5. Vertebral Column. Human beings belong to a vast group animals, the vertebrates. In simple terms we say that vertebrates are animals with a backbone. This statement barely touches the surface of the issue. Vertebrates are animals with a bony internal skeleton. Besides, all vertebrates have a fundamental common body plan. The central nervous system is closer to the back than it is to the belly, the digestive tube in the middle and the heart is ventral. The body is made of many segments (slices) built to a common plan, but specialised in different regions of the body. A “coelomic cavity” with its own special plan is seen in the trunk region and has a characteristic relationship with the organs in the trunk. This is by no means a complete list of vertebrate characteristics. Moreover, some of these features may be shared by other animal groups in a different manner. Such a study is beyond the scope of this unit. Vertebrates belong to an even wider group of animals, chordates. It may be difficult to imagine that we human beings are in fact related to some the earlier chordates! However, we do share, at least during embryonic development, an important anatomical structure with all chordates. This structure is the notochord. The notochord is the first stiff, internal support that appeared during the evolutionary story. As we have seen in early embryology, it also defines the axis of the body. The vertebral column evolved around the notochord and the neural tube, and we see a reflection of this fact during our embryonic development. The notochord disappears during further embryonic development, but leaves behind remnants recognisable in adult life. One reason we study the vertebral column at this stage is that it gives us insight into the segmental nature of the body. The segmental plan is also reflected in the pattern of the spinal nerves which emerge from the vertebral column. At the same time we also recognise that the vertebral column itself is a marvellous functional mechanism, with its joints and muscles. The human vertebral column : Overview. Anterior Posterior Lateral The vertebral column is bony – it is strong, yet it is flexible, as it comprises a number of smaller bones. Its successively C lower parts bear the weight of the trunk above and the column transmits the weight of the body to the lower limbs. It contains and protects the spinal cord, a delicate part of the central nervous system. The thoracic part supports the cage of ribs, which in turn is a part of the mechanism of breathing. The flexibility of the column allows some degree of T movement, brought about by muscles. Originally comprising 33 bones, in the fully developed state the vertebral column has 29 bones. Seven of these are in the neck (cervical vertebrae : cervix = neck), twelve in the thorax (thoracic) and five the in the small of the back (the lumbar L region). Below the lumbar region, the sacrum in the posterior part of the pelvis is formed by the fusion of five vertebrae; the coccyx (the ‘tail’) contains four fused vertebrae. The vertebrae of each region are designated by the initial S letter of the region and the number of a particular vertebra in the region. Thus cervical vertebrae are numbered C1 to C7 from above downwards, thoracic vertebrae T1 to T12, lumbar Co Fig. 1. Three views of the vertebral column. vertebrae from L1 to L5. Though the sacrum is a fused mass, boundaries of individual vertebrae are visible on its anterior surface as horizontal lines and the individual ‘pieces’ are numbered S1 to S5. The vertebrae of the coccyx (coccygeal vertebrae) are not numbered individually. The abbreviation used is ‘Co’, in order to avoid confusion with the ‘C’ of cervical vertebrae. Since the tail is rudimentary in humans, the coccygeal region does not amount to much, either anatomically or functionally. Nevertheless, an injured coccyx can be a source of great pain, or at least discomfort! Variation in numbers of vertebrae is possible among individuals. This is more common in the lumbar and the sacral region. 1 Curvatures of the vertebral column. The vertebral column in an articulated skeleton shows curvatures in a lateral view. In the neck the curvature is convex forwards. The thoracic part of the column is concave forwards. The lumbar region again has an anterior convexity and the sacrum is concave forwards. These alternating curvatures are said to contribute to the ‘springy’ nature of the column. In the uterus the foetus is curled up and the entire column is concave forwards. An anterior concavity is therefore described as a primary curvature – the first curvature of the column. After birth some parts change their curvatures. These are known as secondary curvatures. At birth a child cannot hold the head steady. By the age of three months the muscles at the