2.2. Overview: Spinal Cord

Overview of the Spinal Cord

The spinal cord is the second component of the CNS and is a highly organised tube protected by the bones of the vertebral column. The spinal cord is continuous with the medulla, the most inferior (or “caudal”) part of the brainstem. You can see this in Figure 2.2.1. The spinal cord role is to allow the brain to receive sensory information from and send motor information to almost every region of the body. However, the spinal cord is also responsible for some basic processing of action potentials – this is performed by interneurons (see Section 1.5. Reflex Arcs).

Divisions of the Spinal Cord

The spinal cord, like the vertebral column, can be grouped into five distinct structural divisions. Starting at the medulla and moving inferiorly, these are the cervical, thoracic, lumbar, sacral and coccygeal divisions. When looking at the spinal cord as a whole, there are some important features to note. Each of the five spinal divisions contains a number of spinal segments. Each spinal segment gives off a pair of spinal nerves at that level (one to the left side of the body and one to the right side). In total, there are 31 pairs of spinal segments and therefore 31 pairs of spinal nerves. You can read more about spinal nerves in Section 3.1. Spinal Nerves. Now take a look at Figure 2.2.1. and note that the…

Cervical division has 8 spinal segments and gives off 8 pairs of spinal nerves.
Thoracic division has 12 spinal segments and gives off 12 pairs of spinal nerves.
Lumbar division has 5 spinal segments and gives off 5 pairs of spinal nerves.
Sacral division has 5 spinal segments and gives off 5 pairs of spinal nerves.
Coccygeal division has 1 spinal segment and gives off 1 pair of spinal nerves.

The familiar tube-like appearance of the spinal cord, referred to here as the ‘spinal cord proper’, only exists from C1 – L1/L2. Below L1/L2, the spinal cord proper tapers off to become the conus medullaris, but the spinal nerves do continue downwards in the vertebral column in what looks like a horse’s tail. This bunch of spinal nerves is therefore called the cauda equina.

Contents of the Spinal Cord

As mentioned before, the spinal cord is a major relay centre between the peripheral nerves and the brain. As such, a large proportion of the spinal cord contains ascending, sensory nerve fibres (running to the brain) and descending, motor nerve fibres (running to skeletal muscles, smooth muscle and glands in the periphery). Nerve fibres responsible for the same or similar functions are grouped together into ‘tracts’ in the spinal cord. For example, the ascending, sensory nerve fibres that carry the sensation of pain and temperature from the skin to the brain are all found in the spinothalamic tract.

Figure 2.2.2. shows the main tracts of nerve fibres and their locations in a transverse cross-section of the spinal cord. These tracts and their roles are noted below too. As you can see, the name of the tracts has two parts; the first part is where the tract is coming from, the second part is where the tract is heading to.

The main ascending, sensory tracts are:

Dorsal column medial lemniscus (DCML) tract – carries fine touch and proprioception signals to the brain. (Note that proprioception is our sense of knowing where our body is in 3D space, e.g. when we hold our left arm out horizontally, we know its position relative to the rest of the body.) This tract decussates in a bundle called the medial lemniscus in the medulla.
Spinothalamic tract (direct anterolateral system) – carries pain and temperature signals to the brain. This tract decussates at the level of the spinal cord, normally around the level that the neurons enter the spinal cord.
Spinocerebellar tract – carries proprioception signals to the brain.

The main descending, motor tracts are:

Corticospinal tracts – carry motor information that allows the conscious control of skeletal muscles. These mostly decussate to the other side of the body at a site in the medulla called the pyramids.
Subconscious motor tracts – involved in the subconscious monitoring of motor activity and making adjustments as necessary. Although I have called these tracts subconscious motor tracts, they do have specific names and functions...(1) Vestibulospinal tracts - vestibular nuclei of pons and medulla > spinal cord (for stabilising head movements and posture); (2) Reticulospinal tracts - reticular formation of pons and medulla > spinal cord (for balance and excitation of respiratory muscles); (3) Rubrospinal tract - red nucleus of midbrain > spinal cord (for exciting flexors of the upper limb); (4) Tectospinal tract - superior colliculus of midbrain > cervical region of spinal cord (for head and neck movements linked to sight).

Clinical Top Tip:

Cord Transection

If the spinal cord is completely severed horizontally, it is known as a complete cord transection. This could occur in a car crash where there is sudden deceleration. All of the spinal tracts are severed, resulting in complete paralysis from the level of the injury and downwards. For example, a complete cord transection at T10 would mean that a person would lose sensory and motor function in both legs, but they would retain sensory and motor function in their arms. The higher the transection, the more severe the paralsysis.

The Goofy Anatomist