Physiology of Muscle Contractions

Gross Anatomy of Muscles

• Entire Muscle (covered by epimysium)

• Bundles of fascicles (covered by perimysium)

• Fascicles are composed by muscle fibres (covered by endomysium)

• Muscle fibres can be broken down into myofibrils 

• Myofibrils are composed of sarcomeres

• Sarcomeres are made up of actin (thin) and myosin (thick) filaments

What are The types of Muscle contractions

Isometric Contraction: muscle tension without a change in muscle length 

Isotonic Contraction: muscle tension with a change in muscle length, can be either:

• Concentric: Muscle shortens while contracting

Eccentric: Muscle lengthens while contracting

Key stuff about muscle contraction

• Muscles always pull during a contraction, they never push 

• A muscle will shorten and move an object if the load is light

• A muscle will stay the same length if the load is greater than the muscle strength

How does a muscle contract

In order for a muscle to contract a signal is sent from your brain to the muscle through the motor unit

What is resting membrane potential

• The sodium-potassium pump moves 3 sodium ions out of the cell and 2 potassium ions into the cell

• Establishes and maintains a concentration gradient, with higher sodium outside and higher potassium inside the cell

• Resting membrane potential, -70 mV

What are facilitating Action Potentials

• Depolarization: When a neuron receives a signal, voltage-gated sodium channels open, allowing sodium ions to rush into the cell, causing depolarization

• Repolarization: As the membrane potential reaches a certain threshold, voltage-gated potassium channels open, allowing potassium ions to flow out of the cell, and sodium channels become inactivated, leading to repolarization 

Restoration of Gradients: Sodium-potassium pump restores the ion gradients by pumping sodium out and potassium back in

What is the Sliding Filament theory

Actin & Myosin filaments slide past each other, so that the two types of filaments overlap to a greater degree

What are Myosin Corssbridges

• Myosin Crossbridges - small bridges that extend from myosin filaments to actin filaments

• These “crossbridges” are actually the heads of the myosin filaments (they look like mini golf clubs)

• Over & over again, these crossbridges attach, rotate, detach, and reattach in rapid succession 

• Process results in sliding (or overlapping) of thin & thick filaments (to a greater degree), which shortens the sarcomere

• As this process occurs simultaneously in sarcomeres throughout the fibre, the muscle cell shortens

Contracting Sarcomere

Where does Energy from Contractions come from

• Release of calcium ions is critical “trigger” in this complex process of muscle contraction (calcium facilitates the interaction of myosin and actin molecules)

• Energy source behind the release of calcium (so muscle can contract) is ATP

ATP also used to detach myosin from actin molecule (so muscle can relax)

As work of muscle increases, more and more ATP is used up and must be replaced through food metabolism (eating and digesting food)

Atp use to Relax muscle

What is Excitation-Contraction Coupling

1. Nerve impulse transmitted from nervous system, through neuromuscular junction, to muscle fibre

2. Electrical signal (impulse from nerve) transmitted by chemical means at the neuromuscular junction

3. Here, the nerve terminal releases acetylcholine across synapse to muscle fibre4.  Transverse Tubulae System transmits signal down into muscle fibre

   5. Change takes place in electrical properties of tubulae

   6. This change causes a rapid release of calcium ions (energy is provided by ATP), which sets off a series of other chemical reactions that lead to shortening/contraction of muscle fibre

What are Calcium Ions

• Release of calcium ions is critical “trigger” in this complex process of muscle contraction 

• On actin filaments there are troponin and tropomyosin molecules (proteins)

• These proteins “inhibit” or regulate the interaction of actin and myosin filaments

• If calcium is not present, then actin and myosin do not interact

• When calcium is present it interacts with troponin, tropomyosin then moves to not be blocking actin binding sites, thereby allowing actin and myosin to interact and muscle to contract

Calcium’s Interaction on Troponin and Tropomyosin

to allow for muscle contraction