Muscles Flashcards
how many skeletal muscles’ do humans have
~660 skeletal muscles
muscle mass in adults, for female and male
females 13-30 kg;
males 18-40 kg
what is the average amount and length and diameter of muscle fibres
~100,000 fibres
D: 10 -100 micrometre
L: 1-500 mm (Harris et al 2005)
how does muscle determine identity
through voice
what is the skeletal muscle a source of and what process does it help
source of amino acids, homeostasis…
different muscle fibres shape: pectoralis major
convergent
different muscle fibres shape: orbicularis oris
circular
different muscle fibres shape: deltoid
multipennate
different muscle fibres shape: sartorius
parallel
different muscle fibres shape: biceps brachii
fusiform
different muscle fibres shape: extensor digitorum longus
unipennate
different muscle fibres shape: rectus femoris
bipennate
role on connective tissue
maintains integrity of
the muscle and is important in
transmission of force
roles of tendons
Muscles connect to bones by tendons
role of epimysium
Muscles are enclosed by epimysium
(connective tissue)
what is a muscle fascicles and perimysium
Groups of fibres form muscle fascicles
surrounded by perimysium
what is a endomysium
Each muscle fibre is enclosed by
endomysium which consists of extracellular matrix (made of collagen)
how do Structural intracellular proteins of muscle
fibre link to endomysium and why are they important
Structural intracellular proteins of muscle
fibre link to the endomysium via focal
adhesions (important for force
transmission)
what are sarcolemma
Muscle fibres (10-100
μm) are covered by
sarcolemma (≈7.5 nm
thick
what are t tubules
T-tubules are invaginations
of sarcolemma
what is Sarcoplasmic reticulum
is a network of tubes
surrounding myofibrils
what is Sarcoplasm
is fluid
enclosed within the fibre
what are myofibrils
( ~1 μm) are
packed bundles of
myofilaments running along
the fibre
name 2 types of myofilaments
thick (darker) and thin (lighter) filaments
what do thick filaments consist of
C protein, H
protein, M protein, myomesin
what do thin filaments consist of
actin, tropomyosin and
troponin complex (TN-T,
TN-I, TN-C
how are myofilaments arranged within myofibrils
a series of
repeating units, the
sarcomere, which is basic
contractile unit of muscle
what happens in the sarcomere unit
One end of thin filaments projects into the sarcomere while another connects the Z line
* Thin filaments contain troponin and tropomyosin proteins that participate in blocking
and unblocking of thin filaments. Because of this function troponin and tropomyosin are
referred to as regulatory proteins.
* Thick filaments (myosin) are in the centre of sarcomere and overlap thin filaments from
both sides
what 6 proteins is the myosin molecule compose of
- two myosin heavy chains (MyHC)
- one essential myosin light chain (MLC) on each MyHC
- one regulatory myosin light chain (MLC) on each MyHC
what is MyHC role
MyHC contains ATP- and actin-binding sites (essential for muscle contraction)
what is the tole of titin
acts as a molecular spring permitting return of
stretched sarcomeres to “optimal” length
3 different contraction types
concentric, isometric, eccentric
what happens during concentric contractions
(also on a force-velocity relationship )
muscle
shortens and
generates force
(least amount of force generated, longest velocity)
what happens during isometric contractions
(also on a force-velocity relationship )
muscle
generates force,
length remains
constant (middle for both velocity and force)
what happens during eccentric contractions
(also on a force-velocity relationship )
muscle
generates force while
lengthening (most amount of force generated, shorter velocity)
how long is the delay between action potential and initiation of contraction
~20 ms delay
how long does it take until the peak force to develop
30ms
example of sarcomeres in
series add up
-10-cm fibre contains ~4,000,000
sarcomeres
– Each sarcomere shortening by 1 μm
shortens the fibre to 6 cm
describe the steps of the cross bridge cycle step 1
- ATP hydrolysis
increases affinity of
myosin for actin
describe the steps of the cross bridge cycle step 2
- Following Ca++ release from SR myosin binds actin
Dissociation of Pi results in
5-10 nm displacement
(power stroke) followed
by ~2pN force (strong
binding)
ADP dissociation ends the strong binding phase yet cross-bridge remains
attached
describe the steps of the cross bridge cycle step 3
- ATP binds to myosin
decreasing its affinity
for actin
describe the steps of the cross bridge cycle step 4
- Myosin dissociates
from actin, ATP is
hydrolysed
what part of the cycle contributes to concentric contractions
“power stroke”
cross-bridges, smaller pool of contribution
what part of the cycle contributes to eccentric contractions
all associated cross- bridges contribute
- larger pool of contributions
