The Muscular System: Structure, Function, and Nomenclature

The Muscular System

Overview of Skeletal Muscle Function

The muscular system is responsible for producing movement, maintaining posture, and stabilizing joints. Skeletal muscles typically work in groups and are arranged in antagonistic pairs, allowing for coordinated movement and control.

• Group Actions: Muscles perform movements together, with one group performing an action and another group reversing it.

• Antagonistic Pairs: For every muscle or group that performs an action (e.g., flexion), there is an opposing muscle or group that performs the opposite action (e.g., extension).

• Examples: Flexor-extensor, abductor-adductor pairs.

How Skeletal Muscles Produce Movement

Muscle Attachments and Actions

Skeletal muscles produce movement by exerting force on tendons, which pull on bones. Most muscles span a joint, and contraction changes the angle or position of one bone relative to another.

• Origin: The attachment site on the stationary bone.

• Insertion: The attachment site on the bone that moves.

• Belly: The fleshy part of the muscle between the origin and insertion.

Interactions of Skeletal Muscles

Functional Roles

Muscles are classified based on their roles during movement:

• Prime Mover (Agonist): The main muscle responsible for a specific movement (e.g., biceps brachii for forearm flexion).

• Antagonist: The muscle that opposes the action of the agonist (e.g., triceps brachii for forearm extension).

• Synergists: Assist the prime mover by adding force or reducing unnecessary movement.

• Fixators: Specialized synergists that stabilize the origin of the prime mover.

During most movements, agonists and synergists contract while antagonists relax and are stretched. The roles of these muscles can change depending on the movement.

Naming Skeletal Muscles

Criteria for Muscle Nomenclature

Muscles are named based on several criteria, which often describe their location, shape, size, orientation, number of origins, attachment sites, or action.

• Location: Indicates the region of the body (e.g., tibialis anterior is located on the anterior tibia).

• Shape: Describes the muscle's form (e.g., deltoid is triangular, trapezius is trapezoidal).

• Size: Terms like maximus (largest), minimus (smallest), longus (longest), brevis (short), major (large), and minor (small) are used.

• Direction of Fibers: Rectus (straight), transverse (across), oblique (at an angle).

• Number of Origins: Biceps (two origins), triceps (three), quadriceps (four).

• Location of Attachments: Named for their origin and insertion points (e.g., sternocleidomastoid originates from the sternum and clavicle and inserts on the mastoid process).

• Action: Named for the movement they produce (e.g., flexor carpi radialis flexes the wrist).

Fascicle Arrangement in Skeletal Muscles

Types and Functional Implications

Fascicles are bundles of muscle fibers, and their arrangement affects muscle shape and function.

• Parallel: Fascicles run parallel to the long axis (e.g., sartorius).

• Fusiform: Spindle-shaped with a central belly (e.g., biceps brachii).

• Pennate: Fascicles attach obliquely to a central tendon (unipennate, bipennate, multipennate).

• Convergent: Fascicles converge toward a single tendon (e.g., pectoralis major).

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

Range of Motion (ROM): Muscles with long fibers (parallel, fusiform) have a greater ROM. Power: Muscles with more fibers (convergent, pennate) generate more force.

Lever Systems and Leverage

Biomechanics of Muscle Action

Muscles, bones, and joints form lever systems that determine the efficiency and effectiveness of movement.

• Lever: A rigid bar (bone) that moves on a fixed point (fulcrum, usually a joint).

• Fulcrum: The fixed point around which the lever moves.

• Resistance (Load): The force that opposes movement (e.g., weight of a body part or object).

• Effort: The force applied by muscle contraction to move the load.

Motion occurs when the effort exceeds the resistance, resulting in isotonic contraction.

Leverage: The mechanical advantage gained by a lever. Power is increased when the effort is farther from the fulcrum, while range of motion is increased when the effort is closer to the fulcrum.

Summary Table: Muscle Naming Criteria

Criterion	Example	Description
Location	Tibialis anterior	Located on the anterior tibia
Shape	Deltoid	Triangular shape
Size	Gluteus maximus	Largest muscle of the gluteal group
Direction of Fibers	Rectus abdominis	Fibers run straight
Number of Origins	Biceps brachii	Two origins
Attachment Sites	Sternocleidomastoid	Originates from sternum and clavicle, inserts on mastoid process
Action	Flexor carpi radialis	Flexes the wrist

Major Skeletal Muscles

Students should be familiar with the names, origins, and insertions of major muscles as described in laboratory exercises.

Key Equations

• Lever Law (Mechanical Advantage):

$\text{Effort} \times \text{length of effort arm} = \text{Load} \times \text{length of load arm}$

$\text{(Force} \times \text{distance)} = \text{(Resistance} \times \text{distance)}$

Additional Information

• Details of different leverage system types are not required for Exam 3.

• Refer to laboratory PowerPoint slides for more examples of muscle naming conventions.