Muscle Anatomy and Physiology Overview

Muscle Anatomy

• A muscle is composed of multiple fascicles.

• A muscle fiber is a muscle cell.

tendon vs ligament

-tendons connect muscles to bones

-ligaments connect bones to other bones, providing stability to joints.

Muscle Structure

• Epimysium: Surrounds the entire muscle.

• Perimysium: Surrounds each fascicle (bundle of fibers).

• Endomysium: Surrounds individual muscle fibers.

Muscle Fiber Composition

• The sarcolemma is the cell membrane of a muscle cell.

• Muscle cells contain multiple nuclei and mitochondria.

• Myofibrils: Extend throughout the muscle fiber and contain units called sarcomeres.

Triad Structure

• A triad consists of:

  • 1 T-tubule

  • 2 Terminal cisternae of the sarcoplasmic reticulum

• The T-tubule propagates the action potential into the muscle fiber

• terminal cisternae store calcium for muscle contraction.

 Active transport is when the cell uses ATP (energy) to move molecules against their concentration gradient — from low → high concentration.

Sliding Filament Model

• Thick filament: Composed of myosin.

• Thin filament: Composed primarily of actin.

  • Regulatory proteins include tropomyosin and troponin.

Regulation of Contraction

• Tropomyosin and troponin bind calcium; their binding exposes actin binding sites, allowing contraction to occur.

Sarcomere as Contractile Unit

• The sarcomere is the functional and contractile unit of muscle tissue.

  • Key structural components:

  • Z-discs: Mark the ends of each sarcomere.

  • M-line: The midline of the sarcomere.

  • H-zone: Zone that disappears during contraction.

Motor Units

• A motor unit is defined by its connections to muscle fibers.

  • Small motor units: Involved in fine motor control with fewer fibers. (less than 5 fiber muscles)

  • Large motor units: Involved in powerful contractions with many fibers. (thousands of motor units)

Recruitment of Motor Units

• Recruitment occurs from small to larger motor units depending on the force required for a movement.

  • Examples of movements:

  • Writing with a pen requires small motor units.

  • Riding a bicycle requires larger motor units.

Muscle contraction vs power- wave summation, increase #  of action potential, increase # of action potential, more power brought into muscle, muscle contracts, when it reaches to full contraction= tetany (full muscle contraction)

• Muscle contraction is influenced by several factors, including the frequency of stimulation, which leads to wave summation. As the number of action potentials increases, a greater force is generated in the muscle fibers; consequently, the muscle contracts more powerfully. This process continues until a full contraction, known as tetany, is achieved, where the muscle exhibits sustained contraction without relaxation. 

• -isometric- Muscle contractions where the muscle length does not change, even though tension is generated, allowing for stabilization of body parts or holding objects in place.  

  Isotonic- muscle contractions involve changes in muscle length while maintaining constant tension, allowing for movement, such as lifting weights or walking, whereas isometric contractions occur when muscle length remains unchanged despite tension development, as seen in holding a plank position.

• concentric- the muscle shortens while generating force, allowing for movements such as lifting an object. In contrast, eccentric contractions occur when the muscle lengthens under tension, which can be observed when lowering a weight in a controlled manner.

• vs eccentric (not a contraction, very slow relaxation)  

• Eccentric - occurs when the muscle lengthens while contracting, allowing for controlled movement against resistance, which can lead to muscle soreness if done excessively.

• -Force of any skeletal muscle= pull, where power occurs due to the active shortening or lengthening of muscle fibers in response to load.

  
  

Synapses and Neuromuscular Junction

• A synapse is where a neuron connects to another neuron, a muscle, or a gland.

• The neuromuscular junction connects a motor neuron to a muscle fiber.

  • Components:

  • Synaptic knob: Nerve-side of the junction.

  • Motor end plate: Muscle-side of the junction.

  • Synaptic cleft: Gap between the neuron and muscle fiber.

Action Potential and Muscle Contraction

• An action potential from a nerve causes voltage-gated calcium channels in the synaptic knob to open, resulting in the release of acetylcholine.

