1
Q
What are the 2 types of muslce tissue?
A
- striated muscle
- smooth muscle
2
Q
What are the 2 subdivisions of striated muscle?
A
- skeletal muscle
- cardiac muscle
3
Q
Comparison between Skeletal, Cardiac & Smooth muscle
A
4
Q
What are the 2 major contractile fibers components in muscle cells?
A
- actin
- myosin
5
Q
Which muscles control voluntary contraction?
A
skeletal muscles
6
Q
Which muscles control involuntary contraction?
A
cardiac + smooth muscles
7
Q
Why are connective tissues investments important in skeletal muscles?
A
they provide vascular + neural elements to muscle cells
8
Q
Epimysium
A
surrounds an entire muscle
- forms aponeuroses (skeletal muscle <–> muscle)
- forms tendons (skeletal muscle <–> bone)
9
Q
Perimysium
A
**surrounds fascicles of muscle cells **(small bundles)
10
Q
Endomysium
A
surrounds individual muscle cells
- composed of reticular fibers + external lamina
11
Q
a muscle cell (*muscle fiber) *is also called…
A
a multinucleated syncytium
12
Q
muscle fibers develop from…?
A
… the fusion of small, individual muscle cells called
myoblasts
13
Q
Describe a muscle cell as seen through a light microscope.
A
- multinucleated
- polygonal shape (transvers view)
- long, cylindrical (longitudinal view)
- enveloped by external lamina + reticular fibers
- diameter of 10-100 um (micrometers)
- nuclei lie just beneath plasma membrane
14
Q
What is a good way to differentiat between connective tissue and muscle fibers?
A
muscle cells = cellular elements
connective tissue = extracellular products of CT cells
15
Q
what is a muscle cell plasmalemma also know as?
A
Sarcolemma
(plasma membrane + external lamina + surrounding reticular lamina)
16
Q
What are the 3 types of muscle fibers?
A
- Red fibers
- White fibers
- Intermediate fibers
17
Q
Red Muscle Fibers
(Type 1)
A
- Slow contraction
- do NOT fatigue easily
- High myoglobin content
- High # of mitochondria
- High in oxidative enzymes
- Low in ATPase
18
Q
White Muscle Fiber
(Type 2B)
A
- Fast contraction
- fatigue easily
- Low myoglobin content
- Low # of mitochondria
- Low in oxidative enzymes
- High in ATPase + phosphorylation
19
Q
Intermediate Muscle Fiber
(Type 2A)
A
- **Intermediate **myoglobin content
- ** Intermediate **# of mitochondria
- ** Intermediate **# of oxidative enzymes
- ** Intermediate** amount of ATPase
20
Q
Red Fibers:
What type of contraction?
Do they fatigue easily?
Method of ATPase production?
A
Red Fibers:
- slow + repetitive contraction
- do not fatigue easily
- oxidative phosphorylation (ATPase)
21
Q
White Fibers:
What type of contraction?
Do they fatigue easily?
Method of ATPase production?
A
White Fibers:
- fast contraction
- fatigue easily
- anerobic glycolysis (ATPase)
22
Q
Intermediate Fibers:
What type of contraction?
Do they fatigue easily?
Method of ATPase production?
A
Intermediate Fibers:
- fast contraction
- NOT easily fatigued
- oxidative phosphorylation + anerobic glycolysis (ATPase)
23
Q
What is Myoglobin ?
A
- a protein similar to hemoglobin
- binds O2
24
Q
What can change the type of muscle fibers?
Which type of fiber can change (and why would it)?
A
change in innervation can change a fiber type
- if a RED fiber becomes denervated, its innervation can be replaced by WHITE fiber innervation, changing it into a WHITE fiber.
25
Which fibers are *slow-twitch motor units** *** and are integral to back posture?
red fibers
| (less MUSCLE TENSION than white)
26
Which fibers are *fast-twitch motor units* and are inteegral for fine, precise movements in the digits?
white fibers
| (more NEUROMUSCULAR JUNCTIONS than red)
27
Skeletal Muscle Structure
- "banding pattern" *(alignment of **myofibrils**)*
--\> dark "**A bands**"
--\> light "**I bands**" *(bisected by **Z discs**)*
28
What does the intermediate filament **desmin **assist with?
