When we think about the nervous system we usually think almost exclusively about the brain.
Focusing on this organ is logical because of its special relevance, but it is often forgotten that the nervous system is precisely one system, that is, a set of interrelated elements. In other words, not everything is the brain. Moreover, within the nervous system there are two major divisions, central nervous system and autonomic nervous system.
In addition to the king organ, another large component can be found in the central nervous system: the spinal cord, through which most of the body’s innervations pass .
What is the spinal cord?
The spinal cord is the most caudal part of the central nervous system, starting at the spinal bulb and ending at the lumbar area.
It is the lower part of the neuro-shaft, slightly flattened and asymmetrical in shape, which, like the brain, is strongly protected as it is surrounded by the spine. It also enjoys the protection of the meninges and the cerebrospinal fluid, which prevent most of the damage produced by the surrounding elements.
In a sense, it is a part of the nervous system that is not totally anatomically separate from the brain, but many of the elements involved in the brain work together in the spinal cord. However, it is possible to identify the beginning of this structure just below the brain stem.
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 fibres through the spine. The transmission of information does not generally take place through a single neuron, but as a rule, the neurons that make up the different nerves in the body make one or several intermediate synapses, either within the medulla itself or outside it (as with nerve ganglion neurons).
The spinal cord receives both afferences and efferences , that is, 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, it should be borne 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 a meningeal envelope, cerebrospinal fluid, and a differentiation between white and gray matter.
Although the division into vertebrae has more to do with the configuration of the spine, i.e. the bony protection of the marrow which in turn serves as a support for the body position, it may be useful to take it into consideration in order to locate the location of the parts of the marrow that innervate the various body areas.
Most human beings are born with a total of 33 vertebrae , including seven cervical vertebrae, twelve thoracic, five lumbar, five sacral and four coccygeal. As we develop, the number decreases as the lower ones fuse to form the sacral and coccygeal bones, with only the first 24 being considered vertebrae, ending at L5 or lumbar 5. The point where the cord ends can vary from person to person, usually culminating between the L1 and L3 vertebrae.
In general, the nerve connections correspond to the area where they are located. Thus, in the part of the spine located between the thoracic vertebrae are the nerve connections that innervate the thorax, and so on. With regard to the nerves that connect to the medulla, we have a total of thirty-one pairs, being eight cervical, twelve thoracic, five lumbar, five sacral and one coccygeal. One 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 extremities.
Between the C4 and T1 vertebrae there is a slightly wider area than the rest of the spine. 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 spine, a thickening can be observed, between the T11 and L1 vertebrae, called lumbosacral intumescence. This is the part of the spine that innervates the lower extremities, and which together with the so-called ponytail connects to the parts of the body located at the lower end.
With respect to the recently mentioned horsetail, which receives its name due to the similarity of its shape with the tail of that 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 spine, so the areas below the lumbar zone must project their nerve endings to the spinal nerves located below it.
Parts of the spine
It has been observed that the marrow 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 spine we find both grey and white substance . However, the arrangement is reversed, with the white substance located on the outside and the grey substance on the inside of the cord. Generally the transmission of information is given in an ipsilateral way, that is, the right side of the body is treated by the left side of the spinal cord while the left side is worked with the right side.
The grey 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 the neurons accumulate, centres of information processing (although as they are not in the brain this processing is very superficial).
The grey substance is structured in different horns or antlers, the main ones being the ventral horn, the dorsal horn and the intermediate zone. There is also the lateral shaft, but only in the thoracic area and the beginning of the lumbar.
The dorsal horn is in charge of receiving the information from the systems innervated by the medulla to . In other words, it is the part of the spinal cord that ensures that the external or internal stimulation detected by the receptors can be sent to the brain.
The ventral shaft of the spine, unlike the dorsal shaft, has the main function of emitting information to the nerves, causing the body to react to external or internal stimuli. It is through it that voluntary movement is exercised.
As for 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 distant areas.
Although it only appears in the thoracic area and part of the lumbar, 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 fundamental role in homeostasis, the process by which the organism establishes a balance or harmony between different areas of the body so that the set of organs works in a healthy and coordinated way.
The white matter is formed mainly by the axons of the neurons, interconnecting the medulla and the brain . It is organized into different fibers that are named after the areas they connect to, and can be ascending or descending. In the medulla there are three columns, the dorsal, lateral and ventral.
The dorsal spine is mainly formed by afferent somatic type fibers. In other words, as with the dorsal horn in the grey matter, they are responsible for transmitting sensory information, from the brain to the spine 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 the efferent type , carrying the motor commands given by the brain.
Thus, in general the distribution of the white and grey substance of the spinal cord is reversed in relation to what happens in the brain: while in the latter the white substance predominates in the interior and the grey substance predominates in the more superficial layers, here it is the other way around.
Spinal Cord Functions
The importance of this part of the central nervous system is beyond doubt. One only needs to look at the effects that damage 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 arches.
In summary, the main functions that make this section of the nervous system so relevant are the following .
1. Transmission of sensory and motor information
The spinal cord is the relay nucleus of the neurons and nerve fibers present in most parts of the body. This means that both when the brain gives the order for an action to be performed (e.g. kicking a ball) and when a part of our body perceives some stimulus (a caress on the arm), the information passes first to the spinal cord, which will send the information to the muscles or the brain for processing.
Thus, the spinal cord acts as an elevator for afferent information and efferent information.
- You may be interested in: “Afferent and efferent pathways: the types of nerve fibers”
2. Information processing
While it is in the brain that the stimulation becomes conscious, the marrow makes a quick judgment of the situation in order to determine whether to just send the information to the brain or to trigger emergency action even before it arrives.
Thus, as far as mental processes are concerned, it allows the emergence of a type of shortcut in which 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 a performance without the information having yet been transmitted to the brain. These actions are what we know as reflexes. To exemplify, we can think of putting a hand in the fire accidentally: the hand is immediately removed, unplanned and without the information having yet been passed on to the brain.
The function of reflexes is clear: to offer a quick 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 gained, which is very valuable in the case of an animal attack or when one can receive injuries from a fall or burns. In this way, actions are programmed into the design of the spinal cord, and are always executed in the same way.
These types of functions fit in with the logic that governs the nervous system in general (and therefore the spinal cord as well): speed is often more important than precision or considerations about whether or not to perform 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.
However, for babies 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 survival acts such as sucking on breast milk. In this case we are talking about primitive reflexes, the absence of which may be a sign of illness.
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