Human Physiology: Muscles

Each myosin molecule has binding sites for what molecules?

ATP

Muscle fibers depend on a continuous supply of ATP. How do the fibers in the different types of muscle generate ATP?

Muscle fibers generate ATP by energy transfer from phosphocreatine. Oxidative fibers - use oxygen to make ATP from glucose and fatty acids Glycolytic fibers - get ATP primarily from anaerobic glycolysis

A single contraction-relaxation cycle in a skeletal muscle fiber is known as a(n) _____.

twitch

Compare and contrast isotonic contraction and isometric contraction:

Isotonic contraction moves a load. Isometric contraction creates tension without moving a load.

Compare and contrast temporal summation in motor neurons and summation in skeletal muscles:

Motor neuron temporal summation determines whether or not the neuron fires an action potential. Muscle cell summation increases force of contraction.

Compare and contrast action potentials in motor neurons and action potentials in skeletal muscles:

Both result from inward sodium current and outward potassium current through voltage-gated channels. Motor neuron action potential triggers ACh release. Muscle action potential triggers calcium release from the sarcoplasmic reticulum.

What are the three anatomical elements of a neuromuscular junction?

The SYNAPSE of a SOMATIC MOTOR NEURON and a SKELETAL MUSCLE FIBER

Briefly explain the functions of titin and nebulin.

Titin stabilizes the position of the contractile filaments and its elasticity returns stretched muscles to their resting strength. Nebulin helps align the actin filaments of the sarcomere.

Compare and contrast a twitch and tetanus:

Twitch: a single contraction-relaxation cycle Tetanus: contraction with little to no relaxation

When the circular muscle layer of the intestine contracts, what happens to the shape of the tube? When the longitudinal layer contracts, what happens to the shape?

contraction of the circular layer decreases the diameter of a tube. contraction of the longitudinal layer shortens the tube.

What is the response of a muscle fiber to an increase in the firing of the somatic motor neuron?

increased motor neuron firing rate causes summation in a muscle fiber, which increases the force of contraction

The modified endoplasmic reticulum of skeletal muscle is called the _____. Its role is to sequester _____ ions.

sarcoplasmic reticulum, calcium

Temporal summation in neurons means that the _____ of the neuron increases when two depolarizing stimuli occur close together in time.

strength of the graded potential

Summation in muscle fibers means that the _____of the fiber increases with repeated action potentials.

tension

Compare and contrast the source and role of calcium in skeletal and smooth muscle contraction:

Skeletal muscle: calcium is released from the sarcoplasmic reticulum Smooth muscle: its found in extracellular fluid and sarcoplasmic reticulum

Compare and contrast slow-wave and pacemaker potentials:

Slow-wave potentials: cycles of depolarization and repolarization in smooth muscle cells. Pacemaker potentials: repetitive depolarizations to threshold in some smooth muscle and cardiac muscle

One way that scientists study muscles is to put them in a state of rigor by removing ATP. In this condition, actin and myosin are strongly linked but unable to move. On the basis of what you know about muscle contraction, predict what would happen to these muscles in a state of rigor if you (A) added ATP but no free calcium ions, (B) added ATP with a substantial concentration of calcium ions.

(A) adding ATP allows crossbridges to detach. If insufficient calcium is available, the muscle will relax (B) with ATP and calcium, the muscle will continue in the contraction cycle until it is completely contracted

T-tubules allow _____ to move into the interior of the muscle fiber.

action potentials

List the steps of skeletal muscle contraction that require ATP

1. ATP binding - myosin dissociates from actin. 2. ATP hydrolysis - myosin head swings and binds to a new actin. 3. Release of Pi initiates the power stroke

List six proteins that make up the myofibrils. Which protein creates the power stroke for contraction?

1. Actin (contractile) 2. Myosin (contractile) 3. Tropomyosin (regulatory) 4. Troponin (regulatory) 5. Titin (giant accessory) 6. Nebulin (giant accessory) Myosin

What is the motor end plate, and what kinds of receptors are found there? Explain how neurotransmitter binding to these receptors creates an action potential.

1. The region of muscle fiber where the synapse occurs 2. Contains ACh receptors 3. When the AP arrives at the axon terminal, acetylcholine released into the synapse binds to ACh receptor channels on the motor end plate. When the ACh-gated channels open, they allow sodium and potassium to cross the membrane (much more sodium than potassium). The net positive charge to the muscle fiber depolarizes the membrane, creating an end-plate potential (EPP). Normally, EPP's always reach threshold and initiate a muscle action potential.

Fast-twitch oxidative-glycolytic fiber characteristics:

1. has some myoglobin 2. is also called red muscle 3. uses a combination of oxidative and glycolytic metabolism

Slow-twitch oxidative fiber characteristics:

1. has the most blood vessels 2. has some myoglobin 3. is also called red muscle 4. has the most mitochondria

1. Fast-twich glycolytic fiber characteristics:

1. largest in diameter 2. uses anaerobic metabolism, thus fatigues quickly 3. is used for quick, fine movements

During contraction, the _____ band remains a constant length. This band is composed primarily of _____ molecules. Which components approach each other during contraction??

