Week 9 - Exercise & Muscles: Explain 3

 Week 9 - Exercise & Muscles: Explain 3
Longer Term Structural Changes

As you likely know from personal experience, the more you exercise the easier that exercise becomes.  But why is that?  What changes about your muscle cells that allows them to adjust to new demands?
Angiogenesis and addition of mitochondriaStructural Changes in Aerobic Exercise

Recall that, if you are practicing an exercise that lasts more than a few minutes at a time, your muscle cells are probably relying on aerobic respiration to generate ATP.  Initially, it might be difficult to sustain your efforts during that exercise.  Your muscles will become fatigued as they struggle to produce enough ATP to sustain the activity and you will feel out of breath.  Over time, though, your muscles will develop new features to allow them to handle the exercise.  Quite sensibly, they will gain more of exactly the things they need to more effectively perform aerobic respiration!  Angiogenesis will occur to bring more blood, including its oxygen supply, to your muscles.  In addition, mitochondria within your muscle cells will reproduce to provide more sites for aerobic respiration.
Structural Changes in Anaerobic Exercise

Addition of sarcomere proteinsOn the other hand, if you are performing an exercise with very heavy weights, your muscles are probably not relying heavily on aerobic respiration for ATP.  Those exercises probably don't last more than a minute or so before you take a rest, so your cells can rely entirely on CP and glycolysis to generate ATP.  In that situation, gaining more blood vessels and mitochondria wouldn't really make sense.  What your muscles need is more power, not more oxygen and mitochondrial activity.

It is thought that myofibrils develop small tears when muscles are asked to lift very heavy weights.  Those tears and the inflammation that accompanies them like accounts for the stiffness and soreness that comes a day or so following the start of a new weight lifting program.  In response, your muscles once again attempt to add the required material to handle the exercise - in this case, more contractile proteins!  It's the pulling of actin by myosin that creates force in your muscles, so adding more acting and myosin will add more force.  It will also make your muscles thicker, which accounts for your more muscular appearance if you continue to lift heavier and heavier weights.  Remember that muscle cells don't divide a whole bunch after we're born, so muscles don't grow bigger due to muscle cell division.  Rather, the existing cells pack themselves fuller and fuller of sarcomeres, which makes the cells (and therefore your muscles) thicker.
A Time to Think & Put the Pieces Together

Before moving on, take a moment to think about the above information in relation to our hypothetical friend's question.  Our friend started a new workout program where "she cycles quickly through various dumbbell exercises for a few minutes before resting and repeating later."

    What type of exercise do you suppose your friend is performing?  Aerobic?  Anaerobic?  Could you make the case that it could be either?
    Based on the above question, what changes could your friend expect to see in her muscles over the coming months?  Why would those changes occur

Nervous System Stimulation Patterns

 Nervous System Stimulation Patterns

Earlier in this online module we explored how neurons stimulate muscle cells to contract through a series of activities at the neuromuscular junction.  But how frequently do muscle cells receive those signals and how do cells change their contraction activities if they get multiple signals?
Stimulation Pattern 1: Summation

Most muscle cells do not fully contract themselves based on receiving just one, isolated stimulus from a neuron.  Rather, that single stimulation only causes a small, partial contraction.  To more completely shorten a muscle cell and maintain that contraction requires multiple consecutive stimuli over the neuromuscular junction.  The more consecutive stimuli, the shorter the muscle cell will become and the longer the contraction will last.

 Stimulation Pattern 2: Motor Units

Consider a single muscle, like the biceps brachii.  When the nervous system wishes for the biceps brachii to contract, it generally would not instruct every muscle cell in the biceps brachii to simultaneously contract.  If it did that, the contraction would be quite strong, but would not last very long.  The muscle cells would all tire relatively quickly at the same time and then the entire muscle would need to relax before it could contract again.

