Special Sub-Topic: What Makes You Twitch?
|A neurone is separated from the skeletal muscle by the neuromuscular junction. This is a chemical synapse and so information is transferred via neurotransmitters. What name is given to areas of neurotransmitter release?|
Active zones. The type of neurotransmitter released can vary, some acting as excitatory signals and some acting as inhibitory signals. Acetylcholine is a ubiquitous neurotransmitter at the neuromuscular junction. With skeletal muscle, acetylcholine has an excitatory effect.
|The neurotransmitters released from the neurone terminus (the post-synaptic bulb) are usually transported in a structure whose formation is encouraged by the action of the protein, clathrin. Examples of this structure include endosomes and lysosomes. What structure is this?|
A vesicle. Vesicles are formed from the budding and pinching off of a small portion of plasma membrane. Whether this occurs on the cytosolic face of the membrane (endocytosis) or the extracellular side (exocytosis) partially determines the vesicle's function. A bare vesicle consists of the cargo (in this case the neurotransmitter), surrounded by a phospholipid bilayer. Fusion of the vesicle and muscle membranes allows the neurotransmitters to be released.
|Once the neurotransmitters reach the sarcolemma (the membrane of the skeletal muscle), they bind to proteins which open as a response, allowing sodium ions to rush into the muscle fibre. What name is given to these proteins?|
Ion channels. More specifically, these structures are ligand-gated ion channels, since their opening is dependent upon the attachment of two or more neurotransmitter molecules.
For more information about plasma membrane proteins, please take my quiz, "How a Cell Communicates".
|The entry of sodium ions (Na+) into the cell causes a small depolarization, which causes more sodium ions to enter, eventually leading to a full action potential at the sarcolemma. What is this an example of?|
Positive feedback. The entry of Na+ ions via ligand-gated ion channels causes a small depolarization, which activates (and opens) voltage-gated ion channels. These channels are much more widespread and so lead to a large influx of sodium ions and, therefore, an action potential.
Positive feedback is also seen in blood clotting - effects brought about by platelet aggregation initially act to encourage further aggregation.
|The action potential which is generated on the sarcolemma (muscle membrane) travels down a structure known as a T-tubule, eventually activating the Ca++ (calcium ion) release channel. On which intracellular component is the Ca++ release channel found?|
Sarcoplasmic reticulum. The sarcoplasmic reticulum is a special type of smooth endoplasmic reticulum which acts as a calcium store in muscle cells. Action potentials which travel down T-tubules interacts with a voltage sensitive protein, which activates the Ca++ release channel.
|The Ca++ release channel greatly increases the concentration of calcium ions within muscle fibres. The role of the Ca++ ions is to cause a conformational change, allowing muscle contraction to occur. What name is given to the tropomyosin associated protein which binds calcium ions in skeletal muscles?|
Troponin. Troponin in skeletal muscle fibres consists of three subunits: I, C (which binds calcium) and T (which binds tropomyosin).
Calcium plays a vital role in the contraction of all three types of muscle (skeletal, cardiac and smooth). Like skeletal, cardiac muscle involves calcium binding to troponin, whereas the ions bind to myosin light chain kinase in smooth muscle.
|The binding of calcium leads to a conformational change of the protein, tropomyosin. This change allows an actin-myosin cross-bridge to form. Which of the following best describes this cross-bridge?|
A myosin head bound to an actin binding site. Actin filaments are joined by Z-lines (structures which define the boundaries of the muscle subunits known as sarcomeres). In a resting state, actin binding sites are covered by tropomyosin, but due to troponin binding to calcium ions, the actin binding sites are exposed, allowing myosin heads to bind.
Skeletal muscle has a striated appearance, with actin forming light bands, and the thicker myosin forming dark bands.
|The loss of a molecule of ADP (adenosine diphosphate) from the actin-myosin cross-bridge gives the first major step in muscle contraction, as the two actin and myosin filaments slide over one another, shortening the muscle fibre. What is this action called?|
A power stroke. More specifically, the ADP molecule is located on the myosin head.
It is important to remember that the actin and myosin filaments never shorten, but that they slide over one another due to the power stroke. This shortens the myofibrils which collectively make up a muscle fibre.
During contraction, the length of a sarcomere (the distance between two Z-lines) decreases.
|In order for further contraction or relaxation of the muscle to occur, the actin-myosin cross-bridge must be broken. Which molecule is directly responsible for this, its absence in death usually leading to rigor mortis?|
ATP. ATP joins to the myosin head, breaking the cross-bridge, and is then hydrolysed (to ADP), moving the myosin head back into its original position. The myosin head is therefore ready to make a new cross-bridge, which will result in an even greater shortening of the muscle fibre.
|As long as calcium levels within muscle fibres are high (and there is sufficent energy), a muscle will keep contracting until the fibres exist in their shortest possible form. How are calcium ions removed from the cytoplasm (sarcoplasm), allowing muscles to relax?|
The action of protein pumps. Calcium ions are returned to the lumen of the sarcoplasmic reticulum by the action of the calcium ATPase pumps.
The decrease in intracellular calcium ions will mean that tropomyosin will once again cover the actin biding sites, preventing further cross-bridge formation. The muscle fibres can then be extended to their original length by the action of an antagonistic muscle.
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