- holding myofibrils in alignment
(during embryonic development = **Desmin + vimentin**)
- connects _cytoskeleton_, _nucleus_, _motor end plates_, and _mitochondria_ to myofibrils
* (distributes force of contraction --\> protects structural integrity)*
29
What does the** plectin** assist with?
- helps hold myfibrils in alignment
- tethers adjacent **Z discs** to each other
30
what is a **sarcomere**?
- regular repeating region between successive Z disks
## Footnote
**functional unit of contraction**
31
What is the **sarcomeric reticulum (SR)**?
**modified SER that surrounds myofilaments + forms a meshwork around each myofibril**
- forms a pair of dilated *terminal cisternae* (encircle myofibrils at junction of each A and I band)
_- regulates muscle contraction:_
--\> sequestering calcium ions (**relaxation**)
--\> releasing calcium ions (**contraction**)
32
What is a **triad**?
specialized complexes consisting of a narrow central **T tubule** flanked on each side by **terminal cisternae of the SR**
**-** located at the **A–I junction**
**-** help provide **_uniform contraction_**
33
What are **myofilaments?**
thick filaments (15 nm in diameter and 1.5 um long)
**+**
thin filaments (7 nm in diameter and 1.0 um long)
- lie parallel to the long axis of the myofibril
- responsible for the sarcomere banding pattern
34
What are **satellite cells**?
(myoblast-like)
**_regenerative cells_**
**- differentiate**
**- fuse with one another**
**- form skeletal muscle cells as needed**
- probably left over from embryonic development
- lie within basal lamina of skeletal muscle cells
35
What is **myostatin?**
- protein manufactured + released by muscle cells
**- restricts the size of individual skeletal muscle cells**
* (so that muscle cells do not get too long or broad)*
- member of the tumor growth factor B(beta) superfamily
36
**A bands**
**- anisotropic with polarized light**
*(different properties when observed in different directions)*
**- usually stain dark**
- contain both **thin and thick filaments**, which overlap and interdigitate
* (Six thin filaments surround each thick filament)*
37
**I bands**
**- isotropic with polarized light**
*(same properties when observed in different directions)*
**- appear lightly stained**
**- They contain only thin filaments**
38
**H bands**
- light regions t**ransecting A bands**
- consist of **thick filaments** only.
39
**M lines**
- narrow, dark regions at the center of H bands
- formed by several cross-connections (M-bridges) at the centers of adjacent thick filaments
40
**Z disks (lines)**
- dense regions **bisecting each I band**
- contain **a(alha)-actinin** and **Cap Z** proteins
* (bind to thin filaments + anchor them to Z disks with help of nebulin)*
* -* peripherally located Z disks anchored to regions of sarcolemma ("costameres") by **vinculin** and **dystrophin**
41
Thin filaments are composed of...?
- F-actin
- tropomyosin
- troponin
- associated proteins
42
**F-actin**
| (thin filament)
**polymer of G-actin monomers arranged in a double helix**
**-** each monomer possesses an **active site** that can **interact with myosin**
- diameter of 5–7 nm
- exhibit polarity, having a (+) and a (-) end
* (plus end tethered to cap Z of the Z disk; minus end, capped by tropomodulin, is located at the H band)*
**(-) end = growing end of the F-actin**
- loses and gains back G-actin molecules at both its plus and minus ends *(turn over rate is very slow = a few days)*
43
**Tropomyosin**
| (thin filament molecule)
- 40 nm in length
- bind head to tail, forming filaments that are located in the grooves of the F-actin helix
44
**Troponin**
| (thin filament proteins - 3)
- associated with each tropomyosin molecule
- composed of:
--\> **Troponin T (TnT)** forms the tail
*(binding the troponin complex to tropomyosin)*
--\> **Troponin C (TnC)** 4 binding sites for calcium
*(may be related to calmodulin)*
--\> **Troponin I (TnI)** binds to actin
*(inhibits interaction of myosin and actin)*
45
Identify the following structures from the image:
- M
- S
- Sr
- M = myofibrils
- S = sarcomeres
- Sr = sarcoplasm (cytoplasm of muscle cell)
46
**Nebulin**
| (thin filament protein)
- long, inelastic protein
- 2 nebulin molecules wrap around each thin filament
(assist in anchoring it to the Z disk)
47
**Tropomodulin**
- caps the (-) end of each thin filament
- prevents addition of more G-actin molecules to *growing end*
48
Thick Filaments contain...?
