Histology of Cardiac and Smooth Muscle Flashcards
(111 cards)
1
Q
Describe the shape and main characteristic of muscle fibers.
A
Elongated cells containing a great number of cytoplasmic filaments.
2
Q
What primordial tissue do muscle fibres originate from? Summarize the differentiation process.
A
Mesoderm
Differentiation occurs by a process of cell lengthening and synthesis of myofibrillar proteins.
3
Q
Name the three types of muscle tissue
A
- Skeletal striated muscle
- Smooth muscle
- Cardiac muscle
4
Q
Answer the following about the three classes of muscle:
A) Voluntary or involuntary?
B) Striated or non-striated?
A
Skeletal striated muscle
* Striated
* Voluntary
Smooth muscle
* Non-striated
* Involuntary
Cardiac muscle
* Striated
* Involuntary
5
Q
Describe the cross-sectional appearance (nucleus, myofibrils) of the three classes of muscle fibres.
A
Skeletal striated muscle
*Peripherally located nuclei
* Myofibrils are seen in cross-section
Smooth muscle
* Centrally located nucleus
* Myofibrils are not seen in cross-section
Cardiac muscle
* Centrally located nucleus
* Myofibrils are seen in cross-section
6
Q
Describe the differences in contraction of each muscle class.
A
Skeletal striated
* Contraction is quick, forceful and usually under voluntary control
Smooth muscle
* Contraction is slow and not subject to voluntary control
* Wall of GI tract, blood vessels, uterus
Striated cardiac muscle
* Contraction is involuntary, vigorous and rhythmic
7
Q
What is the principal adaptation of each muscle class in response to increased workload, demand or stress?
A
Skeletal striated muscle:
* Hypertrophy (increase in size)
Smooth muscle
* Hyperplasia (increase in number)
Cardiac muscle
* Hypertrophy (increase in size)
8
Q
What is the most abundant muscle type in our body?
A
Skeletal muscle
9
Q
How are skeletal muscle fibres attached to connective tissue?
A
They are attached to connective tissue collagen by junctional complexes, which are associated with the infolding of the sarcolemma (plasma membrane).
10
Q
What are the names of the plasma membrane and cytoplasm of muscle fibres?
A
Plasma membrane = sarcolemma
Cytoplasm = sarcoplasm
11
Q
What type of junction is formed by junctional complexes?
A
Musculotendinous junction
12
Q
Name the 3 layers of connective tissue that organize skeletal muscle fibres.
A
Epimysium
Perimysium
Endomysium
13
Q
Describe the junctional complexes.
A
At the ends of the striated skeletal cell are finger-like projections: they are made of actin filaments that insert into dense structures located on the inner side of the plasma membrane. The external side of this plasma membrane is covered by basement membrane which is attached to the collagen fibres of the tendon (aponeuroses).
14
Q
What is the endomysium? Name its 3 components
A
Thin layer of connective tissue that surrounds each individual muscle fibres.
It consists of 3 main components:
- Basement membrane (basal lamina), which lies directly against the fibre’s sarcolemma
- Reticular elastic fibrils (collagen type III) and elastic fibres
- Capillaries, embedded in the endomysium
15
Q
What is a fascicle?
A
Bundles of skeletal muscle fibres grouped together within a muscle.
16
Q
What is the perimysium?
A
It is the layer of connective tissue that surrounds skeletal muscle fascicles (bundles of fibres).
17
Q
What is the epimysium?
A
It is the layer of dense connective tissue that surrounds the entire skeletal muscle.
18
Q
What is the length and diameter of skeletal muscle fibres?
A
1-40mm in length and 10-100um in diameter.
19
Q
Explain why skeletal muscle fibres are multinucleated?
A
They arise from the embryonic fusion of uninucleated myoblasts.
20
Q
What are satellite cells?
A
Specialized small stem cells located between the sarcolemma and basement membrane of skeletal muscle fibres.
21
Q
What is the role of satellite cells?
A
When needed, the satellite cell can double its nucleus by mitotic division and transfer one nucleus across the sarcolemma into the muscle fibre, while the second nucleus remains in the satellite cell.
22
Q
What are the 3 major organelles of skeletal muscle fibres?
