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The Spinal Cord

For us to be able to sense and interact with the world, sensory information needs to travel up the spinal cord to the brain and then commands need to find their way down the spinal cord to our body.

Both the spinal cord and the brain are parts of the Central Nervous System (CNS) while the nerves branching out of the spinal cord are part of the Peripheral Nervous System (PNS).

The function of the PNS is to receive important information from the environment while the job of the CNS is to process the information and act on it.

The spinal cord is about 45 cm long and 1 cm thick and consists of the cervical, thoracic, lumbar, sacral and coccygeal segments. Each spinal cord segment innervates a corresponding dermatome which is an area of skin supplied by a single spinal nerve (for example, the area on the back of the neck is supplied by the C3 segment).

Spinal Nerve Roots. Drawing by My-MS / CC

Cervical nerves go above cervical vertebrae. Notice that there are 8 cervical segments in the spinal cord but only 7 cervical vertebrae. This means that starting from T1 spinal cord nerves go below the corresponding vertebrae.

There is an easy way to memorise number of spinal cord segments. Think of them as your meal times:

Breakfast at 8am (8 cervical segments)
Lunch at 12pm (12 thoracic segments)
Dinner at 5pm (5 lumbar and 5 sacral segments)

There are two enlargement along the spinal cord that contain a large number of neurons innervating upper and lower limbs. The first one is called the cervical enlargement (C4 to T1) with the corresponding nerves forming the brachial plexus supplying the upper limbs. If your Latin is a bit rusty, brachial means “related to arm”, plexus means “a network of nerves”. The lumbosacral enlargement (L2 to S3) provides nerves of the lumbosacral plexus innervating the lower limbs.

Did you notice that the spinal cord is shorter than the spinal column and ends around the level of the L1 vertebra? It actually stops growing once you turn five. Because the spinal cord is so short, the nerves supplying the lumbar and sacral parts of the spinal column are very long. They look like a horse’s tail and are aptly named cauda equina. The spinal cord ends as the conus medullaris that tapers into a slender filament called the filum terminale.

Spinal cord and denticulate ligament, conus medullaris, cauda equina and filum terminale / Bourgery, Jean Marc (Antique Anatomy)

The spinal cord is attached to to the dural sheath in 21 points by a denticulate ligament. Nothing can replace seeing the real thing. Here’s a great video that shows the spinal cord in all its glory.

Inside the spinal cord you can see a dark butterly-shaped area. This gray matter area is called “the gray column” and packs cell bodies of motor neurons ventrally and interneurons that accept inputs from the terminating axons on the dorsally. The lighter area surrounding the gray matter consists mostly of axons running in, out of, up or down the spinal cord. These mostly myelinated axons give this area its lighter colour. Because most of the axonal fibres run into the brain the higher we go along the spinal cord the more white matter we are going to see. The amount of gray matter increases in the areas of the cervical and lumbosacral enlargements. There is a narrow central canal (do not confuse with the spinal canal) in the middle of the spinal cord filled with the CSF fluid.

Vertebra / Bourgery, Jean Marc (Antique Anatomy)

Below is an illustration of the spinal cord section. You can see its gray matter divided into the dorsal horn, ventral horn and the lateral horn (which is only present in the thoracic region and upper lumbar segments of the spinal cord). The rest of the spinal cord is white matter divided into columns. Gray and white commissures are the where fibres decussate. The dorsal part of the cord is marked by a shallow median sulcus while the ventral part has a deeper anterior median fissure.

Spinal cord section (white and gray matter).

I took a picture of a stained spinal cord section, as seen below. The white matter consists mostly of myelinated axons (myelin is seen as white space surrounding the purple stained axons and glial cells) while the gray matter has a lot more cell bodies that stain darker. You can also see a layer of large ependymal cells surrounding the central canal (these cells produce and move the cerebrospinal fluid).

A cross section of the spinal cord near the central canal.

Spinal Cord Facts

It can function independently of the brain

A knee jerk reaction when your leg flies forward when the doctor taps you just below the knee is an example of the spinal cord acting without the brain’s interference.

It has memory for pain

If you stub your toe really hard the spinal cord will more readily carry pain signals up the sensory neurons of the injured tie. This can last for a few days. Ouch.

You need the spinal cord to sweat

The spinal cord transmits commands to your sweat glands. Quadriplegics cannot sweat and have to cool themselves down. By the way, 86 out of 100 people with spinal injuries are men. Car accidents are the main cause of the injuries.

Stem cells may allow us to repair spinal injuries

Axons in the CNS heal very poorly, that’s why people rarely recover completely even after a partial paralysis. Stem cell repair of the spinal injuries is currently in the clinical trial but we’ve already seen some success stories.

Protection of the Spinal Cord

The spinal cord is very well protected from damage. It passes through the spinal canal which is the cavity within the vertebrae. Between the vertebrae the spinal cord is protected by ligaments.

