Neuromuscular Lab

### Learning Objectives and Discussion Questions - Neuromuscular Lab

#### Learning Objectives

1. Explain how a whole muscle is compartmentalized

- A whole muscle is divided into muscle fibers, which are individual muscle cells (**myocytes**). These fibers are grouped into fascicles, which are bundles of muscle fibers surrounded by connective tissue. Multiple fascicles make up the entire muscle, allowing for organized contraction and force distribution.

2. Explain the basics of excitation-contraction coupling

- Excitation-contraction coupling (ECC) refers to the process by which an action potential leads to muscle contraction:

1. Alpha motor neurons transmit an action potential (AP) to the muscle fiber.

2. The signal reaches the neuromuscular junction (NMJ), where acetylcholine (ACh) is released and binds to ACh receptors on the muscle fiber membrane (**sarcolemma**).

3. This generates an end-plate potential, which travels down t-tubules, activating voltage-gated Ca²⁺ channels.

4. Ca²⁺ is released from the sarcoplasmic reticulum into the cytoplasm, initiating contraction by binding to troponin, which leads to actin-myosin cross-bridge formation.

3. Define what a motor unit is

- A motor unit consists of a motor neuron and all the muscle fibers it innervates. Each muscle fiber is innervated by only one motor neuron, but a motor neuron can control multiple muscle fibers. The number of fibers within a motor unit determines its function:

- Small motor units → Fine motor control (e.g., eye muscles)

- Large motor units → Greater force generation (e.g., quadriceps)

4. Explain the size principle

- The size principle states that motor units are recruited from smallest to largest depending on the force needed.

- Low-force movements recruit smaller, more fatigue-resistant motor units (slow-twitch fibers).

- As force demand increases, larger, fast-twitch motor units are recruited for greater power output.

5. Define electromyography (EMG)

- EMG records the electrical activity of skeletal muscles. It can be measured via:

- Surface EMG (electrodes on the skin)

- Intramuscular EMG (needle electrodes inside the muscle)

6. Distinguish between raw and integrated EMG

- Raw EMG: Displays the raw electrical signals (irregular squiggly lines).

- Integrated EMG: Processes raw EMG data to provide a smoother, more meaningful signal representing muscle activation over time.

7. Define maximal volitional contraction (MVC)

- MVC is the greatest amount of force a person can voluntarily exert using a muscle or group of muscles.

8. Observe differences in EMG activity at various intensities of muscle contraction and relate to motor unit recruitment (Part 1)

- As contraction intensity increases, more motor units are recruited, leading to greater EMG activity.

9. Observe differences in EMG activity across time during an isometric contraction and relate to motor unit recruitment (Part 2)

- During prolonged isometric contractions, motor units fatigue, and additional units must be recruited to maintain force.

10. Observe differences in force generation before and after a prolonged muscle contraction (Part 2)

- After sustained contraction, muscle fatigue reduces force output due to decreased motor unit firing rates and metabolic depletion.

11. Relate differences in EMG activity during different abdominal exercises to identify which is most effective (Part 3)

- EMG analysis can determine which abdominal exercises produce the highest muscle activation, identifying the most effective exercise.

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### Discussion Questions and Answers

1. Why can’t EMG signals be directly compared across different individuals?

- EMG signals vary between individuals due to differences in muscle size, skin thickness, fat distribution, electrode placement, and physiological factors (e.g., motor unit recruitment patterns).

2. What are some diseases in which EMG (intramuscular or surface) testing would be utilized?

- Neuromuscular disorders, such as:

- Amyotrophic lateral sclerosis (ALS): Decreased motor unit recruitment.

- Myasthenia gravis: Reduced neuromuscular transmission efficiency.

- Muscular dystrophy: Abnormal muscle activation patterns.

- Peripheral neuropathy: Impaired nerve conduction to muscles.

3. How might those EMG recordings differ from a non-diseased recording?

- Diseased EMG recordings may show:

- Reduced amplitude (weaker muscle activation).

- Irregular or absent motor unit recruitment patterns.

- Increased fatigue (e.g., rapid decline in signal strength).

4. For Part II, explain how and why the second MVC was different from the first. What are some physiological explanations?

- The second MVC would be lower due to:

- Muscle fatigue: Depletion of ATP, accumulation of metabolic byproducts (lactic acid, inorganic phosphate).

- Reduced motor unit recruitment: Fatigue reduces motor neuron firing rate.

- Neuromuscular junction inefficiency: Less effective synaptic transmission of signals.

5. For Part III, why were there differences in the EMG recordings for the three elbow flexion exercises?

- Different biceps curl variations target different muscle activation patterns:

- Supinated curl (palms up) → Maximizes biceps brachii activation.

- Hammer curl (neutral grip) → Engages brachioradialis more.

- Prone curl (palms down) → Increases brachialis activation.

6. Which other muscles were performing elbow flexion if not the biceps brachii?

- Brachialis: Lies underneath the biceps, primary elbow flexor.

- Brachioradialis: Contributes during neutral and pronated grips.

- Forearm flexors: Assist with grip and stabilization.

Let me know if you need any clarifications!