EXAM 3 EXERCISE PHYS content

what is skeletal muscle?

Skeletal muscle is organized in a hierarchical, nested structure, each level contributing to its overall function

Whole Muscle

The entire organ (e.g., the biceps brachii), surrounded by a connective tissue layer called the epimysium.

Fascicles

Bundles of muscle fibers within the whole muscle, surrounded by the perimysium.

   Muscle Fiber (Muscle Cell):

A single, long, cylindrical, multinucleated cell. It is surrounded by the endomysium. Each fiber contains hundreds to thousands of myofibrils.

Myofibril:

The rodlike organelle running the length of the fiber. It is the contractile apparatus of the cell.

Sarcomere:

The fundamental, repeating contractile unit of the myofibril, bordered by Zdiscs (or Zlines). It is the functional unit of muscle contraction.

 Thick Filaments

Composed primarily of the protein myosin

Myosin

A motor protein with a tail and a globular head. The heads form crossbridges that bind to actin and generate force

Thin Filaments:

Composed primarily of actin, along with regulatory proteins.

Actin

Globular protein (Gactin) that polymerizes into long chains (Factin). It contains binding sites for myosin heads

Tropomyosin:

A long, ropelike protein that lies in the groove of the actin helix. At rest, it blocks the myosin binding sites on actin.

 Troponin

A complex of three subunits:

TnC

binds calcium ions

Tnl

inhibits the actin myosin interaction

TnT

binds to tropomysin

Titin

A giant, elastic protein that anchors the thick filament to the Zdisc. It acts as a molecular spring, providing passive tension and elasticity.

Nebulin

An inelastic protein that runs alongside the thin filament, thought to regulate its length.

       αActinin

The primary protein of the Zdisc, crosslinking the actin filaments from adjacent sarcomeres.

summary of muscle orginizaition

Whole Muscle → Fascicles → Muscle Fiber → Myofibrils → Sarcomeres (composed of Myosin, Actin, Tropomyosin, Troponin, Titin).

Fiber Pennation

refers to the angle at which muscle fibers attach to the tendon

Parallel (Fusiform) Fibers:

Fibers run parallel to the muscle's long axis.

Force

Lower total crosssectional area, Higher. Pennation allows more fibers to be packed into a given volume, increasing the physiological crosssectional area (PCSA), which is the primary determinant of force production.

Shortening Velocity

Higher, as all sarcomeres pull directly in line with the tendon

Pennate (Uni, Bi, Multipennate) Fibers

Fibers attach to the tendon at an angle (pennation angle).

Tradeoff in pennation 

Pennate muscles are stronger but slower, while parallelfibered muscles are faster but weaker for a given muscle volume.

shortening velocity

Lower. Because each fiber pulls at an angle, not all of its contractile force and speed is transmitted directly to the tendon. A component of the force is used to "deform" the connective tissue, making the overall muscle contraction slower.

what are the 3 sterps involved in Neural signal and excitation contraction coupling 

1.  An action potential arrives at the neuromuscular junction, releasing acetylcholine.

2.  The action potential spreads along the sarcolemma and down the Ttubules.

3.  This triggers the release of Calcium ions (Ca²⁺) from the sarcoplasmic reticulum (SR).

The chemical and mechanical cycle steps

4.  Ca²⁺ Binding: Ca²⁺ binds to TnC on the thin filament.

5.  Troponin/Tropomyosin Shift: This binding causes a conformational change in the troponin complex, pulling tropomyosin away from the myosinbinding sites on actin.

6.  CrossBridge Formation: The energized myosin head (with ADP + Pi bound) can now bind to actin, forming a crossbridge.

7.  Power Stroke: Myosin releases ADP + Pi and undergoes a conformational change, pivoting its head and pulling the thin filament toward the center of the sarcomere (the Mline). This is the forcegenerating step.

8.  ATP Binding: A new molecule of ATP binds to the myosin head, causing it to detach from actin.

9.  ATP Hydrolysis: Myosin hydrolyzes ATP to ADP + Pi, reenergizing the myosin head ("cocking" it) so it can repeat the cycle.

See table in nnotes

make sure you can fill out the table

Endurance Athletes (e.g., Marathon Runners, Cyclists):

Exhibit a very high proportion of Type I fibers (>80% in some muscles). This provides superior fatigue resistance and efficient aerobic energy production.

   Power/Sprint Athletes (e.g., Sprinters, Weightlifters

Exhibit a very high proportion of Type II fibers (especially Type IIx). This allows for rapid, highforce generation.

MiddleDistance Athletes (e.g., 800m runners):

): Often have a more balanced mix, with a significant proportion of Type IIa fibers, which offer a blend of speed and fatigue resistance.

