Muscles Flashcards
(85 cards)
1
Q
Three Types of Muscle Tissue
A
- Skeletal muscle tissue:
- Cardiac muscle tissue:
- Smooth muscle tissue:
2
Q
DESCRIBE Skeletal muscle tissue:
hint: VASP
A
** *VASP**
- Attached to bones and skin
- Striated
- Voluntary (i.e., conscious control)
- Powerful
- Primary topic of this chapter
3
Q
DESCRIBE Cardiac muscle tissue:
Hint: ISOh
A
**ISOh*
- Only in the heart• Striated
- Involuntary
4
Q
DESCRIBE _ Smooth muscle_ tissue:
HINT: INN
A
_*INN_
• In the walls of hollow organs, e.g., s_tomach, urinary bladder, and airways_
- *• Not striated**
- *• Involuntary**
5
Q
Special Characteristics
of Muscle Tissue
Hint: CEEE
A
*CEEE
- Excitability (responsiveness or irritability): ability to receive and respond to stimuli
- Contractility: ability to shorten when stimulated
- Extensibility: ability to be stretched
- Elasticity: ability to recoil to resting length
6
Q
Muscle Functions are?
Hint: HMMS
A
** *HMMS**
- Movement of bones or fluids (e.g., blood)
- Maintaining posture and body position
- Stabilizing joints
- Heat generation (especially skeletal muscle)
7
Q
IN Skeletal Muscle
_Each muscl_e is served by
one_____, one ______, and one or more _____.
Hint: ANV
A
_*ANV_
1 artery, 1 nerve,** 1+ **veins
8
Q
• Connective tissue sheaths of skeletal muscle are?
Hint: PEE
A
**• Epimysium:
• Perimysium:
• Endomysium: **
9
Q
WHAT Connective tissue sheaths of skeletal muscle has
dense regular connective tissue surrounding entire muscle?
A
**
• Epimysium **
10
Q
WHAT Connective tissue sheaths of skeletal muscle has
fibrous connective tissue surrounding fascicles (groups of muscle fibers)??
A
**• Perimysium: **
11
Q
WHAT Connective tissue sheaths of skeletal muscle has
fine areolar connective tissue surrounding each muscle fiber?
A
**• Endomysium **
12
Q
2 Ways Muscles attach:
A
**• Directly
• Indirectly **
13
Q
• Muscles attach when the
_epimysium of muscle is fused to the periosteum of bone or perichondrium of cartilage
_
A
**
Directly **
14
Q
• Muscles attach when the
—connective tissue wrappings extend beyond the muscle as a ropelike tendon or sheetlike aponeurosis?
A
**• Indirectly **
15
Q
Microscopic Anatomy of a Skeletal Muscle Fiber are?
A
- Cylindrical cell 10 to 100 μm in diameter, up to 30 cm long
- Multiple peripheral nuclei
- Many mitochondria
- Glycosomes for glycogen storage, myoglobin for O2 storage
- Also contain m_yofibrils, sarcoplasmic reticulum, and T tubules_
16
Q
DESCRIBE
Myofibrils
A
- Densely packed, r**odlike** elements
- ~80% of cell volume
- Exhibit striations: p_erfectly aligned repeating_ series of_ **dark A bands and light I bands **_
17
Q
DESCRIBE
_Sarcomere
_
A
- Smallest contractile unit (functional unit) of a muscle fiber
- The region of a myofibril between two successive Z discs
- Composed of thick and thin myofilaments made of contractile proteins
18
Q
Features of a
Sarcomere are?
A
• Thick filaments: run the entire length of an A band
- Thin** filaments: run the **length of the I band** and **partway** into the **A band
- Z disc: coin-shaped sheet of proteins that anchors*** the ***thin filaments*** and ***connects myofibrils to one another
- H zone: lighter midregion where filaments do not overlap
- M line: line of protein myomesin that holds adjacent thick filaments together
19
Q
Ultrastructure of Thick Filament
is Composed of?
A
protein myosin
20
Q
• Myosin tails contain?
