ANATOMY OF THE FETAL PIG

Excitation-Contraction Coupling: Muscle Contraction – From action potential through muscle relaxation.

Activity at the Neuromuscular Juncion 

An action potential travels out through a motor neuron from the axon hillock through the axon (neuron fiber) to a skeletal muscle cell. An action potential is a conducted change in permeability of the membrane that results from the rapid depolarization and repolarization. When the action potential reaches the synaptic knob, vesicles release the neurotransmitter acetylcholine (ACh) into the synaptic cleft. ACh receptors in the motor end plate bind ACh and initiate an action potential in the muscle fiber’s sarcolemma. Acetylcholinesterase at the motor end plate rapidly hydrolyzes the ACh. The action potential travels throughout the muscle fiber traveling down “T” tubules to myofibrils. The action potential initiates the opening of calcium-release channels of the cisternae of the sarcoplasmic reticulum causing a sharp rise in the concentration of calcium ions in the sarcoplasm. Rapidly, an ATP-driven calcium pump in the membrane of the sarcoplasmic reticulum removes the calcium ions from the sarcoplasm back into the interior of the sarcoplasmic reticulum, where much of it binds to the protein called calsequestrin.

The Sliding Filament Theory

The repeating contractile unit of the myofibril is the sarcomere, which is composed of thick (myosin) and thin (actin) filaments. Thin filaments also have the proteins Troponin (I, T and C subunits) as well as a long rod-shaped protein called tropomyosin; both of which are involved with contraction. The myosin heads (cross bridges) have an ATPase site, which the ATP will bind with high affinity. ATP is hydrolyzed to ADP and phosphate, but these remain bound to the myosin crossbridge. When Ca++ is low in the sarcoplasm, tropomyosin keeps the myosin head (cross bridge) from binding actin, but when Ca++ is above 1 microMolar, it will bind to the troponin C subunit. This causes a change where tropomyosin will uncover the myosin binding sites on the actin portion of the thin filament. Pi is released from the myosin-ADP-Pi complex and the “power stroke” occurs pulling the thin actin filament along the thicker myosin filament. This moves the actin molecules toward the M-line, shortening the sarcomeres. ADP and Pi is released by the myosin crossbridge. Myosin then wants to bind ATP more than actin so myosin releases actin and the muscle relaxes. Immediately, the ATP is hydrolyzed by ATPase back to ADP + Pi and the head (crossbridge) is primed (“cocked”) for the next contraction.

General Dissection Photos

The following frames are samples of general tissue types. Expect 2-3 microscope slides of tissue to identify.

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