The Sinuvertebral Nerve: A nerve of mystery

[문형철]

http://www.chirogeek.com/000_Disc_Anatomy.htm

아무래도 허리디스크에 관한 책을 쓰려면 이 사이트의 지식은 완전히 숙독한 뒤로 미루는 것이 좋겠다.

참 지식의 습득이란 맑은 샘물을 퍼내는 것과 같이 끝도 없는 작업인 모양이다. 

panic bird.... 

Sinuvertebral nerves — The sinuvertebral nerves are sensory nerves that innervate various structures within the spine, such as ligamentous structures, the dura, periosteum, and blood vessels [10]. They originate distal to the dorsal root ganglia and extend to communicate with branches from radicular levels both above and below the level of entry,

as well as the contralateral side, making it difficult to localize pain from involvement of these nerves. Irritation of the sinuvertebral nerves may result in low back pain. Because they arise distal to the nerve root, however, involvement of sinuvertebral nerves or their branches without involvement of the rest of the nerve root is not considered to be radicular in nature.

 

Basic Disc Anatomy:

The intervertebral discs may be thought of as soft and tough pads that separate the bones (vertebrae) of the spine from one another. 

Their basis function is three-fold:

1) they act as a ligament by holding the vertebrae of the spine together;

2) they act as a shock absorber, which helps carry the downward weight (axial load) of the body while the human is in an up-right position; and

3) they act as pivot point, which allows the spine to bend, rotate and twist.

There are 23 discs in the human spine: 6 in the neck (cervical region), 12 in the middle back (thoracic region), and 5 in the lower back (lumbar region). This page shall focus on the lumbar spine; however, the thoracic and cervical spines are similar in make-up.

The disc is made up of three basic structures: the nucleus pulposus, the annulus fibrosus (aka anulus fibrosus) and the vertebral end-plates. Although their composition percentage differs, the latter three structures are made of three basic components: proteoglycan (protein), collagen (cartilage), and water. We will learn all about these structures below.

Figure #1 depicts a Front view (AP) of the lumbar spine. Here we can see how the discs (blue) lie in between every vertebrae. Spinal nerves (yellow) have emerged from between every two vertebrae and travel down the lower limbs to innervate (give life to) the skin and muscle. Note how the sciatica nerve is formed within the pelvis by branches from the last three lumbar spinal nerves. It is this giant nerve that causes so much trouble in many of us chronic pain sufferers. (more detail here)

Figure #2 Shows a cut-away posterior view of the lumbar spine. Now we can better visualize how the sciatic nerve is formed and see just how close the spinal nerve roots come to the intervertebral disc. Any herniation of the posterior disc may compress the spinal nerve root and result in severe lower back pain and lower limb pain (sciatica). For more information on sciatica please visit my 'Sciatica Page'.

Axial Disc Anatomy and Neurology:

The Nucleus Pulposus (#1) is the water-rich, gelatinous center of the disc, which is under very high pressure when the human is up-right--especially in the seated position. It has two main functions: to bear or carry the downward weight of the body and to act as a 'pivot point' from which all movement of the lower trunk occurs. It's third function is to act as a ligament and bind the vertebrae together.

The Anulus Fibrosus (#2) is a much more fibrous structure that that of the nucleus pulposus ("nucleus") and has a much higher collagen content and lower water content when compared to the nucleus. Its job is to corral or contain the pressurized nucleus, which would love to squirt out [like toothpaste from a tube] because of the high pressure that's upon it. It is made of 15 to 25 concentric sheets of collagen, (a cartilage like substance) called the Lamellae (#9) .

