Back, Vertebral Column, and Spinal Cord Anatomy PDF

Back, vertebral column, and spinal cord Gillian Moritz, PhD [email protected] 1 Learning Objective Clinical Relevance 1. Name the parts of a typical vertebra and describe the features of each regional type; understand the anatomy of the atlas and the axis and how these two vertebrae contribute to...

Back, vertebral column, and spinal cord Gillian Moritz, PhD [email protected] 1 Learning Objective Clinical Relevance 1. Name the parts of a typical vertebra and describe the features of each regional type; understand the anatomy of the atlas and the axis and how these two vertebrae contribute to head and neck movements. Physical exam; evaluation of back pain (e.g. osteoarthritis, herniated nucleus pulposus); evaluation of injury during trauma (e.g. fracture); evaluation of congenital anomalies (e.g. spina bifida, spondylolisthesis); execution/evaluation of imaging studies 2. Describe the organization (bones, joints, ligaments), function, and movements of the vertebral column. Physical exam; evaluation of back pain (e.g. osteoarthritis, herniated nucleus pulposus); evaluation of injury during trauma (e.g. fracture); evaluation of congenital/aquired anomalies (e.g. spondylolisthesis, torticollis); execution/evaluation of imaging studies Physical exam; evaluation of back pain (e.g. lordosis with pregnancy); evaluation of congenital anomalies (e.g. scoliosis); execution/evaluation of imaging studies 3. Discuss normal and abnormal curvatures of the vertebral column: primary, secondary, kyphosis, lordosis, scoliosis. 4. Understand the intervertebral disc structure and relate its structure to disc herniation. Physical exam; evaluation of pain; evaluation of imaging studies 5. Describe spondylolysis and spondylolisthesis. Physical exam; evaluation of back pain (e.g. pediatric versus adult); evaluation of congenital anomalies (e.g. scoliosis); execution/evaluation of imaging studies 6. Describe the general structure and organization of the spinal cord and the organization of the meninges. Foundational knowledge; procedures (e.g. lumbar puncture); administration of anesthesia; evaluation of musculoskeletal problems (e.g. weakness, atrophy); evaluation of pain; evaluation of malignancy (e.g. meningioma); evaluation of imaging studies Adapted from AAA competencies 2 Learning Objective 8. Describe the clinical significance of lumbar puncture and contrast it with an epidural anesthesia. Compare the anatomical layers that must be penetrated in the two techniques. Clinical Relevance Foundational knowledge; procedures (e.g. lumbar puncture); administration of anesthesia Foundational knowledge; physical exam (e.g. reflexes, dermatomal 9. Review the internal organization of the spinal cord on a basic level, what is found in gray matter, what is found in white matter, rashes); evaluation of musculoskeletal problems (e.g. weakness, atrophy); evaluation of neural problems (e.g. neuropathy, pain, paresthesia, referred the basic anatomy of a dorsal root and a ventral root. pain) Evaluation of neural problems (e.g. impaired motor function, pain 10. Describe the importance of the blood supply to the spinal cord and understand what part of the cord can be damaged in an sensation, and temperature sensation with preserved proprioception and light touch sensation) anterior spinal artery syndrome. 11. Recognize the difference between extrinsic and intrinsic back muscles, compare their innervations and describe the basic movements carried out by the intrinsic group. Physical exam; evaluation of musculoskeletal problems (e.g. strains, spasms); evaluation of back pain 12. Recognize the basic pattern of muscles in the suboccipital triangle and their innervations. Physical exam; evaluation of musculoskeletal problems (e.g. strains, spasms); evaluation of neck pain/mobility Adapted from AAA competencies 3 Back problems and neural ramifications Fractures may cause compression of nerve roots or spinal cord Spondylolisthesis may damage cord, roots or exiting spinal nerve Herniating discs may put pressure on exiting nerve roots Osteophytes may impinge on the exiting spinal nerve There is an enormous burden of illness related to back problems. 4 The back Gilory, Fig. 1.1A, 4.18 Be aware that as you learn about the back, you will likely come across the term “true back”. This terminology came about because some of the muscles of the back develop in the region of the back (“true” or intrinsic back muscles = epaxial muscles), while others develop in other areas and migrate into the back (extrinsic back muscles). Another way to think about it is that if a structure (muscle or skin) is innervated by dorsal rami, it is part of the true back (i.e. epaxial muscles receive innervation from the dorsal primary rami DPR). We will go over this in more detail during the embryology lecture on MSK development. The axial skeleton includes the skull, vertebral column, ribs, and sternum (not the pelvis). 