The Muscular System

Overview

Modified from Elaine N. Marieb & Katja Hoehn’s Human Anatomy & Physiology, Tenth Edition
Author: Iryna McGuire

Skeletal Muscles

Origin: Attached to stabilized bone.
Insertion: Moveable end of the muscle, attached to the bone being pulled. Insertion is pulled towards origin.

Functional Groups of Skeletal Muscles

Prime Movers (Agonists): Provide the major force for producing a specific movement.
Antagonists: Oppose or reverse a particular movement.
Synergists: Add force to a movement and reduce undesirable/unnecessary movement.

Naming Skeletal Muscles

Location: Associated with a bone or body region.
Shape: E.g., deltoid muscle (deltoid meaning "triangle").
Relative Size: Terms such as maximus (largest), minimus (smallest), and longus (long).
Direction of fibers or fascicles: E.g., rectus (fibers running straight), transversus, oblique (fibers at angles to an imaginary defined axis).
Number of origins: E.g., biceps (2 origins), triceps (3 origins).
Location of attachments: Named according to point of origin or insertion.
Action: E.g., flexor (muscles that flex) or extensor (muscles that extend).

Muscle Mechanics: Arrangement of Fascicles

Circular: Fascicles arranged in concentric rings (e.g., orbicularis oris).
Convergent: Fascicles converge towards a single tendon insertion (e.g., pectoralis major). Muscle fibers can pull in various directions based on stimulation.
Parallel: Fascicles are parallel to the long axis of a straplike muscle (e.g., sartorius).
Fusiform: Spindle-shaped muscles with parallel fibers (e.g., biceps brachii).
Pennate: Short fascicles attach obliquely to a central tendon running the length of the muscle (e.g., rectus femoris).

Movements of the Body

Flexion: Decreasing angle; bending a joint.
Extension: Increasing angle; straightening a joint.
Abduction: Moving away from the midline.
Adduction: Moving towards the midline.
Rotation: Circular movement around an axis.
Circumduction: Circular movement encompassing flexion, extension, abduction, and adduction.
Elevation/Depression: Moving body parts superiorly or inferiorly, respectively.
Protraction/Retraction: Moving a body part forward or backward, usually concerning the mandible.
Pronation: Radius rotates over ulna.
Supination: The radius and ulna are parallel.
Dorsiflexion: Lifting the foot towards the shin.
Plantar Flexion: Pointing the toes.
Inversion/Eversion: Movements of the foot.

Major Skeletal Muscles of the Body

Grouped by function and location. (slide 13 on ppt)
For each muscle:
- Name and Description: Information reflective of the name.
- Origin and Insertion: Typically, a joint between these points.
- Action: Insertion moves toward the origin, best learned through practical application.
- Innervation: Named according to the major nerve supplying the muscle.

Muscles of the Head

Two Groups:
- Muscles of Facial Expression
- Muscles of Mastication and Tongue Movement

Muscles of Facial Expression

Inserts into the skin and are crucial for nonverbal communication.
All are innervated by cranial nerve VII (facial nerve).

The Scalp

Epicranius (occipitofrontalis): Comprises frontal and occipital bellies, connected by cranial aponeurosis. Alternate actions pull the scalp forward and backward.

The Face

Important examples include:
- Zygomaticus (minor and major): Smiling muscle.
- Orbicularis oris: Muscle encircling the mouth.
- Orbicularis oculi: Muscle encircling the eyes.
- Risorius: Pulls the corners of the mouth.
- Mentalis: Wrinkles the chin.
- Buccinator: Compresses the cheek.

Muscles of Mastication

Involves four pairs of muscles, all innervated by cranial nerve V (the trigeminal nerve).
Prime Movers of Jaw Closure:
- Temporalis
- Masseter (both are responsible for closing the jaw).

Muscles of Tongue Movement

Three Muscles:
- Genioglossus: Protraction.
- Hyoglossus: Depresses the tongue.
- Styloglossus: Retraction.
All are innervated by cranial nerve XII (hypoglossal nerve).

Muscles of the Neck

Two triangles:
- Anterior Triangle: Suprahyoid and Infrahyoid muscles; involved in swallowing and head flexion.
- Posterior Triangle: Includes scalene muscles for side flexion, splenius capitis for extension and rotation, and levator scapulae for rotation.
- Sternocleidomastoid: muscle separates the posterior triangle from the anterior triangle of the neck.
- Platysma: A superficial muscle that stretches across the neck, responsible for tensing the skin and aiding in facial expressions such as frowning.

