Spinal cord

Generality

Together with the brain, the spinal cord forms the central nervous system (CNS).
Extremely complex structure, it has inside two areas rich in neurons, called gray matter and white matter.

In contrast to what happens in the brain, gray matter is surrounded by white matter in the spinal cord.
The spinal cord has several functions. In fact, it presents neurons with sensory properties and neurons with motor properties. In addition, pairs of mixed nerves, known as spinal nerves, originate from the gray matter.
There are 31 pairs (or pairs) of spinal nerves, like the number of segments that ideally divide the spinal cord.
Protecting this fundamental organ are the vertebrae of the spinal column and the meninges.

Central nervous system (CNS)

In vertebrates, the central nervous system (CNS) is the most important component of the entire nervous system. In fact, it deals with analyzing the information that comes from the internal and external environment of the organism and elaborating the most appropriate responses (to the aforementioned information).
To perform all its functions correctly, it uses the peripheral nervous system (SNP): the latter transmits to the CNS all the information collected inside and outside the organism and spreads all the processing originating in the organism to the periphery. central nervous system.

What is the spinal cord?

The spinal cord is, together with the brain, one of the two nerve structures that make up the central nervous system (CNS).
In fact, in addition to dealing with the transmission of nerve signals originating in the brain, it is also able to process an autonomous motor response, better known as a spinal reflex.
Like the brain, the spinal cord has two areas rich in neurons called gray matter and white matter; unlike, however, in the case of the brain, these two areas are located in exactly the opposite way: in the spinal cord the gray matter is found internally and the white matter is located externally.

NEURONS AND NERVES: SOME IMPORTANT DEFINITIONS

Before continuing with the description of the spinal cord, it is worth reviewing what neurons and nerves are.
Neurons are the cells of the nervous tissue. Their task is to generate, exchange and transmit all those (nerve) signals that allow muscle movement, sensory perceptions, reflex responses, etc.
Generally, a neuron consists of three parts: a body (where the cell nucleus resides), dendrites (which are equivalent to receiving antennas) and axons (or extensions that act as nerve signal diffusers).
A bundle of axons makes up a nerve.
Nerves can carry information in three ways:

• From the SNC to the periphery. Nerves with this property are called efferent. The efferent nerves control the movement of the muscles, hence the motor sphere.
• From the periphery to the SNC. Nerves with this ability are called afferents. The afferent nerves signal to the CNS what they have detected in the periphery, therefore they perform a sensory function.
• From the SNC to the periphery and vice versa. Nerves with this dual property are defined as mixed. Mixed nerves cover, at the same time, motor functions and sensory functions.

Please note: nerve and nerve fiber are not exactly the same. By nerve fiber, we mean an axon covered by its sheath.
A set of nerve fibers can make up a nerve.

Anatomy and physiology

Premise: given the complexity of the topic and the considerable number of names and definitions, it was decided to treat anatomy hand in hand with functions (ie physiology), without separating the two themes, in such a way as to simplify the most spinal cord problems.

The spinal cord is a cylindrical nerve structure, housed inside a canal of the spinal column and ideally subdivided into four regions: the cervical region, the thoracic region, the lumbar region and the sacral region.
On average 45 centimeters long in men and 43 centimeters in women, it has a variable diameter, ranging from 13 mm in the cervical region and in the lumbosacral region (the so-called "bulges") to 6.4 mm in the thoracic region.
Proceeding from top to bottom, the spinal cord begins from an area called the foramen magnum (or foramen magnum) and ends at the level of the second lumbar vertebra (although it has some extensions that reach the sacro-coccygeal region). where it originates - that is, in the foramen magnum - it is closely connected to the brain stem or, better, to the portion of the latter better known as medulla oblongata.
From the point of view of the nervous composition, the spinal cord is a decidedly very complex element. This is why gray matter and white matter will be analyzed separately, in their most important details. Here, we will limit ourselves only to describing what emerges from a cross section of the spinal cord:

