Muscle System Overview

Introduction to the Muscular System

The muscular system is responsible for producing movement, maintaining posture, and generating heat in the human body. Muscles work by contracting, which pulls on bones or other structures to create movement. There are three main types of muscle tissue: skeletal, cardiac, and smooth, each with distinct structures and functions.

Muscle Action and Body Movement

Relationship Between Muscle Action and Body Movement

Muscles can only pull; they never push. Body movement results from the coordinated action of muscle groups, where one muscle group performs an action and another undoes it. The attachment points of muscles to bones are called the origin (stationary) and insertion (movable). During contraction, the insertion moves toward the origin.

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

• Antagonist: Muscle that opposes or reverses a movement.

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

• Fixator: Stabilizes the origin of the prime mover.

Muscle Attachments: Origin and Insertion

The origin is the attachment point that does not move during contraction, usually more proximal. The insertion is the attachment point that moves during contraction, usually more distal. Muscles attach to bones via tendons, and movement occurs at the joint between origin and insertion.

Muscle Naming Criteria

Muscles are named based on location, shape, size, direction of fibers, number of origins, location of attachments, and action. For example, the sternocleidomastoid muscle is named for its origin (sternum and clavicle) and insertion (mastoid process).

Fascicle Arrangement and Muscle Shape

Fascicle arrangements determine muscle shape and function:

• Circular: Concentric rings (e.g., orbicularis oris)

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

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

• Pennate: Short fascicles attach obliquely to a central tendon (e.g., rectus femoris)

Levers in the Muscular System

Most skeletal muscles use leverage to move loads. The components of a lever system include:

• Lever: Rigid bar (bone)

• Fulcrum: Fixed point (joint)

• Effort: Force applied by muscle contraction

• Load: Resistance moved by the effort

There are three classes of levers, each with different arrangements of load, fulcrum, and effort, affecting mechanical advantage and movement efficiency.

Muscle Tissue Types

Smooth Muscle Tissue

Smooth muscle is found in the walls of most hollow organs (except the heart). It is not striated and lacks sarcomeres but contains overlapping thick and thin filaments. Smooth muscle cells are spindle-shaped and can contract slowly and synchronously due to gap junctions. They are innervated by the autonomic nervous system, with neurotransmitters released from varicosities into diffuse junctions.

• Longitudinal layer: Fibers run parallel to the organ's long axis, shortening the organ when contracted.

• Circular layer: Fibers run around the organ's circumference, constricting the lumen when contracted.

• Regulation: Controlled by nerves, hormones, or local chemicals. One neurotransmitter can have different effects in different organs.

Major Muscles: Attachments and Functions

Muscles of Facial Expression and Mastication

Facial muscles insert into the skin, allowing for a wide range of expressions. Major muscles include:

• Muscles of the scalp

• Muscles of the face

Prime movers of jaw closure: temporalis and masseter. Grinding movements: pterygoids. Chewing role: buccinator. Tongue movements: genioglossus, hyoglossus, styloglossus.

Muscles of the Neck and Back

The sternocleidomastoid divides the neck into anterior and posterior triangles. The erector spinae group (iliocostalis, longissimus, spinalis) and quadratus lumborum extend the trunk and maintain posture.

Muscles of Respiration

Breathing involves two phases: inspiration (inhaling) and expiration (exhaling). The diaphragm and external intercostals are primary inspiratory muscles. Expiration is passive but can involve internal intercostals and abdominal muscles during forced expiration.

Muscles of the Abdominal Wall

Four paired muscles form the lateral and anterior abdominal wall:

• Rectus abdominis

• External obliques

• Internal obliques

• Transversus abdominis

These muscles support trunk movement and assist in functions such as urination, defecation, and childbirth.

Muscles of the Pelvic Diaphragm

The pelvic diaphragm supports pelvic organs, seals the inferior outlet of the pelvis, and resists increased intra-abdominal pressure. Major muscles include the levator ani and coccygeus.

Muscles of the Shoulder and Arm

Shoulder muscles stabilize and move the scapula, increasing the range of arm movements. Major muscles include:

• Pectoralis major

• Latissimus dorsi

• Deltoid

• Rotator cuff muscles: supraspinatus, infraspinatus, teres minor, subscapularis

Muscles of the Arm

The arm is divided into anterior (flexors) and posterior (extensors) compartments:

• Biceps brachii, brachialis, brachioradialis: Flex and supinate the forearm

• Triceps brachii: Powerful forearm extensor

Muscles of the Thigh

Thigh muscles are grouped as anterior (flexors), medial (adductors), and posterior (extensors):

• Iliopsoas, tensor fasciae latae, rectus femoris: Thigh flexors

• Hamstrings (biceps femoris, semitendinosus, semimembranosus): Thigh extensors and knee flexors

• Adductors (magnus, longus, brevis), pectineus, gracilis: Adduct and medially rotate thigh

Muscles of the Leg

Leg muscles control movements at the ankle and toes:

• Anterior compartment: Tibialis anterior, extensor digitorum longus, extensor hallucis longus (dorsiflexion and toe extension)

• Posterior compartment: Gastrocnemius, soleus, tibialis posterior (plantar flexion)

Summary Table: Muscle Types and Functions

Key Equations

• Force of Muscle Contraction:

• Mechanical Advantage (Levers):

Additional info:

• Muscle strength can decline rapidly with disuse (atrophy).

• Muscle fiber types (slow-twitch vs. fast-twitch) affect contraction speed and endurance.