what is a hallmark of msucle fatigue
Slowing relaxation is one of the
hallmarks of muscle fatigue
following repeated contractions
what is the cause of slowing relaxation of muscles
a decrease in concentration ATP
leading to impaired dissociation of
these metabolites from the myosin
describe Force-velocity relationship
speed of contraction decreases
with increasing resistance
state all the connective tissue
endomysium, perimysium, epimysium, tendons
what is the role of connective tissue
Muscle contractile force is transmitted through connective
tissue
role of one alpha motor neuron
innervates a group of
muscle fibres
- divides into terminals that form neuromuscular junctions on muscle fibres
what do nerve fibres do
(axons) conduct
action potentials (APs) at a fast
rate (~40-80 m/s)
where do terminals of motor nerves form from
neuromuscular junctions
(also called motor end plates)
what is the key functions of neuromuscular junctions
is transmission of activation
from nerve to muscle fibres
what does the arrival of AP do at a nerve terminal : step 1
triggers release of acetylcholine (Ach)
into the synaptic cleft
what does the arrival of AP do at a nerve terminal : step 2
- Ach is made from acetyl-CoA released
from mitochondria and choline in nerve
terminals
what does the arrival of AP do at a nerve terminal : step 3
- Released Ach binds to receptors on the
muscle fibre and triggers influx of Na+
followed by generation of action potential
(AP) in muscle fibres
what does the arrival of AP do at a nerve terminal : step 4
- Ach acts for a short time since it is
degraded by Ach esterase enzyme
* Choline is taken up by nerve terminals
for re-synthesis of Ach
what happens when there is low levels of Ach
Myasthenia gravis
Eyelids are dropped due to muscle
weakness
* Injection of a drug which blocks Ach
esterase helps to regain muscle
strength: eyelids open
what causes Myasthenia gravis and what is used in operations to help
autoimmune disease due to low levels of Ach or Ach receptors
Drugs blocking neuromuscular transmission are muscle relaxants used during
operations
what is Neuromuscular electrical stimulation (NMES) used for
functional and therapeutic applications in subjects
with spinal cord injury or stroke
what does contraction force depend on and what happens to the force-freq relationship
frequency of action
potentials
change in
frequency of AP results in
proportional change of
force
what freq does the muscle fibres activate
at 5-100 Hz
frequencies in voluntary
contractions
what is a tetanic contraction and how is it generated
Continuous stimulation at
100 Hz causes a sustained
increase in intracellular
Ca2+ - [Ca2+]I and generates
tetanic contractions
(smooth contractions)
what does a decrease in [Ca2+] mean
results in lower contractile force – one of the mechanisms of muscle fatigue
how are muscle fibres divided into
subdivided in
two main types: type
I and type II
–
how is type II muscle fibres divided
the type II is further
divided into IIA & IIX
– rodent muscle also
contain large numbers of type IIB fibres
what is not a homogenous tissue
skeletal muscles
what are the 2 main factors separating the fibre types
1 – isoform of expressed myosin
2 – metabolic profile (mitochondrial content, capacity of glycolytic enzymes)
how does being encoded by different genes affect myosin isoforms
Different functional properties:
ATPase activity: Type I < Type IIA < Type IIX
Function: fibre shortening velocity Type I < Type IIA < Type IIX
what the difference between type I and II muscles fibres
type 1 has more mitochondria
less glycolytic enzymes
…
what the difference between type I and IIA/IIX muscles fibres
mitochondria
type I >IIA >IIX
glycolytic enzymes
type I < IIA < IIX
functional role of fibre types
Muscle composed of different fibre types differ in contractile properties
Slower contraction and relaxation in type I fibre dominated muscle
what determines the speed of AP
how quickly the calcium can go from sarcoplasmic reticulum out of sarcoplasm
how are MyHcs encoded in mammals and in humans
MyHCs are encoded by the same set of homologous genes
* However, not all MyHCs are expressed in human
muscles
No type 2B MyHC is expressed in human locomotory muscles
what are the Factors affecting proportion of fibre types
genetic factors, adaptation to physical activity or lack of it
on average how much muscle fibres does vastus lateralis consists of
~50% of type I fibres
(but there is a broad range of variation ~20% to >80% of type I fibres)
what are the consequences of variation of fibre type composition
Prevalence of certain fibre type favours some athletic activities
increasing proportion of type II fibres is associated with prevalence to weight gain and cardiovascular risk
which muscle fibres favours endurance events and which for speed/power events
type I – endurance events,
type II – speed and power events