The lamellae are arranged in a special configuration which makes them extremely strong and easily able to contain that pressurized nucleus pulposus. The Spinal Nerves Roots (aka: Nerve Roots) (yellow and labeled L5, S1, S2, S3, and S4) and Spinal Nerves (L4 yellow) are extensions of the brain and spinal cord. Like super highways the spinal nerves and nerve roots are constantly carrying electric messages--INCLUDING PAIN SIGNALS--to and from the brain. The nerve roots exit the spine through bony holes called the Intervertebral Foramen (aka: IVF) (Red Zone) . As the nerve roots 'peal-off' from the cauda equina (one sensory nerve root and one motor nerve root), they head for their respective IVF. (Noteworthy is the fact that the two delicate nerve root passs very close to the back of the disc.) Once within the IVF the two nerve roots merge into one Spinal Nerve. The Spinal nerves are called "mixed nerves" for they contains both sensory nerve fiber (aka: afferent) and motor nerve fiber (aka: efferent). After leaving the spine through the IVF, the nerve split into a posterior division (Dorsal Ramus) and an anterior division (Ventral Ramus).

The Dorsal Ramus connects to the muscle and skin over the lower back, butt and Facet Joint (#5). The Ventral Ramus combine in the pelvis and form the giant Sciatic Nerve and Lateral Femoral Cutaneous Nerve that in turn connect to all the skin and muscle of the lower limbs. (See my 'Sciatica Page' for more information)

As we will learn below, the ventral ramus has a "recurrent branch" that connects to the back of the disc, as well as laterally to the sympathetic nervous system (Grey Ramus Communicans); this special nerve is called the Sinuvertebral nerve (SN) (see below).

 In the lumbar spine (below the L1 disc level) there is no spinal cord. Instead the nerve roots hang like a horses tail in an enclosed sac, which is called the Thecal Sac (red stars). The thecal sac, which protects the dangling nerve roots, is made up of two distinct but tightly bound layers called the dura mater and arachnoid mater.

A clear fluid called Cerebral Spinal Fluid is also found within the thecal sac. This fluid protects the nerve roots from pressure injury and also supplies nutrients.

Note how the nerve roots (yellow - S1 - S5), which are collectively called the Cauda Equina (#4), often have a highly organized within the thecal sac.

Because of this arrangement, which always puts the lower level nerve root in front, it is possible for a large compressive posterolateral L4 disc herniation to irritate/compress both the S1 and/or L5 roots. This may explain why disc herniations do NOT always match their exact dermatomal distribution; i.e., a disc herniation at L4 may clinically present as nerve root pain (aka: radicular pain, sciatica) and dysfunction of both the L4 and/or L5 distributions!

The Epidural Space (#8) is the space between the bony neural canal and the thecal sac; or the space outside of the thecal sac. In reality, this space is filled with blood vessels and fat and is grossly oversimplifed in figure #9. Note worthy is the fact that this is the region where 'epidural steroid injections' are placed.

The Facet Joints (#5) (aka: zygapophyseal joints) of the spine are where the vertebrae articulate (join) with each other. Actually, the gap between the inferior and superior articular processes is the true facet joint (white region). Collectively the inferior and superior articular processes and the facet joint are called the Zygapophyseal Joints or articular pillars. These joints help carry the axial load of the body and limit the range of motion of the spine. They also make up the back border of the intervertebral foramen and may physically compress and trap the exiting nerves secondary to degenerative thickening (sclerosis); this condition is called lateral canal stenosis.

The Ring Apophysis (#6) is the 'naked bone' of the outer periphery of the vertebral bodies. The very outer fibers of the disc (Sharpey's Fibers) anchor themselves into this region. Bone spurs (aka: Osteophytes) may arise from the ring apophysis as the result of the later stages of Degenerative Disc Disease (DDD) and/or Osteoarthritis (Spondylosis). Specifically, osteophytes arise from the prolonged 'pulling and tugging' of 'Sharpey's Fibers' at their anchor points.

The Posterior Longitudinal Ligament (PLL) (#7) is a strong ligamentous tissue which courses down the anterior aspect of the vertebral canal and is attached to the outer fibers of the anulus fibrosus. This highly innervated (supplied with pain carrying nerve fiber) tissue is the last line-of-defense the posterior neural tissue has against the irritating and inflammatory effects of nucleus pulposus.

