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Ch. 9-12 - Flashcards

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Class:BIOL 1134 - Anatomy & Physiology I Lab
Subject:Biology
University:Midwestern State University
Term:Fall 2013
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Microscopic Structure of Smooth Muscle Fibers Peristalsis: The alternating contraction and relaxation of these two layers mixes substances in the lumen and squeezes them through the origin's internal pathway
Innervating nerve fibers, which are part of the automatic (involuntary) nervous system, have numerous bulbous swellings, called varicosities
Varicosities: release neurotransmitter into a wide synaptic cleft in the general area of the smooth muscle cells; such junctions are called diffuse junctions
Comparing the neural input to skeletal and smooth muscles, you could say the skeletal muscle gets priority mail while smooth muscle gets bulk mailings
The SR lacks a specific pattern relative to the myofilaments
T Tubules are absent, but the sarcolemma has multiple caveolae, pouchlike infoldings that sequester bits of extracellular fluid containing a hgh concentration of Ca close to the membrane
When calcium channels in the caveolae open, Ca influx occurs rapidly
Although the SR does release some of the calcium that triggers contraction, most Ca entrers through calcium channels directly from the extracellular space
Contraction ends when cytoplasmic calcium is actively transported into the SR and out of the cell
No striations in smooth muscle therefore no sarcomeres
Do not contain interdigitating thick and thin filaments, but the myosin filaments are a lot shorter than the actin filaments and the type of myosin contained differs from skeletal muscle

The proportion and organization of smooth muscle myofilaments differ from skeletal muscle in the following ways: Thick filaments are fewer but have mysoin heads along their entire length: the ratio of thick to thin filaments is much lower in smooth muscle; However, thick filaments of smooth muscle contain actin-gripping myosin heads along their entire length. The myosin heads are oriented in one direction on one side of the filament and in the opposite direction on the other side
No troponin complex in thin filaments:  a protein called calmodulin acts as the calcium binding site
Thick and thin filaments arranged diagonally: the smooth muscles cells contract in a twisting way so that they look like tiny corkscrews
Intermediate filament-dense body network: contain a lattice-like arrangement of noncontractile intermediate filaments that resist tension; attach at regular intervals to cytoplasmic structures called dense bodies- these dense bodies, which are also tethered to to the sarcolemma, act as anchoring points for thin filaments and therefore correspond to Z discs of skeletal muscle. Forms a strong, cable-like intracellular cytoskeleton that harnesses the pull generated by the sliding of the thick and thin filaments. 

Contraction of Smooth Muscle Most cases, adjacent smooth muscle fibers exhibit slow, synchronized contractions, the whole sheet responding to a stimulus in unison
This synchronization reflects electrical coupling of smooth muscle cells by gap junctions, specialized cell connections
Electrically isolated from one another, each stimulated to contract by its own neuromuscular junction
By contrast, gap junctions allow smooth muscles to transmit action potentials from fiber to fiber
Some smooth muscle fibers in the stomach and small intestine are pacemaker cells: once excited, they act as "drummer" to set the pace of contraction for the entire muscle sheet
Pacemakers depolarize spontaneously in the absence of external stimuli
Contraction of Smooth Muscle Contraction in smooth muscle is like contraction in skeletal muscle in the following ways:
Actin and myosin interact by the sliding filament mechanism
The final trigger for contraction is a rise in the intracellular calcium ion level
ATP energizes the sliding process

During excitation-contraction coupling, the tubules of the SR release Ca but as mentioned above, Ca also moves into the cell from the extracellular space via membrane channels
Calcium activates myosin by interacting with a regulatory molecule called calmodulin, a cytoplasmic calcium-binding protein
Calmodulin, in turn, interacts with a kinase enzyme called myosin kinase or myosin light chain kinase, which phosphorylates the myosin, activating it
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Contraction of Smooth Muscle Smooth muscle relaxes when intracellular Ca levels drop- but getting smooth muscle to stop contracting is more complex

