Study Notes: Chapter 9 - The Muscular System (Anatomy & Physiology)

The Muscular System

Overview of Skeletal Muscles

The muscular system consists of all contractile tissues in the body, including skeletal, cardiac, and smooth muscle. This section focuses on skeletal muscle, which is responsible for voluntary movements and is attached to bones via tendons.

• Structure of Skeletal Muscle: Skeletal muscles are made up of muscle cells called muscle fibers.

• Associated Structures: Muscles also contain blood vessels (supply muscle cells with oxygen and nutrients), nerves (coordinate muscle contraction), and connective tissue (provides support and organization).

Gross Anatomy of a Skeletal Muscle

Skeletal muscles are organized by connective tissue (CT) sheaths that support and compartmentalize muscle fibers.

• Epimysium: Dense irregular CT surrounding the entire muscle.

• Perimysium: CT surrounding bundles of muscle fibers called fascicles.

• Endomysium: Fine areolar CT surrounding each individual muscle fiber.

Example: The biceps brachii muscle is surrounded by epimysium, with fascicles grouped by perimysium, and each muscle fiber wrapped in endomysium.

Connective Tissue Sheaths of Skeletal Muscle

Connective tissue sheaths (epimysium, perimysium, endomysium) provide structural support, transmit force, and allow for the passage of nerves and blood vessels.

Muscle Mechanics: Importance of Fascicle Arrangement and Leverage

Muscle force and speed are influenced by the arrangement of fascicles and the mechanics of levers.

• Fascicle Arrangement: Determines the range of motion and power of a muscle.

• Lever Systems: Muscles act on bones as levers to produce movement.

Fascicle Arrangements

Muscle fascicles can be arranged in various patterns, affecting muscle function.

• Circular: Fascicles arranged in concentric rings (e.g., orbicularis oris).

• Parallel: Fascicles parallel to long axis of muscle (e.g., sartorius).

• Convergent: Broad origin, fascicles converge toward single tendon (e.g., pectoralis major).

• Pennate: Fascicles attach obliquely to a central tendon (unipennate, bipennate, multipennate; e.g., rectus femoris).

• Fusiform: Spindle-shaped muscles with parallel fibers (e.g., biceps brachii).

Muscle Shapes and Patterns

Muscle shape is determined by the arrangement of fascicles and affects the muscle's range of motion and power.

• Circular: Controls openings (e.g., mouth, eyes).

• Parallel: Allows for greater shortening but less power.

• Convergent: Versatile movement directions.

• Pennate: Greater power, less range of motion.

• Fusiform: Combination of power and range.

Muscle Mechanics: Lever Systems

Muscles use bones as levers to move loads. The arrangement of the fulcrum, effort, and load determines the mechanical advantage or disadvantage.

• Lever: Rigid bar (bone) that moves on a fixed point called the fulcrum (joint).

• Effort: Force applied by muscle contraction.

• Load: Resistance moved by the effort.

Equation:

$\text{Effort} \times \text{Effort Arm} = \text{Load} \times \text{Load Arm}$

Mechanical Advantage and Disadvantage

• Mechanical Advantage: Effort farther from fulcrum than load; allows small effort to move large load.

• Mechanical Disadvantage: Effort nearer to fulcrum than load; allows load to be moved rapidly over large distance.

Classes of Lever Systems

Levers are classified based on the relative positions of the fulcrum, effort, and load.

Class	Fulcrum Position	Example
First-class	Fulcrum between effort and load	Seesaw, scissors, neck muscles
Second-class	Load between fulcrum and effort	Wheelbarrow, standing on tiptoe
Third-class	Effort between fulcrum and load	Tweezers, biceps brachii

Naming Skeletal Muscles

Muscles are named based on several criteria:

• Location: Body region (e.g., temporalis).

• Shape: Deltoid (triangle), trapezius (trapezoid).

• Size: Maximus (largest), minimus (smallest), longus (long).

• Direction of fibers: Rectus (straight), transversus (fibers at right angles).

• Number of origins: Biceps (2 origins), triceps (3 origins).

• Location of attachments: Named according to origin and insertion (e.g., sternocleidomastoid).

• Action: Flexor, extensor, adductor, etc.

Actions and Interactions of Skeletal Muscles

Muscles can only pull; they never push. Movement results from the coordinated action of several muscles.

• Prime mover (agonist): Main muscle responsible for movement.

• Antagonist: Opposes or reverses a particular movement.

• Synergist: Assists prime mover, adds force, or reduces unnecessary movement.

• Fixator: Stabilizes the origin of the prime mover.

Studying Muscles

Major skeletal muscles are grouped by function and location. Each muscle is described by its shape, origin, insertion, action, and innervation.

• Over 600 skeletal muscles in the body.

• Grouped by function and location.

• Information for each muscle includes: shape, location, origin/insertion, action, innervation.

Muscles of the Head and Neck

Muscles in this region are responsible for facial expression, mastication, and movement of the head and neck.

• Facial Expression: Muscles such as zygomaticus, orbicularis oris, buccinator, and platysma are involved in facial movements.

• Mastication: Muscles like masseter and temporalis are involved in chewing.

• Extrinsic Eye Muscles: Six muscles control eye movement, including superior rectus, inferior rectus, lateral rectus, and medial rectus.

Additional info:

• Muscle diagrams and labeled images provide visual identification of major muscles.

• Students are encouraged to learn muscle names, locations, actions, and innervations for exam preparation.