Human Muscular System: Anatomy and Function
Muscle Anatomy
In human anatomy, the muscular system is the set of over 600 muscles in the body whose main function is to generate movement, whether voluntary or involuntary. Muscle is one of the body tissues of humans and other animals. It is contractile in nature, meaning it is characterized by its ability to contract, usually in response to nervous stimulation. The structural and functional unit of muscle is the muscle fiber. It is a very small, thread-like structure consisting of complex proteins.
To understand how muscle contraction occurs, one must understand the organization of the cells that form part of the muscle tissue. The muscles are a set of bundles or bundles of fibers. They are like the wires of a cable and are confined within a sheath of connective tissue called the perimysium. Each muscle cell or fiber is composed of many fibers called myofibrils, each made of thick and thin contractile myofilaments. Each thick myofilament contains several hundred myosin molecules of protein, which have ATP-binding sites. Thin filaments contain two strands of the protein actin. Also, in the myofilaments, there are accessory proteins: troponin, which has binding sites for calcium, and tropomyosin.
The actin and myosin filaments are organized into structures called sarcomeres, which are the basic units of muscle contraction. The sarcomeres are joined at their ends by an interweaving of filaments called the Z line. During muscle contraction, these rows of interdigitating filaments slide past each other, that is, they alternate with the ends overlapping, crossed by bridges that act as wheels. The power requirement in this movement comes from dense mitochondria surrounding myofibers.
Main Functions of Muscles
- Voluntary movement led by the cerebral cortex produced by involuntary reflexes
- Maintain body posture
- Contract to generate motion that, in turn, produces heat
- Perform vital functions
For muscles to function correctly, they need oxygen and glucose. The more work the muscles do, the more oxygen and glucose they need.
Characteristics of Muscles
- Excitability: The ability to respond to stimuli
- Contractility: The ability to shorten or contract
- Extensibility: The ability to extend and stretch
- Elasticity: The ability to change shape and length
Types of Muscles
Muscles are divided into smooth and striated, which is further divided into cardiac and skeletal. Depending on how they are controlled, they can be:
- Voluntary: Controlled by the individual
- Involuntary or visceral: Controlled by the central nervous system
- Autonomic: Their function is to contract regularly without stopping
- Mixed: Controlled by both the individual and the nervous system, e.g., eyelids
Smooth Muscle
Smooth muscle is found in the lining of the stomach, intestines, uterus, gastrointestinal tract, viscera, and blood vessels. It is an involuntary muscle controlled by the autonomic nervous system. Its cell shape is elongated, fusiform, tapered, with sharp points, no stretch marks, and mononuclear. It has a slow speed of contraction, although it shortens much more than striated muscle. Although it is not well adapted for running or flying, it contracts extremely well and has a greater capacity to remain contracted. It often contracts in response to simple stretching, and the contraction tends to be prolonged. It is well suited for tasks such as regulating blood pressure by sustained contraction of the walls of arterioles.
Cardiac Muscle
Cardiac muscle is located in the heart walls. It is an involuntary muscle controlled by the autonomic nervous system. The fibers are joined end to end and branch and reunite to form complex networks. They are cylindrical, elongated, and have striations. It has one or two nuclei per fiber. The speed of contraction is intermediate, as is the ability to stay contracted. Another characteristic of heart muscle tissue is the presence of intercalated disks, which are specialized junctions between the fibers. This muscle contracts abruptly, driving the rhythm of blood with each contraction.
Skeletal Muscle
Skeletal muscle is attached to the skeleton. It is voluntary and is controlled by the central nervous system. Its fibers are elongated, up to 2 or 3 cm, cylindrical, with blunt ends. It has stripes. Each skeletal muscle fiber has many nuclei, an exception to the generalization that cells contain a single nucleus. Its contraction is faster compared with other soft tissues and is less able to maintain contraction compared with other tissues. It forms the large muscle masses attached to the bones of the body. When this muscle is stimulated by a single, brief stimulus, it undergoes a single fast-twitch spasm called a simple twitch. These do not occur except in laboratory experiments.
Neurotransmitters Associated with Muscle Contraction and Muscle Contraction Cycle
- In a motor unit, a motor neuron that stimulates the muscle is functionally connected to about 150 muscle fibers. When a motor neuron is stimulated, it releases the neurotransmitter acetylcholine in the synaptic cleft, the space between the motor neuron and each muscle fiber.
- The acetylcholine binds to receptors on each muscle fiber, causing an electrical change called depolarization along the sarcolemma. This depolarization generates an electrical impulse or action potential.
- The action potential spreads along the sarcolemma through the membranes of T tubules, which are invaginations of the plasma membrane of the muscle fiber. It also passes through the sarcoplasmic reticulum.
- The action potential changes the permeability of the sarcoplasmic reticulum, and this change opens protein channels, allowing the output of calcium ions, which are then free in the cytoplasm.