Understanding the Anatomy of the Spine

The spinal cord is a complex and crucial part of the human body. Not only does it provide support for us as we walk around, it is a vital conduit for information, sending signals and instructions to and from the brain as well as handling reflex responses.

When you decide to reach out and open a door, feel the pain of a pinprick or recoil because you touched something hot, your spine will have been involved in a very specific way. Damage to this area of the human body can have catastrophic consequences with sensation and motor movement lost below the level where an injury occurs.

  • The spinal column houses the spinal cord which is a part of the central nervous system.
  • The spinal cord stretches from the foramen magnum at the base of the skull down to the L1/L2 lumbar vertebrae.
  • The spinal cord is 40 to 50 cm in length and about 1.5 cm in diameter. It weighs around 35 grams.
  • It is protected by bones called vertebrae which form a chain link down the centre of the back as well as three membrane layers called meninges and cerebrospinal fluid.

Spinal Cord Regions

Anatomy of the Spine
Anatomy of the Spine

The spinal cord is divided into distinct anatomical sections. The cervical cord is in the neck area, followed by the thoracic in the upper back, lumbar and finally the sacral and coccygeal towards the buttocks. Each of these sections has its own pair of nerves, 31 in total, that fan out to the rest of the body carrying messages back and forth to the brain.

For example, the thoracic section has 12 pairs of nerves which contain both sensory and motor nerve fibres. The spinal nerves are numbered according to the vertebrae they emerge from so you would have T1, T2, T3 etc.

Internal Structure of the Spinal Cord

The spinal cord is the nerve network that transmits information to and from the brain and it is connected to the brain stem at the base of the skull.

If you cut the spinal cord in two, you will see that it has grey matter (comprising neuronal cells) on the inside and white matter (the axons or pathways) on the outside. The grey matter has a distinctive butterfly shape and is actually pink because of the high number of blood vessels. The proportion of grey to white matter increases the lower down the spine you travel because there are fewer nerve fibres or axons in the white matter.

Within the white matter, there are various tracts called the anterior, lateral and posterior funiculi. Ascending tracts are involved in communicating information from the periphery of the body (for example, the hands and feet) to the brain. Descending tracts send information in the opposite direction. The spinal cord not only transmits information to and from various parts of the body but can modify it as well.

There is a healthy blood supply to the roots of the spinal nerves, delivered through vertebral and spinal arteries. Each artery is matched by a similar vein.

External Structure of the Spinal Cord

The external structure of the spinal cord is divided into segments that affect different parts of the body. Each of the nerve pairs are numbered according to the vertebra from which they originate. For the cervical region, for example, you have eight pairs of nerves labelled C1 through to C8. Each nerve has an anterior and posterior root which have different functions. The anterior root is involved in transmitting motor information while the posterior handles sensory messages. These nerve networks spread out to all parts of the body, sending and receiving information.

This is not the only role the spinal cord performs, however. It can also generate reflex responses without involving the brain at all. A monosynaptic reflex involves just one motor and one sensory nerve which are used to respond to a stimulus by initiating movement in a single muscle. A polysynaptic reflex involves several sets of nerves affecting a number of muscle groups and is a lot more complex.

Meninges

Besides the protective layer of bone provided by the vertebral column, the spinal cord is also wrapped in three membranes, the meninges. The closest to the spine is the pia mater, above this is the arachnoid mater (so called because it is like a spider’s web) and, on the outside, is the dura mater. Between the pia and arachnoid maters is a space that is filled with cerebrospinal fluid. The primary function is to protect the spinal cord but these layers also allow the passage of blood and cerebrospinal fluid.

Summary

The anatomy of the spine is highly complex, which is understandable seeing as it is such an integral part of the human body, responsible for so many things. It helps us move, stand, walk, swim and feel, and acts as a connecting pathway between our brain and the rest of our body.