• Acetylcholine binds to chemically gated channels on the muscle fiber, allowing for sodium and potassium movement, leading to the generation of an action potential in the muscle.

• Threshold must be reached for muscle contraction to occur.

• Neurotransmitters= is Ach

• What goes through the voltage gated channel, what ion? Calcium

Excitation- contraction coupling:

This process involves the release of calcium ions from the sarcoplasmic reticulum, which bind to troponin, leading to a conformational change in the actin filament that allows myosin cross-bridges to attach and initiate contraction.

steps:

1. Nerve impulse (action potential) travels down the motor neuron.

2. At the neuromuscular junction (NMJ), the neuron releases acetylcholine (ACh) into the synaptic cleft.

3. ACh binds to receptors on the motor end plate (the muscle cell membrane).

4. This causes sodium (Na⁺) to rush into the muscle fiber → the sarcolemma depolarizes (electric charge spreads).

5. The electrical impulse travels along the sarcolemma and down the T-tubules.

🧠 Key idea:
This step turns a nerve signal (electrical) into a muscle signal (electrical).
No movement yet — just the “go” signal being sent inside the muscle cell.

⚠ Exam Alert:

• Neurotransmitter: Acetylcholine (ACh)

• Destroyed by: Acetylcholinesterase (AChE) (to end the signal)

• Channel opened first: Sodium (Na⁺)

1. The action potential in the T-tubule triggers the sarcoplasmic reticulum (SR) to open its calcium channels.

2. Calcium ions (Ca²⁺) flood into the sarcoplasm (the cytoplasm of the muscle fiber).

3. Ca²⁺ binds to troponin, a regulatory protein on the thin filament (actin).

4. Troponin changes shape → moves tropomyosin away from the myosin-binding sites on actin.

💡 Result:
The actin “binding sites” are now uncovered, and myosin heads are free to grab on.

⚠ Exam Alert:

🧩 Visual Tip:
Imagine the calcium as the key that unlocks the actin–myosin connection.
No calcium → tropomyosin blocks the sites → no contraction.

💪 3⃣ Contraction — The Sliding Filament Theory

Now the actin and myosin filaments physically move, causing the muscle to shorten.

Steps (the Cross-Bridge Cycle):

1. Cross-Bridge Formation: Myosin heads attach to exposed binding sites on actin. Myosin= pulling (repeated steps)
   → (requires calcium availability).

2. Power Stroke: Myosin head pivots, pulling actin toward the M line (center of

   sarcomere). (DONT BURN ATP HERE) 
   → ADP + Pi are released.

3. Detachment: A new ATP binds to myosin → myosin head detaches from actin. 

4. Reactivation: ATP is broken down (hydrolyzed) → myosin head “cocks” back into position, ready to attach again. (BURN ATP) 

When cycle 

💡 This cycle repeats as long as:

• Calcium remains high in the cytoplasm, and

• ATP is available.

🧠 Key takeaway:
When billions of sarcomeres shorten together, the whole muscle fiber shortens = contraction.

💤 Relaxation Phase

When the signal stops:

1. Nerve stops releasing ACh → AChE breaks it down.

2. Calcium channels in SR close; Ca²⁺ is pumped back into SR using ATP.

3. Troponin–tropomyosin complex covers actin binding sites again.

4. Myosin can’t bind → sarcomeres return to resting length.

⚠ Exam Alert:

🔋 Energy Use Summary

💀 Clinical Ties

🧠 Quick Recap Mnemonic:

Excitation: Nerve sends signal
Coupling: Calcium released → actin sites uncovered
Contraction: Myosin pulls actin → muscle shortens

• Relaxation: Calcium reabsorbed → actin sites covered, muscle returns to resting state.

• You see active transport in two key places:

  1. Sodium–Potassium Pump (Na⁺/K⁺ ATPase):

     • Constantly pumps 3 Na⁺ out and 2 K⁺ in to maintain the resting membrane potential.

     • Keeps the inside of the cell more negative than the outside.

     🧩 Purpose: Resets the electrical gradient after every action potential (so the muscle or neuron can fire again).

