MUSCULAR SYSTEM

Question 1

EXICTABILITY

Answer 1

Ability to receive and respond to stimuli

Question 2

CONTRACTILITY

Answer 2

Ability to contract when stimulated

Question 3

EXTENSIBILITY

Answer 3

Ability to be stretched or extended

Question 4

ELASTICITY

Answer 4

Ability to recoil to resting length

Question 5

TENDON

Answer 5

A tendon or sinew is a tough high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone and is capable of withstanding tension and transmit the mechanical forces of muscle contraction to the skeletal system.

Question 6

AGONIST OR PRIME MOVER

Answer 6

contracts to cause an action

Question 7

ATAGONIST

Answer 7

stretches & yields to the action of the agonist

Opposes he movement and relaxes.

Question 8

SYNERGIST

Answer 8

contracts to stabilise intermediate joint

prevents unwanted movements

Question 9

FIXATOR

Answer 9

stabilises the origin of the agonist

Holds structures in position

Question 10

FASCICLES

Answer 10

A muscle is made up of many bundles of muscle fibres (‘cells’)
Each bundle is known as a Fascicle

Question 11

EPIMYSIUM

Answer 11

is the outer layer, encircling the entire muscle. It consists of dense irregular connective tissue.

Question 12

PERIMYSIUM

Answer 12

surrounds the bundles of fibres (fascicle)

also a layer of dense irregular connective tissue, but it surrounds groups of 10 to 100 or more muscle fibres, separating them into bundles called fascicles (FAS-i-kuls = little bundles).

Question 13

ENDOMYSIUM

Answer 13

Endomysium surrounds individual muscle fibres

penetrates the interior of each fascicle and separates individual muscle fibres from one another. The endomysium is mostly reticular fibres.

Question 14

MUSCLE FIBRE (CELLS)

Answer 14

Long, cylindrical cell with multiple nuclei just beneath the sarcolemma (cell membrane)
➢ 10 to 100 micro m in diameter, some up to 30 cm long
➢ Usual organelles present
➢ Plus: myofibrils, sarcoplasmic reticulum, and T- tubules

Question 15

MYOFIBRIL

Answer 15

➢Densely packed rod-like organelles
➢Contain bundles of contractile proteins
➢Striations are due to a repeating pattern of dark and light bands

At high magnification, the sarcoplasm appears stuffed with little threads. These small structures are the myofibrils (mī-ō-FĪ-brils; myo- = muscle; -fibrilla = little fibre), the contractile organelles of skeletal muscle (figure 10.2c). Myofibrils are about 2μm in diameter and extend the entire length of a muscle fibre. Their prominent striations make the entire skeletal muscle fibre appear striped (striated).

Question 16

SACROMERE

Answer 16

➢ A sarcomere is the smallest contractile unit of a muscle fibre
➢ Sarcomeres line up end-to-end in series
➢ Contraction of the series of sarcomeres leads to contraction of
myofibrils and therefore muscle cells
➢ Striations are due to the arrangement of the contractile proteins (myofilaments)…. Actin and Myosin

The filaments inside a myofibril do not extend the entire length of a muscle fibre. Instead, they are arranged in compartments called sarcomeres (SAR-kō-mērs; -mere = part), which are the basic functional units of a myofibril (figure 10.3a). Narrow, plate-shaped regions of dense protein material called Z discs separate one sarcomere from the next. Thus, a sarcomere extends from one Z disc to the next Z disc.p

Question 17

MYOSIN

Answer 17

Myosin (MĪ-ō-sin) is the main component of thick filaments and functions as a motor protein in all three types of muscle tissue. Motor proteins pull various cellular structures to achieve movement by converting the chemical energy in ATP to the mechanical energy of motion, that is, the production of force. In skeletal muscle, about 300 molecules of myosin form a single thick filament. Each myosin molecule is shaped like two golf clubs twisted together (figure 10.4a). The myosin tail (twisted golf club handles) points towards the M line in the centre of the sarcomere. Tails of neighbouring myosin molecules lie parallel to one another, forming the shaft of the thick filament. The two projections of each myosin molecule (golf club heads) are called myosin heads. The heads project outward from the shaft in a spiraling fashion, each extending towards one of the six thin filaments that surround each thick filament.

The myosin head will attach to the thin filament (actin) to form a cross-bridge. Cross-bridging allows the muscle to shorten and produce force!
Fig 10.4 Structure of thick and thin filaments.

