|Cardiac Muscle: the muscle of the heart. This muscle is involuntary and contractions are caused by pacemaker cells within the heart that send signals to the muscles at regular intervals. Under a microscope cardiac muscle looks branched, has striations due to organization of the myosin and actin fibers, and each mucle fiber has 1 or 2 nuclei6.|
Figure 1: Cardiac muscle under a microscope. The branching and light vertical lines of the striations can be seen. Nuceli are less present than in the skeletal muscle image. (Courtesy of Carolina Biological Supply Company)
|Smooth Muscle: the muscle found in the digestive tract, bladder, uterus, and blood vessel walls along with many other places. Smooth muscle is involuntary with its contractions controlled by the autonomic nervous system (the part of the nervous system below consciousness). Under a microscope smooth muscle appears smooth, non-striated, and each muscle fiber has a single nucleus6.|
Figure 2: Smooth muscle under a microscope. Each nuclei is associated with only one fiber. (Image thanks to Wikimedia Commons)
composes the voluntary muscles in the body. The well known muscles,
such as the biceps, triceps, quadriceps, and pectorals, are all
skeletal muscle. Contracting skeletal muscle is controlled by the
conscious nervous system. Under the microscope skeletal muscle appears
smooth, striated, and with multinucleated muscle fibers6.|
Figure 3: Skeletal muscle under a microscope. The compacted and multinucleated fibers can be seen. The striations are difficult to observe in this image. (Personal work)
|The most basic skeletal muscle unit is called a sarcomere. Multiple sarcomeres are put together end to end to build a myofibril. The myofibrils are surrounded by something called the sarcoplasmic reticulum11, which contains a lot of calcium ions which will be important when we talk about how the muscle contracts. Many of these bound myofibrils together compose a muscle fiber, and many muscle fibers are bound together with membranes into compartments that together compose the entire muscle6.|
Figure 4: Microanatomy of muscle. Actin and myosin are the myofilaments that compose the sarcomere, many of which combine to form the myofibril, many of which together form muscle fibers, many of which together form the muscle. (Image thanks to MIT OpenCourseWare)
-Muscle contraction begins with a signal to contract from the nervous system.
-This signal is sent via a motor neuron to the muscle.
-The signal travels the length of the motor nerve as an electrical signal to the end of the neuron where it becomes a chemical signal in the form of a neurotransmitter released into the space between the end of the neuron and the muscle receptors.
-If the signal is strong enough, the muscle fiber is depolarized, resulting in the release of calcium ions from the sarcoplasmic reticulum.
-The calcium ions bind to troponin which alters the shape of the tropomyosin to expose binding sites on the actin fibers for the myosin heads to bind to.
-One of the phosphate groups is cut from the ATP bound to the myosin head.
-The energy released from this allows the myosin head to bend, pulling the actin fiber with it, contracting the muscle11.
-The ADP is then released from the myosin and a new ATP binds where it was, causing the myosin head to release the actin binding site and return to the "cocked" formation.
-While all of this is going on, the calcium ions are being actively transferred back into the sarcoplasmic reticulum, freeing the troponin and allowing everything to return to their relaxed conformations4,6.
Figure 5: schematic of the process of muscle contraction (Image thanks to Hank van Helvete and Wikimedia Commons)