A
- Myofibrils
- Mitochondria
- Smooth endoplasmic reticulum
23
Q
What are the 3 types of muscle fibres mixed to varying degrees in each skeletal muscle? How are they distinguished?
A
Red, white and intermediate muscle fibres.
They are distinguished based on the amount of mitochondria and myoglobin, an oxygen-binding protein, in each type of fibre.
24
Q
Which fibre type contains more mitochondria and myoglobin?
a) red fibres
b) intermediate fibres
c) white fibres
Which type contains the least mitochondria and myoglobin?
A
Most: a) red fibres
Least: c) white fibres
25
Which fibre type is smaller than the other?
a) red fibres
b) white fibres
a) red fibres are smaller than white fibres
26
Describe how the 2 types of skeletal muscle fibres (red, white) functions. Where are they more prevalent?
Red fibres
* Slow-twitch motor units
* Resistant to fatigue
* Postural muscles of the limbs and back
White fibres
* Fast-twitch motor units
* Fatigue rapidly
* Give strong contractions
* Leg and arm muscles, chicken breast
27
Within the sarcoplasm of skeletal muscle fibres are several hundred to several thousand...
myofibrils (1-2 um in diameter) organized in parallel bundles
28
Each myofibril is made up of...
* 1500 thick filaments (myosin)
* 3000 thin filaments (F-actin)
29
Name the two types of contractile proteins in skeletal muscle.
Myosin
Actin
30
What is a sarcomere?
The sarcomere is the fundamental contractile unit of striated muscle, that enables muscle contraction. It extends from Z line to Z line (2-3 um).
31
What are Z lines? What are M lines?
Z lines: Boundaries of a sarcomere where thin filaments (actin) are anchored.
M lines: At the centre of the sarcomere, where thick filaments (myosin) are anchored.
32
What produces the characteristic appearance of striated muscle under the microscope?
Regularly spaced light and dark bands of each sarcomere.
33
What is the A band? Does it appear anisotropic or isotropic under the microscope?
Region of the sarcomere spanning the entire length of thick filaments, including the overlap with thin filaments.
It appears anisotropic (i.e. dark) under the microscope.
34
What are I bands? Do they appear anisotropic or isotropic under the microscope?
Regions of the sarcomere flanking the A bands, made up of thin filaments and cut down their middle by the Z lines (no overlap with thick filaments).
35
Each sarcomere contains a central ... which is flanked by...
Each sarcomere contains a central A band which is flanked by two "half" I bands.
36
What is the H zone?
The region immediately adjacent to each M line, where thin and thick filaments do not overlap (thick filaments only).
37
Which sarcomere regions change during muscle contraction? Which stay the same?
During muscle contraction, the thin and thick filaments slide past each other, either increasing or decreasing the overlap.
During contraction, the A bands will remain the same size, while the I band and H zone will decrease in size.
38
Describe the composition of thick filaments.
Thick filaments consist of ~400 myosin molecules.
Each myosin molecule is a dimer of tightly intertwined heavy polypeptide chains, i.e. the body or tail.
Each of the two polypeptide chains is attached to a globular head via a short elastic stalk.
39
What type of myosin makes up the body/tail of myosin molecules?
What type of myosin makes up the elastic stalk and head of myosin molecules?
Body/tail: L-meromyosin (LMM)
Elastic stalk and head: H-meromyosin (HMM)
*Note: The elastic stalk consists of the S-2 subtype of HMM and the head consists of the S-1 subtype of HMM.
40
A pair of .... is associated with each of the two globular heads of myosin dimers (thick filaments), for a total of...
A pair of light polypeptide chains (L1 and L2), for a total of 4 light chains associated with a double-head of a myosin dimer.
41
What type of binding sites are on the myosin heads of thick filaments (3)
* Specific binding sites for actin
* ATP receptor site
* Catalytic site capable of hydrolyzing ATP
42
What protein helps anchor thick filaments to the Z line?
Thin elastic filaments composed of the protein titin.
43
What are the 3 structural proteins that make up actin filaments? What is their ratio in skeletal muscle?
Actin (major structural protein)
Tropomyosin and troponin (regulatory proteins).
7:1:1
44
Describe the composition of thin filaments.
Each F-actin (fibrous) polymer is composed of ~300 G-actin monomers (globular subunits) that form a long chain.
