A&P Lecture Exam III: Motor Units, Muscle Contraction, and ATP

Define muscle tension

the force exerted on an object by a contracting muscle

Define load

the opposing force exerted on the muscle by the weight of the object to be moved

What is a motor unit?

1 motor neuron and all the muscle fibers it innervates

What types of movements are performed by small motor units?

exert fine control

What types of movements are performed by large motor units?

large, less precise movements

How are the fibers of a particular motor unit arranged within a muscle? What is the functional significance of this?

they are spread throughout the muscle — not clustered together; allows for even distribution of force generation across the muscle

What is a muscle twitch?

simplest form of contraction - a muscle
fiber’s response to a single action potential; muscle fiber contracts quickly then relaxes

What is a myogram?

graphical recording of muscle activity

What are the three phases of a muscle twitch?

latent period, period of contraction, and period of relaxation

Muscle Twitch: Latent Period

1st few milliseconds following stimulation;
excitation-contraction coupling is occurring; cross bridges begin to cycle, but muscle tension is not yet measurable

Muscle Twitch: Period of Contraction

cross bridges are active; myogram tracing rises to a peak; period lasts 10-100ms

Muscle Twitch: Period of Relaxation

final phase lasting 10-100ms; Ca2+ is
being pumped back into the SR; number of active cross bridges is declining; muscle tension declines to 0

Twitch contractions in _______ muscles are rapid and brief.

extraocular

Twitch contractions in __________ and ______ muscles are slow and long lasting.

gastrocnemius; soleus

All muscles ________ faster than they _________.

contract; relax

Muscle responses are graded by __________ and/or ___________.

changing frequency of stimulation; changing strength of stimulation

Define wave (temporal) summation

2+ stimuli are received in rapid succession

Summation occurs in response to changes in stimulus ____________.

frequency

Physiologically, what causes the increase in the strength of the contraction?

larger motor units with higher-threshold motor neurons

What will happen if the frequency of stimuli continues to increase?

• Relaxation time between twitches becomes shorter and shorter

• Concentration of Ca2+ in the cytosol rises higher and higher

• Degree of summation becomes greater and greater

Define unfused tetany

a sustained, quivering contraction

Define fused tetany

contractions fuse into 1 smooth, sustained
contraction plateau

Define recruitment.

also called multiple motor unit summation; stimuli of
increasing voltage are delivered, and more muscle fibers are called into play; controls the force of contraction more precisely

Physiologically, what causes recruitment?

size principle

Motor units with the _________ muscle fibers are
recruited first – they are controlled by the ________,
most _________ motor neurons

smallest; smallest; excitable

Motor units with ________ muscle fibers are recruited next and _________ ________ increases

larger; contractile strength

The largest motor units, containing _____, _______
muscle fibers, are controlled by the ______ and least excitable (________ ________) neurons - these units are only activated when the most __________ contraction is necessary

large; coarse; largest; highest threshold; powerful

Define subthreshold stimulus

stimulus is not strong enough -- no contractions
are seen

Define threshold stimulus

stimulus is strong enough to cause 1st observable
contraction

Define maximal stimulus

strongest stimulus that increases contractile force – all motor units are recruited

Explain the size principle.

When a muscle needs to generate force, motor units (groups of muscle fibers controlled by a single nerve) are recruited from smallest to largest, meaning smaller, slower-twitch muscle fibers are activated first, followed by larger, faster-twitch fibers only when more force is required.

Explain how the size principle helps us to do things like filling a pot with water in the sink.

When you first pick up the empty pot, only small motor units are activated, providing just enough force to hold it comfortably. As water is added and the pot becomes heavier, the nervous system progressively recruits larger motor units, gradually increasing the grip strength to maintain a stable hold.

What is asynchronous activation? Why is it important?

Helps to prolong a strong contraction and delay/prevent muscle fatigue.