- approximately 250 myosin II molecules
* (arranged in an antiparallel fashion)*
- three associated proteins:
--\> myomesin
--\> titin
--\> C protein
49
**Myosin II**
- composed of 2 identical heavy chains + 2 pairs of light chains
- resembles a double-headed golf club
\* at least 18 different subtypes of myosin\*
50
**Myocine II - Heavy Chains**
- long rod like “tail” and a globular “head”
**tails:**
- wind around each other in an (alpha)-helical configuration
- function in self-assembly of myosin molecules into bipolar thick filaments
**heads:**
- actin-binding sites of the heads function in contraction
51
**Myosin II - light chains**
- two types; one molecule of each type is associated with the globular head of each heavy chain
52
**Digestion of myosin**
1. enzyme _trypsin_ cleaves myosin into:
- **light meromyosin** (part of the tail portion)
- **heavy meromyosin** (the two heads and the remainder of the tail)
2. enzyme _papain_ cleaves the heavy meromyosin releasing the
**short tail (S2 fragment)**
**+ two globular heads (S1 fragments)**
These S1 fragments have ATPase activity but require interaction with actin to release the noncovalently bound adenosine diphosphate (ADP) and Pi
53
**Myomesin**
- protein at the M line that cross-links adjacent thick filaments
- maintains their spatial relations
54
**C protein**
- binds to thick filaments in the vicinity of M lines along much of their lengths
## Footnote
*(between M line and end of thin filament in vicinity of the A–I junction)*
**\*This region of the A band is referred to as the C zone\***
55
**Titin**
- large **linear protein** that displays axial periodicity (reoccuring)
- forms an **elastic lattice** that parallels the thick and thin filaments,
- two titin filaments anchor each thick filament to the Z disk
* (maintains their architectural relationships to each other)*
\>\> amino terminal of the titin molecule spans entire thickness of the Z disk and binds to a-actinin and Z proteins.
\>\> within the Z disk, titin overlaps with other titin molecules from the neighboring sarcomere and probably forms bonds with them or with unidentified linker proteins
\>\> The carboxyl terminal of the titin molecule spans the entire M line and overlaps with titin molecules from the other half of the same sarcomere, and binds to the protein myomesin
\>\> Within the I band, in the vicinity of the Z disk, titin interacts with thin filaments
\>\> Within the A band, titin interacts with C protein
56
General features of cardiac muscle cells:
- **contract spontaneously**; display a rhythmic beat (modified by hormonal + neural stimuli -\> sympathetic + parasympathetic)
- may **branch at ends** = connect w. adjacent cells
- contain **one central nucleus** (occasionally two)
- contain **glycogen granules** (esp. at poles of nucleus)
- sarcoplasm is **rich in myoglobin**
- have **thick + thin filaments** arranged in **poorly defined myofibrils**
- **cross-banding pattern**
- do NOT regenerate; injuries repaired by formation of fibrous CT
(**scar tissue fr. fibroblasts)**
57
Structural components of cardiac muscle cells:
- T tubules **larger** than skeletal m. + **linked** by external lamina
(invaginate fr. the sarcolemma at Z discs)
- SR is **poorly defined** + contributes to **dyad** formation
(dyad = T tubule + one SR profile)
_- **Calcium ions: **_
--\> relaxation = Ca2 leaks into sarcoplasm @slow rate = automatic rhythm
--\> Ca2 enters cardiac muscle cells fr. extracellular environment
*(voltage-gated Ca2 channels of T tubules and sarcolemma)*
--\> Ca2 released fr. SR + corbular sarcoplasmic reticulum via ryanodine receptors *(response to Ca2 entering channels)* = cause contraction of cardiac muscle.
--\> force of contraction directly dependent on availability of
Ca2 in sarcoplasm
*(basal cardiac contraction, **only 50%** of available calcium-binding sites of TnC are occupied)*
58
How does SR presnt in the vacinity of Z discs in cardiac muscle?
- as small, basketlike saccules known as **corbular sarcoplasmic reticulum**
(region rich in Ca2-release channels (junctional feet) --\> analogous to the SR terminal cisternae)
59
Functional anatomy of cardiac muscle:
–Striated
–Short branched cells
–Uninucleate
–Intercalated discs
–T-tubules larger + over z-discs
60
How is injured cardiac tissue repaired?
