5L skeletal muscles and adaptiation to exercise training Flashcards
What is skeletal muscle?
40-50% of total body mass
major site of metabolic activity
largest protein reservoir
What are the 3 main functions of skeletal muscle?
1) locomotor activity
2) postural behaviour
3) breathing
4) other
What is the structure of skeletal muscle?
muscle consists a number of muscle fibres lying parallel to one another and held together by CT
What is a muscle fibre? (myofibre)
a single skeletal muscle cell
multinucleated
large, elongated, and cylindrically shaped
fibres can extend entire length of muscles
What is myofibril?
- contractile elements of muscle fibre
regular arrangement of thick and thin filaments (myosin and actin) - alternating dark (the A bands) and light bands (I bands) giving appearance of striations
What is the difference between a myofibre and myofibril?
Myofibers are the muscle cells itself.
Myofibrils are long protein cords in the sarcoplasm
NWhat is a nerve?
a enclosed cable-like bundle of nerve fibres or axons
provides a common pathway for the electrochemical nerve impulses transmitted along each fo the axons
What is the strength of contraction determined by?
number of fibre stimulated
frequency of stimulation
what is a motor unit?
a motor neuron (nerve cell) and all of the fibres that it innervates
all fibres are of the SAME fibre type
group of fibres activated via the same neuron
all muscle fibres of one particular motor unit are always the Same fibre type
precise (fine) movements
consists of a large number of motor units and few muscle fibres
less precise movements (gross)
carried out by muscles composed of fewer motor units with many fibres per unit
What is intra-muscle cooordination?
how much do you need?
mainly highly trained power athletes are able to activate up to 85% of their available muscle fibres simultaneously
(untrained)
trained athletes have not only a large = muscle mass than untrained individuals, but can also exploit a large number of muscle fibres
trained individuals can further increase strength only by increasing muscle diameter
What is summation?
two successive ap if the twitch tension resulting form the first AP does not declines to zero before second twitch arrives, the twitch forces summate
What is tetanus?
a frequncy of APs - a rise in the contractile force of a single muscle (fibre) until force reaches a plateau called the tetanic tension. This is the maximum tension that can be produced by a fibre.
Slow twitch Fibres
swuited for repeated contractiosn durign activities requireing a force output of < 20-25% of max force output
- smaller motor units
first activated
examples: lower power activities, endurance events
Muscle fibres Type:
the greater the slow twitch fibre content of a muscle…
the lower the force producing capactiy
the slower the contraction speed
the greater the endurance characteristics of the muscle
*based on typical size
Fast twitch fibres
significant greater “force” and speed generating capability than slow twitch fibres (size and speed)
well suited for activities involved high power
activated after ST
larger motor units
Muscle Fibre type:
the greater the fast twitch fibre content of a muscle..
1) greater the force output capacity
2) the greater the overall speed of contraction
3) the greater the fatigability will be when the muscle has been maximally activated
* based on typical size
Properties of type 1 muscle fibres
red
slow oxidative (SO)
type I myosin
contractile: slow
properities of type 2B
White
fast glycolytic
fastest contractile
structural aspects of SO
muscle fibre diameter: dmall
mito density: High
capillary density: high
myoglibin content: high
structural aspects of FG
Muscle fibre diameter: large
mito density: low
capillary density: low
myglobin content: low
functional aspects of SO
twitch time: slow
relaxation time: slow
force production: low
fatigability: resistant
Functional aspects of FG
twitch time: fast
relaxation time: fast
force production: high
fatigability: most fatigable
metabolic aspects of SO
pc stores: low glycogen stores: low TG stores: high myosin ATPase activity: low glycolytic enzyme activity: low oxidative enzyme activity: high
metabolic aspects of FG
pc stores: high glycogen stores: high TG stores: low myosin ATPase activity: high glycolytic enzyme activity: high oxidative enzyme activity: low
Testosterone
directly effects muscle growth by binding to receptors on the surface of muscle cells and amplifying the biochemical signals in muscle tissue that result in protein synthesis.
overall increase in muscular size, strength and recovery from exercise.
more muscle but does not change the fibre type
thyroid hormone
key hormoen wrt fibre type
increases mitos; mosin heavy change switch (type one decresases type 2 increases
hyperthyroid
atrophy and muscle weakness
can exercise training change muscles fibre types in humans?
it depends upon how one defines fibre types
increased neuromuscular activity/overloadinging
endurance exercise
repeated stimulation stretch
move from fast to slow
easier
decrease neuromuscular activity, unloading
detraining, immobilization, microgravity
harder to do but more inconveniences
moving from slow to fast
What is muscular strength?
the ability of a muscle or muclr group to exert focrce against a resitance
commonly measured as a maximal value
force and strength are synonymous
factors influencing the force of muscle contractions:
- the individuals state of health
- the individuals training status
- joint angles
- muscle cross-sectional area
- speed of movement (myosin heavy chain composition)
- muscle fibre type (influences forces)
- age
- sex
Force-velocity curve
at any given velocity the trained can apply more forces
against any given load the trained can move (an object) faster
How do we get stronger?
myostatin, alpha actinin3
neural - motor unit recruitment
hypertrophy - PS pathways, testosterone, IGH - 1 insulin
Genes that define “skeletal muscle phenotype”
include:
AMPK
Calcineurin
Peroxisome proliferator - activated receptor gamma
(PPAR gamma) coactivator 1 alpha(PGC-1alpha)
Type of contractions
in response to physiological demands, intracellular calcium concentration is elevated, activating
calcineurin/nuclear factor of activated T cells (NFAT_ and MEF2/HDAC signaling pathways.