5 Vertebral column 7 cervical 12 thoracic 5 lumbar 5 sacral (fused) 4 coccygeal (fused) dorsal Schuenke Vol. 1 2nd Ed. Fig. 9.101B The vertebral column consists of 33 vertebrae that are subdivided into five regions: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal. The vertebrae in the sacral and coccygeal regions are fused, forming the sacrum and coccyx, respectively. The vertebrae are separated by intervertebral discs. Curvatures Moore 4.17; Netter 150. The vertebral column is not straight – it curves anteriorly and posteriorly in the sagittal plane (thus the anterior and posterior parts of vertebrae and intervertebral discs are not always uniform). The primary curvature of the fetus (concave anteriorly) remains in the thoracic and sacral regions. Secondary curvatures (convex anteriorly) begin to form before birth in the cervical and lumbar regions, however they don’t fully develop until the baby holds it head up and learns to stand. 7 Abnormal curvatures (1) **Clinical correlation Osmosis 2022 kyphosis (hunchback) – exaggerated thoracic curvature (e.g. due to osteoporosis of anterior parts of vertebral bodies or compression fractures) lordosis (swayback) – exaggerated lumbar curvature due to anterior rotation of the pelvis; is associated with obesity and can occur temporarily in pregnancy While the above terms are used to describe abnormal curvatures, it has recently become common in clinical practice to use them in both normal and pathological states. For example, “normal kyphosis” vs. “exaggerated kyphosis”. 8 Abnormal curvatures (2) **Clinical correlation Figures: Moore (6th) p. 481; www.eurospine.org. scoliosis – lateral curvature (i.e. in the coronal plane) usually due to an imbalance of forces on the spine (e.g. asymmetry in muscle strength or development of vertebrae) 9 Practice question #1 10 Typical vertebra vertebral foramen vertebral arch vertebral body Gilroy, Atlas of Anatomy, 2nd ed, Fig. 2.5B A typical vertebra is made up of two basic parts: a vertebral body and a vertebral arch. The fusion of these two basic parts creates an opening in the middle of the vertebra called the vertebral foramen. 11 Parts of the vertebral arch (1) lamina spinous process transverse process pedicle You should be familiar with these features of the vertebral arch. 12 Parts of the vertebral arch (2) superior articular process vertebral body inferior vertebral notch spinous process inferior articular facet Gilroy Fig. 2.13A Some additional features shown best in a lateral view. 13 Atlas and axis superior view of atlas (C1) anterior view of axis (C2) Gilroy, Fig. 2.7C, 2.8B The first (C1 or atlas) and second (C2 or axis) cervical vertebrae have unique features designed to support the skull and facilitate its movement. We will learn more about these vertebrae in the head & neck unit. 14 Typical cervical vertebra uncinate process bifid spinous process transverse foramen Gilroy, Fig. 2.9C The typical cervical vertebra has a bifid spinous process and a foramen in each of the transverse processes (called the transverse foramen or foramen transversarium). In all but C7, the transverse foramen conducts the paired vertebral arteries that are traveling towards the head. Typical cervical vertebrae also have two uncinate processes, which are bony margins that project from the lateral edges of the superior surface of the vertebral body. 15 Vertebra prominens C7 has a particularly prominent spinous process (easily palpable), thus this vertebra is often called “vertebra prominens”. Realize that the features of vertebrae gradually change. 16 Typical thoracic and lumbar vertebra thoracic vertebra lumbar vertebra Gilroy, Fig. 2.11A, 2.13A Thoracic vertebrae have costal facets for articulation with ribs, and relatively long sloping spinous processes. They are also said to resemble a giraffe in profile. Lumbar vertebrae are characterized by large, sturdy vertebral bodies and short, blunt spinous processes. Their large size reflects the fact that they support more weight than the cervical and thoracic vertebrae. The lumbar vertebra look more like a moose. 17 Sacrum and coccyx sacral canal sacrum anterior and posterior sacral foramina sacral hiatus coccyx anterior posterior In the sacral region, the five vertebrae are fused into a single structure known as the sacrum. Anterior and posterior sacral foramina serve the same purpose as the intervertebral foramen between vertebral bodies. The sacrum articulates superiorly with L5, laterally with the pelvic bones and inferiorly with the coccyx. The coccyx is a small bone that consists of 3-4 coccygeal vertebrae, which are typically fused. Recall from the nervous system overview lecture that the vertebrae, sacrum and coccyx are stacked together to form the vertebral column. Successive vertebral foramina form the vertebral canal; the portion of the canal that extends into the sacrum is called the sacral canal. The sacral canal ends at the sacral hiatus. Superiorly the vertebral canal is continuous with the cranial cavity via the foramen magnum. 