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Spinal Cord

An elongated and almost cylindrical part of the central  nervous system
It is suspended in the vertebral canal
Surrounded by the meninges and CSF

Above,  it is continuous with the medulla oblongata
Extends from the upper border of atlas to the lower border of L1    
The tapered, lower end of the spinal cord is called the conus medullaris.
After the spinal cord tapers out, the spinal nerves continue to branch out, forming the cauda equina.
The pia mater that surrounds the spinal cord,  projects directly downward, forming a slender filament called the filum terminale, which connects the conus medullaris to the back of the coccyx. - stabilizes the entire spinal cord.
spinal cord : 45 cm
As thick as a little finger
Link between the brain and the rest of the body

Incompletely divided into two equal parts by fissures
Anteriorly fissure : shallow
Posteriorly : deep narrow septum  -  posterior median septum
Grey matter in the centre
White matter in the surrounding supported by neuroglia

Grey matter
H shaped arrangement
Two anterior, two posterior and two lateral horns
Central transverse portion is called the transverse commissure
Central commissure contains the spinal canal containing CSF
Grey matter contains sensory cells, lower motor neurone cells and connector neurones linking sensory and motor neurones - spinal reflexes
Where an impulse passes from one nuerone to another neurone the junction is called a synapse and there is a chemical transmitter

Posterior columns of grey matter
Composed of cell bodies of the lower motor neurones
Which are stimulated by upper motor neurone fibres
Or by the cell bodies of connector neurones (reflex arc)
The posterior root ganglia (spinal ganglia)
Cell bodies lie outside the cord
On the pathway of sensory nerves
All sensory nerves pass through these ganglia

White matter
Arranged in three columns or tracts
These tracts are formed by:
1. Sensory nerve fibres ascending to the brain
2. Motor nerve fibres descending from the brain
3. Fibres of connector neurones
4. Spinal Nerves
31 pairs of spinal nerves pass through the intervertebral foramina
Each spinal nerve is formed by the union of anterior and posterior roots as soon as they come out of the intervertebral foramina

Regions of the Spinal Cord
The spinal cord is divided into four different regions: the cervical, thoracic, lumbar and sacral regions.
Two enlargements of the spinal cord can be visualized: The cervical enlargement, which extends between C3 to T1; and the lumbar enlargements which extends between L1 to S2.

The cord is segmentally organized. There are 31 segments, defined by 31 pairs of nerves exiting the cord.
8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal nerve. (NOTE : CERVICAL VERTEBRAE ARE 7 IN  NUMBER AND CERVICAL NERVES ARE 8)
Dorsal and ventral roots enter and leave the vertebral column respectively through intervertebral foramen at the vertebral segments corresponding to the spinal segment.

Emergence of the spinal nerves
  The first cervical nerve emerges from the vertebral canal between the occipital bone and the atlas,
The eighth cervical nerve issues between C7 and T1.
The first Intercostal nerve comes out through intervertebral foramen below T1 vertebra, T2 under T2 vertebra and so on.

Blood supply of the spinal cord:
Branches of the vertebral, deep cervical, intercostal, and lumbar arteries contribute to three arteries that run the length of the spinal cord;
Anterior spinal artery
Two posterior spinal arteries.
The anterior spinal artery arises at the level of the foramen magnum by the junction of two branches, one from each vertebral artery.
Each posterior spinal artery arises from the posterior inferior cerebellar artery at the same level.
Generally the proportion of flow is greatest from the radicularis magna "feeder" artery to the thracolumbar region. In abnormal situations
( e.g. high take-off) the iliac artery branch may supply the lower thoracolumbar region of the cord entering by way of the intervertebral foramen in the vicinity of L4-5

21 pairs of segmental radicular arteries supply the nerve roots
Half of them contribute to the spinal arteries.
The largest is the great anterior radicular artery of Adamkiewicz (radicularis magna),
It supplies the lower thoracic and upper lumbar parts of the cord.
It usually arises from a lower intercostal or a high lumbar artery anywhere between L4  & T8
It makes a major contribution
Spinal injury or aortic surgery may compromise the blood supply of the lower part of the spinal cord.
The  other segmental radicular arteries are small
Venous drainage
Venous drainage of the spinal cord is by internal and external venous plexuses
Anterior sulcal vein
Posterior sulcal vein
And thence to the vertebral veins, intercostal veins and lumbar veins

* * * * * * *

. The Spinal Nerves
(Nervi Spinales)