Instead, your nervous system would only instruct certain "motor units" to contract.  As the diagram below shows, a single motor unit consists of a selection of muscle cells within a larger muscle that always contract together.  Any given muscle would have lots of different motor units.  Your nervous system could have the motor units take turns, telling some to contract while others relax, so you could have a smooth and sustained contraction of the muscle over a long period of time.  The nervous system can also stimulate the proper number of motor units for the amount of force needed.  If you lift something extremely heavy, your nervous system might indeed stimulate nearly all motor units at once.  However, that sort of contraction would not be sustainable for very long.

 A Time to Think & Put the Pieces Together

Before moving on, take a moment to think about the above information in relation to our hypothetical friend's question.  Our friend started a new workout program where "she cycles quickly through various dumbbell exercises for a few minutes before resting and repeating later."

    How will the frequency of nervous system stimulation change as your friend initially lifts a weight, holds it up, and then relaxes her arms back down?
    What patterns of motor unit activity might you observe in your friend's arm muscles during the workout?  How would that vary depending on whether your friend is lifting light weights vs. extremely heavy weights?

Strategy 3: Aerobic Respiration

 Strategy 3: Aerobic Respiration

A third strategy for ATP generation is aerobic respiration.  This is a more extensive set of chemical reactions that requires the participation of the mitochondrion.  The reactions require oxygen to proceed and the process is much slower than CP usage or glycolysis.  However, aerobic respiration is advantageous in that it more efficiently harnesses the stored energy in nutrient molecules to produce 30+ ATP molecules per cycle.  As a result, aerobic respiration can sustain contractions for hours.

 A Time to Think & Put the Pieces Together

Before moving on, take a moment to think about the above information in relation to our hypothetical friend's question.  Our friend started a new workout program where "she cycles quickly through various dumbbell exercises for a few minutes before resting and repeating later."  Consider the intensity and duration of this workout.

    What ATP generating strategies would her muscles initially use when beginning the workout?  Why?
    What ATP generating strategies might her muscles use later on in the workout?  Why?
Copilot
Certainly! Let’s break down our friend’s workout and analyze the ATP generation strategies her muscles would employ:

Initial Workout Phase (Beginning):
When our friend starts her workout, her muscles need rapid energy to power the initial contractions.
The first strategy her muscles would use is creatine phosphate (CP). CP provides a quick burst of energy by transferring its phosphate group to ADP, forming ATP.
Since CP reserves are limited (lasting only a few seconds), it’s ideal for short bursts of intense activity, such as the initial moments of her dumbbell exercises.
Mid-Workout Phase:
As our friend continues cycling through exercises, her muscles will transition to another strategy: glycolysis.
Glycolysis occurs in the cytoplasm and doesn’t require oxygen. It breaks down glucose (from blood or muscle reserves) to produce ATP.
Although glycolysis is faster than aerobic respiration, it’s still not sustainable for extended periods.
Our friend’s muscles can generate 2 ATP molecules per glucose molecule during glycolysis.
Lactic acid, a byproduct of glycolysis, accumulates but doesn’t directly cause muscle fatigue.
Later Workout Phase (Sustained Activity):
As the workout progresses, her muscles will shift toward aerobic respiration.
Aerobic respiration occurs in the mitochondria and requires oxygen.
Although slower, it’s highly efficient, producing over 30 ATP molecules per glucose molecule.
Aerobic respiration can sustain contractions for hours, making it ideal for endurance activities.
Our friend’s muscles will rely on this strategy to maintain energy levels during sustained exercise.
In summary, her muscles initially use CP for quick bursts, switch to glycolysis for short-term energy, and eventually rely on aerobic respiration for prolonged activity. The intensity and duration of her workout determine which strategy predominates at different stages. 🏋️‍♀️💪

Feel free to explore more about these fascinating energy pathways and adapt your own exercise routines accordingly!

muscle explain 1

 Muscles require energy both to prepare for a contraction and then to sustain a contraction after stimulation.  Muscle cells have a few different strategies available for generating ATP to power contractions.  Of special relevance to our friend with her new interest in exercise, those energy harnessing strategies change depending on the type and length of exercise.
Strategy 1: CP

