Muscle (Exam II)

Question 1

What is the distinction between a structural syncytium and a functional syncytium? Is skeletal muscle a structural or a functional syncytium? Cardiac muscle? Smooth muscle?

Answer 1

A structural syncytium (a “true” syncytium) is a multinucleated cell formed by the fusion of previously independent cells. Skeletal muscle is a structural syncytium since each mature cell is formed by the fusion of uninucleate myoblasts during development. The syncytiotrophoblast of the placenta is another example of a structural syncytium.

In contrast, a functional syncytium is a group of separate cells that function in a coordinated way, as if they were all part of one single cell. In order to do this they must be joined together by communicating junctions such as gap junctions that allow direct cell-to-cell passage of the ions and small signaling molecules that control the coordinated activity. A ciliated epithelium where the cilia beat in synchronous waves is a functional syncytium, as is cardiac muscle where the cells contract in a coordinated fashion.

In most locations in the body, smooth muscle cells are connected to one another by gap junctions to form a group of cells that function as a syncytium.

Question 2

What specific name is given to the layer of connective tissue that immediately surrounds each muscle cell? Each fascicle (bundle) of muscle cells? An entire muscle?

Answer 2

The connective tissue that surrounds each individual muscle cells is the endomysium. It is composed mainly of fine reticular fibers. It contains small capillaries and nerve terminals. Perimysium is the connective tissue that surrounds groups of muscle cells and thus forms a bundle or fascicle of cells. It is best developed in skeletal muscle, but even there it is not always clearly evident. Nerve bundles and vessels larger than capillaries tend to travel in the perimysium. The connective tissue that surrounds an entire muscle is the epimysium. In gross anatomy the epimysium of a skeletal muscle would be called the investing fascia of the muscle. Epimysium is not well defined around cardiac or smooth muscle and the term is rarely used with reference to them.

Question 3

What is the relationship between a myofibril, a sarcomere, and a myofilament?

Answer 3

The thick (myosin) and thin (actin) filaments are myofilaments. In striated muscle they are arranged to form the A band and I band of the sarcomeres.

The sarcomere is the functional unit of striated muscle extending between two adjacent Z-lines (A sarcomere would therefore extend from the mid-point of one I band to the mid-point of the next, with an A-band between). Many sarcomeres joined end-to-end form a myofibril. There are many myofibrils within the cytoplasm of a single skeletal or cardiac muscle cell.

Thus a myofilament is the smallest of the structures listed in this question. It takes many myofilaments to make a sarcomere, many sarcomeres to make a myofibril, and many myofibrils to fill the cytoplasm of a single muscle cell.

Question 4

During contraction of skeletal or cardiac muscle, do the I bands shorten or remain constant in length? The A bands?

Answer 4

The I bands shorten as the thin filaments overlap more and more with the thick filaments of the A bands. An A band remains constant in length since its length is determined by the length of the thick filaments, which does not change.

Question 5

Suppose you were looking at a cross section of a sarcomere by EM. If you saw only thick filaments, and these filaments appeared to be cross-linked to one another, you could conclude that this cross section had passed through what part of the sarcomere (Z line, I band, zone of thick-thin overlap in the A band, H zone, or M-line)?

Answer 5

This section passed through the M-line.

In the Z line or the I band you would see only thin filaments, not thick.

In the zone of overlap in the A band, there would be both thick and thin filaments.

In the H zone, there would be only thick filaments, but they would not be cross-connected.

Only in the M-line would you see cross-connected thick filaments and no thin filaments. The cross connections are formed by accessory proteins such as myomesin that hold the thick filaments in register with one another.

Question 6

In skeletal and cardiac muscle, is the sarcoplasmic reticulum located between myofibrils or between neighboring muscle cells?

Answer 6

The sarcoplasmic reticulum is the equivalent of the smooth endoplasmic reticulum (SER) in skeletal and smooth muscle cells.

As is always the case with SER, it is located in the cytoplasm the cells rather than in the extracellular space between cells. In striated muscle it closely surrounds each of the myofibrils in the sarcoplasm.