Two fibrous actin (F-actin) polymers wind around each other to form a double-stranded coil.
Tropomyosin is also a double-stranded twisted molecule that runs down the to grooves of the F-actin helix.
45
In thin filaments, 1 molecule of tropomyosin extends over...
1 molecule of tropomyosin extends over 7 G-actin monomers.
46
Name the three troponin proteins (thin filaments) and describe what each is bound to.
Troponin-I (TN-I): binds actin
Troponin-T (TN-T): binds tropomyosin
Troponin-C (TN-C): binds TN-T and TN-I together
47
What is the role of the troponin complex in the absence of free Ca2+ (relaxed striated muscle).
It keep the actin-tropomyosin filaments in a configuration that masks myosin-binding sites on the thin filaments. Therefore, it plays an inhibitory role, because the globular myosin heads cannot bind the actin filaments in this configuration.
48
What is Nebulin? What are its 3 functions?
Nebulin is a giant protein molecule that is part of the muscle cell's cytoskeleton (like titin). Two strands of Nebulin wrap around the entire length of the actin filaments.
* It regulates the assembly of the actin filament
* It stabilizes the length of the actin filament
* It helps anchor the actin filament to the Z line
49
What protein makes up the Z line? What is its function?
Alpha-actinin
It binds titin and nebuli to anchor the thin and thick filaments to the Z line.
50
What comprises the sarco-tubular apparatus?
It comprises the sarcoplasmic reticulum and T-tubules (transverse tubules).
51
What are T-tubules (transverse tubules)?
They are invaginations of the muscle cell membrane (sarcolemma) that occur at the junctions of the A and I bands. They run perpendicular to the myofibrils. They provide an extension of the unit membrane and extracellular space in the interior of the muscle fibres.
52
Describe the sarcoplasmic reticulum.
Network of canals running in parallel with the sarcomere filaments, made of the smooth endoplasmic reticulum.
It is bounded on either side (but is not continuous with) a transverse tubule. At each end (next to the T-tubules) the SR tubules expand into terminal cisternae.
53
How many terminal cisternae does each sarcoplasmic reticulum have?
Two, one at each end.
54
What is a "triad" in skeletal muscle fibres?
A T-tubule with its two adjoining terminal cisternae.
55
T-tubules are connected to...
the sarcolemma
56
What is a neuromuscular junction?
Synaptic junction between a motor neuron and muscle fibre.
57
What is the motor endplate?
The specialized region of the sarcolemma (muscle fibre plasma membrane) at the neuromuscular junction.
58
As the axon of a motor neuron approaches the muscle fibre, how does its configuration change? (3)
1. Its myelin coat ends
2. The cytoplasm of the surrounding Schwann cells thins
3. The axon branches and invaginates into small depressions in the muscle fibre
59
Where is the synaptic cleft (neuromuscular junction)? Describe it.
The synaptic cleft is located on the motor endplate of the muscle fibre. It has many invaginations that increase the surface are of the muscle fibre in this region.
60
Name the 2 important constituents of the axon terminal of motor neurons that synapse onto muscle fibres.
It contains many synaptic vesicles and mitochondria.
61
What neurotransmitter is released at the neuromuscular junction? Describe this process.
Acetylcholine.
Acetylcholine is released into the cleft and binds nicotinic acetylcholine receptors of the sarcolemma, which leads to muscle contraction.
62
How does the release of acetylcholine trigger muscle contraction?
When acetylcholine binds the receptors of the sarcolemma, it causes a local increase in the permeability of the sarcolemma, allowing Na+ ions into the cell and K+ ions out of the cell. Since more Na+ ions enter the cell than K+ ions leave, the motor endplate becomes depolarized. The process is propagated through the T-tubules.
63
Name the 3 layers of the heart wall.
1. Epicardium
2. Myocardium
3. Endocardium
64
What is the pericardium? Name its layers.
The pericardium is a sac that surrounds and protects the heart.
External layer: Fibrous pericardium
Internal layer: Serous pericardium
The serous pericardium in turn has 2 layers:
* Parietal lamina (in contact with the fibrous pericardium)
* Visceral lamina i.e. epicardium (covers the heart and parts of blood vessels)
65
Describe the myocardium.
Thickest part of the heart wall, made up of cardiac muscle fibres.