- formation of fibrous connective cissue produced by **fibroblasts**
## Footnote
**= _scar tissue_**
61
good to know
good to know
62
Function of cardiac valves is...?
to prevent backflow
63
**Atrioventricular Valves**
- Prevent backflow to the atria
- Prolapse is prevented by the chordae tendinae
(Tensioned by the papillary muscles)
64
the function of semilunar valves is...?
- Prevent backflow into ventricles
65
These structures are **more abundant** in cardiac muscle, and **lie parallel to the I bands** and often are **adjacent to lipids**.
mitochondria
66
Where are atrial granules found and what is their function?
- present in the atrial cardiac muscle cells
- contain precursor of atrial natriuretic peptide
(acts to _decrease resorption of sodium and water in the kidneys_)
**= reducing body fluid volume and blood pressure**
67
Identify the muscle tissues:
A- Skeletal
B- Smooth
C- Cardiac
68
Intercalated disks
- complex steplike junctions
- form end-to end attachments b/t adjacent cardiac muscle cells
- _ transverse portion of disks runs across muscle fibers @right angles + possesses three specializations:_
1. **fasciae adherentes** (analogous to zonula adherentes) actin filaments attach to these
2. **desmosomes** (macula adherentes)
3. **gap junctions**
69
Skeletal Muscle (comparison)
- long, cylindrical shape
- many, **peripheral nuclei**
- striated
- has **triads** (T tubules & 2 SE) at A-I junctions
- no gap junctions
- sarcomeres
- restricted regeneration
- voluntary contraction
70
Cardiac Muscle (comparison)
- blunt-ended, branched shape
- 1 or 2 central nuclei
- striated
- has **dyads **( T tubule + 1 SE) at Z discs
- gap junctions at **intercalated discs**
- has sarcomeres
- no regeneration
- no voluntary constriction
71
Smooth Muscle (comparison)
- short, spindle shaped
- one central nucleus
- **not striated**
- has caveolae (but no T tubules) + some SER
- gap junctions in sarcolemme = "nexus"
- no sarcomeres
- extensive regeneration
- no voluntary contraction
72
**Huxley’s sliding-filament model**
| (muscle contraction)
- contraction = thick + thin filaments do not shorten but increase their overlap
- thin filaments slide past thick filaments + penetrate more deeply into the A band (remains constant in length)
- I bands + H bands shorten as Z disks are drawn closer together
73
Initiation of muscle contraction: Step 1
**Depolarization + release of Ca2+ triggers binding of actin and myosin**
**= contraction**
1. sarcolemma depolarizes at myoneural junction
2. T tubules convey wave of depolarization to myofibrils.
3. Voltage-sensitive dihydropyridine (DHP) receptors alter their conformation (membrane depolarization)
4. Ca2+ released into cytosol at A–I junctions via Ca2+-release channels (junctional feet, ryanodine receptors) of the SR terminal cisternae
* (opened by activated DHP receptors)*
As long as the **Ca2+ level is sufficiently high**, the **contraction cycle will continue**
74
Initiation of muscle contraction: Step 2
**Activation of Actin by Ca2+**
**- resting state:** myosin-binding sites on thin (actin) filaments are partially covered by tropomyosin
- TnI is bound to actin and hinders myosin–actin interaction
**- active state:** Ca2+ binding by TnC = conformational change
(breaks the TnI–actin bond)
- tropomyosin shifts slightly + uncovers the myosin-binding sites
75
Relaxation of muscle contraction (step 3)
**Ca2+ concentration in the cytosol is reduced enough that TnC loses its bound Ca2+**
- tropomyosin returns to its resting position, covering actin’s binding sites + restoring the resting state
- relaxation depends on Relaxation depends on a Ca2
76
What is a Motor Unit?