MRI AXIAL ANATOMY

Okay, lets see if you have learned anything. Figure #8 is an MRI over-head view (axial view) of the real anatomy of the L4 disc and neural structures that you have learned above. Can you identify the structures?

#1) Disc and vertebral body of L4.

#2) Exiting L4 nerve root.

#3) Traversing L5 nerve root. (you can also see the traversing S1 roots as well more posteriorly.)

#4) Thecal sac of the cauda equina.

#5) Facet joint.

#6) Errector Spinalis Muscle.

Remember, this is an MRI T2-Weighted Axial View. You would not be able to see the nerve root on either the proton density view or the T1-Weighted view.

SLAB-LAB: NERVE ANATOMY IN AND AROUND THE DISC:

In reality, things don't look so 'nice and neat' within the human body. The below picture demonstrates what real nerve roots look like:

Figure. #15 is a back view (posterior to anterior) of a real human cadaver lumbar spine. The back part of the vertebrae (lamina & spinous processes) have been removed in order to see the dural sac (aka: thecal sac); the dural sac has been sliced open in order to see the dangling nerve roots of the cauda equina.

Note: the cauda equina is only seen below the level of L2. Above L2, we have the more familiar spinal cord.

#1: This ball-like structure is the ultra-sensitive Dorsal Root Ganglion (DRG) that contains the sensory nerve cell bodies. (the motor nerve cell bodies live out of harms way and are found in the dorsal horn of the spinal cord.) The DRG is found within the protective bony intervertebral foramen (cut away in this photo) and can be pinched/irritated from 'far lateral disc herniations' and/or lateral canal stenosis.

#2) This is the famous 'spinal nerve root' and is the number one target of disc herniation. A good sized paracentral disc herniation often will compress this adjacent structure and 'might', if coupled with an inflammatory reaction, ignite the nerve root into 'anger' and sciatica.

#5) As the spinal nerves leave the spine and head-out into the body to do their respective jobs, they temporarily join into a mixed spinal nerve (#5). After this brief marriage, they spilt and become the smaller dorsal primary rami (#7) (which supplies the skin and muscle of the back) and the ventral primary rami (#6). The ventral primary rami (aka: anterior primary rami) of the bottom three nerve roots (L4, L5, and S1), merge within the pelvis to form the giant 'sciatic nerve', which not only causes so many of us grief when irritated but, importantly, gives life to the skin and muscle below the knees.

Inside the dural sac, you can see the free hanging motor nerve root (#4) and the sensory nerve root (#3). These nerve roots connect into the real spinal cord about at the L1 level. Pain signals travel along the sensory nerve root and register 'PAIN' within our brains in the sensory cortex, among other places.

The Sinuvertebral Nerve: A nerve of mystery

The Sinuvertebral Nerves (SN), is a mixed nerve as well. It carries both autonomic fiber (sympathetic) and sensory (afferent) fiber. The sensory portion, which has the capability to carry the feeling of PAIN, arises from the outer 1/3 of the posterior anulus fibrosus (yellow balls) and PLL (#7). It then spilts and attaches to both the dorsal ramus and the grey ramus communicans, although this nerves anatomy and course seems to be quite anomalous.

Of importance is the fact that if irritated, the nerve ending within the disc have the potential to generate both back pain and/or lower limb pain (Discogenic Pain). This lower limb pain-referral has been greatly studied by Ohnmeiss et al. and is quite an interesting phenomenon. Discogenic Sciatica is the term I have given this referred discogenic pain. It is believed that the sinuvertebral nerve-endings are 'sensitive' to the irritating effects of degenerated nucleus pulposus, that may be introduced into the outer region of the anulus from a grade three anular tear. (see may pages on Anular Tears for more information.)

