The spine is one of the most important parts of your body. Without it, youcould not keep yourself upright or even stand up. It gives your body structureand support. It allows you to move about freely and to bend with flexibility.The spine is also designed to protect your spinal cord. The spinal cord is acolumn of nerves that connects your brain with the rest of your body, allowingyou to control your movements. Without a spinal cord, you could not move anypart of your body, and your organs could not function. This is why keeping yourspine healthy is vital if you want to live an active life.
Anatomy
What exactly is the spine? Your spine is made up of 24 small bones (vertebrae)that are stacked on top of each other to create the spinal column. Between eachvertebra is a soft, gel-like cushion called a disc that helps absorb pressureand keeps the bones from rubbing against each other. Each vertebra is held tothe others by groups of ligaments. Ligaments connect bones to bones; tendonsconnect muscles to bones. There are also tendons that fasten muscles to thevertebrae. The spinal column also has real joints (just like the knee or elbowor any other joints) called facet joints. The facet joints link the vertebraetogether and give them the flexibility to move against each other.
Each vertebra has a hole in the center, so when they stack on top of each otherthey form a hollow tube that holds and protects the entire spinal cord and itsnerve roots. The spinal cord itself is a large collection of nerve tissue thatcarries messages from your brain to the rest of your body. In order for yourbody to function, you need your nerves. The spine branches off into thirty-onepairs of nerve roots. These roots exit the spine on both sides through spaces(neural foramina) between each vertebra.
The spine itself has three main segments: the cervicalspine, the thoracicspine, and the lumbarspine. The cervical is the upper part of the spine, made up of seven vertebrae(bones). The thoracic is the center portion of the spine, consisting of 12 vertebrae.The lower portion of the spine is called the lumbar spine. It is usually madeup of five vertebrae, however, some people may have six lumbar vertebrae. Havingsix vertebrae does not seem to cause a problem. Below the lumbar spine is thesacrum. The sacrumis actually a group of specialized vertebrae that connects the spine to thepelvis. During development (those nine months before birth), these vertebraegrow together or fuse creating one large "specialized" vertebral bone that formsthe base of your spine and center of your pelvis. The nerves that leave thespine in the sacral region control the bowel and bladder functions and givesensation (feeling) to the crotch area.
The normal spine has an "S"-like curve when looking at it from the side. Thisallows for an even distribution of weight. The "S" curve helps a healthy spinewithstand all kinds of stress. The cervical spine curves slightly inward, thethoracic curves outward, and the lumbar curves inward. Even though the lowerportion of your spine holds most of the body's weight, each segment relies uponthe strength of the others to function properly.
Now let us look at the specific parts that make up your spine.
Vertebrae
The individual bones of the spine are the vertebrae. These are the buildingblocks of the spinal column. The vertebrae protect and support the spinal cord.They also bear the majority of the weight put upon your spine. The body of eachvertebra is the large, round portion of bone. The body of each vertebra is attachedto a bony ring. When the vertebrae are stackedone on top of the other, this ring creates a hollow tube where the spinalcord passes through.
The bony ring attached to the vertebral body consists of several parts. First,the laminae extend from the body to cover the spinal canal, which is the holein the center of the vertebrae. Second, the spinous process is the bony portionopposite the body of the vertebra. You feel this part if you run your hand downa person's back. Then there are two transverse processes (little bony bumps),where the back muscles attach to the vertebrae. Finally, the pedicle is a bonyprojection that connects to both sides of the lamina.
The vertebra, like all bones, has an outer shell called cortical bone thatis hard and strong. The inside is made of a soft, spongy type of bone that iscalled cancellous bone.
Intervertebral Disc
The intervertebraldiscs are flat, round "cushions" that act as shock absorbers between eachvertebra in your spine. There is one disc between each vertebra. Each disc hasa strong outer ring of fibers called the annulus, and a soft, jelly-like centercalled the nucleus pulposus.
The annulus is the disc's outer layer and the strongest area of the disc. Italso helps keep the disc's center intact. The annulus is actually a strong ligamentthat connects each vertebra together.
The mushy nucleus of the disc serves as the main shock absorber. The nucleusis made up of tissue that is very moist because it has high water content. Thewater content helps the disc act like a shockabsorber - somewhat like a waterbed mattress.