Energy Efficiency of Smooth Muscle Contraction:
Can maintain the same contractile tension for prolonged periods at less than 1% energy cost
Part of the striking energy economy of smooth muscle is the sluggishness of its ATPases compared to those in skeletal muscle
Smooth muscle myofilaments may latch together during prolonged contractions, saving energy in that way as well
May maintain that latch state even after myosin is dephosphorylated
The smooth muscle in small arterioles and other visceral organs routinely maintains a moderate degree of contraction, called smooth muscle tone, day in and day out without fatiguing
Smooth muscle has low energy requirements, and as a rule, it makes enough ATP via aerobic pathways to keep up with the deman
Contraction of Smooth Muscle: Regulation of Contraction Can be regulated by nerves, hormones, or local chemical changes
Neural regulation: neurotransmitter binding generates an action potential, which is coupled to a rise in calcium ions in the cytosol; some types of smooth muscle respond to neural stimulation w/ graded potentials (local electrical signals) only
Hormones and local chemical factors: depolarize spontaneously or in response to chemical stimuli that bind to G protein-linked receptors 
Several chemical factors cause smooth muscle to contract or relax without an action potential by enhancing or inhibiting Ca entry into the sarcoplasm; they include certain hormones, histamine, excess carbon dioxide, low pH, and lack of oxygen
the direct response to these chemical stimuli alters smooth muscle activity according to local tissue needs and probably is most responsible for smooth muscle tone

Special Features of Smooth Muscle Contraction Response to Stretch: the increased tension persists only briefly, and soon the muscle adapts to its new length and relaxes, while still retaining the ability to contract on demand; this stress-relaxation response allows a hollow organ to fill or expand slowly to accommodate a greater volume without causing strong contractions that would expel its contents
Length and Tension Changes: stretches much more and generates more tension than skeletal muscles stretched to a comparable extent; can contract when it is anywhere from half to twice its resting length
Hyperplasia: smooth muscles can divide to increase their numbers
Types of Smooth Muscle: Unitary Smooth Muscle Commonly called visceral muscle
Are arranged in opposing (longitudinal and circular) sheets
Are innervated by varicosities of autonomic nerve fibers and often exhibit rhythmic spontaneous action potentials
Are electrically coupled by gap junctions and so contract as a unit (for this reason recruitment is not an option in unitary smooth muscle)
Respond to various chemical stimuli


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Types of Smooth Muscle: Multi Unit Smooth Muscle The smooth muscles in the large airways to the lungs and in large arteries, the arrector pili muscles attached to hair follicles, and the internal eye muscles that adjust pupil size and allow the eye to focus visually are all examples of multi unit smooth muscle
Consists of muscle fibers that are structurally independent of one another
Is richly supplied with nerve endings, each of which forms a motor unit with a number of muscle fibers
Responds to neural stimulation with graded contractions that involve recruitment
Innervated by the autonomic (involuntary) division and also responds to hormones
Developmental Aspects of Muscles Muscle tissue develops from embryonic mesoderm cells called myoblasts.
Several myoblasts fuse to form a skeletal muscle fiber.
Smooth and cardiac cells develop from single myoblasts and display gap junctions
For the most part, specialized skeletal and cardiac muscle cells lose their ability to divide but retain the ability to hypertrophy. Smooth muscle regenerates well and undergoes hyperplasia
Skeletal muscle development reflects the maturation of the nervous system and occurs in head-to-toe and proximal-to-distal directions. Natural neuromuscular control reaches its peak in midadolescence


Developmental Aspects of Muscles Women's muscles account for about 36% of their total body weight and men's for about 42%, a difference due chiefly to the effects of male hormones on skeletal muscle growth.
Skeletal muscle is richly vascularized and quite resistant to infection, but in old age, skeletal muscles become fibrous, decline in strength, and atrophy. Regular exercise can offset some of these changes.
Satellite cells, myoblast -like cells associated with skeletal muscle, help repair injured fibers and allow limited regeneration of dead skeletal muscle

Developmental Aspects of Muscles: Homeostatic Imbalance Muscular Dystrophy: refers to a group of inherited muscle-destorying disease that generally appear during childhood; the affected muscles initially enlarge due to deposits of fat and connective tissue, but the muscle fibers atrophy and degenerate
The most common and serious form is Duchenne muscular dystrophy, which is inherited as a sex-linked recessive disease; it is caused by a defective gene for dystrophin 

