What is muscle tissue specialized for? What are the 3 histologically distinct types of muscle?
1. specialized for contractility and excitability in order to perform movements. muscle cells are elongate and have membrane organelles specialized for contraction.
2. skeletal: attach to bone and responsible for movement of body parts
smooth: blood vessels, GI tract
Muscle contraction is also a source of ____.
Muscle tissue can be striated or non-striated and voluntary or involuntary. What types of muscle fall into what categories?
skeletal: striated voluntary
cardiac: striated involuntary
smooth: non-striated involuntary
note: striated=light and dark bands on microscopic level
voluntary: under cerebral cortical control
involuntary: under autonomic nervous system control
What are the levels of structural organization of skeletal muscle? What is the functional unit of muscle?
1. muscle (for example deltoid) is composed of numerous muscle fasicles that are composed of bundles of muscle fibers/muscle cells. muscle fibers/muscle cells/myofibrils are composed of myofibrils (organelle with repeating sarcomere units) that is composed of myofilaments (actin and myosin myofilaments
What are characteristics of skeletal muscle cells? (include discussion of nuclei, organelles, cytoplasm) How are they formed?
1. each muscle cell/muscle fiber/myofiber is a multinucleated cell with numerous peripherally located nuclei just beneath the plasma membrane. the plasma membrane in muscle is called the sarcolemma. cytoplasm, called sarcoplasm in muscle, is dominated by contractile filaments but is also rich in mitochondria. the sarcoplasmic reticulum is a specialized form or smooth ER and surrounds myofilaments (actin and myosin). functions in Ca2+ storage
2. muscle cells are formed from the fusion of precursor cells called myoblasts which results in the long multinucleated fibers
What are the 3 connective tissue layers and what portions of muscle do they surround? What is their purpose?
epimysium: dense irregular connective tissue that surrounds all of the fasicles of a muscle (the entire muscle)
perimysium: thinner layer of dense irregular connective tissue that surrounds individual fasicles
endomysium: fine layer of reticular fibers that surround individual muscle fibers (cells)
connective tissue layers maintain the 3D structure of muscles and also serve as a route for nerve and circulatory supply to muscle tissue
How many skeletal muscle fiber types are there? How are they characterized?
3 diff skeletal muscle fiber types based on differences in contractile speed, metabolic profile, and fiber morphology. most muscles have all 3 types but proportion depends on function of a particular muscle
Myofibrils are composed of myofilaments. What are myofilaments?
individual polymers of myosin (thick filaments) and actin with its associated proteins (thin filaments)
What are the 3 major proteins of thin filaments?
1. actin: a globular protein that polymerizes in a double helical structure forming the thin filament
2. tropomyosin: protein that also forms a double helical structure that lies in the groove of the actin helix. acts with the troponin complex to regulate myosin-actin binding
3. troponin complex: composed of 3 globular subunits
- TnI: actin binding protein that prevents binding of actin to myosin
- TnT: tropomyosin binding protein that holde troponin complex to actin
- TnC: calcium binding protein, essential step in initiation of muscle contraction
What are the 3 major proteins of thick filaments?
1. myosin heavy chain: large protein with 2 portions-an alpha helical "rod" portion and a globular "head" protion containing ATP and actin binding sites
2. myosin light chain 1
3. myosin light chain 2
light chain phosphorylation helps regulate calcium stimulation of contraction. is more important in smooth muscle
Myofilaments are linked into a highly structured organization called the ____, a unique structural feature of striated muscle. ____ (same as previous blank) line up end to end along the length of the muscle fiber in small parallel bundles called ____. The highly organaized arrangement of thick and thin filaments in a specific banding pattern forms the basis for muscle contraction
True or false: There are additional accessory proteins that regulate spacing, attachment, and alignment of myofibrils.
Give definitions of each and be sure to discuss which change sizes during contraction and whether or not they contain overlap of filaments:
A (anisotropic) band
I (isotropic) band
Z-line or disc
A-band: darker stained band seen in histological slides (dArK=A band) that is equivalent to the length of the myosin (thick) filament. It contains both the thick filament and the portion of the thin filament that overlaps. The extent of overlap depends on the degree of muscle contraction. Stays the same length regardless of muscle contraction!!!
I-band: lighter stained band (light=I-band) seen in histological slides and is equivalent to the portion of thin filaments in adjacent sarcomeres that are not overlapping with thick filaments. Shorten during contraction and elongate during muscle stretch!!!