Facet Joint
The facets are the "bony knobs" that meet between each vertebra to form thefacet joints that join your vertebrae together. There are two facet joints betweeneach pair of vertebra, one on each side. They extend and overlap each otherto form a joint between the neighboring vertebra facet joints. Without the facetjoints, you would not have flexibility in your spine, and you could only movein very straight and stiff motions.
The facetjoints are what are known as synovial joints. A synovial joint, such asthe knee or elbow, is a structure that allows movement between two bones. Ina synovial joint, the ends of the bones are covered with a material called articularcartilage. This material is a slick spongy material that allows the bones toglide against one another without much friction.
Surrounding the facet joint is a watertight sack made of soft tissue and ligaments.This sack creates what is called the "joint capsule". The ligaments are softtissue structures that hold the two sides of the facet joint together. The ligamentsaround the facet joint combine with the synovium to form the joint capsule thatis filled with fluid (synovial fluid). This fluid lubricates the joint to decreasethe friction, just like oil lubricates the moving parts of a machine.
Neural Foramen
The neural foramen is the opening between every two vertebrae where the nerveroots exit the spine. The nerve roots travel through the foramen to reach therest of your body. There are two neural foramina between each pair of vertebrae- one on each side. Without the foramen, nerve signals could not travel to andfrom the brain to the rest of your body. Without nerve signals, your body wouldnot be able to function.
Spinal Cord and Nerve Roots
The spinal cord is a column of millions of nerve fibers that run through yourspinal canal. It extends from the brain to the area between the end of yourfirst lumbar vertebra and top of your second lumbar vertebra. At the secondlumbar vertebra, the spinal cord divides into several different groups of fibersthat form the nerves that will go to the lower half of the body. For a smalldistance, the nerves actually travel through the spinal canal before exitingout the neural foramen. This collection of nerves is called the cauda equinawhile it is still inside the spinal canal.
A protective membrane called the duramater covers the spinal cord. The dura mater forms a watertight sack aroundthe spinal cord and the spinal nerves. Inside this sack, the spinal cord issurrounded by spinal fluid.
The nerve fibers in your spinal cord branch off to form pairs of nerve rootsthat travel through the small openings (foramina) between your vertebrae. Thenerves in each area of the spinal cord connect to specific parts of your body.This is why damage to the spinal cord can cause paralysis in certain areas andnot others -- it depends on which spinal nerves are affected. The nerves ofthe cervical spine go to the upperchest and arms. The nerves in your thoracic spine go to your chestand abdomen. The nerves of the lumbar spine then reach to yourlegs, bowel, and bladder. These nerves coordinate and control all the body'sorgans and parts, and let you control your muscles.
The nerves also carry electrical signals back to the brain that allow you tofeel sensations. If your body is being hurt in some way, your nerves signalthe brain that you have been hurt. Damage to the nerves themselves can causepain, tingling, or numbness in the area where the nerve travels.
Paraspinal Muscles
The paraspinalmuscles refer to the muscles next to the spine. They support the spine andare the motor for movement of the spine. Your joints allow flexibility and yourmuscles allow mobility. There are many small muscles in the back - each controllingsome part of the total movementbetween all the vertebrae and the rest of the skeleton. These muscles canbe injured directly, such as when you have a pulled muscle or muscle strainof the back muscles. The muscles can also cause problems indirectly, such aswhen the muscles are in spasm after injury to other parts of the spine.
When you experience a muscle spasm, it is because your muscle tightens up andwill not relax. These spasms usually occur as a reflex - meaning that you cannotcontrol the contraction of these muscles. When any part of the spine is injuredincluding: a disc, ligaments, bones, or muscles, the muscles automatically gointo spasm to reduce the motion around the area. This protective mechanism isdesigned to protect the injured area.
When muscles are in spasm they produce too much of the chemical, lactic acid.Lactic acid is a waste product produced by the chemical reaction inside musclecells that must occur to allow the muscle to contract. If the muscle cell cannotrelax, too much lactic acid builds up inside the muscles. The buildup of lacticacid causes a painful burning sensation.
The main reason that lactic acid builds up inside the muscle cells is thatwhen the muscles contract, the small blood vessels traveling through the musclesare pinched off (just like a tube pinched between your thumb and finger). Whenthe muscle relaxes, the lactic acid is eventually washed away by fresh bloodflowing into the muscle as the blood vessels open up.
