Muscle tissue is a soft tissue responsible for producing movement, maintaining posture, and generating heat. Its ability to contract allows for everything from the blink of an eye to the stride of a runner. Muscles work with the skeletal system to create locomotion and support the body’s internal organs.
The Three Types of Muscle Tissue
The human body contains three types of muscle tissue, each with a unique structure, location, and method of control. Skeletal muscle is the most abundant type, making up about 40% of total body mass. As a voluntary muscle, its contraction is under conscious control, receiving signals from the nervous system to produce movements like walking or lifting. These muscles work in pairs to move limbs and maintain posture.
Found within the walls of internal organs like the digestive tract, uterus, and blood vessels, smooth muscle operates without conscious direction. This involuntary muscle is arranged in layered sheets that facilitate functions like moving food through the intestines or regulating blood flow. Its contractions are automatic and managed by the autonomic nervous system, allowing organs to perform their duties without our awareness.
The third type, cardiac muscle, is found exclusively in the walls of the heart. Like smooth muscle, it is involuntary, contracting rhythmically and continuously to pump blood throughout the body. The cells of cardiac muscle are branched and interconnected, forming a network that ensures the heart beats in a coordinated manner.
The Structure of Skeletal Muscle
A single skeletal muscle has a hierarchical structure. The entire muscle is wrapped in a protective sheath of connective tissue called the epimysium. This outer layer allows the muscle to contract while maintaining its structural integrity and separates it from other tissues.
Inside the muscle are bundles of muscle fibers known as fascicles, each encased in a connective tissue layer called the perimysium. This bundled organization helps distribute contraction force and provides pathways for nerves and blood vessels. The arrangement is similar to how a large rope is made of smaller, tightly bound cords.
Within each fascicle are the individual muscle fibers, which are the muscle cells. Each elongated cell is surrounded by a layer of connective tissue called the endomysium. This sheath contains the capillaries and nerve endings that supply each cell. All three connective tissue layers merge at the ends of the muscle to form a tendon, which anchors the muscle to bone.
The Microscopic Engine of Muscle
Inside a single muscle fiber is the microscopic machinery for contraction. The cytoplasm of a muscle cell, the sarcoplasm, is filled with long structures called myofibrils that run the length of the fiber. Their collective shortening is what causes the entire muscle to contract.
Each myofibril is composed of repeating functional units called sarcomeres. The sarcomere is the contractile unit of skeletal muscle, and its organized structure gives the muscle its striated, or striped, appearance. A sarcomere is defined as the segment between two Z-discs, which anchor protein filaments in place.
The sarcomere is built from two protein filaments: actin and myosin. Actin forms thin filaments anchored to the Z-discs, while myosin forms thick filaments in the center. The overlapping arrangement of these filaments allows them to interact and generate the force for muscle contraction.
How Muscles Contract
The process of muscle contraction is governed by the sliding filament theory. This process begins when a signal from a motor neuron travels to a skeletal muscle fiber. The arrival of this signal triggers the release of calcium ions from an internal storage structure called the sarcoplasmic reticulum.
Calcium ions initiate the contraction. In a resting muscle, regulatory proteins block the sites on actin filaments where myosin can bind. Calcium binds to these proteins, causing them to shift and expose the binding sites, allowing myosin heads to attach and form cross-bridges.
Using energy from adenosine triphosphate (ATP), the myosin heads pull the actin filaments toward the center of the sarcomere. This action is similar to oars pulling a boat, as the myosin heads repeatedly bind, pull, and detach to slide the actin filaments. This sliding motion shortens the sarcomere, and when thousands of sarcomeres shorten simultaneously, the entire muscle fiber contracts.
Major Skeletal Muscle Groups and Their Actions
The body has over 600 skeletal muscles, grouped by location and action. In the upper body, major muscles include:
- Pectoralis major: Located in the chest, these muscles are responsible for pushing objects away from the body.
- Latissimus dorsi: Large muscles of the back that perform the opposite action, pulling objects toward the body.
- Deltoids: These muscles cap the shoulders and are used for lifting the arm.
- Biceps: Found on the front of the upper arm, these muscles bend the elbow.
- Triceps: Located on the back of the upper arm, these muscles straighten the elbow.
The core muscles provide stability for the trunk. The rectus abdominis allows you to bend forward, while the obliques on the sides of the torso are for twisting. These muscles work together to support the spine and transfer force.
In the lower body, large muscle groups power locomotion:
- Gluteus maximus: The largest muscle in the body, it is the primary muscle for extending the thigh when climbing stairs or standing up.
- Quadriceps femoris: A group of four muscles on the front of the thigh that works to straighten the knee.
- Hamstrings: Located on the back of the thigh, these muscles bend the knee.
- Gastrocnemius: The calf muscle is responsible for pointing the toes downward, an action used in walking and running.