Skeletal Muscle Physiology Part II

What are the steps of muscle contraction?

1. a motor neuron releases ach, which binds to receptors in the motor end plate

2. this causes an AP to travel down the T-tubules

3. when the AP reaches the SR, it releases Ca2+

4. Ca2+ binds to troponin, which moves tropomyosin off binding sites

5. cross bridges form and contraction occurs

What are the steps of muscle relaxation?

1. AP ceases

2. Ca2+ is actively pumped back into the SR

3. AChE degrades all ach in the synaptic cleft

4. troponin relaxes, and tropomyosin covers actin sites again

What are the three different muscle fiber types?

• Type I

• Type IIa

• Type IIx

Type I muscles

• aka slow twitch (SO)

• strengths: resistant to fatigue and can be used for endurance activities

• weaknesses: contract slowly and do not produce a lot of force

• appear dark in color due to increased capillary density

Type IIa muscles

• aka fast twitch oxidative-glycolytic (FOG)

• strengths: contract fast, can be used for longer activities, and semi-fatigue resistant

Type IIx muscles

• aka fast twitch glycolytic (FG)

• strengths: contract the fastest and produce the most amount of force

• weaknesses: fatigue easily and can only be used for short events

• appear white in color due to low blood flow

What muscle type has the largest motor neuron? Why does this make sense?

• Type IIx muscles have the largest motor neuron

• the larger the motor neuron = more muscle fibers recruited = more force produced

What muscle type has the highest mitochondrial density? Why does this make sense?

• Type I muscles have the highest mitochondrial density.

• the more mitochondria = the more ATP that can be produced oxidatively = the longer a moderate exercise can be maintained

What muscle type has the highest capillary density? Why does this make sense?

• Type I muscles have the highest capillary density.

• A greater capillary density = more oxygen delivered during exercise = supports aerobic energy pathways

What is the major fuel source of Type I, Type IIa, and Type IIx muscles?

• Type I: triglycerides

• Type IIa: PCr and glycogen

• Type IIx: PCr and glycogen

Fiber types CANNOT change, but…

• type II fibers can become more aerobic with endurance training

• and type I fibers can become more anaerobic with resistance training

True or False: Type I and Type II fibers can be found in the same skeletal muscle. They are often spread out, not organized by type.

true

True or False: Everyone has the same ratio of skeletal muscle fibers in their muscles.

false, each person has different ratios

Synapse

• a junction between two neurons or a neuron and a muscle

• facilitates the transmission of nerve impulses

Neuromuscular junction

• the site of communication between a neuron and a muscle

• consists of a synapse between an alpha-motor neuron and muscle fiber

What does an alpha-motor neuron consist of?

a single motor neuron and all the muscle fibers it innervates

The more motor units recruited = ____ the contractile force

the higher

How can we increase force production?

• by increasing the frequency (or how often) a muscle is stimulated to contract

• by increasing motor unit recruitment

• by making sure muscles are at an optimal length for contraction

Twitch

the smallest contractile response of a muscle fiber to a single stimulus

Summation

• a series of twitches that occur one after the other

• results in an increase in force production, b/c each twitch builds off the previous one

Tetanus

peak force of a muscle fiber (or motor unit) as a result of repeated APs

How does increasing motor unit recruitment result in greater force production?

recruiting more motor units that are bigger in size = greater force

Size Principle of motor unit recruitment:

the minimum number of motor units are recruited based on exercise intensity (recruited from small to large)

What is the order that motor units are recruited in?

• smallest (Type I) motor units are recruited first

• midsized (Type IIa) motor units are recruited second

• large (Type IIx) motor units are recruited last

What does this graph tell us?

• Type I muscles contract at the lowest stimulation (20% of contraction needs)

• Type IIa muscles contract at a moderate stimulation (40% of contraction needs)

• Type IIx muscles contract at the highest stimulation (70% contraction needs)

Length-tension relationship

• suggests that muscles have an optimal overlap length of actin and myosin for force production

• too little or too much overlap results in a decrease in force production

Functions of titan

• scaffolding: provides structural support for the sarcomere

• bidirectional molecular spring: prevents overstretching of myofibrils and provide recoil

• passive force enhancement: as muscle length stretches in passive muscle, titan contracts

• possible residual force enhancement: increases isometric force following an eccentric phase

• modulator of contraction: assists with active contraction

Active tension

• the force produced by a muscle during contraction

• there is an optimal overlap length of actin and myosin —> that allows for maximal force production.

Passive tension

• the force generated by a muscle when it is stretched

• is activated when muscle length is stretched 60% or higher

• caused by titan

Total tension

= active tension + passive tension

What are the takeaways from this graph?

• there is an optimal overlap length for muscles with small and large PCSAs that allow them to produce maximal force

• muscles with larger PCSAs produce more force / tension

What are the takeaways from this graph?

• there is an optimal muscle length for tension

• longer fibers can produce tension over a greater range of lengths

• shorter fibers can produce tension over a shorter range of lengths

• tension decreases as we move away from optimal muscle length

Eccentric contractions

• the muscle lengthens while producing force

• ex: lowering weight on a bicep curl / squat

• 
  

Concentric contractions

• a muscle shortens while producing force

• ex: lifting weight during a bicep curl

Do eccentric or concentric muscle contractions produce the most force? Why?

eccentric muscle contractions produce the most force, because titan adds passive tension = greater total tension and force produced

What is the relationship between eccentric contraction force and velocity?

eccentric contractions produce maximal force at high velocities

What is the relationship between concentric contraction force and velocity?

concentric contractions produce maximal force at lower velocities

Why does force decrease as velocity increases during concentric contractions?

b/c some slow twitch fibers fall behind and can’t contribute to high velocity movements

All fiber types (Type I, IIa, IIx) contribute to slower velocity movements =

more force produced

What is the relationship between force, velocity, and PCSA?

• the higher the PCSA = the more fibers that can contribute to force production = greater force at slower velocities

• the smaller the PCSA = fewer fibers available for force production = lower force at slower velocities

What is the relationship between force, velocity, and fiber length?

given that PCSA is constant, longer muscle fibers can produce more force at higher shortening velocities

On average, muscle strength differs between men and women. Why?

men have more muscle mass

No sex strength disparity exists when muscle is expressed per unit of…

CSA

How does strength change in children?

the older children get, the more muscle mass they have —> and the stronger they become

At what age does a woman have peak muscle mass?

around 20 years

At what age does a man have peak muscle mass?

around 25-30 years

__, __, and __ require myelination.

• strength

• power

• skill

Why are children often uncoordinated and not-very strong?

b/c their neurons are still developing myelin, so signals sent to muscle may be slow or imprecise —> leading to slow rxn times and uncoordinated movements

Strength and neuromuscular function ___ with age, especially around 50-60 years old.

decrease

Decreases in muscle mass and strength as one ages can be combatted by ____.

resistance training

Strength trained men at 80 yrs. have the strength of an untrained ___ year old.

30

True or False: Both untrained and resistance trained adults lose strength as they age.

True, but untrained men lose strength at a faster rate

Resistance training benefits

• increased bone density

• more muscle mass

• less fat