• The gray matter occupies the center of the section and has all the appearance of a butterfly or, if you prefer, of the letter "H". By comparing several cross sections, performed in different points, at least a couple of things are evident: the butterfly shape and size vary from region to region and the gray matter / white matter ratio grows as it progresses from the cervical region to the sacral region.
• The white matter resides in the periphery, all around the gray matter.
• Exactly in the center, there is a very small canal filled with the so-called liquor (or cerebrospinal or cerebrospinal fluid). Briefly, the functions of the CSF are: to provide protection from possible trauma, to nourish the central nervous system (favoring the exchanges between it and the blood), to regulate the intracranial pressure and to the spinal cord and to receive waste products as if it were a way for the their removal.

differentiates between gray matter neurons and white matter neurons

Readers are reminded that the difference between gray matter and white matter essentially lies in the type of neurons present within one and the other: gray matter, unlike white matter, contains only neurons devoid of myelin.
Myelin is a whitish insulating substance, mainly composed of lipids and proteins, which increases the conduction of the nerve signal.
In the central nervous system and in the peripheral nervous system, the production of myelin is entrusted to the neurons constituting the glia (or cells of the glia): precisely to the oligodendrocytes, in the case of the CNS, and to the Schwann cells, in the case of the SNP.

As with the brain, pairs of nerves (exactly 31 pairs), called spinal nerves, are also born from the spinal cord. This topic also deserves to be explored in one of the next sub-chapters.

Vertebral column and meninges

As mentioned, the spinal cord runs inside a canal in the spinal column.
The backbone of the human body, the vertebral column is a bony structure of about 70 centimeters, made up of 33-34 vertebrae stacked on top of each other.
Its function towards the spinal cord is essentially to protect it from traumatic insults that could affect its good health.

The sections of the spine:

• Cervical: 7 vertebrae
• Dorsal (or thoracic): 12 vertebrae
• Lumbar: 5 vertebrae
• Sacral: 5 vertebrae
• Coccygea: 4/5 vertebrae

Other elements with a protective function towards the spinal cord (and the whole central nervous system) are the meninges.
Three in number, the meninges are in fact membranes that stand between the spinal cord and the vertebral bone lining (N.B: in the case of the brain, they are between it and the skull).
Proceeding from the outside towards the inside, the names of the meninges are:

• Tough mother. Very thick membrane, it does not adhere completely to the vertebrae, but is separated from them by an area rich in adipose tissue and venous blood vessels, called the perdural space (or epidural space).
• Arachnoid. So named because it consists of a web-like tissue, it is divided from the innermost menynx by a space known as the subarachnoid space. In the subarachnoid space resides the cerebrospinal fluid (which is the one taken during lumbar punctures).
• Pious mother. Very thin membrane, it contains the arterial vessels that supply the spinal cord and brain.

SEGMENTAL ORGANIZATION OF THE SPINAL CORD

In addition to the organization into regions, the spinal cord is also divided into 31 segments.
Proceeding from top to bottom, there are 8 cervical segments (C1-C8), 12 thoracic segments (T1-T12), 5 lumbar segments (L1-L5), 5 sacral segments (S1-S5) and a coccygeal segment ( Co1).

For the avoidance of doubt, it is important to point out that the segments of the spinal cord and the sections of the spine coincide approximately. There is, therefore, no exact correspondence, but almost.
When we talk about the spinal nerves, we will see that each segment corresponds to a pair of spinal nerves.

Gray substance

In each wing of the butterfly that forms the gray matter, three regions populated by neurons can be recognized:

1. the dorsal horn
2. the lateral horn
3. the ventral horn.

If the medulla is observed from top to bottom (longitudinal section), these three regions form elements that are called with the term columns.
In the three horns mentioned (6 in all, if both wings are considered), the cell bodies of different types of neurons take place - including motor neurons, interneurons and neuroglia cells - and a fair number of demyelinated axons (i.e. devoid of myelin ).
All these neurons organize themselves into two large groups of cells; groups that the experts have called with the terms of nuclei and laminae. There are different types of cores, each with its own specific function, and 10 sheets, also with their own specific task. For a matter of complexity of the subject, cores and sheets will not be dealt with further.

