The Human Anatomy Human anatomy organs

The Human Anatomy  Human anatomy organs

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Published by admin on 9th January 2011

We all think of man as being upright and indeed sometimes apply moral values to this posture! However, simple observation will show that the spine is not straight but has a number of curves. The low back or lumbar region is slightly hollowed so that it is curved backwards. This is known technically as the lumbar lordosis. The back of the chest is curved slightly forwards and the neck points slightly forwards. In the various problems that arise in the spine this normal pattern may change. The curves may be lost or exaggerated. There may be an abnormal curvature forwards – a kyphosis, a sideways twist – a scoliosis, or there may be a sharp kink. Not only do these abnormal postures develop due to various types of disease, but also they may make things worse by altering the ways in which the spine works and placing stresses on parts of the spine not designed to deal with them.

The spine consists of a column of bony blocks known as vertebrae standing one on top of the other. In the neck there are seven which are known as cervical vertebrae; there are twelve in the back of the chest – the dorsal or thoracic vertebrae; and five in the back – the lumbar vertebrae. The bottom or fifth lumbar vertebra (L5) is directly attached to the sacrum or tail bone. The sacrum is the back part of the pelvis, a ring of bone providing support for the spine and trunk.

All the vertebrae have similar basic structures but with considerable variations at each level, reflecting the different functions at each site. For example, the neck is remarkably flexible and only has to bear the weight of the skull. The cervical vertebrae are very lightly built and shaped so as to allow a considerable range of movement between them. In contrast the back has to bear the weight of the trunk and transmits forceful movements of the body. The lumbar vertebrae are much thicker and tougher structures with relatively limited movements allowed between them.

Each vertebra consists of a cylindrical part in front known as the vertebral body. This has flat upper and lower surfaces known as the vertebral end-plates. The vertebral body is convex in the front and flattened behind. This flattened area forms the front part of the canal down which the spinal cord and nerve roots pass. The vertebral arch surrounds the rest of the canal giving protection to the structures within. Each vertebra is joined to those above and below by joints between the vertebral bodies and between the arches.

The intervertebral disc lies between the vertebral end-plates. It is a cushion of tissue that normally provides a springy and movable connection between the bones. Three types of movement occur at this cushion – flattening under loads, bending, and twisting. However, unlike a cushion the disc has a very definite structure that is all-important when things go wrong. Basically the disc consists of two parts: a central area known as the nucleus pulposus and an outer ring, the annulus fibrosus.

In his drawings of the spine Vesalius showed the structure of the disc and realized that it was complex. He knew that the outermost layer is different from the inner material but he did not get the details absolutely right. The nucleus consists of jelly-like material containing a few tangled fibres and many large molecules known as proteoglycans. The nucleus has the property of all gelatinous materials, namely that it can be squashed and then alters in shape but its total volume will remain constant. The nucleus is kept in shape by the outer annular ring and the vertebral end-plates above and below.

This allows it to transmit the enormous loads that may be carried by the spine. It is the nature of the proteoglycans in the nucleus that they are constantly trying to suck in water and swell. This tendency is resisted by pressures produced by the weight of the body. When we sleep lying horizontally at night this swelling process is unimpeded. It is a fact that we are slightly taller when we first rise in the morning than at the end of the day. This process is carried to the extreme in spacemen who after several days of weightlessness may grow by a couple of inches. As their space suits are individually and very precisely designed this produced considerable difficulties and a lot of complaints from the spacemen before the cause was appreciated.

The annulus fibrosus which surrounds the nucleus has quite a different structure. It consists mainly of fibres of collagen. This is a fibrous protein which is uniquely designed for tensile or stretching strength. A collagen fibre is considerably stronger than a steel wire of the same dimensions. The collagen fibres of the annulus are attached around the edge of the vertebral end-plate and spiral obliquely upwards and downwards to the end-plates above and below. They cross over and interweave one another in a complex fashion so forming an extremely strong network that surrounds the nucleus. When vertical loads are placed on the spine the nucleus is squashed and slightly flattened and the annular ring will expand slightly, but nevertheless will not give way. Indeed, the annulus is so strong that under extreme load it is the vertebral end-plates that will fracture rather than the annulus itself. This criss-cross arrangement of annular fibres allows relatively easily the movements of bending forwards and backwards and to the side. However, twisting movements are more difficult. This is one of the reasons why back problems are more likely to arise with this type of activity.

