Muscle (Exam II)

Question 1

What is a sarcoplasm, sarcolemma and sarcoplasmic reticulum?

Answer 1

Sarcoplasm - cytoplasm of muscle cells
Sarcolemma - plasma membrane of muscle cells
Sarcoplasmic Reticulum - highly specialized smooth endoplasmic reticulum of a muscle cell that regulates calcium flow.

Question 2

What does sarcoplasm contain?

Answer 2

Myoglobin and glycogen.

Question 3

List some important characteristics of Skeletal Muscles

Answer 3

Multiple, peripherally located nuclei (hyolemmal nuclei)
Striations are seen when cut in longitudinal section
Voluntary muscles

Question 4

How is skeletal muscle formed?

Answer 4

Derived from mesoderm. 
Mesenchymal cells (myoblasts) align and fuse together forming multinucleated tubes called Myotubes. 
Myotubes differentiate forming functional myofilaments and the nuclei are displaced against the membrane.

Question 5

How are satellite cells formed?

Answer 5

Cells that do not differentiate and remain as mesenchymal cells are called satellite cells or myosatellite cells.

Question 6

Describe the breakdown of a muscle belly (the different components).

Answer 6

Muscle –> Muscle Fasicles –> Muscle Fibers –> Muscle Fibrils –> Myofilaments

Question 7

What is epimysium, perimysium and endomysium?

Answer 7

Epimysium - surrounds muscles (dense irregular CT layer)
Perimysium - surrounds muscle fasicles (dense CT)
Endomysium - surrounds muscle fibers (myofibers) (reticular fibers CT)

Question 8

What are myofilaments? How can you see them?

Answer 8

Myosin and Actin - contractile elements

Only seen in Electron Microscopy

Question 9

What is the A-band, I-band, H-zone, Z-line, M-line, Sarcomere?

Answer 9

A-band - Dark Band - myosin and potentially actin
I-band - Light Band - actin only (thin)
H-zone - Myosin only
M-line - middle line, where myosin is anchored
Z-line - where actin is anchored
Sarcomere - smallest contractile element of skeletal muscle from Z-line to Z-line.

Question 10

What covers the myosin binding sites on actin until contraction?

Answer 10

Tropomyosin

Question 11

What is G actin vs. F actin?

Answer 11

G actin - disassembled components of actin thin filament

F actin - assembled components of thin filaments (troponin and tropomyosin)

Question 12

Describe the steps of muscle contraction

Answer 12

1. Binding of calcium to troponin C (TnC)
2. Conformational change in tropomyosin, exposes the myosin binding site on actin.
3. Myosin head binds to actin - ATP –> ADP as myosin head moves.
4. Actin slides over myosin
5. Shortening of entire muscle fiber

Question 13

What is a transverse tubule?

Answer 13

T-tubule.
Deep invagination of the sarcolemma - only found in skeletal and cardiac muscle.
Invaginations allow depolarization of membrane to quickly penetrate the interior of the cell allowing Calcium release from the sarcoplasmic reticulum.

Question 14

What are the 2 types of skeletal muscle fiber types?

Answer 14

Type I - Red Muscle - Slow Twitch

Type II - White Muscle - Fast Twitch

Question 15

Describe Type I, “Red”, Slow Twitch Muscle.

Answer 15

Dark color. 
Rich in myoglobin - to help transfer O2, accounts for dark color. 
Aerobic metabolism - fatigue resistant.
High fat, low glycogen content. 
Many mitochondria.

Question 16

Describe Type II, “White”, Fast Twitch Muscle

Answer 16

Anaerobic metabolism - more prone to fatigue.
Low fat, high glycogen.
Less myoglobin and a few mitochondria

Question 17

List some important characteristics of Cardiac Muscle

Answer 17

Single cell with one central nucleus.
Cross striated.
Has intercalated discs - gap junctions, desmosomes
Has sarcoplasmic reticulum
Has many mitochondria - aerobic - requires a lot of O2

Question 18

What are intercalated discs?

