The Muscular System

Overview of Muscle Tissue

Muscle tissue is composed of contractile cells and is classified into three types: skeletal, cardiac, and smooth muscle. This section focuses on skeletal muscle, which is responsible for voluntary movements of the body.

• Skeletal muscle: Voluntary, striated, attached to bones.

• Cardiac muscle: Involuntary, striated, found in the heart.

• Smooth muscle: Involuntary, non-striated, found in walls of hollow organs.

Muscles can only pull, never push. Movement is produced by the coordinated action of muscle groups.

Functional Groups of Skeletal Muscles

Skeletal muscles are organized into functional groups based on their roles in movement:

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

• Antagonist: Opposes or reverses the action of the prime mover.

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

• Fixator: A type of synergist that stabilizes the origin of the prime mover.

The same muscle may act as a prime mover, antagonist, or synergist depending on the movement.

Naming Skeletal Muscles

Muscles are named according to several criteria, often combined for descriptive clarity:

• Location: Associated bone or region (e.g., Temporalis over temporal bone).

• Shape: Distinctive form (e.g., Deltoid is triangular).

• Size: Relative size (e.g., Maximus = largest, Minimus = smallest, Longus = long).

• Direction of fibers: Orientation (e.g., Rectus = straight, Transversus = right angles, Oblique = angled).

• Number of origins: (e.g., Biceps = two origins, Triceps = three origins).

• Location of attachments: Origin and insertion (e.g., Sternocleidomastoid attaches to sternum, clavicle, and mastoid process).

• Action: Movement produced (e.g., Flexor, Extensor).

Example: Extensor carpi radialis longus (extends wrist, located on radius, long muscle).

Fascicle Arrangements

Fascicles are bundles of muscle fibers whose arrangement determines muscle shape and function.

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

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

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

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

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

Pennate muscles are further classified:

• Unipennate: Fascicles attach to one side of tendon (e.g., Extensor digitorum longus).

• Bipennate: Fascicles attach to both sides (e.g., Rectus femoris).

• Multipennate: Multiple fascicles insert into one tendon (e.g., Deltoid).

Fascicle arrangement affects range of motion and power. Long, parallel fibers allow greater movement but less power; pennate muscles are powerful but have limited range.

Lever Systems in Muscle Action

Most skeletal muscles use lever systems to move bones. A lever consists of:

• Lever: Rigid bar (bone).

• Fulcrum: Fixed point (joint).

• Effort: Force applied by muscle contraction.

• Load: Resistance moved (bone, tissue, weight).

Levers operate at mechanical advantage (power) or disadvantage (speed):

• Mechanical advantage: Load close to fulcrum, effort far; small effort moves large load.

• Mechanical disadvantage: Load far from fulcrum, effort close; load moves rapidly, greater range.

Three classes of levers:

Levers can be described mathematically:

Major Skeletal Muscles of the Body

There are about 600 skeletal muscles, grouped by function and location. Muscles are described by their location, origin and insertion, action, and innervation.

• Origin: Attachment to immovable bone.

• Insertion: Attachment to movable bone.

• Action: Movement produced by contraction.

• Innervation: Nerve supply.

Tips for learning muscles:

• Use muscle names to infer location and function.

• Relate muscle attachments to actions.

• Identify muscles on diagrams and feel for contractions.

Muscles of the Head and Neck

Facial Expression Muscles

Muscles of facial expression insert into skin, not bone, and are important for nonverbal communication. All are innervated by cranial nerve VII (facial nerve).

• Two groups: muscles of the scalp and face.

Mastication and Tongue Movement

Muscles of mastication are innervated by cranial nerve V (trigeminal nerve):

• Prime movers of jaw closure: Temporalis and Masseter

• Grinding movement: Pterygoids

• Chewing role: Buccinator

Muscles promoting tongue movements:

• Genioglossus

• Hyoglossus

• Styloglossus

Neck and Throat Muscles: Swallowing

The sternocleidomastoid divides the neck into anterior and posterior triangles. Anterior muscles are classified as suprahyoid and infrahyoid based on their relation to the hyoid bone.

• Suprahyoid muscles: Form floor of oral cavity, anchor tongue, elevate hyoid bone, move larynx during swallowing.

• Infrahyoid muscles: Depress hyoid bone and larynx during swallowing and speaking.

Swallowing involves coordinated action of tongue, buccinator, pharynx, and epiglottis.

Head Movements and Trunk Extension

• Anterolateral neck muscles: Move the head.

• Intrinsic back muscles: Extend trunk and maintain posture.

