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The spinal cord is essentially the headquarters of the peripheral nervous system (PNS). It is responsible for the transmission of information between the PNS and the central nervous system (CNS). Curiously, with the spinal cord alone, many autonomic functions and even voluntary movements can occur. Not all actions of the body necessarily need the brain to be carried out! Within the spinal cord are nerve tracts that carry sensory information to regulate motor control. These nerve tracts even play a role in consciousness and awareness. Here, we’ll take a look at the anatomy and functions that make up the purpose of the spinal cord.
Neuronal Pathways in the Spinal Cord
There are many different collections of nerve tracts that run up and down the spinal cord, inputting information to the thalamus, cerebral cortex, and cerebellum. These tracts are responsible for the transmission of sensory information as it relates to pain, temperature, touch, pressure, proprioception, and more. Once the action potentials, or neuronal transmissions, reached their ultimate destination in the brain, the signal can then be interpreted to create and execute a motor plan.
For example, let’s look at the pathway for pain and temperature: starting at the bottom of the spinal cord (or particular vertebral level, depending on where sensory information is being transmitted from), sensory neurons transmit neuronal information into the spinal cord at the point of decussation.
(Decussation is the crossing over of nerves from one side of the spinal cord’s midline to the other. The particular bundles of nerve fibers that do this crossing are called commissures.) The information must first have synapsed onto an interneuron in order to enter the spinal cord and decussate.
(A quick distinction: An interneuron is known as such because the whole of its anatomical structure – cell body and axon – is located within the CNS, in the spinal cord and/or brain. On the other hand, a ganglion is a collection of neuron cell bodies outside of the CNS. Their axons then travel into the CNS for transmitting neuronal signals.)
The masses of interneurons within the spinal cord are what make up the gray matter, the butterfly shape in the center of the spinal cord – remember that this gray matter is made up of cell bodies. The white matter surrounding the butterfly is a mass of myelinated nerve fibers.
After the information is passed through a synapse in either the dorsal or ventral horns (gray matter), the interneuron decussates across the spinal cord and ascends the spinal tract, to ultimately pass this information to the brain. Once the interneurons make the transition of traveling either up or down to transmit information, they then become known as intersegmental neurons. Otherwise, they are called intrasegmental neurons – staying within the same singular segment.
The bundle of nerve fibers that travel up and down the spinal cord is called the spinothalamic tract. This tract passes through the medulla oblongata on its way up to the thalamus (most sensory information passes through the thalamus before anywhere else in the brain). The tract then exits the thalamus, where it transitions into mostly white matter (myelin-sheathed axons) to ultimately synapse in the primary sensory level of the cerebral cortex.
Anatomy of the Vertebrae and Spinal Cord
The vertebrae, along with the cerebrospinal fluid (CSF) which flows throughout the central canal along the entire length of the spinal cord, are essential to the safety structure of the spinal cord. The CSF supplies a buffer of sorts that absorbs any shock that may result from possible impact, while the vertebrae surround the spinal cord, maintaining its structure and providing a hard exterior for increased protection.
The spine – the poster portion of the vertebral column – is made up of the posterior, or dorsal, parts of the vertebrae called spinous processes (the pokey parts of the vertebrae that create the bumps you see on your back when you bend over). The main bodies of the vertebrae are on the anterior, or ventral, side of the vertebral column. The center of the vertebral column through which the spinal cord passes is known as the vertebral foramen, or the vertebral canal.
Surrounding the spinal cord and the brain are three meninges, or membranes, called the dura mater (the outermost layer), the arachnoid (middle layer), and the pia mater (the innermost membrane). These, in addition to the CSF, function not only for the protection of the spinal cord, but also the regulation of the neuronal environment by removing toxins, circulating nutrients, and containing blood vessels vital to the organs of the nervous system. The space between the pia mater and the arachnoid membrane is called the subarachnoid space and is also filled with CSF.
Nerves of the Spinal Cord
The spinal nerves, the lifeline of communication for the PNS, originating from the spinal cord are – listed from the top, down, or superior to inferior – 8 pairs of cervical nerves (C1-C8), 12 pairs of thoracic spinal nerves (T1-T12), 5 pairs of lumbar spinal nerves (L1-L5), 5 pairs of sacral spinal nerves (S1-S5), and one single coccygeal nerve, all of them being “mixed nerves,” both sensory and motor.
The nerves emerging from the spinal cord are not simply extensions of the spinal nerve, rather, a “reorganization” of the axons of those nerves that ultimately follow different routes. When axons of different spinal nerves join they form what is called a systemic nerve. Systemic nerves are formed at four places along the entirety of the vertebral column – these sites are referred to individually as a nerve plexus (“plexus” is typically used in reference to describe a network of nerve fibers that have no associated cell bodies).