The SER of any cell type has the ability to sequester calcium, but in skeletal and cardiac muscle this function is highly developed and essential to the process of contraction.

As a result, the SER in striated muscle is very extensive, has a particular form and arrangement relative to the myofibrils, and is given the special name “sarcoplasmic reticulum”

Question 7

Into which type of junction in an intercalated disk do the actin filaments of cardiac muscle insert? This junction is analogous to what component of the junctional complexes that are found between epithelial cells?

Answer 7

The actin filaments of the I band in cardiac muscle insert into the fascia adherens of the intercalated disk. The fasciae adherentes are located on the transverse portions of each disk.

This type of junction is analogous to the zonula adherens of a junctional complex between epithelial cells, which also serves as an attachment site for actin filaments.

The two junctions also resemble one another when seen in thin sections, but differ in their 3-dimensional shape. The zonula adherens is a belt-like structure (zonula = belt in Latin) that runs completely around the epithelial cell near its apical end, binding it to all cells that border on it laterally. In contrast, the fascia adherens is a broad irregularly shaped plaque that unites the ends of two adjacent cardiac muscle cells.

Question 8

Name two characteristic features visible by EM in smooth muscle that distinguish it from skeletal or cardiac muscle.

Answer 8

Two diagnostic features of smooth muscle cells in electron micrographs are dense bodies and caveolae.

• Dense bodies often appear to be small separate structures that are either associated with the plasma membrane or free in the cytosol, but they are actually elongated, branching structures that extend from the membrane throughout the cytoplasm. They are analogous in function to Z lines, in that actin filaments and other components of the cytoskeleton are anchored to them.
• Caveolae are numerous small vesicular invaginations of the plasma membrane that allow calcium to enter smooth muscle cells via calcium channels in their membrane. They are also involved in releasing calcium from nearby SER cisternae into the cytosol.

Question 9

If you were looking at a cross section of the colon, would the individual smooth muscle cells of the inner circularly arranged muscle layer be cut longitudinally or in cross section?

Answer 9

They will be cut longitudinally, while the cells of the outer longitudinal layer will be cut in cross section. If you were looking at a longitudinal section through the colon rather than a cross section, the appearance of the muscle cells in the two layers would be reversed (cells of the inner circular layer cut in cross section, cells of the outer longitudinal layer cut longitudinally). The appearance of the cells depends on their orientation relative to the plane of section through the organ.

Question 10

Skeletal Muscle Organization

Answer 10

Thick & thin myofilaments form a sarcomere Sarcomeres are arranged end-to-end form a myofibril Many myofibrils are arranged side by side in the cytoplasm of each skeletal muscle cell. A skeletal muscle cell is also called a muscle fiber Many muscle cells are bundled together by connective tissue (perimysium) to form a muscle fascicle. Many muscle fascicles are bundled together by connective tissue to form a single muscle

Summary: Myofilaments → Sarcomeres → Myofibrils → Cells (Fibers) → Fascicles → Muscles

Question 11

Connective Tissue in Skeletal Muscle

Answer 11

• Endomysium: Loose connective tissue that surrounds individual muscle cells. Consists mainly of reticular fibers
• Perimysium: Dense irregular connective tissue that surrounds a muscle fascicle; contains the larger blood vessels
• Epimysium: Dense irregular connective tissue that surrounds an entire muscle (= the investing fascia of the muscle)

Question 12

Development, growth and regeneration of muscle cells

Answer 12

Mesenchymal stem cells form myoblasts → Myoblasts cease dividing, fuse end to end to form myotubes. Immature myotubes have central nuclei. As contractile apparatus (myofilaments) and complex membrane structure (sarcoplasmic reticulum and t-tubules) develop, nuclei displaced to periphery.

Satellite cells represent persistent stem cells. They are found between muscle fibers and have single nucleus.

