Muscle tissue converts chemical energy into physical force to enable movement and maintain bodily functions. Among the three types of muscle tissue—skeletal, cardiac, and smooth—striated muscle is responsible for both voluntary locomotion and the involuntary pumping of the heart. The term “striated” refers to the distinct, striped pattern visible under a microscope, an appearance linked to the tissue’s organized ability to contract.
The Microscopic Structure of Striations
The striped appearance of striated muscle arises from a highly organized internal structure within muscle cells, known as muscle fibers. Each muscle fiber contains numerous cylindrical myofibrils, which are the fundamental contractile elements. These myofibrils are composed of repeating functional units called sarcomeres, the smallest units capable of contraction.
The sarcomere is the segment between two Z-lines, which act as anchoring points. The alternating light and dark bands result from the precise overlap of two main types of protein filaments. Thick filaments are composed of myosin, while thin filaments are made up of actin, troponin, and tropomyosin. The dark A-bands correspond to the length of the thick myosin filaments, while the lighter I-bands contain only the thin actin filaments.
Skeletal and Cardiac Muscle Types and Placement
Striated muscle tissue is categorized into two types: skeletal muscle and cardiac muscle. Skeletal muscle is typically attached to the skeleton by tendons, generating movement, posture, and maintaining body heat. These cells are long, cylindrical, and multinucleated, containing many nuclei situated beneath the cell membrane.
Cardiac muscle is found exclusively in the walls of the heart, forming the myocardium, and its primary function is to pump blood. Unlike skeletal fibers, cardiac cells are shorter, branched, and usually contain a single, centrally located nucleus. A distinguishing feature is the presence of specialized junctions called intercalated discs, which connect individual cells. These discs contain gap junctions that allow electrical signals to pass rapidly, enabling the heart muscle to contract as a unified unit.
How Striated Muscle Generates Movement
Movement is generated by the physical shortening of striated muscle tissue, explained by the sliding filament theory. This theory describes how thin and thick filaments interact within the sarcomere to produce contraction without the filaments themselves changing length. Contraction begins when the nervous system sends a signal, triggering the release of stored calcium ions (Ca²⁺) into the muscle cell cytoplasm.
The released calcium ions bind to troponin on the thin actin filaments. This binding shifts tropomyosin away from the binding sites on the actin, allowing the globular heads of the thick myosin filaments to attach, forming a cross-bridge. The energy source for this mechanical action is adenosine triphosphate (ATP).
Once the cross-bridge forms, the myosin head pivots in a power stroke, pulling the thin actin filament toward the center of the sarcomere. This action requires the hydrolysis of ATP to release energy. A fresh ATP molecule must then bind to the myosin head, causing it to detach and break the cross-bridge. The myosin head recocks, ready to bind to a new site if calcium remains present, shortening the sarcomere and contracting the muscle fiber.
Initiating Muscle Contraction
The initiation of contraction differs significantly between the two types of striated muscle. Skeletal muscle is under voluntary control, requiring conscious thought and input from the somatic nervous system. The signal from a motor neuron is transmitted to the muscle fiber at the neuromuscular junction. This triggers a cascade of events that release the calcium ions required for the sliding filament process.
In contrast, cardiac muscle operates under involuntary control, regulated primarily by the autonomic nervous system. The heart possesses specialized cells, such as those in the sinoatrial node, which act as intrinsic pacemakers. These cells spontaneously generate electrical impulses, causing the heart to beat rhythmically without external nerve input. While the autonomic nervous system can modulate the rate and strength of contractions, the fundamental rhythm is self-generated.