Amazingly, the sinuvertebral nerve also innervates (connects to) the disc above and below! So, the sinuvertebral nerve of the L4 disc also innervates the L5 and L3 disc. This may help explain why a L4 disc herniation/anular tear may clinically present with some signs of L5 and/or L3 involvement as well. It also carries autonomic nerve fiber to the blood vessels (not shown) of the epidural space. Sympathetic nerves control how the blood vessels function (vasomotor & vasosensory).

 Although rare, injury to these sympathetic nerves may cause RSD symptoms in the patients lower limbs; this usually would occur following surgery. (This may explain why I have a very slight case of RSD in my left foot following surgery - since my doc spent an hour cutting his way through a 'nest' of epidural vessels during my micro-discectomy)

The exact pain-pathway (how pain travels from the disc to the spinal cord) of discogenic pain is another fascinating and controversial subject. It seems that the sensory pathway from the sinuvertebral nerves into the spinal cord, does NOT take the 'expected' route in every patient. Some research (101) has demonstrated that pain-signals travel from the disc, re-enter the IVF (via sinuvertebral nerve) and DRG at the SAME level. Other, more recent research has indicated that pain-signals travel from the disc, through the sinuvertebral nerve, through the Gray Rami Communicans, into the Sympathetic Trunk (ST), up the sympathetic chain to the L2 vertebral level (yes I said L2), through the gray rami communicans, into the L2 dorsal rami, into the L2 IVF, and into the L2 DRG (80, 81). The latter pain pathway is why some investigators believe that lower level disc herniations may present as L2 dermatomal pain (groin region) in some patients!

To drive-home my point that the pain path from the disc does NOT always re-enter at the same vertebral level, I present the 2004 randomized controlled investigation by Oh and shim (26). In an incredibly well designed outcome study, the latter investigators demonstrated that by 'cutting' (RF neurotomy) the Gray Ramus Communicans, the majority of chronic discogenic pain sufferers achieved substantial relief of their pain and avoid fusion surgery. (26) This proves that at least some of the incoming pain signals were traveling toward the sympathetic trunk (which is on the anterior side of the vertebra) and NOT re-entering the spinal nerves at the same level.

A Committee Error?

Another interesting oddity about the design of the nervous system is the fact that 'the committee' decided to put the delicate sensory nerve cell bodies within the IVF and NOT the within spinal cord, which is where the motor nerve cell bodies are located. The Dorsal Root Ganglion (DRG), which houses these sensory nerve cell bodies, is seen as a tiny bulging structure within the IVF. This structure is 'super-sensitive' to compression (because it houses all these sensory nerve cell bodies) and can cause extreme back and leg pain if compressed and irritated by discal material and/or bony outgrowth (stenosis). The placing of these sensory nerve cell bodies in such close proximity to the disc and within such a narrow bony tunnel (the IVF) was not 'the committees brightest idea! You see if you damage the axon (nerve fiber) of a nerve, the chances are quite good for recovery, but if you damage the 'brain of the nerve fiber' (nerve cell body) the nerve's chances of recovery are much less. This explains why patients often never recover completely from that tingling burning, and numbness following a major attack of sciatica (disc herniation-induced radicular pain and dysfunction).

The View from the Side: (the Sagittal view)

Fig. #2 Is a sagittal view (aka: lateral view) of the 'motion segment'; Two vertebrae which are 'sandwiching' the intervertebral disc.

The disc [which is made of a anulus fibrosis (blue) and the nucleus pulposus (green)] is made up of three distinct areas: 1) The nucleus pulposus (green), which is a water rich (due to proteoglycan aggrecan & aggrecate molecules which trap and hold water within the disc) gel in the center of the disc; 2) The anulus fibrosis (blue), which is the fibrous outer portions of the disc that is made up of type I collagen; and  3) The vertebral end-plates (yellow), which are cartilaginous plates that attach the discs to the vertebrae and supply food (nutrients) to the inner 2/3 of the anulus and entire nucleus pulposus.

