Lecture 1- Structure and Muscle Function

Exercise Physiology

The Study of how our bodies structures and functions are altered when we are exposed to acute bouts of exercise, a challenge to homeostasis

Exercise Physiologist

Someone who has a general understanding of the scientific basis underlying exercise induced physiological responses

Thre types of Muscle tissue

Skeletal

Cardiac

Smooth

Smooth muscle

Involuntary

Controlled my ANS

Located in the walls of blood vessels and throughout internal organs

Cardiac

Controlled by the ANS and endocrine systems

Skeletal

VOluntary muscle

COntrolled consciously by the somatic NS

Over 600 different skeletal muscles in the human body

Structure of a single muscle fiber

Transverse tubules

Sarcoplasmic reticulum

T-Tubules

Mitochondira

Myofibril

Sarcoplasm

Plasmalemma

Terminal cisternae

Plamalemma

(Cell membrane)

Fuses with tendon

Conducts AP

Maintains pH, transports nutrient

Satellite cells

Muscle growth, development

Response to injury, immobilization, training

Sarcoplasm

Cytoplasm of muscle cell

(Glycogen storage, myoglobin)

Transverse tubules

Extensions of plamalemma

Carry AP deep into muscle fiber

Sarcoplasmic reticulum

Calcium storage

Myofibrils

Muscle > Fasciculi > muscle fiber > myofibril

Hundreds to thousands per muscle fiber

Sarcomeres

Basic contractile element of skeletal muscle

Sarcomeres

A bands: Dark stripes

I Bands: Light striped

H-zone: Middle of A band

M- Line: Middle of H zone

Protein filaments

Actin (Thin filaments:

Myosin: Thick filaments

Myosin

Two intertwined filaments with globular heads

Titan

Stabilizer for myosin

Actin

Three proteins

Actin : Contians myosin binding site

Tropomyosin: Covers active site at rest

Troponin: Anchored to actin, moves tropomyosin

Motor units

Single motor neuron and all fibers it innervates

More operating motor units + more contractile force

Neuromuscular junction

Site of communication b/w neuron and muscle

Consists of synapse b/w motor neuron and muscle fiber

ACH

Acetycholine

Skeletal muscle contraction (Excitation contaction coupling)

1. AP starts

2. Ap arrives at axon terminal, releases ACH

3. ACH crosses synapse, binds to ACH receptors on plasmalemma

4. AP travels down plamalemma, T tubulues

5. Triggers CA release from SR

6. Ca enables actin myosin contraction

Role of CA in Muscle contraction

Ap arrives at SR from T tubule

• Causes mass release of CA into sarcoplasm

Ca binds to troponin on thin filament

• Troponin- Ca complex moves tropomyosin

Myosin binds to actin, contraction occurs

Sliding filament theory:

Process of actin myosin contraction

Contracted state

Myosin head pulls actin toward sarcomere center(Power stroke)

Filaments slide past each other

Sarcomeres, myofibril, muscle fiber all shortens

After power stroke ends: Myosin detaches friom active site

Myosin head rotates back to original position

Myosin attached to another active site farther down

Process continues until

Z disk reaches myosin filaments or AP stops

Energy for muscle contration

ATP

Binds to myosin head ( ATP > ADP + P+ energy)

Muscle relaxation

Ap ends, electrical stimulus of SR stops

Ca pumped back into SR

Muscle fiber types

Type I - 50% of all fibers in an average muscle

Peak tension in 110ms (Slow twitch)

Type II - Peak tension in 50 ms (fast twitch)

Type IIa

Type IIx

Type IIc

Slow twitch (Type I)

High aerobic (Oxidative) capacity and fatigue resistance

Low anaerbic (glycolytic) capactiy and motor unit strength

Slow contractile speed and myosin ATPase

Low SR development

Fast twitch (TypeII)

Moderate (Oxidative) capactiy and fatigue resistance

High anaerobic( GLycolytic) capacity and motor unit strength

Fast contractile speed and myosin ATPase

High SR development

FAst twitch ( Type IIx)

Low aerobic (oxidative) capactiy and fatigue resistance

High anaerobic (glycolytic) capactiy and motor unit strength

FAst contractile speed and myosin ATPase

High SR development

SIngle muscle fiber physiology

Peak power is different b.w muscle fiber types

All fiber types tend to reach their peak power at 20% peak force

Type I fibers during exercise

High aerobic endurance

Can maintain exercise for prolonged periods