# Lecture 5: Back - Muscles and Nerves
## Unit 1.3: Human Gross Anatomy I - Musculoskeletal System
---
## Learning Objectives
By the end of this lecture, students will be able to:
1. Classify the muscles of the back into superficial, intermediate, and deep (intrinsic) groups
2. Identify the origins, insertions, actions, and innervations of major back muscles
3. Describe the organization of the erector spinae and transversospinalis muscle groups
4. Explain the anatomy of the suboccipital triangle and its contents
5. Describe the organization and distribution of spinal nerves and the posterior rami
6. Correlate muscle function with clinical presentations
---
## Overview of Back Muscles
The muscles of the back are organized into three groups based on their embryological origin, location, and innervation. Understanding this organization helps predict the nerve supply and function of each muscle group and provides a framework for clinical reasoning about back pain and muscle dysfunction.
The superficial back muscles, also called extrinsic muscles, derive from limb bud mesoderm and migrated to the back during development. Despite their posterior location, these muscles act primarily on the upper limb and shoulder girdle. They are innervated by ventral rami of spinal nerves or by the accessory nerve.
The intermediate back muscles are also extrinsic, derived from body wall mesoderm. They function primarily in respiration and are innervated by intercostal nerves, which are ventral rami.
The deep back muscles, also called intrinsic or true back muscles, derive from epaxial mesoderm and are the only muscles that act primarily on the vertebral column. They are innervated exclusively by posterior (dorsal) rami of spinal nerves. This consistent innervation pattern is an important distinguishing feature.
The thoracolumbar fascia is a strong, multilayered sheet of connective tissue that covers the deep back muscles. In the lumbar region, it has three layers that enclose the erector spinae and quadratus lumborum muscles. The thoracolumbar fascia provides attachment for several important muscles including the latissimus dorsi, internal oblique, and transversus abdominis, linking the upper limb, back, and abdominal wall into an integrated functional unit.
Panel A: Cross-sectional view through lumbar region showing superficial layer with trapezius and latissimus dorsi with broad aponeurosis in purple. Panel B: Intermediate layer showing serratus posterior muscles in blue, deep layer showing erector spinae in the paravertebral gutter in red. Panel C: Transversospinalis muscles deep to erector spinae closer to midline, thoracolumbar fascia enclosing deep muscles with three layers demarcated. Panel D: Vertebra at center with spinous and transverse processes, quadratus lumborum within fascia layers, innervation pattern labels indicating ventral versus dorsal rami.
---
## Superficial Back Muscles
The superficial back muscles connect the upper limb and shoulder girdle to the axial skeleton. They include the trapezius, latissimus dorsi, levator scapulae, and rhomboid muscles. Their presence on the back reflects their developmental migration from the limb during embryogenesis.
The trapezius is a large, flat, triangular muscle covering the posterior neck and upper back. It originates from the external occipital protuberance, the nuchal ligament, and the spinous processes from C7 to T12. The muscle fibers converge laterally to insert on the lateral third of the clavicle, the acromion, and the spine of the scapula. The muscle is functionally divided into three parts with different actions. The upper fibers elevate the scapula and extend the neck. The middle fibers retract the scapula, pulling it toward the spine. The lower fibers depress the scapula and rotate it so the glenoid cavity faces upward, essential for raising the arm above horizontal. The trapezius is innervated by the spinal accessory nerve (cranial nerve XI), which provides motor function, while the cervical spinal nerves C3 and C4 provide proprioceptive sensation.
The latissimus dorsi is the widest muscle of the back, forming a broad sheet covering the lower back and sides. It originates from an extensive area including the spinous processes of T7 through T12, the thoracolumbar fascia, the iliac crest, and the lower three or four ribs. The muscle fibers converge and spiral as they pass laterally to insert on the floor of the intertubercular groove of the humerus. The latissimus dorsi is a powerful extensor, adductor, and medial rotator of the arm. It is the primary muscle for pulling movements such as climbing and swimming, and assists in forced expiration by depressing the ribcage. It is innervated by the thoracodorsal nerve (C6-C8), a branch of the posterior cord of the brachial plexus.
