Anatomy Chapter 9: Muscular System

Explain how a motor nerve impulse can trigger a muscle contraction

- the brain sends a message through nerve through Action Potential. - Causes Acetlycholine (nerurotransmitter or chemical messenger) to spill into the synaptic cleft and bind to muscle respecters on the cell membrane - causes sodium to run into the cell called depolarization, this helps reform action potential - It then travels through the T-tubules and uses them through the cell to deliver messages to entire cell that it is time to move as it moves, it causes calcium to release from the sarcoplasmic reticulum - calcium binds to troponin and tropomyosin complex which causes them to shift and expose active site - the myosin heads can now attach to the active site to form a cross bridge and pull the actin strand toward the M line to shorten the muscle - ATP allows release of cross bridge. And powers the reaction.

The network of membranous channels in the cytoplasm of muscle fibers is the __Sarcoplasmic reticulum__. The other channels found within the cytoplasm are the _transverse tubules__ _cisternae_.

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What is a cross-bridge?

A cross-bridge is a myosin head attached to an actin binding site. These can bend slightly and pull on the actin filament. The head then can release, straighten, combine with another binding site further down the actin filament, and pull again. Causing the muscle to contract. This is called the sliding fillament mechanism.

Define prime mover.

A prime mover is a muscle that provides most of the force for a particular body movement. For instance, abduction of the arm by the deltoid.

Describe a single muscle fiber.

A skeletal muscle fiber is a single muscle cell. Each fiber forms from many undifferentiated cells that fuse during development. The resulting multinucleated muscle fiber is a thin, elongated cylinder with rounded ends that attach to the connective tissues associated with a muscle. Just beneath the muscle cell membrane (sarcolemma), the cytoplasm (sarcoplasm) of the fiber contains many small, oval nuclei and mitrochondria.

What is the function of a synergist?

A synergist is when muscles work together, like muscles from nearby also contract to help with an action. For example, when the deltoid muscle abducts the arm, the supraspinatus, a muscle that originates on the scapula helps with the abduction

Explain how parts of the upper limb form a first-class lever and a third-class lever.

A third-class lever is formed when the upper limb bends, the forearm bones represent the rigid bar, the elbow joint is the fulcrum, the hand is moved against the resistance provided by the weight, and the force is supplied by muscles on the anterior side of the arm. Because the parts of this lever are arranged in the sequence resistance-force-fulcrum, it is a third-class lever. When the upper limb strengthens at the elbow, the forearm bones again serve as the rigid bar, the hand moves against the resistance by pulling on a rope to raise weight, and the elbow joint as the fulcrum. A muscle on the posterior side of the arm supplies the force. Because the parts of the lever are arranges resistance-fulcrum-force, it is a first-class lever

Distinguish between a twitch and a sustained contraction.

A twitch is the contractile response of a single muscle fiber to a muscle impulse. A sustained contraction is the force of individual twitches combining by the process of summation.

Explain how ATP is used during a muscle contraction.

ATP is released and the myosin head breaks the ATP down to ADP and phosphate which creates energy. This energy powers the myosin head (which has formed a cross-bridge with the actin) to bend backwards and pull the thin filament, causing the muscle to contract. The cross-bridge cycle will continue as long at ATP is present and action potentials release AC(h) at that junction.

Describe acetylcholine and how it helps to stimulate a muscle contraction.

Acetylcholine is the neurotransmitter that motor neurons use to control skeletal muscle contraction. ACH is synthesized in the cytoplasm of the motor neruron and is stores in synaptic vesicles near the distal end of its axon. When it reaches the end of the axon, the vesicles release acetylcholine into the synaptic cleft. The Acetylcholine diffuses rapidly across the synaptic cleft and binds to protein molecules (receptors) in the muscle fiber membrane, increasing the membrane permeability of sodium ions

Define all-or-none response.

All-or-none response is a response that occurs completely or not at all, such as a neuron generating an action potential in response to a stimulus of threshold strength

What halts the action of acetylcholine?

An enzyme called acetylcholinesterase rapidly decomposes acetylcholine that is remaining in the synapse. The results in the relaxation of the muscle fibers.

Isometric

An isometric contraction is when tension within the muscles increases, but the muscles remain the same length, like when someone is pushing against a wall.

Isotonic

An isotonic contraction is when the muscles remain taut, the attached ends pull closer together, like when a person lifts an object

What is the function of an antagonist?

Antagonists work to oppose an action. These muscles can cause movement in the opposite direction. The antagonist of the muscles that raise the upper limb can lower the upper limb, or the antagonist of the muscles that bend the upper limb can straighten it. Smooth body movement depend upon the antagonists' relaxing to a degree and giving way to the agonists action.

Compare the characteristics of smooth muscle contraction and skeletal muscle contraction.