For the initial energy needs during a contraction, muscles can take advantage of a quick form of stored energy in the form of creatine phosphate (CP).  At rest, the muscle cell uses some of its ATP to add a phosphate group to creatine, creating CP.  As a result, muscle cells have a reserve of CP ready to use when energy demands grow.  The phosphate group on CP can very quickly be transferred to ADP, creating ATP to power contractions.  Only one ATP molecule is created for each CP molecule used and CP is depleted after a matter of seconds.

 Strategy 2: Glycolysis

Beyond 15 seconds or so of contraction, muscles will need another strategy besides CP use to generate ATP.  One option is glycolysis.  Glycolysis is a series of chemical reactions in the cytoplasm that use energy stored within carbohydrates from the blood or from muscle cell reserves to generate ATP.  This is still a relatively fast set of chemical reactions that does not require oxygen and it produces 2 ATP molecules per glucose molecule used.  It also produces lactic acid, which itself can be converted back to glucose and used in glycolysis.  It turns out lactic acid is not a primary cause of muscle fatigue or pain as was popularly believed.  Since glycolysis only produces 2 ATP molecules per cycle, it is not efficient enough to sustain contractions beyond a couple minutes.

 Strategy 3: Aerobic Respiration

A third strategy for ATP generation is aerobic respiration.  This is a more extensive set of chemical reactions that requires the participation of the mitochondrion.  The reactions require oxygen to proceed and the process is much slower than CP usage or glycolysis.  However, aerobic respiration is advantageous in that it more efficiently harnesses the stored energy in nutrient molecules to produce 30+ ATP molecules per cycle.  As a result, aerobic respiration can sustain contractions for hours.

 A Time to Think & Put the Pieces Together

Before moving on, take a moment to think about the above information in relation to our hypothetical friend's question.  Our friend started a new workout program where "she cycles quickly through various dumbbell exercises for a few minutes before resting and repeating later."  Consider the intensity and duration of this workout.

    What ATP generating strategies would her muscles initially use when beginning the workout?  Why?
    What ATP generating strategies might her muscles use later on in the workout?  Why?

Muscle Responses to Exercise- Week 9 - Exercise & Muscles: Engage/Explore

 Muscle Responses to Exercise

Having used the context of neuromuscular toxins to clarify how our muscles are stimulated to contract, we will take a brief look at one other area of muscle activity this week - how muscles respond to exercise.

More specifically, we will seek to...

    Explain to a friend how her muscles respond in the short-term and long-term to exercise at the cellular and molecular levels.

Our Scenario...

DumbellsPretend you have a friend who has recently gotten very excited about fitness and exercise. Now all she wants to talk about is what’s happening in her body during workouts! She’s started a new weight lifting program for her arms where she cycles quickly through various dumbbell exercises for a few minutes before resting and repeating later. She asks you to help her understand what’s happening in her muscle cells during and after those workouts.

Before we explore some of the physiology underlying this scenario, think about how you would respond to your friend now.  Submit a couple sentences here to discuss what you think you already know about this scenario.  Feel free to discuss what you are uncertain about or what questions you think you need to answer yourself to respond to your friend.

Your response here will not be graded for correctness.

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Sodium ions play a critical role in muscle cell depolarization.Calcium ions are essential for muscle contraction.During normal muscle contraction, sodium channels open, allowing sodium ions to flow into the muscle cell.

Latrotoxin effects affect cell membrane which sodium and calcium currents depolarize the membrane that allow influx of sodium and calcium ions .Binding to Neuronal Receptorsdisrupt normal neuromuscular junction function

Increase calcium ion [Ca2+] ion resulting in increase release ACH  . Excessive release of acetylcholine (ACH) at the neuromuscular junction,   resulting in increase muscle contraction of skeletal muscle and cardiac muscle.