs

  Organs include muscle tissue, blood vessels, nerve fibers, and connective tissue The three connective tissue wrappings are:    Epimysium – an overcoat of dense regular CT that surrounds the entire muscle Perimysium fibers called – fibrous CT that surrounds groups of muscle

fascicles

Endomysium – fine sheath of CT composed of reticular fibers surrounding each muscle fiber Human Anatomy Spring 2012 7

Skeletal Muscle: Nerve and Blood Supply

 Each muscle is served by at least one nerve, artery, and vein  Each skeletal muscle fiber is supplied with a nerve ending that controls contraction – a neuromuscular junction  Contracting fibers require continuous delivery of oxygen and nutrients via arteries  Wastes must be removed via veins Human Anatomy Spring 2012 8

Skeletal Muscle: Attachments

 Muscles span joints and are attached in at least two places  When muscles contract the movable bone, the muscle’s insertion moves toward the immovable bone – the muscle’s origin (i.e., origin is stationary — flawed concept, but customary )  Muscles attach:   Directly – epimysium of the muscle is fused to the periosteum of a bone Indirectly (more common) aponeurosis – CT wrappings extend beyond the muscle as ropelike tendon or sheetlike Human Anatomy Spring 2012 9

Microscopic Anatomy of a Skeletal Muscle Fiber

 Each fiber is a long, cylindrical cell with multiple  nuclei just beneath the sarcolemma Fibers are 10 to 100  m in diameter, and up to hundreds of centimeters long  Each cell is a syncytium produced by fusion of myoblasts (embryonic cells) 

Sarco

plasm has a unique oxygen-binding protein called

myoglobin

 Fibers contain the usual organelles plus myofibrils, sarcoplasmic reticulum, and T tubules Human Anatomy Spring 2012 10

Myoblasts Syncytium Note satellite cells

Human Anatomy Spring 2012 Muscle Cell Formation 11

Sarcoplasmic Reticulum (SR)

     SR is an elaborate smooth endoplasmic reticulum that mostly runs longitudinally and surrounds each myofibril Paired terminal cisternae form perpendicular cross channels Functions in the regulation of intracellular calcium levels Elongated tubes called

T tubules

each A band –I band junction penetrate into the cell’s interior at T tubules associate with the paired terminal cisternae to form triads

Mitochondria Terminal cisterna Sarcolemma Sarcolemma Myofibril Sarcoplasm Myofibrils Thin filament Thick filament Internal organization of a muscle fiber.

Note the relationships among myofibrils, sarcoplasmic reticulum, mitochondria, triads, and thick and thin filaments.

Triad Sarcoplasmic reticulum T tubules

Human Anatomy Spring 2012 12

T Tubules

 T tubules are continuous with the sarcolemma  They conduct impulses to the deepest regions of the muscle  These impulses signal for the release of Ca 2+ from adjacent terminal cisternae

Mitochondria Terminal cisterna Sarcolemma Sarcolemma Myofibril Sarcoplasm Myofibrils Thin filament Thick filament Internal organization of a muscle fiber.

Note the relationships among myofibrils, sarcoplasmic reticulum, mitochondria, triads, and thick and thin filaments.

Triad Sarcoplasmic reticulum T tubules

Human Anatomy Spring 2012 13

Myofibrils

 Myofibrils are densely packed, rodlike contractile elements  They make up most of the muscle volume  The arrangement of myofibrils within a fiber is such that a perfectly aligned repeating series of dark A bands and light I bands is evident Human Anatomy Spring 2012 14 Figure 9.2b

Sarcomeres

 The smallest contractile unit of a muscle  The region of a myofibril between two successive Z discs  Composed of myofilaments made up of contractile proteins  Myofilaments are of two major types – thick and thin Human Anatomy Spring 2012 15

Myofilaments: Banding Pattern

 Thick filaments – extend the entire length of an A band  Thin filaments – extend across the I band and partway into the A band  Z-disc – coin-shaped sheet of proteins (connectins) that anchors the thin filaments and connects myofibrils to one another Human Anatomy Spring 2012 16

Myofilaments: Banding Pattern

 Thin filaments do not overlap thick filaments in the lighter H zone  M lines appear darker due to the presence of the protein

desmin

 Elastic filaments of protein titin Human Anatomy Spring 2012 17

Myofibril Z line

Ultrastructure of Myofilaments:Thick Filaments

Sarcomere H band M line The structure of M line thick filaments

 Each myosin molecule has a rodlike tail and two globular heads  Tails – two interwoven, heavy polypeptide chains  Heads – two smaller,

