Muscular System Notes

I. The Muscular System

  • Muscles are responsible for all types of body movement

    • they contract or shorten and are the machine of the body

  • Three basic muscle types are found in the body

    • Skeletal muscle

    • Cardiac muscle

    • Smooth muscle

II. Function of Muscles

  1. Support the body

  2. Allow for movement by making bones and other body parts move

  3. Maintain constant body temperature

  4. Assist in movement of cardiovascular veins and lymph

  5. Protect internal organs and stabilizes joints

III. Anatomy of a Skeletal Muscle

A. Organization of Skeletal Muscle

B. Coverings of Skeletal Muscle

  • Skeletal muscles are organs

    • They contain muscle fibers, nerves and blood vessels

    • Connective tissue membranes separate each muscle structure

  • Fascia - layer of fibrous tissue that separates muscles from each other and from the skin

  • Coverings from largest to smallest:

    • Epimysium - covers the entire skeletal muscle

    • Perimysium - surrounds a bundle of muscle fibers (fascicle)

    • Endomysium - surrounds a single muscle fiber

C. Skeletal Muscle Attachments

  • Epimysium blends into a connective tissue attachment, the tendon

    • Tendon - cord-like structure

  • Sites of muscle attachment

    • Bones, Cartilages, and connective tissue coverings

A. Microscopic Anatomy of Muscle Fiber (muscle cell)

  • Cells are multinucleate

    • Nuclei are just beneath the membrane

  • Sarcolemma - specialized plasma membrane

  • Sarcoplasmic reticulum - specialized smooth endoplasmic reticulum involved with muscle contraction

  • Myofibril

    • Bundles of myofilaments

    • Myofibrils are aligned to give distinct bands

      • Light band = I band

      • Dark band = A band

  • Sarcomere

    • Contractile unit of a muscle fiber

  • Organization of a sarcomere:

    • Thick filaments = myosin protein

    • Thin filaments = actin protein

    • Myosin and actin overlap somewhat in the sarcomere

  • Myosin filaments have heads (extensions) that can “grab” onto actin forming a crossbridge

I. Physiology of Muscle Contraction

  • Skeletal muscles must be stimulated by a nerve (motor neuron) to contract

A. Transmission of Nerve Impulse to Muscle

Step 1: Nerve releases a neurotransmitter (acetycholine)

Step 2: Neurotransmitter causes muscle cell membrane gates to open

Step 3: Ions (Na+ and K+) exchange places causing the sarcoplasmic reticulum to release Ca2+

Step 4: This release of Ca2+ starts the muscle contraction as the actin filaments slide past the myosin filaments

B. The Sliding Filament Theory of Muscle Contraction

  • Sliding Filament Model - a muscle contracts when the thin filament in the muscle fiber slides over the thick filament

  • Activated by ATP and calcium (Ca2+) ions

Step 1: An influx of Ca2+ causes thick myosin filaments to form crossbridges with the thin actin filament by exposing the binding site on actin.

Step 2: The crossbridges change shape as it pulls on filaments which slides towards the center of the sarcomere in the power stroke

  • The distance between the Z line decreases, shortening the muscle

Step 3: The crossbridges detach from the actin filament when ATP bonds to myosin head

Step 4: The myosin head gets ready to bond to actin again using ATP energy

  • The cycle is repeated on another site of the actin filament

C. Contraction of a Skeletal Muscle

  • Muscle fiber contraction is “all or none”

  • Within a skeletal muscle, not all fibers may be stimulated during the same interval

  • Different combinations of muscle fiber contractions may give differing responses

  • Graded responses - different degrees of skeletal muscle shortening

  • Rapid stimulus = constant contraction or tetanus

D. Muscle Response to Strong Stimuli

  • Muscle force depends upon the number of fibers stimulated

    • More fibers contracting results in greater muscle tension

  • Muscle can continue to contract unless they run out of ATP or Ca2+

    • One molecule of ATP supplies enough energy for one actin and myosin cross-bridge

II. Energy for Muscle Contraction

  • Muscles use stored ATP for energy

    • Bonds of ATP are broken to release energy

    • Only 4-6 seconds worth of ATP is stored by muscles

Three ways for muscle to make energy (ATP)

  1. Creatine Phosphate

  • Creatine phosphate is a high-energy compound and is the fastest way to make ATP available for muscles

  • Used for activities lasting less than 15 seconds

  • Anaerobic (no oxygen needed)

  • Reaction:

    • Creatine phosphate + ADP → creatine + ATP

  • Creatine phosphate is made when a muscle is at rest

  1. Cellular Respiration

  • Mitochondria uses glucose molecules to make ATP in the presence of oxygen

    • Provides most of a muscle’s ATP

  • Aerobic (needs oxygen)

  • Used for activities lasting hours

  • Reaction;

  • C6H12O6 + 6O2 → 6CO2 + 6H20 + energy

  • 1 glucose = 36 ATP

  1. Anaerobic Respiration/Fermentation

  • Reaction that breaks down glucose without using oxygen

  • Used for activities lasting 30-60 seconds

  • Anaerobic (no oxygen needed)

  • Reaction:

  • Glucose → pyruvic acid + 2 ATP → lactic acid

    • Lactic acid is also produced causing pain in the muscle

  • Heavy breathing after exercise is a sign of oxygen deficiency

  • A marathon runner is exhausted after crossing the finish line because they have depleted not only their oxygen but their glucose as well

  • It takes up to two days to replace all of the glucose in the muscles and glycogen in the liver