To further increase the strength of the anulus fibrosus, individual sheets of collagen are layered throughout the anulus. There sheets of collagen are called lamellae (black curved lines within blue). The very outer lamellae (Sharpey's fibers), unlike the inner lamellae, are anchored into the solid bony periphery (Ring Apophysis) of each vertebral body. This is the region that 'osteophytes' or bone spurs typically like to form. (Click here to see a real axial view of a 'motion segment'.)

Disc Physiology 101:

The normal human intervertebral disc, which is considered the largest avascular structure in the human body, is made up of two main components, proteoglycan and collagen (type I and type II). The anulus is mostly made of collagen, which is a tough fibrous tissue similar to the cartilage that is found in the knee, and the nucleus is made mostly of proteoglycan. Proteoglycans, which are produced by disc cells that resemble chondrocytes, are extremely important for disc function (see next paragraph) and are what 'trap' and hold water molecules (H20) within the tissue of the disc. In fact both the disc and anulus are comprised mainly of water, i.e., the nucleus is 80% water, and the anulus is 65% water. Proteoglycans are the building blocks of the aggrecan molecule which is the true 'water trap' of the disc.  Aggrecans combine within the disc on strands of hyaluronan acid to form huge structures called 'Aggregates'. These super water-filled proteoglycan aggregates are what give the healthy young disc its amazing strengths and pliability, in fact a well hydrated disc is often even stronger than the bony vertebral body. Fig. #3: Here we have the healthy disc of a teenager (cadaver). The water content is extremely high as you can even see by the 'glistening' appearance of the nucleus (which is the gray center of the white disc).

Disc Function:

In order for a disc to function properly, it MUST have high water content; this is especially true of the nucleus.   A well hydrated (with water) disc is both strong and pliable.   The nucleus pulposus needs to be strong and well hydrated to do its job (axial load), for it is this structure that supports or carries the lion's share of the axial load (downward weight of body) of the body.   With an undamaged anulus, strongly corralling a fully hydrated nucleus, the disc can easily support even the heaviest of bodies!   As the disc dehydrates (loses water) the disc loose ability to support the axial load of the body (loses hydrostatic pressure); this causes a 'weight bearing shift' from the nucleus, outward, onto the anulus, outer vertebral body, and zygapophyseal joints (facets).   Now, we have an 'over-load' on the anulus (which may trigger other destructive biochemical reactions) which, if severe and/or is imposed upon a genetically inferior anulus, will result in pathological DDD. ( see below)  

Hydration also is important with respect to disc nutrition.   As we have already mentioned, nutrients (which all living tissue needs in order to survive) must diffuse (soak) through the discal tissue in order to reach the hungry disc cells.   This diffusion process is much faster and easier IF the diffusing tissue has a high water content.   We may use 'swimming' as an analogy: It's easier to swim through the water, than through the sands of a desert.   The sands of the desert would be a dehydrated disc, and the water would be a hydrated disc.   So, water and disc hydration are one of the key factors for a normally functioning spine and well feed disc.

So, we've learned WHY disc hydration is so important.   Now it time to learn HOW this disc hydration is accomplished:

Water is held within the disc by tiny sponge-like molecules called proteoglycan aggrecans.   These 'super sponges' have an amazing ability to attract and hold water molecules (324), and can in fact hold over 500 times their own weight in water; this   gives the non-dehydrated disc the tremendous 'hydrostatic pressure' which is needed to bear the axial load of the body.   Amazingly, the aggrecans water absorption is so powerful that over night (non-axial loading) the height of the disc and the body will actually measurably increase due to the discs engorgement with water.   This phenomenon is called 'Diurnal Change' and is only present in non-degenerated discs.

Disc cells, particularly the chondrocyte-like cells of the nucleus and inner anulus, manufacture proteoglycan aggrecan molecules.   Like little factories, they create, replace and rebuild aggrecan molecules.   As long as the disc cells have food (glucose), building material (amino acids) and oxygen all is well in disc-land.   It is also important for them of have a non-acidic working environment, which is taken care of, since wastes are diffused out of the disc the same way nutrients diffuse in.   In the living disc up to 100 aggrecans combine on a long piece of 'hyaluronan acid' to form giant proteoglycan aggregate molecules.   It's these aggregates that are found within the disc in the real world.