Titin

light polypeptide chains called

cross

Hinge Myosin head Myosin tail

bridges

A single myosin molecule detailing the structure and movement of the myosin head after cross-bridge binding occurs

Human Anatomy Spring 2012 18 Figure 9.3a, b

Ultrastructure of Myofilaments:Thick Filaments

 Thick filaments are composed of the protein

myosin

 Myosin heads contain:  2 smaller, light polypeptide chains that act as cross bridges during contraction  Binding sites for actin of thin filaments  Binding sites for ATP  ATPase enzymes Human Anatomy Spring 2012 19

Ultrastructure of Myofilaments: Thin Filaments

 Thin filaments are chiefly composed of protein

actin

 Each actin molecule is a helical polymer of globular subunits called

G actin

 The subunits contain the active sites to which myosin heads attach during contraction  Tropomyosin (filamentous protein) and troponin are regulatory subunits bound to actin

Actinin Z line Titin Myofibril Sarcomere H band The attachment of thin filaments to the Z line Troponin site Nebulin Tropomyosin G actin molecules

Human Anatomy Spring 2012

the organization of G actin, troponin, and tropomyosin F actin strand M line Z line

Arrangement of Filaments in a Sarcomere

 Longitudinal section within one sarcomere Human Anatomy Spring 2012 21 Figure 9.3d

Sliding Filament Mechanism of Contraction

 Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree  In the relaxed state, thin and thick filaments overlap only slightly  Upon stimulation, myosin heads bind to actin and sliding begins (interactive physiology page 17)  Each myosin head binds and detaches several times during contraction, acting like a ratchet to generate tension and propel the thin filaments to the center of the sarcomere  As this event occurs throughout the sarcomeres, 22

Human Anatomy Spring 2012 23

The Sliding Filament Theory

Human Anatomy Spring 2012 Figure 9.7 Changes in a Sarcomere 24

Human Anatomy Spring 2012 25

Sarcomere Structure

Human Anatomy Spring 2012 Figure 9.4b Sarcomere Structure 26

Regulation of Contraction

 In order to contract, a skeletal muscle must:  Be stimulated by a nerve ending (NMJ)  Propagate an electrical current, or action potential

,

along its sarcolemma (T-tubules)  Have a rise in intracellular Ca 2+ levels, the final trigger for contraction (SR) Human Anatomy Spring 2012 27

Nerve Stimulus of Skeletal Muscle

 Skeletal muscles are stimulated by motor neurons of the somatic nervous system  Axons of these neurons travel in nerves to muscle cells  Axons of motor neurons branch profusely as they enter muscles  Each axonal branch forms a neuromuscular junction with a single muscle fiber Human Anatomy Spring 2012 28

Neuromuscular Junction

 The neuromuscular junction is:  Axonal endings, which have small membranous sacs (synaptic vesicles) that contain the neurotransmitter

acetylcholine

(ACh

)

 The motor end plate of a muscle, which is a specific part of the sarcolemma that contains ACh receptors that helps form the neuromuscular junction  Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the

synaptic cleft

Human Anatomy Spring 2012

Motor neuron Axon Path of action potential Motor end plate Neuromuscular synapse Myofibril

29

Neuromuscular Junction

Human Anatomy Spring 2012 30 Figure 9.8a, b

Neuromuscular Junction

 When a nerve impulse reaches the end of an axon at the neuromuscular junction:   Voltage-regulated calcium channels open and allow Ca 2+ to enter the axon Ca 2+ inside the axon terminal causes axonal 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 Human Anatomy Spring 2012 31

Neuromuscular Junction

Human Anatomy Spring 2012 32 Figure 9.8c

Human Anatomy Spring 2012 33

Summary

Human Anatomy Spring 2012 34

Excitation-Contraction Coupling

Human Anatomy Spring 2012 35

Motor Unit: The Nerve-Muscle Functional Unit

  A motor unit is a motor neuron and all the muscle fibers it supplies The number of muscle fibers per motor unit can vary from four to

Spinal cord Axon terminals at neuromuscular junctions Motor unit 1 Motor unit 2 Motor neuron cell body Muscle Nerve Motor neuron axon Muscle fibers

several hundred  Muscles that control fine movements (fingers, eyes) have small motor units Human Anatomy Spring 2012 36

Motor Unit: The Nerve-Muscle Functional Unit

 Large weight-bearing muscles (thighs, hips) have large motor units

Spinal cord Axon terminals at neuromuscular junctions Motor unit 1 Motor unit 2

 Muscle fibers from a motor unit are spread throughout the muscle; therefore, contraction of a single motor unit causes weak contraction of the entire muscle