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 Microscopic Structure of Smooth Muscle FibersPeristalsis: The alternating contraction and relaxation of these two layers mixes substances in the lumen and squeezes them through the origin's internal pathway
Innervating nerve fibers, which are part of the automatic (involuntary) nervous system, have numerous bulbous swellings, called varicosities
Varicosities: release neurotransmitter into a wide synaptic cleft in the general area of the smooth muscle cells; such junctions are called diffuse junctions
Comparing the neural input to skeletal and smooth muscles, you could say the skeletal muscle gets priority mail while smooth muscle gets bulk mailings
The SR lacks a specific pattern relative to the myofilaments
T Tubules are absent, but the sarcolemma has multiple caveolae, pouchlike infoldings that sequester bits of extracellular fluid containing a hgh concentration of Ca close to the membrane
When calcium channels in the caveolae open, Ca influx occurs rapidly
Although the SR does release some of the calcium that triggers contraction, most Ca entrers through calcium channels directly from the extracellular space
Contraction ends when cytoplasmic calcium is actively transported into the SR and out of the cell
No striations in smooth muscle therefore no sarcomeres
Do not contain interdigitating thick and thin filaments, but the myosin filaments are a lot shorter than the actin filaments and the type of myosin contained differs from skeletal muscle

 The proportion and organization of smooth muscle myofilaments differ from skeletal muscle in the following ways:Thick filaments are fewer but have mysoin heads along their entire length: the ratio of thick to thin filaments is much lower in smooth muscle; However, thick filaments of smooth muscle contain actin-gripping myosin heads along their entire length. The myosin heads are oriented in one direction on one side of the filament and in the opposite direction on the other side
No troponin complex in thin filaments:  a protein called calmodulin acts as the calcium binding site
Thick and thin filaments arranged diagonally: the smooth muscles cells contract in a twisting way so that they look like tiny corkscrews
Intermediate filament-dense body network: contain a lattice-like arrangement of noncontractile intermediate filaments that resist tension; attach at regular intervals to cytoplasmic structures called dense bodies- these dense bodies, which are also tethered to to the sarcolemma, act as anchoring points for thin filaments and therefore correspond to Z discs of skeletal muscle. Forms a strong, cable-like intracellular cytoskeleton that harnesses the pull generated by the sliding of the thick and thin filaments. 

 Contraction of Smooth MuscleMost cases, adjacent smooth muscle fibers exhibit slow, synchronized contractions, the whole sheet responding to a stimulus in unison
This synchronization reflects electrical coupling of smooth muscle cells by gap junctions, specialized cell connections
Electrically isolated from one another, each stimulated to contract by its own neuromuscular junction
By contrast, gap junctions allow smooth muscles to transmit action potentials from fiber to fiber
Some smooth muscle fibers in the stomach and small intestine are pacemaker cells: once excited, they act as "drummer" to set the pace of contraction for the entire muscle sheet
Pacemakers depolarize spontaneously in the absence of external stimuli
 Contraction of Smooth MuscleContraction in smooth muscle is like contraction in skeletal muscle in the following ways:
Actin and myosin interact by the sliding filament mechanism
The final trigger for contraction is a rise in the intracellular calcium ion level
ATP energizes the sliding process

During excitation-contraction coupling, the tubules of the SR release Ca but as mentioned above, Ca also moves into the cell from the extracellular space via membrane channels
Calcium activates myosin by interacting with a regulatory molecule called calmodulin, a cytoplasmic calcium-binding protein
Calmodulin, in turn, interacts with a kinase enzyme called myosin kinase or myosin light chain kinase, which phosphorylates the myosin, activating it
 Contraction of Smooth MuscleSmooth muscle relaxes when intracellular Ca levels drop- but getting smooth muscle to stop contracting is more complex