Question 18

ORIGIN

Answer 18

During muscular contraction:
One end of the muscle is attached to a structure (usually bone) that remains stationary. This is known as the Origin of the muscle

Question 19

INSERTION

Answer 19

The opposite end of the muscle that is moved by the contraction is known as the insertion

Question 20

TRIPOPIN

Answer 20

Blue part of the thing filament.

Tripopin “locks” tropomyosin in place!

Contractile proteins (myosin and actin) generate force during contraction; regulatory proteins (troponin and tropomyosin) help switch contraction on and off.

Smaller amounts of two regulatory proteins— tropomyosin (trō-pō-MĪ-ō-sin) and troponin (TRŌ-pō-nin)— are also part of the thin filament. In relaxed muscle, myosin is blocked from binding to actin because strands of tropomyosin cover the myosin-binding sites on actin. The tropomyosin strands in turn are held in place by troponin molecules. You will soon learn that when calcium ions (Ca2+) bind to troponin, it undergoes a change in shape; this change moves tropomyosin away from myosin-binding sites on actin and muscle contraction subsequently begins as myosin binds to actin.

Question 21

TROPOMYOSIN

Answer 21

Smaller amounts of two regulatory proteins— tropomyosin (trō-pō-MĪ-ō-sin) and troponin (TRŌ-pō-nin)— are also part of the thin filament. In relaxed muscle, myosin is blocked from binding to actin because strands of tropomyosin cover the myosin-binding sites on actin. The tropomyosin strands in turn are held in place by troponin molecules.

When muscle fibres are stimulated, tropomyosin moves out of the way, and active binding sites on actin are exposed.

Question 22

SLIDING FILAMENT MECHANISM

Answer 22

During muscle contraction and relaxation, the actin and myosin filaments slide past each other this is known as the Sliding Filament Mechanism.

Cross-bridges are formed and broken several times over – Ratchet Mechanism – to propel the actin filament towards the centre of the sarcomere.

Myosin “heads” bind to the actin (forming cross-bridges) and pull the actin towards the centre of each sarcomere.
The Z disks move closer together and the sarcomere shorten.

Question 23

Z DISC

Answer 23

Narrow, plate-shaped regions of dense material that separate one sarcomere from the next.

Question 24

ACTIN

Answer 24

Yellow protein part of thin filament.

Contractile protein that is the main component of thin filament; each actin molecule has a myosin-binding site where myosin head of thick filament binds during muscle contraction.

Thin filaments are anchored to Z discs (see figure 10.3b). Their main component is the protein actin (AK-tin). Individual actin molecules join to form an actin filament that is twisted into a helix (figure 10.4b). On each actin molecule is a myosin-binding site, where a myosin head can attach.

Smaller amounts of two regulatory proteins— tropomyosin (trō-pō-MĪ-ō-sin) and troponin (TRŌ-pō-nin)— are also part of the thin filament. In relaxed muscle, myosin is blocked from binding to actin because strands of tropomyosin cover the myosin-binding sites on actin. The tropomyosin strands in turn are held in place by troponin molecules.

Question 25

RATCHET MECHANISM

Answer 25

During muscle contraction, the heads of myosin myofilaments quickly bind and release in a ratcheting fashion, pulling themselves along the actin myofilament. … The sarcomere and the sliding filament model of contraction: During contraction myosin ratchets along actin myofilaments compressing the I and H bands.

Question 26

SACROLEMMA

Answer 26

Plasma membrane of a muscle cell

Question 27

Sarcoplasmic Reticulum (SR)

Answer 27

→Interconnecting tubules surrounding each myofibril

→Regulates intracellular Ca levels; stores Ca and releases it when the muscle fiber is stimulated

Question 28

Transverse Tubules (T-Tubules)

Answer 28

→Tiny invaginations of the sarcolemma that tunnel in from the surface toward the center of each muscle fiber
→T-tubules are open to the outside of the fiber and are filled with interstitial fluid

Question 29

SOMATIC MOTOR NEURONS

Answer 29

The nerve cells that stimulate a muscle to contract are the Somatic Motor Neurons.

Each somatic motor neuron has a threadlike axon that extends from the brain or spinal cord to a group of skeletal muscle fibres (see figure 10.9d). The axon of a somatic motor neuron typically branches many times, each branch extending to a different skeletal muscle fibre.

Question 30

ACETYLCHOLINE (Ach)

Answer 30

A neurotransmitter liberated by many peripheral nervous system neurons and some central nervous system neurons. It is excitatory at neuromuscular junctions but inhibitory at some other synapses.

