Back and Autonomic Nervous System

VERTEBRAL COLUMN

Embryology

During week 4, sclerotome cells ofthe somites (mesoderm) migrate medially to surround the spinal cord and notochord. After proliferation ofthe caudal portion ofthe sclerotomes, the vertebrae are formed, each consisting ofthe caudal part of one sclerotome and the cephalic part ofthe next.

Vertebrae

The vertebral column is the central component ofthe axial skeleton which func­ tions in muscle attachments, movements, and articulations ofthe head and trunk.

•The vertebrae provide a flexible support system that transfers the weight of the body to the lower limbs and also provides protection for the spinal cord.

•The vertebral column (Figure III- 1-1) is composed of 32-33 vertebrae (7 cervical, 12 thoracic, 5 lumbar, and the fused 5 sacral, and 3-4 coccy­ geal), intervertebral disks, synovial articulations (zygapophyseal joints) and ligaments.

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Section Ill • Gross Anatomy

SPINAL MENINGES

The spinal cord is protected and covered by 3 connective tissue layers within the vertebral canal: the pia mater, dura mater, and arachnoid (Figure III-1-5).

Ventral root of spinal nerve

Spinal nerve

Internal vertebral venous plexus

Figure 111-1-5. Cross-Section of Vertebral Canal

Pia Mater

The piamater is tightly attached to the surface of the spinal cord and provides a delicate covering of the cord.

•The spinal cord, with its covering of pia mater, terminates at the LI or

L2 vertebral levels in the adult.

•There are 2 specializations of the pia mater that are attached to the spinal cord:

-The denticulate ligaments are bilateral thickenings of pia mater that run continuously on the lateral sides of the midpoint of the cord. They separate the ventral and dorsal roots of the spinal nerves and anchor to the dura mater.

-The filum terminale is a continuation of the pia mater distal to the

lower end of the spinal cord. The filum terminale is part of the cauda equina which is composed of ventral and dorsal roots of lumbar and sacral nerves that extend below the inferior limit of the spinal cord.

Dura Mater

The dura mater is a tough, cylindrical covering of connective tissue forming a dural sac which envelops the entire spinal cord and cauda equina.

• The dura mater and dural sac terminate inferiorly at the vertebra level.

•Superiorly, the dura mater continues through the foramen magnum and is continuous with the meningeal layer of the cranial dura.

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Chapter 1 • Back and Autonomic Nervous System

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Section Ill • Gross Anatomy

AUTONOMIC NERVOUS SYSTEM

The autonomic nervous system (ANS) is concerned with the motor innervation of smooth muscle, cardiac muscle, and glands of the body.

Anatomically and functionally, the ANS is composed of 2 motor divisions: ( 1 ) sympathetic and (2) parasympathetic (Figure III- 1 -9). In both divisions, 2 neurons formglian autonomic pathway.

• Pregan onicneuronshave their neuronal cell bodies in the CNS (formed by neuroectoderm); their axons exit in cranial and spinal nerves.

•Postganglionic neurons have cell bodies in autonomic ganglia in the peripheral nervous system (PNS) (formed by neural crest cells)

Target

Figure 111-1-9. Autonomic Nervous System

Sympathetic Nervous System

The preganglionic cell bodies of the sympathetic nervous system are found in the lateralhorn gray matter ofspinal cord segments TI-12 (14 segments).

The postganglionic cell bodies of the sympathetic system are found in one of

2types of motor ganglia in the PNS:

•Chain or paravertebral

•Collateral or prevertebral (found only in abdomen or pelvis)

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Section Ill • Gross Anatomy

ChapterSummary

Vertebral Column

•The vertebral column is composed ofa series ofcervical, thoracic, lumbar, sacral, and coccygeal vertebrae connected by intervertebral disks and ligaments. The disks consist of an outer core of fibrocartilage, the annulus fibrosus, and an inner part-the nucleus pulposus-which develop from the notochord. Herniation ofthe nucleus pulposus is usually posterolateral where it can compress a spinal nerve at the intervertebral foramen.

•The spinal nerve exits the vertebral column at the intervertebral foramen. The foramen is bound superiorly and inferiorly by the pedicles of the vertebrae, anteriorly by the vertebral bodies and intervertebral disks, and posteriorly by the zygapophyseal joint.

•The spinal cord is covered by 3 protective layers of meninges: dura mater, arachnoid, and pia mater. The dura and dural sac terminate inferiorly at the second sacral vertebra, and the spinal cord terminates at the second lumbar vertebra. The cauda equina fills the lower part of the dural sac and contains the filum terminate and the ventral and dorsal roots ofthe lumbar

and sacral spinal nerves. Between the arachnoid and pia is the subarachnoid space that contains cerebrospinal fluid (CNS), and between the dura mater and the vertebrae is the epidural space, which contains fat and a plexus of veins. Spinal taps are performed at the level ofthe L4 vertebra (located at the horizontal level ofthe iliac crest) to avoid puncturing the spinal cord.

Autonomic NervousSystem

•The autonomic nervous system (ANS) provides visceral motor innervation to smooth muscle, cardiac muscle, and glands. The ANS is divided into

2 divisions: sympathetic (thoracolumbar) and parasympathetic (craniosacral). The peripheral distribution ofthese 2 divisions consists of

2 neurons: preganglionic neuron (cell bodies in the CNS) and postganglionic neuron (cell bodies in motor ganglia in PNS).

•Sympathetic preganglionic cell bodies are found in the lateral horn of the gray matter of spinal cord segments Tl-L2. These synapse with

postganglionic cell bodies located in either chain (paravertebral) ganglia or collateral (prevertebral) ganglia. Sympathetics to the body wall, head, and thoracic viscera synapse in the chain ganglia. Sympathetics to the foregut and midgut (thoracic splanchnic nerves: T5-T12) and to the hindgut and pelvic viscera (lumbar splanchnic nerves: L1-L2) synapse in collateral ganglia. Interruption of sympathetic innervation to the head results in ipsilateral Homer's syndrome.

•Parasympathetic preganglionic neuron cell bodies are located in the brain stem nuclei of cranial nerves Ill, VII, IX, X, or in the gray matter of the spinal cord segments S2-S4 (pelvic splanchnics). The preganglionic

neurons synapse with postganglionic neurons in terminal ganglia scattered throughout the body. Parasympathetics to the head originate in cranial nerves Ill, VII, and IX; those to the thorax, foregut, and midgut originate in cranial nerve X; and those to the hindgut and pelvic viscera originate in the S2-S4 cord segments.

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