The levator scapulae lies deep to the trapezius along the lateral neck. It originates from the transverse processes of C1 through C4 and inserts on the medial border of the scapula from the superior angle to the root of the spine. As its name suggests, it elevates the scapula and also rotates the glenoid cavity inferiorly. It assists in laterally flexing the neck when the scapula is fixed. The levator scapulae is innervated by the dorsal scapular nerve (C5) and direct branches from C3 and C4.
The rhomboid minor and rhomboid major are thin, rhomboid-shaped muscles lying deep to the trapezius. The rhomboid minor originates from the nuchal ligament and spinous processes of C7 and T1, inserting on the medial border of the scapula at the level of the spine. The rhomboid major originates from the spinous processes of T2 through T5 and inserts on the medial border below the spine of the scapula. Both muscles retract the scapula and rotate the glenoid cavity inferiorly. They also stabilize the scapula against the thoracic wall. Both are innervated by the dorsal scapular nerve (C5).
Panel A: Left side showing complete superficial layer with trapezius divided into upper, middle, and lower portions extending from skull and spine to shoulder girdle. Panel B: Latissimus dorsi spanning from lower spine and iliac crest sweeping upward to humerus with fiber direction arrows. Panel C: Right side showing muscles deep to trapezius including levator scapulae, rhomboid minor, and rhomboid major with action arrows. Panel D: Innervation labels showing accessory nerve for trapezius, thoracodorsal nerve for latissimus, dorsal scapular nerve for rhomboids and levator scapulae.
---
## Intermediate Back Muscles
The intermediate back muscles comprise the serratus posterior superior and serratus posterior inferior. These thin muscles lie between the superficial muscles and the true back muscles, and their primary function relates to respiration rather than back movement.
The serratus posterior superior lies deep to the rhomboid muscles in the upper back. It originates from the nuchal ligament and the spinous processes of C7 through T3, passing inferolaterally to insert on ribs 2 through 5, lateral to their angles. When it contracts, it elevates these ribs and may assist inspiration. It is innervated by the intercostal nerves T2 through T5.
The serratus posterior inferior lies deep to the latissimus dorsi in the lower back. It originates from the spinous processes of T11 through L2 and passes superolaterally to insert on ribs 9 through 12, lateral to their angles. It depresses the lower ribs and may assist forced expiration or stabilize the lower ribs during diaphragmatic contraction. It is innervated by the intercostal nerves T9 through T12.
Current research suggests these muscles may function more as proprioceptive sensors than as primary movers, providing feedback about rib cage position and movement during respiration.
Panel A: Serratus posterior superior in blue in upper thoracic region with origin from lower cervical/upper thoracic spinous processes, fibers running inferolaterally to ribs 2-5. Panel B: Arrows indicating upward pull on ribs for inspiration function, intercostal nerve branches innervating the muscle. Panel C: Serratus posterior inferior in green in lower thoracic/upper lumbar region with fibers running superolaterally to ribs 9-12, arrows indicating downward pull. Panel D: Erector spinae muscles visible deep to both muscles, thoracolumbar fascia between muscles, ribs numbered for reference with respiratory function labels.
---
## Deep (Intrinsic) Back Muscles Overview
The deep back muscles are the true muscles of the back, deriving from epaxial mesoderm and functioning primarily to move and stabilize the vertebral column. All are innervated by posterior rami of spinal nerves, which is the defining characteristic of this group. These muscles extend from the sacrum to the skull and are organized into layers based on their depth and orientation.
The deep back muscles are organized into three layers. The superficial layer of deep muscles consists of the splenius muscles. The intermediate layer is the erector spinae, the largest muscle mass of the back. The deep layer comprises the transversospinalis muscles, along with the small segmental muscles.
The primary actions of the deep back muscles are extension of the vertebral column when acting bilaterally and lateral flexion to the same side when acting unilaterally. The deeper muscles, particularly the transversospinalis group, are also important for rotation and for proprioceptive control of vertebral position.