The spinal nerves spring from the medulla spinalis, and are transmitted through the intervertebral foramina. They number thirty-one pairs, which are grouped as follows: Cervical, 8; Thoracic, 12; Lumbar, 5; Sacral, 5; Coccygeal, 1.       1
  The first cervical nerve emerges from the vertebral canal between the occipital bone and the atlas, and is therefore called the suboccipital nerve; the eighth issues between the seventh cervical and first thoracic vertebræ.       2
Nerve Roots.—Each nerve is attached to the medulla spinalis by two roots, an anterior or ventral, and a posterior or dorsal, the latter being characterized by the presence of a ganglion, the spinal ganglion.       3
  The Anterior Root (radix anterior; ventral root) emerges from the anterior surface of the medulla spinalis as a number of rootlets or filaments (fila radicularia), which coalesce to form two bundles near the intervertebral foramen.       4
  The Posterior Root (radix posterior; dorsal root) is larger than the anterior owing to the greater size and number of its rootlets; these are attached along the posterolateral furrow of the medulla spinalis and unite to form two bundles which join the spinal ganglion. The posterior root of the first cervical nerve is exceptional in that it is smaller than the anterior; it is occasionally wanting.       5
  The Spinal Ganglia (ganglion spinale) are collections of nerve cells on the posterior roots of the spinal nerves. Each ganglion is oval in shape, reddish in color, and its size bears a proportion to that of the nerve root on which it is situated; it is bifid medially where it is joined by the two bundles of the posterior nerve root. The ganglia are usually placed in the intervertebral foramina, immediately outside the points where the nerve roots perforate the dura mater, but there are exceptions to this rule; thus the ganglia of the first and second cervical nerves lie on the vertebral arches of the atlas and axis respectively, those of the sacral nerves are inside the vertebral canal, while that on the posterior root of the coccygeal nerve is placed within the sheath of dura mater.       6
Structure (Fig. 638).—The ganglia consist chiefly of unipolar nerve cells, and from these the fibers of the posterior root take origin—the single process of each cell dividing after a short course into a central fiber which enters the medulla spinalis and a peripheral fiber which runs into the spinal nerve. Two other forms of cells are, however, present, viz.: (a) the cells of Dogiel, whose axons ramify close to the cell (type II, of Golgi), and are distributed entirely within the ganglion; and (b) multipolar cells similar to those found in the sympathetic ganglia.       7
  The ganglia of the first cervical nerve may be absent, while small aberrant ganglia consisting of groups of nerve cells are sometimes found on the posterior roots between the spinal ganglia and the medulla spinalis.       8
  Each nerve root receives a covering from the pia mater, and is loosely invested by the arachnoid, the latter being prolonged as far as the points where the roots pierce the dura mater. The two roots pierce the dura mater separately, each receiving a sheath from this membrane; where the roots join to form the spinal nerve this sheath is continuous with the epineurium of the nerve.       9

FIG. 796– A portion of the spinal cord, showing its right lateral surface. The dura is opened and arranged to show the nerve roots. (Testut.) (See enlarged image)
Size and Direction.—The roots of the upper four cervical nerves are small, those of the lower four are large. The posterior roots of the cervical nerves bear a proportion to the anterior of three to one, which is greater than in the other regions; their individual filaments are also larger than those of the anterior roots. The posterior root of the first cervical is an exception to this rule, being smaller than the anterior root; in eight per cent. of cases it is wanting. The roots of the first and second cervical nerves are short, and run nearly horizontally to their points of exit from the vertebral canal. From the second to the eighth cervical they are directed obliquely downward, the obliquity and length of the roots successively increasing; the distance, however, between the level of attachment of any of these roots to the medulla spinalis and the points of exit of the corresponding nerves never exceeds the depth of one vertebra.       10

  The roots of the thoracic nerves, with the exception of the first, are of small size, and the posterior only slightly exceed the anterior in thickness. They increase successively in length, from above downward, and in the lower part of the thoracic region descend in contact with the medulla spinalis for a distance equal to the height of at least two vertebræ before they emerge from the vertebral canal.       11

FIG. 797– Distribution of cutaneous nerves. Ventral aspect. (See enlarged image)
  The roots of the lower lumbar and upper sacral nerves are the largest, and their individual filaments the most numerous of all the spinal nerves, while the roots of the coccygeal nerve are the smallest.       12
  The roots of the lumbar, sacral, and coccygeal nerves run vertically downward to their respective exits, and as the medulla spinalis ends near the lower border of the first lumbar vertebra it follows that the length of the successive roots must rapidly increase. As already mentioned (page 750), the term cauda equina is applied to this collection of nerve roots.       13

FIG. 798– Distribution of cutaneous nerves. Dorsal aspect. (See enlarged image)
  From the description given it will be seen that the largest nerve roots, and consequently the largest spinal nerves, are attached to the cervical and lumbar swellings of the medulla spinalis; these nerves are distributed to the upper and lower limbs.