Biology 3.8 Flashcards
(20 cards)
Skeletal muscle
Posture, locomotion, voluntary and conscious movement. Long, thin cell called muscle fibre in bundles called fascicles. Groups of fascicles make a muscle. Striated pattern of dark and light stripes under microscope.
Cardiac muscle
Spontaneous contraction of the heart. Physiologically similar to skeletal muscle. Exclusive to the heart, highly organized cells with striations. Fibers are short, extensively branched, mono-nucleated. Short cardiac fibres are connected by gap junctions at ends referred to as intercalated disks. Action potentials made spontaneously by the heart spread rapidly through electrical synapses that travel via the gap junctions
Smooth muscle
Involuntary control from autonomic nervous system. Contraction can occur at blood vessels, bronchi, uterus. Actin and myosin filaments lack a highly ordered structure with more circular muscle cells. Lack striations, generate less force than skeletal muscle. Contraction is similar to that of skeletal muscle because it’s activated by calcium release by sarcoplasmic reticulum (SR). Present in structures controlled by ANS including blood vessels, reproductive system
Myofibril
Cylindrical bundles that fill the multinucleate muscle fibre cell. Striated due to light I-bands and dark A-bands. I-band has perpendicular dark line called Z-line/Z-disk that delineate the sarcomere, the contractile unit of skeletal muscle. During contraction, sarcomere shortens, I and A-bands overlap and Z-lines get closer. Lighter central region of the A-band is the H-zone that has a central perpendicular dark strip called the M-line
Two types of filaments in a myofibril
Actin and myosin
Actin
Thin filaments in myofibril. Extends across I-band and partially across A-band at relaxed state. In most cell types, important for cell motility and in the cytoskeleton. In myofibril, 2 chains of actin subunits are arranged into a double helix. Covered in troponin and tropomyosin proteins which cover the binding sites from myosin
Myosin
Thick filaments in myofibril. Extend from one ends of A-band to the other. Filamentous protein with bulbous protruding heads at the end. 2 myosin bundles attached at M-line makes a single think filament
T-tubule system and the Sarcoplasm
Sarcolemma is plasma membrane of muscle cell and invaginated by T-tubules all over the muscle fibres. Action potential spreads fast with t-tubule system to initiate coordinated contraction of sarcolemma. Sarcoplasm is cytoplasm of muscle cell and contains sarcoplasmic reticulum (SR) that stores Ca2+ ions that are released in response to stimulation from nervous system
Sliding filament model
When sarcomere contracts, Z-lines are closer but fibre length is the same. Has 6 steps.
Sliding filament model process step 1/6
Attachment: At rest, myosin head is in a “snapped back” position where it’s tightly bound
Sliding filament model process step 2/6
Release: When ATP binds to myosin, the head changes conformation to release actin filament
Sliding filament model process step 3/6
Cocked: When ATP is hydrolyzed, myosin head is “cocked” forward and bound to ADP+Pi
Sliding filament model process step 4/6
Binding sites exposed by calcium: At rest, myosin binding sites on actin are covered by tropomyosin. During contraction initiation, Ca2+ binds to troponin, causing tropomyosin to expose binding sites
Sliding filament model process step 5/6
Formation of cross bridges: Myosin heads are now able to bind to actin filaments
Sliding filament model process step 6/6
Power stroke and ADP+Pi release: When cross bridges are formed, a conformation change in head occurs to release tension like a spring and slides actin filament to centre of sarcomere. Once thin filament is pulled, myosin head releases ADP+Pi and cycle restarts. If ATP not replenished, muscles lock in state of contraction called rigor mortis.
Neuromuscular junction
For voluntary movement to be carried out, neurones need to communicate with skeletal muscle cells through specialized synapses called neuromuscular junctions. Has 4 steps.
Neuromuscular junction process step 1/4
Action potential generates release of acetylcholine: When action potential from neuron reaches neuromuscular junction, the neuron releases neurotransmitter acetylcholine from presynaptic terminal to cross the synaptic cleft
Neuromuscular junction process step 2/4
Action potential generated on sarcolemma and throughout T-tubules: Acetylcholine reaches postsynaptic membrane of muscle cell called the end plate where it binds to receptors on the sarcolemma and induces opening of Na+ channels. Results in influx of Na+ that depolarizes the muscle cell and causes action potential to spread through t-tubule system of skeletal muscle.
Neuromuscular junction process step 3/4
Sarcoplasmic reticulum releases Ca2+: In response to action potential, sarcoplasmic reticulum (ER of muscle cell) releases Ca2+ ions into muscle fiber
Neuromuscular junction process step 4/4
Myosin cross bridges form: Ca2+ binds to troponin on actin filaments, moving tropomyosin away from binding sites, allowing myosin heads to attach to actin binding sites. If ATP available, muscle contraction begins