• The posterior or dorsal horns (N.B: the back of the spinal cord looks in the direction of our back) contain sensitive nerve fibers, which process information from the periphery (proprioceptive sensitivity, exteroceptive sensitivity, etc.).
• In the lateral horns, the neurons that control the pelvic and visceral organs are housed. The lateral horns are present only in the segment of bone marrow that goes from the eighth cervical segment (C8) to the second lumbar segment (L2).
• Finally, the anterior or ventral horns (N.B: the belly of the spinal cord looks in the direction of our abdomen) host nuclei of motor neurons, which are the neurons that innervate the skeletal muscles.

Finally, to complete the anatomical-functional picture of the gray matter, we note the presence of two swellings, the result of a concentration of nerve cells, one at the level of the cervical segments and another at the level of the lumbosacral segments.
The cervical swelling (o intumescentia cervicalis) contains the neurons that innervate the upper limbs of the body; it resides approximately at the height of the nerves of the brachial plexus, exactly between the 4th cervical segment (C4) and the 1st thoracic segment (T1).
The lumbosacral swelling (o intumescentia lumbalis), on the other hand, contains the neurons innervated the lower limbs; this is located approximately in correspondence with the nerves of the lumbosacral plexus, between the II lumbar segment (L2) and the III sacral segment (S3).

Figure: gray matter and white matter of the spinal cord. As for the gray matter, note the position of the ventral horns and the dorsal horns.
As for the white matter, pay attention to the position of the ascending and descending beams.

White substance

In the white matter, around a "wing of the central butterfly, 3 symmetrical regions can be recognized (therefore 6, considering both wings); these regions, observed along their longitudinal axis, form the so-called cords. In the dorsal position, resides the cord. posterior (or precisely dorsal); in an intermediate position, the lateral cord takes place; finally, in the ventral position, it houses the anterior cord (or ventral).
Within the various cords, there are three different types of nerves:

• The so-called bundles or ascending tracts.
  These nerve elements carry sensitive information from the periphery to the central nervous system, precisely to the nuclei of the brainstem, cerebellum and dorsal part of the thalamus.
  In the dorsal cords, we find the bundles (or fascicles) known with the name of gracile and cuneato; in the lateral cords, the neospinathalamic tracts and the spinocerebellar tracts (distinct in anterior and posterior) take place; finally, in the ventral cords, the paleospinothalamic bundles, the spino-olive bundles, the spinoreticular tracts and the spino-tectal tracts lodge.
• The so-called bundles or descending tracts.
  These nerve elements transmit information of a motor nature, originating in the CNS (precisely in the cerebral cortex and in the nuclei of the brainstem).
  Among the most important ascending bundles are the corticospinal bundle, the rubrospinal bundle, the medial and lateral vestibulospinal bundle, the medial and lateral reticulospinal bundle and the tectospinal bundle.
• The nerve fibers responsible for coordinating flexor reflexes.
  We speak of a flexor reflex when, after a painful stimulation, the part of the body involved moves away.
  A classic example of a flexor reflex is the one that occurs when you place your foot on a nail or take a burning coal in your hand: the answer consists, respectively, of withdrawing the affected limb and opening the hand to leave the "hot object.

Function of the main ascending beams (or tracts)

White matter cord Street (or bundle) Function Dorsal cord Gracile and wedge-shaped They deal with conscious proprioceptive sensitivity and tactile perception, conveying information regarding the sense of pressure, vibration, position and movement. Side cord Via neospinothalamica (or lateral spinothalamic) It mainly carries thermal (i.e. temperature) and pain (or nociceptive) information.
Anterior and posterior spinocerebellar tracts They carry information regarding unconscious proprioceptive sensitivity and some aspects of skin sensitivity. Ventral cord Paleospinothalamic pathway It carries tactile information about temperature and pain to the nuclei of the brainstem and the diencephalon. Spino-olive beam It deals with the transport of proprioceptive and tactile information. Spinoreticular beam It deals with the transmission of deep tactile and pain information. Via spino-tectale It mainly carries thermal, nociceptive and itch and tickling information.