Disc Nutrition:

The intervertebral disc is the largest avascular structure in the human body. The reason for this is because it has no direct blood supply like most other body tissue. Nutrients (food) for the disc are found within tiny capillary beds (black arrows) that are in the subchondral bone, just above the vertebral end-plates . This subchondral vascular network 'feeds' the disc cells of the all important nucleus and inner anulus through the diffusion process. The Figure on the left shows the 'disc feeding setup' for disc. Note that the outer anulus has its oven blood supply that is embedded within the very outer anulus. This is a much more efficient system and nutrients don't have to diffuse very far to find their hungry disc cells. The 'more direct' blood supply of the outer anulus is why tears of the outer 1/3 of the anulus will heal/scar shut with the passage of time, which unfortunately is not true of the rest of the disc. Research has indicated that disc tears will not heal in the inner zones of the disc - probably because of the avascular nature of the inner two thirds of the disc. Note the nutrients (pink balls) diffuse directly into the tissue of the outer anulus, where as the nucleus and inner anulus has a much longer diffusion route that is block by the vertebral end-plates.  Note how the nutrients (pink balls) are released from the blood vessels (red) in the subchondral bone just under the vertebral end-plates. These nutrients must 'diffuse' or soak their way through the vertebral end-plates and into the disc. This 'diffusion method' is how the cells of the disc get the nutrients oxygen, glucose, and amino acids which are required for normal disc function and repair. This poor blood/nutrient supply to the disc is one of the main reasons that the disc ages and degenerates so early in life. (Read my Disc Degeneration page for more information.)

The 'diffusion feeding process' is enhanced somewhat by a phenomena called 'Diurnal Change'. Our discs have the ability to expand and compress over the course of a day. As we start the day our discs, like squeezing out a sponge, will compress and dehydrate because of the gravity and physical activity which place axial loads upon the discs. In fact a healthy disc will shrink down some 20% (104), which in turn decreases our height by 15 to 25mm (194,441,815). As we sleep and decompress our spines, our discs swell with water plus nutrients and expand back to their fully hydrated state. This tide-like movement of fluids in and out of the disc will help with the movement of nutrients into the avascular center of the disc. (Click here to learn more on Diurnal Change).

Super Advanced Anatomy & Physiology:

The Nucleus Pulposus:

The nucleus pulposus is a hydrated gelatinous structure located in the center of each intervertebral disc that has the consistency of toothpaste. Its main make-up is water (80%). Its solid/dry component make-up are proteoglycan (65%), type II collagen fiber (17%) and a small amount of elastin fibers . Collectively the proteoglycans and the collagen are called the 'nuclear matrix'. The cells of the disc, which produce the water holding proteoglycan molecules are very similar to chondrocytes seen in articular cartilage and are also held within the matrix.

Proteoglycans are found in several structural forms within the disc but the most important 'arrangement' is called a proteoglycan aggrecan. These aggrecans main function is to trap and hold water, which is what gives the nucleus its strength and resiliency. Like a 'super sponge', aggrecans can trap and hold over 500 times their weight in water!

The nucleus has two functions. The first is to bear most of the tremendous axial load coming from the weight of the body above and second to 'stand-up' the lamellae of the anulus - so that the anulus can reach its full weight baring potential. In order for proper weight bearing the nucleus and the anulus MUST work hand in hand.

The Anulus Fibrosis:

The anulus is the outer portion of the disc that surrounds the nucleus. It is made up of 15 to 25 collagen sheets which are called the 'lamellae'. The lamellae are 'glued' together with a proteoglycans. These sheets encircle the disc and, in concert with the nucleus, give the disc tremendous axial load strength. 

