BY: S T E P H A N I E M A RT E L L A

THE CHEMISTRY OF A MUSCLE CONTRACTION

VOCABULARY

• Muscle Fiber- Single muscle cell.• Sarcolemma- Plasma membrane of a muscle

fiber.• Sarcoplasm- Cytoplasm of a muscle fiber.• Sarcoplasmic reticulum- Smooth endoplasmic

reticulum in a muscle fiber.• Terminal Cisternae- Expanded ends of the

sarcoplasmic reticulum that are in contact with the transverse tubules.

VOCABULARY CONTINUED…

• Transverse (T) Tubule- Narrow, tubular extensions of the sarcolemma into the sarcoplasm, contacting the terminal cisternae; wrapped around myofibrils.• Myofibrils- Organized bundles of myofilaments:

cylindrical structures as long as the muscle fiber itself.• Thick Filament- Fine protein myofilament

composed of bundles of myosin.• Thin Filament- Fine protein myofilament

composed of actin, troponin, and tropomyosin.

VOCABULARY CONTINUED…

• Actin- Double stranded contractile protein.• Tropomyosin- Double stranded regulatory protein.• Troponin- Regulatory protein that holds

tropomyosin in place and anchors to actin.• Titin- Filaments of an elastic protein.• Sarcomere- Functional contractile unit of a

skeletal muscle fiber.

MOLECULAR STRUCTURE OF THICK AND THIN FILAMENTS

• When calcium is present it binds to troponin, changing its shape.

• Upon Troponin changing its shape it simultaneously moves the tropomyosin molecules to which they are attached. This exposes the active site on the actin molecule allowing myosin to bind to actin.

• The myosin heads make an “attach, pivot, detach” motion, moving the actin towards the M line in the sarcomere.

• Upon calcium leaving, the tropomyosin goes back to covering the active site on the actin molecule.

STRUCTURE OF A SARCOMERE

• I band contains thin filaments and titin.

• H zone is only thick filaments in a relaxed muscle, in a contracted muscle thin filaments are pulled into the H zone.

• The M line serves as an attachment site for thick filaments and is a meshwork of protein.

• The Z line serves as an attachment site for thin filaments and is a thin protein structure.

• The A band contains the entire thick filament.

NEUROMUSCULAR JUNCTIONS

• Muscle contraction begins when a nerve impulse stimulates an impulse in a muscle fiber.• Each muscle fiber has one motor neuron which

transmits the effect of a nerve impulse to the muscle fiber at a neuromuscular junction.• A neuromuscular junction is where a motor

neuron meets a skeletal muscle fiber.

NEUROMUSCULAR JUNCTIONS CONTINUED…

• Synaptic knob- expanded tip of an axon.• Nerve impulse travels down the axon and into the

synaptic knob, which contains synaptic vesicles filled with molecules of acetylcholine (ACh).

• Motor end plate- specialized region of sarcolemma.• Synaptic cleft- narrow space that separates the

synaptic knob and motor end plate. • ACh Receptors- When in the motor end plate they

open the receptor doors.• Acetylcholinesterase (AChE) breaks down molecules of

ACh that are released into the synaptic cleft to prevent the muscle from always being stimulated.

NEUROMUSCULAR JUNCTIONS CONTINUED…

• Nerve impulse at synaptic knob causes synaptic vesicles to release ACh into the synaptic cleft. ACh attaches to receptors in the motor end plate causing the receptor to open and release sodium ions (Na+).• The sodium ions cause an impulse to be spread

across the sarcolemma and into the muscle fiber through the T-Tubules.• As long as ACh keeps the receptor doors open the

muscle impulse continues.

NEUROMUSCULAR JUNCTIONS CONTINUED…

• The T-Tubules are between Terminal Cisternae, which are calcium reservoirs. The nerve impulse spread along the T-Tubule allows calcium ions to leak out of the Terminal Cisternae into the sarcoplasm. As I mentioned earlier, when the calcium is present it binds to the troponin which ends up causing the active site on the actin molecule to be open for myosin to attach and therefore contracting the muscle.

MUSCLE CONTRACTION

• Muscle contraction happens in less than a second even though it’s such a complicated and very interesting process.• Here is a video to better explain.• http://www.youtube.com/watch?v=ZscXOvDgCmQ

WORK CITED

• McKinley, Michael, and Valerie Dean O'Loughlin. Human Anatomy. 3rd ed. N.p.: McGraw-Hill Companies, 2012. Print.