Two of these nerve plexuses are located at the cervical level, one at the lumbar level, and one at the sacral level.
- Cervical Plexus: made up of axons from spinal nerves C1-C5, axons of this plexus innervate the posterior neck and head and also connects to the phrenic nerve (controls the diaphragm).
- Brachial Plexus: made up of axons from spinal nerves C4-T1, this plexus innervates the arms. The radial nerve comes from this plexus and gives rise to the axillary nerves that innervate the armpit. (More on this another time.)
- Lumbar Plexus: comprised of all the lumbar spinal nerves and innervates the pelvic region and anterior leg. A major nerve from this plexus is the femoral nerve.
- Sacral Plexus: this plexus comes is composed of the lower lumbar nerves, L4 and L5, and sacral nerves, S1 to S4. The most prominent nerve to arise from this plexus is the sciatic nerve.
Inside the Spinal Cord
Let’s do one of those image interpretation tests: When you look at the center of the spinal cord, what do you see? Most people see this shape as either an “H” or a butterfly – I’m here to tell you that if you see an “H,” you need to work on your handwriting. The wings of the butterfly make up the portion of the gray matter known as the lateral gray horn. This gray matter is surrounded by white matter that doesn’t go by any particular name but is responsible for transmitting information up and down the spinal cord, to and from the brain.
There are two branches that project from either side of the spinal cord: the branch on the anterior, or ventral, side is known as the ventral root, and the one on the dorsal, or posterior side, is called the dorsal root. Located within the dorsal root is a structure called the dorsal root ganglion. Sensory information is passed through the dorsal root ganglion after passing through the point at which the two branches meet. This joining of the dorsal and ventral branches creates a spinal nerve that can be both types of sensory nerves: general sensory and motor (because of this, they are called “mixed nerves”).
(The dorsal root ganglion functions as the “sensory branch” of the spinal nerve and is therefore known as the “spinal ganglion” as well. Within this ganglion are the cell bodies of somatic and visceral sensory nerve fibers. The ventral root, on the other hand, functions as the motor branch of the spinal nerve, sending signals down from the brain based on information gathered from sensory neurons.)
The anatomical regions of the gray matter butterfly each are known as the following:
- The horns extending toward the dorsal side of the butterfly are called the dorsal gray horns. This is where sensory nerve fibers synapse onto interneurons.
- There are two processes in between the two butterfly wings pointing outward called the lateral gray horns. Now, these are quite unique in that they are only located at the thoracic and lumbar levels of the spinal cord. They are also the home of autonomic neuron cell bodies.
- Lastly, the horns that point toward the ventral side of the spinal cord are known as the ventral gray horns. This is where the cell bodies of somatic motor neurons, also known as alpha motor neurons or lower motor neurons, are located.
- In the very center of the gray butterfly is the central canal. As previously mentioned, through this canal runs CSF which distributes nutrients and removes toxins from the neuronal environment and provides a buffer for any possible traumatic impact to the spinal cord or vertebral column. CSF goes on to distribute throughout the rest of the body by exiting the central canal at the bottom of the spinal cord.
- The area just above the central canal is called the posterior gray commissure, where the nerve fibers decussate (remember, this is to cross from one side of the midline to the other). Opposite of this is the anterior gray commissure.
Inside the spinal nerve (the point at which the dorsal and ventral roots join) are both sensory neurons, which send information to the CNS, as well as motor neurons, which send commands out to the skeletal muscles (effectors) from the brain or spinal cord, depending on the type of movement and muscle to which the command is being sent. The sensory neurons pass through the dorsal root branch, and the motor neurons pass through the ventral root branch.
As the transitional structure between the CNS and PNS, the spinal cord is another absolutely essential part of the nervous system that has many different functions in distributing information for sensory and motor functions, including continuous autonomic processes. The extent of its reach in functionality throughout the entire human body is enormous and still being explored!
References
- Biga, L. M., Dawson, S., Harwell, A., Hopkins, R., Kaufmann, J., LeMaster, M., … Matern, P. (n.d.). 13.3 spinal and cranial nerves – Anatomy & physiology. Retrieved from https://open.oregonstate.education/aandp/chapter/13-3-spinal-and-cranial-nerves/
- Fink. (2013, January 17). The spinal cord & spinal tracts; part 1 by Professor Fink [Video file]. Retrieved from https://www.bing.com/videos/search?q=youtube+the+spinal+cord&view=detail&mid=7D51F1D2918F665B2CC97D51F1D2918F665B2CC9&FORM=VIREFink. (2013, January 17). The spinal cord & spinal tracts; part 2 by Professor Fink [Video file]. Retrieved from https://www.bing.com/videos/search?q=youtube+the+spinal+cord&&view=detail&mid=D0413958C915777BBCEBD0413958C915777BBCEB&&FORM=VDRVRV