18 Intervertebral foramen intervertebral foramen Access to the vertebral canal is provided by intervertebral foramina, which are formed when a superior vertebral notch of one vertebra meets an inferior vertebral notch on an adjacent vertebra. It’s important to note that if adjacent vertebrae become closer together (e.g. if the intervertebral disc became thinner or worn down) then the size of lateral the intervertebral foramen could decrease and its contents might be compressed. 19 Movements extension flexion lateral flexion rotation Movements of the vertebral column include flexion, extension, lateral flexion (aka lateral bending) and rotation. Movement of the trunk can be allowed or restricted by many factors: 1. The thickness of the intervertebral discs 2. The orientation of the articular facets 3. The attachment of ribs 4. The size, elasticity and orientation of the muscles of the back, and abdominal wall Cervical region is most mobile (3 degrees of freedom) Thoracic less due to ribs, and thin discs (2 degrees of freedom) Lumbar allows considerable flexion and extension (1 degree of freedom) 20 Joints of the vertebra joints between vertebral bodies joints between articular processes Joints (sometimes referred to as zygapophyses) exist between the bony parts of the vertebral column, and these are held together by ligaments and intervertebral discs. 21 Intervertebral discs (1) nucleus pulposus anulus fibrosus Gilroy Fig. 2.18 The joint between two vertebral bodies is a symphysis (two bones connected by cartilage). The cartilage that connects the bones here is the intervertebral disc, which consists of an outer layer of fibrocartilage (anulus fibrosus) and a gelatinous core (nucleus pulposus). Discs increase in size from cervical to lumbar regions. No disc between the atlas and axis. 22 Intervertebral discs (2) Non-weight-bearing Weight-bearing Movement Figure: Moore’s Clinically Oriented Anatomy 4.9. Intervertebral discs act as shock absorbers and allow for movement of the vertebral column. Nucleus pulposus cushions axial loads. 23 Herniated disc **Clinical correlation **Clinical correlation: A “herniated or ruptured disc” is a common source of back pain. In this condition, the anulus fibrosus tears (this can be due to trauma or degenerative changes with age) and the nucleus pulposus protrudes through the torn area. Typically the protrusion occurs in a posterolateral direction because the anulus is thinner in that area, and no support is provided by the longitudinal ligaments. The prolapsed nucleus pulposus may compress spinal nerve roots causing back pain. Herniated discs are most commonly seen in the lumbar region; approximately 95% occur between L4 and L5 or L5 and S1. 24 How spinal nerves exit the vertebral column nerve C1 emerges between skull and C1 vertebra nerves C2-C7 emerge superior to their corresponding vertebra nerve C8 creates a problem! nerve T1 and below emerge inferior to their corresponding vertebra Spinal nerves exit the vertebral column through the intervertebral foramina. The cervical spinal nerves exit the vertebral column superior to their same numbered vertebra – e.g. the C5 nerve passes through the intervertebral foramen between the C4 and C5 vertebrae. However, since there is an eighth cervical nerve but no C8 vertebra, the C8 nerve passes superior to the T1 vertebra and the T1 nerve must pass inferior to the T1 vertebra. Consequently, all the remaining spinal nerves pass inferior to their corresponding vertebra. Nerves impinged by herniated disc L4 “Intervertebral disc(s) herniate into central canal, affecting the inferior nerves (e.g. herniation of L4/5 disc affects L5 spinal nerve, but not L4)” -first aid (2018) spinal nerve L4 L5 herniated disc between L4-5 spinal nerve L5 **Clinical correlation Spinal nerves in the cervical region exit above the vertebra with the same number. Spinal nerves in the thoracic, lumbar and sacral levels exit below the vertebra with the same number. This should make sense if you think about the rather large opening created by the inferior intervertebral notch -- exiting nerve fibers can avoid the herniation (the shallow superior notch creates a smaller opening) 26 Disc herniation Level of herniation L3 L4 nerve root L5 S L5 nerve root L4 L5-S1 disc L5 S Numbness Weakness Atrophy Reflexes minor changes uncommon in knee and ankle reflexes, but internal hamstring reflex diminished or absent gastrocnemius and soleus