Muscles of the Thoracic Wall

Includes:
- Pectoralis Major
- Pectoralis Minor
- Serratus Anterior
Intercostal Muscles:
- External Intercostals: Aid inhalation.
- Internal Intercostals: Aid exhalation.

Muscles of the Abdominal Wall (Anterior)

Comprises:
- Rectus Abdominis: Flexes the trunk.
- External Oblique: Rotates the trunk.
- Internal Oblique: Also rotates the trunk.
- Transversus Abdominis: Provides core stability.
Key features:
- Linea Alba: Midline connective tissue.
- Linea Semilunaris: Curved line of the rectus abdominis.
- Tendinous Intersections: Segments of muscle.

Back Muscles

Deep Muscles:
- Splenius Capitis: Offers head extension, rotation, and lateral bending.
- Splenius Cervicis: Facilitates head extension and rotation, as well as stabilization of the cervical spine.
- Erector Spinae: Extends the back and aids lateral bending. Includes:
  - Spinalis (only thoracic)
  - Longissimus
  - Iliocostalis
Superficial Muscles:
- Latissimus Dorsi: Arm movements.
- Trapezius: Stabilizes and moves the scapula.
- Rhomboid Major/Minor: Retract the scapula.
- Levator Scapulae: Elevates scapula.

Shoulder Muscles

Includes:
- Deltoid: Key muscle for arm movement.
- Teres Major: Assists in arm movement.
- Rotator Cuff Muscles:
  - Supraspinatus
  - Infraspinatus
  - Teres Minor
  - Subscapularis

Upper Arm Muscles

Anterior Compartment (Flexors):
- Biceps Brachii: Flexes the elbow.
- Coracobrachialis: Assists in flexion and adduction.
- Brachialis: Primary elbow flexor.
Posterior Compartment (Extensors):
- Triceps Brachii: Extends the elbow; has three heads.

Forearm Muscles

Anterior Compartment:
- Superficial Flexors:
  - Pronator Teres
  - Flexor Carpi Radialis
- Intermediate Flexors:
  - Flexor Digitorum Superficialis
- Deep Flexors:
  - Flexor Digitorum Profundus
  - Flexor Pollicis Longus
  - Pronator Quadratus
  - Includes Palmaris Longus and Flexor Carpi Ulnaris.
Posterior Compartment:
- Superficial Extensors:
  - Brachioradialis
  - Extensor Carpi Radialis Longus/Brevis
  - Extensor Carpi Ulnaris
  - Extensor Digiti Minimi
  - Extensor Digitorum
- Deep Extensors:
  - Anconeus
  - Supinator
  - Abductor Pollicis Longus
  - Extensor Pollicis Brevis/Longus
  - Extensor Indicis.

Intrinsic Muscles of the Palm

Divided into three groups:
- Thenar Group: Muscles of the thumb.
- Hypothenar Group: Muscles of the little finger.
- Intermediate Group: Muscles that flex fingers.

Gluteal Region Muscles

Superficial:
- Gluteus Maximus: Abducts and laterally rotates the thigh.
- Gluteus Medius: Medially rotates the thigh.
- Gluteus Minimus: Assists with medial rotation.
- Tensor Fasciae Latae: Also medially rotates the thigh.

Muscles of the Pelvic Floor

Pelvic Diaphragm
Urogenital Diaphragm
Functions include:
- Sealing the inferior pelvic outlet.
- Supporting pelvic organs.
- Assisting in fecal release.
- Resisting increased intra-abdominal pressure.

Thigh Muscles

Anterior Compartment (extension of the knee):
- Quadriceps:
  - Sartorius
  - Rectus Femoris
  - Vastus Medialis
  - Vastus Lateralis
  - Vastus Intermedius.
Medial Compartment (adductors):
- Pectineus
- Adductor Longus
- Adductor Brevis
- Adductor Magnus
- Gracilis.
Posterior Compartment (flexors of the knee):
- Hamstrings:
  - Biceps Femoris
  - Semitendinosus
  - Semimembranosus.

Muscles of the Leg

Anterior Compartment (dorsiflexion and toe extension):
- Tibialis Anterior
- Extensor Hallucis Longus
- Extensor Digitorum Longus.
Lateral Compartment (eversion and plantarflexion):
- Fibularis Longus
- Fibularis Brevis.
Posterior Compartment (plantarflexion):
- Superficial:
  - Plantaris
  - Gastrocnemius
  - Soleus.
- Deep:
  - Popliteus
  - Tibialis Posterior
  - Flexor Hallucis Longus
  - Flexor Digitorum Longus.