What is muscle tone? What creates it? Why is it important?

the constant, slightly contracted state of all muscles; it is due to spinal reflexes; Keeps muscles firm, healthy, and ready to respond

Define hypotonia

decreased muscle tone; floppy appearance

Define hypertonia

too much muscle tone

Isotonic contraction

muscle changes in length and moves load; Once enough tension is generated to move the load, tension remains relatively constant

Isotonic contractions can either be __________ or ________.

concentric; eccentric

Concentric contractions

muscle shortens and does work; Ex: biceps brachii contraction to pick up a book

Eccentric contractions

muscle lengthens and generates force – 50% stronger contractions than concentric; Ex: quadriceps contractions while walking downstairs

Isometric contractions

muscle tension develops, but the load is not
moved – load is greater than the tension the muscle can develop; Ex: attempting to lift a piano with 1 hand, holding a plank or wall sit

In isometric contractions, ____ _______ are formed and generate _____, but they do not slide the ____ __________.

cross bridges; force; thin filaments

Why can eccentric contractions be stronger than concentric?

Because in eccentric contractions, muscle force is less than the resistance

Why do we need ATP to contract muscle?

It is the only source of energy for contractile activities. It supplies energy to move and detach cross bridges, operate the calcium pump in the SR, and operate the Na+ -K+ pump in the plasma membrane.

List the 3 methods we use to regenerate ATP.

1) Direct phosphorylation of ADP by
creatine phosphate (CP)

2) Anaerobic pathway: glycolysis and lactic
acid formation

3) Aerobic respiration

Which of these methods of regeneration of ATP is the fastest? Which creates the most ATP?

Direct phosphorylation of ADP by
creatine phosphate (CP); aerobic respiration

Write out the steps of direct phosphorylation of ADP.

CP donates a phosphate to ADP to instantly form ATP; Muscle fibers have enough ATP + CP reserves to power the muscle cell for 15s; CP reserves are replenished during rest or inactivity

Creatine Phosphate (CP)

unique high energy molecule stored in muscle fibers

Creatine Kinase

enzyme that carries out the transfer of phosphate

Define glycolysis

(“Sugar-Splitting”) = the 1st step in glucose breakdown; Process does not require Oxygen; Glucose is broken into 2 pyruvic acid molecules; 2 ATPs are generated for each glucose broken down

Define lactic acid

It is used as fuel by the liver, kidneys, and heart; It is converted back to pyruvic acid or glucose by the liver

How long can we power a muscle cell using the glycolysis pathway?

can provide most of the ATP needed for 30-40 seconds of strenuous muscle activity

Define aerobic respiration

A chemical process in which oxygen is used to make energy from carbohydrates (sugars)

Where does aerobic respiration take place?

in mitochondria

What is the chemical equation for aerobic respiration?

Glucose + O2 -> CO2 + H2O + ATP

In aerobic respiration, what are the ingredients for ATP?

oxygen and glucose

What happens to the CO2 produced in aerobic respiration?

It diffuses out of the muscle and into blood – it will be removed by the lungs

List the fuel sources for aerobic respiration in the order that they are used.

As exercise begins, muscle glycogen provides most of the fuel; Shortly thereafter, blood borne glucose, pyruvic acid from glycolysis, and free fatty acids are the major fuel sources

When Oxygen is available, which ATP production pathway is used?

aerobic respiration

Define aerobic endurance

the length of time a muscle can continue to contract using aerobic pathways

Define anaerobic threshold

the point at which muscle metabolism converts to anaerobic glycolysis

Which pathway would power a gymnast performing a vault? How about a triathlete completing an Olympic distance triathlon?

ATP and CP pathway; aerobic respiration

What is muscle fatigue?

the physiological inability to contract despite continued stimulation

What might cause muscle fatigue?

• Ionic Imbalances: changes in the levels of K+, Na+, and Ca2+ can disrupt membrane
  potentials

• Increased inorganic phosphate (Pi) from CP and ATP breakdown may interfere with
  calcium release from the SR

• Decreased ATP and increased Mg2+ act on voltage-sensitive proteins in the T Tubule and decrease Ca2+ release from the SR

• Decreased glycogen

What RARELY causes muscle fatigue?

Lack of ATP and rise of lactic acid

What must happen for a muscle to return to its pre-exercise state?

• Oxygen reserves must be replenished

• Accumulated lactic acid must be
  reconverted to pyruvic acid

• Glycogen stores must be replaced

• ATP and creatine phosphate reserves must be resynthesized

Define excess postexercise oxygen consumption (EPOC).

the extra amount of oxygen that the
body must consume for these restorative
processes – formerly called the Oxygen Debt

How does an Oxygen deficit occur?

when the body's demand for oxygen during physical activity exceeds the amount of oxygen that can be supplied