Synergists - muscles working together

Cervical Spine

Flexion

          Sternocleidomastoid (bilaterally)
          Anterior scalene (bilaterally)
          Longus capitis (bilaterally)
          Longus coli (bilaterally)

Extension 
 
          Trapezius - upper fibers (bilaterally)
          Levator scapula (bilaterally)
          Splenius capitis (bilaterally)
          Rectus capitis posterior major
          Rectus capitis posterior minor
          Oblique capitis superior
          Semipinalis capitis
          Longissimus capitis (assists)
          Longissimus cervicis (assists)
          Lliocostalis cervicis (assists)

Rotation - unilaterally to the same side

          Levator scapula
          Splenius capitis
          Splenius cervicis
          Rectus capitis posterior major
          Oblique capitis inferior
          Longus colli
          Longus capitis
          Longissimus capitis (assists)
          Longissimus cervicis (assists)
          Iliocostalis cervicis (assists)

Rotation - unllaterally to the opposite side

          Trapezius - uppper fibers
          Sternocleidomastoid 
          Anterior scalene
          Middle scalene
          Posterior scalene

Lateral Flexion - unilaterally to the same side

          Trapezius - upper fibers
          Levator scapula
          Splenius capitis
          Splenius cerviicis
          Sternoceidomastoid
          Longus capitis
          Longus colli
          Anterior scalen (with ribs fixed)
          Middle scalen (with ribs fixed)
          Posterior scalene (with ribs fixed)
          Longissimus capitis (assists)
          Longissimus cervis (assists)
          lliocostalis cervicis (assists)