Intrinsic Muscles of the Foot

Functions:
- Help to flex, extend, abduct, and adduct toes.
- Support arches of the foot; some leg tendons assist.
- Dorsal Group: Extensor Digitorum Brevis.
- Plantar Group: Consists of four layers of muscles (flexors, abductors).

Nerves Associated with Upperand Lower Limb Muscles

Radial Nerve: Innervates triceps brachii, anconeus, and muscles of the posterior forearm.
Musculocutaneous Nerve: Innervates biceps brachii, brachialis, and coracobrachialis muscles.
Median Nerve: Innervates the flexor compartment of the forearm (exceptions include the flexor carpi ulnaris and flexor digitorum profundus). Also innervates the thenar muscles and the lateral two lumbricals.
Ulnar Nerve: Innervates the medial half of the flexor digitorum profundus, flexor carpi ulnaris, and hypothenar muscles. Also involved with interossei muscles.
Femoral Nerve: Innervates quadriceps femoris, sartorius, and iliacus. Sensory function includes the hip joint capsule and the anteromedial thigh.
Obturator Nerve: Innervates gracilis, adductor longus, adductor brevis, and adductor magnus (partially). Provides sensory for the skin of the middle part of the medial thigh.
Sciatic Nerve: Splits into the tibial nerve and common peroneal nerve; it innervates the hamstrings partially and other muscles in the leg.

Conclusion

The muscular system comprises a complex network of muscles operating in functional groups, with specific names reflecting their roles, origins, and insertions defined by anatomical landmarks, enabling precise movement and function in the human body.

Summary Notes

Learn what the origin and insertion of a muscle mean. What happens with the origin and insertion of a muscle during muscle contraction?
- During muscle contraction, the origin, which is the fixed point of attachment, remains stable, while the insertion, the movable attachment, is pulled towards the origin, resulting in coordinated movement.
What is an agonist?
- An agonist is a muscle that is primarily responsible for generating a specific movement or action at a joint, often working in opposition to an antagonist muscle.
What is an antagonist?
- An antagonist is a muscle that opposes the action of the agonist, helping to control the movement and stabilize the joint during motion.
What is a synergist?
- A synergist is a muscle that assists the agonist in performing a movement, providing additional force or stability to enhance the overall action.
Learn the major principles of skeletal muscle naming.
- The major principles of skeletal muscle naming are based on several criteria, including the location of the muscle, its size, shape, direction of fibers, number of origins, and its action. These principles help in identifying muscles and understanding their functions.
What does the term biceps mean?
- Biceps means "two heads" or "two origins," which refers to the muscle having two points of attachment at one end.
What does the term rectus mean?
- The term rectus means "straight" in Latin, indicating that these muscles are arranged in a straight line along the body's midline or axis.
Learn the types of fascicle organization of skeletal muscles.
- The types of fascicle organization of skeletal muscles include parallel, pennate, circular, and convergent patterns, each contributing to the muscle's range of motion and strength.
Learn all the muscles that originate on the scapula (muscles of the rotator cuff). What bone do those muscles insert into?
- The muscles that originate on the scapula include the supraspinatus, infraspinatus, teres minor, and subscapularis, all of which insert into the humerus, facilitating shoulder stability and movement. The bone these muscles insert into is the greater and lesser tubercles of the humerus, which enhance the effectiveness of arm rotation and lifting.
What muscle is parallel to the linea alba?
- The muscle that is parallel to the linea alba is the rectus abdominis, which plays a crucial role in flexing the vertebral column and stabilizing the core.
Learn the muscles of the quadriceps group.
- The muscles of the quadriceps group include:
- Rectus femoris: The main muscle that flexes the thigh at the hip and extends the knee.
- Vastus lateralis: Located on the outer thigh, it extends the knee and stabilizes the patella.
- Vastus medialis: Known for its ability to stabilize the patella during movement and extend the knee.
- Vastus intermedius: Situated beneath the rectus femoris, it also aids in knee extension.
Learn the muscles of the hamstring group.
- Biceps femoris: Composed of two heads, it flexes the knee and rotates the leg laterally.
- Semitendinosus: Located in the middle of the hamstring group, this muscle assists in flexing the knee and extending the hip.
- Semimembranosus: The most medial of the hamstrings, it also contributes to knee flexion and hip extension.
Review the skeletal muscle names and actions that we covered in our lectures and labs.
- Quadriceps femoris: A group of four muscles located in the thigh, primarily responsible for extending the knee and stabilizing the patella.
- Gastrocnemius: This muscle forms the bulk of the calf and is responsible for plantarflexing the foot at the ankle joint and flexing the knee.
- Tibialis anterior: Located at the front of the lower leg, it acts to dorsiflex the foot and invert the ankle.