In response to workload, ATP is depleted activated AMPK
AMPK
sustained depression of energy change
[atp] decrease < – > {ADP] increase [AMP] increases –> AMPK –> mitochondiral biogensesis, Glut 4, HK (increased oxidative metabolism, not MHC)
has many roles
Calcineurin
a ca2+/calmodulin activate phosphatase
- implicated in enrve activity - dependent fibre type specification in skeletal muscle
controls the phopshyrlation state of transcription factor NFAT
alow for its translocation to the nucelus
- leading to the activation of slow type msucle proteins in cooperation with myocyte enhancer factor 2 (MEF2)
causes a change from fast to slow muscles
PGC -1 aplha
transcriptional co-activator PGC-1 alpha drives the formation of slow-twitch muscle fibres
regulates the genes involved in energy metabolism
regulator of mitochondrial biogenesis and function
activated in endurance exercise in human skeletal muschle
major factor that regulated muscle fibre type determination
serves as a target for calcineurin singling
PGC-1alpha + nuclear receptor PPAR-gamma
permits the interaction of this protein with multiple trnasciption factors
Neural response
the first measurable effect is an increase in the neural drive stimulating muscle contraction.
within just a few days, an untrained individual can achieve measurable strength gains (without hypertrophy) resulting from learning to use the muscle
increased strength with NO hypertrophy
development of maximal strength through increased intra-muscular coordination (the abiltiy to recruit more motor units)
Hypertrophy
is the increase of the size of an organ or in a select area of the tissue
how you get bigger =
rate of protein synthesis vs. rate of protein degradation
MPS = muscle protein synthesis
factional synthetic rate
protein accretion
PS = PD = Mass
PS > PD increase mass
increase PS = increase mass
decrease degration = increase mass
hormonal responses
controls myofibre size and maturation by idnirect and direct actions
does not directly affect the proliferation and differentiation of hte cells that make the muscle in the first place
testoersterone
insulin
growth hormone (IGF-1)
insulin and myofibres
myofibres have more insulin receptors tahn type 1 IGF receptors, thus insulin is nexessary for myofiber hypertrophy. (nexessary to store protein in muscle)
resting muscle and insulin
the resting muscle needs insulin to increase in size, thus following a meal, when insulin is secreted, excess nutrients can be stored in muscle. If insulin is low, protein stores are mobilized (protein degradation increases)
myofibres and contracting muscle
contracting msucles DO NOT need insulin for glucose uptake; thus nutrietns in the cirulation can enter muscle to supply energy for contraction without eating a meal and getting the subsequent rise in circuatling insulin
insulin
eating keeps insulin high and therefore keeps protein synthesis high
insulin and GH
in response to IGF, the AKt/mTOR signalling pathway is activated. Activation of mTOR by AKT promotes protein synthesis and increases muscle mass –> hypertrophy
Foxo is an inhibiting pathway
hypertophy (presynthetic/anabolic) pathways
GH - IGF signalling
AKT - mTOR pathway
P13k - Akt-GSK3beta pathway
ca2+
Growth hormone (IGF-1)
stimualtes igf-1
gorwht factor with a configuration which closely resembles insulin –> increase protein synthesis
produced in the liver and skeletal muscles by GH
many of the actions fo the GH are through IGF-1
p70s6 kinase
targets s6 ribosomal protein –> phosphoraltyion –> protein synthesis in the ribosome –> increase type II area after training
training for strength
low reps with high loads
training for endurance
high reps low loads
repeated muscle contraction
PGC-1 activates that does not activate fox O therefore causes protein degration inhibition and then pgf also icnrease slow fibres in the mito –> aerobic cpaacity increases
local IGF-2 inhibits fox O and activates AKT–> MTOR which increase protein synthess _->icnrease fibre size
Satelitte cells
a distinct sub-population of myoblasts that fail to differentiate and remain associated with the surface of the developing fibre as quiescent “muscle cells”
are precursors to primary myoblasts
muscle stem cells
surrounded by basal lamina
join existing fibre or replace a damged cell
types of muscle contraction: static
isometic strength training
limitations:
- strengethens muscle at a specific pt in ROm
time consuming
Tpyes of muscle contraction: dynamic
plyometric training
uses explosive humps to mobilize the stretch-recoil properties of msucle
ballistic resistance
Muscle soreness and stiffness
delyed onset msucel soresness (DOMS)
lengthening contractiosn produce muscle soreness
hyperplasia
increase the nubmers of cells
Hyperplasia or hypertrophy
mostly hypertrophy
physiological effects of strength training
increased muscle mass and size of muscle fibres
increased utilization and coordination of motor units
incrased strength of tendons, ligaments and bones
increased storage of fuel in and blood supply to muscles
improvmenets in blood - fat levels and biochemical processes.
