Axial Musculature: Head, Neck, and Trunk Study Notes

Axial Musculature

Introduction

The axial musculature includes the muscles of the head, neck, and trunk. These muscles provide strength, stability, and facilitate specialized functions such as facial expression, mastication (chewing), breathing, and posture. Understanding their attachments, actions, and innervation is essential for comprehending their roles in human movement and physiology.

Muscles of the Head

Facial Expression Muscles

Muscles of facial expression originate on the skull and insert into the skin, allowing for a wide range of facial movements. Their unique insertions enable the skin to move more than in other body regions, creating expressions.

• Orbicularis oris: Circular muscle that closes and protrudes the lips.

• Orbicularis oculi: Circular muscle that closes the eyelids.

• Occipitofrontalis: Raises the scalp and eyebrows; consists of frontal and occipital bellies connected by the epicranial aponeurosis.

• Zygomaticus major and minor: Elevate the corners of the mouth (smiling/laughing).

• Risorius: Draws the angle of the mouth laterally (grinning).

• Levator labii superioris: Raises and protrudes the upper lip.

• Mentalis: Protrudes the lower lip and elevates the chin (pouting).

• Depressor labii inferioris: Depresses the lower lip.

• Buccinator: Compresses the cheek, assists in chewing, whistling, and blowing.

Muscles of Mastication

These muscles are responsible for moving the mandible during chewing and speaking.

• Masseter: Elevates the mandible to close the mouth; primary muscle for chewing.

• Temporalis: Elevates and retracts the mandible.

• Buccinator: Assists in holding food between teeth during chewing.

Table: Muscles of the Head

Muscle Name	Primary Action	Origin	Insertion	Innervation
Frontalis	Raises eyebrows, draws scalp anteriorly	Galea aponeurotica	Subcutaneous tissue of eyebrows	Facial nerve (VII)
Temporalis	Elevates (closes) mandible	Temporal fossa	Coronoid process and ramus of mandible	Trigeminal nerve (V)
Masseter	Elevates (closes) mandible	Zygomatic arch	Angle and ramus of mandible	Trigeminal nerve (V)
Orbicularis oculi	Closes eyelid	Frontal and maxilla on medial margin of orbit	Skin of eyelid	Facial nerve (VII)
Orbicularis oris	Closes lips	Modiolus of mouth	Skin near lips	Facial nerve (VII)
Risorius	Abducts corner of mouth	Fascia of masseter, zygomatic bone	Modiolus at angle of mouth	Facial nerve (VII)
Mentalis	Protrudes lower lip	Anterior mandible	Skin of chin below lower lip	Facial nerve (VII)
Buccinator	Compresses cheek	Maxilla and mandible near molar teeth	Orbicularis oris	Facial nerve (VII)
Zygomaticus (major & minor)	Elevates corner of mouth	Zygomatic bone	Superolateral angle of mouth, upper lip muscles	Facial nerve (VII)
Depressor labii inferioris	Depresses inferior lip	Mandible	Muscles and skin of inferior lip	Facial nerve (VII)
Levator labii superioris	Elevates superior lip, flares nostril	Maxilla and zygomatic bones	Muscles of superior lip	Facial nerve (VII)

Muscles of the Neck and Back

Anterior Neck Muscles

These muscles assist in swallowing and speech by controlling the larynx and hyoid bone. They are categorized by their position relative to the hyoid bone.

• Digastric: Elevates the hyoid bone and depresses the mandible.

• Mylohyoid: Forms the floor of the oral cavity, elevates the hyoid, and depresses the tongue.

• Omohyoid: Depresses the hyoid bone.

Lateral and Posterior Neck Muscles

These muscles balance, move, and rotate the head. Unilateral contraction rotates the head, while bilateral contraction flexes or extends it.

• Sternocleidomastoid: Abducts, rotates, and flexes the head; divides the neck into anterior and posterior triangles.

• Platysma: Depresses the mandible and lower lip, tenses skin of the neck.

• Scalene muscles (anterior, middle, posterior): Flex and rotate the neck, elevate ribs during deep inhalation.

• Levator scapulae: Elevates the scapula and assists in neck movement.

Muscles of the Back

The back muscles stabilize and move the vertebral column. The erector spinae group is the primary extensor, controlling flexion, lateral flexion, and rotation of the vertebral column.

• Iliocostalis: Lateral column of erector spinae.

• Longissimus: Intermediate column.

• Spinalis: Medial column.

Table: Muscles of the Neck and Back

Muscle Name	Primary Action	Origin	Insertion	Innervation
Levator scapulae	Elevates scapula, abducts and rotates neck	Vertebrae C1-4	Upper vertebral border of scapula	Spinal nerves C3-5, posterior scapular nerve
Scalenes	Flex and rotate neck, elevate ribs 1 and 2	Transverse process of cervical vertebrae	Ribs 1 and 2	Spinal nerves C3-8
Sternocleidomastoid	Abduct, rotate, and flex head	Sternum, clavicle	Mastoid process of temporal bone	Spinal nerves C2-4, accessory nerve (XI)
Omohyoid	Depress hyoid bone	Superior border of scapula	Hyoid bone	Spinal nerves C1-3
Platysma	Depress inferior lip, open jaw	Fascia covering pectoralis major and deltoid	Mandible and skin of inferior face	Facial nerve (VII)
Digastric	Open mouth; depress mandible	Inferior, distal margin of mandible, mastoid notch of temporal	Hyoid bone	Trigeminal (V) and facial (VII) nerves
Mylohyoid	Elevate hyoid bone and tongue	Inferior margin of mandible	Hyoid bone	Trigeminal nerve (V)
Erector spinae	Extend and rotate vertebral column and head	Nuchal ligament, thoracic/lumbar vertebrae, sacrum, ilium, ribs 3-12	All ribs, thoracic vertebrae, temporal bones	Dorsal rami of spinal nerves

Muscles That Position the Pectoral Girdle

The pectoral girdle consists of the clavicle, scapula, and proximal humerus, stabilized by muscles on the anterior and posterior thorax. These muscles allow for a wide range of arm movements.

