Anatomy of the back: spine and back muscles
The back is the body region between the neck and the gluteal regions. It comprises the vertebral column (spine) and two compartments of back muscles; extrinsic and intrinsic. The back functions are many, such as to house and protect the spinal cord, hold the body and head upright, and adjust the movements of the upper and lower limbs.
By the way, have you heard about the myth of Sisyphus, a guy being forced to eternally roll an enormous rock up a hill only for it to roll down when it nears the top? Many would compare anatomy studying to his struggle. But we at Kenhub have a different approach, so in this page we’ve covered the back anatomy in an easy to understand mode for you.
|Bones||Vertebral column: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral (sacrum), 3-5 coccygeal (coccyx) vertebrae|
|Joints||Intervertebral discs, zygapophyseal joints|
|Muscles||Extrinsic (Superficial) muscles
Intrinsic (Deep) muscles
|Nerves||Dorsal rami of the spinal nerves, intercostal nerves, cervical plexus, brachial plexus, accessory nerve (CN XI)|
|Arteries and veins||Vertebral, ascending cervical, occipital, posterior intercostal, subcostal, lumbar, lateral sacral, deep cervical arteries and veins|
|Function||Protection of the spinal cord, absorption of mechanical force, maintains the posture of the body and head, coordination of limbs movements, assists respiration|
- Back muscles
- Nerves and vessels
- Functions of the back
The vertebral column (spine) is the bony core of the back. It is formed by a chain of 33 interconnected vertebrae and their intervening joints. It forms the axial skeleton together with the skull and rib cage. Numerous muscles, ligaments and tendons support the spine, providing it with flexibility and a great range of motion.
The vertebral column is formed by a series of vertically aligned vertebrae. Vertebral alignment produces 4 curvatures of the vertebral column; cervical lordosis, thoracic kyphosis, lumbar lordosis and sacral kyphosis. Lordoses are concave posteriorly, while kyphoses are concave anteriorly. Located centrally within the vertebral column is the vertebral foramen (spinal canal), through which the spinal cord passes.
Vertebrae are the structural constituents of the spine. There are 33 vertebrae in total; seven cervical vertebrae in the neck, twelve thoracic vertebrae in the torso and five lumbar vertebrae in the lower back. Five sacral and 3-5 coccygeal vertebrae are fused into the sacrum and coccyx bones, respectively.
Each typical vertebra consists of a body, an arch and three processes that stem from the arch (spinous, transverse and articular). The vertebral body is the main weight bearing structure of the spine, while the arch and processes provide numerous muscle and ligament attachment points. The arch is connected to the body by bony pedicles, together these structures bound the vertebral foramen. The superior and inferior margins of each pedicle are convex toward its midline, forming superior and inferior vertebral notches. Notches of adjacent vertebrae bound the intervertebral foramen, providing an outlet for the spinal nerves and vessels.
Although having the same parts, the vertebral groups are distinct from each other in terms of their appearance. This is all related to their function. For example, cervical vertebrae have the widest vertebral foramen and the smallest bodies, while lumbar vertebrae have the largest bodies and a very narrow vertebral canal. This is because the lumbar vertebrae bear much more weight than the cervical. Vertebrae also differ in the appearance of their processes and other anatomical features.
Try out our special, customizable, quiz to test your knowledge about the bones of the spine!
Adjacent vertebrae are connected by two intervertebral joints:
- Intervertebral discs - fibrocartilaginous joints (symphysis) interposed between the vertebral bodies.
- Zygapophyseal joints - (z joints or facet joints) formed by the superior and inferior articular processes of adjacent vertebrae.
The column joins with the skull superiorly at the two craniovertebral joints; atlantooccipital and atlantoaxial joints. The former is between the occipital bone and atlas (C1 vertebra), while the latter is between the atlas and axis (C1 and C2 vertebrae). Inferiorly, the vertebral column articulates with the pelvis at sacroiliac joints.
Costovertebral joints anchor the ribs to the thoracic cage through articulations between each rib and its adjacent thoracic vertebrae. The joint of head of rib (costocorporeal joint) is the articulation between the rib head and vertebral body. This articulation is present at all thoracic levels (T1-T12) however, at T2-T9 levels the rib articulates with 2 adjacent vertebral bodies and the intervening disc while at T1, T10-T12 levels the articulation is with a single vertebra. Costotransverse joints is the articulation between the rib neck and tubercle with the adjacent vertebral transverse process. This joint is present only at T1-T10 levels.
Muscles of the back act upon these joints producing the movements of the spine. The movements of the spine are flexion, extension, lateral flexion (sidebending) and rotation. Ligaments of the vertebral column support its joints and stabilize them during spinal movements. The anterior and posterior longitudinal ligaments (ALL, PLL) extend along the entire length of the spine, primarily supporting the intervertebral joints. Accessory ligaments also support the spine, these are the ligamenta flava, interspinous, supraspinous, nuchal and intertransverse ligaments. Other specialised ligaments are involved in supporting the atlantooccipital and atlantoaxial joints.
To learn more about the spine anatomy, function and distinctive anatomy of each group of vertebrae, check out our study materials.
The back muscles are anatomically layered into superficial (extrinsic) and deep (intrinsic) muscles. The extrinsic back muscles are located in the back, but act to produce movements of the shoulder and assist respiration. The intrinsic back muscles are found deeper to the extrinsic muscles, separated from them by the thoracolumbar fascia. They act exclusively upon the joints of the vertebral column.
