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Chapter 1: Structure and Function

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Joseph White

on 16 October 2014

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Transcript of Chapter 1: Structure and Function

Overview
Functional Anatomy of Skeletal Muscle
- Muscle Fibers
- Myofibrils
- Muscle Fiber Contraction

Skeletal Muscle and Exercise
- Muscle Fiber Types
- Muscle Fiber Recruitment
- Use of Muscles

Chapter 1: Structure and Function
of Exercising Muscle

Involuntary
- Cardiac

- Smooth
Voluntary
- Skeletal
Plasmalemma (cell membrane)
- Fuses with tendon
- Conducts action potential
- Maintains pH, transports nutrients
- True cell boundary
Satellite cells
- Located between the plasmalemma
and the basement membrane
- Muscle growth, development
- Response to injury,
immobilization, training
Sarcolemma
- Composed of the plasmalemma and
the basement membrane
Single Muscle Fiber (Muscle Cell)
10 - 120 micrometers
Sarcoplasm
- Cytoplasm of muscle cell
- Contains:
- ATP, CP
- glycogen
- fat
- minerals
- soluble proteins and enzymes
- Myoglobin - oxygen-binding compound
Transverse Tubules
- Interconnected extensions of the
plasmalemma that extend laterally
through the muscle
-

Allow AP's to transmit rapidly to
individual myofibrils
Sarcoplasmic Reticulum
- Longitudinal network of fibers that run
parallel to the muscle fibers
- Storage site for Calcium
Myofibrils
- Each muscle fiber (cell) contains several
hundred to several thousand myofibrils

- Runs the entire length of the fiber

- Made up of basic contractile elements called
sarcomeres

- ~ 90% of cell volume is myofibrillar, remaining
10% is SR and mitochondrial volume
Basement Membrane
- Loose collection of glycoproteins and
collagen network
- Freely permeable
What happened?
Myosin
- Thick Filament: 2/3 of all protein in
skeletal muscle
- 9 Isoforms (slow and fast twitch)
* provide different energy transduction
kinetics and crossbridge turnover rates
Thin Filaments
Actin
- Composed of 'beads' of G-actin
that polymerize into F-actin
- Contains a myosin binding site
Tropomyosin
- Regulatory protein
- Covers actin's myosin binding
sites under resting conditions
Troponin
- TnC, TnT, TnI
Nebulin
- Anchors actin
Titin
- Extends half a sarcomere
- Template for sarcomere assembly
- Contains elastic elements that
contribute to passive force of myofibrils
Alpha Motor Neuron
- Nerve cell that connects with
and innervates many muscle
fibers
Motor Unit
- Consists of an alpha motor
neuron and all the muscle
fibers it directly innervates
Neuromuscular Junction
- synapse between the alpha
motor neuron and a muscle
fiber

- communication between
nervous system and muscle
Excitation - Contraction Coupling
- Process of events that triggers a muscle
fiber to contract

- Initiated by an action potential (nerve impulse)
from the brain or spinal cord to an alpha motor
neuron
E-C Coupling Sequence
1. Action Potential (AP) starts in brain

2. AP arrives at axon terminal, releases
neurotransmitter acetylcholine (ACh)

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

4. AP travels down plasmalemma, T-tubules

5. Triggers Calcium release from sarcoplasmic
reticulum (SR)

5. Calcium enables actin-myosin contraction
- Filament is composed of ~ 200
myosin molecules
Sarcomere
- Basic functional unit of a
myofibril


- Each myofibril is composed
of numerous sarcomeres
joined end to end at each
Z-disc
Sliding Filament Theory
Single Muscle Fiber Physiology -->
* Peak force generally occurs at ~20% of peak force
Muscle Relaxation
- Crossbridge activation is a function of
intracellular Ca++ concentration

- After contraction Ca++ is pumped back into the
SR by an ATP - dependent process
- Ca++ remains in SR until a new AP arrives
(Vo) single-fiber contractile velocity
Muscle Fiber Determination
- Mainly genetically determined, changing
little from childhood to middle age
- Alpha motor neuron innervation determines
fiber type
- More sarcomeres in series
= greater change in length
- More sarcomeres in parallel
= more force
- Transition model

I <-> IIA <-> IIX <-> IIB
- Exercise training can shift to more
oxidative fibers
- Inactivity (bed rest) and aging can shift
more to IIX/B
Skeletal Muscle Composition
- 75% water

- 5% inorganic salts and substrates

- 20% mixed proteins

- 12% myofibrillar
- 8% enzymes, membrane proteins,
transport channels, and other proteins
Muscle Fiber Types
Types of Contraction
- Muscles shorten, remain at the same length
or lengthen during contraction, depending on
the external load placed on the muscle relative
to the amount of force developed by the fibers
Why is the energy cost of concentric
contractions higher?
1. Elastic Components of muscle
2. Myosin - Actin bonds are broken
by the higher force development,
wherein concentric contractions
ATP is solely responsible for
breaking the bonds
-->
More ATP use!
Series Elastic Component
- Most important for assisting
force development
Parallel Elastic Component
- Gives structure and protection
to the muscle
Length - Tension Relationship
Lo = Optimal length
Contractile Properties of Isolated Muscles
- Pt = peak twitch tension

- TPT = time to peak tension

- 1/2 RT = half relaxation time
Fast vs. slow twitch defined by:
Twitch speed determined by:
- Myosin ATPase activity (MHC isoform)

- SR content
~ = weak binding

circle = strong binding

f = force
Fast - I.R. = internal rectus (eye muscle)

Intermediate - G = gastrocnemius

Slow - S = soleus
Fast Twitch Fibers
- High ATPase activity on MHC

- More developed SR
- Shorter TPT & 1/2RT

Isotonic Contraction (Force Constant, Length Changes)
- Greater loads produce slower speeds
but greater tension

- Strength training can increase maximum
isometric tension but not maximal
unloaded velocity

- A stronger muscle can move a given
(same absolute) isotonic load at a greater
velocity
Size Principle
- The frequency of motor unit use, called
recruitment, is directly related to the size
and ease of triggering an AP in the soma
- Fiber recruitment is usually determined not
by the speed of a movement but rather by
the force necessary to perform the
movement
Rate Coding
- Describes the process by which the tension of a
given motor unit can vary from that of a twitch to
that of tetanus by increasing the frequency of
stimulation of that motor unit
Mitochondria
- ATP production by oxidative
phosphorylation
- Muscle fibers are multinucleated


- 100 - 200 nuclei per mm of length


- Likely that gene expression programs
of myonuclei vary along fiber

Rate of Activation vs. Relaxation
Rate of Activation dependent on:
1. Rate of Calcium release

2. Binding of Calcium to troponin C

3. Rate of crossbridge cycling
Rate of Relaxation
1. Release of Calcium from troponin c

2. Rate of Calcium uptake by SERCA
Sarcoplasmic endoplasmic reticulum calcium ATPase (SERCA)
Type 1 = black
Type IIa = white
Type IIx = gray
* Specific Tension is the same across fiber type

Specific Tension
= amount of force relative to
the physiological cross-sectional area
Muscle Tone
- A partial state of contracture
Full transcript