Nerve Motor neuron cell body Motor neuron axon Muscle Muscle fibers

Human Anatomy Spring 2012 37

Neuromuscular Junction

Human Anatomy Spring 2012 38

Muscle Tone

 Muscle tone:  The constant, slightly contracted state of all muscles, which does not produce active movements  Keeps the muscles firm, healthy, and ready to respond to stimulus  Spinal reflexes account for muscle tone by:  Activating one motor unit and then another  Responding to activation of stretch receptors in muscles and tendons Human Anatomy Spring 2012 39

Force of Contraction

 The force of contraction is affected by:  The number of muscle fibers contracting – the more motor fibers in a muscle, the stronger the contraction  The relative size of the muscle – the bulkier the muscle, the greater its strength Human Anatomy Spring 2012 40 Figure 9.19a

Force of Contraction

 Frequency of stimulation  Length-tension relationships   Series-elastic elements – the noncontractile structures in a muscle Degree of muscle stretch – muscles contract strongest when muscle fibers are 80 120% of their normal resting length Human Anatomy Spring 2012 41 Figure 9.19a

Length/Tension Relationship

Sarcomeres greatly shortened Sarcomeres at resting length Sarcomeres excessively stretched 75% 100% 170% Optimal sarcomere operating length (80% –120% of resting length)

Maximum force generated between 80-120% of resting length, interactive physiology page 16 Human Anatomy Spring 2012 Think of cross-bridge mechanism for explanation 42

Human Anatomy Spring 2012 43

Muscle Fiber Type: Functional Characteristics

 Speed of contraction – determined by speed in which ATPases split ATP  The three types of fibers are

slow

intermediate and

fast

and  ATP-forming pathways  Oxidative fibers – use aerobic pathways  Glycolytic fibers – use anaerobic glycolysis  These two criteria define three categories – slow oxidative fibers, fast oxidative fibers, and fast glycolytic fibers Human Anatomy Spring 2012 44

Human Anatomy Spring 2012 45

Human Anatomy Spring 2012 46

Training

 Slow, oxidative respond to endurance training. Diameter changes little.

 Fast, oxidative respond to strength and power training. Diameter increases.

 Intermediate can take on characteristics of fast or slow, depending on type of training.

In what birds do you expect to find FT? And ST?

Human Anatomy Spring 2012 47

Muscle Hypertrophy

 Exercise causes:  An increase in the number of mitochondria  An increase in the activity of muscle spindles  An increase in the concentration of glycolytic enzymes  An increase in the glycogen reserves  An increase in the number of myofibrils  The net effect is an enlargement of the muscle (

hypertrophy

)  Disuse causes atrophy:  A decrease in muscle size  A decrease in muscle tone

Developmental Aspects

 Muscle tissue develops from embryonic mesoderm called myoblasts  Multinucleated skeletal muscles form by fusion of myoblasts forming a syncytium  The growth factor

agrin

stimulates the clustering of ACh receptors at newly forming motor end plates  As muscles are brought under the control of the somatic nervous system, the numbers of fast and slow fibers are also determined Human Anatomy Spring 2012 49

Developmental Aspects

 Cardiac myoblasts do not fuse but develop gap junctions at an early embryonic stage  Most smooth muscle follows the same pattern of gap junctions rather than fusion Human Anatomy Spring 2012 50

Developmental Aspects: After Birth

 Muscular development reflects neuromuscular coordination  Development occurs head-to-toe, and proximal-to-distal  Peak natural neural control of muscles is achieved by midadolescence  Athletics and training can improve neuromuscular control Human Anatomy Spring 2012 51

Developmental Aspects: Male and Female

    There is a biological basis for greater strength in men than in women Women’s skeletal muscle makes up 36% of their body mass Men’s skeletal muscle makes up 42% of their body mass  These differences are due primarily to the male sex hormone

testosterone

With more muscle mass, men are generally stronger than women  Body strength per unit muscle mass, however, is the same Human Anatomy Spring 2012 52

Homeostatic Imbalance: Age Related

 With age, connective tissue increases and myofibrils, glycogen and myoglobin decrease  Muscles become stringier and more sinewy  By age 80, 50% of muscle mass is lost (sarcopenia), and myosatellite cells decrease  Regular exercise reverses sarcopenia  Aging of the cardiovascular system affects every organ in the body  Atherosclerosis may block distal arteries, leading to intermittent claudication and causing severe 53 pain in leg muscles Human Anatomy Spring 2012

