Muscles of the Neck & Trunk: Structure, Function, and Innervation

Muscle Groups of the Neck & Trunk

Epaxial vs. Hypaxial Muscles

The muscles of the neck and trunk are classified into two major groups based on their embryological origin and anatomical location: epaxial and hypaxial muscles.

• Epaxial Muscles: Located dorsal (posterior) to the vertebral column. These muscles are primarily responsible for extension and stabilization of the vertebral column.

• Hypaxial Muscles: Located ventral (anterior) to the vertebral column. These muscles are involved in flexion, lateral bending, and movements of the trunk and neck, as well as respiration.

Superficial vs. Deep Muscles: Muscles can also be categorized by their depth:

• Superficial Muscles: Located closer to the skin; often responsible for gross movements.

• Deep Muscles: Located closer to the bones and joints; often responsible for fine motor control and stabilization.

Neck Muscles

Superficial Muscles of the Neck

The superficial muscles of the neck are easily palpable and visible. They play key roles in head movement and stabilization.

• Sternocleidomastoid: Originates from the sternum and clavicle, inserts on the mastoid process of the temporal bone. Responsible for flexion, rotation, and lateral bending of the neck.

• Trapezius: Extends from the occipital bone to the lower thoracic vertebrae and out to the scapula. Elevates, retracts, and rotates the scapula; also assists in neck extension.

Example: Turning your head to the side involves contraction of the sternocleidomastoid on one side and relaxation on the other.

Deep Muscles of the Neck

Deep neck muscles are involved in fine movements and stabilization of the cervical spine.

• Semispinalis Capitis: Extends and rotates the head.

• Splenius Capitis and Cervicis: Extend, rotate, and laterally flex the head and neck.

• Scalene Muscles (Anterior, Middle, Posterior): Elevate the first two ribs during inspiration and assist in lateral flexion of the neck.

Cervical Plexus

Structure and Function

The cervical plexus is a network of nerves formed by the anterior rami of the first four cervical spinal nerves (C1–C4). It provides sensory and motor innervation to parts of the neck, shoulders, and diaphragm.

• Phrenic Nerve: Arises from C3–C5; innervates the diaphragm, essential for breathing.

• Ansa Cervicalis: Supplies the infrahyoid (strap) muscles, which depress the hyoid bone and larynx during swallowing and speech.

Intrinsic Muscles of the Back

Erector Spinae and Transversospinal Groups

The intrinsic muscles of the back are responsible for maintaining posture and controlling movements of the vertebral column.

• Erector Spinae Group: Includes iliocostalis, longissimus, and spinalis. These muscles extend and laterally flex the vertebral column.

• Transversospinal Group: Includes semispinalis, multifidus, and rotatores. These muscles stabilize and rotate the vertebral column.

Articulation of the Ribs with the Vertebrae

Rib-Vetebrae Joints

Ribs articulate with the thoracic vertebrae at two main points:

• Costovertebral Joint: Head of the rib articulates with the body of the vertebra.

• Costotransverse Joint: Tubercle of the rib articulates with the transverse process of the vertebra.

Example: The head of rib 8 articulates with the bodies of T8 and T9, while its tubercle articulates with the transverse process of T8.

Intercostal and Abdominal Muscles

Similarities and Functions

Both intercostal and abdominal muscles are involved in respiration and trunk movement.

• Intercostal Muscles: Located between the ribs; include external, internal, and innermost intercostals. They assist in breathing by elevating or depressing the ribs.

• Abdominal Muscles: Include external oblique, internal oblique, transversus abdominis, and rectus abdominis. They compress abdominal contents, flex and rotate the trunk, and assist in forced expiration.

Trunk Movements and Responsible Muscles

Types of Movements

Trunk movements include flexion, extension, lateral bending, and rotation. Different muscle groups are responsible for each movement.

Movement	Primary Muscles
Flexion	Rectus abdominis, Psoas major
Lateral Bending	External & Internal obliques, Quadratus lumborum
Rotation	External & Internal obliques

Anatomy of the Abdominal Rectus Sheath

Structure and Clinical Relevance

The rectus sheath is a fibrous compartment that encloses the rectus abdominis muscle. Its structure varies above and below the arcuate line.

• Above the Arcuate Line: The sheath has both anterior and posterior layers formed by the aponeuroses of the abdominal muscles.

• Below the Arcuate Line: The sheath is only anterior, as all aponeuroses pass in front of the rectus abdominis.

Clinical Note: The absence of a posterior sheath below the arcuate line makes this area more susceptible to hernias.

Ligaments and Injury: Whiplash

Anterior Longitudinal Ligament

Whiplash injuries often affect the anterior surface of the vertebral bodies.

• Ligament: The anterior longitudinal ligament runs along the anterior surface of the vertebral bodies.

• Function: Prevents hyperextension of the vertebral column.

• Compensation: If damaged, muscles such as the sternocleidomastoid and longus colli may compensate for increased strain.

Example: In a car crash, rapid acceleration-deceleration can stretch and tear this ligament, leading to instability and pain.

Summary Table: Neck & Trunk Muscle Groups

Region	Superficial Muscles	Deep Muscles	Innervation
Neck	Sternocleidomastoid, Trapezius	Semispinalis capitis, Splenius capitis/cervicis, Scalenes	Cervical plexus, Accessory nerve (CN XI)
Back	Trapezius	Erector spinae, Transversospinal group	Dorsal rami of spinal nerves
Thorax	External intercostals	Internal/innermost intercostals	Intercostal nerves
Abdomen	External oblique	Internal oblique, Transversus abdominis, Rectus abdominis	T7–L1 spinal nerves

Additional info: The above notes expand on brief points with academic context, definitions, and clinical relevance for Anatomy & Physiology students.