#121 Striated muscles

Striated muscles are muscles attached to the skeleton. They are neurogenic - they contract when stimulated to do so by impluses that arrive via motor neurones.

Structure of a striated muscle

- a muscle contains many muscle fibres

- muscle fibres are made up of specialized cells called syncytium

• Sarcolemma: cell surface membrane
• Sarcoplasm: cytoplasm // large numbers of mitochondria packed between myofibrils to perform aerobic respiration and produce ATP required for muscle contraction
• Sarcoplasmic reticulum (SR): cell surface has large numbers of protein pumps to transport Ca²⁺ into cisternae of SR
• T-tubules: deep infoldings into the interior of the muscle fibre

Myofibril

• Striations: stripes on a muscle fibre, produced by the regular arrangement of many myofibrils. Each myofibril is made up of parallel groups of thick filaments that lie between groups of thin filaments.

Structure of thick and thin filaments

Thick filaments: made of myosin – a fibrous protien with a globular head that points away from the M-line

Thin filaments:

• actin: globular protein. Many actin molecules link together to form a chain. 2 chains twisted together form the thin filament
• tropomyosin (fibrous protein) twisted around the 2 chains
• troponin: attached to actin chain at regular intervals

How muscles contract

1. Muscle contracts; Ca²⁺ released from stores in SR and binds to troponin

2. Troponin molecules change shape

3. Troponin and tropomyosin move to different positions on the thin filament to expose myosin-binding sites on the actin chain; Cross-links form between the thick and thin filaments

4. Myosin heads tilt and pull actin filaments towards the sarcomere centre

5. ATP hydrolysis forces heads to let go of actin

6. Heads spring back and the process repeats so long as:

• troponin and tropomyosin molecules don’t block the binding site
• muscles have a supply of ATP

Stimulating muscles to contract:

1. action potential arrives at presynaptic neurone

2. stimulates opening of voltage-gated channels for Ca²⁺ to diffuse into cytoplasm

3. Ca²⁺ cause vesicles containing acetylcholine (ACh) to move towards the presynaptic membrane

4. Vesicle fuses with the membrane, Ach is released and diffuses across the neuromuscular junction; Ach temporarily binds to receptor proteins on the sarcolemma; causes chemically-gated ion channels for Na⁺ to open

5. Na⁺ diffuse into the sarcolemma à depolarizes membrane à generates action potential that spreads along the membrane

6. Depolarization of sarcolemma spreads down to T-tubules

7. Channel proteins open: Ca²⁺ diffuses out of SR

8. Ca²⁺ binds to troponin. Tropomyosin moves to expose myosin-binding sites on actin filament; Myosin heads form cross-bridges with thin filaments à sarcomere shortens.

Providing ATP for muscle contraction

1. aerobic respiration in mitochondria

2. lactic fermentation in sarcoplasm

3. creatine phosphate: is stored in the sarcoplasm and acts as an immediate source of energy when ATP in the sarcoplasm runs out

- when the demand for energy is slowed down or stopped, ATP molecules recharge creatine

- when the demand for energy is high, but no ATP is spare to recycle creatine

Syllabus 2016-2018 15.1  Control and co-ordination in mammals The nervous system provides fast communication between receptors and effectors. Transmission between neurones takes place at synapses. a)   compare the nervous and endocrine systems as communication systems that  co-ordinate responses to changes in the internal and external environment  b)   describe the structure of a sensory neurone and a motor neurone c)   outline  the roles of sensory receptor cells in detecting stimuli and stimulating the transmission of nerve  impulses in sensory neurones (a suitable example is the chemoreceptor cell found in human taste buds) d)   describe the functions of sensory, relay and motor  neurones in a reflex arc e)   describe and explain the transmission of an action potential in a myelinated neurone and its initiation from a resting potential (the importance of sodium and potassium ions in impulse transmission should  be emphasised) f) explain the importance of the myelin sheath (saltatory conduction) in determining the speed of nerve  impulses and the refractory period in determining their frequency g)   describe the structure of a cholinergic synapse and explain how it functions, including the role of calcium  ions h)   outline  the roles of synapses in the nervous system in allowing transmission in one direction  and in allowing connections between one neurone and many  others (summation, facilitation and inhibitory synapses are not required) i) describe the roles of neuromuscular junctions, transverse system tubules and sarcoplasmic reticulum in stimulating contraction in striated muscle j) describe the ultrastructure of striated muscle with particular reference to sarcomere structure k)   explain the sliding filament  model  of muscular contraction including the roles of troponin,  tropomyosin, calcium  ions and ATP. l) explain the roles of the hormones FSH, LH, oestrogen and progesterone in controlling changes in the ovary and uterus during the human menstrual cycle m)  outline  the biological basis  of contraceptive pills containing oestrogen and/or progesterone