Spinal cord: anatomy, parts and functions.

A fundamental part of our nervous system that has multiple roles.

When we think of the nervous system we tend to think almost exclusively of the brain.

Focusing on this organ is logical due to its special relevance, but it is often forgotten that the nervous system is precisely a system, i.e. a set of interrelated elements. In other words, the brain is not everything. In fact, within the nervous system there are two major divisions, the central nervous system and the autonomic nervous system.

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

What is the spinal cord?

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

It is the lower part of the neuroaxis, slightly flattened and asymmetrical cylindrical in shape, which, like the brain, is strongly protected by being surrounded by the spinal column. It is also protected by the meninges and cerebrospinal fluid, which prevent most damage from the elements.

In a sense, it is a part of the nervous system that is not completely separate anatomically from the brain, but many of the elements involved in the brain work at the same time in the spinal cord. However, it is possible to identify the beginning of this structure just below the brainstem.

This part of the nervous system is the point of connection between the brain and the rest of the organism.The vast majority of nerve fibers pass through the medulla. The transmission of information does not generally occur through a single neuron, but rather, as a general rule, the neurons that make up the different nerves of the body make one or more intermediate synapses, either within the medulla itself or outside it (as with the neurons of the nerve ganglia).

The spinal cord receives both afferents and efferents, i.e., it has both neurons in the spinal cord and neurons in the spinal ganglia.that is to say, it has both neurons that receive information from the receptors of the different organs and structures and others that send information and orders to these areas.

On the other hand, we must bear in mind that the spinal cord is not simply a kind of conduit through which nerves from all parts of the body travel to the brain and vice versa; its 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 an envelope of meninges, cerebrospinal fluid and a differentiation between white matter and gray matter.

Neuroanatomical configuration

Although 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 body position, it may be useful to take it into consideration in order to locate the location of the parts of the spinal cord that innervate the different body areas.

Most human beings are born with a total of 33 vertebrae, including seven cervical, twelve thoracic, five lumbar, five sacral and four coccygeal vertebrae.including seven cervical vertebrae, twelve thoracic vertebrae, five lumbar vertebrae, five sacral vertebrae and four coccygeal vertebrae. As we develop, the number is reduced as the lower ones fuse to form the sacral and coccygeal bones, and only the first 24 are considered vertebrae, ending at L5 or lumbar 5. The beginning of the spinal cord is located a little before it is covered by the spinal column, being attached to the spinal bulb. The point where the cord ends may vary from one person to another, generally culminating between the L1 and L3 vertebrae.

In general, the nerve connections of the medulla correspond to the area where they are located. Thus, in the part of the medulla located between the thoracic vertebrae are the nerve connections that innervate the thorax, and so on. Regarding the nerves that connect with the medulla, we have a total of thirty-one pairs, being eight cervical, twelve thoracic, five lumbar, five sacral and one coccygeal. A point to note is the presence of two areas in which the medulla is somewhat wider, because in these areas are the nerve connections with the limbs.

Between the C4 and T1 vertebrae there is an area somewhat wider than the rest of the medulla. This area, known as the cervical intumescence, is thicker because this is where the nerve connections to the upper extremities are located.

Towards the lower end of the cord there is a thickening between the T11 to L1 vertebrae called the lumbosacral intumescence. This is the part of the medulla that innervates the lower extremities, and which, together with the so-called horse tail, connects with the parts of the body located at the lower end.

Regarding the recently mentioned horse's tail, which receives its name due to the similarity of its shape with the tail of said animal, it is the set of nerve fibers that connect with the spinal nerves. This shape is due to the fact that the spinal cord is shorter than the spinal column, so the areas below the lumbar region must project their nerve endings to the spinal nerves located below it.

Parts of the medulla

It has been observed that the medulla 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 medulla we find both gray matter and white matter.. However, the arrangement is reversed, the white matter being located in an external position and the gray matter in the internal part of the medulla. Generally, the transmission of information occurs ipsilaterally, i.e. the right side of the body is treated by the left side of the spinal cord while the left side is worked by the right side.

Gray substance

The gray substance has this coloration because it is a set of somas or nuclei of neurons, 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 being in the brain this processing is very shallow).

The gray matter is structured in different horns or horns, the main ones being the ventral horn, the dorsal horn and the intermediate zone. There is also the lateral horn, but only in the thoracic area and the beginning of the lumbar area.

The dorsal horn is in charge of receiving the information from the systems innervated by the medulla ****.. In other words, it is the part of the medulla that ensures that external or internal stimulation detected by the receptors can be sent to the brain.

The ventral horn of the medulla, in contrast to the dorsal horn, has as its main function to send information to the nerves, causing the organism to react to external or internal stimuli. It is through it that voluntary movement is exercised.

Regarding the intermediate zone, there are many interneurons, which are those whose main function is to serve as a link between two other neurons. They are bridges connecting distal zones.

Although it only appears in the thoracic and part of the lumbar region, the lateral horn 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 fundamental role in homeostasis, the process by which the organism establishes a balance or harmony between different areas of the body so that the organs as a whole function in a healthy and coordinated manner.

White matter

The white matter is formed mainly by the axons of neurons, interconnecting the medulla and the brain.. It is organized in different fibers that receive the name of the areas with which they connect, being able to be ascending or descending. In addition, this grouping of neuronal projections is visible to the naked eye, mainly due to the contrast between their paler color compared to the surrounding parts of the nervous system (hence the name "white matter").

Three columns, the dorsal, lateral and ventral, can be found in the medulla and are visible without a microscope. The dorsal column consists mainly of somatic afferent fibers. In other words, as is the case with the dorsal horn in the 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 tracts and fascicles, which tend to be of efferent type, transporting the motor commands given by the brain to the spinal cord.The ventral and lateral columns are tracts and fascicles, which tend to be of the efferent type, carrying the motor commands given by the brain, so they go "from top to bottom".

Thus, in general, the distribution of white matter and gray matter in the spinal cord is reversed with respect to what happens in the brain: whereas in the latter the white matter predominates in the interior and the gray matter in the more superficial layers, 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 only necessary to observe the effects of damage in this area to understand that it is a fundamental section for normal functioning. The spinal cord is more than just 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 pass through the brain, as we shall see).

In summary, 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 relay nucleus for the neurons and nerve fibers present in most parts 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 caress on the arm), the information passes first to the spinal cord, the information passes first to the medulla, which will send the information to the muscles or the brain for processing. for processing.

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

2. Information processing

While it is in the brain that the stimulation becomes conscious, the medulla makes a quick judgment of the situation in order to determine whether only to send the information to the brain or to trigger an emergency action even before it arrives.

Thus, in terms of mental processes, it allows the emergence of a type of shortcut in which the information does not have to wait to be processed by higher instances 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 been without the information having yet been transmitted to the brain. These actions are what we know as reflexes. To exemplify this, we can think of accidentally putting a hand in the fire: the hand is withdrawn immediately, unplanned and without the information having been passed to the brain.

The function of the reflexes is clear: to offer rapid reaction to potentially dangerous situations. As the sensory information already produces a response when it reaches the spinal cord, without having to wait to be captured by the brain, time is saved, something very valuable in case of an animal attack or when you can receive injuries from falls or burns. In this way, actions programmed into the design of the spinal cord are performed, and are always executed in the same way.

These types of functions fit with the logic that governs the nervous system in general (and therefore also the spinal cord): speed is often more important than accuracy or considerations of whether or not performing a certain action fits in with social norms. The time we gain from this can save us a lot of trouble when our physical integrity is at stake. when our physical integrity is at stake.

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

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