Spinal Segment
Back specialists sometimes look at a spinal segment to understand and explainhow the whole spine works. A spinal segment is made up of two vertebrae attachedtogether by ligaments, with a soft disc separating them. The facet joints fitbetween the two vertebrae, allowing for movement, and the neural foramen betweenthe vertebrae allow space for the nerve roots to travel freely from the spinalcord to the body.
The spinal segment allows us to focus on the repeating parts of the spinalcolumn to better understand what can go wrong with the various parts of thespine. Sometimes problems in the spine involve only one spinal segment, whileother times the problems involve multiple segments.
Each spinal segment is like a well-tuned part of a machine. All of the partsshould work together to allow weight bearing, movement, and support. When allthe parts are functioning properly, all spinal segments join to make up a remarkablystrong structure called the spinal column. When one segment deteriorates tothe point of instability, it can lead to problems at that segment causing painand other difficulties.
Now that you know the parts of the spine, let us look at the spine itself, whichhas three main segments - the lumbar, thoracic, and cervical spines.
Lumbar Spine
The lowest part of the spine is called the lumbarspine. This area has five vertebrae. However, sometimes people are bornwith a sixth vertebra in the lumbar region. The base of your spine (sacrum)is a fusion of many bones, and when one of them forms as a vertebra rather thanpart of the sacrum, it is called a transitional (or sixth) vertebra. This occurrenceis not dangerous and does not appear to have any serious side effects.
The lumbar spine's shape has what is called a lordotic curve. The lordoticshape is like a backwards "C". If you think of the spine as having an "S"-likeshape, the lumbar region would be the bottom of the "S". The vertebrae in thelumbar spine area are the largest of the entire spine, so the lumbar spinalcanal is larger than in the cervical or thoracic parts of the spine. Becauseof its size, the lumbar spine has more space for the nerves to move about.
Low back pain is a very common complaint for a simple reason. Since the lumbarspine is connected to your pelvis, this is where most of your weight bearingand body movement takes place. Typically, this is where people tend to placetoo much pressure, such as: lifting up a heavy box, twisting to move a heavyload, or carrying a heavy object. Such repetitive injuries can lead to damageto the parts of the lumbar spine.
Thoracic Spine
The thoracicspine is made up of the middle 12 vertebra of the spine. These vertebraeconnect to your ribs and form part of the back wall of the thorax (the ribcagearea between the neck and the diaphragm). This part of the spine has very narrow,thin intervertebral discs, so there is much less movement allowed between vertebraethan in the lumbar or cervical parts of the spine. It also has less space inthe spinal canal for the nerves. The thoracic spine's curve is called kyphoticbecause of its shape, which is a regular "C"-shaped curve with the opening ofthe "C" in the front.
Cervical Spine
The cervicalspine is made up of the first seven vertebrae in the spine. It starts justbelow the skull and ends just above the thoracic spine. The cervical spine hasa lordotic curve (a backward "C"-shape) - just like the lumbar spine. The cervicalspine is much more mobile than both of the other spinal regions - think aboutall the directions and angles you can turn your neck.
Unlike the rest of the spine, there are special openings in each vertebra inthe cervical spine for the arteries (blood vessels that carry blood away fromthe heart), as well as the spinal canal that carries the spinal cord. The arteriesthat run through these openings bring blood to the brain.
Two vertebrae in the cervical spine, the atlas and the axis, differ from theother vertebrae because they are designed specifically for rotation. These twovertebrae are what allow your neck to rotate in so many directions, includinglooking to the side.
The atlas is the first cervical vertebra - the one that sits between the skulland the rest of spine. The atlas does not have a vertebral body, but does havea thick forward (anterior) arch and a thin back (posterior) arch, with two prominentsideways masses.
The atlas sits on top of the second cervical vertebra - the axis. The axishas a bony knob called the odontoid process that sticks up through the holein the atlas. It is this special arrangement that allows the head to turn fromside to side as far as it can. Special ligaments between these two vertebraeallow a great deal of rotation to occur between the two bones.
Though the cervical spine is very flexible, it is also very much at risk forinjury from strong, sudden movements, such as whiplash-type injuries. This highrisk of harm is due to: the limited muscle support that exists in the cervicalarea, and because this part of the spine has to support the weight of the head.This is a lot of weight for a small, thin set of bones and soft tissues to bear.Therefore, sudden, strong head movement can cause damage.
Copyright © 2003 DePuy Acromed.
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