Muscle Contraction Flashcards
What are the 3 types of muscles?
Cardiac, smooth and skeletal
Where is smooth muscle found?
The walls of blood vessels and the gut.
Where is skeletal muscle found?
Bulk of body muscles in vertebrates, attached to the bone acts under voluntary conscious control.
Myofibrils
Tiny muscle fibres which build together to form individual muscles
Muscle structure
Separate cells fused together into muscle fibres -> share nuclei and sarcoplasm
Where is sarcoplasm largely found?
Around the circumference of the fibre
Sarcoplasm consists largely of
Mitochondria and E.R.
What two types of protein filament are Myofibrils made of?
- Actin, thinner and consists of 2 strands twisted around one another
- Myosin, thicker and consists of long rod-shaped fibres with bulbous heads that project to the side
Light bands
Isotropic bands -> appear lighter because actin and myosin filaments don’t overlap in this region
Dark bands
Anisotropic bands -> actin and myosin filaments overlap in this region
What is at the centre of each anisotropic band?
A lighter-coloured region called the H-zone
What is at the centre of each isotropic band?
A line called the Z-line
Sacromere
The distance between adjacent Z-lines. When a muscle contracts, sacromeres shorten and the pattern of light and dark bands changes.
What are two other important proteins found in muscle?
- Tropomyosin, forms a fibrous strand around actin filament
2. Troponin, a globular protein involved in muscle contraction
Slow-twitch fibres
Contract more slowly and provide less powerful contractions over a longer period of time
What are the adaptations of slow-twitch fibres?
- A large store of myoglobin
- A supply of glycogen to provide a source of metabolic energy
- A rich supply of blood vessels to deliver oxygen and glucose
- Numerous mitochondria -> atp
Fast-twitch fibres
Contract more rapidly and produce powerful contractions but only for a short period
What are the adaptations of fast-twitch fibres?
- Thicker and more numerous myosin filaments
- High conc. of enzymes involved in anaerobic respiration
- A store of phosphocreatine, a molecule that can rapidly generate ATP from ADP in anaerobic conditions and so provide energy for muscle contraction
Neuromuscular junction
Where a motor neurone meets a skeletal muscle fibre
Why are there many neuromuscular junctions?
Ensures that contraction of a muscle is rapid and powerful when it’s simultaneously stimulated by action potentials.
Motor unit
All muscle fibres supplied by a single motor neurone act together as a single functional unit
Benefits of the motor unit
Arrangement gives control over the force that the muscle exerts i.e. if only slight force is needed, only a few units are stimulated
What ensures that the muscle is not-over stimulated?
Acetylcholinesterase breaks down acetylcholine into choline and ethanoic acid which diffuse back into the neurone.
What happens to a sarcomere when a muscle contracts?
The I-bands become narrower, Z-lines closer together (sarcomere shortens), H-zone becomes narrower
Composition of Myosin
- A fibrous protein arranged into a filament made up of several hundred molecules
- A globular protein formed into two bulbous structures at one nd
Actin
A globular protein whose molecules are arranged into long chains that are twisted around one another to form a helical strand
Tropomyosin
Long tin threads that are wound around actin filmanets
Sliding filament mechanism of muscle contraction
Actin and myosin filaments slide past one another during muscle contraction
Outline Muscle stimulation
- An a.p. reaches many neuromuscular junctions simultaneously, causing Ca+ channels to open -> Ca+ into synaptic knob
- Ca+ causes the synaptic vesicles to fuse with the presynaptic membrane and release acetylcholine into the synaptic cleft
- Acetylcholine diffuses across the synaptic cleft and binds with receptors on the postsynaptic membrane, causing it to depolarise
Outline Muscle contraction
- A.p. travels deep into fibre through a system of T-tubules that branch throughout sarcoplasm
- Tubules in contact with sarcoplasmic reticulum which has actively absorbed Ca+ from sarcoplasm
- A.p. opens the Ca+ channels on the E.R. and Ca+ ions flood into the sarcoplasm down a diffusion gradient
- Ca+ cause the tropomyosin molecules that were blocking binding sites on the actin filament to pull away
- ADP attached to myosin heads means they are now in a state to bind to the actin filament and form a cross-bridge
- ATP attaches to each myosin head, causing it to detach from actin filament
- Ca+ active the enzyme ATPase, hydrolyses breakdown of ATP. -> provides energy for myosin head to return to its original position
- Myosin head + ADP -> reattaches itself further along actin filament
Outline muscle relaxtion
- When nervous stimulation ceases, Ca+ are a.t. back into E.R. using energy from hydrolysis of ATP
- Reabsorption of Ca+ allows tropomyosin to block the actin filament again
- Myosin heads become unable to bind to actin filaments and contraction ceases.
What is energy needed for in muscle contraction?
Movement of myosin heads, reabsorption of Ca+ into E.R. by active transport
How is phosphocreatine used for ATP?
Stored in muscle and acts as a reserve supply of phosphate, available immediately to combine with ADP and so re-form ATP.