Question 13

The Sarcomere

Answer 13

• Specialized smooth endoplasmic reticulum.
• Forms membranous meshwork around each myofibril. At junction of A and I bands, membrane dilates and flattens (terminal cisternae).
• Functions to sequester calcium between contractions (longitudinal portions) and allow its release in preparation release into the cytoplasm in preparation for contraction (at the terminal cisternae).
• Have ryanodine receptors = gated Ca²⁺ release channels
• It runs from one Z-line to the next and contains: an A band in the center of the sarcomere and half of an I band at each end of the sarcomere

Question 14

I-Band of Sarcomere

Answer 14

• Light-staining band
• Contains the parts of thin filaments not overlapped by thick filaments
• Bisected by Z-line, which anchors the actin filaments
• Its length decreases during contraction

Question 15

A-Band of Sarcomere (including H-zone, Bare zone, and M-line).

Answer 15

• Dark staining band of constant length
• Length corresponds to the length of the thick filaments
• Also contains the part of the thin filaments that overlaps the thick filaments.
• At EM level the A-band also contains:
  
  • H-zone: Lighter staining region at center of A band Contains only thick filaments (i.e., is the region of the thick filaments that doesnʼt overlap with thin) Length changes during contraction
  • Bare zone: An even lighter staining region within the H zone Contains shafts of thick filaments but no myosin heads Constant in length
  • M-line: A darker line at the center of bare zone Site of cross-connections between thick filaments Constant in length

Question 16

T-tubules in skeletal muscle

Answer 16

• Invaginations of sarcolemma into interior of cell.
• Extend transversely into muscle cell to surround individual myofibrils (at junction of A and I bands in mammalian skeletal muscle).
• Function to conduct nerve impulse into cell, transferring it to the terminal cisternae of sarcoplasmic reticulum
• Have dihydropyridine (DHP) receptors = depolarization-sensitive transmembrane channels organized into tetrads

Question 17

Triad in skeletal muscle

Answer 17

• A T-tubule, together with a pair of terminal cisternae of the SR
• Located at A and I band junction in mammalian skeletal muscle.
• In muscles of some other vertebrates (i.e. amphibians), located at Z-line.
• Ryanodine and Dihydropyridine receptors closely opposed in the triad
• Site of excitation-contraction (EC) coupling

Question 18

Skeletal Muscle Cells: description, nucleation, and structure.

Answer 18

• Mature fibers are long, unbranched, cylindrical cells
• Multinucleated, with peripheral nuclei
• Each cell is a syncytium formed by fusion of uninucleate cells (myoblasts)

Question 19

Thick Filaments of skeletal muscle

Answer 19

• Each thick filament contains 200-300 myosin molecules.
• Each myosin molecule has elongated tail and globular head.
  
  • Head region contains actin-binding site and ATP-binding site.
  • Aggregate tail-to-tail forming central bare zone.
  • Aggregate head to tail on each side of the bare zone.
• Held together by cross bands formed by Myomesin and C protein at midpoint of A band to form M-line.
• Thick filaments also held in place by a core of titin, an accessory protein that extends from M-line to Z-line.

Question 20

Thin Filaments on the sarcomere

Answer 20

• Originate at Z-line
• Project towards center of two adjacent sarcomeres
• Constitute I band and extend some distance into A band

Question 21

Thin Filaments (composition) of skeletal muscle

Answer 21

• Actin filaments are highly polarized with plus end binding to Z-line (via α-actinin) minus end extending to the M line.
• Nebulin runs the length of the thin filament and assists α-actinin in binding thin filament to Z-line
• Tropomyosin = long pencil shaped proteins form long filaments that run in grooves between F-actin molecules.
• Troponin, binds to tropomyosin. It has 3 subunits:
  
  • TnT binds to tropomyosin
  • TnC binds calcium
  • TnI binds actin, thus inhibiting actin - myosin interaction.
• Length of filament likely regulated by nebulin in conjunction with capping protein called tropomodulin f. Linked to cell membrane and external lamina (equivalent to a basal lamina) by protein complex that includes dystrophin.

Question 22

Clinical Correlation: Muscular Dystrophy

Answer 22

Muscular dystrophy is attributed to mutations of the genes that encode for proteins of the dystrophin-glycoprotein complex that links dystrophin to the cell membrane (via the extracellular matrix proteins laminin and agrin).

