Deep Muscles of the Back
When I refer to the muscles of the back, I am referring to those muscles that develope around the vertebral column and which acts directly on the vertebral column for some of its movements. What are the movements of the vertebral column? 1) flexion, 2) extension, 3) rotation, and 4) lateral flexion. The deep back or intrinsic muscles of the back help perform 2, and 3, above.

The superficial muscles of the back are the various parts of the erector spinae muscles and have already been pointed out in previous lessons. These were the:

  • iliocostalis lumborum, thoracis and cervicis
  • longissimus thoracis, cervis and capitis
  • spinalis
  • splenius cervicis and capitis
  • semispinalis cervicis and capitis

The deepest muscles of the back are much smaller than the ones making up the erector spinae and will cross only 1, 2 or 3 vertebrae as they pass from their originis to their insertions. These deeper layers along the vertebrae are the:

The remaining deep muscles of the back are actually grouped as the suboccipital triangle because they form a small triangle just inferior to the occipital part of the skull. These muscles attach to either the base of the skull, the atlas or the axis and serve to rotate and extend the head upon the atlas and the axis.
As with other parts of the body, you should always gain an understanding of the skeletal background of a region before you start to relate vessels, nerves and muscles to it. The image to the right shows you that the deep back skeletal region is the vertebral column and the bse of the skull above. You can also see that the deep back muscles have their origins and insertions from and to parts of the vertebrae and the base of the skull. These attachments are represented in the image as red for origins and blue for insertions. The details of the origins and insertions for individual muscles groups are represented below in a table.

Now lets start adding the muscles from the deepest layer to the more superficial one.
The deepest layer of deep back muscles is the rotator group.

Origin: transverse processes

Insertion: lamina of vertebra above

Action: Mainly postural muscles

Nerve: dorsal primary rami of spinal nerves
Just superficial to the rotator muscles are the deepest layer of multifidus muscles. These muscles fill the groove along side the spines of the vertebrae.

Origin: from below, posterior surface of sacrum, posterior tubercles of lumbar vertebrae, transverse processes of the remaining vertebrae.

Insertion: full length of the spinous process of the vertebra above

Action: postural in action

Nerve supply: posterior primary rami of spinal nerves.
The superficial multifidus group of muscles are variable in length and may span 2 or 3 vertebrae before inserting into the spinous processes.

Origin: from below, posterior surface of the sacrum, posterior tubercles of the lumbar transverse processes, and the transverse process of the vertebrae above.

Insertion: full length of the spinous processes of the second or third spinous processes above.

Action: postural in action

Nerve supply: posterior primary rami of spinal nerves in the different regions of the muscles.

The Suboccipital Region
There is a small space located just beneath the base of the skull which includes the atlas and axis (first 2 cervical vertebrae). These bones form the background for describing the suboccipital triangle. The importance of this triangle is probably the fact that the vertebral artery winds through the region and can be prone to excess pressure when a person turns his/her head around to look backward.
The skeleton of the suboccipital region is shown in the figure to the right. As you can see, the following structures form the background for the other structures in the region:
  • base of skull and superior nuchal line
  • posterior arch of the atlas (C1) and its tubercle and transverse process
  • spine and transverse process of the axis (C2)

The deepest structures in the suboccipital triangle are the 1st cervical nerve and the vertebral artery. These are shown in the next 2 diagrams.
The vertebral artery is one of the first branches of the subclavian artery as it passes through the neck. From its origin, it enters the foramen in the transverse process of the 6th cervical vertebra and passes upward through all foramina transversaria until it reaches the top of the posterior arch of the atlas. At this point it lies in a groove there and then enters the cranial cavity through the foramen magnum. One of its primary supplies is the visual cortex in the occipital lobe of the cerebrum.
There are 2 nerves in the suboccipital region. The first cervical nerve arises from the spinal cord as a motor nerve and passes above the posterior arch of the atlas but deep to the vertebral artery. It will supply the muscles of the suboccipital triangle.

Arising from the 2nd cervical nerve is its posterior primary ramus which travels below the posterior arch of the atlas. It a fairly large sensory nerve called the greater occipital nerve and supplies the posterior part of the scalp.
The rectus capitis minor arises from the tubercle on the posterior arch of the atlas and rises to attach to the medial 1/3 of the superior nuchal line and the skull beneath.

Action: extend the skull
The rectus capitis major muscle arises from the spinous process of the axis and passes upwards to attach to most of the superior nuchal line. It overlies the rectus capitis minor. This muscle forms the upper and medial wall of the suboccipital triangle.

Action: extension of skull
As you can see, the inferior oblique muscle arises from the spine of the axis and passes upward and laterally to attach to the transverse process of the atlas. This muscle forms the lower lateral border of the suboccipital triangle.

Action: turns the face towards the same side.
The superior rectus oblique arises from the transverse process of the atlas and passes upward to the lateral part of the superior nuchal line. It forms the upper and lateral border of the suboccipital triangle.

Action: bends the head backwards and to the same side.
Finally, you can see what the suboccipital triangle like when all of the structures are in place.

Note the boundaries of the triangle:

  • rectus capitis posterior major (medial) (RCM)
  • inferior rectus oblique (inferior and lateral) (IRO)
  • superior rectus oblique (super and lateral) (SRO)

The triangle, itself, is represented by the dotted line.

Notice the structures that are located in the depth of the triangle:

  1. posterior arch of the atlas
  2. vertebral artery
  3. C1 (posterior ramus)

All of the muscles of the suboccipital triangle are supplied by branches of the posterior ramus of C1. There apparently is no sensory root to C1.

Since the vertebral artery takes a circuitous course to get to the brain, and it is in a confined region between the base of the skull and the posterior arch of the atlas, it may be compresses when the head is rotated to the left or the right. This is the movement that one uses when backing a car out of a garage or driveway. The vertebral artery is also a common artery for arteriosclerosis to occur with plaque build up and narrowing of the artery. Persons with advanced arteriosclerosis may mention to a doctor that they see spots when they back their car up. When the artery is already narrowed by plaque and further constriction by rotating the head, there is momentary loss of blood supply to the visual cortex. Hence, spots. This suggestion by the patient may alert you to arteriosclerotic disease if it hasn't been identified yet.
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Intrinsic muscles of the back
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