---
## Splenius Muscles
The splenius muscles form the superficial layer of the deep back muscles. They lie just deep to the serratus posterior superior and the rhomboids, wrapping around the deeper muscles of the neck like a bandage, which gives them their name (splenius means bandage in Latin).
The splenius capitis originates from the nuchal ligament and the spinous processes of C7 through T4. Its fibers run superolaterally to insert on the mastoid process of the temporal bone and the lateral part of the superior nuchal line of the occipital bone. When both splenius capitis muscles contract together, they extend the head and neck. When one contracts unilaterally, it produces lateral flexion and rotation of the head to the same side. The splenius capitis is innervated by posterior rami of C2 through C4.
The splenius cervicis lies inferior to the splenius capitis. It originates from the spinous processes of T3 through T6 and inserts on the transverse processes of C1 through C3. It has the same actions as splenius capitis but affects the cervical spine rather than the head. It is also innervated by posterior rami of C2 through C4.
The splenius muscles are active during many daily activities involving head position and neck movement and can be a source of tension headaches when chronically contracted.
Panel A: Lateral view showing splenius capitis running from lower cervical/upper thoracic spinous processes upward and laterally to mastoid process and lateral superior nuchal line. Panel B: Splenius cervicis running from mid-thoracic spinous processes to upper cervical transverse processes with fiber direction arrows. Panel C: Posterior view showing both muscles wrapping around deeper muscles like bandages, covering semispinalis and erector spinae beneath. Panel D: Action arrows showing bilateral extension of head/neck and unilateral lateral flexion and rotation to same side, innervation by posterior rami C2-C4 labeled.
---
## Erector Spinae
The erector spinae is the largest muscle mass of the back, forming the prominent longitudinal bulge visible on either side of the spinous processes. It occupies the "gutter" between the spinous processes medially and the transverse processes and angles of the ribs laterally. The muscle extends from the sacrum and iliac crest to the skull.
The erector spinae is organized into three vertical columns running parallel to the spine, named from lateral to medial: iliocostalis, longissimus, and spinalis. A helpful mnemonic is "I Love Standing" (Iliocostalis, Longissimus, Spinalis from lateral to medial). Each column is further divided into regional parts.
All three columns share a common origin from a broad tendon attached to the posterior sacrum, the medial iliac crest, the sacroiliac ligaments, and the lumbar spinous processes. From this common origin, the muscle fibers diverge as they ascend.
The iliocostalis is the lateral column, inserting primarily on the ribs. The iliocostalis lumborum arises from the common tendon and inserts on the angles of the lower six ribs. The iliocostalis thoracis arises from the lower ribs and inserts on the upper ribs. The iliocostalis cervicis arises from the upper ribs and inserts on the transverse processes of the lower cervical vertebrae.
The longissimus is the intermediate column, inserting on transverse processes and adjacent ribs. The longissimus thoracis arises from the common tendon and inserts on the transverse processes of all thoracic vertebrae and adjacent ribs. The longissimus cervicis arises from the upper thoracic transverse processes and inserts on the cervical transverse processes. The longissimus capitis arises from the upper thoracic transverse processes and lower cervical articular processes and inserts on the mastoid process.
The spinalis is the medial column, running between spinous processes. The spinalis thoracis arises from the spinous processes of the upper lumbar and lower thoracic vertebrae and inserts on the spinous processes of the upper thoracic vertebrae. The spinalis cervicis is often poorly developed or absent. The spinalis capitis usually blends with the semispinalis capitis.
The actions of the erector spinae are extension of the vertebral column and maintenance of erect posture when acting bilaterally, and lateral flexion to the same side when acting unilaterally. The erector spinae is innervated segmentally by posterior rami of spinal nerves at each level.
Panel A: Intact erector spinae as large muscle mass in paravertebral gutter from sacrum to skull, common tendon origin at sacrum and iliac crest. Panel B: Three columns differentiated showing iliocostalis lateral attaching to ribs, longissimus middle attaching to transverse processes, spinalis medial attaching to spinous processes with mnemonic labeled. Panel C: Regional divisions showing iliocostalis lumborum/thoracis/cervicis and longissimus thoracis/cervicis/capitis from sacrum to mastoid. Panel D: Arrows indicating bilateral extension action and unilateral lateral flexion action, posterior rami innervation labeled at each level.
