Skeletal muscle cellular structure & contraction Flashcards
Somatic vs. autonomic motor/efferent pathways
Somatic motor system
- voluntary
- On-off control- so when no signal, muscle is relaxed
- Innervates skeletal system
Autonomic motor system
- Targets both cardiac and smooth muscle
- Involuntary
- Includes both sympathetic and parasympathetic divisions
- Work in a push-pull control (contraction or relaxation)
Muscle physiology
Composed of muscle cells specialized for contraction using microfilaments and motor proteins
Three types of muscle, each with unique morphology and function
1. skeletal muscle
2. cardiac muscle
3. smooth muscle
Skeletal muscle physiology
- Body movements; Both light and strong force
- Striated- regular repetitive units in cell
- Regular repeated movements
- Controlled by somatic motor system
Cardiac muscle physiology
- Responds to changes in contraction and relaxation; has the ability to pump blood
- Striated
- Regular repeated movements within muscle
- Controlled by autonomic motor system
Smooth muscle physiology
- Organs can change size (eg. Bladder) so need to be able to stretch and still perform function
- Not striated
- Controlled by autonomic nervous system
Neuromuscular junction (NMJ)
In skeletal muscle (between neuron and muscle) and therefore somatic motor system
Each muscle fiber receives a single synaptic input at the NMJ which is known as the motor endplate
An activated motor neuron releases acetylcholine which binds to its nicotinic receptor (a ligand-gated Na channel)
Results in an endplate potential
Endplate potential
Graded potential in the muscle fiber due to acetylcholine binding to its receptor on the muscle fiber
Ratio of AP in motor axon and muscle fiber
Always a 1:1 ratio
1AP from motor neuron will always result in 1AP in muscle fiber
Muscle Development
Myocyte (muscle cell) make up the muscle fibers
Multinucleated because during development, the muscle fiber is formed by many myoblasts fusing together
Muscle fiber content terms
- Sarcoplasm= cytoplasm
- Filled with long cylindrical filamentous bundles called myofibrils (contractile units) - Sarcoplasmic reticulum= sER
- Sarcolemma= plasma membrane
Contractile units of muscle fiber
Sarcomeres make up Myofibrils
Muscle tissue section
Repetitive light-dark bands
- Dark= A band (middle is H band)
- Light= I bands
- Middle of light band has a Z band
Z-bands
The boundary of sarcomeres which are the smallest contractile units in muscle fiber
What is a Sarcomere?
- Smallest functional unit of a muscle fiber
- Bounded by Z-lines
Muscle organ structure
- Multiple bundles of multiple muscle fibers
- Each muscle fiber has many myofibrils with I bands, A bands, Z lines that combine to make many sarcomeres
Components of a sarcomere
2 types of myofilaments
- Thick filament
- Made of myosin - Thin filament
- Made of actin
Three components of Thin filament
- Myofilament made of G-actin polymer
- Tropomyosin
- Troponin complex
Two components of thick filament
- Polymer of myosin
- Myosin head which is an ATPase (breaks down ATP)
Sliding Filament Theory
The thick and thin filaments do not change their length, but instead slide past each other
The myosin heads of the thick filaments are attached to the thin filaments and pulling the sarcomere causing the sarcomere to shorten and generate force
Cross-bridge cycling
- Involves myosin heads attaching to actin filaments, pulling them and then detaching
- Propelled by energy from ATP hydrolysis
Steps of cross-bridge cycling
- When muscle is relaxed:
- Thin actin filament -Tropomyosin covers the binding sites on actin
- Thick myosin filament- a molecule of ADP and phosphate remains attached to myosin from the previous contraction - During a contraction, calcium binds to troponin of thin filament causing the repositioning of tropomyosin which exposes the myosin binding sites on actin filaments
- Myosin heads bind to actin filaments
- Myosin heads spring forward in a power stroke, pulling the actin filaments. This stroke causes ADP and phosphate to be released
- Myosin is released from actin. New molecule of ATP binds to myosin, causing it to be released from the actin filament
- ATP is hydrolyzed, providing energy to cock the myosin filaments in a recovery stroke to set up for repeating the process
- Repeat cycle!
Where does the muscle get the calcium needed for cross-bridge cycling?
Receives from the sarcoplasmic reticulum in response to an action potential
T-tubules (transverse tubule)
Invaginations of the sarcolemma that pass into the muscle fiber near the ends of the A band
Location of sarcoplasmic reticulum in a muscle fiber
Each muscle fiber has multiple myofibrils and each myofibril is surrounded by sarcoplasmic reticulum. This location allows for the timely delivery of Ca2+ to the sarcomeres in the myofibrils of a muscle fiber
What connects the sarcoplasmic reticulum and the T-tubules?
They are separated by a gap but are linked by two proteins: Ryanodine receptor (RYR) and Dihydropyridine receptor (DHPR) that are physically connected in a triad structure
Allows for the connection between plasma membrane where the action potential is and the sarcoplasmic reticulum so that communication can occur for release of calcium
Dihydropyridine receptor (DHPR)
A specialized voltage-sensitive Ca2+ receptor found in skeletal muscle on the T- tubule
Location of Ryanodine receptor (RYR)
Sarcoplasmic reticulum- both near T tubules and DHPR, and away from T-tubules
Ways that calcium is released to sarcomeres
- An action potential (depolarization) causes dihydropyridine receptor (DHPR) to change shape which pulls open the ryanodine receptor (RYR) and releases Ca2+ ions from the sarcoplasmic reticulum to the cytoplasm
- Ca2+-induced Calcium release- after step one and the initial release of calcium, this build up of calcium will cause the other RYR not near the T-tubules to open and release more Ca2+
Terminating contraction in muscle
- Calcium needed for contraction of muscle and cross-bridge cycling process so to terminate contraction, need reuptake of calcium
- The calcium is removed by active transport after the action potential ends
Timeline of Skeletal muscle contraction
- Action potential- shortest; few ms
- Rise in Ca2+= ~80ms
- Twitch force= ~180ms