A
**
• 2 interwoven, heavy polypeptide chains**
21
Q
• Myosin heads contain:
A
- 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
22
Q
Ultrastructure of Thin Filament
A
- Twisted double strand of fibrous protein F actin
- F actin consists of G (globular) actin subunits
- G actin bears active sites for myosin head attachment during contraction
- Tropomyosin and troponin***: regulatory proteins ***bound to actin
23
Q
DESCRIBE
Sarcoplasmic Reticulum (SR)
A
- Network of smooth endoplasmic reticulum surrounding each myofibril
- Pairs of terminal cisternae form perpendicular cross channels
- Functions in the regulation of intracellular Ca2+ levels
24
Q
DESCRIBE
T Tubules
A
- Continuous with the sarcolemma
- Penetrate the cell’s interior at each A band–I band junction
- Associate with the paired terminal cisternae to form triads that encircle each sarcomere
25
Explain
**_Triad Relationships_**
* **T tubules** ***_conduct impulses deep into muscle fiber_***
* I**_ntegral proteins protrude into the intermembrane space_** from T tubule and SR cisternae membranes
* **_T tubule proteins: voltage sensors_**
* **_SR foot proteins:_** g***_ated channels that regulate Ca2+ release from the SR cisternae_***
26
**What is**
***_Contraction?_***
* The generation of force
* **_Does not_** necessarily ***_cause shortening of the fiber_***
* **_Shortening occurs**_ when _**t_*_ension generated by cross bridges on the thin filaments exceeds forces opposing shortening_***
27
DESCRIBE
**_Sliding Filament Model of Contraction_**
* In the ***_relaxed state***_, _***thin and thick filaments slightly overlap_***
* **During contraction,** _m_**_yosin_ *_heads bind to actin, detach, and bind again, to propel the thin filaments toward the M line_***
* As ***_H zones shorten and disappear_***, ***_s_**_arcomeres shorten, muscle cells shorten, and the whole muscle shortens_***
28
WHAT are
_Requirements_ for _Skeletal Muscle Contraction_
## Footnote
**1. Activation: **
**2. Excitation-contraction coupling:
• Final trigger: a brief rise in intracellular Ca2+ levels**
29
WHAT is-
**_neural stimulation at a
neuromuscular junction_**
**_ _**
** Activation **
30
What is
**_Generation and propagation of an action potential along the sarcolemma?_**
## Footnote
**_ _**
**
Excitation **
31
Events at the **_Neuromuscular Junction_**
• Skeletal muscles are stimulated by somatic motor neurons
• Axons of motor neurons travel from the central nervous system via nerves to skeletal muscles
• Each axon forms several branches as it enters a muscle
• Each axon ending forms a neuromuscular junction with a single muscle fiber
Neuromuscular Junction
• Situated midway along the length of a muscle fiber
• Axon terminal and muscle fiber are separated by a gel-filled space called the synaptic cleft
• Synaptic vesicles of axon terminal contain the neurotransmitter acetylcholine (ACh)
• Junctional folds of the sarcolemma contain ACh receptors
Events at the Neuromuscular Junction
• Nerve impulse arrives at axon terminal
• ACh is released and binds with receptors on the sarcolemma
• Electrical events lead to the generation of an action potential
Events at the Neuromuscular Junction
• Skeletal muscles are stimulated by somatic motor neurons
• Axons of motor neurons travel from the central nervous system via nerves to skeletal muscles
• Each axon forms several branches as it enters a muscle
• Each axon ending forms a neuromuscular junction with a single muscle fiber
Neuromuscular Junction
• Situated midway along the length of a muscle fiber
• Axon terminal and muscle fiber are separated by a gel-filled space called the synaptic cleft
• Synaptic vesicles of axon terminal contain the neurotransmitter acetylcholine (ACh)
• Junctional folds of the sarcolemma contain ACh receptors
Events at the Neuromuscular Junction
• Nerve impulse arrives at axon terminal
• ACh is released and binds with receptors on the sarcolemma
• Electrical events lead to the generation of an action potential
32
DESCRIBE
**_Destruction of Acetylcholine_**
* **_ACh effects**_ are quickly _**terminated by_** the enzyme ***_acetylcholinesterase_***
* ***_Prevents_*** ***continued muscle fiber contraction in the absence of additional stimulation***
33
Events in Generation of an Action Potential?
