The spinal cord Is a tubular bundle containing a long, thin structure of nerve tissue and supporting cells. This region of the body comprises a large part of the organism. Specifically, it slides from the Medulla bulb of the Trunk of the brain (Brain) to the lumbar region.
The main function of the spinal cord consists of transmitting nerve impulses to the 31 pairs of nerves of the medulla oblongata. In this way, the spinal cord is the region responsible for communicating the encephalon to the body.
Communication between agency and brain Is performed by two main transmission mechanisms: the afferent function that sends nerve impulses from the trunk, neck and limbs to the brain and efferent function that carries signals from the brain to different regions of the body.
The spinal cord is one of the structures of the body that has a greater study and analysis on both its anatomy and its main functions. It is established that it is one of the most important and committed regions of the body.
Characteristics of the spinal cord
Evolutionarily, the spinal cord is the first region of the nervous system Appear. It is a necessary structure to integrate the bodily functions, to communicate them with the cerebral functioning and to relate them with the outside world.
For this reason, not only primates but all vertebrate beings are characterized by having a spinal cord in their body.
In this sense, there are areas of the skin called Dermatomas , Which are organized as organized segments. These segments contain their representation in the spinal cord.
Thus, depending on the excitatory or inhibitory processes present in the spinal cord, the different segments of the skin originate primary responses or medullary reflexes. These reflexes are characterized by always producing the same response to the same stimuli, without requiring more processors.
An example of this basic functioning of the spinal cord would be the transmission of pain that causes a puncture in the skin. The fact of receiving damage in a specific cutaneous region automatically translates into a sensation of pain that is transmitted to the brain.
Thus, in general, the spinal cord is a set of functional segments with both afferent (body to brain) and efferent (brain to body) connections. Specifically, at present eight cervical segments are distinguished, twelve dorsal, five lumbar and six sacrococcygeal.
The cervical segments mainly control the neck, the diaphragm and the upper extremities. In contrast, the dorsal segments control the thorax and abdomen, the lumbar segments the lower extremities and the sacrococcygeal segments regulate the functioning of the pelvis and sphincters.
Parts of the spinal cord - Anatomy
Anatomically, the spinal cord has two main elements of study: its external anatomy and its internal anatomy.
External anatomy refers to the qualities of the superficial regions of the spinal cord, while the internal anatomy refers to the structures and substances that the spinal cord holds inside.
In this sense, it should be noted that the spinal cord is a highly complex structure. It has multiple elements both inside and outside, as well as multiple properties that are scientifically relevant.
The study of the anatomical properties of the spinal cord has allowed to increase the knowledge about the characteristics of this delicate structure of the organism.
Also, it has allowed to identify the functioning of the spinal cord and to detect the possible injuries or affections that can occur in this part of the organism.
External anatomy of the spinal cord
Spinal column with spinal cord.
First of all, it should be noted that the spinal cord is the most extensive nervous tissue in the human body. In fact, the axons of the Neurons Which houses inside can reach up to one meter long, being much larger than brain neurons.
Approximately, the spinal cord weighs about thirty grams in total, and in its complete development can reach a length of between 40 and 45 centimeters.
The length of the spinal cord appears to be somewhat higher in men (45 centimeters) than in females (43 centimeters). This is due to the fact that the men's organism tends to be somewhat higher than that of women.
The spinal cord is located within the intravertebral bone called the spinal canal, which is located from the foramen magnum to the first or second lumbar vertebra.
In this way, the spinal cord of a newborn reaches the lumbar vertebrae three and in the embryos the spinal cord is found until the base of the coccyx of the body. Based on these data, it becomes clear that the spinal cord is one of the first regions of the body to be formed.
The spinal cord presents a cylindrical shape in the upper and ventral cervical segments. In contrast, it adopts an ovoid shape with a transverse diameter greater than the obverse in the lower cervical and thoracic segments.
On the other hand, it must be taken into account that the spinal cord is an asymmetrical structure in most people. That is, this tends to be larger in the right hemisphere of the individuals.
Other important elements about the external anatomical properties of the spinal cord are: the faces and the membranes.
Faces
Externally, the spinal cord has two faces and two main edges. Specifically, it contains an anterior face, a posterior face and two lateral edges.
The anterior aspect of the spinal cord contains, in its midline, an anterior medial groove, which borders laterally with the anterior collateral grooves. These anterior collateral grooves are the apparent origins of the motor or efferent nerve roots of the spinal nerves.
The posterior surface also has an intermediate posterior sulcus, which extends through a septum until reaching the central gray matter. The posterior aspect of the spinal cord is bordered on the sides by the posterior collateral grooves, which correspond to the apparent origins of the sensory nerve roots of the spinal nerves.
On the other hand, the spinal cord has two main thickenings (regions where it increases its diameter). One is located in the cervical region while the other is located in the lumbar region.