There are several forms of the disease, but all will lead to progressive muscle weakness and cell death.

Question 23

Summary of Z-line proteins and structures

Answer 23

• α-actinin, assisted by nebulin, anchors thin filaments to the Z-line.
• Ends of titin proteins insert into Z-line, anchoring thick filaments.
• Intermediate filaments (e.g. desmin and vimentin), secure Z-lines in adjacent myofibrils.
• Near sarcolemma, Z-lines attached to inner aspect of membrane by aggregations of intermediate filaments and some microtubular structures

Question 24

Neuromuscular Junctions (Motor Endplates)

Answer 24

• Terminations of motor neurons on skeletal muscle cells
• Neurons that innervate skeletal muscle cells are called somatic motor neurons The axon branches near its end (terminal arborization)
  
  • NOTE: One neuron plus all the muscle cells innervated by it is known as a motor unit.
• Each muscle cell is innervated by one motor endplate
• Each axonal branch ends in a motor endplate
• Axon loses its myelin sheath at the motor endplate, but remains partially covered by a Schwann cell
• Endplate has many mitochondria in the axon terminal
• Releases acetylcholine (ACh) from round, clear synaptic vesicles into the synaptic cleft
• The motor endplate sits in a depression in the muscle cell sarcolemma called the primary synaptic cleft
• Secondary synaptic clefts are formed by junctional folds of the sarcolemma ACh receptors are located at the crests of the junctional folds
• Subneural clefts (secondary synaptic clefts) are the spaces created between junctional folds
• External lamina of muscle cell (equivalent to a basal lamina) extends into synaptic cleft & subneural clefts
• Acetylcholinesterase is localized in external lamina

Question 25

Clinical Correlate: Amyotrophic lateral sclerosis

Answer 25

Amyotrophic lateral sclerosis, or ALS (commonly known as Lou Gehrig’s Disease) affects motor neurons in the brain and anterior horn of the spinal cord. It causes progressive loss of motor control. This results in grouped atrophy of muscle fibers, as motor units are successively deinnervated.

Question 26

Clinical Correlate: Myasthenia gravis

Answer 26

Myasthenia gravis is an autoimmune disease in which auto-antibodies block acetylcholine receptors at the neuromuscular junction. Results in a loss of junctional folds and widened synaptic cleft, which reduces the effectiveness of muscle fibers, leads to progressive muscle weakness

Question 27

What happens to sarcomere during muscle contraction?

Answer 27

During contraction, thin filaments slide past thick filaments.

• H-band and I-band get smaller
• Zone of overlap gets larger
• Z lines move closer together
• A-band width remains constant

Question 28

Sliding Mechanism of Skeletal Muscle

Answer 28

1. Action potential passes along motor neuron axon to reach motor end plate.
2. Acetylcholine in synaptic vesicles released from NMJ by exocytosis, and binds to its receptor in the sarcolemma.
3. Depolarization signal transmitted into fiber through T-tubules and transferred to terminal cisternae of sarcoplasmic reticulum.
4. Calcium ions released through voltage gated calcium release channels Calcium binds to Troponin C, causing change in configuration of troponintropomyosin complex (i.e. tropomyosin is pushed deeper into groove).
5. Exposes the myosin-binding site.
6. Myosin heads bind to actin filament, and hydrolysis of ATP occurs.
7. Hydrolysis of ATP results in a change in the position of the myosin heads, so thin filaments are pulled passed the thick filaments.
8. This cross-bridge cycle repeats itself numerous times as contraction occurs. Active cross bridges function asynchronously, so that there is smooth and continuous contraction of the whole muscle.
9. Once stimulating impulses end, calcium pumps in sarcoplasmic reticulum force calcium ions back into terminal cisternae. As calcium ion levels in cytosol drop, TnC loses its bound Ca²⁺, and moves back into its original position - blocking active site of myosin binding, and ending the contraction

Question 29

Clinical Correlate: Rigor Mortis

Answer 29

After death, no oxygen is available to make ATP. Calcium pumps in sarcoplasmic reticulum can no longer operate, so calcium ions diffuse into cytosol. Here they are free to bind with troponin, thus allowing cross-bridges to form between myosin and actin proteins. But in the absence of ATP, the cycle cannot continue, i.e. myosin and actin cannot decouple = muscular contraction = rigor mortis. Eventually due to decomposition the myosin heads degrade and the muscles can relax. The degree of rigor mortis can be used in forensic pathology in determining time since death.

