Spinal cord: anatomy, parts and functions

When we think of the nervous system, we usually think almost exclusively of the brain.

Focusing on this organ makes sense due to its special relevance, but it is often forgotten that the nervous system is precisely a system, that is, a set of interdependent elements. In other words, not everything is the brain. In addition, within the nervous system there are two main divisions, the central nervous system and the autonomic nervous system.

In addition to the royal organ, in the central nervous system we can also find another important component: the spinal cord, through which most of the body’s innervations pass.

    What is the spinal cord?

    The spinal cord is the most complete part of the central nervous system, starting in the medulla oblongata and ending in the lumbar region.

    It is the lower part of the neuroaxis, slightly flattened cylindrical and asymmetrically shaped, which, like the brain, is strongly protected by being surrounded by the spine. It also benefits from the protection of the meninges and cerebrospinal fluid, which prevent most damage from the elements in the environment.

    In a way, it is a part of the nervous system that is not completely anatomically separated from the brain, but many elements involved in the latter act simultaneously in the spinal cord. However, it is possible to identify the start of this structure just below the brainstem.

    This part of the nervous system is the connection point between the brain and the rest of the body., Passing the vast majority of nerve fibers through the spinal cord. The transmission of information does not usually take place by a single neuron, but as a rule, the neurons that make up the various nerves in the body make one or more intermediate synapses, either within the spinal cord itself or within outside of it (as with nerve ganglion neurons).

    The spinal cord receives both afferents and efferentsIn other words, it has both neurons which receive information from receptors of different organs and structures and others which send information and commands to these areas.

    On the other hand, it should be borne in mind that the spinal cord is not just some sort of conduit through which nerves move from all parts of the body to the brain and vice versa; the composition and functions are more complex than they appear, and it is even possible to find similar elements between this part of the nervous system and the brain. For example, in both structures we find a meningeal envelope, cerebrospinal fluid and a differentiation between white matter and gray matter.

    neuroanatomical configuration

    While the division into vertebrae has more to do with the configuration of the spine i.e. the bony protection of the spinal cord which in turn serves as a support for the position of the body, it may be helpful to consider it. the parts of the spinal cord that innervate different areas of the body.

    Most humans are born with a total of 33 vertebrae, Including seven cervical vertebrae, 12 thoracic, five lumbar, five sacral-four coccygeal. As we grow the number is reduced by fusing the bottom to form the sacrum and coccygeal bones, becoming vertebrae only the first 24, ending with the L5 or lumbar 5. The beginning of the spinal cord la spinal cord is located a little before it is covered by the spine, being attached to the medulla oblongata. The point where the spinal cord ends can vary from person to person, usually peaking between the L1 and L3 vertebrae.

    Usually, the corresponding nerve connections in the spinal cord correspond to the area where they are located. So, in the part of the spinal cord between the thoracic vertebrae are the nerve connections innervating the thorax, etc. As for the nerves that connect to the spinal cord, we have a total of thirty-one pairs, eight cervical, 12 thoracic, five lumbar, five sacral and one coccygeal. A point to note is the presence of two areas in which the spinal cord is slightly wider, because in these areas are the nerve connections with the limbs.

    Between the C4 and T1 vertebrae there is an area slightly larger than the rest of the spinal cord. This area, known as cervical intumescence, is thicker because there are the nerve connections that connect to the upper extremities.

    Towards the lower end of the spinal cord, a thickening can be observed, ranging from the T11 vertebra to the L1 vertebra, called lumbosacral intumescence. It is the part of the spinal cord that innervates the lower extremities and the one next to the so-called ponytail connects to the body parts located at the lower end.

    As for the recently mentioned ponytail, which gets its name from the resemblance of its shape to the tail of this animal, it is the set of nerve fibers that connect to the spinal nerves. This shape is due to the fact that the spinal cord is shorter than the spine, so the areas below the lumbar region project their nerve endings to the spinal nerves below.

    Parts of the spinal cord

    It has been observed that the spinal cord has different nerve connections that innervate different areas of the body. However, it may be of interest to analyze the internal structure of the spinal cord.

    As in the brain, in the spinal cord we find gray and white matter. However, the arrangement is reversed, with the white matter located in an external position and the gray in the internal part of the spinal cord. Usually the transmission of information occurs ipsilaterally i.e. the right side of the body is processed by the left side of the spinal cord while the left side is worked with the right side.

    grey matter

    Gray matter has this coloring because it is a collection of somas or neural nuclei, which project their axons to other areas. That is to say, it is in these areas where the bodies of neurons accumulate, centers of information processing (although not in the brain, this processing is very brief).

    The gray matter is structured into different horns or lances, the main ones being the ventral lance, the dorsal lance and the intermediate zone. There is also the lateral rod, but only in the thoracic region and at the beginning of the lumbar.

    The dorsal shaft is responsible for receiving information from the systems innervated by the spinal cord. In other words, it is the part of the spinal cord that is responsible for the external or internal stimulation detected by receptors that can be sent to the brain.