ankle reflex diminished or absent L3 nerve root L4 L4-L5 disc Pain L4 nerve root L5 nerve root over sacroiliac joint, hip, lateral thigh and leg lateral leg, first 3 toes over sacroiliac joint, hip, postero-lateral thigh and leg to heel back of calf, lateral heel foot to toe dorsiflexion of big toe and foot; difficulty walking on heels; foot drop possible plantarflexion of big toe and foot may be affected; difficulty walking on toes S1 nerve root **Clinical correlation Figure adapted from Netter’s Clinical Anatomy. Clinical picture of herniated discs at the L5 and S1 vertebral levels. Notice in a L5 disc herniation, the pattern of pain, numbness, weakness, atrophy, and reflexes is distinct from a disc impinging on the S1 nerve root. 27 Normal MRI & herniated disc 28 Practice question #2 29 Facet joints superior articular process how they fit together inferior articular process lateral posterior Gilroy, Fig. 2.20B The vertebrae also articulate with one another at the synovial facet (zygapophyseal) joints. These occur between the superior articular process of one vertebra and the inferior articular process of another vertebra. The figure on the right is a posterior view of the facet joint. The orientation of the facet joints is different in each region of the vertebral column to facilitate different types of movement. For example, in the thoracic region rotation is the primary movement, while in the lumbar region flexion and extension predominate. 30 Spondylolysis Stress fracture of pars interarticularis- **Clinical correlation Netter 155 Sponylolysis is a congenital defect or an acquired stress fracture of the lamina (the pars interarticularis specifically) that presents with no slippage of adjacent articulating vertebrae. In a radiographic image, this fracture looks like a “Scottie dog” with a collar (the collar represents the side of fracture). 31 Spondylolysis on x-ray Scotty dog = fracture of pars interarticularis, portion forming dog’s neck where collar belongs Moore and Dalley B 4.16 32 Sports injuries Repeated stress to the spine can lead to spondylolysis Bilateral spondylolysis can cause spondylolithesis Really common sports injury in young kids. 33 Spondylolisthesis **Clinical correlation Spondylolisthesis is a bilateral defect (complete dislocation, or luxation) resulting in an anterior displacement of the L4 or L5 vertebral body and transverse process. The radiographic appearance of this defect is that of a dog with a broken neck (i.e. appears ‘decapitated’) 34 Ligaments (1) Gilroy, Fig. 2.20B The anterior longitudinal ligament is a continuous band found on the anterior aspect of the vertebral bodies. It is important for preventing hyperextension (over extension) of the vertebral column. The posterior longitudinal ligament is a continuous band found on the posterior aspect of the vertebral bodies (and thus anterior to the vertebral canal). It limits flexion of the vertebral column. 35 Ligaments (2) ligamentum nuchae supraspinous ligament interspinous ligament Gilroy, Fig. 2.28A The supraspinous ligament runs along the tips of the spinous processes from C7sacrum; it limits flexion of the vertebral column. In the cervical region, the ligamentum nuchae takes the place of the supraspinous ligament; it attaches superiorly to the external occipital protuberance and helps to support the weight of the head. The spaces between adjacent spinous processes are filled by the interspinous ligaments. 36 Ligamentum flavum ligamentum flavum posterior sagittal The ligamentum flavum is named for its yellow color due to a high concentration of elastic fibers. This ligament connects adjacent laminae and forms part of the posterior wall of the vertebral canal (fills in the gaps especially in the lumbar region). 37 Practice question #3 38 Practice question #4 39 Atlas, axis, and occipital ligaments tectorial membrane cruciate ligaments (transverse and longitudinal) alar ligaments dens posterior longitudinal ligament Thieme, Atlas of Anatomy The dens of the axis is attached to the occipital bone by the alar ligaments which limit rotation. The capsule of the atlanto-occipital joint adds strength. Cruciate ligaments are comprised of a strong transverse and two weaker superior and inferior parts. All of the above are covered posteriorly by the tectorial membrane as a continuation of the posterior longitudinal ligament. 