---
## Transversospinalis Muscles
The transversospinalis muscles form the deep layer of intrinsic back muscles, lying deep to the erector spinae. Their distinguishing feature is their oblique orientation, running from the transverse processes upward and medially to the spinous processes. This orientation makes them effective rotators of the vertebral column. The group includes three muscles distinguished by the number of vertebral segments they span.
The semispinalis muscles span four to six vertebral segments and are the most superficial of the transversospinalis group. The semispinalis thoracis arises from the transverse processes of the lower thoracic vertebrae (T6-T10) and inserts on the spinous processes of the upper thoracic and lower cervical vertebrae (C6-T4). The semispinalis cervicis arises from the transverse processes of the upper thoracic vertebrae (T1-T6) and inserts on the spinous processes of the cervical vertebrae (C2-C5). The semispinalis capitis is the largest and most powerful of the group, arising from the transverse processes of C7-T6 and the articular processes of C4-C6, and inserting on the occipital bone between the superior and inferior nuchal lines. This large muscle is a major extensor of the head.
The multifidus muscles span two to four vertebral segments and lie deep to the semispinalis. They are present throughout the spine but are most developed in the lumbar region, where they play a crucial role in stabilizing the lumbar vertebrae. Each fascicle arises from the sacrum, ilium, transverse processes, or mamillary processes and passes upward and medially to insert on the spinous process two to four levels above. The multifidus is increasingly recognized as important for lumbar stability, and its atrophy is associated with chronic low back pain.
The rotatores muscles span only one to two segments and are the deepest of the transversospinalis group. They are most developed in the thoracic region. The rotatores longi span two segments, arising from a transverse process and inserting on the base of the spinous process two levels above. The rotatores breves span one segment, inserting on the spinous process of the immediately superior vertebra. Despite their name, the rotatores may function primarily as proprioceptive sensors rather than prime movers for rotation.
The actions of the transversospinalis muscles are extension of the spine when acting bilaterally and rotation to the opposite side when acting unilaterally. All are innervated by posterior rami of spinal nerves.
Panel A: Erector spinae removed revealing transversospinalis group with semispinalis most superficial spanning 4-6 segments from transverse to spinous processes, semispinalis capitis prominent in cervical region. Panel B: Multifidus intermediate spanning 2-4 segments most developed in lumbar region, rotatores deepest spanning 1-2 segments most developed in thoracic region. Panel C: Cross-sectional inset showing oblique fiber direction from transverse toward spinous process, comparison diagram of segment spans for each muscle. Panel D: Actions labeled showing bilateral extension and unilateral rotation to opposite side, lumbar multifidus highlighted with note about stability importance and atrophy in low back pain.
---
## Segmental Deep Back Muscles
The smallest and deepest back muscles span only single vertebral segments. They include the interspinales, intertransversarii, and levatores costarum. These muscles may function primarily as proprioceptive organs rather than as prime movers.
The interspinales are short paired muscles connecting the spinous processes of adjacent vertebrae. They are best developed in the cervical and lumbar regions where they assist extension and likely provide sensory feedback about spinal position. They are innervated by posterior rami.
The intertransversarii connect the transverse processes of adjacent vertebrae. In the cervical region, they form anterior and posterior groups. They assist lateral flexion and stabilization and are innervated by anterior and posterior rami depending on location.
The levatores costarum are found only in the thoracic region. They arise from the tips of the transverse processes from C7 to T11 and pass inferolaterally to insert on the rib below, between the tubercle and angle. They elevate the ribs during inspiration and may assist in rotation. They are innervated by posterior rami.
---
## Suboccipital Region
The suboccipital region lies deep to the semispinalis capitis and the trapezius, at the junction of the skull and the upper cervical vertebrae. It contains four small muscles that provide fine control of head position and the important suboccipital triangle.
