Smooth muscle
In today's lab you will be using microscopes to examine the differences between different types of vertebrate muscle tissues.
Smooth muscle has a simple structure with a single nucleus per cell and cells arranged in sheets. Actin and myosin, the basic contractile structures of muscle cells are arranged in loose networks. Cells are connected by gap junctions, which allow ion flow between muscle cells. This results in a wave-like contraction down the sheet of smooth muscle cells as the nerve signals travel through the sheet. Smooth muscle is found in the walls of contractile organs such as the intestines, in blood vessels and in the walls of the bladder, stomach and other hollow organs. Smooth muscle contraction is usually controlled by the nervous system and is involuntary. While contraction is slower than other types of muscle, smooth muscle can maintain steady levels of contraction for a much longer period than cardiac and skeletal muscle. Figure 1 shows part of a cross section of an intestine with a layer of smooth muscle cells arranged horizontally (the outside of the intestine is at the top and the lumen is at the bottom) and an enlarged image of a smooth muscle layer showing nuclei.
Fig. 1. Smooth muscle layer in a cross section of a small intestine (left) and enlarged smooth muscle layer showing nuclei (right).
Skeletal muscle
Skeletal muscle is striated (i.e. it has a striped appearance, enlargement, Fig. 2 right) due to the regular arrangement of actin and myosin filaments. The highly ordered array of actin and myosin filaments, each unit of which is called a sarcomere, is repeated many times to form larger structures called myofibrils. Myofibrils are very long relative to their width and are the narrowest structures that are visible under our compound microscopes within an individual muscle fiber. Because the actin/myosin arrays in adjacent myofibrils are aligned, the repeating pattern of the assembly can be seen across the entire muscle fiber as striations. Many myofibrils are bundled together to form a muscle fiber. A muscle fiber is a single muscle cell which has multiple nuclei due to the fusion of multiple cells during development to create a single fiber. These nuclei are grouped on the surface of the fibers. Skeletal muscle is associated with the skeleton and facilitates movement by contraction. Its contraction is controlled by the nervous system and is usually voluntary.
Fig. 2. Skeletal muscle fibers showing multiple nuclei (left) and striations (right).
Cardiac muscle
Cardiac muscle is also striated. Cardiac muscle cells are branched and have one nucleus. Cells are connected end to end by intercalated discs. The intercalated disks provide support and contain gap junctions which allow nerve signals to be transmitted between two adjacent muscle cells. This allows signals to travel very quickly throughout the heart causing the heart to pump. Cardiac muscle is found in the heart and is myogenic, or self-excitable. No outside signal is required to stimulate contraction in cardiac muscle.
Fig. 3. Cardiac muscle, with an example intercalated disk (indicated by arrows) and branched cells.
