Lecture 4 Flashcards
What are 3 key traits of the sarcoplasmic reticulum? What is a myofibril made of? What is a sarcomere?
- Surrounds each myofibril
- A single T-tubule and the 2 terminal cisternae form a triad
- SR stores Ca+2 when muscle not contracting
When stimulated, calcium released into sarcoplasm
SR membrane has Ca+2 pumps that function to pump Ca+2 out of the sarcoplasm back into the SR after contraction
Made up of a series of sarcomeres joined end-to-end
the smallest contractile unit of the muscle fiber
What are the 3 types of myofilaments?
- Actin: Thin Filament
- Myosin: Thick Filament
- Titin: Elastic Filament (“springy” protein)
These filaments make up the contractile machinery of muscle
What is tropomyosin? What is troponin?
• Tropomyosin
– Rod-shaped protein that occupies the groove between the twisted strand of actin molecules
– Blocks the myosin binding sites on the G-actin molecules
• Troponin – A complex of three globular proteins. • One is attached to the actin molecule • One is attached to tropomyosin • One contains a binding site for calcium
What are the main structures associated with myosin?
– Crossbridges
• Composed of a rod-like tail and two globular heads
• Interacts with actin during contraction.
• Contain binding sites for both actin and ATP
– The enzyme ATP-ase is located at the ATP binding site for hydrolysis of ATP
What does titin do?
– Connects myosin to the Z-lines in the sarcomere
– It is very elastic
• Able to stretch up to 3 times its resting length
– Important molecule because it is responsible for muscle flexibility
What is the role of calcium in muscle contraction?
Electrical impulse travels through T-tubules and sarcoplasmic reticulum. The SR releases large quantities of Ca+2 into the sarcoplasm. – Bind with troponin – lifts tropomyosin from actin – active sites are open – myosin cross-bridges attach – Muscles can contract
What is the sliding filament model?
It is a cycle of repetitive events that causes actin and myosin myofilaments to slide over each other, contracting the sarcomere and generating tension in the muscle.
What is excitation-contraction coupling?
Excitation-contraction coupling refers to the sequence of events by which an AP (an electrical event) in the sarcolemma of the muscle cell initiates the sliding of the myofilaments, resulting in contraction (a mechanical event).
What are 3 key characteristics of motor neurons?
- muscle fibers innervated by large (alpha) myelinated nerves
- Motor neurons originate from spinal cord
- nerve ending ends at neuromuscular junction
What is a motor unit?
Motor Unit = all the muscle cells controlled by one motor neuron
- Each muscle fibre receives a single axon terminal from a motor neuron
- Each axon can have collateral branches to innervate an equal # of fibres
What are the 3 structural features of a chemical synapse?
- Synaptic cleft
- Signal passes from pre-synaptic to post synaptic cell
- neurotransmitter
What is a neuromuscular junction? What are the 3 key features of a neuromuscular junction?
Where the nerve (motor neuron) meets the muscle
• Synapse between motor neuron and muscle fibre
– neuromuscular cleft
• Motor end plate
– Pocket formed around motor neuron by sarcolemma
• Acetylcholine
– Causes end-plate potential (EPP)
– Depolarizes muscle fibre
What is the 4 step simplified process of muscle contraction?
Muscle contraction (simplified): •Alpha motor neurons release Ach
- Ach binding to receptors evokes action potential
- Action potential (excitation) triggers Ca2+release, leads to fibre contraction
- Relaxation, Ca2+ levels lowered by reuptake
What is an analogy for the crossbridge cycle? What are the 3 specific steps?
- Analogous to rowing a boat through water
- Oar paddle =crossbridge
- Linking of thick filament to thin filament = oar contact with water
- Power stroke: myosin head moves, propelling thin filament toward centre of muscle = movement of oar propelling boat
- Thick and thin filaments detach = oar breaks contact with water
- Myosin head returns to the initial position = oar moved to new position, cycle starts again