## Footnote
* *
1. Local depolarization (end plate potential):
2. Generation and propagation of an action potential:
3. Repolarization:**
** **
34
Events in Generation of an Action Potential
Describe **_Local depolarization (end plate potential):_**
**_ _**
**• ACh binding opens chemically (ligand) gated ion channels**
**• Simultaneous diffusion of Na+ (inward) and K+ (outward)**
**• More Na+ diffuses, so the interior of the sarcolemma becomes less negative**
* *• Local depolarization – end plate potential
* *
** **
35
Describe ***_Generation and propagation_*** of an action potential:
* End plate potential spreads to adjacent membrane areas
* Voltage-gated Na+ channels open
* Na+ influx decreases the membrane voltage toward a critical threshold
* If threshold is reached, an action potential is generated
* Local depolarization wave continues to spread, changing the permeability of the sarcolemma
* Voltage-regulated Na+ channels open in the adjacent patch, causing it to depolarize to threshold
36
Events in Generation of an Action Potential
***_ _***
Describe
***_ Repolarization:_***
**• Na+ channels close and voltage-gated K+ channels open**
**• K+ efflux rapidly restores the resting polarity**
**• Fiber cannot be stimulated and is in a refractory period until repolarization is complete**
**• Ionic conditions of the resting state are restored by the Na+-K+ pump**
37
DESCRIBE
**_*Excitation-*Contraction (E-C) Coupling_**
* Sequence of events by which transmission of an AP along the sarcolemma leads to sliding of the myofilaments
* **Latent period**: *_T**_i_**me when E-C coupling events occur & Time between AP initiation and the beginning of contraction_*
38
**_ Events of_** Excitation-Contraction (E-C) Coupling
## Footnote
• ***AP is propagated along sarcomere to T tubules***
***• Voltage-sensitive proteins stimulate Ca2+ release from SR***
***• Ca2+ is necessary for contraction***
39
Role of Calcium (Ca2+) in Contraction
At ***_low_*** intracellular Ca2+ concentration:
* Tropomyosin blocks the active sites on actin
* Myosin heads cannot attach to actin
* Muscle fiber relaxes
40
Role of Calcium (Ca2+) in Contraction
• At ***_higher_*** intracellular Ca2+ concentrations:
** • Ca2+ binds to troponin**
**• Troponin changes shape and moves tropomyosin away from active sites**
**• Events of the cross bridge cycle occur**
**• When nervous stimulation ceases, Ca2+ is pumped back into the SR and contraction ends**
41
DESCRIBE
_**Cross Bridge Cycle
**_
* **Continues as long as the Ca2+ signal and adequate ATP are present**
* Cross bridge formation—high-energy myosin head attaches to thin filament
* Working (power) stroke—myosin head pivots and pulls thin filament toward M line
* Cross bridge detachment—ATP attaches to myosin head and the cross bridge detaches
* “Cocking” of the myosin head—energy from hydrolysis of ATP cocks the myosin head into the high-energy state
42
Review Principles of Muscle Mechanics
## Footnote
1. Same principles apply to contraction of a single fiber and a whole muscle
2. Contraction produces tension, the force exerted on the load or object to be moved
3. Contraction does not always shorten a muscle:
• **Iso*_metric***_ contraction: _***n_*_o shortening; muscle tension increases but does not exceed the load_**
• **Iso*_toni_***c contraction: **_muscle shortens because muscle tension exceeds the load_**
4. Force and duration of contraction vary in response to stimuli of different frequencies and intensities
Review Principles of Muscle Mechanics
1. Same principles apply to contraction of a single fiber and a whole muscle
2. Contraction produces tension, the force exerted on the load or object to be moved
3. Contraction does not always shorten a muscle:
• Iso**_metric_** contraction: no shortening; muscle tension increases but does not exceed the load
• Isotonic contraction: muscle shortens because muscle tension exceeds the load
4. Force and duration of contraction vary in response to stimuli of different frequencies and intensities
43
What is
The ***_Nerve-Muscle Functional Unit_***
• ***_Motor unit_*** = *_a motor neuron and all (four to several hundred) muscle fibers it supplies_*
## Footnote
44
**• Small motor units in muscles that controls? **
** fine movements (fingers, eyes)**
45
• ***_Large motor_*** units Control
** weight-bearing muscles (thighs, hips) **
46
STUDY
## Footnote
***_• Muscle fibers from a motor unit are spread throughout the muscle so that a single motor unit causes weak contraction of entire muscle_***
* _**• Motor units in a muscle usually contract asynchronously; helps prevent fatigue
* *_*
Study
## Footnote
**_• Muscle fibers from a motor unit are spread throughout the muscle so that a single motor unit causes weak contraction of entire muscle_**
**_• Motor units in a muscle usually contract asynchronously; helps prevent fatigue_**
47
Describe
Muscle Twitch?