Both are triggered by membrane impulses and release of calcium ions, reflect reactions of myosin and actin, and both use energy from ATP molecules. Smooth muscle does not contain troponin, the protein that binds to calcium ions in skeletal muscle. Instead, it uses the protein calmodulin, which binds to calcium ions released when fibers are stimulated. Also, much of the calcium necessary for smooth muscle contraction diffuses into the cell form the extracellular fluid. Hormones affect smooth muscle, like in childbirth when the hormone oxytocin stimulated smooth muscles in the wall of the uterus to contract.

What is the Calcium ion's role in a muscle contraction

Calcium ions are released from the sarcoplasmic reticulum. The calcium ions bind to the troponin and tropomyosin complex which causes them to shift. This shift allows the myosin heads to bind to the active site forming a cross-bridge, which then the myosin heads pull the actin strand toward the M line and shortens the muscle.

How does cardiac muscle differ from skeletal muscle?

Cardiac muscle is found only in the heart. The cisternae of the sarcoplasmic reticulum of a cardiac muscle fiber are less developed and store less calcium than those of skeletal muscle. The transverse tubules of cardiac muscle are larger than those in skeletal muscle, and they release many calcium ions into the sarcoplasm in response to a single muscle impulse. The extracellular calcium partially controls the strength and enables the cardiac fibers to contract longer than skeletal muscle fibers can. Cardiac muscle fibers also have, at opposing ends, cross-bands called intercalated disks that are complex membrane junctions that include desmsomes and gap junctions. Cardiac muscle fibers are also sell-exciting and rythmatic.

How does creatine phosphate supply energy for muscle contraction?

Creatine phosphate is the initial source of energy that is available to regenerate ATP from ADP and phosphate. It includes a high-energy phosphate bond. It is more abundant in muscle fibers than ATP, but it cannot directly supply energy to a sell. However, it stores energy released from mitochondria. When sufficient ATP is present, an enzyme in the mitochondria promotes the synthesis of creatine phosphate, which stores the excess energy in its phosphate bond

How is cardiac muscle similar to skeletal muscle?

Each cell of cardiac muscle contains a single nucleus and many filaments of actin and myosin similar to those in skeletal muscle. Also, cardiac muscle has a well-developed sarcoplasmic reticulum, a system of transverse tubules, and many mitochondria.

Describe a neuromuscular junction.

Each cell that a neuron controls is connected functionally to the end of an axon. This site of functional connection is called a synapse. The neurons communicate with the cells they are controlling by releasing chemicals, called neurotransmitters, at the synapse. The molecules then diffuse a short distance to the cell being controlled, where they have a specific effect. The neurons that control effectors are called motor neurons. The synapse where the motor neuron axon and a skeletal muscle fiber meet is the neuromuscular junction.

Why is oxygen necessary for muscle contraction?

Hemoglobin carries oxygen from the lungs in the blood. When it reaches the muscle, the oxygen is transferred from the hemoglobin to the myoglobin in the muscle fiber. Oxygen is necessary for muscle contraction because without it there would be no aerobic reactions of cellular respiration

How does the muscle continue to contract in the absence of oxygen?

In the absence of sufficient oxygen, glycolysis leads to lactic acid accumulation. The lactic acid threshold, the lactic acid diffuses out of the muscle fibers and is carried in the bloodstream to the liver

What is the function of intercalated discs?

Intercalated disks help join cells and transmit the force of contraction from cell to cell, the intercellular junction of the fused membranes of intercalated disks allows ions to diffuse between cells. This allows muscle impulses to travel rapidly from cell to cell. When one portion of cardiac muscle is stimulated, the impulse passes to other fibers of the network, and the whole structure contracts as a unit.

Troponin-

Is a component of the thin filament that is a regulatory protein. A change in shape is made when calcium ions bind to it. This change moves tropomyosin away from myosin-binding sites on actin molecules. Muscle contraction subsequently begins as the myosin binds to actin.

Fascia

Layers of dense connective tissue that separate and hold in position an individual skeletal muscle from adjacent muscles.

Define motor unit

Motor neurons have highly branched nerve fibers connected to the motor end plate of a single muscle fiber. When the motor neuron is stimulated, the impulse is carried to the muscle fibers attached to the branches. The motor unit is all the muscle fibers attached to the motor neuron as well as the motor neuron itself.

What is meant by muscle fatigue? What causes it?

Muscle fatigue is when a muscle is exercised for a prolonged period of time and may loose its ability to contract. A number of things may cause this, including decreased blood flow, ion imbalances across the sarcolemma from repeated stimulation, and psychological loss of the desire to continue the exercise. However, muscle fatigue is most likely to arise at lease in part from accumulation of hydrogen ions due to lactic acid formation in the muscle from anaerobic ATP production.

Define muscle tone.

Muscle tone is the ongoing contraction of some fibers in otherwise resting skeletal muscles.

Myosin-

Myosin is a contractile protein that makes up the thick filament. A myosin molecule has two myosin heads and a tail that bind to myosin-binding sites on actin molecules of a thin filament during muscle contractions.

Distinguish between the origin and insertion of a muscle.

