Back and Anterior Neck Muscles Lecture PDF

**Lecture 3 - Back and Anterior Neck Muscles - SYNCH**\ \ So today we\'re going to be talking about the muscles that attach to the spine and, in some cases, move it; in other cases, do not. Okay. And we\'ll also look at how the spine generates movement as a result of these muscles. So the first thi...

**Lecture 3 - Back and Anterior Neck Muscles - SYNCH**\ \ So today we\'re going to be talking about the muscles that attach to the spine and, in some cases, move it; in other cases, do not. Okay. And we\'ll also look at how the spine generates movement as a result of these muscles. So the first thing that's important to note is that when we look at the muscles of the back, we're actually going to classify them in two different categories: extrinsic and intrinsic. If the muscle is classified as an extrinsic back muscle**, it attaches to the spine but does not move it.** It simply uses the spine and its bony landmarks as an attachment point, and then it inserts on the scapula, shoulder blade, or the arm. So these muscles move the scapula or the shoulder joint. They're called extrinsic because they generate movement primarily in the shoulder joint complex; they just use the back as a way to anchor themselves. They also help hold the upper limb against the axial skeleton. Remember, your axial skeleton is your foundation, your relatively immobile core. The only bony connection of the upper limb to the axial skeleton is **between the clavicle and the sternum**, a very small joint, so the rest is held in place by muscle. Trapezius, latissimus dorsi, levator scapula, rhomboid minor, and rhomboid major all attach along the axial skeleton and then insert elsewhere on the scapula or upper limb. This picture here just shows you the broad, shared origin of some of these big muscles. You can see a large white structure that indicates fascia, which often serves as a tendon-like attachment. In the lumbar region, this is referred to as the thoracolumbar fascia. It's a broad, **shared attachment for some extrinsic and also many intrinsic back muscles.** They rely on each other for strength and stability, so if one of them becomes weak, another has to work harder to maintain the stability of that area. Next, we have the intermediate extrinsic muscles (These muscles too do not aid in movement). These muscles attach along the spinous processes of the spine and then to the rib cage---specifically the upper or lower ribs. They are serratus posterior superior and serratus posterior inferior. Someone thought they looked like the serrated edges of a saw when viewed in dissection, hence "serratus." These are thin, weak muscles that are sometimes referred to as accessory respiratory muscles because serratus posterior superior pulls the upper ribs up and serratus posterior inferior pulls the lower ribs down, thereby increasing space in the thorax during a deep breath in. They are not major muscles of respiration but serve a supporting role in proprioception rather than motor functions.\ \ The serratus posterior superior lies deep to the rhomboid muscles.\ The serratus posterior inferior lies deep to the latissimus dorsi. Both these muscles are innervated by intercostal nerves running within the ribcage. These muscles degenerate with age. That covers our extrinsic muscles (third layer down). Now we get into the intrinsic back muscles, which are responsible **for moving the spine**. We subdivide these into several groups: 1. **Superficial Layer: Spinotransverse Group** Splenius Capitis and Splenius Cervicis 2. **Intermediate Layer: Sacropinalis Group** Erector Spinae (Spinalis, Longissimus, Iliocostalis) 3. **Deep Layer: Transversospinalis Group** Semispinalis, Multifidus and Rotatores 4. **Deepest Layer: Inter-Segmental group\ **(interspinales, intertransversarii, levatores costarum) - Deepest muscles found intervertebral **Spinotransversales Group** "Spino" indicates these muscles begin on spinous processes, and "transverse" indicates they run up (superiorly) and out toward transverse processes or the skull. The two primary muscles here are **splenius cervicis** and **splenius capitis**. 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 rotation to the same side (ipsilateral rotation). **Sacrospinalis (Erector Spinae) Group - INTRINSIC** These muscles anchor at the sacrum and run vertically up the spine. Often called the "erector spinae," they help keep the spine erect. They can be divided into three columns: 1. **Iliocostalis** (most lateral, running from ilium up to the ribs) 2. **Longissimus** (middle column, runs up to the head/neck region) 3. **Spinalis** (most medial, primarily thoracic region near the spinous processes) All three share a broad common origin around the sacrum and iliac crest, often via the thoracolumbar fascia aka deep fascia - **Bilateral action**: extension of the spine (they hold you upright). - **Unilateral action**: ipsilateral lateral flexion (side-bending). - Note that only the portion of longissimus that attaches to the head (longissimus capitis) can contribute to head motions. **Transversospinales Group** The name indicates these muscles go from transverse processes to spinous processes of higher vertebrae. They are found more deeply, in the "gutter" between the transverse and spinous processes. There are three sets: 1. **Semispinalis** (spans several vertebrae; found in thoracic, cervical, and up into the head region as semispinalis thoracis, semispinalis cervicis, semispinalis capitis) 2. **Multifidus** (most developed in the lumbar region) 3. **Rotatores** (best seen in the thoracic region) All can extend the spine when working bilaterally, but they also produce **contralateral rotation** when working unilaterally (the 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** These are the 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) They primarily serve to stabilize individual vertebral segments rather than produce large-scale movements. **Suboccipital Triangle (Posterior Neck/Head)** Beneath the occiput (back of the skull), at the atlas (C1) and axis (C2), there are four small muscles: - **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; the major and inferior oblique can also rotate (C2 involvement). The **vertebral artery** and the **suboccipital nerve** run through this triangle. Tension here is a common source of suboccipital headaches. **Anterior Neck Muscles** On the front of the neck, we have both superficial and deep layers. **Superficial** 1. **Platysma** - A broad, thin muscle running from fascia over the pectoralis major and deltoid area up into the skin of the lower face. It has no significant bony attachments. It helps depress the mandible and tense the skin of the neck. Innervated by the facial nerve (a cranial nerve). 2. **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)** 1. **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. 2. **Rectus capitis anterior** and **Rectus capitis lateralis** - Attach between C1 (atlas) and the occipital bone. - They allow subtle flexion (anterior) and lateral flexion (lateralis) of the head on the neck at the atlanto-occipital joint. **Lateral Neck Muscles** - **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. - Often referred to as "guy wires" because they help stabilize the neck. **Innervation Overview** After exiting each intervertebral foramen, a spinal nerve splits into a **dorsal (posterior) ramus** and a **ventral (anterior) ramus**. The dorsal rami supply the intrinsic back muscles (all the layers that move the spine itself), while the ventral rami supply most anterior/lateral trunk and limb muscles. A few muscles (e.g., sternocleidomastoid, trapezius, platysma) receive named cranial or peripheral nerve innervation (e.g., accessory nerve, facial nerve), but the majority of deep back and anterior neck muscles are innervated by branches of the spinal nerves in the form of dorsal or ventral rami. That's our overview of back and anterior neck muscles and their innervation. The key takeaway is to understand which groups attach where, which actions they perform, and how they work together---both bilaterally for extension or flexion and unilaterally for lateral flexion or rotation (ipsilateral or contralateral).