Spinal cord

What the spinal cord is

The spinal cord is a long, cylindrical column of nervous tissue that continues downward from the base of the brain. It begins at the foramen magnum, the large opening in the bottom of the skull, where it is continuous with the lowest part of the brainstem. From there, it runs down through the center of the spine.

In an adult, the spinal cord is surprisingly modest in size: roughly 17 to 18 inches (about 42 to 45 cm) long and, at its widest, about as thick as your thumb. Yet almost every sensation you feel from the neck down, and almost every movement you make from the neck down, depends on it.

The cord is not perfectly even in thickness. It has two noticeable swellings:

• The cervical enlargement, in the lower neck, where the nerves serving the arms connect to the cord.
• The lumbar enlargement, near the bottom of the cord, where the nerves serving the legs connect.

These enlargements make sense when you think about it. The arms and legs contain a great deal of skin and muscle that must be monitored and controlled, so more nervous tissue is needed at the levels that serve them.

How the spinal cord is protected

Nervous tissue is soft and delicate, roughly the consistency of firm gelatin, so the body surrounds the spinal cord with several layers of protection. From the outside in:

1. The vertebral column. The stacked vertebrae form a bony tunnel called the vertebral canal (or spinal canal). Each vertebra contributes a ring of bone, and together they create a flexible, armored tube.
2. The epidural space. Between the bone and the outermost membrane lies a cushion of fat and small veins. (This is the space into which an “epidural” anesthetic is injected, for example during childbirth.)
3. The meninges. Three protective membranes wrap the spinal cord. They are continuous with the same three membranes that wrap the brain:
   • Dura mater (“tough mother”): the thick, durable outer layer.
   • Arachnoid mater (“spider-like mother”): a delicate middle layer with fine, web-like strands.
   • Pia mater (“gentle mother”): a very thin layer that clings directly to the surface of the cord.
4. Cerebrospinal fluid (CSF). Between the arachnoid and pia mater is a space filled with cerebrospinal fluid. CSF acts as a shock absorber, lets the cord float gently within the canal, and helps deliver nutrients and remove wastes.

A memory aid for the order of the meninges from outside to inside is “DAP”: Dura, Arachnoid, Pia.

Where the spinal cord ends (and why it is shorter than the spine)

Here is a fact that surprises many students: the spinal cord does not run the full length of the spine. In adults, it usually ends around the level of the first or second lumbar vertebra (L1–L2), roughly at the level of the bottom of the rib cage in the back.

Why? Early in fetal development, the cord and the vertebral column are about the same length. But as development continues, the bones of the spine keep growing longer while the cord grows much more slowly. The cord appears to “rise” within the canal, ending higher and higher, until in adulthood it stops near L1–L2.

The lower end of the cord tapers into a cone shape called the conus medullaris. Below that point, the vertebral canal is filled with a bundle of long nerve roots that continue downward to reach their exit points in the lower lumbar spine and sacrum. This bundle looks remarkably like a horse’s tail, and its Latin name means exactly that: cauda equina. A thin anchoring strand called the filum terminale continues from the tip of the cord down to the coccyx, holding the cord in place.

This arrangement has two practical consequences:

• When medical professionals perform a lumbar puncture (spinal tap), they insert the needle below L2. At that level, the needle enters the fluid-filled space among the loose, floating nerve roots of the cauda equina rather than piercing the cord itself.
• Because the cord is shorter than the spine, the nerve roots for the low back, pelvis, and legs must travel downward inside the spinal canal for some distance before they exit. As you will see later, this long route past several intervertebral discs is one reason the lower back is such a common site for nerve root problems.

A brief look inside

If you could slice the spinal cord crosswise, like cutting a carrot into coins, you would see a butterfly-shaped inner core surrounded by an outer region.

• The butterfly-shaped core is where processing happens. Incoming sensory messages arrive here, are sorted, and are passed along, and outgoing motor commands begin here. The “wings” of the butterfly that point toward the back receive sensory information. The “wings” that point toward the front send out motor commands.
• The outer region contains the pathways that carry messages up and down the cord, between the body and the brain.

For our purposes, the most important point is the general rule this creates: sensory information enters the back of the cord, and motor commands leave the front of the cord. A simple way to remember it is to picture someone tapping you on the back (sensation) and you stepping forward (movement). You will see this rule again when we look at nerve roots.

The two main jobs of the spinal cord

1. The spinal cord is a two-way communication highway. Sensory signals travel up the cord to the brain; motor signals travel down from the brain and out to the body. When a client feels your hands on their calf, that information enters the spinal cord and travels upward to the brain, where it becomes a conscious sensation. When the client wiggles their toes to show you where it hurts, the command travels down the cord and out to the muscles of the foot.

2. The spinal cord is a reflex center. Some responses are so urgent that the body does not wait for the brain to consider them. A reflex is a rapid, automatic, involuntary response to a stimulus, and many reflexes are handled right in the spinal cord.

The route a reflex follows is called a reflex arc. It has five basic parts:

1. A receptor detects a stimulus. (For example, sensors in your fingertip detect the heat of a hot pan.)
2. A sensory pathway carries the signal into the spinal cord.
3. An integration center in the cord processes the signal.
4. A motor pathway carries a command back out.
5. An effector, usually a muscle, responds. (You pull your hand away.)

All of this can happen before the pain message has even finished reaching your brain, which is why you often jerk your hand back before you consciously think “ouch.”

A familiar example is the knee-jerk (patellar) reflex. When a physician taps the tendon just below the kneecap, the quadriceps is briefly stretched, and the spinal cord immediately signals the quadriceps to contract, kicking the lower leg forward. Physicians test reflexes like this one because a weak or absent reflex can point to a problem with a particular level of the spinal cord or its nerve roots.

Clinical connection: spinal cord injury

Because nearly all signals between the brain and the body below the neck must pass through the spinal cord, damage to the cord can interrupt sensation and movement below the level of the injury. Injury in the thoracic or lumbar region may cause paraplegia, affecting the legs and lower trunk. Injury in the cervical region may cause quadriplegia (also called tetraplegia), affecting all four limbs and the trunk. Very high neck injuries can even affect breathing, because the nerve supply to the diaphragm comes from the neck.

Clients living with spinal cord injury can benefit greatly from massage, but therapists must adapt. These clients may be unable to feel pressure or report discomfort in affected areas, may be prone to pressure sores and circulatory problems, and may be at risk of a dangerous condition called autonomic dysreflexia: a sudden, severe rise in blood pressure, often triggered by a stimulus below the level of injury such as a full bladder or a tight piece of clothing. Work in close communication with the client and, when appropriate, their health care team.