Cervical thickening is called cervical intumescence and is found between the fourth cervical vertebra and the first vertebra of the trunk. The thickening is formed by the roots of nerves that transmit sensitivity and motor action from the upper limbs.
Lumbar thickening is called lumbosacral lump and is located between the eleventh vertebra of the trunk and the first lumbar vertebra. In this case, the thickening is due to the roots of nerves that allow to transmit sensitivity and motor action to and from the lower extremities.
Finally, in the lower portion the sides of the spinal cord become significantly thinner, and then end up as a cone tip in the coccyx region. This latter region of the marrow is called the terminal cone.
In the lateral pates, the spinal cord has as a fixation element to dentate ligaments. In contrast, in the inferior part the marrow continues with the terminal filum, which extends to the dural sac at the level of the second vertebra of the sacrum.
Membranes
The spinal cord contains three membranes that wrap around its structure. These are: the pia mater, the arachnoid and the dura mater.
A) Piamadre
The pia mater is an internal meningeal that protects both the brain and spinal cord. It is waxed from the nervous structures and is responsible for upholstering the convolutions of the brain.
Likewise, the pia mater generates choroidal formations, which are applied against the ependymal membrane of the ventricles.
A space filled with cerebrospinal fluid called the subarachnoid space is located above the pia mater. Above this space, there is the most homogeneous and distinguishable part of arachnoid, which forms a thin, transparent and lax network that can not be introduced into the grooves of the spinal cord.
B) Arachnoids
Arachnoids are an intermediate meningeal that also protects both the Encephalon Such as the spinal cord. It is located just below the dura and its main function is to distribute the cerebrospinal fluid , Which circulates through the subarachnoid space.
This membrane is formed by an external and homogeneous sheet, as well as an inner areolar layer containing large meshes and constituting the subarachnoid space.
The outer lamina of the arachnoid adhere directly to the dura mater. The subarachnoid cavity is cylindrical and surrounds the spinal cord and its roots along the entire length of the vertebral canal (to the bottom of the dural sacrum).
C) Dura mater
Finally, the dura mater is the outermost membrane of the marrow. It constitutes a hollow cylinder that is formed mainly by a fibrous wall, thick, solid and little extensible.
The outer surface of the dura is regularly rounded and responds to the bony walls and ligaments of the vertebral canal. The back of the outer surface of this membrane is in contact with the posterior longitudinal ligament. Instead, in the lateral sense, it extends around each spinal nerve.
The inner surface of the dura is smooth and polished, and corresponds to the arachnoid. Its upper end continues without net limits with the cranial dura mater. Its lower end constitutes the dural sac fundus, which stops between the second and third sacral vertebrae.
Internal anatomy of the spinal cord
Internally, the spinal cord is mainly composed of regions of White matter And regions of gray matter.
Transversally, the marrow contains throughout its length and in its different divisions a large area of gray matter. This region adopts an"H"or butterfly shape.
Around the gray matter region, the spinal cord contains another region consisting of White matter . In this way, the spinal cord that characterizes by possessing gray substance in the center and white substance in the peripheral regions.
This organization is important because it forms an inverse structure to that of the brain. That is, the brain regions are characterized by having white matter in the central areas and gray matter in the peripheral regions, however, the spinal cord has a contrary organization.
The internal and posterior extensions of the spinal cord are relatively thin. These extensions are called posterior horns and reach, practically, the posterior groove.
For its part, the previous extensions are wide and rounded. They are called anterior horns and reach the brain regions.
The three-dimensional arrangement of both the anterior and posterior horns makes it possible to form a series of columns running through the spinal cord and forming the anterior and posterior gray columns.
At the functional level, the posterior horns are responsible for performing somato-sensitive activities. They are composed of sensory neurons that receive the impulses that reach the posterior roots.
In this sense, the main function of the posterior horns (those furthest from the skull) is to receive stimuli and transmit them to the brain regions.
The anterior horns, on the other hand, are functionally somato-motor. Is it so Formed by motor neurons Whose axons exit the anterior roots.
On the other hand, in the upper thoracic and lumbar segments a small lateral horn is located. This emerges from the junction of the anterior horn with the posterior horn and is characterized by containing sympathetic visceral neurons.
Finally, in the lateral part of the base of the posterior horn of the upper cervical segments is a region called Reticular formation . This formation is characterized by containing white matter and mixed gray matter.
1- Gray substance
The gray matter of the spinal cord is a region that is composed primarily of bodies of neurons and supporting cells. This region contains two anterior gray horns and two gray posterior horns, which are united by a gray commissure.
The gray commissure of the spinal cord is in turn divided by a posterior region and an anterior region. This division of the commissure is made by a small central hole called the ependymal conduit or spinal cord.
In the thoracic and lumbar region of the spinal cord, gray lateral horns are detected that have a wedge shape. These horns are formed by the sums of the neurons of the sympathetic autonomous system.