66
Describe the endocardium.
Thin membrane consisting of endothelium, connective tissue and smooth muscle tissue (innermost layer of the heart wall).
67
Which layer of the heart wall contains cardiac muscle fibres?
The myocardium.
68
What is between the visceral pericardium (epicardium) and the myocardium? (3)
* Mesothelium
* Subepithelial layer of connective tissue containing nerves, nerve ganglia and veins
* Adipose tissue
69
The endocardium is continuous with...
the endothelium of large blood vessels.
70
Name 3 differences in the appearance of cardiac muscle fibres compared to skeletal muscle fibres.
* Cardiac muscle fibres are shorter than skeletal muscle fibres
* Cardiac muscle fibres can branch
* Cardiac muscle fibres have a single central nucleus
71
What is the name of the junctional complexes that attach cardiac muscle cells to one another?
Intercalated disks
72
The sarcolemma of cardiac muscle fibres is surrounded by...
a basement membrane (basal lamina) and reticular fibrils.
73
Describe the sarcoplasm of cardiac muscle fibres.
It contains fewer and larger myofibrils than skeletal muscle. These myofibrils diverge around the central nucleus, leaving the perinuclear poles (regions around the nucleus) devoid of myofibrils but rich in mitochondria and Golgi apparatus.
74
Where are mitochondria located in cardiac muscle fibres and why?
Large mitochondria are packed between the myofibrils to bring the energy source close to the contractile proteins.
75
Describe the two parts that make up intercalated disks in cardiac muscle.
1. Transverse part
* Forms the physical connection
* Consists of desmosomes and adherens junctions called fascia adherens
2. Lateral part
* Forms the communicating part
* Consists of a gap junction
76
Explain the role of the gap junctions in intercalate disks of cardiac muscle fibres.
Gap junctions provide a means of passing ionic signals from one cell to another and allow the heart to have a synchronous beat
77
What regulates synchronization in the heart?
Modified cardiac myocytes called Purkinje fibres that are organized into nodes.
78
How do the T tubules in cardiac muscle fibres compare to skeletal muscle?
The T tubules in cardiac muscle are also located on the Z-lines, but the are much wider and the basement membrane lines the entire interior of the tubules.
79
How does the sarcoplasmic reticulum in cardiac muscle fibres compare to skeletal muscle?
The SR tubules are much wider in cardiac muscle, while the terminal cisternae are smaller and not as consistent.
80
Why is the SR and T tubule complex considered a diad in cardiac muscle?
Because only one larger terminal cisternae is associated with each T tubule (rather than 2 as in skeletal muscle).
81
Cardiac muscle fibres of the atria contain...
secretory granules (atrial granules) rich in atrial natriuretic factor and brain natriuretic factor.
82
Name the 2 diuretic hormones found in cardiac muscle fibres of the atria and their effects on the body.
Atrial natriuretic factor
Brain natriuretic factor
* Inhibit renin secretion in the kidney
* Relax vascular smooth muscle
83
When do brain natriuretic factor levels increase in the blood?
In pathological conditions where there is congestive heart failure.
84
What is the name of "adult" cardiac muscle fibres?
Cardiomyocytes
85
Why does scar tissue replace damaged cardiac fibres? What is the consequence?
Once mature, cardiac myocytes can no longer divide, so when injuries occur from heart attacks (myocardial infarctions), scar tissue replaces the damaged cell and disrupts normal heart function.
86
What type of muscle is in the ciliary body of the eye?
Smooth muscle
87
How is smooth muscle organized and how does it contract?
* Organized in layers and sheets
* Contracts in spontaneous fashion under the control of the autonomic nervous system or hormones
88
Describe the appearance of smooth muscle cells
* Long and spindle shaped
* Eosinophilic sarcoplasm
* Single, central, sausage shaped, elongated nucleus (appears corkscrew shaped when contracted)
* No cross-striations, but contraction bands are visible
89
Each smooth muscle cell is surrounded by basement membrane except at the site of...
gap junctions, which connect smooth muscle fibres to one another
90
What surrounds the basement membrane of smooth muscle cells?
Reticular fibres and elastic fibrils (collagen type III and IV secreted by muscle cells).
91
What is the equivalent of T tubules in smooth muscle?
