Human Physiology L8

Tension

the force exerted on an object by a contracting muscle

Load

the force exerted on the muscle by an object

How does a muscle shorten?

the tension exerted by a muscle must be greater than the force exerted by the load

Isotonic Contractions

the muscle length changes and joints move but the tension of the muscle remains the same

Isometric Contractions

a force/load is applied on the muscle without initiating muscle length change or joint movement

Concentric Contractions

a type of isometric contraction where the muscle length shortens

Eccentric Contractions

a type of isometric contraction where the muscle length lengthens

Twitch

the response of a muscle fiber to a single action potential

Latent Period

the delay caused by excitation-contraction coupling that encompasses the period from the action potential to the onset of a contraction. It accounts for the time for excitation contraction coupling and for enough cross bridges to form and move the load applied on the muscle; this is a brief period of isometric contraction.

Contraction Time

the interval spanning from the beginning of tension development at the end of the latent period to the peak tension on the muscle

Relaxation Time

the period after peak muscle tension is reached until the tension declines back to zero. Where heavier loads come to rest before all the cross-bridges have detached.

Which lasts longer: a twitch or action potential?

a twitch, it lasts 100ms+ while an action potential only lasts 1-2ms

Summation

an increase in muscle tension where the muscle does not relax due to repeated twitches in a short period of time

Tetanus

a maintained contraction due to consistent Ca2+ availability because action potentials right after one another maintain the release of calcium and therefore keep binding sites open and available.

What protein is responsible for the spring-like tension/elastic properties of relaxed muscle fibers?

titin

What type of tension is in a relaxed muscle?

passive tension

What type of tension is in a contracted muscle?

active tension

How does the amount of action tension change with the length of the muscle fiber?

more active tension as the length of the muscle fiber increases

Optimal Length

the length at which muscle fibers develop the greatest isometric active tension

How much ATP is readily available to skeletal muscles upon beginning a contraction?

a minimal amount

What is the order of the methods to supplying ATP to muscles given a contraction?

Creatine Phosphate, Oxidative phosphorylation, Glycolysis

This method skeletal muscle energy and metabolism is where ATP is stored in small quantities by the muscles, serving as an initial source of ATP so that the other methods of making ATP have the opportunity to produce ATP. This process involves a phosphate carrier which rapidly transfers a phosphate to ADP that subsequently yields a single ATP (and one other element).

Creatine Phosphate

This method skeletal muscle energy and metabolism is where the cell uses oxygen during moderate exercise to produce most of the ATP. The initial source of glucose is glycogen stored in the muscles ( first 5-10 minutes of exercise), the blood glucose and fatty acids, then just fatty acids.

Oxidative Phosphorylation

This method skeletal muscle energy and metabolism is where it provides exponentially growing amounts of ATP during strenuous exercise. This method is rapid, anaerobic, uses glycogen stored in the muscle and glucose stored in the blood, and creates lactic acid as a byproduct.

Glycolysis

How is lactic acid metabolized?

with oxygen, it is broken down by the body

Do muscles still consume oxygen after exercise?

yes, because Creatine Phosphate and Glycogen must be replaced. This is why breathing takes longer to slow down after increasing intensity exercises.

What happens to the muscle when there is continued stimulation?

the muscle will lose its ability to develop and maintain tension

How does a muscle regain its tension?

with breaks, the longer the break, the more tension that can be developed

What form of exercise occurs when fatigue develops rapidly but the recovery is fast?

short term, intense exercise

What form of exercise occurs when fatigue develops slowly and the recovery is also slow?

long duration, low intensity

Short term, intense exercise results in what?

blood vessel compression, a buildup of ADP, phosphate, Potassium Imbalance, H+MG2+. This decreases the rate of Ca release and reuptake, decreases sensitivity of actin to Ca and inhibits the binding of myosin.

Long duration, low intensity exercise results in what?

Ca+ channels become leaky and the elevated calcium levels in the cytoplasm activate proteases that break down contractile proteins and the decrease in muscle glycogen supply. To recover from this the body must build glycogen reserves and repair the muscle.

How are fibers classified?

Based on their maximal velocities of shortening (fast or slow twitch) and the major pathway they use to form ATP (oxidative or glycolytic).

What factors are altered between fast and slow twitch fibers?

The type of myosin that each fiber has, and the subsequent speed of membrane excitation, excitation-contraction coupling, and ATP production that results from that. The type of myosin in the type of muscle fibers determine the rate of cross bridge formation (maximal shortening velocity).

What are the traits of oxidative fibers?

Generally slow twitch muscles: lots of mitochondria, high capacity for oxidative phosphorylation, dependent on blood flow, contain myoglobin, and are used for long term contractions.

What are the traits of glycolytic fibers?

Generally fast twitch muscles: few mitochondria, high concentration of glycolytic enzymes, high stores of glycogen, used for quick bursts of energy, do not have myoglobin.

What are the types of skeletal muscle fibers?