A-band, myosin Z-discs approach each other during contraction

Myosin hydrolyzes ATP to ADP and Pi. Enzymes that hydrolyze ATP are collectively known as

ATPases

Which neurotransmitter is released by somatic motor neurons?

Acetylcholine

Explain the roles of troponin, tropomyosin, and calcium in skeletal muscle contraction.

Calcium is responsible for turning the muscle contraction on and off. 1.Calcium is released by the sarcoplasmic reticulum and one protein of the complex troponin binds to the calcium. 2. The calcium/troponin complex pulls tropomyosin completely away from the actin's myosin-binding sites 3. This "on" position enables the myosin heads to form strong, high-force crossbridges and carry out their power strokes. 4. The power stroke moves the moves the actin filaments 5. Contractile cycles repeat as long as the binding sites are uncovered For muscle relaxation to occur, calcium concentrations must decrease.

Compare and contrast fast-twitch oxidative-glycolytic, fast-twitch glycolytic, and slow twitch muscle fibers:

FTOG: smaller, some myoglobin, use both oxidative and glycolytic metabolism, more fatigue-resistant FTG: largest, rely primarily on anaerobic glycolysis, least fatigue resistant ST: develop tension more slowly, maintain tension longer, the most fatigue resistant, depend primarily on oxidative phosphorylation, more mitochondria, greater vascularity, large amounts of myoglobin, smallest in diameter

Define muscle fatigue. Summarize factors that could play a role in its development. How can muscle fibers adapt to resist fatigue?

Fatigue: a reversible state in which a muscle can no longer generate or sustain the expected force. May involve changes in ion concentrations, depletion of nutrients, or excitation-contraction coupling. Increasing size and number of mitochondria or increasing blood supply are adaptations that can help to resist fatigue.

Explain the sliding filament theory

Overlapping actin and myosin filaments of fixed length slide past one another in an energy-requiring process, resulting in muscle contraction.

Explain how you vary the strength and effort made by your muscles in picking up a pencil versus picking up a full gallon container of milk.

The body uses different types of motor units and recruits different numbers of motor units. Small movements use motor units with fewer muscle fibers; gross movements use motor units with more fibers.

What events are taking place during the latent period before contraction begins?

The events of the latent period include creation of the muscle action potential, release of calcium from the sarcoplasmic reticulum, and diffusion of calcium to the contractile filaments.

The dense bodies that anchor smooth muscle actin are analogous to what structure in a sarcomere?

dense bodies are analogous to Z disks

If potassium concentration increases in the extracellular fluid surrounding a cell but does not change significantly in the cell's cytoplasm, the cell membrane (depolarizes or hyperpolarizes) and becomes (less/more negative)

depolarize, less negative

What is the difference in how contraction force is varied in multi-unit and single-unit smooth muscle?

multi-unit smooth muscle increases force by recruiting additional muscle fibers; single-unit smooth muscle increases force by increasing calcium entry

How do pacemaker potentials differ from slow wave potentials?

pacemaker potentials always reach threshold and create regular rhythms of contraction. slow wave potentials are variable in magnitude and may not reach threshold each time

The two functional types of smooth muscle are _____ and _____.

single unit (visceral) and multi-unit

Which type of muscle tissue is controlled only by somatic motor neurons?

skeletal

Which two muscles are striated?

skeletal and cardiac

The three types of muscle tissue found in the human body are______,______ and _____. Which type is attached to bones, enabling it to control body movement?

skeletal, smooth, cardiac. Skeletal.

In the sliding filament theory of contraction, what prevents the filaments from sliding back to their original position each time a myosin head releases to bind to the next actin binding site?

the crossbridges do not all unlink at one time, so while some myosin heads are free and swiveling, others are still tightly bound

One study found that many world-class athletes have muscle insertions that are farther from the joint than in the average person. Why would this trait translate into an advantage for a weight lifter?

the leverage is better and a contraction creates more rotational force

How does the nervous system increase the force f contraction in a muscle composed of many motor neurons?

the nervous system increases the force of contraction by recruiting additional motor neurons

Which part of contraction requires ATP? Does relaxation require ATP??

the release of myosin heads from actin requires ATP binding. Energy from ATP is required for the power stroke. Relaxation doesn't directly require ATP, but relaxation cannot occur unless calcium is pumped back into the sarcoplasmic reticulum using a calcium-ATPase

Name an elastic fiber in the sarcomere that aids relaxation

titin is an elastic fiber in the sarcomere

How does an action potential in a muscle fiber trigger a calcium signal inside the fiber?

transduction of the electrical signal into a calcium signal requires two key membrane proteins. The t-tubule membrane contains a voltage-sensing L-type calcium channel called a dihydropyridine (DHP) receptor. These specialized ion channels are mechanically linked to calcium release channels in the adjacent sarcoplasmic reticulum (SR). The calcium release channels are also known as ryanodine receptors or RyR. When the depolarization of an action potential reaches a DHP receptor, the receptor changes conformation. The conformation change opens the RyR calcium release channels in the SR. Stored calcium then flows down its electrochemical radient into the cytosol where it initiates contraction.

What happens to contraction if a smooth muscle is placed in a saline bath from which all calcium has been removed?

without ECF calcium, contraction decreases because smooth muscle depends on ECF calcium for contraction