Summary
Highlights
There are three types of muscles: skeletal, cardiac, and smooth. Skeletal muscles are voluntary, attached to bones, and striated. Cardiac muscles are involuntary, found in the heart, striated, and possess auto-rhythmic contraction. Smooth muscles are involuntary, found in hollow organs and blood vessels, and are non-striated.
The muscular system is responsible for movement, maintaining posture, facilitating cellular respiration, producing body heat, constricting organs and vessels (smooth muscles), and the rhythmic contraction of cardiac muscles.
Key functional properties include contractility (ability to shorten with force), extensibility (ability to stretch beyond normal resting length), elasticity (ability to recoil to original length), and excitability (ability to respond to stimuli).
Skeletal muscles make up approximately 40% of body weight, are attached to the skeletal system, and are striated. A muscle is an organ composed of muscle cells, nerves, and connective tissues. Muscles are covered by epimysium, fascicles by perimysium, and individual muscle cells by endomysium.
Muscles are composed of myofibrils, which contain myofilaments: actin (thin) and myosin (thick). Sarcomeres, the basic functional units of skeletal muscles, are formed by the alternating arrangement of actin and myosin. Z discs connect myofilaments. Light-staining areas (I bands) are actin, and dark-staining areas (A bands) are myosin.
Actin myofilaments involve troponin (binds to calcium, essential for muscle function) and tropomyosin (blocks myosin binding sites). Motor neurons stimulate muscles, and the neuromuscular junction is the synapse between a neuron and a muscle fiber. A motor unit is a group of muscle fibers stimulated by one motor neuron.
The sliding filament model explains muscle contraction: actin and myosin filaments slide past each other, shortening the sarcomere and the muscle. ATP supplies energy for contraction, converting to ADP, with energy stored in myosin heads. Rigor mortis occurs post-mortem due to a lack of ATP, causing muscles to become stiff.
Muscle fatigue is a temporary reduction in work capacity due to overworked and damaged muscle cells. Muscle soreness arises from structural damage and the buildup of byproducts from ATP usage after intense exercise. Oxygen deficit, or excess post-exercise oxygen consumption, refers to the time it takes for breathing to return to normal after exercise.
Smooth muscles are non-striated because their myofilaments are not arranged in sarcomeres. Cardiac muscles are branched, have one nucleus per cell, and are found in the heart. Their contraction is autorhythmic (involuntary yet steady) and connected by intercalated discs, functioning as a single unit.
Tendons connect muscles to bones; broad, sheet-like tendons are called aponeuroses, and those in the wrist and ankle are retinacula. The origin is the muscle attachment point with the least movement, while the insertion is where most movement occurs. The muscle belly is the segment between the origin and insertion. Synergists are muscles working together for the same motion, while antagonists oppose each other.
Facial muscles are responsible for expressions and mastication (chewing). Thoracic muscles include external intercostals (elevate ribs for inspiration), internal intercostals (depress ribs for forced expiration), and the diaphragm (moves during quiet breathing). Abdominal wall muscles like the rectus abdominis compress the abdomen, and the external abdominal obliques are on the sides.
The trapezius (shoulders/upper back) extends the neck and head. The pectoralis major (chest) elevates the ribs for breathing. The deltoid is found around the shoulder. Triceps (three heads) extend the elbow, while biceps (flexing muscle) flex both the elbows and shoulders. The latissimus dorsi (lower back) extends the shoulder.
Hip and thigh muscles include the iliopsoas (hip movement), gluteus maximus (buttock), and gluteus medius (thigh rotation). The gracilis adducts the thigh and flexes the knee. The hamstring is composed of three structures: biceps femoris, semimembranosus, and semitendinosus. The vastus lateralis, medialis, and intermedius extend the knee. Forearm muscles are also listed.
The tibialis anterior inverts the foot. The gastrocnemius is the calf muscle, moving the foot and leg. The soleus also flexes the foot.