The posterior portion of the anulus if further strengthened by the 'posterior longitudinal ligament'. This structure is the final barrier between the disc and the delicate spinal cord, and nerve roots.

The biochemical make up is similar to that of the disc only in different proportions. The anulus is 65% water, with the collagen, both type I and II making up 55% of the dry weight, and proteoglycans (mostly the larger aggregate type - 60%) making up 20% of the dry weight. 10% of the anulus also contain 'elastic fiber' that are seen near where the anulus attaches into the vertebral end-plate.

The lamellae are made up of both Type I (very strong type) and Type II collagen fiber. The very outer lamellae are almost all Type I. As we move inward toward the nucleus the more Type II is seen and less Type I. The very inner layers are very hard to distinguish from the nucleus. There is not a clear boundary between the nucleus and the anulus. 

A simply amazing fact about the lamellae design is that the collagen fibers that make-up each lamellae all run parallel at a 65 degree angle to the sagittal plane. Even more amazing is the fact that the each lamellae are flipped so that the 65 degree angle alternates between every lamellae, one to the right then one to the left. This design greatly increases the shear strength of the anulus and makes it had for cracks to develop through the layers of the anulus. This is just amazing if you think about it!! Brilliant design!

The function of the anulus is to help the nucleus support the axial weight from the body. The anulus does need some help from the nucleus in order to achieve its strongest configuration. It relies on the nucleus to push it outward which keeps the lamellae from collapsing inward. The nucleus must keep a very high hydrostatic pressure to achieve this. We saw what happens when the nucleus losses hydrostatic pressure under the 'Disc Degeneration' page. Bogduk used the analogy of a rolled up telephone book standing on end, to describe how strong the anulus could be when the nucleus holds the inner lamellae or phonebook in a rolled up position. If you unroll the phone book our 'on end phone book' it would not longer be able to support much axial loading.

The Vertebral End-Plates:

Both the top and the bottom of each vertebrae (spinal bones) are capped with a thin ¾ millimeter cartilaginous pad called the 'Vertebral End-Plate' (Figure #1).  Despite their name, these end-plates are NOT attached to the subchondral bone of the vertebrae but are instead strongly interwoven into the anulus of the disc (156, 388). It is for this reason, as well as strong morphological similarities, that the vertebral end-plates are considered part of the disc and NOT part of the vertebral body.

The biochemical morphology of the end-plates is extremely similar to that of the disc: Water, proteoglycans, collagen and cartilage cells (chondrocytes). The concentration scheme of these components also mirrors that of the disc: The center of the end-plate is mostly water and proteoglycan. As we move outward toward the periphery, more and morecollagen is seen with less and less proteoglycans. This similar biochemical makeup and distribution scheme helps the diffusion of nutrients between the subchondral bone of the vertebra and the depths of the disc.

The very outer rim of the vertebrae is NOT covered by the end-plate, which leaves a ring of exposed bone on the periphery of the top and bottom of each vertebra. This exposed peripheral area is called the 'Ring Apophysis' and is often a site for the development of spur formation associated with the degeneration process. 

References:

12) Marchand F, et al. (1990) "Investigation of the laminate structure of lumbar disc anulus fibrosus." Spine - 1990; 15:402-410

80) Nakamura SI, Takahashi K, Takahashi Y, et al. "Origin of nerves supplying the posterior portion of lumbar Intervertebral discs in rats." Spine 1996; 21:917-924

81) Nakamura SI, Takahashi K, Takahashi Y, et al. "The afferent pathways of discogenic low-back pain. Eval‎uation of L2 spinal nerve infiltration." J Bone Jont Surg. 1996;78:606-612

101) Bogduk N, et al. "The nerve supply to the human lumbar intervertebral disc." J Anat 1981; 132:39-56

26) Oh WS, Shim JC. "A randomized controlled trial of radiofrequency denervation of the ramus communicans nerve for chronic discogenic low back pain." Clin J Pain 2004; 20(1):55-60