Skeletal muscles

Question 1

Describe the 3 types of muscles

Answer 1

• Smooth muscle + cardiac muscle contract without conscious control
• Smooth muscle found in walls of internal organs (stomach, intestines, blood vessels)
• Cardiac muscle only found in the heart
• Skeletal/striated muscles are attached to skeleton by tendons + are used to move bone. Under conscious control. Pairs of muscle contract + relax to move bones at a joint - antagonistic pairs. Contracting muscle is agonist + relaxing muscle is antagonist

Question 2

Structure of skeletal muscle fibres

Answer 2

• Made up of large bundles of long cells called muscle fibres
  Cytoplasm- Sarcoplasm
  Cell membrane- Sarcolemma which folds inward in places to spread electrical impulses through sarcoplasm
  Mitochondria- To carry out respiration + provide ATP needed for muscle contraction
  Have many nuclei (multinucleate)
  Sarcoplasmic reticulum - stores and releases calcium ions needed for muscle contraction
  Myofibrils- long cylindrical organelles which are highly specialised + made of 2 proteins: myosin + actin which allow the muscle to contract

Question 3

Ultrastructure of a myofibril

Answer 3

• Each myofibril contains bundles of thick (myosin) + thin (actin) filaments which slide past each other to make muscles contract
• Made up of repeating short units called sarcomeres
• End of each sarcomere are marked with a dark Z line within the light I band. I band is light as it contains actin filaments only
• Dark A band found in middle + is dark as it contains both actin + myosin filaments. In centre of A band is a lighter stripe known as H zone
• M line is middle of myosin filaments. It is in the centre of the H zone

Question 4

Slow twitch muscle fibres

Answer 4

Slow twitch:
- Darker in colour as high concentrations of myoglobin (red proteins that store O2)
- Contract slowly, slow to fatigue- used for endurance (long distance running ie. marathon runner)
- Energy is released slowly from aerobic respiration ( getting constant supply of O2)
- Lots of mitochondria to provide ATP + lots of blood vessels to reduce diffusion pathway for O2 CO2

Question 5

Fast twitch muscle fibres

Answer 5

• Lighter in colour - contain less myoglobin. Lots of glycogen which can be broken down to release glucose - need a lot as anaerobic respiration releases little ATP
• Contract quickly, fatigue quickly -used for short bursts of speed + power (sprinting)
  -Energy is released quickly through anaerobic respiration
• Few mitochondria + blood vessels

Question 6

Structure of neuromuscular junction

Answer 6

• Neuromuscular junction is the synapse between a motor neurone + cell membrane of muscle fibre (sarcolemma)
• Signal passing across synapse causes muscle fibre to contract
• Work in same way as a cholinergic synapse
• Action potential triggers calcium ions to diffuse into presynaptic neurone + causes the release of ACh by exocytosis into synaptic gap
• ACh binds to receptors on sarcolemma causing Na+ ion channels to open, depolarising the membrane
• If theshold potential is reached, an action potential is initiated in muscle fibre
• Impulse travels along sarcolemma down T-tubules which allow transmission of action potential into sarcoplasm
• This depolarisation causes Ca+ ions to be released leading to contraction of muscle fibres

Question 7

Compare cholinergic synapse + neuromuscular junction

Answer 7

• Cholinergic synapse is between neurones, neuromuscular junction between motor neurones + muscles
• Cholinergic can be excitatory+ inhibitory but neuromuscular junction is excitatory only
• In cholinergic action potential is triggered in postsynaptic neurone, in neuromuscular junction, action potential is triggered in sarcolemma + travels down T tubules
• ACh bind to receptors on postsynaptic membrane but in neuromuscular junction, ACh binds to receptors on sarcolemma