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muscular sytem

a muscle is composed of many cells.

  • muscle: group of muscle cells with the same origin, insertion, and function.

  • fascicles: muscle cells wrapped with connective tissues (fascia).

muscle fibres (muscle cells):

  • long muscle cells.

  • vary in length from mm to 30cm.

  • multinucleate.

  • packed with myofibrils, which are long cylindrical structure that contain proteins actin and myosin.

the muscle contractile unit is the sarcomere.

  • arrangement of filaments give rise to striated appearance of skeletal muscle.

  • z lines: attachment points for sarcomere.

  • a sarcomere is a segment of myofibrils from one z line to another.

  • sarcomere: contractile unit

    • myosin: forms thick filaments.

    • actin: forms thin filaments.

muscle contraction:

  • myosin and actin drive contraction.

  • myosin has heads that pull on actin.

  • ATP powers myosin movement.

  • actin forms a twisted scaffold.

  • myosin binds on actin when stimulated.

  • sarcomere shortens, muscle contracts.

isotonic contractions: muscle shortens, while maintaining a constant force. movement occurs.

isometric contractions: force is generated, muscle doesn’t shorten. no movement.

slow twitch fibres:

  • contract slowly.

  • make ATP as needed by aerobic metabolism.

  • many mitochondria.

  • well-supplied blood vessels.

  • stores very little glycogen.

  • “red” muscle

  • used for endurance activities.

fast twitch fibres:

  • contract quickly.

  • rapidly break down ATP

  • fewer mitochondria

  • little to no blood vessel.

  • store a lot of glycogen

  • “white” muscle.

  • capable of anaerobic metabolism

  • used for brief high-intensity activities,

why rations differ:

  • genetics: your genetic make up determines the primary ration of slow twitch and fast twitch fibres.

  • muscle location: specific muscles influence the fibre ratios.

  • training: physical activity can change the characteristics and effectiveness of muscle fibres.

cardiac muscle:

  • sarcomere arrangement of thick and thin filaments.

  • striated appearance.

  • cardiac muscles are joined by intercalated disks

    • have gap junctions allowing cells to electrically stimulate the next one.

  • pacemaker cells dictate the rate of contractions of the whole heart.

smooth muscle:

  • filaments arranged in criss-crossed bundles, not sarcomeres.

  • no striations

  • smooth muscles joined by gap junctions, allowing cells to activate each other.

cardiac and smooth muscle cells respond to stimulation from the autonomic nervous system, which can modify the degree of muscles.

diseases and disorders:

  • muscular dystrophy:

    • modified dystrophin proteins enable leakage of calcium into cells.

    • extra calcium activates enzyme that destroy muscle proteins.

    • muscle weakening and wasting.

    • muscle mass is replaced with fibrous connective tissue.

    • life expectancy: approx. 30 years.

  • muscle cramps: often caused by dehydration and ion imbalance.

  • pulled muscles: result from overstretching of a muscle, fibre tears apart.