Para spinal muscles

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paraspinal muscles anatomy origin insertion innervation function

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Paraspinal Muscles

The paraspinal muscles (also called intrinsic or deep back muscles) are the group of muscles running on either side of the vertebral column, enclosed within the thoracolumbar fascia. They are the primary extensors and stabilizers of the spine.

Layers of Back Musculature

The muscular support of the spine is organized into three layers (- Textbook of Family Medicine 9e):

Layer	Type	Muscles
Superficial	Long, polysegmental	Erector spinae (chief extensors)
Middle	Short, polysegmental	Multifidus
Deep	Small, intersegmental	Rotatores, interspinales, intertransversarii

1. Erector Spinae (Superficial Paraspinal Group)

The erector spinae is the largest paraspinal muscle group. It is divisible into three columns, from lateral to medial (- Fischer's Mastery of Surgery 8e):

A. Iliocostalis (Lateral Column)

Most lateral of the three erector spinae columns
Three parts: iliocostalis lumborum, thoracis, cervicis
Origin: Iliac crest, sacrum, lower ribs
Insertion: Angles of ribs, transverse processes of cervical vertebrae
Action: Extension, lateral flexion of vertebral column; assists respiration

B. Longissimus (Intermediate Column)

Largest of the three columns; occupies the interval between spinalis and angles of lower ribs
Three parts: longissimus thoracis, cervicis, capitis
Origin: Transverse and articular processes of lumbar/thoracic/cervical vertebrae
Insertion: Transverse processes of superior vertebrae, ribs (between tubercle and angle), mastoid process (capitis)
Action: Extension and lateral flexion of spine; extends and rotates head (capitis)

C. Spinalis (Medial Column)

Smallest of the three, adjacent to spinous processes
Three parts: spinalis thoracis, cervicis, capitis
Origin: Spinous processes of T11-L2 (thoracis); C7-T1 and nuchal ligament (cervicis)
Insertion: Spinous processes of T2-T8 (thoracis); C2-C4 (cervicis); occipital bone (capitis)
Action: Extension of spine; lateral flexion

2. Transversospinalis Group (Intermediate/Deep)

These muscles run obliquely from transverse processes (below) to spinous processes (above) - hence the name. Their main role is spinal stabilization.

A. Semispinalis

Spans 4-6 vertebral levels
Three parts: semispinalis capitis, cervicis, thoracis
Origin: Transverse processes of T6-T10 (thoracis); T1-T6 (cervicis); articular processes C4-C7 + transverse processes T1-T6 (capitis)
Insertion: Spinous processes C6-T4 (thoracis); spinous processes C2-C5 (cervicis); between nuchal lines of occipital bone (capitis)
Innervation: Semispinalis capitis - greater occipital nerve (C2) and C3; others - posterior rami of adjacent spinal nerves

B. Multifidus

Spans 2-4 vertebral levels
Runs the full length of the spine, most prominent in the lumbar region
Origin: Sacrum, posterior iliac spine, mammillary processes of lumbar vertebrae, transverse processes of thoracic vertebrae, articular processes of C4-C7
Insertion: Spinous processes 2-4 levels above
Function: Primary stabilizer of individual vertebral segments; especially important for lumbar spine stability
Innervation: Medial branches of posterior rami of spinal nerves

C. Rotatores

Shortest of the transversospinalis group; most developed in thoracic region
Rotatores breves: Origin - transverse process; insertion - lamina/spinous process of vertebra 1 level above
Rotatores longi: Origin - transverse process; insertion - 2 levels above
Function: Bilateral = extension; unilateral = contralateral rotation
Innervation: Medial branches of posterior rami of spinal nerves

3. Segmental (Deepest) Muscles

Muscle	Location	Function
Interspinales	Between adjacent spinous processes	Fine extension control
Intertransversarii	Between adjacent transverse processes	Lateral flexion; stabilization

Innervation - Key Principle

All deep back (paraspinal) muscles are innervated by the dorsal (posterior) primary rami of spinal nerves (- Miller's Review of Orthopaedics 9e). This segmental innervation is why paraspinal muscles can be used for electromyographic localization of nerve root lesions - each segmental level innervates the corresponding paraspinal muscle.

Blood Supply

Dorsal branches of posterior intercostal arteries (thoracic region)
Lumbar arteries (lumbar region)
Lateral sacral arteries (sacral region)

Clinical Relevance

Condition	Paraspinal Involvement
Lumbar strain	Erector spinae and multifidi most often strained; presents with painful paraspinal spasm (- Textbook of Family Medicine 9e)
Disc degeneration / LBP	Paraspinal atrophy (especially multifidus) is common and contributes to instability
Paraspinal EMG	Denervation in paraspinals localizes lesion to nerve root (proximal to dorsal ramus takeoff)
Erector spinae plane block	Regional anesthesia targeting the erector spinae fascia at the transverse process for chest/abdominal surgery (- Barash Clinical Anesthesia 9e)
Posterior spinal surgery	Subperiosteal dissection of paraspinal musculature off spinous processes is the standard approach (- Miller's Review of Orthopaedics 9e)
Paraspinal muscle atrophy on MRI	Loss of cross-sectional area, especially multifidus, is strongly associated with chronic low back pain and intervertebral disc degeneration

Summary

The paraspinal muscles form a hierarchical system:

Superficial (erector spinae): Power generators for spinal extension and lateral flexion
Intermediate (transversospinalis - semispinalis, multifidus): Span multiple segments; control inter-segmental motion
Deep (rotatores, interspinales, intertransversarii): Fine segmental stabilizers

All are innervated by dorsal rami of spinal nerves. The multifidus is the most studied clinically due to its role in lumbar stability and its atrophy in chronic low back pain.

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