Z-line/disc: serves as an anchor for thin filaments. defines boundaries of sarcomere (sarcomere is from one Z-line to the next)
M-line: located at center of sarcomere. where myosin filaments are anchored by accessory proteins
H-zone: distance between end of actin molecules from opposite ends of sarcomere (no overlap of actin & myosin). Shorten during contraction and elongate during muscle stretch!!!
Summary: During contraction, the I and H bands shorten and the Z-lines move together thus shortening the entire sarcomere. The opposite occurs with stretch or lengthening. The lenght of the A-band never changes.
Identify the bands and zones of a sarcomere in this electron micrograph.
Identify the stuctures in this electron micrograph of muscle.
What are transverse tubules (T-tubules)? Where on the myofibrils are they aligned? What organelle are they closely associated with and why? What is a triad?
T-tubules are tubular invaginations of the sarcolemma and are normally aligned with myofibrils at the A-I band junction. they are closely associated wit the terminal cisternae of the sarcoplasmic reticulum, where Ca2+ ions essential for muscular contraction are stored. The close association of T-tubules and the sarcoplasmic reticulum allow for quick contraction of myofibrils in the muscle fiber despite the fibers' large diameter i.e. allows for changes in the membrane electrical potential to extned into the center of the cell and by virtue of the close linkage with T-tubules, with terminal cisternae of sarcoplasmic reticulum which then release Ca2+
triad: triple structure seen in cross section consisting of 2 terminal cisternae flanking a T-tubule at the
A-I band jucntion
What are the names for the synaptic connection btwn a motor neuron terminal and a muscle?
neuromuscular junction (NMJ)/motor end plate
What are junctional folds? What is their purpose? What receptors are located there and what is their ligand? What effect does this ligand have on the muscle?
folding of the sarcolemma at the NMJ to increase surface area. contains acetylcholine receptors for release of the neurotransmitter aceytlcholine (Ach) from neurons. Ach causes contraction and is responsible for a trophic effect on the target muscle cell. reason why you get muscle atrophy/weakness in diseases involving motor end plate such as myasthenia gravis (autoimmune disease that attacks Ach receptors-when antibody binds them, are internalized and destroyed).
What is a motor unit? Where in the body will you find a small vs a large motor unit?
motor unit: a motor neuron (from spinal cord) and all of the muscle fibers it innervates. a motor neuron may branch and innervate a small number of muscle fibers (1 neuron to ~10 fibers) for fine motor control as in the digits of the hand or may have large motor units (1 neuron to ~2000 muscle fibers). large motor units are associated with gross postural control as in erector spinae muscles, abdominal muscles
Loss of stimulation to the muscle cell at the NMJ leads to muscle atrophy called _____ atrophy.
What are muscle spindles? Are they afferent or efferent receptors? What is their structure? What reflex are they the basis for?
muscle spindles are a type of sensory receptor in skeletal muscle that detect the amount of stretch in a muscle (length and rate of length changes). muscle spindles have an outer connective tissue capsule, within which an inner capusle containing 2 types of modified muscle cells (intrafusal fibers) lies. Afferent sensory fibers (Ia afferent) wrapped around intrafusal fibers are stimulated by change in the length of intrafusal fibers in the spindle. efferent fibers modulate the sensitivity of the spindle by adjusting the tension in the intrafusal fibers, thus mantaining sensitivity to stretch during contraction. spindle apparatus is the basis for the afferent limb of the stretch reflex (deep tendon reflex/DTR)
Muscle mass is a balance btwn myofibril protein ______ and ______.
synthesis and degradation
During development, myoblasts differentiate from what type of cell?
myotomal cells of somites of the paraxial mesoderm
Early myoblasts fuse to form ____ that then mature into skeletal muscle fibers. The _____ (same as previous blank) orginally have centrally located nuclei which then become displaced to the periphery by newly synthesized ____. Differentiation of myoblasts is associated with transcription factors that upregulate ____ (same as previous) gene transcription
True or false: One can increase the number of muscle fibers (muscle cells) they have with exercise.
False. the number of muscle fibers is determined prenatally
What is hypertrophy as it relates to muscle growth and what is the mechanism? How does longitudinal growth of muscles occur with skeletal growth?
1. hypertrophy, or circumfirential muscle growth during development occurs by increasing the size of myofibrils and myofibrils per muscle fiber
2. longitudinal growth occurs via adding new sarcomeres at the ends of myofibrils so that muscle cells increase in length with skeletal growth
True or false: Skeletal muscle is adaptive to use throughout life and occurs by changes in fiber size as well as fiber type.
What are factors affecting hypertrophy?
