Skeletal muscle structureAQA A-Level Biology: Revision notes
Section 1
Antagonistic pairs and the skeleton
Skeletal muscles are attached to bones by tendons. A muscle can only pull when it contracts, so movement at a joint needs pairs of muscles acting in opposite directions: antagonistic pairs.
- Biceps contracts and triceps relaxes: elbow bends (flexion)
- Triceps contracts and biceps relaxes: elbow straightens (extension)
The skeleton is incompressible, so it acts as a rigid lever and fixed point against which muscles pull, transferring the force into movement.
A relaxing muscle does not push the bone back; the other muscle of the pair contracts and pulls.
Section 2
Gross structure of skeletal muscle
A whole skeletal muscle is made of bundles of muscle fibres held together by connective tissue and supplied with blood vessels and nerves. The muscle is attached to bones by tendons at each end.
Each muscle fibre is a single, very long cell that formed by the fusion of many cells, so it is multinucleate. Skeletal muscle is also called striated (striped) muscle because of the banding of its myofibrils.
Section 3
Microscopic structure of a muscle fibre
A muscle fibre has specialised parts:
- Sarcolemma: the cell surface membrane, which folds inwards as T-tubules so that depolarisation spreads quickly through the fibre
- Sarcoplasm: the cytoplasm, containing many mitochondria (ATP) and stored glycogen
- Sarcoplasmic reticulum: a specialised endoplasmic reticulum that stores and releases calcium ions
- Many parallel myofibrils, long cylinders of protein filaments that contract
The large number of mitochondria, myofibrils and nuclei adapt the fibre for powerful, repeated contraction.
Section 4
Ultrastructure of a myofibril
A myofibril is made of repeating units called sarcomeres, each between two Z lines.
- Actin: thin filaments, attached to the Z lines
- Myosin: thick filaments, in the centre of the sarcomere (held at the M line)
The pattern of bands under the electron microscope:
- I band: light, actin only
- A band: dark, the whole length of the myosin filaments (including overlap with actin)
- H zone: lighter region in the centre of the A band, myosin only
Example: sarcomere 2.4 µm, A band 1.6 µm, so I band material = 2.4 − 1.6 = 0.8 µm in total.
Section 5
Slow and fast skeletal muscle fibres
Slow (slow-twitch) fibres
- Location: muscles used for posture and endurance, e.g. back and calf muscles
- Many mitochondria, much myoglobin (red), rich capillary supply
- Aerobic respiration; contract slowly but resist fatigue
Fast (fast-twitch) fibres
- Location: muscles used for rapid, powerful movement, e.g. biceps, thigh and eyelid muscles
- Thicker myosin filaments, high concentration of glycolytic enzymes, large store of phosphocreatine
- Fewer mitochondria, less myoglobin (paler); contract rapidly and powerfully but fatigue quickly
Match structure to function: many mitochondria and myoglobin with endurance, thick myosin and phosphocreatine with power.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Skeletal muscle structure
- A physiotherapist is explaining arm movement to a patient who is recovering from an elbow injury. She uses the biceps muscle on the front of the upper arm and the triceps muscle on the back of the upper arm, both of which are attached by tendons to bones of the forearm and shoulder.The biceps and triceps are described as an antagonistic pair. Explain how they work together to bend and then straighten the elbow.2 marks
- In a relaxed sarcomere from a mammalian skeletal muscle, the distance from one Z line to the next is 2.4 µm. The dark A band is 1.6 µm long. In the centre of the A band is a lighter region, the H zone, which is 0.4 µm long.Explain why the A band appears darker than the I band in an electron micrograph.2 marks
- A physiologist takes muscle biopsies from the thigh of a marathon runner and of a sprinter. About 80% of the fibres in the runner's sample are slow fibres, compared with about 25% in the sprinter's sample. The sprinter's fast fibres have thicker myosin filaments, a large store of phosphocreatine, high concentrations of glycolytic enzymes and few mitochondria.Explain why a high proportion of slow fibres is an advantage in the leg muscles of a marathon runner.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).