Muscles of the Neck and Vertebral Column: Head Movements and Trunk Extension

Chapter 10: The Muscular System

Muscles of the Neck and Vertebral Column: Head Movements and Trunk Extension

This section covers the major muscles responsible for movements of the head and trunk, focusing on their anatomical locations, functions, and groupings. Understanding these muscles is essential for comprehending posture, locomotion, and head movement in human anatomy.

Functional Groups of Muscles

• Anterolateral neck muscles: Move the head and originate from the axial skeleton.

  • Sternocleidomastoid: Agonist of head flexion; also laterally rotates the head.

  • Scalenes: Elevate the first two ribs, flex and rotate the neck.

• Intrinsic muscles of the back: Originate or associate with the vertebrae, extend the trunk, and maintain posture and the normal curvature of the spine.

  • Splenius (capitis and cervicis): Extend the head.

  • Erector spinae: Largest muscle group for trunk extension and lateral flexion.

    • Iliocostalis: Group of muscles responsible for extension and lateral flexion of the vertebral column.

    • Longissimus (thoracis, cervicis, capitis): Extend and laterally flex the column; capitis extends the head and turns the face toward one side.

    • Spinalis: Extends the vertebral column.

  • Semispinalis (regions: thoracis, cervicis, capitis): Extends vertebral column and head, rotates head and column to opposite side.

  • Quadratus lumborum: Laterally flexes the column.

Key Muscles and Their Functions

• Sternocleidomastoid: Originates from the sternum and clavicle, inserts on the mastoid process. Responsible for flexion and rotation of the head.

• Scalenes: Three muscles (anterior, middle, posterior) located on the lateral neck; assist in neck flexion and elevate ribs during forced inspiration.

• Splenius capitis and cervicis: Located on the posterior neck; extend and rotate the head.

• Erector spinae: A group of three columns (iliocostalis, longissimus, spinalis) running along the vertebral column; primary extensors of the back and maintain posture.

• Semispinalis: Deep to the erector spinae; extends vertebral column and head, rotates them to the opposite side.

• Quadratus lumborum: Located in the lower back; laterally flexes the vertebral column.

Table: Major Muscles of the Neck and Vertebral Column

Muscle	Location	Primary Action
Sternocleidomastoid	Anterolateral neck	Flexes and rotates head
Scalenes (anterior, middle, posterior)	Lateral neck	Flexes neck, elevates ribs
Splenius capitis/cervicis	Posterior neck	Extends and rotates head
Iliocostalis	Lateral vertebral column	Extends and laterally flexes vertebral column
Longissimus	Middle vertebral column	Extends vertebral column, extends and rotates head
Spinalis	Medial vertebral column	Extends vertebral column
Semispinalis	Deep posterior vertebral column	Extends and rotates vertebral column and head
Quadratus lumborum	Lower back	Laterally flexes vertebral column

Definitions of Key Terms

• Flexion: Decreasing the angle between two body parts (e.g., bending the neck forward).

• Extension: Increasing the angle between two body parts (e.g., straightening the back).

• Lateral flexion: Bending the body part to the side.

• Rotation: Moving a body part around its axis (e.g., turning the head).

Examples and Applications

• Posture maintenance: The erector spinae group is essential for maintaining an upright posture and the normal curvature of the spine.

• Breathing: The scalenes elevate the first two ribs during deep inspiration, aiding in breathing.

• Head movement: The sternocleidomastoid allows for nodding and turning the head side to side.

Additional info:

• The muscles described are critical for everyday movements such as looking around, sitting upright, and breathing efficiently.

• Injury or dysfunction in these muscles can lead to postural problems, neck pain, or limited range of motion.