**• Response of a muscle to a single, brief threshold stimulus**
**• Simplest contraction observable in the lab (recorded as a myogram)**
48
WHAT are the
_**Three phases of a twitch**:_
* **Latent period:** _events of excitation-contraction coupling_
* **Period of contraction:** _cross bridge formation; tension increases_
* **Period of relaxation:** _Ca2+ reentry into the SR; tension declines to zero_
49
Explain
**_Muscle Twitch Comparisons_**
**_Different strength and duration of twitches are due to variations in metabolic properties and enzymes between muscles _**
50
Graded Muscle Responses
**_Responses are graded by?_**
* Variations in the degree of muscle contraction
* Required for proper control of skeletal movement
1. ***Changing*** the ***_frequency of stimulation_***
2. **Changing** the ***_strength of the stimulus_***
51
\*STUDY
Response to Change in Stimulus Frequency
• _A single stimulus_ results in a single contractile response—a muscle twitch
• _Increase frequenc_y of stimulus (muscle does not have time to completely relax between stimuli)
• _Ca2+ release stimulate_s further contraction → temporal (wave) summation
• _Further increase_ in stimulus frequency → unfused (incomplete) tetanus
• If _stimuli are given quickly enou_gh, fused (complete) tetany results
\*STUDY
Response to Change in Stimulus Frequency
• A single stimulus results in a single contractile response—a muscle twitch
* Increase frequency of stimulus (muscle does not have time to completely relax between stimuli)
* Ca2+ release stimulates further contraction → temporal (wave) summation
* Further increase in stimulus frequency → unfused (incomplete) tetanus
* If stimuli are given quickly enough, fused (complete) tetany results
52
STUDY
Response to Change in Stimulus Strength
• Threshold stimulus: stimulus strength at which the first observable muscle contraction occurs
* Muscle contracts more vigorously as stimulus strength is increased above threshold
* Contraction force is precisely controlled by recruitment (multiple motor unit summation), which brings more and more muscle fibers into action
* Size principle: motor units with larger and larger fibers are recruited as stimulus intensity increases
STUDY
## Footnote
Response to Change in Stimulus Strength
• Threshold stimulus: stimulus strength at which the first observable muscle contraction occurs
* Muscle contracts more vigorously as stimulus strength is increased above threshold
* Contraction force is precisely controlled by recruitment (multiple motor unit summation), which brings more and more muscle fibers into action
* Size principle: motor units with larger and larger fibers are recruited as stimulus intensity increases
53
**• Isotonic contractions are either?**
** concentric**
**or**
**eccentric**
54
\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_contractions—the ***_muscle shortens and does work_***
** • Concentric **
55
\_\_\_\_\_\_\_\_\_\_\_\_\_\_contractions—the ***_muscle contracts as it lengthens_***
**
• Eccentric **
56
STUDY
Isotonic Contractions
* Muscle changes in length and moves the load
* Isotonic contractions are either concentric or eccentric:
* The load is greater than the tension the muscle is able to develop
* Tension increases to the muscle’s capacity, but the muscle neither shortens nor lengthens
STUDY
Isotonic Contractions
* Muscle **_changes in length and moves the load_**
* Isotonic contractions are ***_either concentric or eccentric_***:
* The l***_oad is greater than the tension_*** the **_muscle is able to develop_**
* **T_ension increases_** to the **_muscle’s capacity,_** but the ***_muscle neither shortens nor lengthens_***
57
During Muscle Metabolism: What happens in **_Energy for Contraction? _**
**• ATP is the only source used directly for contractile activities**
**• Available stores of ATP are depleted in 4–6 seconds**
**• ATP is regenerated **
58
**
• ATP is regenerated by?**
**• Direct phosphorylation of ADP by creatine phosphate (CP)**
**• Anaerobic pathway (glycolysis)**
**• Aerobic respiration**
59
During Anaerobic Pathway what happens
a**_t 70% of maximum contractile activity_**?
