3. Back and Neck Muscles - SYNCH

Back Muscles

• Muscles that attach to the spine either move it or use it as a point of attachment, which dictates how the spine generates movement.
• Back muscles are classified as extrinsic or intrinsic.

Extrinsic Back Muscles

• Attach to the spine but do not move it.
• Use the spine and its bony landmarks as an attachment point.
• Insert on the scapula, shoulder blade, or the arm.
• Move the scapula or the shoulder joint.
• Generate movement primarily in the shoulder joint complex.
• Use the back as a way to anchor themselves.
• Help hold the upper limb against the axial skeleton.
• Muscles that attach along the axial skeleton: trapezius, latissimus dorsi, levator scapula, rhomboid minor, and rhomboid major
• Insert elsewhere on the scapula or upper limb.
• Some muscles have shared origin; a broad, shared attachment for some extrinsic and many intrinsic back muscles
• Rely on each other for strength and stability.
• If one muscle becomes weak, another has to work harder to maintain stability.
• Intermediate extrinsic muscles attach along the spinous processes of the spine and then to the rib cage.
• Serratus posterior superior
• Serratus posterior inferior.
• Serratus muscles are thin, weak muscles and referred to as accessory respiratory muscles.
• Serratus posterior superior pulls the upper ribs up.
• Serratus posterior inferior, pulls the lower ribs down to increase space in the thorax during a deep breath in.
• Serratus serve a supporting role in proprioception rather than motor functions.
• Serratus posterior superior lies deep to the rhomboid muscles.
• Serratus posterior inferior lies deep to the latissimus dorsi.
• Innervated by intercostal nerves running within the ribcage.
• Degenerate with age

Intrinsic Back Muscles for Moving the Spine

• Superficial Layer: Spinotransverse Group
• Intermediate Layer: Sacropinalis Group; Erector Spinae (Spinalis, Longissimus, Iliocostalis)
• Deep Layer: Transversospinalis Group; Semispinalis, Multifidus and Rotatores
• Deepest Layer: Inter-Segmental group (interspinales, intertransversarii, levatores costarum)
• Deepest muscles are found intervertebral.

Spinotransversales Group

• Begin on spinous processes (“spino”).
• Run up (superiorly) and out toward transverse processes or the skull (“transverse”).
• Muscles include:
• Splenius cervicis: runs up to the cervical spine
• Splenius capitis attaches to the skull (caput = head).
• Bilateral action (right and left sides working together): extends the head/neck.
• Unilateral action (one side working at a time): lateral flexion and ipsilateral rotation.

Sacrospinalis (Erector Spinae) Group - INTRINSIC

• Anchor at the sacrum and run vertically up the spine.
• Help keep the spine erect.
• Three columns:
• Iliocostalis (most lateral, running from ilium up to the ribs)
• Longissimus (middle column, runs up to the head/neck region)
• Spinalis (most medial, primarily thoracic region near the spinous processes)
• All three share a broad common origin around the sacrum and iliac crest.

Sacrospinalis (Erector Spinae) Actions

• Bilateral action: extension of the spine (hold you upright).
• Unilateral action: ipsilateral lateral flexion (side-bending).
• Longissimus portion that attaches to the head (longissimus capitis) can contribute to head motions.

Transversospinales Group

• Muscles go from transverse processes to spinous processes of higher vertebrae.
• Found more deeply, in the "gutter" between the transverse and spinous processes.
• Three sets:
• Semispinalis (spans several vertebrae; found in thoracic, cervical, and up into the head region as semispinalis thoracis, semispinalis cervicis, semispinalis capitis)
• Multifidus (most developed in the lumbar region)
• Rotatores (best seen in the thoracic region)
• All can extend the spine when working bilaterally.
• Also produce contralateral rotation when working unilaterally (right side rotates you left, and vice versa).
• Multifidus and rotatores, especially in the lumbar and thoracic areas, are also key stabilizers of the vertebral column.

Segmental Group

• Deepest back muscles, found between adjacent vertebrae:
• Interspinales (run between spinous processes)
• Intertransversarii (run between transverse processes)
• Levator costarum (attaches from transverse processes down to the ribs, helping stabilize/elevate ribs)
• Primarily serve to stabilize individual vertebral segments rather than produce large-scale movements.

Suboccipital Triangle Muscles

• Located beneath the occiput (back of the skull), at the atlas (C1) and axis (C2).
• Rectus capitis posterior minor (posterior tubercle of C1 to occiput)
• Rectus capitis posterior major (spinous process of C2 to occiput)
• Obliquus capitis superior (transverse process of C1 to occiput)
• Obliquus capitis inferior (spinous process of C2 to transverse process of C1)
• Collectively, they fine-tune movements at the atlanto-occipital and atlanto-axial joints.
• The two rectus capitis posterior muscles and obliquus capitis superior can extend the head
• Major and inferior can also rotate (C2 involvement).
• The vertebral artery and the suboccipital nerve run through this triangle
• Tension here result in suboccipital headaches.

Anterior Neck Muscles

• Contain superficial and deep layers

Superficial Neck Muscles

• Platysma
• Broad, thin muscle running from fascia over the pectoralis major and deltoid area up into the skin of the lower face. No significant bony attachments.
• Depresses the mandible and tense the skin of the neck.
• Innervated by the facial nerve
• Sternocleidomastoid (SCM) Originates from the sternum and clavicle, inserts on the mastoid process of the temporal bone behind the ear.
• Bilateral action: flexes the cervical spine (though a small upper portion can extend the head).
• Unilateral action: ipsilateral lateral flexion of the neck and contralateral rotation of the head.
• Also can elevate the sternum during forced inspiration (labored breathing).
• Innervated by the spinal accessory nerve.

Deep Anterior Neck Muscles

• Longus capitis and Longus colli
• These run along the anterior cervical spine.
• Bilateral action: flexion of the neck (longus colli) or head (longus capitis).
• Unilateral action: lateral flexion and ipsilateral rotation.
• Rectus capitis anterior and Rectus capitis lateralis
• Attach between C1 (atlas) and the occipital bone.
• Allow subtle flexion (anterior) and lateral flexion (lateralis) of the head on the neck at the atlanto-occipital joint.
• Anterior scalene, Middle scalene, Posterior scalene.
• Run from the transverse processes of the cervical vertebrae to the first rib (anterior and middle) or second rib (posterior). -Bilateral action: help elevate the first or second rib during inspiration, increasing thoracic volume.
• Unilateral action: lateral flexion of the neck.
• Referred to as "guy wires" because they help stabilize the neck.

Innervation Overview

• After exiting each intervertebral foramen, a spinal nerve splits into dorsal and ventral rami.
• Dorsal rami supply the intrinsic back muscles, while the ventral rami supply most anterior/lateral trunk and limb muscles.
• Muscles (e.g., sternocleidomastoid, trapezius, platysma) receive named cranial or peripheral nerve innervation (e.g., accessory nerve, facial nerve).
• Most deep back and anterior neck muscles are innervated by branches of the spinal nerves in the form of dorsal or ventral rami.
• Key takeaway is understanding which groups attach where, which actions they perform, working both bilaterally (extension or flexion) and unilaterally (lateral flexion or rotation).