L1: Muscle Tissue

Question 1

Describe muscle tissue

Answer 1

capable of contraction
composed of muscle cells and CT (carries blood vessels and nerves, each muscle cell is supplied with capillaries and nerve fiber).
Muscle cells are elongate (therefore they are termed fibers) and lie in parallel arrays (with the longitudinal axis of the muscle).

Question 2

3 types of muscle fibers

Answer 2

skeletal, cardiac, smooth

Question 3

What is skeletal (striated)

Answer 3

striated (striped in appearance under microscope), voluntary (under conscious control)

Question 4

What is cardiac muscle in brief?

Answer 4

striated, involuntary

Question 5

smooth muscle in brief

Answer 5

non-striated, involuntary

Question 6

skeletal muscle fibers

Answer 6

very long compared with most other cells, up to several cm long, 10-100 um in diameter

multinucleate, nuclei are located peripherally

development: mesenchymal cells > myoblasts (proliferative) (fuse to make multinucleate cells) > myotubule > muscle cells

Question 7

Describe the arrangement of fibers with skeletal muscle (content).

Answer 7

similar to tendon arrangements.

blood vessels, lymph vessels, and nerves penetrate muscle with perimysium (each cell has BLN access)

endomysium contains capillaries and nerve fibers

Question 8

striation ultrastructure of skeletal muscles

Answer 8

fibers > myofibrils > myofilament

proteins are actin (thin filaments) and myosin (thick filaments)
Also tropomyosin and tropinin are associated with thin filaments - regulate when the muscle contracts

Question 9

sarcomere

Answer 9

the smallest contractile unit of skeletal muscle, bounded by Z lines, 2-3 um long

Question 10

Z lines

Answer 10

disc-like structures to which actin filaments attach on both sides; composed of alpha-actinin and dense amorphous matrix

Question 11

A-band

Answer 11

“anisotropic” band: birefringent in polarized light (as light source rotated 360 degrees becomes light-dark-light); signifies greater than 1 molecular species present (in this case, myosin and actin filaments)

Question 12

I-band

Answer 12

isotropic band: maintains darkness in polarized light; signifies a singular molecular species is present (actin filaments)

Question 13

H-zone

Answer 13

pale central region in A-band due to absence of thin filaments; outer portions of A band with both filaments (so therefore mostly thick filaments present)

Question 14

M-line

Answer 14

Thick filaments interconnected by cross-linking fine radial filaments, acts to maintain regular spacing and arrangement of thick filaments.

Question 15

What are myofilaments?

Answer 15

thin and thick filaments that make up a sarcomere

Question 16

Thin myofilaments, aka Actin.

Answer 16

composed mainly of F-actin (polymer of globular G-actin subunits) in two-stranded double helix (1 um in length)
(string of pearls, curls like DNA)
Associated with actin double helic is a long slender filament of tropomyosin that lies in the groove between the 2 F-actin strands

Troponin (globular protein) is attached to tropomyosin at regular intervals.

Question 17

Thick myofilaments, aka myosin.

Answer 17

composed of myosin (1.5 um long) arranged in bundle.

structurally has smooth central region with projections at each head. Each myosin molecule looks like a golf club with a shaft and a head.

Question 18

Describe the 7 steps of the sliding filament theory.

Answer 18

1. Myosin ATPase splits ATP and ADP+ Pi providing energy for “cocking” myosin head (stored energy in cocked position).
2. Stimulation causes release of Ca from sarcoplasmic reticulum (muscle ER)
3. Ca binds to troponin (C subunit)
4. Structural change in troponin removes tropomyosin from actin-myosin binding site
5. myosin head contacts actin molecule.
6. Upon contact, inorganic phosphate released concurrent with release of stored energy in myosin head causing “backward rowing” motion of myosin head and sliding of actin along myosin.
7. Another ATP molecule becomes bound to myosin causing release from actin, splitting ATP causes return of head to cocked state, and reattachment to actin. Ca present.

Repeat process until contraction is attained.

Question 19

Describe the innervation of skeletal muscle.

Answer 19

Every muscle fiber is supplied with a motor neuron

Generally one motor neuron innervates several muscle fibers (Motor unit- all muscle fibers innervated by a single neuron)

Each motor unit has a different stimulus threshold, when the threshold stimulus is attained get all or none contraction of all fibers within motor unit.

Graded contraction possible by activating different numbers of motor units.

