9.1 Structure of Skeletal Muscles

Gross Anatomy of a Skeletal Muscle

Skeletal muscles are composed of specialized cells and connective tissue layers that organize and support muscle fibers. Understanding their structure is essential for grasping how muscles function and generate movement.

• Skeletal Muscle Fibers: Muscle cells, called skeletal muscle fibers, are long and cylindrical. Each is surrounded by a thin layer of connective tissue called the endomyseum.

• Fascicle: Groups of 10–100 muscle fibers are bundled together into a fascicle, which is surrounded by another connective tissue layer called the perimysium.

• Epimysium: All fascicles in a muscle are surrounded by the epimysium, a thicker connective tissue sheath. The epimysium is continuous with the fascia, the most superficial connective tissue sheath.

• Tendons: Connect muscles to bones or other structures, transmitting the force of muscle contraction.

• Blood and Nerve Supply: Skeletal muscles are richly supplied with blood vessels and nerves, essential for function and control.

• Voluntary Control: Skeletal muscle is voluntary, meaning it is under conscious control and stimulated by the nervous system.

Fascicle Patterns and Muscle Shapes

Muscles are classified by the arrangement of their fascicles and their overall shape, which influences their function and movement.

• Parallel Muscle: Fascicles are evenly spaced and run parallel to the muscle's long axis; e.g., Sartorius.

• Convergent Muscle: Broad at one end and tapers to a single tendon; e.g., Pectoralis Major.

• Circular Muscle: Fascicles encircle an opening; act as sphincters; e.g., Orbicularis Oculi.

• Fusiform Muscle: Thicker in the middle, tapered at ends; e.g., Biceps Brachii.

• Pennate Muscle: Fascicles attach to a tendon at an angle, resembling a feather.

  • Unipennate: Fascicles feather out from one side of a tendon; e.g., Flexor Pollicis Longus.

  • Bipennate: Fascicles feather out from both sides of a tendon; e.g., Rectus Femoris.

  • Multipennate: Several tendons with fascicles arranged at multiple angles; e.g., Deltoid.

Naming Muscles

Muscles are named based on several criteria, which often describe their characteristics or functions.

• Size: Terms like major, minor, longus (long), brevis (short), and vastus (broad).

• Location: Directional terms such as superior, inferior, medial, lateral; anatomical regions (e.g., Pectoralis Minor).

• Attachment: Named for structures to which they attach (e.g., Sternocleidomastoid).

• Function: Terms like flexor, extensor, levator, adductor, abductor.

• Other Vocabulary: Some names use common vocabulary; e.g., profundus means "deep" as in Flexor Digitorum Profundus.

9.1 Functions of Skeletal Muscles

Muscle Tension and Movement

The primary function of skeletal muscle is to contract and generate force, known as muscle tension, which produces movement and heat.

• Muscle Tension: Enables body movements (actions) and generates heat as a byproduct of ATP conversion to mechanical energy.

• Heat Production: Explains phenomena such as shivering when cold.

Functional Groups of Muscles

• Agonists (Prime Movers): Provide most of the force for a movement.

• Antagonists: Oppose the action of agonists, allowing for controlled movement.

• Synergists: Work with agonists to guide and refine movement.

• Fixators: Stabilize a bone or joint, anchoring it to allow efficient movement and reduce injury risk.

Muscle Origin and Insertion

• Origin (O): The attachment site that remains relatively stationary during movement.

• Insertion (I): The attachment site that moves during contraction.

• Example: Biceps brachii originates on the scapula (stationary) and inserts on the radius (moves).

• Some muscles can switch origin and insertion depending on the movement (e.g., Iliopsoas during hip flexion vs. trunk flexion).

Lever Systems Used in Body Movements

Muscles and bones interact as lever systems to produce movement. A lever system consists of four components:

• (1) Lever: The rigid bar (bone).

• (2) Load: The object or body part being moved.

• (3) Force: The effort applied by muscle contraction.

• (4) Fulcrum: The joint or hinge point.

Types of Levers

• First-Class Lever: Fulcrum is between the load and the force; e.g., Atlanto-Occipital Joint.

• Second-Class Lever: Load is between the fulcrum and the force; uncommon in the body; e.g., Metatarsophalangeal Joints.

• Third-Class Lever: Force is between the fulcrum and the load; most common in the body; e.g., Elbow Joint (biceps brachii).

Mechanical Advantage and Disadvantage

• Mechanical Advantage: Lever systems use a small force to move a large load.

• Mechanical Disadvantage: Lever systems require a greater force to move the load, but allow for greater speed and range of motion.

9.2 Muscles of Facial Expression

Overview

Muscles of facial expression typically insert into skin or connective tissue, allowing them to change the shape of the face rather than move bones.

• Orbicularis Oculi Muscle: Circular muscle around the eye; closes eyelids (blinking, winking).

• Zygomaticus Major and Minor: Muscles at the corners of the mouth; assist with smiling.

• Orbicularis Oris Muscle: Encircles the mouth; controls movements for eating, drinking, whistling, and speech.

• Buccinator Muscle: Pulls the cheek inward; important for sucking, chewing, and whistling.

• Platysma Muscle: Broad, flat muscle in the superficial neck; produces an open-mouthed, "jaw-dropping" look and tightens the skin of the neck.

Additional info:

• Some explanations and examples were expanded for clarity and completeness.

• Table formatting was not required as no tables were present in the provided images.