Tutor profile: Joan C.
Subject: Basic Chemistry
Explain what is meant by the term nucleophilic substitution.
A nucleophile is a molecule that is electron-rich, therefore it donates electrons to electron-poor sites. In this case, it attacks a positively charged electrophile to replace a leaving group (such as Cl- or Br-).
Explain how skeletal muscle contracts.
Activation of skeletal muscle occurs through the propagation of an action potential down a motor neuron, causing the release of acetylcholine into the synaptic cleft. Acetylcholine causes depolarization within the sarcolemma which spreads to the t-tubules to reach the muscle fibres, and in turn triggers the release of calcium ions from the sarcoplasmic reticulum, which is the internal calcium store. This calcium ion release allows for the shortening of the muscle fibres by allowing sarcomere contraction. In order for sarcomere contraction to occur, cross-bridge formation between the actin and myosin filaments take place upon calcium release. Prior to calcium release, troponin and tropomyosin are two block complexes on the actin filaments which physically prevent the binding of the myosin heads. Upon calcium ion release, the calcium ions bind to troponin, and this causes tropomyosin to move off the actin filaments to reveal the active site for myosin. As a result, the myosin heads form a cross-bridge with the filaments. For contraction to occur, this requires the sliding of the actin and myosin filaments. In its relaxed state, ATP molecules are attached to the myosin head, preventing the formation of the cross-bridge. When calcium is bound to troponin, ATP hydrolysis occurs, allowing the myosin heads to change their position so that the are able to move along to the next binding site on the actin filament. Therefore, it is able to bind to a new actin binding site, and adopts its original conformation. By adopting its original conformation, this creates the sliding mechanism as the actin is dragged along the myosin. Once calcium detaches from troponin, the active site is blocked once again by tropomyosin, and ATP reattaches to the myosin head, breaking the cross-bridge formation until the next calcium ion comes along and binds to troponin.
Define the structures that border the subclavian triangle. Describe the main contents of this triangle and discuss their function and clinical relevance.
The subclavian triangle is a tiny triangle located just above the clavicle in the posterior portion of the neck. Also known as the supraclavicular triangle, it is bordered by the clavicle, inferior belly of the omohyoid muscle, and the posterior border of the sternocleidomastoid muscles. Within this triangle, nerves and blood vessels are primarily located here - more specifically, the brachial plexus, subclavian artery and vein, proximal and inferior end of the external jugular vein, and the phrenic nerve. Underneath the pre-vertebral fascia, we also have splenius capitis, levator scapulae, and the anterior, posterior and middle scalene muscles that form the floor of the subclavian triangle. Although this is a small triangle in the neck, it is very important. Within the triangle, the brachial plexus and subclavian artery are confined to an area between the anterior and middle scalene muscles, and rib I. The phrenic nerve, located underneath the pre-vertebral fascia and on scalenus anterior's surface, is a branch of the cervical plexus and primarily innervates the diaphragm. Other branches are present in the subclavian triangle to innervate other areas of the neck and scalp. A local anaesthetic can be applied to the posterior border of the sternocleidomastoid muscle, thereby blocking the junction of the cutaneous branches of the cervical plexus. On the other hand, the subclavian vein is located outside of this triangular area (also known as the scalenus triangle). This vein is often used as an entry point into the venous system of the body. As this is relatively superficially located, it is largely vulnerable to any damage. Slicing through this vein can result in cyanosis, drawing air into the vein, and this can interrupt and prevent blood flow to the right atrium of the heart.
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