FORM & FUNTION (Skeletal Muscle)

Question 1

Monosynaptic reflex:

Answer 1

1. Signal received by stretch receptor
2. Signal transmitted by sensory neuron
3. Glutamate is released in synapse
4. Signal is transmitted by the motor neuron
5. ACh is released in the synapse
6. Muscle contraction

Question 2

Somatic NS innervates:

Answer 2

-skeletal muscle
-voluntary movement
-‘on and off’

Question 3

Autonomic NS innervates:

Answer 3

-cardiac muscle
-smooth muscle
-“push-pull” control (parasympathetic and sympathetic)
-postganglionic fiber innervates the muscles

Question 4

Muscle tissue is composed of:

Answer 4

-muscle cells specialized for contraction using microfilaments and motor proteins
*different types with unique morphology and function

Question 5

Types of muscle tissues:

Answer 5

Striated:
-skeletal (somatic motor)
-cardiac (autonomic motor)
Non-striated
-smooth (autonomic motor)

Question 6

Striated muscle:

Answer 6

-regular repetitive arrangements

Question 7

Skeletal muscle cell body:

Answer 7

-in ventral horn

Question 8

each muscle fiber receives a single synaptic input at:

Answer 8

-the NMJ: motor end plate

Question 9

Activated motor neuron:

Answer 9

-releases Ach
-Ach binds to its nicotinic receptor (ligand-gated Na+ channel)
*End plate potential

Question 10

End plate potential:

Answer 10

-GP in the muscle fiber due to Ach binding to its receptor on the muscle fiber

Question 11

Simple synapse at NMJ: 1AP from motor axon:

Answer 11

-get 1 AP in muscle fiber

Question 12

Muscle development:

Answer 12

-myoblasts fuse to form a skeletal muscle fiber

Question 13

Muscle fiber:

Answer 13

-sarcoplasm
-sarcoplasmic reticulum
-sarcolemma
-myofibrils

Question 14

Sarcoplasm:

Answer 14

-cytoplasm
-filled with long cylindrical filamentous bundles=myofibrils

Question 15

Sarcoplasmic reticulum (SR):

Answer 15

-sER
-intracellular membrane network that plays a critical role of intracellular Ca2+ concentration

Question 16

Sarcolemma:

Answer 16

-plasma membrane

Question 17

Myofibrils:

Answer 17

*contractile units
-tubes of filaments bundles
-diameter of 1-2 micrometer
-each of them are surrounded by SR

Question 18

A band:

Answer 18

-dark bands
-where there is thick filament (myosin)

Question 19

I band:

Answer 19

-light bands
-where there is no thick filaments (myosin)

Question 20

Z-line:

Answer 20

-marks the edges of the sarcomere

Question 21

Sarcomere:

Answer 21

-smallest functional unit of the muscle fiber
-2.5 micrometers long in resting muscle
-bounded by Z-lines

Question 22

M-line:

Answer 22

-middle line of the sarcomere (middle of myosin)
-in between the Z-lines

Question 23

H zone:

Answer 23

-space of only myosin
>no actin overlap

Question 24

2 types of microfilaments in sarcomeres:

Answer 24

-thick=myosin (‘heads’)
-thin=actin
*sliding filament theory

Question 25

Sliding filament theory:

Answer 25

-thick and thin filaments do NOT change their length, but instead SLIDE past each other
-myosin heads of thick filaments are attached to the thin filaments and pull the sarcomere
-sarcomere shortens=generating force

Question 26

Thin filament:

Answer 26

-myofilament made of G-actin polymer
-tropomyosin
-troponin complex

Question 27

Thick filaments:

Answer 27

-polymer of myosin
-myosin head is an ATPase

Question 28

Cross-bridge cycling:

Answer 28

-process that drives muscle contraction at the molecular level
-involves myosin heads attaching to actin filaments, pulling them and then detaching
*propelled by energy from ATP hydrolysis

Question 29

Cross bridge cycling steps:

Answer 29

1. When muscle is relax, tropomyosin (thin coil) cover the binding site of action
   >Molecule of ADP and P remains attached to myosin from the previous contraction
2. During contraction, calcium binds to troponin which causes tropomyosin to reposition.
   >Exposes myosin binding sites on actin filaments
3. Myosin heads bind to actin filaments
4. Myosin head springs forward “powerstroke” pulling the actin filaments (ADP and P released from myosin)
5. Myosin is released from actin when ATP binds
6. ATP is hydrolyzed, providing energy to “cock” the myosin filaments (recovery stroke)
7. Repeated

Question 30

New ATP binds to myosin:

Answer 30

*essential to release the cross-bridges
-not broken down yet

Question 31

At rest: sarcomere

Answer 31

-need calcium release from SR to initiate contraction
-calcium reuptake to Sr to terminate contraction

Question 32

Transverse tubule (T-tubule):

Answer 32

-invaginations of sarcolemma
-pass into the muscle fiber near the ends of the A band

Question 33

SR and T-tubule:

Answer 33

-separated by a gap
-linked by two proteins

Question 34

2 proteins linking SR and T-tubule:

Answer 34

1. Ryanodine receptor (RYR)
2. Dihydropyridine receptor (DHPR)

Question 35

RYR:

Answer 35

-ryanodine receptor
-on the SR

Question 36

DHPR:

Answer 36

-dihydropyridine receptor
-on the T-tubule
-a specialized voltage-sensitive Ca2+ receptor found in skeletal muscle
-changes shape in response to AP

Question 37

DHPR changes shape due to AP:

Answer 37

-when changes shape it pulls RYR open and releases Ca2+ ions from the SR to the cytoplasm

Question 38

2 triggers for RYR opening:

Answer 38

1. DHPR and sarcolemma depolarization
2. Ca2+ induced Ca2+ release: activation by Ca2+

Question 39

Summary of skeletal muscle excitation-contraction coupling:

Answer 39

-RYR and DHPR are physically connected in a ‘triad’ structure
-sarcolemma depolarization causes DHPR activation which causes RYR opening, SR Ca2+ release, actomyosin interaction and contraction
*source of Ca2+ for contraction is from SR not influx via DHPR
-contraction ends when Ca2+ concentration drops due to reuptake into the SR

Question 40

Timeline for muscle excitation-contraction coupling:

Answer 40

-AP: few ms
-increase Ca2+: 80ms
-twitch force: 180ms