The spinal cord is a long cylinder of nervous tissue with subtle cervical and lumbar (lumbosacral) enlargements. The enlarged segments contribute to the brachial and lumbosacral plexuses. In the above image, showing a brain and spinal cord from a neonatal pig, the spinal cord and spinal roots are enveloped by dura mater.
The spinal cord is divided into spinal cord segments. Each segment gives rise to paired spinal nerves. Dorsal and ventral spinal roots arise as a series of rootlets. A spinal ganglion is present distally on each dorsal root. The canine spinal cord has 8 cervical, 13 thoracic, 7 lumbar, 3 sacral and 5 caudal segments. The following table compares species.
Spinal cord Segments and spinal Roots
Dural mater (dm) is reflected to expose segments and roots in a length of equine spinal cord. The arrow points to the dorsal median sulcus. The orange pic (asterisk) marks the dorsolateral sulcus, where dorsal roots enter the spinal cord. Each spinal cord segment gives rise to right and left dorsal and ventral spinal roots. Each spinal root is composed of rootlets (r). The dorsal root (DR) and the ventral root (VR) unite to form a spinal nerve (SN). A spinal ganglion (SG) is located distally on each dorsal root. Colored pics mark: spinal ganglia (red), the separation between dorsal and ventral roots (black), and the location of the denticulate ligament. The specimen rests on white cardboard.
Spinal cord segments in different species (for reference purposes):
Dog: 8 cervical; 13 thoracic; 7 lumbar; 3 sacral; & 5 caudal = 36 total
Cat: 8 cervical; 13 thoracic; 7 lumbar; 3 sacral; & 5 caudal = 36 total
Bovine: 8 cervical; 13 thoracic; 6 lumbar; 5 sacral; & 5 caudal = 37 total
Horse: 8 cervical; 18 thoracic; 6 lumbar; 5 sacral; & 5 caudal = 42 total
Swine: 8 cervical; 15/14 thoracic; 6/7 lumbar; 4 sacral; & 5 caudal = 38 total
Human: 8 cervical; 12 thoracic; 5 lumbar; 5 sacral; & 1 coccygeal = 31 total
The spinal cord and spinal roots are enveloped by meninges and housed within the vertebral canal. The epidural space, situated between the wall of the vertebral canal and the spinal dura mater, contains a variable amount of fat. Within dura mater, the spinal cord is suspended by bilateral denticulate ligaments and surrounded by subarachnoid space filled with cerebrospinal fluid. Dorsal and ventral spinal roots unite to form spinal nerves which exit the vertebral canal at intervertebral foramina. An intervertebral foramen is formed by adjacent vertebrae and by the intervertebral disc joining the vertebrae.
The spinal cord and spinal roots are enveloped by meninges and housed within the vertebral canal. The epidural space, situated between the wall of the vertebral canal and the spinal dura mater, contains a variable amount of fat. Within dura mater, the spinal cord is suspended by bilateral denticulate ligaments and surrounded by subarachnoid space filled with cerebrospinal fluid. Dorsal and ventral spinal roots unite to form spinal nerves which exit the vertebral canal at intervertebral foramina. An intervertebral foramen is formed by adjacent vertebrae and by the intervertebral disc joining the vertebrae.
As a result of differential growth of the spinal cord and vertebral column, most spinal cord segments are positioned cranial to their nominally corresponding vertebrae. However, spinal segment length is variable along the spinal cord in our domestic mammals. Segments become progressively shorter from the C3 to T2. Then they elongate so that segments at the thoracolumbar junction are within nominally corresponding vertebrae. Thereafter, segments progressively shorten until the cord terminates in a terminal filament of glia. (The term "conus medullaris" refers to the cone-shaped cord region between the lumbosacral enlargement and the glial filament.)
Since spinal nerves exit the vertebral canal at nominally corresponding intervertebral foramina, spinal roots must elongate when spinal cord segments are displaced cranially. The term cauda equina (horse tail) refers to caudally streaming spinal roots running to intervertebral foramina in the sacrum and tail. Damage to the cauda equina affects pelvic viscera and the tail. Cauda equina epidural anesthesia (putting anesthetic into the epidural space to block conduction in spinal roots) is a common obstetrical procedures in cattle.
Because vertebrae can be palpated and visualized in ordinary radiographs, unlike spinal segments, it is clinically useful to know locations of spinal cord segments relative to vertebrae. Typically (for most dogs) the cervical enlargement is centered at the C6-7 intervertebral disc; spinal segments of the thoracolumbar junction are within nominally corresponding vertebrae; the sacral segments are within vertebra L5; and the functional spinal cord terminates at the L6-7 vertebral junction. (Termination is about one vertebra further caudally in small dogs, less than 7 kg.)