• Serratus anterior: Abducts scapula, moves it away from the spine.

• Pectoralis minor: Depresses the scapula.

• Trapezius: Extends and abducts the head, rotates and adducts scapula.

• Rhomboid major and minor: Adduct the scapula.

• Pectoralis major: Flexes, adducts, and medially rotates the humerus.

• Latissimus dorsi: Extends, adducts, and medially rotates the humerus.

Table: Muscles of the Pectoral Girdle

Muscle Name	Primary Action	Origin	Insertion	Innervation
Pectoralis major	Flex, adduct, medially rotate humerus	Clavicle, sternum, ribs 1-7	Crest of greater tubercle of humerus	Medial/lateral pectoral nerves
Pectoralis minor	Depress glenoid cavity	Ribs 3-5	Coracoid process of scapula	Medial/lateral pectoral nerves
Trapezius	Extend/abduct head, rotate/adduct scapula	Occipital bone, nuchal ligament, vertebrae C7-T4	Clavicle, acromion, spine of scapula	Accessory nerve (XI)
Latissimus dorsi	Extend, adduct, medially rotate humerus	Vertebrae T7-12, L1-5, ilium, ribs 10-12	Intertubercular sulcus of humerus	Thoracodorsal nerve
Rhomboid major	Adduct scapula	Spines of vertebrae T2-5	Medial border of scapula	Dorsal scapular nerve
Rhomboid minor	Adduct scapula	Nuchal ligament, spines of vertebrae C7-T1	Medial border of scapula	Dorsal scapular nerve

Muscles of the Thorax

Intercostal Muscles

These muscles facilitate breathing by altering the size of the thoracic cavity.

• External intercostals: Elevate the ribs during inspiration.

• Internal intercostals: Depress the ribs during forced expiration.

• Innermost intercostals: Assist internal intercostals.

Diaphragm

The diaphragm is the primary muscle of respiration, separating the thoracic and abdominal cavities. Its contraction increases thoracic volume, causing inhalation; relaxation causes exhalation. It also plays a role in increasing intra-abdominal pressure during defecation, urination, and childbirth (Valsalva maneuver).

Table: Muscles of the Thorax

Muscle Name	Primary Action	Origin	Insertion	Innervation
External intercostals	Elevate ribs (inspiration)	Inferior border of ribs 1-11	Superior border of next inferior rib	Intercostal nerves
Internal intercostals	Depress ribs (expiration)	Superior border of ribs 2-12	Inferior border of next superior rib	Intercostal nerves
Diaphragm	Inspiration	Xiphoid process, ribs 7-12, lumbar vertebrae	Central tendon of diaphragm	Phrenic nerve

Muscles of the Abdomen

The abdominal muscles stabilize the trunk, maintain posture, and protect internal organs. They are arranged in layers with different fiber orientations for strength and flexibility.

• Rectus abdominis: Flexes the vertebral column and compresses the abdomen.

• External abdominal oblique: Compresses the abdominal wall, laterally rotates the waist.

• Internal abdominal oblique: Compresses the abdominal wall, laterally rotates the waist.

• Transverse abdominis: Compresses the abdominal wall.

• Quadratus lumborum: Extends and abducts the vertebral column.

Table: Muscles of the Abdomen

Muscle Name	Primary Action	Origin	Insertion	Innervation
External abdominal oblique	Compress abdominal wall, laterally rotate waist	Ribs 5-12	Iliac crest, pubis	Intercostal nerves T7-12
Internal abdominal oblique	Compress abdominal wall, laterally rotate waist	Inguinal ligament, iliac crest	Pubis, ribs 10-12	Intercostal nerves T7-12, spinal nerve L1
Transverse abdominal	Compress abdominal wall	Inguinal ligament, iliac crest, costal cartilages 7-12	Linea alba, pubis	Intercostal nerves T7-12, spinal nerve L1
Rectus abdominis	Flex vertebral column, compress abdominal wall	Crest of pubis, pubic symphysis	Costal cartilages 5-7, xiphoid process	Intercostal nerves T6-12
Serratus anterior	Abduct scapula	Ribs 1-8	Vertebral border and inferior angle of scapula	Long thoracic nerve
Quadratus lumborum	Extend and abduct vertebral column	Iliac crest	Vertebrae L1-4, rib 12	L1-4, T12

Summary Table: Key Muscle Actions and Innervations

Muscle	Action	Innervation
Temporalis	Elevates and retracts mandible	Trigeminal nerve (V)
Masseter	Elevates mandible	Trigeminal nerve (V)
Zygomaticus (major & minor)	Elevates corners of mouth	Facial nerve (VII)
Sternocleidomastoid	Abducts, rotates, flexes head	Accessory nerve (XI), spinal nerves C2-4
Rectus abdominis	Flexes vertebral column, compresses abdomen	Intercostal nerves T6-12

Additional info:

• The Valsalva maneuver is a coordinated action of the diaphragm and abdominal muscles to increase intra-abdominal pressure, aiding in defecation, urination, and childbirth.

• The linea alba is a fibrous structure running down the midline of the abdomen, where the aponeuroses of the abdominal muscles meet.