The consistency of the gray lateral horns is uniform, although the substance surrounding the ependymal duct is somewhat more transparent and soft than the others. This specific region of the gray matter of the spinal cord is known as the central gelatinous substance.
2- White substance
The White matter Of the spinal cord is characterized by surrounding the gray matter. That is, it forms a region that completely surrounds the gray substance within.
The white matter of the spinal cord is formed by the axons of the neurons (not the nuclei). These axons are the parts of the cell that transport the information, so this region is cataloged as a transmission structure.
The white matter of the spinal cord is divided into three major regions: the anterior region, the lateral region and the posterior region.
The entry site of the dorsal root is detected through a dorso-lateral groove, and the entry of the ventral root is determined by a ventro-lateral groove.
These two grooves allow the white matter to be divided into a dorsal funiculus called the lateral funiculus and a ventral funiculus.
Cells and Functions
At the microscopic level, the spinal cord is characterized by different Cell types . This region of the organism has ependymal cells, elongated cells and neurological cells.
These types of cells are organized differently in each region of the spinal cord. The microscopically more interesting areas are the gray matter and the white matter.
Gray matter cells
The gray matter of the spinal cord varies its functioning and the type of neurons that it houses in each zone. In this way, it presents different properties in its dorsal horn, it is an intermediolateral horn, in its ventral horn and in the intermediate zone.
The dorsal horn of the gray matter receives axons from the dorsal ganglia through its posterior region. This transmission of axons of the dorsal ganglia is carried out by the homonymous roots and is characterized to contain mainly sensitive bundles.
In this sense, the dorsal horn of the gray substance comprises the nucleus of the commune of clarke, place where the synapses are realized between the fibers that transmit the deep unconscious sensitivity.
On the other hand, the dorsal horn of the gray matter also contains the rolling gelatinous substance, a region where the synapses of the fibers transmit thermo-analgesic sensitivity.
Finally, the core of the dorsal horn is characterized by synapses of the fibers that transmit the tactile sensitivity.
In the intermediate layer of the gray matter, only upper thoracic and lumbar segments of the spinal cord are found. This region is full of preganglionic neurons.
Finally, the central horn is composed of axons of multipolar motor neurons, and the intermediate zone is characterized by a large number of interneurons.
White matter cells
The white matter of the spinal cord is made up mainly of a large number of nerve fibers, neuroglia and blood vessels.
In the posterior cord of the white matter are the axons of sensitive neurons, whose nuclei are located in the dorsal ganglia. These neurons participate in two modes of conscious proprioception: kinesthesia and epicritic touch.
The posterior cord of the white matter is also characterized by two different beams: the Goll beam in the medial regions and the Burdach beam in the lateral zones.
The lateral cord of the white matter, on the other hand, contains both ascending and descending pathways. Ascending axons are responsible for driving pain, temperature and thick touch. In contrast, the descending fibers are mainly motor neurons, which are responsible for the control of voluntary movements.
Finally, the anterior cord of the white matter also contains ascending and descending pathways. The ascending neurons transmit information espinotectal (reflex movements), espinoolivar (cutaneous sensation) and espinotalámico (thick touch and pressure). The descending pathways contain motoneurons that control movement.
Spinal cord injuries
Spinal cord involvement (myelopathy) is a condition characterized by chronic spinal cord disruption.
This disease is often used to name spinal cord conditions that have not been caused by trauma.
The effects of myelopathy may depend on the degree of damage to the spinal cord, resulting in complete injury (if all symptoms of the disease occur) or incomplete injury (if only a few).
The medullary lesion can generate several symptoms, the main ones being: paralysis or loss of sensitivity in muscles of the trunk, neck and extremities, vesical, anal or seminal sphincter disorders and sympathetic system block, causing hypotension, bradycardia or abdominal distension.
On the other hand, spinal cord injuries, whether due to myelopathy or trauma in the spinal cord regions, vary markedly depending on the affected region. For this reason, it is often critical to detect the injured bone marrow region.
As we have seen, each spinal segment is responsible for performing a series of specific actions related to movement, perception, functioning of the parasympathetic system and control of different organs.
In this sense, lesions in the cervical vertebrae four and seven have been found to cause paralysis of the four extremities, and involvement of the eleventh vertebra of the thorax causes paralysis of the lower limbs.
References
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- Junqué, C. I Barroso, J (2009). Neuropsychology. Madrid, Ed. Synthesis.
- Kaufman, Bard. "Spinal Cord Development and Stem Cells" . Life Map Discovery Compendium . Retrieved 12 Dec 2015 .
- Michael J. Aminoff... [et al.] (2008). Neuropsychology and behavioral neurology.
- Spinal Cord Gross Anatomy". Retrieved December 27, 2015.
- The Science of CSM". Org: an online resource for cervical spondylotic myelopathy . Retrieved 2015-11-05.