Invaginations of the sarcolemma called caveolae.
92
Which muscle cell type is capable of mitosis in response to injury?
Smooth muscle cells
93
Where can you find the longest smooth muscle cell fibres?
In the gravid uterus, where the smooth muscle develops extensively during pregnancy.
94
What are myoepithelial cells?
Epithelial cells around sweat glands, mammary glands and salivary glands that show characteristics of smooth muscle cells.
95
The sarcoplasm of smooth muscle cells 3 types of filaments...
1. Thin filaments
2. Intermediate filaments
3. Thick filaments
*Note that these do not impart any visual pattern on the cells and are not cross striated*
96
Describe thin filaments in smooth muscle cells.
Thin filaments
* Originate from dense patches that are either membrane-associated or free in the sarcoplasm
* Dense patches are similar to Z lines and contain alpha actinin
* One end of thin filaments is free in the sarcoplasm
* They contain actin and tropomyosin
97
Describe intermediate filaments in smooth muscle cells
* Extend between and attached to dense patches
* Not directly involved in contraction
* Desmin is a component of all intermediate filaments
* Vimentin is a component of intermediate filaments in vascular smooth muscle only
98
Describe thick filaments in smooth muscle cells
* Not attached to dense patches
* Lie parallel to thin filaments
* Made up of myosin II
99
Describe how contraction works in smooth muscle.
Myofibrils criss cross the sarcoplasm and contraction occurs according to the sliding filament theory. The cell wrinkles up during contraction and shortens.
100
Describe what happens in smooth muscle fibres upon calcium release from the small SR associated with the caveolae.
1. Calcium is released and binds calmodulin
2. A myosin light chain kinase phosphorylates one of the two light chains of myosin
3. Phosphorylated myosin can interact with actin and contracts the cell
4. Upon dephosphorylation the myosin releases actin
101
Smooth muscle cells can maintain...
a contracted state for long periods of time.
102
Smooth muscle can act like a .... by producing a wave of contraction (e.g. in the gut).
peristaltic pump
103
What is the most common injury of cardiac muscle?
Ischemia, i.e. reduction in blood flow to the heart tissue (injury is caused by lack of oxygen when a coronary artery is occluded).
104
What is myasthenia gravis?
It is a neuromuscular disease leading to muscle weakness and fatigue. It is an autoimmune disorder. Antibodies recognize the nicotinic acetylcholine receptors in the neuromuscular junction and inhibit the effects of acetylcholine.
105
How does a cardiac gap junction help connect cells together?
Via coupled connexons (some on the membrane of each cell), each composed of 6 connexins (connexin 43).
*Note: these gap junctions are calcium channels that help propagate the action potential between cardiac muscle fibres!*
106
What two proteins make up the dense bodies (patches) of the smooth muscle cells?
Alpha actinin
Dystrophin
107
Ehlers-Danlos' Syndrome
* Group of inherited disorders that affect connective tissues
* Primarily affects skin, joints and blood vessel walls
* Severe form: Vascular Ehlers-Danlos Syndrome (may cause the walls of blood vessels, intestines or uterus to rupture)
108
Name two mutations that can cause muscular dystrophy.
1. Mutations of the dystrophin gene (Duchenne and Becker muscular dystrophy)
2. Mutations of the laminin gene
109
Describe muscular dystrophy as caused by mutations in the dystrophin gene
Duchenne and Becker Muscular Dystrophy
* Dystrophin is a protein found in the sacolemma
* Dystrophin links actin of the cytoskeleton of muscle fibres to dystroglycans of the sarcolemma
* Mutations in dystrophin cause lack of mechanical stabilization in muscle fibres during contraction
110
Describe muscular dystrophy as caused by mutations in the the laminin gene
* Laminins are extracellular matrix proteins and a major component of the basement membrane
* Laminins also interact with dystroglycans of the sarcolemma
* Defective laminins can cause muscle to form improperly, leading to a form of muscular dystrophy
111
Regardless of the cause, what are the consequences of muscular dystrophy?
* Muscle weakness and degeneration
* Some muscular dystrophy variants may affect the heart
* Virtually all patients with Duchenne muscular dystrophy (DMD) will develop some degree of cardiomyopathy in adulthood (heart muscle disease)