Hint: **_BOP_**
**_BOP_**
**• _Bulging muscles_ compress blood vessels**
**• _Oxygen deliver_y is _impaired_**
**• _Pyruvic acid_ is _converted_ into l_actic acid
_**
60
During Anaerobic Pathway
_ What happens to ** Lactic acid**_**?**
Hint: **_DUC_**
**DUC**
* **_Diffuses_** into the bloodstream
* **_Used as fue_**l by the **_liver, kidneys, and heart_**
* **_Converted back_** into **_pyruvic acid_** by the liver
61
Study
Aerobic Pathway
* Produces 95% of ATP during rest and light to moderate exercise
* Fuels: stored glycogen, then bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids
Study
Aerobic Pathway
* Produces 95% of ATP during rest and light to moderate exercise
* Fuels: stored glycogen, then bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids
62
***WHAT is Muscle Fatigue***
***_and Occurs when:_***
• ***_Physiological inability to contract_***
• ***Occurs when:***
**• Ionic imbalances (K+, Ca2+, Pi) interfere with E-C coupling**
**• Prolonged exercise damages the SR and interferes with Ca2+ regulation and release**
**• Total lack of ATP occurs rarely, during states of continuous contraction, and causes contractures (continuous contractions)**
63
Muscle Fatigue during
* _Oxygen Deficit_*
* _Extra O2 needed after exercise for?_*
_**Replenishment o**f_
***• Oxygen reserves***
***• Glycogen stores***
***• ATP and CP reserves***
***\*\* Conversion of lactic acid to pyruvic acid, glucose, and glycogen***
64
What happens during
**_Heat Production During Muscle Activity?_**
**• ~ 40% of the energy released in muscle activity is useful as work**
**• Remaining energy (60%) given off as heat**
**• Dangerous heat levels are prevented by radiation of heat from the skin and sweating**
65
• The ***_force of Muscle contraction is affected_*** by:
* Number of muscle fibers stimulated **(recruitment)**
* Relative size of the fibers—hypertrophy of cells increases strength
* Frequency of stimulation—↑ frequency allows time for more effective transfer of tension to noncontractile components
* Length-tension relationship—muscles contract most strongly when muscle fibers are 80–120% of their normal resting length
66
Muscle ***_Velocity and Duration of Contraction_***
* *Influenced by:
* *
Hint **MLR**
* *
1. Muscle fiber type**
**2. Load**
**3. Recruitment**
67
Muscle Fiber Type
Classified according to two characteristics?
## Footnote
1. _Speed of contraction_: slow or fast, according to:
• Speed at which myosin ATPases split ATP
• Pattern of electrical activity of the motor neurons
2. _Metabolic pathways for ATP synthesis:_
• **_Oxidative_** fibers—use **_aerobic pathways_**
• **_Glycolytic_** fibers—use _**ana**erobic **glycolysis**_
68
Muscle Fiber Type has
Three types?