Motor end plates= synapse of motor neuron on sarcolemma

Question 20

Motor end plates

Answer 20

synapse of motor neuron on sarcolemma

Question 21

Motor unit

Answer 21

all muscle fibers innervated by a single neuron

Question 22

Transverse tubules

Answer 22

tubular invaginations of sarcolemma extending deep into muscle fiber.

in humans, enters at A-I junction; amphibians at Z-line.

Function is to conduct stimulus into entire fiber.

Question 23

Sarcoplasmic reticulum

Answer 23

muscle cell equivalent of smooth ER

forms collar-like complex around each myofibril, consisting of connecting tubules and flattened terminal cisternae

A pair of terminal cisternae, in association with a single T-tubule, occur at each A-I junction

SR function= regulation of Ca in myofibrils

Question 24

Describe the cell type and development of cardiac muscle.

Answer 24

Cells (fibers):

1. elongate and cylindrical (80 um long, 15 um wide)
2. May be branched
3. Single central nucleus
4. Striations- same as in skeletal
5. intercalated discs= appear at junction between adjacent cells of muscle. Represent points of cohesion between cells; composed of sarcolemma and cell junctions. Occur at Z-lines.
6. development: mesenchymal cell –> myoblast –> cardiac muscle cell.

Question 25

Describe the “cardiac muscle untrastructure”

Answer 25

Myofibrils branc, anastomose, and are of variable width; not discrete cylinders as in skeletal muscle.

T-tubules less well-developed (wider and fewer) and enter at Z-lines

SR smaller and less complex, more dependent on extracellular Ca

Only endomysium present- no perimyseium or epimysium and no fascicles present.

Question 26

What is meant by saying the heart is myogenic?

Answer 26

demonstrates inherent spontaneous rhythmic activity

Question 27

Sinoatrial node

Answer 27

Located near junction of vena cava with right atrium; consists of specialized mass of cardiac muscle fibers innervated by autonomic NS; functions as pacemaker, each wave of excitation spreading over heart begins here. Spreads via gap junctions between cardiac muscle cells.

Question 28

Atrioventricular node

Answer 28

located near junction of right atrium and ventricle; specialized cardiac muscle fibers innervated by autonomic NS; as excitation wave spreads over atria, it is momentarily delayed here before transmission to ventricles.

Question 29

AV bundle

Answer 29

aka bundle of histawara

Bundle of specialized conducting cardiac muscle fibers; function = passes impulse rapidly from AV node to ventricles. Helps speed impulse b/c ventricles are bigger (via what fibers?)

Question 30

Purkinge fibers

Answer 30

specialized cardiac uscle fibers extending from AV septum and lateral ventricle walls to supply ventricular muscle (base)

Stain more lightly than cardiac muscle cells, are swollen in nuclear region where no myofibrils or striations are present.

Question 31

Describe smooth muscle.

Answer 31

present in walls of tubes (e.g. digestive tract, circulatory system, etc)

contraction is slow process

fibers capable of sustaining partial contraction indefinitely (=tonus)

Question 32

Describe smooth muscle cells (physical, developmental)

Answer 32

Elongate and tapering (40-200 um long)

uninucleate, centrally located, in longitudinal section assume “snake-like” appearance when contracted.

no striations.

development: mesenchymal cell –> smooth muscle cell (direct lineage)

Question 33

Describe smooth muscle ultrastructure

Answer 33

No myofilament arrangement into sarcomeres.

arragenemtn of contractile elements:
-intermediate filaments (tonofilaments) attached to dense bodies (contain alpha-actinin, same protein as in z-lines) distributed throughout sarcoplasm

• bundles of intermediate filaments stretch from one dense body to the next, assumes cable-like system
• sliding of thin filaments over thick filaments harnessed by “cables” so that dense bodies are pulled together = contraction

Question 34

Describe the arrangement of fibers in smooth muscle.

Answer 34

Smooth muscle cells arranged shingle-like in longitudinal-section, held together with elastic and reticular fibers and some collagen.

In cross-section, look for “a light smattering of nuclei in a field of cells”; this appearance is characteristic of smooth muscle

Question 35

Histophysiological types of skeletal muscle: red fibers

Answer 35

high concentration of myoglobin (involved in oxygen uptake from the blood), high numbers of mitochondria, aerobic, slow twitch, fatigue resistant

Question 36

Histophysiological types of skeletal muscle: white fibers

Answer 36

lower myoglobin concentration and lower numbers of mitochondria, glycolytic, fast-twitch, fatigue rapidly

Question 37

Histophysiological types of skeletal muscle: intermediate fibers

Answer 37

intermediate myoglobin concentration and relatively high numbers of mitochondria, fast-twitch, oxidative-glycolytic, fatigue resistant.