FORM & FUNTION (Skeletal Muscle) Flashcards
Monosynaptic reflex:
- Signal received by stretch receptor
- Signal transmitted by sensory neuron
- Glutamate is released in synapse
- Signal is transmitted by the motor neuron
- ACh is released in the synapse
- Muscle contraction
Somatic NS innervates:
-skeletal muscle
-voluntary movement
-‘on and off’
Autonomic NS innervates:
-cardiac muscle
-smooth muscle
-“push-pull” control (parasympathetic and sympathetic)
-postganglionic fiber innervates the muscles
Muscle tissue is composed of:
-muscle cells specialized for contraction using microfilaments and motor proteins
*different types with unique morphology and function
Types of muscle tissues:
Striated:
-skeletal (somatic motor)
-cardiac (autonomic motor)
Non-striated
-smooth (autonomic motor)
Striated muscle:
-regular repetitive arrangements
Skeletal muscle cell body:
-in ventral horn
each muscle fiber receives a single synaptic input at:
-the NMJ: motor end plate
Activated motor neuron:
-releases Ach
-Ach binds to its nicotinic receptor (ligand-gated Na+ channel)
*End plate potential
End plate potential:
-GP in the muscle fiber due to Ach binding to its receptor on the muscle fiber
Simple synapse at NMJ: 1AP from motor axon:
-get 1 AP in muscle fiber
Muscle development:
-myoblasts fuse to form a skeletal muscle fiber
Muscle fiber:
-sarcoplasm
-sarcoplasmic reticulum
-sarcolemma
-myofibrils
Sarcoplasm:
-cytoplasm
-filled with long cylindrical filamentous bundles=myofibrils
Sarcoplasmic reticulum (SR):
-sER
-intracellular membrane network that plays a critical role of intracellular Ca2+ concentration
Sarcolemma:
-plasma membrane
Myofibrils:
*contractile units
-tubes of filaments bundles
-diameter of 1-2 micrometer
-each of them are surrounded by SR
A band:
-dark bands
-where there is thick filament (myosin)
I band:
-light bands
-where there is no thick filaments (myosin)
Z-line:
-marks the edges of the sarcomere
Sarcomere:
-smallest functional unit of the muscle fiber
-2.5 micrometers long in resting muscle
-bounded by Z-lines
M-line:
-middle line of the sarcomere (middle of myosin)
-in between the Z-lines
H zone:
-space of only myosin
>no actin overlap
2 types of microfilaments in sarcomeres:
-thick=myosin (‘heads’)
-thin=actin
*sliding filament theory
Sliding filament theory:
-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
Thin filament:
-myofilament made of G-actin polymer
-tropomyosin
-troponin complex
Thick filaments:
-polymer of myosin
-myosin head is an ATPase
Cross-bridge cycling:
-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
Cross bridge cycling steps:
- 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 - During contraction, calcium binds to troponin which causes tropomyosin to reposition.
>Exposes myosin binding sites on actin filaments - Myosin heads bind to actin filaments
- Myosin head springs forward “powerstroke” pulling the actin filaments (ADP and P released from myosin)
- Myosin is released from actin when ATP binds
- ATP is hydrolyzed, providing energy to “cock” the myosin filaments (recovery stroke)
- Repeated
New ATP binds to myosin:
*essential to release the cross-bridges
-not broken down yet
At rest: sarcomere
-need calcium release from SR to initiate contraction
-calcium reuptake to Sr to terminate contraction
Transverse tubule (T-tubule):
-invaginations of sarcolemma
-pass into the muscle fiber near the ends of the A band
SR and T-tubule:
-separated by a gap
-linked by two proteins
2 proteins linking SR and T-tubule:
- Ryanodine receptor (RYR)
- Dihydropyridine receptor (DHPR)
RYR:
-ryanodine receptor
-on the SR
DHPR:
-dihydropyridine receptor
-on the T-tubule
-a specialized voltage-sensitive Ca2+ receptor found in skeletal muscle
-changes shape in response to AP
DHPR changes shape due to AP:
-when changes shape it pulls RYR open and releases Ca2+ ions from the SR to the cytoplasm
2 triggers for RYR opening:
- DHPR and sarcolemma depolarization
- Ca2+ induced Ca2+ release: activation by Ca2+
Summary of skeletal muscle excitation-contraction coupling:
-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
Timeline for muscle excitation-contraction coupling:
-AP: few ms
-increase Ca2+: 80ms
-twitch force: 180ms