Energy Efficiency of Smooth Muscle Contraction:
Can maintain the same contractile tension for prolonged periods at less than 1% energy cost
Part of the striking energy economy of smooth muscle is the sluggishness of its ATPases compared to those in skeletal muscle
Smooth muscle myofilaments may latch together during prolonged contractions, saving energy in that way as well
May maintain that latch state even after myosin is dephosphorylated
The smooth muscle in small arterioles and other visceral organs routinely maintains a moderate degree of contraction, called smooth muscle tone, day in and day out without fatiguing
Smooth muscle has low energy requirements, and as a rule, it makes enough ATP via aerobic pathways to keep up with the deman
 Contraction of Smooth Muscle: Regulation of ContractionCan be regulated by nerves, hormones, or local chemical changes
Neural regulation: neurotransmitter binding generates an action potential, which is coupled to a rise in calcium ions in the cytosol; some types of smooth muscle respond to neural stimulation w/ graded potentials (local electrical signals) only
Hormones and local chemical factors: depolarize spontaneously or in response to chemical stimuli that bind to G protein-linked receptors 
Several chemical factors cause smooth muscle to contract or relax without an action potential by enhancing or inhibiting Ca entry into the sarcoplasm; they include certain hormones, histamine, excess carbon dioxide, low pH, and lack of oxygen
the direct response to these chemical stimuli alters smooth muscle activity according to local tissue needs and probably is most responsible for smooth muscle tone

 Special Features of Smooth Muscle ContractionResponse to Stretch: the increased tension persists only briefly, and soon the muscle adapts to its new length and relaxes, while still retaining the ability to contract on demand; this stress-relaxation response allows a hollow organ to fill or expand slowly to accommodate a greater volume without causing strong contractions that would expel its contents
Length and Tension Changes: stretches much more and generates more tension than skeletal muscles stretched to a comparable extent; can contract when it is anywhere from half to twice its resting length
Hyperplasia: smooth muscles can divide to increase their numbers
 Types of Smooth Muscle: Unitary Smooth MuscleCommonly called visceral muscle
Are arranged in opposing (longitudinal and circular) sheets
Are innervated by varicosities of autonomic nerve fibers and often exhibit rhythmic spontaneous action potentials
Are electrically coupled by gap junctions and so contract as a unit (for this reason recruitment is not an option in unitary smooth muscle)
Respond to various chemical stimuli


 Types of Smooth Muscle: Multi Unit Smooth MuscleThe smooth muscles in the large airways to the lungs and in large arteries, the arrector pili muscles attached to hair follicles, and the internal eye muscles that adjust pupil size and allow the eye to focus visually are all examples of multi unit smooth muscle
Consists of muscle fibers that are structurally independent of one another
Is richly supplied with nerve endings, each of which forms a motor unit with a number of muscle fibers
Responds to neural stimulation with graded contractions that involve recruitment
Innervated by the autonomic (involuntary) division and also responds to hormones
 Developmental Aspects of MusclesMuscle tissue develops from embryonic mesoderm cells called myoblasts.
Several myoblasts fuse to form a skeletal muscle fiber.
Smooth and cardiac cells develop from single myoblasts and display gap junctions
For the most part, specialized skeletal and cardiac muscle cells lose their ability to divide but retain the ability to hypertrophy. Smooth muscle regenerates well and undergoes hyperplasia
Skeletal muscle development reflects the maturation of the nervous system and occurs in head-to-toe and proximal-to-distal directions. Natural neuromuscular control reaches its peak in midadolescence


 Developmental Aspects of MusclesWomen's muscles account for about 36% of their total body weight and men's for about 42%, a difference due chiefly to the effects of male hormones on skeletal muscle growth.
Skeletal muscle is richly vascularized and quite resistant to infection, but in old age, skeletal muscles become fibrous, decline in strength, and atrophy. Regular exercise can offset some of these changes.
Satellite cells, myoblast -like cells associated with skeletal muscle, help repair injured fibers and allow limited regeneration of dead skeletal muscle

 Developmental Aspects of Muscles: Homeostatic ImbalanceMuscular Dystrophy: refers to a group of inherited muscle-destorying disease that generally appear during childhood; the affected muscles initially enlarge due to deposits of fat and connective tissue, but the muscle fibers atrophy and degenerate
The most common and serious form is Duchenne muscular dystrophy, which is inherited as a sex-linked recessive disease; it is caused by a defective gene for dystrophin 

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