Ch.10-The Muscular System: Structure, Function, and Major Muscles

The Muscular System

Muscle Tissue Overview

The human body contains three main types of muscle tissue, each with distinct structures and functions. The primary focus in the study of the muscular system is on skeletal muscle, which is responsible for voluntary movements.

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

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

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

Additional info: Skeletal muscle fibers are multinucleated and under conscious control, while cardiac and smooth muscle are controlled by the autonomic nervous system.

How Skeletal Muscles Work Together

Skeletal muscles produce movement by contracting (pulling). They cannot push. During contraction, the insertion of the muscle moves toward the origin.

• Antagonist: A muscle that opposes or reverses a particular movement.

Example: During elbow flexion, the biceps brachii is the agonist (prime mover), and the triceps brachii is the antagonist.

Functional Muscle Groups

Muscles often work in groups to coordinate movement. The main roles include:

• Prime Mover (Agonist): Main muscle responsible for a specific movement.

• Antagonist: Muscle that opposes the action of the prime mover.

• Synergist: Assists the prime mover by adding force or reducing unwanted movement.

• Fixator: Stabilizes the origin of the prime mover so it can act more efficiently.

Example: In a biceps curl, the brachialis acts as a synergist, and the muscles of the shoulder girdle act as fixators.

Naming Skeletal Muscles

Skeletal muscles are named based on several criteria:

• Location: e.g., Temporalis (over the temporal bone)

• Shape: e.g., Deltoid (triangular shape)

• Fiber Direction: e.g., Rectus (straight), Oblique (angled)

• Action: e.g., Flexor, Extensor, Adductor

Additional info: Other naming criteria include muscle size (e.g., maximus, minimus), number of origins (e.g., biceps, triceps), and points of attachment.

Fascicle Arrangement

The arrangement of muscle fibers (fascicles) affects muscle function and shape:

• Parallel: Fibers run parallel to the long axis (e.g., Sartorius).

• Pennate: Short fibers attach obliquely to a central tendon; more powerful due to higher fiber density.

• Circular: Fibers arranged in concentric rings; function as sphincters to close openings.

Example: The orbicularis oris is a circular muscle around the mouth.

Lever Systems in the Body

Muscles, bones, and joints form lever systems to produce movement. The components include:

• Lever: The rigid bar (bone).

• Fulcrum: The fixed point (joint).

• Effort: The force applied by muscle contraction.

• Load: The resistance to be moved.

Mechanical Advantage: When the load is closer to the fulcrum, less effort is needed (power lever). Mechanical Disadvantage: When the effort is closer to the fulcrum, movement is faster but requires more force (speed lever).

Classes of Levers

Class	Arrangement	Example in Body
First Class	Fulcrum – Effort – Load (F-E-L)	Neck extension (atlanto-occipital joint)
Second Class	Fulcrum – Load – Effort (F-L-E)	Standing on tiptoe
Third Class	Effort – Fulcrum – Load (E-F-L)	Biceps curl (most common in body)

Additional info: Third-class levers favor speed and range of motion over force.

Major Muscles and Their Actions

Muscles of the Face and Mastication

• Innervation: Muscles of facial expression are innervated by the facial nerve (cranial nerve VII).

• Prime Movers of Mastication: Masseter and Temporalis muscles.

Muscles of Respiration

• Prime Mover of Inspiration: Diaphragm

• Muscles Elevating Ribs: External intercostals

Shoulder and Arm Prime Movers

• Arm Flexion: Biceps brachii

• Forearm Extension: Triceps brachii

• Forearm Flexion: Biceps brachii and Brachialis

Hip and Thigh Movements

• Largest Muscle in Body: Gluteus maximus

• Longest Muscle in Body: Sartorius

• Primary Knee Extensor Group: Quadriceps femoris

• Hamstrings: Responsible for hip extension and knee flexion

Leg and Foot Muscles

• Primary Plantar Flexors: Gastrocnemius and Soleus

• Anterior Leg Muscles: Mainly cause dorsiflexion of the foot

Additional info: Plantar flexion points the toes downward (as in standing on tiptoe), while dorsiflexion raises the toes upward.