40 Rule of thirds **Clinical correlation Moore (5th ed.) p. 510, Fig. B4-13 If the transverse ligament of the atlas is ruptured, it would be possible for the dens to dislocate posteriorly compressing or even transecting the spinal cord. If the dens is fractured, its inferior portion, not held in place by the transverse ligament, could also impact the spinal cord. 41 Dens It is easy to see the dens on an x-ray taken with the mouth open and angled appropriately to position overlying bony structures out of the way. 42 Meninges pia mater arachnoid mater dura mater Gilroy, Atlas of Anatomy, 2nd ed. Fig 4.7 The spinal cord (as well as the brain) is surrounded by three connective tissue layers called the meninges. The dura mater is the thick outermost layer, the arachnoid mater is the middle layer (it resembles a spider web), and the pia mater is the innermost layer that is intimately applied to the spinal cord. Epidural space epidural space fat vertebral venous plexus superior Gilroy, Atlas of Anatomy, 2nd ed. Fig. 4.8 There are spaces (some real, some potential) between the meninges themselves, and also between the vertebral canal and dura. The epidural (extradural) space is the area between the dura mater and the vertebral canal. It is filled with fat and contains the internal vertebral venous plexus (veins that drain the spinal cord and vertebrae). The epidural space begins at the foramen magnum and ends inferiorly at the sacral hiatus. Local anesthesia is often injected into the epidural space (epidural block) to anesthetize the nerve roots (e.g. for childbirth). Dura mater dural sac ends at S2 external filum terminale posterior Gilroy, Atlas of Anatomy, 3rd ed. Fig. 4.11 The dura mater around the spinal cord forms a tube known as the dural (thecal) sac. This sac begins at the foramen magnum where it is continuous with the dura mater around the brain. It extends approximately to the S2 vertebral level (varies between S1-S3), where it is continuous with the outer part of a structure called the external hilum terminale (coccygeal ligament) that anchors the dural sac to the coccyx. The dural sac has sleeve-like projections that surround the spinal nerve roots as they exit the vertebral canal. Subdural “space” and arachnoid arachnoid mater dura mater Image from Suny Downstate (http://act.downstate.edu/courseware/haonline/imgs/00000/0000/400/418.jpg). The subdural space between the dura and arachnoid mater is a potential space – this means that it does not normally exist, however there is potential for it to become a space if something (e.g. blood) accumulates in it. The arachnoid mater is closely associated with the dura mater and lines the dural sac (note – it is not attached to the dura). Subarachnoid space subarachnoid space contains CSF access via lumbar puncture superior Figure: Gilroy Atlas of Anatomy, 2nd ed., Fig. 4.8 The subarachnoid space located between the arachnoid mater and pia mater contains cerebrospinal fluid (CSF). It extends inferiorly as far as the dural sac (S2); accordingly, CSF fluid may be sampled from the subarachnoid space without puncturing the spinal cord. The subarachnoid space of the spinal cord is continuous with the subarachnoid space of the brain. Anesthesia **Clinical correlation Clinicians need to access the vertebral canal for a variety of reasons including administration of anesthesia (spinal or epidural) and obtaining a sample of CSF (lumbar puncture or spinal tap). The region between L2 and S2 is favorable because the spinal cord is not present. More specifically, the lower lumbar region is also preferred because there are gaps between adjacent vertebrae that allow access with a needle. Flexion of the vertebral column widens these gaps, thus this is taken into account when positioning the patient. An epidural block is administered in the epidural space; a “spinal” refers to when anesthesia is injected into the subarachnoid space. With a lumbar puncture, CSF is withdrawn from the subarachnoid space. 48 Practice question #5 49 Anatomy of the spinal cord gray matter cervical white matter thoracic lumbar sacral transverse sections Purves Neuroscience 4th ed. Fig. A5; Sylvius 4 (spinal cord sections) The spinal cord is divided into four segments (a reflection of its development) that correspond to regions of the vertebral column (cervical, thoracic, lumbar, sacral). In the fully developed spinal cord, two distinct areas are visible in the cross-sectional view, the gray matter and white matter. Spinal nerve Netter 4th 180 51 Cell body locations Collection of cell bodies in CNS called nucleus Collection of cell bodies in PNS called ganglion Collection of axons in CNS called tract 52 Practice question #6 From uMich BlueLink 53 Dermatomes Netter 4th 164 Dermatome is defined as the area of skin innervated by a single spinal nerve (specifically the cutaneous branch). We are going to ignore the phenomenon of overlap. 