- Age: reduced ability to hypertrophy with advancing age
• Nutrition: adequate supply of amino acids necessary
• Hormones: testosterone (and to a lesser extent
estrogen), growth hormone, FGF and IGF promote
hypertrophy; cortisol causes muscle breakdown and
• Taking anabolic steroids (including testosterone) as
performance-enhancing drugs increases hypertrophy (but is illegal without prescription)
How do muscle cells regenerate? How well are they able to generate?
satellite cells are responsible for the limited ability of muscle cells to generate and are inside the external lamina of muscle cells. after injury, some become activated, reenter the cell cycle, express myogenic regulator factors, and proliferate. as long as the external lamina remains intact, myoblasts fuse to form myotubes that then mature into a new muscle fiber. if external lamina is disrupted, fibroblasts repair the injured site with resultant scar tissue formation
What is muscular dystrophy?
class of congenital muscle diseases causing severe weakness, atrophy, and destruction of muscle fibers that are caused by genetic mutations in the muscle transmembrane proteins of the dystrophin-glycoprotein complex. In skeletal and cardiac muscles, dystrophin is part of a group of proteins (a protein complex) that work together to strengthen muscle fibers and protect them from injury as muscles contract and relax. The dystrophin complex acts as an anchor, connecting each muscle cell's structural framework (cytoskeleton) with the lattice of proteins and other molecules outside the cell (extracellular matrix). The dystrophin complex may also play a role in cell signaling by interacting with proteins that send and receive chemical signals. progressive degeneration of skeletal muscle fibers places a constant demand on the satellite cells to replace the degenerating fibers. the satellite pool is ultimately exhausted and additional myogenic cells are recruited from bone marrow to supplement available satellite cells
What is disuse atrophy of muscles?
lack of exercise, prolonged bedrest, and cast immobilization leads to decreased muscle protein synthesis an disuse atrophy (occurs within 24 hours of casting!). atrophy also occurs within multiple disease states such as myasthenia gravis
What are the similarities and differences of cardiac and skeletal muscle?
similarities: contractile proteins, striated appearance, ultrastructure of sarcomeres
- 1-2 centrally located nuclei per cell
- presence of intercalated discs
- T-tubules in more prominent association with reduced SR cisternae so diads (instead of triads in skeletal muscle) are formed at the vicinity of the Z-line
- more mitochondria, lipid droplets, glycogen, and myoglobin in the sarcoplasm
- contraction of cardiac muscle is vigorous, rhythmic, and under the conrol of both the autnomic nervous system and circulating hormones
- branching of muscle fibers
How is cardiac muslce contraction controlled?
autonomic nervous system and circulating hormones
What are intercalated discs and what do they contain? What is their purpose?
intercalated discs are highly specialized attachments that joint cardiac muscle fibers together. the structural arrangement of the intercalated discs allows the heart to contract in a wringing fasion. intercalted discs contain:
1. fasciae adherens
2. maculae adherens (desmosomes)
3. gap junctions
the many gap junctions present are responsible for the electronic coupling btwn adjacent cardiac myocytes so the heart muscle contracts as a syncytium
Identify the portions of this cell. What type of cell is it and how can you tell?
presence of a diad at level of/running with Z-line
What are characteristics of smooth muscle? (think of those of cardiac and skeletal and how they are similar or differ). Where is it found? How is it controlled?
smooth muscle cells are fusifom, lack cross-striations and T-tubules, and have a single centrally located nucleus. found in walls of tubular organs, the iris and ciliary body of the eye, scrotum, blood vessels, hair follicles. length can vary from 20 micrometers to 500 micrometers. they are arranged as sheets joined by gap junctions to allow intercellular communication regulating contraction of the entire sheet or bundle of smooth muscle. contain dense bodies. like cardiac muscle, control is both autonomic (adrinergic and cholinergic) and hormonal but NMJs are simple, no T-tubules are present, and innervation tends to modify rather than initiate activity
What are dense bodies (in smooth muscle)?
serve to anchor the actin filaments to the sarcolemma in order to transmit contractile force. interaction of actin and myosin filaments during contraction is somewhat similar to that of skeletal muscle (details given in physiology lecture)
Physical exercise causes hypertrophy of skeletal and cardiac muscle fibers, where the increase in cell volume is due to an increase in size of myofibrils and formation of new myofibrils. Discuss the growth potential of smooth muscle.
smooth muscle has a modest regenerative potential. smooth muscle fibers are capable of hypertrophy and hyperplasia (increase in cell number, skeletal and cardiac not capable of this)
True or false: Cardiac muscle has a high regeneration potential.
False. For the most part, cardiac muscle cannot regenerate. Therefore, myocardial infarcts are replaced by connective tissue scars