**a neuron and every muscle cell it innervates**
- muscle cells of a single motor unit contract in unison
- follow the “all or none law”
77
**myoneural junction**
| (neuromuscular junction)
**a synapse between a branch of a motor nerve axon + a skeletal muscle cell**
**B. synaptic cleft**
- narrow space between presynaptic membrane of axon terminal & the postsynaptic membrane ("motor end plate")
- contains an amorphous external lamina (basal laminalike material) derived from the muscle cell
78
**axon terminal**
- **mitochondria** + **synaptic vesicles** (acetylcholine) + **SER elements**
- axon terminal lacks myelin; has a Schwann cell on nonsynaptic surface
- membrane on axon terminal synaptic surface = presynaptic membrane
79
**synaptic cleft**
## Footnote
- narrow space between presynaptic membrane of axon terminal & the postsynaptic membrane ("motor end plate")
- contains an amorphous external lamina (basal laminalike material) derived from the muscle cell
80
Muscle cell components near the myoneural junction
- junctional folds
- Acetylcholine receptors
- sarcoplasm is rich in mitochondria, ribosomes, and rough endoplasmic reticulum (RER)
81
**junctional folds**
- sarcolemmal invaginations (of the postsynaptic membrane)
- lined by an external lamina + extend inward from synaptic cleft
82
**Acetylcholine receptors**
- located in the postsynaptic membrane
- binding sites for ACh
83
What occurs as the presynaptic membrane is depolarized at a myoneural junction?
- voltage-gated Ca2+ channels open
- extracellular Ca2+ enters into the axon terminal
- Ca2+ triggers synaptic vesicles to release acetylcholine (ACh) in *quanta* into the synaptic cleft
* quantum* = approx. 20,000 acetylcholine molecules
84
What occurs when acetylcholine is released into the synaptic cleft?
- released from synaptic vesicles
- ACh binds to receptors of postsynaptic membrane
= **depolarization of sarcolemma** + **generation of action potential**
- acetylcholinesterase (enzyme) located in external lamina lining junctional folds of motor end plate **degrades acetylcholine =** ending the depolarizing signal
85
What happens to "left over" acetylcholine in the synaptic cleft?
- recycled as **choline **(by acetylcholinesterase) --\> returned to the axon terminal
- recombined with **acetyl coenzyme A (CoA)** *(from mitochondria)* under the influence of **choline acetyl transferase** enzyme
- forms acetylcholine --\> stored in synaptic vesicles
- Membranes of emptied synaptic vesicles = recycled via clathrin-coated endocytic vesicles
86
What does the** "all or none" law** refer to?
an individual muscle cell **either contracts or does not contract.**
***\*there's no half-assing it\****
(all muscle cells of a single motor unit contract in unison)
87
**concentric contraction**
as a muscle contracts the *sarcomeres shorten*
= the **entire muscle becomes shorter**
88
**isometric contraction**
sarcomeres *do not shorten*
= entire **muscle remains the same length**
(e.g. squeezing a hard object)
89
Muscle innervation consists of what types of nerve endings?
## Footnote
- motor nerve endings (**myoneural junctions**)
- two types of sensory nerve endings (**muscle spindles** and **Golgi tendon organs**)
90
_Muscle spindles_ & _golgi tendon organs _function in what sensation?
**proprioception**
*"ability to sense stimuli arising within the body regarding position, motion, and equilibrium"*
91
What happens to the membranes of emptied synaptic vesicles once they've been used?
recycled via **clathrin-coated endocytic** vesicles
92
**muscle spindle (neuromuscular spindle)**
- elongated, fusiform sensory organ in skeletal muscle
- functions primarily as a **stretch receptor**
93
Structure of a muscle spindle:
- bounded by a connective tissue capsule enclosing the fluid-filled **periaxial space** + 8 to 10 modified skeletal muscle fibers (**intrafusal fibers**).
- surrounded by normal skeletal muscle fibers (**extrafusal fibers**)
- anchored via capsule to **perimysium + endomysium of extrafusal fibers**
94
Muscle cells depend on energy from ATP + phosphocreatine.The metabolism of what 2 substances creates this energy?
fatty acids + glucose
95
What is the **primary metabolic substrate** in actively contracting muscles?
**glucose**
96
What are the **five stages** of the skeletal muscle contraction cycle?
1. attachment
2. release
3. bending
4. force generation
5. reattachment
97
Muscle Spindle function
- **muscle stretch** = spindle stretch = stimulates afferent nerve endings; send impulses to the CNS.
(response to both rate - *phasic response* + duration - *tonic response*)
- **depolarization of** **y*(gamma)*-efferent neurons** = stimulates intrafusal nerve endings; rate + duration of stimulation monitored same way as stretching
- **muscle overstimulation**; initiates contraction to counteract stretching
98
What happens during **overstimulation **of the muscles?
results from stretching at **too great a frequency** or **too long a time**
- causes stimulation of a(alpha)-efferent neurons to the muscle = initiating contraction + counteracting the stretching
99
**Golgi tendon organ**
- located in tendons; *counteracts effects of muscle spindles*
- composed of **encapsulated collagen fibers** surrounded by **terminal branches of type Ib sensory nerves**
- stimulated when muscle **contracts too strenuously**, increasing tension on the tendon
- impulses from type Ib neurons **inhibit a*(alpha)*-efferent neurons** = *preventing further contraction*
100
the **lateral portion of intercalated disks** has desmosomes + numerour gap junctions.