54 Blood supply of spinal cord posterior radicular artery anterior spinal artery anterior segmental medullary artery spinal artery posterior intercostal arteries Netter 4th 172 The spinal cord is supplied by branches of the spinal artery. The posterior radicular artery communicates with posterior spinal artery while the anterior medullary artery communicates with anterior spinal artery (*only present at specific levels). The blood supply of the skin and intrinsic back muscles also reflects the segmental nature of development – recurring branches from the descending aorta supply these structures. Branches from the aorta associated with thoracic vertebrae are called posterior intercostal arteries; those near the lumbar vertebrae are called lumbar arteries 55 Blood supply ventral view dorsal view ve nt ra l Netter 171; Gliroy, Atlas of Antomy, 3rd ed., Fig. 4.15A The anterior spinal artery doesn't have enough flow to supply the entire length of the cord – segmental contributions from anterior segmental medullary arteries boost flow. Blockage causes a syndrome known as the anterior spinal artery syndrome. **The T8-L2 region is the origin of the most important (clinically) medullary artery (Adamkiewicz) due to prevalence of aortic aneurysm in the area. 56 Anterior spinal artery syndrome posterior spinal arteries What will be damaged in this syndrome? From what you already know about the spinal cord what are the effects of this damage? CT L anterior spinal artery S spinothalamic tract **Clinical correlation Loss of sensation or muscle strength in a specific location that correlates with either a dermatome or a myotome is considered to be a segmental loss. Compare this with the loss of sensation or muscle strength that starts at the toes and ascends to a particular level called a level loss. Think about situations that might cause a level loss vs a segmental loss. We will explore this concept again in Block 5, but we want you to understand the basic principle now. 57 Venous drainage Netter 173 The most important point about the venous drainage is that the internal vertebral venous plexus communicates with both the venous sinuses of the brain and a venous plexus associated with the pelvis (prostatic venous plexus). It is possible for prostate cancer to metastasize to the CNS 58 Muscles of the back Superficial layer – contains muscles that move the upper extremity extrinsic Intermediate layer – contains muscles of respiration Deep layer – contains the true back muscles that stabilize or move the axial skeleton intrinsic The muscles of the back are classified as extrinsic or intrinsic based on their embryological derivation (whether they developed in the back or developed elsewhere and migrated into the back). Extrinsic back muscles migrated into the back region and are innervated by ventral rami of spinal nerves (trapezius is an exception to this general rule). Intrinsic back muscles are the “true back muscles” that are innervated by dorsal rami of spinal nerves. The back muscles are arranged in three layers as described above. 59 Superficial layer (1) rhomboid minor trapezius rhomboid major latissimus dorsi Gilroy, Fig. 21.21A,B The superficial layer (extrinsic muscles) consists of the trapezius, latissimus dorsi, levator scapulae (not labeled) and rhomboid major and minor. 60 Intermediate layer serratus posterior superior serratus posterior inferior Gilroy, Fig. 3.10A The intermediate layer is comprised of two thin muscles, serratus posterior superior and serratus posterior inferior. They are thought to be accessory respiratory muscles and are not considered clinically relevant. 61 Deep layer (1) Splenius group – splenius capitis, splenius cervicis Erector spinae group – iliocostalis, longissimus, spinalis Transversospinal group – semispinalis, multifidus, rotatores The intrinsic (deep) back muscles are separated from the extrinsic muscles by the thoracolumbar fascia. The muscles are organized in layers, and their classification varies among textbooks. 62 Deep layer (2) posterior Figure: Drake 2.47. The splenius group (splenius capitis and splenius cervicis) contains muscles that extend and rotate the head, and provide support for the small deep muscles of the neck. 63 Deep layer (3) erector spinae Gilroy, Fig. The erector spinae group (iliocostalis, longissimus, spinalis) are the primary muscles that extend the vertebral column; they are also important for controlling flexion against gravity. 64 Deep layer (4) rotatores multifidus Figure: Drake 2.49 The transversospinal group (semispinalis, multifidus, rotatores) are small postural muscles that stabilize the vertebral column and provide proprioceptive feedback. 65 Innervation of the skin of the back dorsal rami Figure: Netter 171. The innervation of the back muscles is listed in the muscle table. The skin of the back is innervated in a segmental fashion by the dorsal rami of spinal nerves. 66 Blood supply of the back Figure: Netter 165. 67 Suboccipital triangle greater occipital nerve rectus capitis minor rectus capitis major superior oblique inferior oblique Netter 4th 178 Suboccipital muscles are important for fine control of head movements. 68