Slow Oxidative Fibers (Type 1), Fast Oxidative Glycolytic Fibers (Type 2a), and Fast Glycolytic Fibers (Type 2x)

What are the characteristics of Slow Oxidative Fibers?

High oxidative capacity, low myosin ATPase activity, small diameter so it develops the least amount of tension, and small diameter neurons.

What are the characteristics of Fast Oxidative Glycolytic Fibers?

High oxidative capacity, high myosin-ATPase activity, intermediate glycolytic activity, intermediate diameter so it develops intermediate tension, and intermediate diameter neurons.

What are the characteristics of Fast Glycolytic Fibers?

High myosin-ATPase activity, high glycolytic capacity, large diameter so it develops the greatest amount of tension, and large diameter neurons.

Why is the tension a fiber can develop related to the diameter of the muscle?

The large the diameter of the muscle, the more cross bridges that can be formed, which results in more tension.

Why no slow glycolytic fibers?

Glycolytic fibers are meant to provide rapid energy and movement, so they lack the number of mitochondria necessary to be slow twitch muscles, as they cannot be sustained for long periods of time.

From fastest to slowest level of fatigue: in what order do the three muscle types fatigue?

Fast Glycolytic fibers, Fast Oxidative Glycolytic fibers, Slow Oxidative fibers

What are motor units?

a single motor neuron and all muscle fibers innervated by it. The same type of fiber is in one motor unit.

Are the proportions of types of muscle fibers the same within each muscle?

Generally no, depending on what a given muscle has been trained to do, the number of different types of muscle fibers will vary. Ex. A sprinter will have more fast twitch muscles and a marathon runner will have more slow twitch muscles.

What are muscles composed of?

individual motor units, each with different types of muscle fibers.

What is Whole Muscle contraction?

the contraction of an entire muscle and all of its fibers. This is done by motor unit recruitment, summation, tetanus, and the overall graded response.

What type of muscle fiber would be most present in the back?

Slow Oxidative fibers because they are needed to maintain posture for a long period of time.

In a boxer, what type of muscle fiber would be most present in the arms?

Fast Glycolytic fibers because they need to react quickly over a short period of time.

In a short distance runner, what muscle fiber would be most present in the legs?

Fast Oxidative Glycolytic fibers because they need to run quickly but have enough mitochondria to run fast for a longer distance than sprinting, for example.

What size motor units are required for fine movement?

small motor units because that lessens the amount of tension that can be produced.

What size motor unites are required for more unrefined/big movements?

large motor unis because that increases the amount of tension that can be produced.

How does the total tension of a muscle develop?

based on the amount of tension developed by each fiber and the number of fibers contracting at any given time.

What factors determine the tension of each muscle fiber?

Action potential frequency (frequency-tension relation), fiber length (length-tension relation), fiber diameter, fiber type, and fatigue

What factors determine the number of active fibers?

number of fibers per motor unit and the number of active motor units.

Recruitment

increasing the number of motor units that are active in a contraction

What determines the order of recruitment?

the neuron diameter (smallest to largest diameter)

What fibers are recruited first?

small diameter fibers because they are fatigue resistant so they are ideal when maximum force use is not necessary

What causes tetanic muscle tension?

the successive recruitment of the three types of motor units

What is the order of muscle fiber recruitment?

Slow Oxidative fibers, Fast Oxidative Glycolytic fibers, then Fast Glycolytic fibers. Smallest diameter to largest diameter, least force used to most forced used.

What determines the shortening velocity of a whole muscle?

The load on the muscle, which determines how many cross bridges need to be formed to make the muscle tension greater than the load, the types of motor units in the muscle, and the number of motor units recruited to work against the load.

What is shortening velocity?

The rate at which a muscle can contract. Fast twitch muscles have the fastest shortening velocity.

Hypertrophy

the increase in the size of muscle fibers, their capacity for ATP production, and the subtype of myosin that they express. This is caused by repeated exercise.

Disuse atrophy

when prolonged inactivity results in the muscles getting smaller in size. Ex. an arm in a cast

Denervation atrophy

shrinkage of paralyzed muscle when nerve remains disconnected. Ex. nerve damage/loss of function

What is poliomyelitis?

A viral disease that destroys motor neurons. This causes paralysis of the skeletal muscle. Severe forms of this cause respiratory failure and death.

What causes muscle cramps?

involuntary tetanic contraction of the skeletal muscles. This is where action potentials fire at very high rates, higher than maximum voluntary contractions, and causes imbalances in the intracellular and extracellular components. This typically occurs because of overexercise, dehydration, or chemical imbalances.

Hypocalcemic tetany

involuntary tetanic contractions/muscular spasms caused by a low concentration of calcium in the blood. In this case, extracellular Ca2+ concentration falls to ~40% of its typical value and its effects, since Ca2+ normally changes in the Sarcoplasmic Reticulum, are directly on the sarcolemma. This is muscle cramps.