Spinal cord Within Vertebral Canal
The following drawing depicts a spinal cord segment within a lumbar vertebra, at the level of an intervertebral disc (nucleus pulposus surrounded by annulus fibrosus). spinal nerves are present bilaterally at intervertebral foramina, dorsal to the disc. An epidural space, containing fat, is evident external to spinal dura mater (blue). The latter is shown surrounding roots on the left; it is removed on the right side. Bilaterally, dorsal and ventral spinal roots (green) unite to form a spinal nerve (yellow) which soon branches. Bilateral thickenings of pia mater (purple), called denticulate ligaments, suspend the spinal cord within the dura mater.
spinal cord In Situ
Left: Cervical transection through an intervertebral disc (nuchal ligament at the top). The spinal cord, surrounded by meninges, is evident within the vertebral canal. Internal vertebral venous sinus is marked by asterisks. (Vertebral a. & v. are visible bilaterally.)
Right: Thoracic vertebra transection. The spinal cord is surrounded by meninges within the vertebral canal. Internal vertebral venous sinus is marked by asterisks.
Canine spinal Cord
Cranial and caudal halves of a canine vertebral column are illustrated, after a laminectomy to expose the spinal cord. Spinal cord segments are labeled, and locations of vertebral bodies separated by intervertebral discs are shown to the right. Dura mater (blue) has been removed except along the right side. The illustrated position relationship of spinal cord segments to vertebrae represents the most common relationship for medium and large dogs (typical variation is half a vertebral length cranial or caudal to that shown). In small dogs (under 7kg) spinal cord segments are positioned more caudally than is shown.
Canine spinal cord — Cranial Half
The cranial half of a canine vertebral column has been drawn after a laminectomy to expose the spinal cord. spinal cord segments are labeled and locations of vertebral bodies separated by intervertebral discs are labeled to the right.
Dura mater (blue) has been removed except along the right side. Dura mater envelops spinal roots including spinal ganglia.
Notice that spinal segments vary in length and that spinal roots must elongate to reach intervertebral foramina where segments are shifted cranially. In the cervical region, notice that the spinal root of the accessory cranial nerve (Accessory r., tan) emerges laterally, between dorsal and ventral roots. The C1 spinal nerve (N.1C, yellow) exits from a lateral foramen, rather than an intervertebral foramen like other spinal nerves. The C8 spinal segment appears to be "extra" (it lacks a nominally corresponding vertebra). Thus, caudal to the cervical region, spinal nerves exit through intervertebral foramina located at caudal margins of nominally corresponding vertebrae.
The C3 segment is the longest. Thereafter, segments progressively shorten in length. After the T2 segment, segments progressively lengthen. The cervical enlargement (C6, 7, 8, & T1) which innervates the thoracic limb (brachial plexus) is centered approximately at the C6-C7 intervertebral disc.
Canine spinal cord — Caudal Half
The cranial half of a canine vertebral column has been drawn after a laminectomy to expose the spinal cord. spinal cord segments are labeled and locations of vertebral bodies separated by intervertebral discs are labeled to the right.
Dura mater (blue) has been removed except along the right side. Dura mater envelops spinal roots including spinal ganglia.
Notice that thoracolumbar spinal segments are long and located within nominally corresponding vertebrae. Thereafter, segments progressively shorten in length and spinal roots elongate as segments shift position cranial to nominally corresponding vertebrae. Sacral and caudal roots streaming caudally are referred to as the cauda equina. Notice that the cauda equina is initially intrathecal (within the main cylinder of spinal dura mater); thereafter, the roots are enveloped by dural sheaths in the epidural space.
The term conus medullaris refers to the cone-shaped region of spinal cord caudal to the lumbosacral enlargement (L4 — S1). The cord terminates approximately at the L6-L7 intervertebral disc. Thereafter a terminal filament of glial tissue continues for some distance. The term caudal ligament refers to the terminal filament enveloped by a dural sheath.
Canine spinal cord Termination
Illustration of a canine spinal cord termination. A laminectomy was performed and dura mater has been reflected to expose spinal cord segments and spinal roots. Dorsal roots are cut on the left side to expose the denticulate ligament (purple). Notice the termination of the denticulate ligament; the cauda equina; and the terminal filament (filum terminale), a glial continuation persisting beyond the functional end of the spinal cord. The illustrated position of the spinal cord termination at the L6-L7 intervertebral disc represents the most common relationship for medium and large dogs (varying plus or minus a half vertebra). In small dogs (under 7 kg) the typical position is one vertebra caudal to that shown.
Spinal Cord—Vertebrae Relationships
It is clinically useful to know the approximate locations of spinal cord segments relative to palpable, radiographically visible vertebrae. One learning strategy is to remember the following four relationships and then interpolate other position relationships as necessary. (The illustrated relationships are the most common for medium and large dogs ( half vertebra). In small dogs the position is one vertebra caudal to that shown.)
A. The cervical enlargement (brachial plexus segments) are centered at the C6-C7 intervertebral disc.
B. At the thoraco-lumbar junction, segments are positioned within nominally corresponding vertebrae.
C. The three sacral segments are located within the L5 vertebra.
D. The spinal cord terminates at the L6-L7 intervertebral disc
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