These facilitate what?
**ionic coupling** between cells
+ aid in coordinating contraction
101
Cardiac muscle behaves as a ......?
**functional syncytium**
* \*uninucleated cells acting in syncronisation as a multinucleated cell = performing same function\* *
* (WARNING: this is Grace's definition, not from a dictionary - but at least I know what it means...\*ahem\* unlike some people)*
102
The thin filiments of cardiac muscle are secured to the Z disk by ...?
**a*(alpha)*-actinin** + **nebulette**
\*nebulette = nebulin-like molecule; extends proximal 25% of thin filament\*
103
What type of tissue is this?
Identify the structures indicated by the **N** and the **arrows**
CARDIAC TISSUE
**N =** nuclei
**arrows =** intercalated disks
104
Cardiac cell connective tissue supports a rich capillary bed that supplies...?
What does this maintain?
- sufficient nutrients + oxygen
- maintains the high metabolic rate
105
What percentage of the energy production of cardiac muscle cells is generated by **aerobic respiration**?
At least **90%**
106
What type of fibers are **purkinji fibers**?
Where are they located?
- modified cardiac muscle cells
- located in **bundle of His**
107
Structure & function of **purkinji fibers**?
- specialized for **conduction**
- contain a few peripheral myofibrils
- large, pale cells; **rich in glycogen + mitochondria**
- form **gap junctions, fasciae adherents, desmosomes** w. cardiac muscle cells
(but not through typical intercalated disks)
108
**Ischemia**
"*Inadequate blood supply* to a local area due to *blockage of blood vessels* leading to that area. Treatment is directed toward increasing the circulation to the affected body area."
109
Functional anatomy of the heart
| (intrinsic conduction system)
- consists of **"pacemaker" cells** + **conduction pathways**
- coordinates contraction of ventricles + atria
110
**Autorhythmic Cells** (Pacemaker Cells)
- smaller than contractile cells
- do NOT contain myofibrils
- no organized sacromere structure
**do not contribute to contractile force of heart**
- unstable membrane potential\*
- myogenic
111
Pacemaker Cells have an unstable membrane potential.
At what electrical charges do these cells "bottom out" and "drift upward" to ?
"bottom out" at** -60mV**
"drifts upward" to **-40mV **= "pacemaker potential"
112
The upward “drift” in pacemaker cells allows the membrane to reach threshold potential (-40mV) by itself.
This "drift" is due to what?
_1. Slow leakage of **K+ out** & faster leakage **Na+ in**_
= slow depolarization
= through If channels (f=funny); open at negative membrane potentials + start closing as membrane approaches threshold potential
_2. **Ca2+ channels opening** as membrane approaches threshold_
= @threshold additional Ca2+ ion channels open
= more rapid depolarization
(deactivate shortly after)
3. **Slow K+ channels open** as membrane depolarizes causing an
efflux of K+ and a repolarization of membrane
113
Smooth Muscle Cells
- non-striated, fusiform
- 20um (sm. blood vessels) - 500um (uterus)
- single nucleus
- actively regenerate
- surrounded by external lamina + reticular fiber network
- arranged in layers, sm. bundles, or helices (in arteries)
114
Smooth Muscle: Nucleus
- centrally located
- may not be visible in each cell *(in cross-section)*
- has corkscrew shape + deeply indented *(in longitudinal sections)*
115
Smooth Muscle: **Mitochondria, RER + Golgi complex**
- concentrated near the nucleus
- involved in synthesis of_ _**_Type III collegen_, elastin, GAGs, external lamina, growth factors**
116
Smooth Muscle: ** Sarcolemmal Vesicles (Caveolae)**
- along periphery of smooth muscle cells
- may function in uptake/release of Ca2+
- SER is sparce; may be associated w. caveolae
117
Smooth Muscle: **contractile filaments**
**actin + myocin**; not organized into myofibrils
- attached to peripheral + cytoplasmic densities; alignes obliquely to longitudinal axis of smooth muscle cells
**Thick filaments**
**Thin filaments**
118
Thick filaments of smooth muscle are composed of what?
