Chapter 8 - Muscle Structure & Force Regulation

Skeletal Muscle

• fasicle (bundle of muscle fibres)

• striated apperance

Muscle Fibers Contain

same organelles as other cells

• mitochondria

• lysosomes

Muscle Fibers Are

multinucleated

Muscle Fibers

• thin, elongated cylinder

• extends length of muscle

• thoudands per muscle

Satellite Cells Help

provide nuclei to muscle fiber

Microfibrils Contain

contractile proteins

• myofilaments

• actin and myosin

• sarcomere

• 85% of fibre content

Sarcomere

functional unit of the entire structure

Satellite Cells

• undifferentiated cells

• involved in muscle growth and repair

• training activates them initiating a divide

Increase Number of Myonuclei

enhances fibers ability to hypertrophy

• myonuclear domain

• each nucleus performs integral functions

In Order To Grow Muscle Tissue

more nuclei are required

T-Tubule System

• trigger calcium release from the SR

• carry eletrical signals into the muscle fibers

Neuromuscular Junction (NMJ)

• AP comes from nervous system

• no physical contact with muscle (synaptic cleft)

What Happens at Neuromuscular Junctions

• AP stimulates release of ACh

• ACh travels across the cleft to open Na+ channels

• generates an end-plate potential

• AP then travels along muscle fibre membrane down T-tubules

Sliding Filament Theory

• shortening of sarcomere

• actin slides over myosin

Sarcomere Shortening Strongest When

you have the most overlap of actin and myosin

Isometic Action

• “between”

• no movement, no sliding

• no advantags of ECC

• no disadvantages of CON

• muscle contracts but does not shorten

Concentric Action

• “weakest”

• muscle produces force while shortening

Concentric Action Steps

ATP required to shorten

• taking up SEC (series elastic component)

• cross bridge “braking effect”

• sliding filament effect (actin slides in same direction as power stroke)

• must overcome load

Eccentric Action

• “strongest”

• muscle produces force while lengthening

Ecentric Action Steps

• “braking effect” helps ECC

• “passive” resistance to stretch (CB stretch while bound to actin → resistive force is added to CB)

• you are just “resting” the load

Motor Units

• 1 MU can innervate: 10 - 2000 muscle fibers

• average MU innervates: 600 muscle fibers

• # of MU innervating a muscle: 100 - 2000

Force Regulation

1. types and number of motor units recruited

2. initial muslce length

3. nature of the neural stimulation of motor units

4. contractile history of the muscle

Types and Number of MU Recruitment

• more motor units = greater force

• fast motor units = greater force

Initial Muscle Length

• “ideal” length for force generation

• increased cross bridge formation

Nature of the Neural Stimulation of Motor Units

frequency of stimulation

• simple twitch

• summation: indiviual muscle twitches add together → increased muscle force

• tetanus: max force a muscle unit can produce

Muscle Twitch

• latent period: before AP reaches muscle to generate force

• contraction: muscle develops force and shortens

• relaxation: muscle relaxes and returns to original length

Size Principal (Henneman’s Principle)

MUs are activated in a specific order

• recruitment is dependent on size of the MN

• smallest to largest

Orderly Recruitment

• 1st recruited are also last de-recruited

• last recruited is 1st de-recruited

Length-Tension Relationship

• less than optimal legnth → weaker

• optimal length → stronger

• more than optimal length → weaker

Muscle Fibre Recruitment

never actually recruit 100% of available muscle fibres

• prevents damage to muscles and tendons

If We Maximally Recruit We Would

likely tear the muscle of the tendon

Contractile History of the Muscle

1. if preceding activity is fatiguing

• subsequent force production is reduced

2. if preceding activity is non-fatiguing

• subsequent force production is enhanced

Post Activation Potentiation - Why?

• phosphorylation of myosin light chains

• more sensitive to Ca2+

• more CBs for a given amount of Ca2+

Force-Velocity Relationship

• as contraction velocity increase, muscle force decreases

• high speed movements → decreased muslce force

• low speed movements → increased muscle force

Power-Velocity Relationship

• power increases as movement velocity increases

• low velocity → low power

• very high velocity → force decreases, limiting power output