One end of a skeletal muscle is usually fastened to a relatively immoveable or fixed part on one side of a joint called the origin of the muscle. The other end of the muscle is connected to a moveable part on the other side of that joint called its insertion. When a muscle contracts, its insertion is pulled toward its origin. The head of a muscle is the part nearest its origin.

List the 3 types of muscles.

Skeletal muscle Smooth Muscle Cardiac Muscle

Explain why skeletal muscle fibers appear striated.

Skeletal muscle fibers appear striated from the abundant, parallel, threadlike structures called myofibrils. These myofibrils play a fundamental role in the muscle contraction mechanism. The organization of the two types of protein filaments (thick and thin) produce an alternating light and dark striation characteristic of skeletal muscle fibers. The striations form a repeating patter of units called sacomeres.

Compare the structure of smooth muscle fibers and skeletal muscle fibers.

Smooth muscle cells are shorter than the fibers of skeletal muscle, and they have single, centrally located nuclei. Smooth muscle cells are elongated with tapering ends and contain filaments of actin and myosin in myofibrils that extend throughout their lings, but these filaments are thin and more randomly distributed than those in skeletal muscle fibers. Smooth muscle fibers lack striations and appear smooth, while skeletal muscle fibers have striations.

Describe summation

Summation is when a muscle fiber is exposed to a series of stimuli of increasing frequency reaches a point when it is unable to relax before the next stimulus in the serious arrives. When this happens, the individual twitches begin to combine, and the contraction becomes sustained. In a sustained contraction, the force of individual twitches combines by the process of summation.

What characteristic of cardiac muscle causes the heart to contract as a unit?

The intercellular junctions of the fused membranes of intercalated disks allows ions to diffuse between the cells. This allows the whole structure to contract as a unit in an all-or-none capacity when one portion of the cardiac muscle network is stimulated, the impulse passes to the fibers of the network.

What is the function of those channels?

The sarcoplasmic reticulum corresponds to the endoplasmic reticulum of other cells. The Transverse tubules extends into the sarcoplasm as invaginations continuous with the sarcolemma and contains extracellular fluid. Each transverse tubule lies between two enarges potions of the sarcoplasmic reticulum called cisternae. These three structures form a triad near the region where the actin and myosin filaments overlap

What substance in muscle seems able to store oxygen temporarily?

The substance in muscle that is able to store oxygen temporarily is myoglobin

Define threshold stimulus.

When an isolated muscle fiber is exposed to a series of stimuli of increasing strength, the fiber remains unresponsive until a certain strength of stimulation called the threshold stimulus.

What is meant by oxygen debt?

When lactic acid accumulates, a person develops oxygen debt that must be repaid at a later time. The amount of oxygen debt includes the amount of oxygen that liver cells require to use the accumulated lactic acid to produce glucose, as well as the amount that the muscle cells require to re-synthesize sufficient ATP and creatine phosphate to restore their original concentrations.

Less than half the energy released by cellular respiration is available for metabolic processes. The rest is lost as ___________

_Body heat_

aponeuroses

a broad, fibrous sheet of connective tissue that attaches muscles to bone and other muscles

Fast-twitch (type II)

fibers are also called white fibers because they have less myoglobin and have a poorer blood supply than red fibers. These fibers have fewer mitochondria and have a reduced respiratory capacity. However, they have more extensive sarcoplasmic reticulum to store and reabsorb calcium ions, and their ATPase is faster than that of red fibers. They contract rapidly, although they fatigue, lactic acid accumulates and as the ATP and the biochemicals to regenerate ATP are depleted

Slow-twitch (type I)

fibers such as those in the long muscles of the back, are often called red fibers because they contain the red, oxygen-storing pigment myoglobin. They contain many mitochondria, which is an adaption for the aerobic reactions of cellular respiration. These fibers have a high respiratory capacity and can generate ATP fast enough to keep up with the ATP breakdown that occurs when they contact. They can contract for long periods without fatiguing

Tendon

is a cordlike or bandlike mass of dense connective tissue that connects a muscle to a bone

Tropomysosin

is also a component of the thin filament that is a regulatory protein. Tropomysosin covers the myosin-binding sites on actin molecules when a muscle fiber is relaxed. This prevents myosin from binding to actin.

Actin

is the main component of the thin filament. Each actin molecule has a myosin binding site where the head of a thick filament (myosin) binds during muscle contraction.

endomysium

layer of connective tissue surrounding each skeletal muscle fiber.

epimysium

outer layer of connective tissue surrounding a skeletal muscle

perimysium-

sheath of connective tissue that encloses a bundle of skeletal muscle fibers or a fasicile.

fascicle

small bundles of muscle fibers

Describe the structure and function of a sarcomere.

the structure of a sarcomere has two main parts. The first are the I bands which are composed of thin actin filaments held by direct attachments to Z lines. The second part consists of the A bands which are composed of thick myosin filaments that overlap thin actin filaments. The function of a sarcomere is essentially what makes the muscle contract