At the back of the spine the vertebral arches are also joined to the arches above and below by the small facet joints. There is one joint on each side at each level so that connecting each pair of vertebrae are three joints: one between the vertebral bodies and two between the arches. The facet joints differ from the intervertebral disc in that they are synovial joints. The opposing joint surfaces are covered by cartilage or gristle and are connected to each other by a fibrous capsule lined by a layer of tissue known as the synovial membrane. Within the joint is a lubricant known as the synovial fluid which allows the joint surfaces to move against each other. This arrangement is very similar to that of the joints in the limbs, although the sizes and shapes of the various joints differ enormously. Any movement between a pair of vertebrae must involve all three joints; it is not possible for one of these joints to move to the exclusion of the others- It may seem trite to point this out but one often hears statements that one of these joints alone is stiff or that manipulation is applied to a single joint, ignoring the movements that must take place at the others.

The vertebrae are also joined to one another by a series of ligaments which ensheath the bodies and connect the arches. These ligaments are relatively flexible and yet have an important role in covering the bony and joint surfaces.

A pile of vertebrae one on top of the other is intrinsically unstable and one can readily imagine how bowing or slipping of the structure could occur. The spine is stabilized by very powerful muscles attached to the vertebrae, the pelvis, and the back of the chest wall. An interesting analogy is with the stays on a yacht which have a remarkably similar function in stiffening and stabilizing the mast. During any exertion these muscles contract so stiffening the spine and enabling it to bear the load.

The spinal cord emerges from the base of the brain and passes down through the vertebral canal behind the vertebral bodies and surrounded by the vertebral arches. During its passage downwards nerve roots separate off to emerge through the invertebral foramina between the vertebral arches at every level. The spinal cord itself finally ends at about the junction between the first and second lumbar vertebrae (Ll/2) and below that level there are only nerve roots in the vertebral canal. The roots emerge from the canal and pass on to their ultimate destinations, uniting and dividing in complex fashions to provide appropriate nerve supplies to all the structures of the human body.

The spinal cord and nerve roots are not in direct contact with the bony vertebral canal but are covered by several protective sheaths. The outermost is known as the dura mater and is a strong fibrous membrane which forms a wide tubular sheath around the spinal cord and the upper part of the nerve roots with tubular prolongations along the nerve roots themselves as they pass out through the intervertebral foramina. Inside the dura is the arachnoid which is a much more delicate membrane loosely investing the spinal cord and nerve roots. The innermost layer is known as the pia and is intimately adherent to the spinal cord and nerve roots. In the course of various back disorders these coverings may become inflammed and thickened with a lot of scar tissue. This is known as arachnoiditis and can be the cause of severe pain in the back and in the areas in the legs that the nerves supply.

The spine itself also has a nerve supply. Tiny branches from the nerve roots will supply the ligaments around the spine, the capsule or surroundings of the facet joints, the dura, arachnoid, and pia coats around the spinal cord and the blood vessels in the bone marrow. Surprisingly they do not occur in the substance of the intervertebral disc but only in the very outermost layers of the annulus fibrosus. As damage to a nerve fibre is a prerequisite to produce the sensation of pain, it is clear that most disorders of the disc alone will be painless. The presence of damage to a disc seen on X-rays does not mean that this has been the cause of back pain. It is only when a disc disorder produces damage to the surrounding structures that pain may develop.

Another point about the nerve supply to the spine is that the nerves from each level combine with one another in extremely complex fashions. This produces the phenomenon known as cross-innervation which means that any nerve may carry information about abnormalities arising at several different levels. For this reason it is often extremely difficult to localize the source of symptoms felt in the back. Pain felt at one site can be due to abnormalities at a quite different area. Considerable circumspection is required when trying to identify the source of the problem in the back pain subject.

Learning human anatomy can be a daunting task and takes a lot of time and persistence. You can learn human anatomy online with free resources such as vAnatomy.com. High quality anatomy models & posters from VisuMedical.com also make excellent study tools to learn human anatomy.

Brendon Mitchell is dedicated to providing world class educational resources to further the study of human anatomy and physiology.

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