Transcript Chapter 17 Autonomic Nervous System
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
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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
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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
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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
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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
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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
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The Sliding Filament Theory
Human Anatomy Spring 2012 Figure 9.7 Changes in a Sarcomere 24
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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
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Motor neuron Axon Path of action potential Motor end plate Neuromuscular synapse Myofibril
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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
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Summary
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Excitation-Contraction Coupling
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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
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Neuromuscular Junction
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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
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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
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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?
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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
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Wheelbarrow Less distance Hotdog tongs most common, least mechanical advantage, more force, more speed/distance
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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
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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
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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