VERTEBRAL COLUMN
Embryology
During the fourth week, sclerotome cells migrate medially to surround the spinal cord and notochord.
After proliferation of the caudal portion of the sclerotomes, the vertebrae are formed, each consisting of the caudal part of one sclerotome and the cephalic part of the next.
The notochord persists in the areas between the vertebral bodies, forming the nucleus pulposus.
The latter, together with surrounding circular fibers of the anulus fibrosis, forms the intervertebral disc.
Cervical Vertebrae
There are seven cervicalvertebrae of which the first two are atypical.
All cervical vertebrae have openings in their transverse processes, the transverse foramina, which, when aligned, produce a canal that transmits the vertebral artery and vein.
Atlas
This is the first cervical vertebra (Cl).
It has no body and leaves a space to accommodate the dens of the second cervical vertebra.
Axis
This is the second cervicalvertebra (C2).
It has a tooth-shaped process, the dens (odontoid process), which articulates with the atlas as a pivot joint.
Movement at this joint allowslateral rotation of the head.
Thoracic Vertebrae
There are 12 thoracic vertebrae.
* The vertebrae have facets on their bodies to articulate with the heads of ribs; each rib head articulates with the body of the numerically corresponding vertebra and the one below it.
* The thoracic vertebrae have facets on their transverse processes to articulate with the tubercles of the numerically corresponding ribs.
Lumbosacral Vertebrae
Unlike the thoracic wall, the bony support of the abdomen is minimal, consisting only of the lumbar vertebrae and portions of the pelvis (the ilium and the pubis).
* There are five lumbar vertebrae, L1 through L5.
* There are five sacral vertebrae, SI through S5, which are fused.
Intervertebral Disks
Each disk has an outer portion, the anulus fibrosus, which is composed of fibrocartilage and fibrous connective tissue, and an inner portion, the nucleus pulposus, which is a semigelatinous fluid with very few,if any, cells.
T1 Herniation of a nucleus pulposus is almost always in a posterolateral direction, passing through a rupture of the anulus fibrosus.
The herniated nucleus often comes to lie in the intervertebral foramen where it may compress a spinal nerve.
Ligaments
The intervertebral disk is reinforced anteriorly and anterolaterally by the anterior longitudinal ligament.
The posterior longitudinal ligament reinforces it posteriorly.
It is not reinforced posterolaterally.
Intervertebral Foramen
The intervertebral foramen is bounded superiorly and inferiorly by the pedicles of the vertebrae.
It is bounded anteriorly by parts of the bodies of the vertebrae and the intervertebral disk.
The articular processes and the zygapophyseal joint bound it posteriorly.
The spinal nerve contained within the intervertebral foramen may be compressed by herniation of the nucleus pulposus or zygapophysealjoint disease.
SPINAL NERVES AND SPINAL CORD
Neuronand Spinal Cord
The basic functional unit of the nervous system is the neuron.
Many different types of neurons are found in the nervous system, and most of them contain three elements: the soma (cell body), dendrites, and an axon.A multipolar neuron is shown below to illustrate the main components of a neuron.
Spinal nerves arise from the spinal cord by way of dorsal and ventral roots.
The dorsal root contains sensory nerve fibers with their cell bodies in the dorsal root ganglion.
The ventral root contains motor nerve fibers with their cell bodies in the gray matter of the spinal cord.
The spinal nerve divides into a dorsal ramus and ventral ramus.
Each ramus carries sensory and motor fibers to the dorsal and ventral parts of the body, respectively.
The dorsal ramus innervates the skin of the back, the deep back muscles, and the zygopophysealjoints. The anterior rami innervate the anterior and lateral portions of the body wall and the limbs.
Meninges
Piamater
The pia mater is fused to the surface of the spinal cord and cannot be separated from it.
External to this is the subarachnoid space,which is filled with cerebrospinal fluid (CSF).
The pressure of this fluid keeps the next layer,the arachnoid) awayfrom the pia mater.
Duramater
The outermost layer is the dura mater.
There normally is no subdural space,but such a space can be created when, for example, bleeding occurs into this space.
External to the dura is the epidural space) which contains fat and a plexus of veins.
The inferior limit of the dural sac and the subarachnoid space is at vertebral level S2.
Cauda equina :
Below the inferior limit of the spinal cord at the level of 11 and L2, but within the subarachnoid space, is the cauda equina.
This is composed of dorsal and ventral roots. When a spinal tap is performed it is typically done at the level of L4 (top of the iliac crest).
The cauda equina is at this level.
AUTONOMICNERVOUSSYSTEM
General Organization
Definition
The autonomic nervous system (ANS)is responsible for the motor innervation of smooth muscle, cardiac muscle, and glands of the body.
The ANSis composed of two divisions: (1) sympathetic and (2) parasympathetic.
In both divisions there are two neurons in the peripheral distribution of the motor innervation.
Preganglionic neuron with the cell body in the central nervous system (CNS).
Postganglionic neuron with the cell body in a ganglion in the peripheral nervous system (PNS).
Sympathetic System
Sympathetic = thoracolumbar outflow.
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