Muscles part 1 Flashcards
Connective tissue and muscle
Connective tissue wraps around individual muscle fiber, then over bundles of fibers, then over bundles of these bundles and so on. It then weaves into the tendon, attaching the muscle to the skeleton
Muscle fiber cells (2)
-Inside muscle fibers, there are tiny thread-like structures called myofibrils that help muscles contract. These myofibrils are wrapped by a special network called the sarcoplasmic reticulum (SR), which stores and releases calcium—a key player in muscle contraction.
-The muscle cell also has tiny tunnels called transverse tubules (T-tubules) that reach deep inside from the cell’s outer membrane. These T-tubules work closely with the SR.
Muscle contraction (3)
-Myofibrils are made up of tiny repeating segments called sarcomeres
-When a nerve signal tells the muscle to move, it travels along the cell’s outer membrane and down the T-tubules. This signal makes the SR release calcium, which then triggers the sarcomeres to contract.
-Picture a person sitting in a limo (myosin). Outside of their limo is a rope (Actin) attached to a wall (Z-disc). The driver pulls on the rope, bringing the limo closer to the wall.
-note these filaments do not shorten, only pull themselves closer, overlapping and shortening the empty space between them
Parts of a sarcomere (5)
From the middle outwards…..
-M-Line - Middle of the A-band, stabilized myosin filaments keeping them properly aligned
-H-zone – region in the middle of the A-band with only myosin.
-A-band – The dark area where actin and myosin overlap
-I-band – The light area with only actin (shortens when muscle contracts).
Z-discs – The boundaries of each sarcomere.
Actin structure
-Each actin filament is made of two actin chains arranged in a loose helix. Regulatory proteins tropomyosin and troponin which aid actin in binding to myosin
Myosin structure
Myosin filaments are made of many myosin molecules. Each myosin molecule has two heavy chains of amino acids, which coil to from a long tail. At the end of each tail are two heads, which have an actin binding site as well as binding site for ATP hydrolysis. These heads are attached to the tails with 2 light amino acid chains for each head. So in total for each molecule there are 2 heavy AA chains, and 4 light
process of muscle contraction (6 steps)
1). the myofibril is in rigor - a state between a power stroke and the binding of a new ATP.
2). ATP binds to myosin - detaching it from actin
3). The ATPase hydrolyzes ATP into ADP and Pi, and the myosin loosely attached to the G-Actin (actin binding site)
4). if Ca is present, then the binging tightens
5). Pi is released, releasing energy that pulls the the actin 10um toward the M-line
6). ADP unbinds, leaving the leaving the myofibril in rigor
Troponin and tropomyosin function
Tropomyosin stops myosin from fully binding to the G-actin of myosin. When Ca is present, it binds to troponin, which in turn moves the tropomyosin out of the way allowing the myosin to bind
Calcium release (8)
1.) Action potential in a motor neuron triggers release of ACh
2). ACh opens Ligand-gated Na+ channels
3). the resulting action potential travels down the membrane of the T-tubule
4). AP reaches the voltage-gated DPHR, opening a Calcium channel, which allows Ca to travel through the cytoplasm to the actin
5). The Ca binds to troponin which moves tropomyosin allowing myosin to bind to the g-actin
6). AChE stops the action potential from continuing
7), Ca channels close
8). Ca pumps remove excess Ca, decreasing the [Ca] in the cytoplasm. Due to the lower [Ca], Ca ions diffuse from the actin back into the cytoplasm
fused tetanus
-When a muscle gets a single quick signal (stimulus) from a nerve, it gives a small, brief contraction called a twitch.
-But if the muscle gets repeated signals very quickly, the twitches start adding up (summation). This makes the muscle contract stronger and more continuously.
-If the signals come fast enough, the muscle can’t relax at all—it stays fully clenched in a smooth, strong contraction called fused tetanus
Isometric contraction vs Isotonic contraction
Isometric = Force without movement (muscle stays the same length). Ie trying to lift a car, you strain but your muscles don’t budge
Isotonic = Movement with tension (muscle changes length while lifting a load).ie lifting up your backpack, you move it easily.
maximum isometric tension
When a muscle is relaxed, it has the greatest amount of tension force. If you shorten/lengthen the muscle, the actin/myosin overlap too much/too little and tension decreases
3 metabolic pathways that supply ATP for muscle contraction
- Creatine phosphate pathway (fast but small yeild)
-Glycolysis (Fast with moderate yield)
-Oxidative phosphorylation (slow but high yield)
Motor unit
Consists of a single motor neuron and all the muscle fibers it supplies. In vertebrates, a single motor neuron connects to multiple fibers, with multiple branches to each fiber, resulting in EPSPs. In Arthropods, they have 2 types a motor neurons, excitatory and inhibitory , which also connect to multiple fibers at multiple places.
Multiunit smooth vs single-unit smooth muscles (3 each)
Multiunit:
-Each fiber contract independently
-requires direct nerve stimulation (nervous system initiates contractions)
-Made for precision ex Eye movement
Single-unit
-Fibers are connected by gap junctions, contract synchronized
-Self-excitable (pacesetter cells initiate contractions, nervous system fine tunes them)
-Made for sustained coordinated activity ex movement of food through intestines