Question 31

Neuromuscular Junction

Answer 31

the synapse between a somatic motor neuron and a skeletal muscle fibre (figure 10.9a). A synapse is a region where communication occurs between two neurons, or between a neuron and a target cell— in this case, between a somatic motor neuron and a muscle fibre

Question 32

Excitation-Contraction Coupling

Answer 32

Action Potential Propagation
➢Once the muscle fibre is stimulated, an action potential is re- generated and is conducted along the sarcolemma and down the T-tubules to reach all of the myofibrils
➢From the T-tubule, the impulse is transferred to the SR
➢ Causes calcium to ‘flood’ out of the SR
Calcium is the trigger for cross bridge formation!

At rest, tropomyosin blocks binding sites on actin
➢ Calcium released from the SR binds to troponin, initiating a
change in shape which moves tropomyosin, uncovering the binding sites
➢ Myosin is free to bind and form cross-bridges!
The events from propagation of the action potential along the sarcolemma to cross-bridge formation are collectively called Excitation-contraction coupling

ATP binds and the myosin head detaches and returns the head to the “cocked” position
The myosin head ‘flicks’ and ‘slides’ the actin
The myosin head now releases the actin and returns to the resting position
The cycle is repeated

Muscular contraction requires large amounts of ATP energy

Question 33

CROSS-BRIDGE

Answer 33

Attachment of myosin to actin to form cross-bridges. The energised myosin head attaches to the myosin-binding site on actin and releases the previously hydrolysed phosphate group. When the myosin heads attach to actin during contraction, they are referred to as cross-bridges.

Question 34

Interaction of Myofilaments is the key to Muscle Contraction!

Answer 34

Remember that this contraction cycle is happening at multiple sites so that the overall length of the sarcomere, and therefore muscle, shortens!

Question 35

Muscle Relaxation

Answer 35

When the nerve impulse stops being sent

1. The calcium release channels on the SR close
2. ATP is used to actively transport Ca2+ back into the sarcoplasmic reticulum
3. Without calcium available to bind to troponin, tropomyosin resumes its original position blocking myosin-binding sites on actin. No cross-bridges are able to be formed and thus no muscle contraction.

Question 36

Energy for contraction and ATP

Answer 36

ATP is an energy carrying molecule used by muscle cells to:

1. Power the contraction cycle (sliding of actin and myosin)
2. Actively transport Ca2+ into the sarcoplasmic reticulum when the muscle is at rest
3. Maintain the resting membrane potential (remember the Na+/K+ pump?)

Question 37

RIGOR MORTIS

Answer 37

After death,

1. High concentration of calcium influx into muscle cells to promote cross bridge formation
2. ATP synthesis stops after breathing stops, but cross bridge detachment requires ATPs
3. Irreversibly cross-linked actin and myosin produce stiffness of rigor mortis (most muscles begin to stiffen 3 to 4 hours after death)

After death, cellular membranes become leaky. Calcium ions leak out of the sarcoplasmic reticulum into the sarcoplasm and allow myosin heads to bind to actin. ATP synthesis ceases shortly after breathing stops, however, so the cross-bridges cannot detach from actin. The resulting condition, in which muscles are in a state of rigidity (cannot contract or stretch), is called rigor mortis (rigidity of death). Rigor mortis begins 3–4 hours after death and lasts about 24 hours; then it disappears as proteolytic enzymes from lysosomes digest the cross-bridges.

Question 38

SYNAPTIC END BULB

Answer 38

At the NMJ, the end of the motor neuron, called the axon terminal, divides into a cluster of synaptic end bulbs (figure 10.9a, b), the neural part of the NMJ. Suspended in the cytosol within each synaptic end bulb are hundreds of membrane-enclosed sacs called synaptic vesicles. Inside each synaptic vesicle are thousands of molecules of acetylcholine (ACh) (as′-ē-til-KŌ-lēn), the neurotransmitter released at the NMJ.

Question 39

SYNAPTIC CLEFT

Answer 39

The narrow gap at a chemical synapse that separates the axon terminal of one neuron from another neuron or muscle fibre (cell) and across which a neurotransmitter diffuses to affect the postsynaptic cell.

Question 40

SKELETAL MUSCLE

Answer 40

Attached to bone or (some facial muscles) to skin.

Single, very long, cylindrical, multinucleute cells with obvious striation.

Question 41

CARDIAC MUSCLE

Answer 41

Walls of the heart.

Branching chains of cells; uni or binucleate; striations.

Question 42

SMOOTH MUSCLE

Answer 42

Unitary muscle in the walls of hollow organs (other than the heart); multi-unit muscle in intrinsic eye muscles, airways and large arteries.

Single fusiform, uninucleate; no striations.