Muscle Tissue Part 2

Anatomy muscl e tissue

46 cards   |   Total Attempts: 182
  

Cards In This Set

Front Back
What is the Z disc (line)
A dense material called the Z disc is found at each end of a sarcomere, separating it from the next sarcomere in line
Where are thin and thick myofilaments located in the sarcomere
Extending from each Z disc towards the middle of the sarcomere are the thin myofilaments. Suspended within the sarcoplasm, between the thin myofilaments, and not attached to the z discs, are the thick myofilaments.
What forms the striations of skeletal muscle
The alternating areas of thin myofilaments, followed by areas of overlapping thin and thick myofilaments, are responsible for the striations seen in skeletal muscle
Actin
The main component of a thin myofilament is actin, each molecule of which looks like a kidney bean. Individual molecules of actin are linked together to form the actin filament that is twisted to form a helical strand. On each molecule of actin within the helical strand is a myosin-binding site upon which the thick myofilaments will attach
Tropomyosin-troponin complex
Also present on the thin myofilament are two regulatory molecules called tropomyosin and troponin. In relaxed muscle, the tropomyosin-troponin complex covers the myosin-binding sites on the actin molecules. This blocks the myosin-binding sites and prevents the attachment of the thick myofilaments, thus preventing contraction of the sarcomere.
Myosin
Each thick myofilament is composed of about 200 molecules of a protein called myosin. A molecule of myosin is shaped like two golf clubs twisted together. The tail of the molecule extends to the center of each sarcomere. The projecting “head,” called a cross bridge, extends out towards the thin myofilaments
Arrangement of molecules
Tails of adjacent myosin molecules lie parallel to each other, forming the “shaft” of the thick myofilament, while the “heads” project around the shaft in a spiraling fashion
Titan
A third component of the sarcomere is the elastin filament (also known as titan). The role of titan is to anchor the thick myofilaments in position and to play a role in recovery of the resting sarcomere length when a muscle cell is stretched or contracted
Describe the sarcoplasmic reticulum and calcium flux
A fluid-filled system of tubules called the sarcoplasmic reticulum encircles each myofibril. In a relaxed muscle cell, the sarcoplasmic reticulum stores calcium ions by sequestering them from the sarcoplasm. Calcium ions released through calcium channels back into the sarcoplasm around the thin and thick myofilaments trigger muscle contraction
Describe transverse tubules
Transverse tubules (T tubules) are tunnel- like enfolding of the sarcolemma. They penetrate the myofiber at right angles to the sarcoplasmic reticulum and the myofilaments. T tubules are open to the outside of the muscle fiber and are therefore filled with extracellular fluid
What is a muscle triad
On both sides of a T-tubule are dilated end sacs of the sarcoplasmic reticulum called the terminal cisternae. A T-tubule, together with its two terminal cisternae, is called a muscle triad
Describe the sliding filament theory of muscular contraction
During muscle contraction, the thick myofilaments attach to, then pull on, the thin myofilaments, causing them to slide inward towards each other. As the cross bridges apply force to the thin myofilaments, the thin myofilaments move towards the center of the sarcomere. This may occur so far that their tips overlap each other. As the thin myofilaments “slide” inward, they pull the Z discs towards each other, and the sarcomere shortens, but the lengths of the myofilaments stay the same. The shortening of the sarcomeres, all in series, causes shortening of the whole muscle fiber, and ultimately of the entire muscle itself, producing force used for work
Describe the electrical and chemical events of skeletal muscle contraction
Acetylcholine released by the motor neuron at the neuromuscular junction diffuses across the synaptic cleft and binds to its receptors on the motor end plate. This binding initiates formation of an electrical message in the sarcolemma that spreads in all directions, passing down the transverse tubules and out into the sarcoplasmic reticulum, causing calcium channels there to open, allowing calcium to diffuse into the sarcoplasm. Calcium does two things: (1) it binds to troponin, causing the tropomyosin-troponin complex to move, exposing the myosin binding sites on the actin; (2) it activates ATP on the myosin heads, causing them to bind to the actin. Binding of myosin heads to actin causes the hinge regions to tilt, pulling the thin myofilaments across the thick myofilaments, thus causing the sarcomeres to shorten and the muscle cells to contract
Describe the electrical and chemical events of skeletal muscle relaxation
Two changes are necessary to permit a muscle to relax after it has contracted. ACh is rapidly broken down in the synaptic cleft by the enzyme acetylcholinesterase (AChE) present on the motor end plate; this stops the generation of the muscle membrane electrical message. Secondly, calcium ions are pumped into the sarcoplasmic reticulum, where they are bound to the molecule calsequestrin, thus removing them from the sarcoplasm. Without calcium, the tropomyosin-troponin complex moves back over the actin, covering the myosin-binding sites. This prevents binding of myosin cross bridges to actin. Release of the actin by the cross bridges allows the thin myofilaments to slip back to their resting position so that the sarcomere resumes its resting length. The muscle cell is now relaxed
What is muscle tone?
Sustained, small contractions of motor units give skeletal muscles a firmness known as muscle tone