Endurance Training affect on fiber type 

Promotes a shift from Type IIx → Type IIa. It can also induce small, but significant, shifts from Type IIa → Type I over long periods. This adaptation increases the muscle's oxidative capacity and fatigue resistance.

Strength/Resistance Training affect on fiber comp

Promotes a shift from Type IIx → Type IIa. It increases the size (hypertrophy) of all Type II fibers but does not create a meaningful shift towards Type I fibers. The result is a muscle dominated by fast, powerful Type IIa fibers

key points about fiber

You cannot turn a Type I fiber into a Type II fiber or vice versa through training alone. However, you can change the characteristics of the subtypes within the fasttwitch group.

Mitophagy is

the selective autophagic degradation of damaged or dysfunctional mitochondria.

Why is mitophagy necessary

Mitochondria are the powerhouses of the cell, but they produce reactive oxygen species (ROS) and can become damaged with use. A buildup of damaged mitochondria is inefficient and can trigger cell death pathways.

Mitophagy role in muscle

quality control, adaption to exercise, metabolic health,

quality control role in muscle

Mitophagy is a constant "housekeeping" process that removes damaged mitochondria, ensuring a healthy, functional mitochondrial pool. This is crucial for maintaining efficient energy production, especially in highly oxidative Type I fibers.

Adaption to exercise role in muscle

: Exercise initially increases mitochondrial damage, triggering a wave of mitophagy. This clears the way for the synthesis of new, healthier mitochondria (mitochondrial biogenesis) in response to training. This process is essential for improving aerobic capacity.

Metabolic Health role in muscle

Proper mitophagy prevents the accumulation of dysfunctional mitochondria, which is associated with insulin resistance and sarcopenia (agerelated muscle loss).

key ideas about mitophagy

is not a destructive process but a vital renewal mechanism that maintains muscle metabolic health, facilitates adaptation to training, and supports longterm function.

Alpha Motorneuron:

A type of lower motor neuron located in the spinal cord and brainstem. Its cell body and dendrites receive signals, and its long axon projects out to skeletal muscle fibers. It is the final common pathway through which the central nervous system initiates skeletal muscle contraction.

   Motor Unit:

Defined as a single alpha motor neuron and all the skeletal muscle fibers it innervates. When the alpha motor neuron fires an action potential, it causes all the muscle fibers in its unit to contract simultaneously. different variants in size 

Small motor units

(e.g., in eye muscles) for fine control: a neuron may innervate only 510 fibers.

Large motor units

(e.g., in quadriceps) for powerful movements: a neuron may innervate over 1000 fibers.

   Motor Pool:

The collection of all alpha motor neurons that innervate a single skeletal muscle. For example, all the alpha motor neurons that send axons to the biceps brachii muscle constitute the biceps motor pool.

EPSPs (Excitatory):

Depolarizing potentials that make the neuron more likely to fire an action potential. They are often triggered by neurotransmitters like glutamate

IPSPs (Inhibitory):

Hyperpolarizing potentials (or oppose depolarization) that make the neuron less likely to fire an action potential. They are often triggered by neurotransmitters like GABA (gammaaminobutyric acid).

ESPS and IPSPs

The neuron integrates all the EPSPs and IPSPs to decide whether to reach the threshold for firing.       These are graded electrical potentials in the dendrites and cell body of a neuron.

Saltatory Conduction:

: The process by which an action potential "jumps" from one Node of Ranvier to the next along a myelinated axon. Myelin acts as an insulator, forcing the action potential to regenerate only at the unmyelinated nodes. This dramatically increases the speed of neural transmission compared to unmyelinated axons.

   Initiation of Muscle Depolarization

When an action potential from the alpha motor neuron reaches the neuromuscular junction, it triggers the release of the neurotransmitter acetylcholine (ACh). ACh binds to receptors on the muscle fiber, opening ion channels and generating an endplate potential. This local depolarization initiates an action potential that propagates along the muscle fiber, leading to excitationcontraction coupling and muscle contraction.

size principle

Motor units are recruited in a fixed, orderly sequence from smallest to largest.

SIze principles Mechanism

Smaller alpha motor neurons have a higher input resistance, meaning a given synaptic current will produce a larger voltage change (depolarization), making them easier to excite. As the demand for force increases, larger and larger motor neurons are recruited.

concequences of size principles

       Lowforce, sustained activities (e.g., maintaining posture) recruit small, slowtwitch (Type I) motor units first. These are fatigueresistant.

       As force requirements increase (e.g., walking, then running), medium, fasttwitch (Type IIa) units are recruited.

       Maximal force efforts (e.g., a vertical jump) recruit the largest, fastfatigable (Type IIx) units.

   Advantage: This provides smooth gradation of force and optimizes energy efficiency by matching the most efficient (fatigueresistant) fibers to the task.