Question 30

Skeletal Muscle Fiber Types

Answer 30

• “Endurance” fibers = Red/Slow twitch/ Oxidative
  
  • Red color due to high oxidative metabolism à numerous mitochondria with lots of cytochromes (colored), increased myoglobin, increased blood supply.
  • Greater fatigue resistance, but lower muscle tension produced.
  • Adapted for movements requiring endurance.
  • Fibers smaller in diameter, with increased numbers of mitochondria, wider Z-lines, and smaller neuromuscular junctions relative to “Strength” fibers.
• “Strength” fibers = White/Fast twitch/Glycolytic
  
  • Generally white due to larger amounts of glycogen, less blood supply and fewer mitochondria.
  • Fatigue quickly, but produce greater muscle tension.
  • Adapted for rapid contraction and precise, fine movements.

Question 31

Characteristics of Cardiac Muscle by Light Microscopy

Answer 31

• Each cell contains 1 or 2 centrally located nuclei
• Cells are joined together end-to-end by intercalated disks, which are unique to cardiac muscle
• Cells may branch
• Average cell diameter is intermediate between skeletal & smooth muscle

Question 32

Intercalated Disks

Answer 32

• Have transverse and longitudinal portions Transverse portions run at right angles to the myofibrils
• Contain fasciae adherentes (singular = fascia adherens) - sites of actin filament attachment to the membrane - sites of end-to-end attachment of cardiac myocytes
• Desmosomes are common on transverse portions, but also present on longitudinal portions
• Longitudinal (lateral) portions run parallel to myofibrils. Gap junctions are common on longitudinal portions

Question 33

Characteristics of Cardiac Muscle by Electron Microscopy

Answer 33

• More mitochondria than skeletal muscle
• They form almost continuous rows between myofibrils T tubules:
• Are at Z-line, not at A-I junction as in human skeletal muscle
• Are wider than in skeletal muscle Sarcoplasmic reticulum although not as well developed as in skeletal muscle
• Terminal cisternae are not continuous along each T tubule
  
  • Results in formation of diads (dyads) more often than triads
  • Diad: Consists of one T tubule & one terminal cisterna of SR

Question 34

Innervation of Cardiac Muscle

Answer 34

• Ordinary cardiac muscle cells are usually not directly innervated
• Autonomic (sympathetic & parasympathetic) fibers innervate the modified cardiac muscle fibers that form the conduction system of the heart (SA node, AV node)
• Autonomic nerve endings are less elaborate than the neuromuscular junctions on skeletal muscle

Question 35

Characteristics of Nodal Fibers (SA/AV Node)

Answer 35

• Smaller in diameter than ordinary cardiac myocytes
• Separated by larger amounts of connective tissue
• Attached to each other in places by gap junctions.

Question 36

Characteristics of Purkinje Fibers

Answer 36

• Larger in diameter than ordinary ventricular myocytes
• Lower density of fibrils
• More glycogen (makes them paler).
• Electrically coupled via gap junctions with each other.
• Form branching bundles running in the deepest part of the endocardium (the subendocardium) toward the apex of the heart Here they form gap junctions with working ventricular myocytes

Question 37

Characteristics of Smooth Muscle by Light Microscopy

Answer 37

• Spindle shaped (fusiform) cells
• Single central nucleus
• Has a distorted or “corkscrew” appearance in contracted cells
• Cells have smallest average diameter of any type of muscle
• Cells are usually packed tightly together
• Can be organized in orderly layers (in tubular organs, e.g., gut) or in interlacing bundles (in spherical organs, e.g. uterus)

