Longissimus muscle

Structure

Longissimus is the most central erector spinae muscle, occupying the intermediate column. Based on their superior attachment, the longissimus muscle fascicles are divided into three parts that span through different segments of the spinal column:

• Longissimus capitis, which is the most cranially positioned.
• Longissimus cervicis is the middle portion of the muscle, located between longissimus capitis and longissimus thoracis.
• Longissimus thoracis, which is the most caudal part and the most prominent component of the erector spinae muscle group. Longissimus thoracis is divided into a lumbar and thoracic portion.

Some sources claim that the lumbar portion of longissimus thoracis is an independent fourth part of the longissimus muscle, named longissimus lumborum. However, it is more widely accepted as a part of longissimus thoracis.

Despite being seemingly different, these parts of the longissimus muscle follow a typical pattern of attachment in which they originate at the transverse elements, and insert into the costal elements of a given vertebral segment. In the cervical region, this pattern is represented by the transverse process and posterior tubercle; at the thoracic segments by the transverse process and the posterior surface of the immediately adjacent rib; and at the lumbar segments by the accessory process and medial half of the transverse process.

Origin and insertion

Longissimus capitis arises from the transverse processes of the first 4-5 thoracic vertebrae (T1-T5), and ascends superiorly across the lateral surface of the semispinalis capitis muscle. Along the way, it attaches to the transverse processes of the last 3-4 cervical vertebrae (C4-C7). Longissimus capitis finally inserts at the lateral surface of the mastoid process of the temporal bone, adjacent to the attachments of the sternocleidomastoid and splenius capitis muscles. 

Longissimus cervicis originates from the transverse processes of the first 5 thoracic vertebrae (T1-T5). It ascends between the tendons of longissimus capitis and longissimus thoracis to insert at the posterior tubercle of the transverse processes of vertebrae C2-C6. 

Longissimus thoracis is divided into a lumbar and thoracic portion, each one having its own set of origins and insertions: 

• Lumbar portion - is usually composed of 5 fascicles, most of which originate from the lumbar intermuscular aponeurosis which is attached to the medial end of the iliac crest. These fascicles ascend and insert into the accessory and transverse processes of vertebrae L1-L4. The inferior most fascicle arises separately from the lumbar intermuscular aponeurosis at the medial part of sacropelvic surface of ilium and the posterior sacroiliac ligament. It inserts into the accessory and transverse processes of vertebra L5.
• Thoracic portion - contains 11-12 fascicles arranged in a tiered fashion across the length of the posterior thoracic wall. Some fascicles originate from the spinous and transverse processes of vertebrae L1-L5 and their supraspinous ligaments. They ascend and insert into the transverse processes of vertebrae T1-T6. Other fascicles originate from the median sacral crest, posterior surface of the sacrum and the posterior iliac crest. They ascend and insert into the transverse processes of vertebrae T7-T12, as well as the angles of adjacent ribs.

Relations

The longissimus muscle is a deep muscle of the back that spans the entire length of the vertebral column. It assumes a central position within the erector spinae group, in between the spinalis and iliocostalis muscles. It is found on either sides of the vertebral bodies, running superficial to the transversospinalis and spinalis thoracis muscles. Longissimus is located deep to the splenius capitis, splenius colli, iliocostalis thoracis, iliocostalis lumborum muscles, and erector spinae aponeurosis.

Close to its insertion point onto the temporal bone, longissimus capitis lies adjacent and deep to the sternocleidomastoid and splenius capitis muscles, which are also attached to the lateral surface of the mastoid process. The occipital artery runs close to this attachment point of the longissimus. Longissimus capitis is also adjacent and superficial to the posterior belly of digastric muscle, which is attached on the deep mastoid notch on its medial surface.

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Innervation

The various parts of longissimus muscle are innervated by branches of the posterior rami of the corresponding regional spinal nerves: 

• Longissimus capitis and cervicis are innervated by the lateral branches of posterior/dorsal rami of cervical spinal nerves. 
• The thoracic part of longissimus thoracis receives innervation from the medial and lateral branches of posterior rami of thoracic spinal nerves. 
• The lumbar part of longissimus thoracis is innervated by the lateral and intermediate branches of posterior rami of lumbar spinal nerves.

Blood supply

The longissimus muscle has an extensive blood supply with a unique arrangement. The arteries supplying it run in grooves between the erector spinae muscle columns and give off branches at various levels: 

• Longissimus capitis and cervicis receive arterial blood from the vertebral artery, deep cervical artery, superficial and deep descending branches of occipital artery and deep branch of the transverse cervical artery.
• Longissimus thoracis is supplied by the dorsal branches of superior intercostal, posterior intercostal, lateral sacral and median sacral arteries.

Venous drainage of the longissimus muscle occurs via the corresponding, similarly named veins.

Function

Generally speaking, longissimus muscle is a  powerful extensor of the vertebral column. However, its exact function depends on the degree of engagement of the different muscle parts. Bilateral contraction of the entire muscle results in extension of the lumbar, thoracic and cervical spine, along with extension of the head and neck. Unilateral contraction of the longissimus muscle results in lateral flexion of the spine on the same side (ipsilaterally). An alternated unilateral contraction of the left and right longissimus aids the walking process by leveling the pelvis and steadying the vertebral column.

Bilateral contraction of only longissimus capitis causes extension of the head and neck only, whereas unilateral contraction causes lateral flexion and rotation of the head ipsilaterally. Through these actions, the longissimus helps to maintain the general posture and the vertebral column curvatures while sitting and standing.

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Longissimus muscle, together with other muscles of the erector spinae group, also has a role in controlling the degree of flexion of the trunk. Flexion of the trunk is initiated by the rectus abdominis and continued under the influence of gravity. When the trunk is fully flexed, further flexion is limited by the passive tension of the longissimus muscle, among other erector spinae muscles. Similarly, lateral flexion is limited by the passive tension of the contralateral erector spinae muscles. 

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