Our rock-rolling Sisyphus must’ve built fabulous back muscles, right? Let’s learn their anatomy to see why are they so important.
Superficial back muscles
Let us introduce you to each of these muscles presented in our diagram.
The four pairs of suboccipital muscles bound the triangular suboccipital region. They include rectus capitis posterior major, rectus capitis posterior minor, obliquus capitis superior and obliquus capitis inferior. These muscles attach between the inferior nuchal line, atlas and axis. The former three pairs act to produce head extension and ipsilateral rotation, while the latter pair causes contralateral rotation of the head. They are all supplied by the posterior ramus of C1 spinal nerve (suboccipital nerve).
The trapezius muscle consists of three parts; descending, transverse and ascending. By attaching between the occipital bone, scapula, vertebrae and clavicle, this muscle bounds the nuchal region. Trapezius is supplied by the accessory spinal nerve (CN XI) and anterior rami of C3-C4 via cervical plexus. It moves the scapula and causes extension, lateral flexion and rotation of the head and neck.
The latissimus dorsi is also multipartial, having vertebral, iliac, costal, scapular and humeral parts. It is innervated by the thoracodorsal nerve. Its functions are internal rotation, adduction and extension of the arm, it also assists respiration.
Levator scapulae extends from C1-C4 vertebrae to the medial border of scapula. It is innervated by the anterior rami of spinal nerves C3-C4 and dorsal scapular nerve. This muscle draws the scapula superomedially, rotates the glenoid fossa of the shoulder joint inferiorly and flexes the neck ipsilaterally.
The rhomboids include rhomboid major and rhomboid minor. The former spans between the T2-T5 vertebrae and medial border of scapula. The latter attaches from the nuchal ligament and C7-T11 vertebrae to the root of the spine of scapula. They are supplied by the dorsal scapular nerve. Both muscles act upon the scapulothoracic joint where they draw the scapula superomedially, rotate the glenoid cavity inferiorly and support the position of scapula.
Serratus posterior consists of two muscles that assist respiration; serratus posterior superior and serratus posterior inferior. The former attaches between the nuchal ligament, vertebrae C7-T3 and ribs 2-4. Its function is to elevate the ribs. The latter spans from vertebrae T11-L2 and ribs 9-12. It acts to depress the ribs. Both muscles are supplied by anterior rami of spinal nerves T9-T12 (a.k.a. Intercostal nerves 9-11 and subcostal nerve).
Deep back muscles
The deep back muscles are divided into three layers; superficial, intermediate and deep.
The superficial layer contains the splenius cervicis and splenius capitis muscles. They extend and rotate the head and neck. The intermediate layer contains the erector spinae muscles, whose many functions include the extension and lateral flexion of the spine, head and neck. The deep layer consists of the transversospinalis muscles. Their functions include extension, lateral flexion and rotation of the head and spine. All intrinsic muscles are supplied by the posterior rami of spinal nerves.
Now, you’ve probably heard that each of these muscles have their own parts. This is often confusing for anatomy students, so we’ll make a simple table just to get you going before you start reading about them further.
|Superficial layer||Splenius muscles:
- Splenius capitis
- Splenius cervicis
|Intermediate layer||Erector spinae muscles:
- Iliocostalis: cervicis, thoracis, lumborum
- Longissimus: capitis, cervicis, thoracis
- Spinalis: cervicis, thoracis
|Deep layer||Transversospinalis muscles:
- Semispinalis: capitis, cervicis, thoracis
- Multifidis: cervicis, thoracis, lumborum
- Rotatores: breves, longi
With so many layers and parts, the deep back muscles are probably the highest level of muscle facts anatomy game. We’ve created these muscle anatomy reference charts to make your muscle study session easier.
To spare you from hours of memorization, we’ve made some pretty cool video tutorials, articles and quizzes about muscles of the back.
Nerves and vessels
Arteries and veins
The back is supplied by the dorsal branches of the vertebral, ascending cervical, deep cervical, occipital, intercostal, subcostal, lumbar and lateral sacral arteries. A venous network of longitudinal channels form the veins of the spine, called basivertebral and intervertebral veins. These empty into two major networks; internal and external vertebral venous plexuses, and ultimately drain into the vertebral and segmental veins. Occipital, deep cervical, intercostal, subcostal and lumbar veins drain the skin and back muscles into the vertebral veins, azygos venous system and inferior vena cava.
Posterior rami of spinal nerves supply innervation to the skin, spine and intrinsic muscles of the back. Supply to the skin of the back follows the standard segmental dermatome pattern, while intrinsic muscles are innervated by adjacent posterior rami of the spinal nerves. Extrinsic muscles are supplied by branches of the cervical and brachial plexus (latissimus dorsi, levator scapulae, rhomboids), by spinal accessory nerve (trapezius) or by the intercostal and subcostal nerves (serratus posterior muscles). Spinal nerves from cervical, thoracic, lumbar and sacral regions of the spinal cord form four plexuses of the peripheral nervous system after exiting the vertebral column. These are cervical (C1-C4), brachial (C5-T1), lumbar (L1-L4) and sacral (L4-S4) plexuses.
Functions of the back
The vertebral column forms the core structure of the trunk. Its functions are to protect the spinal cord, assist respiration, maintain the upright posture of the body and transmit body weight. The curvatures and overall flexibility of the vertebral column enable shock resilience in sagittal and coronal planes, while the intervertebral discs have a high capacity to absorb shock in the transverse plane. The back bones and muscles coordinate the position of the head with the movements of the body, preventing its extreme extension and flexion.