  2. Calcium Pump (Ca²⁺ ATPase):

     • Actively pumps calcium back into the sarcoplasmic reticulum (SR) after contraction.

     • Uses ATP.

     • When calcium levels drop, muscle relaxation occurs.

  ⚠ Exam Alert:

  “What causes muscle relaxation?” → Active transport of Ca²⁺ back into the SR.

  Exocytosis is the process where vesicles fuse with the cell membrane to release substances outside the cell.

  It’s a type of active transport because it requires ATP to move vesicles and membranes.

  ---

  💬 In Muscle & Nerve Cells:

  1. At the Neuromuscular Junction (NMJ):

     • The neuron releases acetylcholine (ACh) into the synaptic cleft through exocytosis.

     • Triggered by calcium entering the axon terminal.

     • ACh then binds to receptors on the muscle membrane to start excitation.

  🧠 Sequence:

  Nerve signal → Ca²⁺ enters neuron → vesicles fuse → ACh released (via exocytosis)

  2. In Other Cells:

     • Used to release hormones, waste products, or enzymes.

  ⚠ Exam Alert:

  “Which process releases ACh into the synaptic cleft?” → Exocytosis

  Hyperpolarization means the inside of the cell becomes more negative than its resting potential.

  Normal resting potential: -70 mV
  Hyperpolarized: -80 mV or lower

  This makes it harder for the neuron or muscle fiber to generate another action potential.

• 💥 In Context: Putting It All Together

  1. Neuron fires → Ca²⁺ enters terminal → Exocytosis releases ACh.

  2. ACh binds to muscle → Na⁺ enters → Depolarization → action potential spreads.

  3. Ca²⁺ released from SR → contraction begins.

  4. Active transport pumps Ca²⁺ back into SR → muscle relaxes.

  5. Na⁺/K⁺ pump restores balance → brief hyperpolarization before resting again.

  ---

  🧩 Where It Happens:

  1. After an action potential, potassium (K⁺) channels stay open a little too long, allowing extra K⁺ to leave the cell.

  2. This causes the membrane to become more negative than normal for a brief time — that’s hyperpolarization.

  3. The Na⁺/K⁺ pump (active transport again!) restores resting conditions.

Phases of Action Potential

1. Depolarization: Sodium channels open, allowing sodium influx.

2. Repolarization: Sodium channels close, potassium channels open, returning to resting membrane potential.

3. Hyperpolarization: Brief dip below resting potential, which makes it harder to generate another action potential.

   • Sodium-potassium pump: Actively restores resting potential.

Crossbridge Cycling

• The process involves several key steps:

  1. Crossbridge formation: Myosin binds to actin.

  2. Power stroke: Myosin pulls actin, causing muscle contraction.

  3. Myosin head release: ATP binds to myosin and releases it from actin.

  4. Reset of myosin head: ATP is hydrolyzed to reset the myosin head for another contraction.

Muscle Metabolism

• Muscle fibers utilize different energy sources:

  • Stored ATP: Lasts for about 5 seconds.

  • Creatine phosphate: Provides energy for an additional 5-10 seconds.

  • Glycolysis: Takes over after 15-30 seconds for continued energy supply before aerobic respiration begins. (break down of glucose) 30 sec to 1 min

  • Aerobic respiration: Kicks in after about 1 minute for efficient ATP production.

After is oxidative phosphate, start producing atp at level of muscle in efficient manner 

• The body does this ( why do we have to burn this to get to aerobic respiration?) aerobic respiration takes time to inarticulate, not convenient to keep it at aerobic burning, stores enzymes to generate atp,and thus relies on glycolysis initially to meet immediate energy demands during high intensity activities.

Muscle Fiber Types

1. Slow Oxidative Fibers:

   • High endurance, less powerful.

   • Dark red color due to blood flow and mitochondria.

   • Found in posture muscles (e.g., back).

2. Fast Oxidative Fibers:

   • Moderate endurance and power.

   • Intermediate in size and color (red), predominant in leg muscles.

3. Fast Glycolytic Fibers:

   • Low endurance, high power, and size.

   • Primarily white due to less blood flow; found in upper body muscles.