back of the neck become stronger, the baby can hold the head. The cervical column gradually becomes convex. When the child sits, stands and walks, the lumbar column similarly attains a forward convexity. An anterior convexity is a secondary curvature. The entire column thus shows alternating primary and secondary curvatures. A generalised vertebra. Vertebrae in different regions have characteristic features. These features are variations of a common plan. This common plan reflects the functions of the column as a whole. The common plan is best studied in a thoracic vertebra, though thoracic vertebrae do have their special features we ignore for the time being. A generalised vertebra is illustrated in fig. 2. Each vertebra has a weight-bearing part, the ‘body’(1), in the form of a block of bone. A curved arch (2) joins the body to form a ring around the spinal cord. Because it protects the spinal cord, the arch is called the neural arch. The cavity enclosed between the body and the arch is the vertebral foramen. In a single vertebra it is like a foramen, but in the entire column, all such foramina form a canal for the spinal cord. Fig. 2. A generalised vertebra. A : Parts numbered. B : Parts named. C : Two neighbouring vertebrae. Note the notches forming the intervertebral foramen (asterisk). The neural arch has bony outgrowths or ‘processes’. The transverse processes (3), as the name suggests, spread transversely. The single process at the posterior end (4) is the spine. The small portion between the transverse process and the body is the pedicle, and the flat part between the transverse process and the spine is the lamina. Running up and down from the arch are columns of bone for forming synovial joints with the neighbouring vertebrae. These are the articular processes (‘5’ and ‘6’). Each of these has a flat area which the actual joint surface – this is the articular ‘facet’. The superior and inferior notches are related to the pedicle. When two or more vertebrae are seen as they are arranged in the body, these notches form a foramen through which a nerve passes. This foramen is the intervertebral foramen. The intervertebral foramen must not be confused with the vertebral canal! The canal houses the spinal cord. In each vertebra, the canal is bounded by the body and the neural arch. The intervertebral foramina are between two vertebrae. They are formed mainly by the notches of adjacent vertebrae, open laterally and allow passage of nerves from the spinal cord. 2 An important note on the use of the term ‘spine’. As we have described above, the spine of a vertebra is a pointed process on the posterior side of a vertebra. This usage comes from the Latin word spina, meaning a thorn. The entire vertebral column is often referred to as the spinal column. This seems logical, as the spines of vertebrae are often seen forming a prominent feature down the midline of the back. Unfortunately, this term ‘spinal column’ tends to be abbreviated as ‘the spine’ when referring to the entire vertebral column. This can lead to confusion. It is safer to reserve the term spine for the spinous process of a vertebra. Regional characteristics. Each region of the vertebral column has special features – some related to their functions, some developmental. o The cervical column allows movements of the head. It also gives attachments to strong muscles which balance the head. o The thoracic column has ribs attached to it. Ribs have movable joints with thoracic vertebrae, as movements of ribs are required for respiration. o The lumbar column bears great weight and has strong muscles and ligaments. Further, each region has some vertebrae which do not conform to the characteristic features. o This is explained by two reasons. One, there is a gradual transition between regions; therefore “borderline” vertebrae have overlapping features. Second, some vertebrae are specialised for specific functions. Vertebrae sharing the characteristic features are said to be typical for a region, others are atypical vertebrae of the region. Be careful with the use of the words “typical” and “atypical”. The phrase “a typical vertebra” can sound like “atypical vertebra”. In the first instance, ‘a’ is an indefinite article. The second phrase, if it is to be grammatically correct, should be “an atypical vertebra”. Vertebral bodies have to bear progressively greater weight in the craniocaudal direction; and show a gradual increase from the top to the bottom of the column. The upper part of the sacrum transmits the weight of the trunk to the lower limbs, and the sacrum and the coccyx show a gradual decrease in size. (Note : anatomical features of vertebrae are best studied in the lab!) Cervical vertebrae. There are seven vertebrae in the neck. Of these, C3 to C6 are typical vertebrae.