Muscle Tissue

Types of Muscle Tissue

Muscle tissue is specialized for contraction and is classified into three main types, each with distinct microscopic and functional characteristics.

• Skeletal Muscle: Voluntary, striated muscle attached to bones; responsible for body movement.

• Smooth Muscle: Involuntary, non-striated muscle found in walls of hollow organs (e.g., intestines, blood vessels).

• Cardiac Muscle: Involuntary, striated muscle found only in the heart; responsible for pumping blood.

Key Point: Microscopic identification relies on the presence or absence of striations and the arrangement of nuclei.

• Skeletal: Multinucleated, peripheral nuclei, striated.

• Smooth: Single, central nucleus, no striations.

• Cardiac: Single or two central nuclei, striated, intercalated discs present.

Muscle Attachments

Definitions

• Origin: The fixed attachment point of the muscle, typically proximal or closer to the trunk.

• Insertion: The movable attachment point, usually distal or farther from the trunk.

Example: For the biceps brachii, the origin is on the scapula, and the insertion is on the radius.

Types of Skeletal Muscles (Functional Roles)

• Prime Movers (Agonists): Muscles primarily responsible for a specific movement.

• Antagonists: Muscles that oppose the action of the agonist.

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

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

Other Structures

• Tendon: A tough, fibrous connective tissue that connects muscle to bone.

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

Major Skeletal Muscles: Identification and Function

Muscles of the Head, Neck, and Face

• Orbicularis oculi: Closes the eyelids.

• Buccinator: Compresses the cheek.

• Orbicularis oris: Closes and protrudes the lips.

• Temporalis: Elevates the mandible (jaw).

• 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

• External and Internal Intercostals: Aid in breathing by moving the rib cage.

• Diaphragm: Main muscle of respiration; contracts to expand the thoracic cavity.

• Rectus Abdominis: Flexes the vertebral column; compresses abdominal contents.

• External & Internal Obliques: Rotate and laterally flex the trunk.

• Erector Spinae Group: Extends and laterally flexes the vertebral column.

• Serratus Anterior: Protracts and stabilizes the scapula.

Muscles that Move the Arm & Shoulder

• Deltoid: Abducts, flexes, and extends the arm.

• Pectoralis Major: Flexes, adducts, and medially rotates the arm.

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

Muscles that Move the Forearm

• 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

• Gluteus Maximus: Extends and laterally rotates the thigh.

• Iliopsoas: Flexes the thigh at the hip.

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

• 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

• Gastrocnemius: Plantar flexes the foot; flexes the knee.

• Soleus: Plantar flexes the foot.

• Tibialis Anterior: Dorsiflexes and inverts the foot.

Summary Table: Major Muscle Groups and Functions