Muscle Tension and Fatigue

• Muscle tension refers to the constant state of semi-contraction in muscles due to repeated exercise.

  • Atrophy: Loss of muscle tissue due to lack of use.

• Muscle tension- just to know, body fast to contract, not fast for relaxation, contract= necessity, repeated exercise= leave muscle in ready position (muscle tone) constant time of ready position

Types of Muscle Contraction

• Isometric: Muscle does not change length, tension increases.

• Isotonic: Muscle changes length while maintaining tension; includes:

  • Concentric: Muscle shortens while contracting.

  • Eccentric: Muscle lengthens while contracting (slow relaxation, not a true contraction).

Differences in Muscle Contraction

• For greater muscle force, more action potentials, not stronger action potentials, are needed.

• Tetany: Full muscle contraction when maximum force is reached with no more contractions.

Muscle Types: Smooth Muscle

• Smooth muscle is involuntary and not normally controlled by conscious thought.

• Multiunit Smooth Muscle: Functions in localized areas (e.g., the eyes).

• Single-unit Smooth Muscle: Coordinates large areas simultaneously (e.g., digestive tract).

• Implications for muscle activation: localized vs. coordinated contraction in the body.

resistance vs endurance- Resistance training focuses on building muscle strength through high-intensity activities, while endurance training emphasizes prolonged, low to moderate intensity exertion to improve stamina and cardiovascular efficiency.

LACTIC ACID CYCLE: The lactic acid cycle refers to the process by which lactic acid is produced in muscles during anaerobic respiration, and subsequently converted back into glucose in the liver, allowing for energy regeneration and the resumption of aerobic metabolism.

steps>

1. Glycogen breakdown: In the absence of sufficient oxygen, glucose stored as glycogen in the muscles is broken down to lactate.

2. Lactic acid diffusion: Lactic acid accumulates in the muscle tissue and enters the bloodstream, where it travels to the liver.

3. Conversion in the liver: The liver converts lactic acid back into glucose through gluconeogenesis, replenishing energy stores.

4. Energy utilization: The regenerated glucose can then be used again by the muscles during aerobic activities, helping to sustain prolonged exercise.

Oxygen debt:  When the body undergoes anaerobic exercise, oxygen intake is insufficient to meet energy demands, leading to the accumulation of lactic acid and other metabolic byproducts.

1. replace oxygen on hemoglobin and myoglobin, 2. replenish glycogen, 3. replenish atp and creatine phosphate,4. convert lactic acid back to glucose

overlap from myosin and actin and how much a muscle can exert, strongest amount of force- is influenced by the number of cross-bridges formed between these two proteins during contraction, which is affected by factors such as muscle fiber type, length-tension relationship, and the frequency of stimulation.

- muscle training, resistance training, endurance, Atrophy definition-   muscle fibers can lead to an increase in the size and strength of the muscle, while endurance training enhances muscular stamina by increasing the efficiency of oxygen usage. Atrophy is defined as the decrease in muscle mass and strength due to disuse or aging.

• hypertrophy: the process of increase in muscle size and strength resulting from resistance training, characterized by an increase in the cross-sectional area of muscle fibers. 

  muscle hypertrophy, and the adaptation of muscle fibers to different types of stress.

hypertrophy, 

-location of smooth muscle- Found in the walls of hollow organs such as the digestive tract, blood vessels, and the bladder, smooth muscle plays a crucial role in involuntary movements.

, where can you find it? Where in the body-  Smooth muscle is primarily located in various systems including the gastrointestinal system, where it aids in peristalsis; the circulatory system, regulating blood flow through vascular contraction; and the urinary system, facilitating the expulsion of urine.

- know types of tissues

- smooth muscle characteristics- make t tubules, involuntary, not voluntary so there are other needs of muscle, not activated by the brain so we have to do it ex, stretch. 

Multi vs single unit, and where you can find it on the body.

Multiunit smooth muscle- eyes or goosebumps arrector pili muscles of skin ex, cover an eye and flash the light (localized affect) need more units to function it 

Single unit smooth muscle (visceral) - digestive tract, most common type,