    The ventral rod of the spinal cord, as opposed to the dorsal, has the main function of transmitting information to the nerves, causing the body to react to external or internal stimuli. Through it the voluntary movement is exercised.

    As for the intermediate zone, interneurons abound, which are those whose main function is to serve as a link between two neurons. They connect bridges between the distal areas.

    Although it appears only in the thoracic region and part of the lumbar spine, the lateral shaft is of great importance, innervating different structures and participating in the sympathetic and parasympathetic systems of the autonomic nervous system. In this sense, it plays a key role in homeostasis, the process by which the body establishes a balance or harmony between different areas of the body so that all the organs function in a healthy and coordinated way.

    white matter

    White matter is made up mainly of axons of neurons, interconnecting the spinal cord and the brain. It is organized into different fibers that receive the name of the zones with which they connect, which can be ascending or descending. In addition, this cluster of projections of neurons is visible to the naked eye, mainly due to the contrast between its lighter color compared to the parts of the nervous system that surround it (hence the name “white matter”).

    In the spinal cord, three columns can be found, the dorsal, lateral and ventral, and they are seen without the need for a microscope. The spine is mainly made up of afferent fibers of the somatic type. In other words, as with the dorsal rod in gray matter, they are responsible for transmitting sensory information from the brain to the medulla and vice versa depending on whether it is ascending or descending.

    The ventral and lateral columns are bundles and fascicles, which tend to be of the efferent type, Carrying the motor commands given by the brain, so that they go “up and down”.

    Thus, in general, the distribution of white matter and gray matter in the spinal cord is reversed compared to what happens in the brain: while in the latter white matter predominates inside and gray the made. here it is the other way around.

    Functions of the spinal cord

    The importance of this part of the central nervous system is beyond doubt. It is enough to look at the effects that have damage in this area to understand that this is a fundamental section for normal operation. And it is that the spinal cord is more than the channel through which the brain communicates with the rest of the body; it can also perform certain automated activities, such as reflex arcs (in which very little time elapses between the capture of a stimulus and the emission of a motor reaction, since this process does not go through the brain, such as we will see).

    In short, the main functions that make this section of the nervous system so relevant are as follows.

    1. Transmission of sensory and motor information

    The spinal cord is the nucleus of relieving neurons and nerve fibers found throughout most of the body. This means that both when the brain gives the order to perform an action (for example kicking a ball) and when a part of our body perceives a stimulus (a stroke on the arm), the information first passes to the spinal cord, which will send the information to the muscles or the brain for you to deal with.

    Thus, the spinal cord acts as an elevator for afferent information and efferent information.

      2. Information processing

      While it is in the brain that the stimulation becomes conscious, the spinal cord quickly judges the situation to determine whether to simply send the information to the brain or trigger an emergency action before it even happens. .

      Thus, in terms of mental processes, it allows the emergence of a type of shortcuts in which information does not have to wait to be processed by higher bodies to generate a response.

      3. Immediate reaction: reflexes

      As we have just seen, sometimes the spinal cord itself produces an action without the information having yet been transmitted to the brain. These performances are what we call reflexes. To illustrate, one can think of accidentally putting a hand on the fire: the hand is withdrawn immediately, unplanned and without the information having yet passed to the brain.

      The function of reflexes is clear: to offer rapid reaction to potentially dangerous situations. Since sensory information is already producing a response when it reaches the spinal cord, without having to wait for it to be picked up by the brain, time is saved, something very valuable in the event of an animal attack or when it may suffer damage. fall injuries or burns. In this way, actions are performed programmed in the conception of the spinal cord, and which are always performed in the same way.

      These types of functions are part of the logic that governs the nervous system in general (and therefore also the spinal cord): often speed is more important than precision or considerations of whether or not performing certain actions corresponds to the requirements or not. social norms. The time we have saved with this can save us a lot of trouble when our physical integrity is at stake.

      However, in the case of babies, there are also reflexes that are lost during the first months after birth and the basic function is not always to react quickly, but to perform acts that promote survival, such as sucking breast milk. In this case, we are talking about primitive reflexes, the absence can be a sign of illness.

      Bibliographical references:

      • Cardinali, DP (2000). Manual of neurophysiology. Diaz de Santos editions.
      • Carpenter, M., Sutin, J., Mascitti, T. and Lorenzo, I. (1990). Human neuroanatomy. Buenos Aires: the Athenaeum.
      • Moore, KL and Agur, AMR (2007). Fundamentals of anatomy with clinical focus. 2nd edition. Editorial Médica Panamericana.
      • Navarro, X. (2002) Physiology of the autonomic nervous system. Journal of Neurology, 35 (6): 553-562.
      • Rexed B. (1954). Cytoarchitectonic Atlas of the Cat Spinal Cord. J Comp Neurol. 100: 297-379.
      • Squire, LR; Floyd Bloom, Nova Scotia (2008). Basic Neuroscience (digitized online by Googlebooks). Academic press.
      • Testut, L .; Latarjet, A. (1969). Treatise on human anatomy. Vol. 2, Angiology-Central Nervous System (9th edition). Checked in.

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