**• *_SLOW & FAST _*oxidative fibers**
**• Fast glycolytic fibers**
69
WHAT HAPPENS
DURING
_Influence of MUSCLE Load_
**↑ load → ↑ latent period,**
**↓ contraction,**
**and**
**↓ duration of contraction**
70
WHAT HAPPENS DURING MUSCLE
_Influence of Recruitment_
**Recruitment → faster contraction **
**and**
**↑ duration of contraction**
71
Effects OF
Aerobic (endurance) exercise
**_Leads to *increased*_***?*
* **Muscle capillaries
* Number of mitochondria
* Myoglobin synthesis**
* Results in greater endurance, strength, and resistance to fatigue
* May convert fast glycolytic fibers into fast oxidative fibers
72
Effects of Resistance Exercise
**_Resistance exercise (typically anaerobic) results in_**?
** • Muscle hypertrophy (due to increase in fiber size)**
**• Increased mitochondria, myofilaments, glycogen stores, and connective tissue**
73
WHAT IS
**_The Overload Principle_**?
** • Forcing a muscle to work hard promotes increased muscle strength and endurance**
**• Muscles adapt to increased demands**
**• Muscles must be overloaded to produce further gains**
74
WHAT IS ***_Peristalsis?_***
## Footnote
**• Alternating contractions and relaxations of smooth muscle layers that mix and squeeze substances through the lumen of hollow organs**
**• Longitudinal layer contracts; organ dilates and shortens**
**• Circular layer contracts; organ constricts and elongates**
75
***_Microscopic Structure_***
OF **_SMOOTH MUSCLE_**
* Spindle-shaped fibers: thin and short compared with skeletal muscle fibers
* Connective tissue: endomysium only
* SR: less developed than in skeletal muscle
* Pouchlike infoldings (caveolae) of sarcolemma sequester Ca2+
* No sarcomeres, myofibrils, or T tubules
76
WHAT IS
***_Innervation of Smooth Muscle_***
**• Autonomic nerve fibers innervate smooth muscle at diffuse junctions**
**• Varicosities (bulbous swellings) of nerve fibers store and release neurotransmitters**
77
DESCRIBE
**_Myofilaments in Smooth Muscle_**
* Ratio of thick to thin filaments (1:13) is much lower than in skeletal muscle (1:2)
* Thick filaments have heads along their entire length
* No troponin complex; protein calmodulin binds Ca2+
* Myofilaments are spirally arranged, causing smooth muscle to contract in a corkscrew manner
• Dense bodies: proteins that anchor noncontractile intermediate filaments to sarcolemma at regular intervals
78
Contraction of
Smooth Muscle
**• Slow, synchronized contractions**
**• Cells are electrically coupled by gap junctions**
**• Some cells are self-excitatory (depolarize without external stimuli); act as pacemakers for sheets of muscle**
**• Rate and intensity of contraction may be modified by neural and chemical stimuli**
**• Sliding filament mechanism**
**• Final trigger is ↑ intracellular Ca2+**
**• Ca2+ is obtained from the SR and extracellular space**
79
STUDY
Contraction of Smooth Muscle
• Very energy efficient (slow ATPases)
• Myofilaments may maintain a latch state for prolonged contractions
Relaxation requires:
• Ca2+ detachment from calmodulin
• Active transport of Ca2+ into SR and ECF
• Dephosphorylation of myosin to reduce myosin ATPase activity
STUDY
Contraction of Smooth Muscle
• Very energy efficient (slow ATPases)
• Myofilaments may maintain a latch state for prolonged contractions
Relaxation requires:
• Ca2+ detachment from calmodulin
• Active transport of Ca2+ into SR and ECF
• Dephosphorylation of myosin to reduce myosin ATPase activity
80
WHAT 2 THINGS HAPPEN
DURING
**_Regulation of Contraction_**
**_1)Neural regulation:_**
• Neurotransmitter binding → ↑ [Ca2+] in sarcoplasm; either graded (local) potential or action potential
