Overview of Muscle Tissue and Major Skeletal Muscles

Muscle Tissue

Types of Muscle Tissue

Muscle tissue is a specialized tissue found throughout the body, responsible for movement and force generation. There are three main types, each with distinct microscopic features and functions:

• Skeletal muscle: Voluntary muscle attached to bones; characterized by striations and multinucleated fibers. Responsible for body movement.

• Cardiac muscle: Found only in the heart; striated, branched fibers with intercalated discs. Involuntary control.

• Smooth muscle: Non-striated, spindle-shaped cells; found in walls of hollow organs (e.g., intestines, blood vessels). Involuntary control.

Example: Skeletal muscle enables walking, cardiac muscle pumps blood, and smooth muscle moves food through the digestive tract.

Muscle Attachments

Definitions

Muscles attach to bones or other structures at specific points, allowing movement at joints.

• Origin: The fixed attachment point of the muscle, typically proximal or less movable.

• Insertion: The movable attachment point, usually distal or more movable.

Example: The biceps brachii originates at the scapula and inserts on the radius, allowing forearm flexion.

Types of Skeletal Muscles

Functional Roles

Skeletal muscles can be classified based on their roles during movement:

• Prime movers (agonists): Muscles primarily responsible for a specific movement.

• Antagonists: Muscles that oppose the action of the prime mover.

• Synergists: Muscles that assist the prime mover in performing its action.

Example: During elbow flexion, the biceps brachii is the prime mover, the triceps brachii is the antagonist, and the brachialis acts as a synergist.

Other Structures

Connective Tissue Elements

Muscles are connected to bones and other structures via specialized connective tissues:

• Tendon: Dense connective tissue that attaches muscle to bone.

• Aponeurosis: Broad, flat sheet of connective tissue that attaches muscles to bones or other muscles.

Example: The Achilles tendon connects the calf muscles to the heel bone.

Major Skeletal Muscles

Muscles of the Head, Neck, and Face

These muscles control facial expressions, mastication, and head movement.

• Orbicularis oculi: Closes the eyelids.

• Buccinator: Compresses the cheek.

• Orbicularis oris: Closes and protrudes the lips.

• Temporalis: Elevates the mandible.

• Masseter: Elevates the mandible; main chewing muscle.

• Sternocleidomastoid: Flexes and rotates the head.

• Trapezius: Elevates, retracts, and rotates the scapula.

Muscles That Move the Trunk

These muscles are involved in respiration, posture, and movement of the vertebral column.

• External and internal intercostals: Aid in breathing by moving the rib cage.

• Diaphragm: Main muscle of respiration.

• Rectus abdominis: Flexes the vertebral column.

• External & internal obliques: Rotate and laterally flex the trunk.

• Erector spinae group: Extends and maintains posture of the vertebral column.

• Serratus anterior: Protracts the scapula.

Muscles That Move the Arm & Shoulder

These muscles control movement of the upper limb.

• Deltoid: Abducts the arm.

• Pectoralis major: Flexes, adducts, and rotates the arm.

• Latissimus dorsi: Extends, adducts, and medially rotates the arm.

Muscles That Move the Forearm

These muscles are responsible for flexion and extension of the elbow.

• Biceps brachii: Flexes the elbow and supinates the forearm.

• Triceps brachii: Extends the elbow.

• Brachialis: Flexes the elbow.

• Brachioradialis: Flexes the forearm at the elbow.

Muscles That Move the Hip and Knee

These muscles are involved in movement of the thigh and leg.

• Iliopsoas: Flexes the hip.

• Gluteus maximus: Extends and laterally rotates the hip.

• Sartorius: Flexes, abducts, and laterally rotates the thigh.

• Quadriceps femoris group: Extends the knee. Includes:

  • Rectus femoris

  • Vastus medialis

  • Vastus lateralis

  • Vastus intermedius

• Adductor group: Adducts the thigh.

• Hamstring muscles: Flex the knee and extend the hip. Includes:

  • Biceps femoris

  • Semimembranosus

  • Semitendinosus

Muscles That Move the Ankle/Foot

These muscles control movement of the foot and toes.

• Gastrocnemius: Plantar flexes the foot.

• Soleus: Plantar flexes the foot.

• Tibialis anterior: Dorsiflexes the foot.

Summary Table: Major Muscle Groups and Their Actions

Muscle Group	Example Muscles	Main Actions
Head & Neck	Orbicularis oculi, Masseter, Sternocleidomastoid	Facial expression, mastication, head movement
Trunk	Diaphragm, Rectus abdominis, Erector spinae	Respiration, trunk flexion/extension
Arm & Shoulder	Deltoid, Pectoralis major, Latissimus dorsi	Arm abduction, flexion, extension
Forearm	Biceps brachii, Triceps brachii	Elbow flexion/extension
Hip & Knee	Gluteus maximus, Quadriceps, Hamstrings	Hip extension, knee flexion/extension
Ankle/Foot	Gastrocnemius, Tibialis anterior	Plantar flexion, dorsiflexion