Developmental Aspects: Regeneration

 Cardiac and skeletal muscle become amitotic, but can lengthen and thicken (

hypertrophy

)  Myoblastlike satellite cells of skeletal muscle show very limited regenerative ability (Cardiac tissue lacks satellite cells)  Smooth muscle has good regenerative ability (

hyperplasia

) Human Anatomy Spring 2012 54

Levers

   F 1 L 1 = F 2 L 2 Mass=Force L=length There are several ways to increase the force efficiency of a lever:  increasing the length of the in-lever arm  decreasing the length of the out-lever  or doing both of the above Human Anatomy Spring 2012 55

See-saw

Human Anatomy Spring 2012

Wheelbarrow Less distance Hotdog tongs most common, least mechanical advantage, more force, more speed/distance

56

57

The Arm is a Lever and Fulcrum System

Figure 12-21b

The Lever-Fulcrum System Amplifies the Load Distance Traveled and the Speed of Movement

58

Figure 12-22

Smooth Muscle

 Composed of spindle-shaped fibers diameter of 2-10  m and lengths of several hundred  m  Lack the coarse CT sheaths of skeletal muscle, but have fine endomysium Human Anatomy Spring 2012 59 Figure 9.23

Smooth Muscle

 Are generally organized into two layers (longitudinal and circular) of closely apposed fibers  Found in walls of hollow organs (except the heart) Human Anatomy Spring 2012 60 Figure 9.23

Innervation of Smooth Muscle

 Most smooth muscle lacks neuromuscular junctions  Innervating nerves have bulbous swellings called

varicosities

 Varicosities release neurotransmitters into wide synaptic clefts called

diffuse junctions

Human Anatomy Spring 2012 61

Microscopic Anatomy of Smooth Muscle

 SR is less developed than in skeletal muscle and lacks a specific pattern (no cisterns)  T tubules are absent  Plasma membranes have pouchlike infoldings called

caveoli

 Ca 2+ is sequestered in the extracellular space near the caveoli, allowing rapid influx when channels are opened  There are no visible striations and no sarcomeres  Human Anatomy Spring 2012 Thin and thick filaments are present 62

Proportion and Organization of Myofilaments in Smooth Muscle

 Ratio of thick to thin filaments (1:2) is much lower than in skeletal (1:6) or cardiac (1:4)  Thick filaments have heads along their entire length  There is no troponin complex Human Anatomy Spring 2012 63 Figure 9.25

Proportion and Organization of Myofilaments in Smooth Muscle

 Thick and thin filaments are arranged diagonally, causing smooth muscle to contract in a corkscrew manner Human Anatomy Spring 2012 64 Figure 9.25

Proportion and Organization of Myofilaments in Smooth Muscle

 Noncontractile intermediate filament bundles attach to dense bodies (analogous to Z discs) at regular intervals Human Anatomy Spring 2012 65 Figure 9.25

Contraction of Smooth Muscle

 Whole sheets of smooth muscle exhibit slow, synchronized contraction  They contract in unison, reflecting their electrical coupling with gap junctions  Action potentials are transmitted from cell to cell  Some smooth muscle cells:  Act as pacemakers and set the contractile pace for whole sheets of muscle  Are self-excitatory and depolarize without external stimuli Human Anatomy Spring 2012 66

Contractile Mechanism

 Actin and myosin interact according to the sliding filament mechanism  The final trigger for contractions is a rise in intracellular Ca 2+  Ca 2+ is released from the SR and from the extracellular space  Ca 2+ interacts with calmodulin and myosin light chain kinase to activate myosin Human Anatomy Spring 2012 67

Special Features of Smooth Muscle Contraction

 Unique characteristics of smooth muscle include:  Smooth muscle tone  Slow, prolonged contractile activity  Low energy requirements  Response to stretch Human Anatomy Spring 2012 68

Response to Stretch

 Smooth muscles exhibits a phenomenon called

stress-relaxation response

in which:  Smooth muscle responds to stretch only briefly, and then adapts to its new length  The new length, however, retains its ability to contract  This enables organs such as the stomach and bladder to temporarily store contents Human Anatomy Spring 2012 69

Types of Smooth Muscle: Single Unit

 The cells of single unit smooth muscle, commonly called visceral muscle:  Contract rhythmically as a unit  Are electrically coupled to one another via gap junctions  Often exhibit spontaneous action potentials  Are arranged in opposing sheets and exhibit stress-relaxation response Human Anatomy Spring 2012 70

Types of Smooth Muscle: Multiunit

 Multiunit smooth muscles are found:  In large airways to the lungs  In large arteries  In arrector pili muscles  In the internal eye muscles  Characteristics include:  Rare gap junctions  Infrequent spontaneous depolarizations  Structurally independent muscle fibers   A rich nerve supply, which, with a number of muscle fibers, forms motor units Human Anatomy Spring 2012 Graded contractions in response to neural stimuli 71

Muscle Comparison Summary

Table 12-3