The suboccipital muscles connect the atlas, axis, and skull. The rectus capitis posterior major arises from the spinous process of the axis and inserts on the lateral part of the inferior nuchal line. It extends the head and rotates it to the same side. The rectus capitis posterior minor arises from the posterior tubercle of the atlas and inserts on the medial part of the inferior nuchal line. It extends the head. The obliquus capitis superior arises from the transverse process of the atlas and inserts on the occipital bone between the nuchal lines. It extends and laterally flexes the head. The obliquus capitis inferior arises from the spinous process of the axis and inserts on the transverse process of the atlas. It rotates the atlas (and therefore the head) to the same side, working in conjunction with the atlantoaxial joint.
The suboccipital triangle is a space bounded by three of these muscles. The superomedial boundary is formed by the rectus capitis posterior major. The superolateral boundary is the obliquus capitis superior. The inferior boundary is the obliquus capitis inferior. The floor of the triangle is formed by the posterior atlanto-occipital membrane and the posterior arch of the atlas. This space is important because it contains the vertebral artery as it passes across the posterior arch of the atlas before entering the skull, and the suboccipital nerve (the posterior ramus of C1), which is the sole motor nerve to the suboccipital muscles.
The greater occipital nerve is the posterior ramus of C2, distinct from the suboccipital nerve. It emerges between the atlas and axis, passes deep to the obliquus capitis inferior, then pierces the semispinalis capitis and trapezius to become cutaneous. It provides sensory innervation to the posterior scalp as far forward as the vertex. Irritation of the greater occipital nerve can cause occipital neuralgia, characterized by shooting pain from the upper neck to the back of the head.
Panel A: Posterior aspect of upper cervical spine showing four suboccipital muscles in different colors with rectus capitis posterior major and minor from axis/atlas to inferior nuchal line. Panel B: Obliquus capitis superior from atlas transverse process to occiput, obliquus capitis inferior from axis spinous process to atlas transverse process. Panel C: Suboccipital triangle with boundaries labeled, floor showing posterior atlanto-occipital membrane and atlas posterior arch, vertebral artery crossing floor and suboccipital nerve emerging within. Panel D: Greater occipital nerve course emerging between atlas and axis, passing deep to obliquus capitis inferior, piercing semispinalis and trapezius to reach scalp.
---
## Spinal Nerves and Posterior Rami
Understanding the organization of spinal nerves and their posterior rami is essential for understanding the innervation of the back and the clinical presentation of back pain.
Thirty-one pairs of spinal nerves exit the vertebral column: eight cervical, twelve thoracic, five lumbar, five sacral, and one coccygeal. Each spinal nerve is formed by the union of an anterior (ventral) root carrying motor fibers and a posterior (dorsal) root carrying sensory fibers. The posterior root contains a dorsal root ganglion housing the cell bodies of sensory neurons. The roots unite within or just outside the intervertebral foramen to form the spinal nerve.
Almost immediately after exiting the foramen, each spinal nerve divides into an anterior (ventral) ramus and a posterior (dorsal) ramus. This is a fundamental division: the anterior rami supply the limbs and the anterolateral trunk, while the posterior rami supply the deep back muscles and the skin of the back.
The posterior rami are generally smaller than the anterior rami. With the exception of C1, each posterior ramus divides into a medial branch and a lateral branch. The distribution of these branches varies by region. In the cervical region, the medial branches are predominantly muscular while the lateral branches are predominantly cutaneous. In the upper thoracic region, this pattern reverses: the lateral branches supply muscle while the medial branches supply skin. In the lower thoracic and lumbar regions, the pattern resembles the cervical region again.
Several posterior rami have clinical significance. The suboccipital nerve (posterior ramus of C1) is purely motor, supplying the suboccipital muscles. It has no cutaneous distribution. The greater occipital nerve (posterior ramus of C2) is the largest posterior ramus and provides sensory innervation to a large area of the posterior scalp. The third occipital nerve (posterior ramus of C3) supplies skin over the upper neck and lower occiput. The superior cluneal nerves arise from the posterior rami of L1 through L3 and supply skin over the upper buttock. The middle cluneal nerves arise from the posterior rami of S1 through S3 and supply skin over the middle buttock.
