Dominic Stone
Striated muscle tissue is a type of muscle tissue that features repeating functional units called sarcomeres, which give it a striped appearance under a microscope. The two types of striated muscle are skeletal muscle and cardiac muscle. Skeletal muscle is the most common type of muscle in the human body, comprising 30% to 40% of total body mass and allowing for a wide range of movements and functions. Cardiac muscle, on the other hand, is found in the walls of the heart and is responsible for heart contractions and pumping blood throughout the body. Both types of striated muscle have a unique structure that enables their respective functions, with skeletal muscle being able to regenerate better than cardiac muscle due to the presence of satellite cells.
| Characteristics | Values |
|---|---|
| Definition | Muscles that can contract and relax independently |
| Appearance | Striped due to light and dark bands |
| Types | Skeletal, cardiac, and smooth |
| Skeletal Muscle Composition | Skeletal muscle fibres, blood vessels, nerve fibres, and connective tissue |
| Cardiac Muscle Composition | Myofibrils and densely packed mitochondria |
| Skeletal Muscle Function | Chewing, swallowing, breathing, movement, posture maintenance |
| Cardiac Muscle Function | Heart contraction, pumping blood |
| Smooth Muscle Function | Involuntary functions like waste removal |
| Skeletal Muscle Structure | Alternating dark (anisotropic or A-bands) and light bands (isotropic or I-bands) |
| Cardiac Muscle Structure | Intercalated discs connect cardiac cells mechanically and electrically |
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Skeletal muscle
Each skeletal muscle is wrapped in epimysium, which provides structural integrity to the muscle despite contractions. The perimysium organises the muscle fibres, which are encased in collagen and endomysium, into fascicles. The perimysium is the middle layer surrounding bundles of muscle fibres, and the endomysium is the innermost layer surrounding individual muscle fibres. The connective tissue covering provides support and protection for the delicate cells and allows them to withstand the forces of contraction.
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Cardiac muscle
Striated muscle tissue is a muscle tissue that features repeating functional units called sarcomeres. Under a microscope, the sarcomeres are visible along the muscle fibres, giving the tissue a striated appearance. The two types of striated muscle are skeletal muscle and cardiac muscle.
The primary function of cardiac muscle is to pump oxygenated blood into circulation by generating sufficient force. The mechanism behind each coordinated contraction involves the cardiac muscle and electrical impulses. These contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones. Aerobic production is the core utilization process; however, the heart may use anaerobic processes in a limited capacity.
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Smooth muscle
Multi-unit smooth muscle, on the other hand, is neurogenic, meaning its contraction must be initiated by an autonomic nervous system neuron. It is found in the trachea, in the iris of the eye, and lining the large elastic arteries.
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Muscle contractions
The process of muscle contraction, called excitation-contraction coupling, begins when an action potential causes depolarization in the myocyte membrane. This depolarization spreads via the transverse (T) tubules, which are invaginations of the muscle cell membrane. The T tubules then undergo a conformational change, causing the opening of nearby ryanodine receptors on the sarcoplasmic reticulum (SR), the storage site for calcium within muscle cells. When calcium is released from the SR, it binds to troponin C, leading to a shift in tropomyosin that allows the myosin heads to attach to the actin filaments, creating a cross-bridge. This cross-bridge cycling begins when ATP binds to an ATP-binding domain on the myosin head.
The contraction produced by the interaction of myosin and actin can be described as a twitch, summation, or tetanus, depending on the frequency of action potentials. The sliding filament theory explains how the protein filaments within each skeletal muscle fiber slide past each other to produce a contraction. In skeletal muscles, muscle tension is greatest when the muscle is stretched to an intermediate length, as per the length-tension relationship.
Cardiac muscle, on the other hand, is under involuntary control and comprises the walls of the heart, allowing blood to be pumped through the body. Smooth muscle is also involuntary and is found throughout the blood vessels, gastrointestinal tract, bronchioles, uterus, and bladder.
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Muscle regeneration
Satellite cells are dormant cells found in healthy skeletal muscle tissue, and they are key to the regeneration process. During muscle regeneration, satellite cells re-enter the cell cycle to multiply and differentiate into myoblasts. This process involves the activation, differentiation, and fusion of satellite cells, leading to the formation of new myofibrils.
The environment of muscle tissue, or "cellular ecology," is crucial for muscle regeneration. Intercellular cross-talk creates an interactive system of cellular networks, including myogenic, endothelial, and fibroadipogenic progenitors, that work together to restore function. The muscle niche, including motor neurons, blood vessels, and the extracellular matrix, plays a critical role in muscle regeneration by influencing the activity of satellite cells.
The regeneration process can be divided into five interrelated and time-dependent phases: degeneration-necrosis, inflammation, regeneration, maturation/remodelling, and functional recovery. The kinetics and amplitude of each phase can vary depending on the nature and intensity of the injury. While muscle regeneration has been observed across different species, the specific mechanisms and interactions involved are still being actively researched to develop effective therapeutic strategies for muscular disorders.
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Frequently asked questions
Muscle striations are the light and dark bands that give skeletal and cardiac muscles their striped appearance. These bands are called isotropic (I-bands) and anisotropic (A-bands) respectively.
The two types of striated muscle are skeletal muscle and cardiac muscle. Skeletal muscle is voluntary and is attached to the bones, enabling movement. Cardiac muscle is involuntary and is found in the walls of the heart, enabling the heart to contract and pump blood around the body.
The main function of striated muscle is to contract and create force. This enables breathing, movement, posture maintenance, and the pumping of blood through the body.
The structure of striated muscle is made up of muscle fibres and connective tissue layers. The fibres are cylindrical with blunt ends and are surrounded by endomysium, perimysium, and epimysium. The muscle fibres contain spindle-shaped nuclei and are composed of myofibrils, which are long strands of proteins called actin and myosin.