Just like the brain, the spinal cord is also protected by three layers called meninges:

The dura matter (“tough mother” in Latin), tough outermost layer
The arachnoid matter that has spider-web appearance
The pia matter (“gentle mother”), delicate innermost layer

The area between the dura matter and the wall of the spinal canal is called the epidural space and is filled with fatty tissue as well as blood vessels. The area between the arachnoid matter and the pia matter is called the subarachnoid space and is filled with the cerebrospinal fluid (CSF).

This fluid provides a few important benefits to the central nervous system:

  • It protects and cushions the delicate nervous tissue
  • It provides buoyancy (significant for the brain as it “floats” in it and doesn’t get crushed under its own weight)
  • It contributes to homeostasis by providing nutrition, circulating active molecules and removing waste

Spinal Nerves

As we saw above, there are 31 pairs of spinal nerves in the human body (8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal). Each spinal nerve is formed by fibres exiting the spinal cord via ventral (belly-side) and dorsal (back-side) roots. Afferent neurons are the neurons that carry sensory information from the nerve endings in the body into the spine via dorsal roots and up the spine into the brain. Efferent neurons send motor commands via ventral roots towards muscles, blood vessels or glands. Note that the dorsal roots have thicker areas called dorsal root ganglia which contain neuronal cell bodies of the dorsal axonal fibres. Ventral root nerves have their cell bodies in the gray matter of the spinal cord.

Notice how the dorsal and ventral roots consist of multiple rootlets joining together.

Formation of spinal nerve from roots of spinal cord. Image by Gray’s Anatomy, public domain

Immediately outside the spinal cord things get a bit complicated. Dorsal and ventral roots join for a short distance only to branch into the smaller posterior or dorsal ramus (ramus = branch) innervating the skin and muscles of the back and the larger anterior or ventral ramus innervating the rest of the body.

Roots and rami of the spinal cord (see terms for usage rights).

Notice that there are two branches out of the anterior ramus. The gray ramus consists of unmyelinated axons while the white ramus contains myelinated fibres. Together they form sympathetic ganglia which are the pathways of the sympathetic nerves that carry information about stress and impending danger to the body. Neurons of the sympathetic chain are responsible for the fight-or-flight response of the nervous system.

Multiple fibres entering the sympathetic chain run up and down forming multiple synapses and this is what makes possible a rapid and massive response to the activation of the sympathetic nervous system.

Sympathetic chain ganglia / Bourgery, Jean Marc (Antique Anatomy)

Sensory information is received via free nerve endings, Pacinian corpuscles, Ruffini endings and other sensory organs (see peripheral nervous system). Sensory information from the posterior side of the body travels via the dorsal ramus, while the rest of the sensory information comes from through the anterior ramus. It then ends up in the spinal cord via the dorsal root.

Sensory information travels to the dorsal root via the dorsal ramus (the back) or ventral ramus (the anterolateral trunk).

Motor commands from the brain descends via the axons in the spinal cord to the appropriate level and end up in the ventral gray horn where the axons synapse with lower motor neurons exiting the spinal cord via the ventral root.

Somatic motor commands travel via axons to the ventral gray horn where they synapse with lower motor neurons before exiting via anterior ramus.

Visceral commands to the internal organs such as the gut, heart, glands and so on travel via the ventral root into the sympathetic ganglion chain via the white ramus.

Motor commands to the internal organs travel via the sympathetic chain (either passing through it, or synapsing in the sympathetic ganglia).

Passing through the white ramus the pre-ganglionic neuron can synapse with a post-ganglionic neuron and reach the target organ (a), travel up or down the ganglion chain and then synapse with the post-ganglionic neuron (b), or proceed to innervate the target organ travelling via one of the splanchnic nerves (c).

Sympathetic connections and ganglia. Image by OpenStax College, CC

Dorsal Roots

Axons carrying sensory information enter the spinal cord via its dorsal roots. Fibres entering the dorsal horn that are slightly more lateral are usually unmyelinated or lightly myelinated fibres (C and Aδ types) of small diameter carrying pain and temperature sensation. These are called the lateral division. They usually cross over via the anterior white commissure and form part of the anterolateral tract.

Thicker myelinated fibres are more medial. They are called the medial division and carry the sensation of discriminative touch, proprioception, pressure and vibration. They ascend along the spinal cord via gracile and cuneate fasciculi of the dorsal column-medial lemniscus pathway.

Dorsal root pathways (medial and lateral divisions).

Neurons in the dorsal root are unipolar with the neuronal body located in the dorsal root ganglion and the axon extending from the receptors in the skin, muscles or other organs all the way to the spinal cord of the brain stem. Many axons ramify (branch out) upon entering the spinal cord and travel up or down the cord. Thicker myelinated fibres travel along the dorsal column (funiculus) often reaching the lower medulla and terminating in the gracile and cuneate nuclei. Thinner fibres branch out and travel only one or two segments up or down in what is known as Lissauer’s or posterolateral tract of white matter that is mixed with the gray matter of the most distal part of the dorsal gray horn.