Answer 18

Attach cardiac muscle cells to each other providing strength and the ability to function as a syncytium.
Consist of:
Transverse Elements: anchor, desmosome (Fascia adherens, macula adherens), serve for strong attachment
Longitudinal Elements: communication, gap junction with ion channels, propagate electrical impulse.

Question 19

What are the 2 types of cardiomyocytes?

Answer 19

Contractile - bulk of myocardium - red

Conductile - purkinje fibers - pale pink - bigger, no cross striations, sometimes 2 nuclei

Question 20

What are purkinje fibers?

Answer 20

Modified cardiac muscle cells with 1 or 2 nuclei, myofibrils are sparse and restricted to periphery of cell
They are arranged in groups, cells are bigger and paler than contractile cardiac muscle cells

Question 21

Describe the important characteristics of smooth muscle cells.

Answer 21

Single, centrally located nucleus. 
No striations, no myofibrils. 
Actin and myosin are still present but are not ordered (no Sarcomere)
Cytoplasmic dense bodies represent anchors for myofilaments
Desmosomes and gap junctions
No T-tubules, No Sarcoplasmic reticulum 
PSNS and SNS innervation 
Contraction is involuntary.

Question 22

Where is single (unitary) smooth muscle found? How do they work? What is their innervation like?

Answer 22

It is found in visceral organs.
Cells behave like a syncytium - contracting in a network
Sparse nerve innervation but cell communicate via multiple gap junctions - pass on quickly, cells behave like a network

Question 23

Where is multi-unit smooth muscle found? How does it work? What is the innervation like?

Answer 23

Found in iris of eye.
More precise contraction.
Individual innervation of each myocyte.
Lack of gap junctions - they function individually

Question 24

What are some of the functions of smooth muscle cells?

Answer 24

Peristalsis
Vascular dynamics (constriction and dilation)
Propulsion (urinary bladder, uterus)
Secretion (minor role)

Question 25

What are the three tunics of luminal organs?

Answer 25

Tunica Mucosa (mucous membane)
Tunica Submucosa 
Tunica Muscularis (1 - innermost circular layer (longitudinal section), 2 - outer longitudinal layer (cross section))

Question 26

What are dense bodies (or electron dense bodies)?

Answer 26

Serve as attachment sites for myofilaments - cytoplasm contains numerous myofilaments

Question 27

What happens to the nucleus in the contracted state of a smooth muscle cell?

Answer 27

Adopts corkscrew appearance.

Question 28

What are myoepithelial cells? Can they be stimulated by hormones? What is another name for them? What is their germ layer origin? Where are these cells located?

Answer 28

Contractile cells but NOT muscle. But DO contain actin and myosin - similar to smooth muscle.
Can be stimulated by hormones.
Basket cells - have basket shape.
Of ectodermal origin (not mesoderm like muscle).
Located in salivary/mammary/lacrimal glands.

Question 29

What are myofibroblasts?

Answer 29

Role in wound contraction - scars contracting down.

Question 30

What are the 2 types of non-muscular contractile cells?

Answer 30

Myoepithelial cells.

Myofibroblasts.

Question 31

Can skeletal muscle repair?

Answer 31

Has limited regeneration capacity.
Satellite cells - can respond to injury in skeletal muscle - they are positioned between basal lamina and sarcolemma of muscle cells.
Satellite cells retain their mitotic potential and thus can accomplish some repair.
Fibroblasts also form connective tissue (scar tissue) as part of the repair process.

Question 32

Can smooth muscle repair?

Answer 32

Has limited regeneration. In addition to some mitotic activity, new smooth muscle cells ma be derived from pericapillary mesenchymal cells.

Question 33

Can cardiac muscle repair?

Answer 33

NO they lack the ability to regenerate - scar tissue formation only