Muscles Flashcards
What are the differences between the 3 types of muscle?
• Skeletal muscle (striated, voluntary)
This is always attached to the skeleton, and is under voluntary control via the motor neurones of the somatic nervous system.
It can be subdivided into red (slow) muscle and white (fast) muscle. The striated appearance is due to the overlap of thick (myosin) and thin (actin filaments).
• Cardiac Muscle
This is special type of red skeletal muscle. It looks and works much like skeletal muscle, but is not attached to skeleton, and is not under voluntary control.
• Smooth Muscle
This is found in internal body organs such as the wall of the gut, the uterus, blood arteries and arterioles.
It is under involuntary control via the autonomic nervous system or hormones.
What is a myofibril and how is it made up?
A myofibril is made of repeating dark and light bands. There is a Z line in the middle of the light band and an M line in the middle of the dark band. Under very high resolution the myofibrils are shown to be made of filaments, there are two types, thick (myosin) and thin (actin)
Is Myosin thick or thin? Tell me a bit about it!
Myosin: Thick filament. Each molecule has a tail with two heads at the end. The tails meet at the M line of the A band. The myosin heads contains ATPase, an enzyme complex that hydrolyses ATP to ADP + Pi.
Is Actin thick or thin? Which other 2 proteins is it associated with?
Actin: thin filament. Chains of globular proteins. 2 chains are twisted round each other. At point of twist, myosin head binding site is located.
Actin is associated with two other proteins
1) Troponin: attached at regular intervals along the chain, can bind calcium ions
2) Tropomyosin: lies in the grooves between the actin chains, blocking the myosin binding site
Tell me about toponin and tropomyosin!
Tropomyosin blocks the myosin binding site on the actin filament when the muscle is at rest. When calcium is released it binds to troponin and this causes the tropomyosin to shift position exposing the binding site for myosin.
Tell me about the I band! (I’ve heard they are good!!)
I-Band/Light band/Isotropic band: light passes through easily.
Contains only actin filaments
Dark Z line running down it, where proteins anchor the actin filaments
Distance between Z lines = a sarcomere
Tell me about the A band!
A-band/Dark band/Anisotropic band:
Actin and myosin overlap
Darker M line down the centre, proteins anchor filaments
What is the H band and where do we find it?
H-band, region in A band that is less dense as no overlap with actin here, only myosin present.
What is the sliding filament theory and explain how it works.
Sarcomeres get shorter when the muscle contracts, so the whole muscle gets shorter. But the dark band, which represents the thick filament, does not change in length. This shows that the filaments don’t contract themselves, but instead they must slide past each other. This is the sliding filament theory
Muscle contraction requires ATP; this can be sourced form a variety of places. Tell me about them!
1) ATP stores in the muscle last for around 2-3 seconds of activity, produced by aerobic respiration
2) Regeneration of ATP from Creatine Phosphate/PC (without respiration). The hydrolysis of PC releases inorganic phosphate and energy which can be used to phosphorylate ADP to ATP. This is a coupled reaction as they occur together
PC gives P + C + energy
ADP + P + energy gives ATP
3) From anaerobic glycolysis. This produces lactic acid and some ATP. The build-up of lactic acid means that this process is short lived. The possible reasons for this are…..
The change in pH (more acidic) affects the enzymes involved in glycolysis
The increases in acidity affect the binding of calcium to troponin.
Explain what happens at the neuromuscular junction.
- Nerve impulse depolarises the presynaptic membrane
- Calcium channels opened and calcium ions enter the presynaptic membrane
- Synaptic vesicles move towards and fuse with, the presynaptic membrane;
- Release of transmitter substance (Ach skeletal muscles or noradrenaline effcetors associated with the sympathetic nervous system) into synaptic cleft
- Diffusion of transmitter substance across cleft
- Attachment to receptor sites on intrinsic protein molecules on post synaptic membrane causes
- Ion gated sodium channels to open and sodium ion influx down concentration gradient
- Causing depolarisation of post synaptic membrane/sarcolemma;
Tell me about fast twitch fibres!
White/fast twitch: (principally releases energy by anaerobic respiration)
Speed of response is more important than sustained contraction
Provide rapid and powerful contractions: hydrolyses ATP rapidly (ATP from CP/ATP or Glycolysis)
Fewer mitochondria
Low density of capillaries
Low myoglobin concentration
Extensive sarcoplasmic reticulum: rapid release and uptake of calcium
More myosin filaments
High concentration of anaerobic enzymes
CP store
Higher concentrations of ATPase than slow twitch so that ATP can be hydrolysed rapidly.
Fatigue quickly and accumulate lactic acid
Tell me about slow twitch fibres
Red/slow twitch/tonic muscle fibres (principally releases energy by aerobic respiration)
Slower, less powerful sustained contractions (endurance muscles)
Many mitochondria (release energy)
Dense capillary network (providing oxygen and glucose)
High myoglobin (oxygen holding component of muscle)
Glycogen store
Small diameter giving short diffusion pathway
High concentration of enzymes involved in aerobic respiration
High resistance to fatigue
Explain the advantage of having both fast and slow twitch fibres
Fast fibres make immediate contraction possible before the blood supply adjusts
Most energy anaerobically generated;
fast fibres used in explosive locomotion;
slow fibres allow sustained contraction
Slow fibres used in maintaining posture/endurance events
Respire / release energy aerobically;
Or too much lactate would accumulate;
Slow twitch fibres adapted to aerobic metabolism;
As have many mitochondria; Site of Krebs’ cycle; And electron transport chain; Much ATP formed; Also are resistant to fatigue.
Describe the role of ATP and Calcium ions in the sliding filament theory
Calcium ions bind to troponin;
Remove blocking action of tropomyosin and exposes actin binding sites;
Calcium activates myosin ATPase and ATP is hydrolysed allowing myosin to form a cross-bridge
ATP is required to break the cross bridge and to remove calcium form the sarcoplasm into the sarcoplasmic reticulum (active transport)
