Locomotion and Movement 2 Flashcards
explain the structure of actin protein
Each actin (thin) filament is made of two ‘F’ (filamentous) actins
helically wound to each other. Each ‘F’ actin is a polymer of monomeric
‘G’ (Globular) actins. Two filaments of another protein, tropomyosin
also run close to the ‘F’ actins throughout its length. A complex protein
Troponin is distributed at regular intervals on the tropomyosin. In the
resting state a subunit of troponin masks the active binding sites for
myosin on the actin filaments
explain the structure of myosin protein
Each myosin (thick) filament is also a polymerised protein. Many
monomeric proteins called Meromyosins constitute one
thick filament. Each meromyosin has two important parts, a globular
head with a short arm and a tail, the former being called the heavy
meromyosin (HMM) and the latter, the light meromyosin (LMM). The HMM
component, i.e.; the head and short arm projects outwards at regular
distance and angle from each other from the surface of a polymerised myosin
filament and is known as cross arm. The globular head is an active ATPase
enzyme and has binding sites for ATP and active sites for actin.
state sliding filament theory
Mechanism of muscle contraction is best explained by the sliding filament
theory which states that contraction of a muscle fibre takes place by the
sliding of the thin filaments over the thick filaments.
what is responsible for initiating the muscle contraction?
(or)
what stimulates muslce contraction
Muscle contraction is initiated by a signal sent by the central nervous
system (CNS) via a motor neuron. A motor neuron alongwith the muscle
fibres connected to it constitute a motor unit. The junction between a
motor neuron and the sarcolemma of the muscle fibre is called the
neuromuscular junction or motor-end plate. A neural signal reaching
this junction releases a neurotransmitter (Acetyl choline) which generates
an action potential in the sarcolemma, when the Ach( acetyl choline) binds to the Ach receptors present in the sarcolemma.
what happens once Action potential is generated?
This spreads through the muscle
fibre and causes the release of calcium ions into the sarcoplasm. Increase
in Ca++ level leads to the binding of calcium with a subunit of troponin on
actin filaments and thereby remove the masking of active sites for myosin.
ATP Hydrolysis takes place in the globular head of the myosin protein due to ATPase enzyme. Utilising the energy from ATP hydrolysis, the myosin head now binds to
the exposed active sites on actin to form a cross bridge. This stage is called cross bridge formation
what is a cross bridge.
The cross-bridge formed from the binding of actin with myosin.
what causes the muscle to contract
The myosin is charged with energy. It uses energy to pulls the Actin filaments towards the centre of A band. This causes the actin filaments to slide over the myosin filament. This is called as sliding phase/ rotation phas.e
what happens to the different components of sarcomere during the sliding phase
The
‘Z’ line attached to these actins are also pulled inwards thereby causing a
shortening of the sarcomere, i.e., contraction. It is clear from the above
steps, that during shortening of the muscle, i.e., contraction, the ‘I’ bands
get reduced, whereas the ‘A’ bands retain the length. The H zone disappears.
what causes the muscle to relax
The
myosin, releasing the ADP and P1
goes back to its relaxed state. A new
ATP binds and the cross-bridge is broken. The ATP is again
hydrolysed by the myosin head and the cycle of cross bridge formation and breakage is repeated causing further sliding. The process continues
till the Ca++ ions are pumped back to the sarcoplasmic cisternae resulting
in the masking of actin filaments. This causes the return of ‘Z’ lines back
to their original position, i.e., relaxation.
the muscle goes into the relaxed state when the action potentialis terminated, as a result of which ca2+ ions return back to the sarcoplasmic reticulum resulting in the decrease of calcium ion concentration, leaving no ca2+ for the troponin to bind with hence closing the myosin bidning sites.
what causes fatigue in muscles
The reaction time of the fibres
can vary in different muscles. Repeated activation of the muscles can lead
to the accumulation of lactic acid due to anaerobic breakdown of glycogen
in them, causing fatigue. M
what are red muscle fibres
Muscle contains a red coloured oxygen storing
pigment called myoglobin. Myoglobin content is high in some of the
muscles which gives a reddish appearance. Such muscles are called the
Red fibres. These muscles also contain plenty of mitochondria which can
utilise the large amount of oxygen stored in them for ATP production.They have less amount of sarcoplasmic reticulum.
These muscles, therefore, can also be called aerobic muscles. It shows sustained contractions
describe white muscles
On the
other hand, some of the muscles possess very less quantity of myoglobin
and therefore, appear pale or whitish. These are the White fibres. Number
of mitochondria are also few in them, but the amount of sarcoplasmic
reticulum is high. They depend on anaerobic process for energy. It is used for rapid, short interval contraction