Question 38

Characteristics of Smooth Muscle by Electron Microscopy

Answer 38

• Has no sarcomeres
• Has dense bodies: some are attached to the plasma membrane and form a branching network extending into the cytoplasm
  
  • Are analogous to Z-lines: Contain attachment plaque proteins (e.g., alpha-actinin)
• Thin filaments & intermediate filaments insert on dense bodies
• Has caveolae (pits) invaginations of the sarcolemma analogous to T tubules
• Lack a well-organized sarcoplasmic reticulum. Sparse SER membranes are located near caveolae
• Smooth muscle filaments include:
  
  • Thin filaments that contain actin but lack troponin
  • Thick filaments that contain myosin & are hard to preserve
  • Networks of intermediate filaments that contain desmin or desmin & vimentin
• Actin-myosin interaction pulls on the network of intermediate filaments, which twist & cause contraction of cell
• Gap junctions are common between smooth muscle cells, especially where the muscle cells are not individually innervated (unitary or single unit smooth muscle)

Question 39

Innervation of smooth muscle

Answer 39

Similar to cardiac conduction fibers: “en passant” swellings of autonomic nervous system axons with neurotransmitter in vesicles.

Question 40

Morphology differences from neuromuscular junction of skeletal muscle to smooth muscle

Answer 40

• In smooth muscle:
  
  • Sarcolemma has no junctional folds
  • Synaptic cleft is much wider than in skeletal muscle
  • A single bouton can innervate several nearby smooth muscle cells
  • Not all smooth muscle cells receive direct innervation
    
    • Cells that are directly innervated form “multiunit” smooth muscle
    • Cells that lack direct innervation are coupled to innervated cells by gap junctions, and form “unitary” or “single unit” smooth muscle where co-ordinated contraction can occur

Question 41

Dense bodies of smooth muscle

Answer 41

• Anchor actin filaments (contain α-actinin)
• Anchor intermediate filaments (composed of proteins such as desmin or vimentin)
• Analogous to Z-lines in skeletal & cardiac muscle
• Some attach to inner face of sarcolemma, others are in the sarcoplasm.
• All these densities may be joined in a network.
• Play role in transmitting contractile forces generated inside cell to the cell surface, thus altering cell shape

Question 42

Caveolae (micropinocytotic vesicles) of smooth muscle

Answer 42

• Invaginations of cell membrane
• Look like pits on sarcolemma
• Associated with underlying vesicles and in proximity to sER.
• Thought to function similar to T-tubule system of skeletal muscle for Ca⁺² entry into cytoplasm.

Question 43

Thin filaments of smooth muscle

Answer 43

• Numerous
• Associated with tropomyosin (different isoform from skeletal muscle).
• No troponin is present, but some smooth muscles have some similar protein (e.g., leiotonin).
• Caldesmon and calponin are smooth muscle-specific proteins associated with thin filaments.
• Thin filaments insert into dense bodies (along with intermediate filaments)

Question 44

Thick filaments of smooth muscle

Answer 44

• Hard to see; usually surrounded by rosettes of thin filaments.
• Composed of a myosin II that is chemically different (both heavy and light chains) from skeletal myosin II, and is organized differently (side-polar rather than bipolar)
  
  • Made of stacks of myosin dimers, tail-to-tail
  • No bare zone in middle

Question 45

Contraction of smooth muscle

Answer 45

1. Calcium mainly from extracellular sources. There is no T-tubule system or sarcomeres
2. Ca binds to calmodulin (not troponin C) to form Ca²⁺ calmodulin complex
3. Phosphorylation of myosin activates actin-binding site of myosin head.
4. In presence of ATP, myosin head bends to cause contraction.
   
   1. Slow ATP hydrolysis rate results in slow sustained contractions in smooth muscle
   2. Can also maintain sustained contraction in a ‘latch state’ (a) Myosin heads remain attached to actin (much like rigor mortis of striated muscle)
5. Change in length of smooth muscle not due to sarcomeres, but possibly due to longer thin filament lengths.
   
