There are so many concepts on the MCAT, and sometimes it can be easy to get lost in how different concepts relate to each other. Notecards, flow charts or large maps, and teach-backs are all ways to make sure you solidify these complicated topics! One strategy to study for the MCAT and consolidate information is approaching biology concepts from multiple perspectives — for example, by levels of organization.
More than 600 muscles are in the human body, taking up around 40% of our body weight. These muscle cells work daily to help you digest, walk, breathe, and read this blog post. Muscles are also a potential topic for the MCAT, and therefore important to read about in preparation.
There are a few basic concepts that are essential to your review. For instance, you should know that the cytoplasm of a muscle cell is the sarcoplasm. The smooth endoplasmic reticulum is the sarcoplasmic reticulum. The plasma membrane of a muscle cell is the sarcolemma.
Organ level organization
To begin your content review of muscle tissue on the MCAT, I would start at the organ level. A skeletal muscle is comprised of fascicles, which are muscle fiber bundles with nerves and blood vessels. Each bundle is made of a cell, a muscle fiber, wrapped in sarcolemma. There are many mitochondria within the muscle cells in order to provide the ATP needed to contract. Sometimes myoblasts (immature muscle cells) fuse and therefore these muscles are multinucleated. Within these muscle fibers are myofibrils with sarcoplasmic reticulum inside. T tubules run along these muscle fibers. The myofibrils are made of microfilaments of molecules called actin and myosin.
Cardiac, skeletal, or smooth
You can also categorize muscles by the three different kinds: cardiac, skeletal, or smooth. Cardiac muscles are striated and contract involuntarily. These are special because they can generate their own spontaneous cardiac rhythm. Skeletal muscles are also striated, but contract voluntarily. Smooth muscles are not striated and contract involuntarily, along a graded, directionless contraction path. Histologically, smooth muscles are rather disorderly compared to cardiac and skeletal muscles.
Another way to approach the muscles is from a microcellular level. Skeletal myofibrils are made of microfilaments of actin and myosin in light and dark bands that form units called sarcomeres. Filaments sliding against each other, in movements generated by the power strokes of the actin heads forming cross bridges to the myosin, create contractions of the muscles.
There are also molecules called troponin and tropomyosin. In a relaxed state, tropomyosin normally blocks the actin binding sites on myosin. In a key element of muscle contraction, when in the presence of calcium, troponin will transmit structural changes to tropomyosin, causing it to detach from the myosin. This frees up the myosin for binding with actin, and therefore allows for continued contraction of the muscle cell.
The regulation of muscle cell contractions are also important, and calcium plays a particularly important role in signaling between nerve and muscle cells. Another significant point is that calcium signaling allows muscle cells in a muscle to coordinate contractions, so that together the larger muscle structure can contract. Calcium is normally stored in high concentrations in the sarcoplasmic reticulum. When a depolarization is spread along the T tubules, it triggers release of calcium from the sarcoplasmic reticulum. Upon calcium binding to troponin, the tropomyosin that previously blocked actin-myosin binding is free. Cross bridging between the actin and myosin is allowed, and muscle contraction can occur.