Structure of skeletal muscle
Muscles = effector organs - respond to nervous stimulation > contract > bring about movement.
Cardiac muscle - found exclusively in heart. / Smooth muscle - found in walls of blood vessels and the gut. / Skeletal muscle - attached to bone and acts under voluntary , conscious control.
Individual muscles made up of millions of tiny muscle fibres called myofibrils > lined up parallel to each other in order to give maximum force.
Muscles could not be made up of individual cells as the junctions between cells would act as a point of weakness - reducing the overall strength.
Separate cells have fused together into muscle fibres - share nuclei and cytoplasm called sarcoplasm - mostly found around the circumference of the fibre. Also contains a large concentration of mitochondria and ER.
Microscopic structure of skeletal muscle
Myofibrils made up two types of protein filament...
Actin - thinner, consists of two strands twisted around one another.
Myosin - thicker, consists of long rod-shaped fibres with bulbous heads that project to the side.
Myofibrils appear striped as there are light-coloured bands which are isotropic bands (I-bands) and darker bands which are anisotropic bands (A-bands). I-bands appear lighter because the actin and myosin filaments do not overlap in this region and A-bands appear darker because the actin and myosin filaments do overlap here. At the centre of each A-band is a lighter region called the H-zone. At the centre of each I-band is a line called the Z-line. Sarcomere = distance between adjacent Z-lines.
Muscle contracts > Sarcomere shortens & pattern of light and dark vands changes.
Tropomyosin - forms a fibrous strand around the actin filament.
Troponin - globular protein involved in muscle contraction.
Slow-twitch muscle fibres
Contract more slowly and provide less powerful contractions over a longer period. Adapted to endurance work. More common in calf muscles - must contract constantly to keep body upright. Suited to this role by being adapated to aerobic respiration in order to avoid a build-up of lactic acid - would cause them to be less effective.
> Large store of myoglobin - bright red molecule that stores oxygen
> Rich supply of blood vessels - deliver oxygen and glucose
> Numerous mitochondria to produce ATP
Fast-twitch muscle fibres
Contract more rapidly and produce more powerful contractions but only for a short period. Adapted to intense exercise and are more common in muscles that need to do short bursts of intense activity - biceps.
> Thicker and more numerous myosin filaments
> High concentration of enzymes involved in anaerobic respiration
> Store of phosphocreatine - molecule that can rapidly generate ATP from ADP in anaerobic conditions - provide energy for muscle contraction.
> Supply of glycogen - source of metabolic energy
Point where a motor neurone meets a skeletal muscle fibre. (synapse that occurs between a neurone and a muscle).
There are many neuromuscular junctions spread throughout the muscle - ensures contraction is rapid and powerful when it is simultaneously stimulated by action potentials. All muscle fibres supplied by a single motor neurone act together as a single functional unit = motor unit. If slight force is needed > few motor units stimulated. Greater force required > larger number of units stimulated.
Nerve impulse received at neuromuscular junction > synaptic vesicles fuse with the presynaptic membrane and release their acetylcholine. Acetylcholine diffuses to postsynaptic memebrane > alters its permeability to sodium ions > sodium ions enter rapidly > membrane depolarised.
Acetylcholine broken down to ensure muscle is not over-stimulated. Resulting choline and ethanoic acid are recombined back in the presynaptic neurone using energy provided by mitochondria.
Evidence for sliding filament mechanism
Would expect to see more overlap of actin and myosin in a contracted muscle than a relaxed one. When a muscle contracts, the following happens to the Sarcomere...
> I-band becomes narrower
> Z-lines move closer together (sarcomere shortens)
> H-zone becomes narrower
> The A-band remains the same width (because determined by length of myosin filaments)
Myosin is made up of: > Fibrous protein arranged into a filament made up of several hundred molecules (tail). > Globular protein formed into two bulbous structures at one end (head).
Actin - globular protein whose molecules are arranged into long chains that are twisted around one another to form a helical strand.
Tropomyosin - forms long thin threads that are wound around actin filaments.
Sliding filament mechanism and muscle stimulation
The bulbous heads of the myosin filaments form cross-bridges with the actin filaments. They attach themselves to binding sites on actin filaments > flexing in unison > pulling actin filaments along the myosin filaments > become detached > Use ATP to return to their original angle and re-attach themselves further along the actin filaments. Repeated up to 100 times a second.
1. AP reaches many neuromuscular junctions simultaneously, causing calcium ion channels to open and calcium ions move into the synaptic knob.
2. Calcium ions cause synaptic vesicles to fuse with the presynaptic membrane and release their acetylcholine into the synaptic cleft.
3. Acetylcholine diffuses across the synaptic cleft and binds with receptors on the postsynaptic membrane, causing it to depolarise.
4. AP travels deep into the fibre through a system of T-tubules that branch througout sarcoplasm.
5. Tubules are in contact with the sarcoplasmic reticulum which has actively absorbed calcium ions from the cytoplasm of the muscle.
6. The AP opens the calcium ion channels on the ER and calcium ions flood into the muscle cytoplasm down a diffusion gradient.
7. The calcium ions cause the tropmyosin molecules that were blocking the binding sites on the actin filament to pull away.
8. The ADP molecule attached to the myosin heads means they are now in a state to bind to the actin filament and form a cross-bridge.
9. Once attached to the actin filament, the myosin heads change their angle, pulling the actin filament along as they do so and releasing a molecule of ADP.
10. An ATP molecule attaches to each myosin head, causing it to detach from the actin filament.
11. Calcium ions activate the enzyme ATPase, which hydrolyses ATP to ADP > provides energy for myosin head to return to its original position.
12. Myosin head reattaches itself further along actin filament - cycle repeated.
Muscle Relaxation and Energy supply
Nervous stimulation ceases > calcium ions actively transported back into ER using energy from hydrolysis of ATP > Reabsorption of calcium ions allows tropomyosin to block the actin filament again > Myosin heads now unable to bind to actin filaments and contraction ceases.
Energy supplied by the hydrolysis of ATP to ADP plus an inorganic phosphate. Energy released needed for...
> Movement of myosin heads > Reabsorption of calcium ions into the ER by active transport
Most ATP is regenerated from ADP during respiration of pyruvate in the mitochondria. Plentiful mitochondria in muscles. Takes time for blood supply to replenish oxygen levels, so phosphocreatine used to rapidly generate ATP anaerobically. Stored in muscle and acts as a reserve supply of phosphate > available to combine with ADP > re-form ATP. Phosphocreatine supply replenished using phosphate from ATP when the muscle is relaxed.