Reflex Arc

In some situations we need to react to external events almost instantaneously. Not stepping on a nail. Moving your hand away from fire. Maintaining balance.

Let’s consider a simple knee jerk reflex. A doctor lightly uses a light hammer to tap just below your kneecap. In response your leg immediately kicks up.

Patellar or knee-jerk reflex. Image from the book of nervous diseases (1901)

This is a familiar example of a useful reflex that helps you maintain balance. When you stand up straight your legs constantly bend up or down at the knees ever so slightly. If the knee bends too much, then the muscle in the thigh contracts bringing the leg back to its original position. All this is done without the brain being aware of what’s going on. A tap on the patellar tendon stretches the quadriceps muscle forcing the muscle spindles to send an impulse into the spinal cord via the dorsal horn. There it synapses directly with a lower motor neuron that forces the quadriceps to stretch in response. At the same time this impulse activates an inhibitory interneuron forcing the hamstring muscle to relax. This is all done in an instant without your brain being aware of it before it’s all over.

Stretch / knee jerk reflex arc.

This is an example of an ipsilateral monosynaptic reflex. It is monosynaptic because it involved only one synapse from the afferent to the efferent fibre (if we ignore the inhibitory interneuron). And it is ipsilateral because the response occurs on the same side of the body. An example of a contralateral reflex would be shifting weight onto the other foot if we step on a sharp object.

Stretch Reflex

Patellar or knee-jerk reflex described above is an ipsilateral monosynaptic stretch reflex that causes muscles to contract in response to a stretch. This is how we maintain balance or hold something still without our hand moving up or down. Bicep reflex, Achilles reflex and other similar reflexes belong to this category.

Muscular Defence Reflex

This is a reflex causing the abdominal muscles to contract in response to sudden mechanical force being applied to the abdomen.

Golgi Tendon Reflex

Golgi tendon receptors react to the tension applied to tendons and force muscles to relax to avoid an injury. It can override the stretch reflex. So if you’re lifting a very heavy weight it may force you to drop it.

Withdrawal reflex

This is another spinal reflex that protects the body by withdrawing from harmful stimuli. For example when heat stimulates the skin the impulse of the sensory neurons trigger interneurons that connect to motor neurons activating flexors and inhibiting extensors.

Crossed extensor reflex

Often balance needs to be maintained following the withdrawal reflex. When you step on a sharp nail, the withdrawal reflex causes the flexors in your leg to contract. The crossed extensor reflex then causes the flexors in the other leg to relax and extensors to contract taking the the weight of the body.

Renshaw Cells

Renshaw cells are inhibitory cells first discovered over half a century ago. These are inhibitory cells that use glycine as their neurotransmitter. Renshaw cells may receive input from multiple alpha motor neurons and synapse on one or more motor neurons.

In patients with tetanus the toxin produced by the bacteria Clostridium tetani inhibits the release of glycine effectively blocking Renshaw cells from inhibiting motor neurons and causing frequent spasms that could be so severe that bone fractures may occur.

Therefore it was suggested that Renshaw cells inhibit motor neurons to cap the amount of muscle contraction. They can also form circuits to favour one muscle group over the other (Renshaw cell bias, as shown below). Their actions can also be consciously or unconsciously modulated via the descending pathways.

Renshaw cells provide recurrent inhibition and bias favouring flexor muscles.

The precise role of Renshaw cells is unclear but it has been suggested that they are important for “the normal function of motor systems that control posture and locomotion” (G. S. Bhumbra). Motor circuits controlling fingers do not have Renshaw cells so it is not clearly exactly how these cells assist in motor functions [3].

Central Pattern Generators

The spinal cord alone can produce locomotion with the help of central pattern generator (CPG) circuits. These are collection of neurons capable of producing periodic impulses without external stimulation. They drive rhythmic activities such as walking, chewing, breathing. Forebrain or the brain stem can influence them or override their activity when necessary. Specifically CPGs responsible for locomotion are found in the lower thoracic and lumbar segments of the spinal cord [4].

A simplified circuit of a central pattern generator.

A simple central pattern generator consists of an excitatory interneuron that keeps firing for a while exciting the corresponding motor neuron and inhibiting the contralateral excitatory interneuron via the inhibitory interneuron. After a while the contralateral neuron starts firing inhibiting the first neuron. Here’s a video of a robot that employs CPG mechanisms for stable locomotion.

Further Reading

  • Ascending Spinal Tracts
  • Descending Spinal Tracts

Sources

  1. John A. Kiernan, Nagalingam Rajakumar. 2014. Barr’s The Human Nervous System: An Anatomical Viewpoint.
  2. David L. Felten, M. Kerry O’Banion, Mary Summo Maida. 2016. Netter’s Atlas of Neuroscience. 3rd edition.
  3. Gardave S. Bhumbra et al. 2014. The Recurrent Case for the Renshaw Cell.
  4. Kiehn O, Butt SJ. 2003. Physiological, anatomical and genetic identification of CPG neurons in the developing mammalian spinal cord.

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