3. Tension Production Energy Use and Muscular Activity

Tension 

pulling force generated as sarcomeres shorten 

Tension of entire muscle depends on

Number of muscle cells contracting

Tension produced in individual muscle fibers (cells) can vary due to

Length-tension relationship
Frequency of stimulation

Length-tension relationship

amount of tension depends on number of cross bridges formed and degree of overlapbetween thick and thin filaments in a muscle fiber.  

Skeletal muscle contracts most forcefully over a _____

narrow range of resting sarcomere length

narrow range of resting sarcomere length: Too short

filaments overlap too much so bridges can form effectively

narrow range of resting sarcomere length: Too long

filaments do not overlap sufficiently, resulting in fewer cross bridges and less tension.

Twitch

cycle of contraction/relaxation produced by a single action potential in a muscle fiber.

Twitch: latent phase

Action potential has occurred but no tension is produced as calcium is released.

Twitch: contraction phase

Calcium binding to troponin allows myosin heads to attach to actin, resulting in muscle contraction.

Twitch: relaxation phase

Calcium is reabsorbed, causing myosin heads to detach and the muscle fiber returns to its resting state.

Most muscular activities involve: 

sustained muscular contractions produced by high frequency of action potentials 

Summation of tension produces

greater force

Summation

repeated stimulation produced before relaxation phase has been completed, causing build up of calcium ions in sarcoplasm

Treppe summation

a phenomenon where muscle contractions increase in strength with repeated stimulation, leading to progressive enhancement of muscle tension.

Wave summation 

occurs when muscle fibers are stimulated repeatedly at a frequency that does not allow complete relaxation, resulting in increased muscle tension.

Incomplete tetanus

tension production rises to peak and relaxation period are very brief

Complete tetanus

is the state where muscle fibers are stimulated at such a high frequency that they do not relax at all between stimuli, resulting in a sustained, maximal contraction.

Motor units

all the muscle fibers innervated by one motor neuron that work together to produce a contraction.

Differences in number and size of motor units in different muscles 

control precision and control of movements produced 

Small motor unit

consists of a single motor neuron innervating a few muscle fibers, allowing for precise control of movements.

large motor unit

consists of a single motor neuron innervating many muscle fibers, which generates stronger but less precise contractions.

Asynchronous motor unit summation

is a process where motor units are activated at different times to produce smoother and more sustained muscle contractions.

Muscle tone 

is the continuous and passive partial contraction of the muscles, important for posture and readiness for action.

Is there a time where there is no motor unit activity in a muscle within a healthy person

No, healthy muscle maintains some level of activity for tone even if no movement is produced

Greater resting muscle tone leads to higher

resting metabolic rate because active muscle cell consume more energy even at rest

Why is it difficult to contract a muscle that has
been overstretched?
A. Little or no calcium can be released from the sarcoplasmic
reticulum if it is stretched
B. Actin becomes detached from the Z disks when muscle
fibers are over stretched
C. Transverse tubules can not conduct action potentials when
distorted by stretching
D. Few if any myosin-actin cross bridges can form when
sarcomeres are overextended
E. The neuromuscular junction becomes detached when a
muscle is too stretched

D. Few if any myosin-actin cross bridges can form when
sarcomeres are overextended

isotonic muscle contraction

tension rises and muscle length changes, Can be concentric or eccentric

Concentric contraction

muscle shortens while generating tension and Z-lines are pulled together

Eccentric muscle contraction

muscle lengthens while generating tension, cross bridges are still active but muscle is being pulled longer by external force

Ex of eccentric contraction 

lowering a weight slowly 

Are there active mechanisms that cause muscle fiber elongation

No, muscles return to resting length due to contraction of opposing muscle groups, gravity, and some elastic recoil of connective tissue

ATP and creatine phosphate (CP) reserves

only last 15 seconds once contraction begins and must be replenished for sustained activity.

Creatine phosphate reserves

rapid energy buffer for high-intensity muscular activity, helping to regenerate ATP during short bursts of effort.

ATP generation- aerobic cellular respiration 

most ATP needed for resting muscle and moderate levels of activity 

ATP generation- aerobic AND anaerobic cellular respiration 

both are needed to generate additional ATP for peak performance

Aerobic metabolism

Uses O2, released CO2 and produces ATP in mitochondria with citric acid cycle (NADH and FADH2 produced) and ETC, which produces ATP and uses O2 as final e- acceptor

Resting muscle fibers rely on aerobic metabolism of

fatty acids to generate steady, long-lasting source of ATP

How are fatty acids metabolized for resting muscle fibers 

absorbed from circulation, broken down through beta-oxidation, to produce 2 acetyl CoA to enter citric acid cycle 

Excess ATP

used to store glucose into glycogen —> create creatine phosphate

Moderately active muscle activity

ATP is generated through aerobic metabolism of glucose from stored glycogen and fatty acids, allowing for sustained energy during exercise.

Anerobic metabolism

if oxygen delivery is too slow, cell rely on pyruvate metabolism to generate ATP through lactic acid fermentation.

Peak activity muscle metabolism 

oxygen cannot diffuse fast enough for mitochondria to meet ATP demand, so glycolysis becomes the primary source of ATP, but excess pyruvate is produced and builds up in cytosol and is converted to lactic acid.

Build up of hydrogen ions in cytosol from excess lactic acid formation

increases cytosol acidity, which inhibits contraction and causes rapid fatigue

Benefits of anaerobic metabolism

includes rapid ATP production and allows for sustained activity in low oxygen conditions.

disadvantages of anaerobic metabolism

inefficient use of glucose
lactic acid decreases intracellular pH

Recovery period 

Rebuild ATP and creatine phosphate reserves
Recycle lactic acid to make pyruvate
Rebuild glycogen reserves 

excess post-exercise oxygen consumption – EPOC

is the increased rate of oxygen intake following strenuous activity to restore the body to its resting state. It helps in the recovery processes like ATP replenishment and lactic acid removal.