   1. Still have thin filaments sliding past thick filaments, but since organized differently than in striated muscle ( anchored on cytoplasmic densities which anchor to sarcolemma), contraction results in the shortening of muscle cell in all directions.

Question 46

Smooth muscle features: collagen synthesis and division

Answer 46

• Can make collagen (type III and IV) other extracellular matrix molecules
• In some locations can also make collagen type I and elastin 1. You see this especially in the tunica media of blood vessels, where there are no fibroblasts.
• Smooth muscle cells can divide

Question 47

Identify the tissue. Explain your reasoning.

Answer 47

Cardiac Muscle.

• One or two central nuclei
• Intercalated disks
• Smaller average diameter than skeletal muscle.

Question 48

Identify the tissue type. Explain your reasoning.

Answer 48

Cardiac muscle. Note the branching.

Question 49

Identify the cellular structure. What tissue is this structure found?

Answer 49

These are caveoli which are found on smooth muscle. They are deep invaginations of the sarcolemma.

Question 50

What type of tissue is this? Explain what is happening in this image.

Answer 50

It is smooth muscle that is contracting. Note the corkscrew nuclei.

Question 51

Identify the cellular structure. Where do you find these structures? What is their purpose?

Answer 51

They are dense bodies which are analogous to z-lines as they anchor thin filaments and intermediate filaments. They are found on smooth muscle.

Question 52

Distinguish the three types of tissue shown here:

Answer 52

In order, these are cross-sections of skeletal muscle, cardiac muscle and smooth muscle. Note the differences in nucleus diameter.

Question 53

Identify the cellular structure shown here. Where do you find these cellular structures?

Answer 53

Intercalated disks. They are found on cardiac muscle.

Question 54

Is this mammalian or non-mammalian muscle? Explain your reasoning.

Answer 54

This is non-mammalian muscle. The triad is at the Z-line (like a frog) when it is supposed to be at the A-I line.

Question 55

What is this structure? How many neurons are typically present?

Answer 55

One motor unit. One axon plus all the muscle fibers that it innervates = a motor unit. Each muscle fiber is innervated by only one motor endplate.

Question 56

Where do you find the cell body of the axons projecting here?

Answer 56

The cell body of the neuron that the axon belongs to is in the ventral horn of the spinal cord (alpha motor neurons). This is the neuromuscular junction.

Question 57

What type of tissue is this? Is it a cross-section or longitudinal section?

Answer 57

This is skeletal muscle. It is a cross-section.

Question 58

What type of tissue is this? How do you know?

Answer 58

Skeletal Muscle.

• Long unbranched cylindrical cells
• Peripheral nuclei
• Wide diameter
• In humans, the sarcomeres are pretty short.

Question 59

Identify the two types of tissue here. How do you distinguish between them?

Answer 59

The left is skeletal muscle while the right is cardiac muscle. Cardiac muscle has more mitochondria.

Question 60

What type of junction is shown here? Where do you typically find these types of junction in muscle?

Answer 60

Gap junctions and they are found in smooth muscle near the sarcolemma to transmit electrical information between cells. Not every smooth muscle is directly innervated.

Question 61

Identify the tissue shown in both a longtiudinal and cross-section.

Answer 61

Smooth muscle.

Question 62

What structure is shown? How do you determine what neurotransmitters are in the veiscle?

Answer 62

This is a sympathetic bouton seen in smooth muscle. The synaptic vesicles of acetylcholine are clear core synaptic vesicles. Large dense core vesicles contain neuropeptides and large neurotransmitters

Question 63

Where do you see this type of muscle on the left? What about on the right?

Answer 63

Left: smooth muscle in layers (like GI tract).

Right: smooth muscle in bundles (contractile; uterus).

Question 64

Where do you find acetylcholine receptors? Where do you find acetylcholinesterase?

Answer 64

The acetylcholine receptors are near the junctional folds.

Acetylcholinesterase is in the external lamina (#4). It is part of the basement membrane.