Muscles of the Thorax and Abdomen

Deep Muscles of the Thorax: Breathing

Breathing consists of inspiration and expiration:

• Inspiratory muscles: Diaphragm and external intercostals enlarge rib cage.

• Expiration: Relaxation of inspiratory muscles and contraction of internal intercostals decrease rib cage size.

Muscles of the Abdominal Wall

Four paired muscles form the lateral and anterior abdominal wall:

• Rectus abdominis

• External obliques

• Internal obliques

• Transversus abdominis

Fascicles run at angles for added strength. All are innervated by intercostal nerves. Actions include lateral flexion, rotation, and compression of abdominal viscera (urination, defecation, childbirth, vomiting, coughing, screaming).

Muscles of the Pelvic Floor and Perineum

Muscles of the pelvic diaphragm support abdominopelvic organs:

• Levator ani

• Coccygeus

Functions:

• Seal inferior outlet of pelvis

• Support pelvic organs

• Lift pelvic floor to release feces

• Resist increased intraabdominal pressure

Muscles of the urogenital diaphragm and superficial perineal space control urination, erection, and defecation.

Muscles of the Shoulder and Upper Limb

Superficial Muscles of the Thorax

Extrinsic shoulder muscles move the scapula and arm, increasing range of motion. Actions include elevation, depression, rotation, protraction, and retraction.

Muscles Crossing the Shoulder Joint

Nine muscles cross the shoulder joint, inserting on and moving the humerus. Actions include flexion, extension, and adduction.

• Prime movers: Pectoralis major, Latissimus dorsi, Deltoid

• Rotator cuff muscles: Supraspinatus, Infraspinatus, Teres minor, Subscapularis

• Synergists: Coracobrachialis, Teres major

Muscles Crossing the Elbow Joint

Arm muscles are divided into anterior (flexors) and posterior (extensors) compartments.

Muscles of the Forearm

Forearm muscles are divided into anterior (flexors) and posterior (extensors), further subdivided into superficial and deep groups. Actions include wrist, finger, and thumb movements, as well as pronation and supination.

• Pronators: Pronator teres, Pronator quadratus

• Supinator: Synergist with Biceps brachii

Most anterior muscles arise from the medial epicondyle; posterior muscles from the lateral epicondyle of the humerus.

Intrinsic Muscles of the Hand

Small muscles within the palm control fine movements of fingers and thumb.

• Thenar eminence: Ball of thumb

• Hypothenar eminence: Ball of pinky

• Midpalmar muscles: Lumbricals and Interossei (extend, abduct, adduct fingers)

Actions: flexion, extension, abduction, adduction, opposition.

Muscles of the Lower Limb

Muscles Crossing the Hip and Knee Joints

Muscles are grouped as anterior, medial, or posterior. Actions include flexion, extension, abduction, adduction, circumduction, and rotation.

• Thigh flexors: Iliopsoas (iliacus and psoas major), Tensor fasciae latae, Rectus femoris, Sartorius

• Thigh extensors: Hamstring muscles

• Adductors: Adductor magnus, Adductor longus, Adductor brevis, Pectineus, Gracilis

• Abductors and rotators: Gluteus maximus, Gluteus medius, Gluteus minimus, Piriformis, Obturator externus/internus, Gemellus

Quadriceps femoris is a powerful knee extensor, formed from four heads, inserting via the patellar ligament into the tibial tuberosity.

Hamstrings cross both hip and knee joints, are prime movers of thigh extension and knee flexion, and are innervated by the sciatic nerve.

Muscles of the Leg and Foot

Leg muscles are divided into anterior, lateral, and posterior compartments by deep fascia.

• Anterior compartment: Toe extensors and ankle dorsiflexors (Tibialis anterior, Extensor digitorum longus, Fibularis tertius, Extensor hallucis longus)

• Lateral compartment: Plantar flexion and eversion (Fibularis longus, Fibularis brevis)

• Posterior compartment: Plantar flexion (innervated by tibial nerve; divided into superficial and deep groups)

Movements include:

• Ankle: Dorsiflexion and plantar flexion

• Intertarsal joints: Inversion and eversion

• Toes: Flexion and extension

Intrinsic Muscles of the Foot

These muscles flex, extend, abduct, and adduct toes, and support the arches of the foot. Extensor digitorum brevis is a dorsal muscle that helps extend toes. Plantar muscles are organized into four layers.

Additional info: The muscular system is essential for movement, posture, and support of body structures. Understanding muscle names, attachments, and actions is critical for clinical and anatomical applications.