skeletal muscles

Why wouldn't all of the muscle fibers in a muscle be innervated by just one motor neuron?

When an action potential arrives at the branching axon terminals of a single somatic motor neuron, ALL of the muscle fibers innervated by that neuron will contract in unison because they are all members of the same motor unit.

What filaments are found in myofibrils?

actin and myosin

What are thin filaments made of?

actin, troponin, tropomyosin

How are reflex arcs different from voluntary movement of skeletal muscles?

-A reflex arc like the knee-jerk reaction does not involve the brain; rather it occurs within the spinal cord. -Voluntary movements differ from reflexes and basic locomotor rhythms in several important ways. By definition they are intentional—they are initiated by an internal decision to act—whereas reflexes are automatically triggered by external stimuli

Why are t-tubules so important? What is a triad?

-Ca2+ release from the SR is triggered by an action potential on the sarcolemma which travels down transverse-tubules (T-tubules) into the fiber's center to reach the triads. -Triads consist of a T-tubule and its two flanking terminal cisternae of the Sarcoplasmic Reticulum within adjacent sarcomeres.

Where does most of the calcium that plays a role in contraction come from? What happens to it once it is released?

-In heart muscle cells, most of the calcium is stored inside a chamber named the sarcoplasmic reticulum. The calcium in the sarcoplasmic reticulum is released during heart muscle contraction and transported back inside the sarcoplasmic reticulum during relaxation.

What is troponin and tropomyosin?

-In resting or non-contracting skeletal muscle, myosin heads cannot bind to actin filaments because the protein Tropomyosin covers up (i.e., occludes) the sites on actin where myosin heads can bind. -Troponin is a complex of three proteins. Ca2+ ions bind to Troponin C, which is a Ca2+ -binding protein. The Ca2+-induced conformational change in Troponin C is transmitted to Troponin T, which pulls Tropomyosin away from the myosin-binding sites on the actin filament. Troponin I binds to actin.

What are I bands or A bands? What is an M line?

-Isotropic or I-bands) are actin filaments projecting in opposite directions from a Z-disc? -Anisotropic or A-bands) are myosin filaments that interdigitate with actin filaments. -M line In high-power micrographs of striated muscle, the thin, dark line in the center of the H band of a sarcomere. It contains myomesin (a protein that connects thick (myosin) filaments), C protein, and creatinine kinase.

sliding filaments

-during muscle contraction the sarcomeres shorten because the actin filaments slide relative to myosin filaments and the distance between adjacent z-discs become shorter

What shortens during contraction? Do the filaments shorten?

-sarcomere -There is no shortening of individual actin or myosin filaments during muscle contraction.

What is a motor unit?

A single spinal motor neuron, and all of the skeletal muscle fibers that it innervates (within a single muscle), are collectively known as a motor unit.

When is ATP used? Is it only needed during contraction? What about relaxation?

ATP is in fact a critical requirement for muscle contraction because it breaks the myosin-actin cross-bridge, freeing the myosin for the next contraction. Without ATP, muscles would remain in their contracted state, rather than their relaxed state.

What are myofibrils?

any of the elongated contractile threads found in striated muscle cells. -contain parallel arrays of actin and myelin filaments

What are thick filaments made of?

myosin -are composed of myosin II dimers (2 protein molecules that function together

relaxtion

relaxation of muscle fibers occurs with myoplasmic [Ca2+] is decreased by the pumping activity of the SERCA

Do you understand the cross-bridge cycle?

• Only one of the two heads of the myosin molecule at a given time binds to actin to form a cross-bridge between myosin and actin filaments. • Myosin filaments pull actin filaments toward one another by these cross-bridges. In the presence of increased intracellular calcium ions, myosin heads go through a repeating cycle of: 1) binding to actin filaments 2) undergoing a power stroke 3) unbinding from actin filaments.

What is SERCA and is it important?

• SERCA stands for Sarcoplasmic-Endoplasmic Reticulum Ca2+ ATPases. • SERCAs are constantly pumping Ca2+ back into the SR lumen.

Skeletal muscle fibers are encased in extracellular material that provide a covering, protection, and support. What is this covering?

Skeletal muscle fibers are encased in extracellular material known as connective tissue. -The periosteum, tendon, fascia, epimysium, perimysium and endomysium are all part of this connective tissue and all are connected to one another. -The covering offers support and protection. Fibers are fragile and this allows them to withstand the forces of contraction.

There are two binding sites on the head of myosin II molecules. What binds here?

The head domain of myosin contains two sites: one is the enzymatic ATP-binding & hydrolysis site and the other is an actin-binding site.

Why is the release of calcium so important for contraction?

Calcium helps regulate muscle contraction. When a nerve stimulates a muscle, the body releases calcium. The calcium helps the proteins in muscle carry out the work of contraction.

CaV1.1

CaV1.1 is an L-type, voltage-gated Ca2+ channel (L-type refers to long- lasting openings). • CaV1.1 serves as the voltage-sensor for E-C coupling in skeletal muscle fibers. • CaV1.1 is also known as the Dihydropyridine Receptor (DHPR) because it binds with high affinity to dihydropyridine drugs such as Nifedipine used clinically to treat hypertension.

Each myofibril has a very small diameter, why would this be important?

Each myofibril has a diameter of 1-2 micrometers. They are created during embryonic development in a process known as myogenesis. Myofibrils are composed of long proteins including actin, myosin, and titin, and other proteins that hold them together.

Each skeletal muscle fiber in an adult human is innervated by how many presynaptic terminals of a somatic motor neuron?

Each skeletal muscle fiber in an adult human is innervated by a single presynaptic terminal of a somatic motor neuron.

RYR1

Ryanodine Receptors (RYR1) are giant, tetrameric Ca2+ channels embedded within the SR bilayer. One RyR1 tetramer, present in skeletal muscle, is mechanically gated (i.e., physically opened) by the cytoplasmic domains of four CaV1.1 channels, which are embedded within the sarcolemma of the T-tubules. Ryanodine receptors are so named because they bind with high affinity to ryanodine, a poisonous chemical found in the South American plant Ryania speciosa (a member of the Willow family).

What are Z discs? Does anything attach to these disks?

The boundaries of a sarcomere (contractile unit) are Z-discs or Z lines. -F-actin filaments, titin, and the nebulin/nebulette system directly attach to the Z-disc, and, because of their importance for sarcomere mechanics, the Z-disc has initially been regarded as important for mechanical stability only

What is a sarcomere?

Thick and thin filaments occur in parallel arrays and are the reason contraction can occur. The basic contractile unit is a sarcomere.