Vocab. Ch. 9 - Skeletal Muscle Tissue

14. Neuromuscular junction

Junction of a somatic motor neuron and the sarcolemma (junctional folds) of a muscle fiber *An elliptical neuromuscular junction, or end plate, is formed between short, curling branches given off from each axon ending and a single muscle fiber. *Many toxins, drugs, and diseases interfere with events at the neuromuscular junction.

11. Myofilaments (filaments)

Small structures that are the muscle equivalents of the actin- or myosin-containing microfilaments *Contractile myofilaments are of two types- thick and thin. *Thick filaments contain bundled myosin molecules; thin filaments contain actin molecules (plus other proteins).

8. Fascicle

a discrete bundle of muscle cells, segregated from the rest of the muscle by a connective tissue sheath *Surrounded by permysium

16. Perimysium

a layer of fibrous connective tissue that surrounds each fascicle *Perimysium, as well as epimysium and endomysium, is a connective tissue sheath of skeletal muscle.

1. Acetylcholine

a neurotransmitter found in small membranous sacs called synaptic vesicles, which are found within the mound-like axon terminal *ACh is released when a nerve impulse reaches a neuromuscular junction. *Upon binding to sarcolemma recpetors, ACh causes a change in sarcolemma permeability leading to a change in membrane potential.

12. Myoglobin

a red pigment that stores oxygen *Myoglobin is similar to hemoglobin, the pigment that transports oxygen in blood. *It's the reason why slow oxidative fiber red, being muscles' oxygen-binding pigment that stores O2 reserves in the cell and helps O2 diffuse through the cell.

26. Striations

a repeating series of dark and light bands, evident along the length of each myofibril *They are obvious stripes in the longest muscles cells: the skeletal muscle fibers. *In an intact muscle fiber, the dark A bands and light I bands are nearly perfectly aligned, giving the cell its striated appearance.

25. Tropomyosin

a rod-shaped protein whose polypeptide strands spiral about the actin core and help stiffen and stabilize it *Successive tropomyosin molecules are arranged end to end along the actin filaments, and in a relaxed muscle fiber, they block myosin-binding sites on actin so that myosin heads on the thick filaments cannot bind to the thin filaments. *Both troponin and tropomyosin help control the myosin-actin interactions involved in contraction.

22. Synaptic cleft

a space filled with a gel-like extracellular substance rich in glycoproteins and collagen fibers that separates the exceedingly close axon terminal and the muscle fiber *Varicosities release neurotransmitter into a wide synaptic celft in the general area of the smooth muscle cells.

6. Endomysium

a wispy sheath of connective tissue that surround each individual muscle fiber *It consists of fine areolar connective tissue.

20. Sarcoplasmic reticulum

an elaborate smooth endoplasmic reticulum *Its interconnecting tubules surround each myofibril the way the sleeve of a loosely crocheted sweater surrounds your arm. *Most SR tubules run longitudinally along the myofibril, communicating with each other as the H zone.

28. T tubule (Transverse tubule)

at each A band-I band junction, an elongated tube formed when the sarcolemma of the muscle cell protrudes deep into the cell interior *The T tubules tremendously increase the muscle fiber's surface area. *Along its length, each T tubule runs between the paired terminal cisterns of the SR, forming triads, successive groupings of the three membranous structures (terminal cistern, T tubule, and terminal cistern).

9. Motor unit

consists of one motor neuron and all the muscle fibers it innervates, or supplies *When a motor neuron fires (transmits an action potential), all the muscle fibers it innervates contract. *The muscle fibers in a single motor unit are not clustered together but are spread throughout the muscle.

7. Epimysium

is an "overcoat" of dense irregular connective tissue that surrounds the whole muscle *Sometimes it blends with the deep fascia that lies between neighboring muscles or the superficial fascia deep to the skin.

4. Axon

long threadlike extensions of motor neurons, which "reside" in the brain or spinal cord, that travel, bundled within nerves, to the muscle cells they serve *Axons of motor neurons extend from the spinal cord to the muscle. *There each axon divides into a number of axon terminals that form neuromuscular junctions with muscle fibers scattered throughout the muscle.

3. Action potential

propagates (moves along the length of the sarcolemmma) in all directions form the neuromuscular junction, just as ripples move away from a pebble dropped into a stream *the end plate potential (local depolarization at the neuromuscular junction) ignites an action potential by spreading to adjacent membrane areas and opening voltage-gated sodium channels there. *As it propagates, the local depolarization wave of the AP spreads to adjacent areas of the sarcolemma and opens voltage-gated sodium channels there.

13. Myosin

protein primarily composing thick filaments (about 16 nm in diameter) *Each myosin molecule consists of two heavy and four light polypeptide chains, and has a rod-like tail attached by a flexible hinge to two globular heads. *The tail consists of two intertwined helical polypeptide heavy chains.

2. Actin

protein that plays a role in motility and shape change in virtually every cell in the body *has kidney-shaped polypeptide subunits, called globular actin (G actin) which bear the active sites to which the myosin heads attach during contraction *actin chiefly compose the thin filaments

10. Myofibril

rod-like structures that compose, along with hundreds to thousands of others, of a single muscle fiber, running parallel to its length *They account for about 80% of cellular volume. *They contain the contractile elements of skeletal muscle cells, the sarcomeres, which contain even smaller rod-like structures called myofilaments.

15. Neurotransmitter

signaling chemicals usually stored in vesicles *Neurotransmitter binding generates an action potential, which is coupled to a rise in calcium ions in the cytosol. *The effect of a specific neurotransmitter on a smooth muscle cell depends on the type of receptor molecules on the cell's sarcolemma.

21. Sliding filament model of contraction

states that during contraction the thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree *When the nervous system stimulates muscle fibers, the myosin heads on the thick filaments latch onto myosin-binding sites on actin in the thin filaments, and the sliding begins. *These cross bridge attachments form and break several times during a contraction, acting like tiny ratchets to generate tension and propel the thin filaments toward the center of the sarcomere

5. Axon terminal

the axon ending of a motor neuron *The axon terminal releases ACh into the synaptic cleft when a nerve impulse reaches the end of an axon. *It, along with the synaptic cleft and junctional folds of the sarcolemma, is included in the neuromuscular juction.

18. Sarcomere

the contractile unit, composed of myofilaments made up of contractile proteins *Sarcomeres, which contain even smaller rodlike structures called myofilaments, are contained by the contractile elements of skeletal muscle cells. *Each sarcomere extends from one Z disk to the next.

19. Sarcoplasm

the cytoplasm of a muscle cell that is similar to the cytoplasm of other cells, but it contains unusually large amounts of glycosomes and myoglobin *Ca2+ levels in the sacroplasm fall as Ca2+ is continually pumped back into the SR by active transport during the aftermath of excitation-contraction coupling.

24. Origin

the immovable or less movable bone in muscle contraction *In the muscles of the limbs, the origin typically lies proximal to the insertion. *Muscle attachments, whether origin or insertion, may be direct or indirect.

23. Insertion

the movable bone in muscle contraction *Muscle attachments, whether origin or insertion, may be direct or indirect.

27. Troponin

the other major protein in thin filaments, is a globular three-polypeptide complex *One of its polypeptides (Tnl) is an inhibitory subunit that binds to actin. *Another (TnT) binds to tropomyosin and helps position it on actin.

17. Sarcolemma

the plasma membrane of muscle cells *The trough-like part of the muscle fiber's sarcolemma that helps form the neuromuscular junction is highly folded. *Like the plasma membranes of all cells, a resting sarcolemma is polarized.