## Footnote
composed of **myosin II**
(each surrounded by ~ 15 thin filaments)
119
In contrast to striated muscle, the heads of the myosin molecules\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_?
all point in the same direction
120
Thin filaments of smooth muscle are composed of what?
## Footnote
**actin**
**caldesmon**
**tropomyosin**
**calponin** *(functions similar to TnT + TnI)*
121
Intermediate filaments of smooth muscle are attached to cytoplasmic densities.
What types are included in **vascular** smooth muscle cells?
What types are included in **nonvasculaar** smooth muscle cells?
vascular = **vimentin + desmin **
nonvascular **= desmin **
122
**Ctyoplasmic densities** in smooth muscle cells are believed to be ...?What do they contain and what is their main function?
- analogous to Z disks
contain: a*(alpha)*-actinin
function: as a filament attachment site
123
Smooth Muscle Gap Junctions
- between smooth muscles
- facilitate **spread of excitment**
- collectively called a "**nexus"**
124
Contraction of smooth muscle
- occurs **more slowly** + **lasts longer** (than skeletal)
...because ATP hydrolysis = slower
- regulated by different mechanism (than skeletal)
125
The contraction cycle in smooth muscle is stimulated by...?
What happens next?
_stimulated by: transient increase in **cytosolic Ca2+**_
- Ca2+ binds to **calmodulin** = alters conformation
- **Ca2+–calmodulin complex** activates enzyme **myosin light-chain kinase** = catalyzes phosphorylation of one light chain of myosin
- presence of Ca2+: inhibitory effect of **caldesmon–tropomyosin complex **are eliminated
- globular head of phosphorylated myosin interacts w. actin; stimulates myosin ATPase = **contraction**
* (myosin in phosphorylated form = contraction cycle continues)*
* (Dephosphorylation of myosin disturbs myosin–actin interaction*
* = relaxation)*
126
In **vascular smooth muscle**, contraction is usually triggered by...?
**a nerve impulse**
(little spread of impulse from cell to cell)
127
In **visceral smooth muscle**, contraction is triggered by...?
**stretching of the muscle itself (myogenic)**
(signal spreads from cell to cell)
128
In the uterus during labor, what triggers smooth muscle contraction?
**oxytocin**
129
In smooth muscle throughout in the body (other than uterus), what triggers muscle contraction?
**epinepherine**
130
What types of nerve fibers stimulate smooth muscle?
Sympathetic + Parasympathetic
- act in antagonistic fashion to stimulate or depress activity
131
Nerve terminales with empty vesicles are considered...?
**"cholinergic"**
- parasympathetic
(secreting acetylcholine)
132
Nerve terminales with vesicles filled with dense granular material are considered to be...?
**"adrenergic"**
- sympathetic
(secreting norepinepherine)
133
Nerve terminals with vesicles containing an opaque content are considered to be...?
**"purinergic"**
(secreting other neurotransmitters in general)
134
nerve endings in smooth muscles that are considered sensory have what two characterstics?
- contain primarily mitochondria
- have no vesicles
135
In certain glands, this type of cell shares the basal laminae of secretory and duct cells
**Myoepithelial cells**
(attached to basal lamina via **hemidesmosomes)**
136
What type of tissue do myoepithelial cells arise form?
What happens when they contract?
_arise from **ectoderm**_
**- **contract to express **secretory material from glandular**
**epithelium into ducts** and out of the gland
137
What type of **contractile, nonmuscle cell** has a basketlike shape and several radiating processes?
myoepithelial cells
138
What do myoepithelial cells contain?
actin
myocine
intermediate filaments
cytoplasmic peripheral densities (attach to filaments)
139
Myoepithelial cells:
1. contraction **generally** occurs via what process?
2. In **lactating mammary glands**, contraction occurs in response to...?
3. In **lacrimal glands**, contraction occurs in response to...?
1. generally = **calmodulim-mediated perocess**
2. mammary glands = **oyxtocin**
3. lacrimal glands = **acetylcholine**
140
What cells resemble fibroblasts, but possess higher amounts of actin and myosin and are capable of contraction?
Myofibroblasts
141
What do myofibroblasts do during wound healing?
may contract to decrease the size of the defect
(**wound contraction**)
BLOCK 2 - Histology (Grace)
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