Exercise Physiology

Introduction Anatomy

Focuses on the structure of the body parts and their interrelationship.

Physiology Study of the body function

Exercise Physiology Study of how body structures and functions are altered when exposed to acute bouts of exercise.

Chronic adaptations to exercise.

Sports Physiology Concepts of exercise physiology to training the athlete and enhancing the athletes sport performance.

Thus it is derived from exercise physiology.

Structure & Function of the Exercising MuscleTypes of muscle

Structure of the Muscle

EpimysiumOuter connective tissue covering muscle

Surrounds the entire muscle holding it together

Perimysium Connective tissue surrounding the fasciculus

Endomysium Surrounds the muscle fiber (10 – 120 micrometer)

Do muscle fibers extend from one end of the muscle to the other?

Muscle bellies often divide into compartments or more transverse fibrous bands (inscriptions).

Muscle Fiber Contents

Plasmalemma

Sarcoplasm

Transverse Tubules

Sarcoplasmic Reticulum

PlasmalemmaPlasma membrane that surrounds the fiber

Apart of a larger unit called the sarcolemma (plasma membrane + basement membrane)

@ the end of the muscle fiber it blends with the tendon which inserts into the bone.

Appears as a series of shallow folds along the surface of the fiber when contracted or rested.

Has junctional folds in the innervation zone at the motor endplate

Assists with maintaining acid – base balance

Transports metabolites from the capillaries into the fiber

Satellite cells (growth and development) located in between plasmalemma and the basement membrane

Sarcoplasm Gelatin like substance that fills the space between the myofibrils

Contains dissolved protein, minerals, glycogen, fats, and necessary organelles.

Differs from the cytoplasm of most cells because it contains large quantity of stored glycogen and oxygen binding compound myoglobin.

Transverse Tubules Extensions of the plasmalemma that passes laterally through the muscle fiber.

Interconnected to allow nerve impulses received to be transmitted rapidly to individual myofibrils.

Provides pathways from outside to its interior, allowing substances to enter and waste products to leave

Sarcoplasmic Reticulum Longitudinal network of tubules

Membranous channels parallel the myofibrils and loop around them

Storage site for calcium

MyofibrilsContractile elements of the skeletal muscle

Appear as long strands of smaller subunits called the sarcomeres.

Skeletal muscle has distinct striped appearance.

SarcomeresBASIC functional unit of a myofibril and the BASIC contractile unit of the muscle.

Each myofibril consists of numerous sarcomere joined end to end at the Z disks

Myofibril

Thick Filament 2/3 of the skeletal muscle protein is myosin, principal protein of the thick filament.

Each myosin molecule composed of two protein strands twisted together.

One end of each strand is folded into a globular head – myosin head

Myosin head protrudes from the thick filament to form cross bridges onto the thin filaments.

Titin – array of fine filaments that stabilizes the myosin from Z to M line.

Thin Filaments Composed of three different protein molecules:

Actin

Tropomyosin

Troponin

One end attached to the Z line and the opposite extending to the center of the sarcomere, lying in space between the thick filaments.

NebulinAnchoring protein for actin

Plays a regulatory role in mediating actin and myosin interactions

Each thin filament contains active binding sites to which myosin heads can bind.

Actin Backbone of the thin filament

Individual actin molecules are globular proteins joined end to end like stands twisted into a helical pattern

TropomyosinTube shaped protein that twists around the actin strands

TroponinAttached at regular intervals to both the actin strands and tropomyosin

Muscle Fiber Contraction

α-motor neuron Neuron that connects with and innervates many muscle fibers.

Single motor neuron and all the muscle fibers it supplies are collectively termed a motor unit.

Communication between the nervous system and muscular system occurs at the neuromuscular junction (gap between the α-motor neuron and the muscle fiber)

Action PotentialAction potentials (AP) are electrical signals propagated from the brain or spinal cord to the α-motor neuron.

From the α-motor neuron dendrites (specialized receptors on the neuron’s cell body) the AP travels down the axon to the axon terminals located close to the plasmalemma.

@ the terminals the nerve ending secretes achetylcholine (Ach) which binds to the receptors on the plasmalemma.

Enough ACh has to bind to the receptors to allow for the AP to be transmitted to the ful length of the muscle fibers.

This open ions gates in the muscle cell membrane and allows sodium to enter, depolarization.

An AP MUST be generated in the muscle before the muscle cell can act.

Role of Calcium in the Muscle Fiber

The AP travels over the T-tubules to the interior of the cell.

The arrival of the electrical charge causes the SR to release large quantity of calcium (Ca) into the sarcoplasm.

@ rest tropomyosin covers the myosin binding sites on the actin, hence preventing the binding of the myosin heads.

Once the Ca ions are released from the SR they bind to the troponin on the actin molecule.

Troponin has a strong affinity for Ca and is believed to initiate the contraction process by moving the tropomyosin molecules off the myosin-binding sites on the actin molecules.

Once lifted off the the myosin heads can attached to the binding sites on the actin molecules.

How does muscle shorten????

Sliding Filament Theory

Once the myosin cross bridges are activated they bind to actin conformational change in the cross bridge causing the myosin head to tilt (power stroke) dragging the thin filament towards the center of the sarcomere pulling of the thin filament past the thick filament shortens the sarcomere and generates force.

Immediately after the myosin head tilts it breaks away from the active site, rotates back to its original position and attaches to a new active site.

Repeated attachments and power strokes cause the filament to slide past one another, hence the term sliding filament theory.

This process continues until the end of the myosin touches the Z disks, or until the Ca is pumped back into the SR

Energy for Muscle Contraction

Muscle contraction is a process that requires energy.

In addition to the binding site for actin, the myosin head contains a binding site for adenosine triphosphate (ATP).

Myosin MUST bind with ATP for muscle contraction to occur, because ATP supplies the needed energy.

Adenosine triphosphatase (ATPase) – located on the myosin head, splits the ATP to adenosine diphosphate (ADP), inorganic phosphate, and energy.

Energy released from the ATP is used to power the tilting of the myosin head.

Thus, ATP is the chemical source of energy for muscle contraction.

End of Muscle Contraction

Muscle contraction will continue as long as calcium is available in the sarcoplasm.

@ the end of muscle contraction, Ca is pumped back into the SR where it is stored until a new AP arrives.

Ca is returned to the SR by an active Ca – pumping system.

This system is energy demanding and hence relies on ATP.

Thus contraction and relaxation phases requires energy.

Once Ca is pumped back into the SR, troponin and tropomyosin return to the resting conformation.

Blocking the linking of myosin cross bridges and actin molecules, and stops the use of ATP.

Resulting in the thick and thin filaments returning to their relaxed original state.

Skeletal Muscle and Exercise