• Response depends on neurotransmitter released and type of receptor molecules
**_2)Hormones and local chemicals:_**
• May bind to G protein–linked receptors
• May either enhance or inhibit Ca2+ entry
81
What are the
**_Special Features of Smooth Muscle Contraction_**?
**_Stress-relaxation response:_**
• Responds to stretch only briefly, then adapts to new length
* Retains ability to contract on demand
* Enables organs such as the stomach and bladder to temporarily store contents
**_Length and tension changes:_**
• Can contract when between half and twice its resting length
82
Special Features of Smooth Muscle Contraction
**_Hyperplasia?
_**
**• Smooth muscle cells can divide and increase their numbers**
• *_Example:_*
**• estrogen effects on uterus at puberty and during pregnancy**
83
2 Types of ***_Smooth Muscle?_***
## Footnote
***_ _***
***_1)Single-unit (visceral) smooth muscle:_***
• Sheets contract rhythmically as a unit (gap junctions)
• Often exhibit spontaneous action potentials
• Arranged in opposing sheets and exhibit stress-relaxation response
***_2)Multiunit smooth muscle:_***
• Located in large airways, large arteries, arrector pili muscles, and iris of eye
• Gap junctions are rare
• Arranged in motor units
• Graded contractions occur in response to neural stimuli
84
_**\*\*STUDY**_ Developmental Aspects
• All muscle tissues develop from embryonic myoblasts
• Multinucleated skeletal muscle cells form by fusion
• Growth factor agrin stimulates clustering of ACh receptors at neuromuscular junctions
• Cardiac and smooth muscle myoblasts develop gap junctions
• Cardiac and skeletal muscle become amitotic, but can lengthen and thicken
• Myoblast-like skeletal muscle satellite cells have limited regenerative ability
• Injured heart muscle is mostly replaced by connective tissue
• Smooth muscle regenerates throughout life
• Muscular development reflects neuromuscular coordination
• Development occurs head to toe, and proximal to distal
• Peak natural neural control occurs by midadolescence
• Athletics and training can improve neuromuscular control
• Female skeletal muscle makes up 36% of body mass
• Male skeletal muscle makes up 42% of body mass, primarily due to testosterone
• Body strength per unit muscle mass is the same in both sexes
• With age, connective tissue increases and muscle fibers decrease
• By age 30, loss of muscle mass (sarcopenia) begins
• Regular exercise reverses sarcopenia
• Atherosclerosis may block distal arteries, leading to intermittent claudication and severe pain in leg muscles
_**\*\*STUDY**_ Developmental Aspects
• All muscle tissues develop from embryonic myoblasts
• Multinucleated skeletal muscle cells form by fusion
• Growth factor agrin stimulates clustering of ACh receptors at neuromuscular junctions
• Cardiac and smooth muscle myoblasts develop gap junctions
• Cardiac and skeletal muscle become amitotic, but can lengthen and thicken
• Myoblast-like skeletal muscle satellite cells have limited regenerative ability
• Injured heart muscle is mostly replaced by connective tissue
• Smooth muscle regenerates throughout life
• Muscular development reflects neuromuscular coordination
• Development occurs head to toe, and proximal to distal
• Peak natural neural control occurs by midadolescence
• Athletics and training can improve neuromuscular control
• Female skeletal muscle makes up 36% of body mass
• Male skeletal muscle makes up 42% of body mass, primarily due to testosterone
• Body strength per unit muscle mass is the same in both sexes
• With age, connective tissue increases and muscle fibers decrease
• By age 30, loss of muscle mass (sarcopenia) begins
• Regular exercise reverses sarcopenia
• Atherosclerosis may block distal arteries, leading to intermittent claudication and severe pain in leg muscles
85
What is **_Muscular Dystrophy_**
and what is **_DMD_**
**• Group of inherited muscle-destroying diseases
• Muscles enlarge due to fat and connective tissue deposits
• Muscle fibers atrophy**
_*Duchenne muscular dystrophy **(DMD):***_
**• Most common and severe type
• Inherited, sex-linked, carried by females and expressed in males (1/3500) as lack of dystrophin
• Victims become clumsy and fall frequently; usually die of respiratory failure in their 20s
• No cure, but viral gene therapy or infusion of stem cells with correct dystrophin genes show promise**