Panel A: Cross-section showing spinal cord with dorsal root and ganglion, ventral root, roots uniting to form spinal nerve, division into anterior and posterior rami. Panel B: Distribution pattern of posterior rami with medial and lateral branches, table showing which branch supplies skin versus muscle at different spinal levels. Panel C: Dermatome map of posterior body surface showing cutaneous territories of posterior rami, greater occipital nerve territory on posterior scalp. Panel D: Cluneal nerves on buttocks with superior from L1-L3 and middle from S1-S3, named nerves labeled including suboccipital, greater occipital, and third occipital.
---
## Clinical Correlations
Low back pain is the most common musculoskeletal complaint in clinical practice. While many causes exist, dysfunction of the deep back muscles, particularly the erector spinae and multifidus, is frequently involved. Studies have shown that the multifidus undergoes atrophy on the affected side in patients with chronic low back pain, and strengthening these muscles is an important component of rehabilitation. The facet joints and their innervation by medial branches of the posterior rami are also important targets for diagnostic and therapeutic interventions.
Whiplash injury results from sudden hyperextension followed by hyperflexion of the cervical spine, typically in motor vehicle accidents. The cervical muscles including the splenius, semispinalis, and multifidus may be strained or torn, and ligaments may be damaged. Symptoms include neck pain, headache, and restricted range of motion. Recovery often requires physical therapy to restore muscle function and flexibility.
Accessory nerve injury causes paralysis of the trapezius muscle. Because this nerve is superficial in the posterior triangle of the neck, it may be damaged by penetrating injuries, lymph node biopsies, or surgical procedures. Patients present with a drooping shoulder, weakness of shoulder elevation and scapular retraction, and difficulty raising the arm above horizontal. The scapula may show medial winging, distinct from the lateral winging seen in serratus anterior paralysis.
Thoracodorsal nerve injury causes weakness of the latissimus dorsi, impairing arm adduction, extension, and medial rotation. Patients have difficulty with pulling movements, climbing, and swimming. The nerve may be injured during axillary surgery or lymph node dissection.
Occipital neuralgia results from irritation of the greater occipital nerve and presents as sharp, shooting, or stabbing pain originating in the upper neck and radiating to the back of the head. It may result from muscle tension, trauma, compression, or inflammation. Treatment options include physical therapy, nerve blocks, and occasionally surgical decompression.
---
## Summary
Back muscles are organized into superficial, intermediate, and deep groups based on their embryological origin and innervation. The superficial muscles (trapezius, latissimus dorsi, rhomboids, levator scapulae) act primarily on the upper limb and are innervated by ventral rami or the accessory nerve. The intermediate muscles (serratus posterior superior and inferior) assist respiration and are innervated by intercostal nerves.
The deep (intrinsic) back muscles are innervated by posterior rami and act on the vertebral column. The erector spinae is the largest back muscle mass, organized into three columns from lateral to medial: iliocostalis, longissimus, and spinalis. The transversospinalis muscles (semispinalis, multifidus, rotatores) run obliquely from transverse to spinous processes and rotate the spine to the opposite side.
The suboccipital region contains four muscles that control fine head movements and the suboccipital triangle, which transmits the vertebral artery and the suboccipital nerve (C1 posterior ramus). The greater occipital nerve (C2 posterior ramus) provides sensory innervation to the posterior scalp.
Posterior rami of spinal nerves innervate the deep back muscles and the skin of the back. Their medial and lateral branches have variable distributions depending on the spinal level.
---
## Key Terms
| Term | Definition |
|------|------------|
| Erector spinae | Three-column muscle group (iliocostalis, longissimus, spinalis) that extends and laterally flexes the vertebral column |
| Transversospinalis | Deep muscles running obliquely from transverse to spinous processes; rotate the spine to the opposite side |
| Suboccipital triangle | Muscular space containing the vertebral artery and the posterior ramus of C1 |
| Posterior ramus | Branch of each spinal nerve that innervates deep back muscles and skin of the back |
| Greater occipital nerve | Posterior ramus of C2; provides sensory innervation to the posterior scalp |
| Thoracodorsal nerve | Branch of the brachial plexus that innervates the latissimus dorsi |
---
*This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*