Function of the main descending beams (or tracts)

Street (or bundle) Function White matter cord Corticospinal bundle It deals with precise voluntary movements. Side cord Rubrospinal bundle It has functions similar to the corticospinal bundle. Medial vestibulospinal bundle It has inhibitory power on the axial muscles of the neck and on the muscles of the upper back. Front cord Lateral vestibulospinal bundle It is excitatory and inhibitory, respectively, for the extensor muscles and for the flexor muscles of the neck, back and limbs. Medial reticulospinal bundle It has excitatory power on the muscles of the trunk and limbs. Front cord and side cord Lateral reticulospinal bundle It is excitatory for the trunk muscles and inhibitory for the neck muscles Side cord Tectospinal bundle It mainly coordinates the movement of the neck muscles. Front cord

Spinal nerves

As anticipated, each segment of the spinal cord corresponds to a pair of spinal nerves.
Spinal nerves are mixed nerves, so they possess both motor and sensory functions.
The nerve cells that make up the spinal nerves are somehow related to the gray matter. To be exact, the motor component of the spinal nerves refers to the ventral horn, while the sensory component derives from the dorsal horn.
The points of emergence of the nerve fibers coming from the ventral horn and the dorsal horn are called, respectively, ventral roots and dorsal roots.
Therefore, as can also be seen from the image below, in its very first section, each spinal nerve is divided into two branches: a branch that contains the axons innervating the skeletal and visceral muscles and a branch that includes the axons. of sensitive nerve cells (NB: the viscerals are present only in the tract of the spinal cord between segments C8 and L2).

It is important to underline that there is a noticeable difference between the two roots: unlike the ventral root, the dorsal root has a small swelling, called the ganglion, inside which all the sensory neuron bodies of the resulting spinal nerve are contained.
The ventral root does not have this peculiarity, as the bodies of motor neurons reside within the gray matter.
Each pair of spinal nerves owes its name to the corresponding spinal cord segment. Thus, the cervical spinal nerves are indicated with the letter C and the numbers from 1 to 8, based on the segment d "belonging; the thoracic spinal nerves with the letter T and the numbers from 1 to 12; the lumbar spinal nerves with the letter L and the numbers from 1 to 5; the sacral spinal nerves with the letter S and the numbers from 1 to 5; finally, the coccygeal pair with the initials Co and the number 1.
At this point, it is necessary to remind readers that the naming of the segments of the spinal cord is closely linked to the vertebrae from which the spinal nerves emerge and not to the vertebrae located nearby.To better understand this concept, it is useful to give some examples: the lumbar spinal nerves originate at the level of the thoracic vertebrae T11 and T12 (the sacral segment of the medulla resides here), but emerge from the vertebral column only at the lumbar level; in a similar way, the sacral spinal nerves arise in correspondence with the first lumbar vertebra, but come out of the column only starting from the sacral portion.

• The sensory nerve cells of the spinal nerves send information about tactile perception, proprioceptive sensitivity, skin temperature and pain to the spinal cord. Once in the spinal cord, this information is sent to the brain and, there, processed.
  On the surface of the body, the signals to be transmitted first to the medulla and then to the brain are the dermatomes. The dermatomes are skin regions innervated by nerve fibers of a specific spinal nerve. In fact, if a given spinal nerve is cut, the sensory capacity of the area of ​​skin that it controls fails.
  This particular property is useful in the diagnostic field, because the loss of sensitivity of a certain dermatome indicates a problem with a specific spinal nerve.
• The motor nerve cells of the spinal nerves reach and stimulate the skeletal muscles.
  In general, the cervical spinal nerves innervate the muscles of the neck, shoulders, arms, hands and diaphragm; the thoracic spinal nerves innervate the trunk and intercostal muscles for breathing; lumbar spinal nerves innervate the muscles of the hips, legs and feet; finally, the sacral spinal nerves innervate the anal and urethral sphincters.
  The table details the various actions of the spinal motor nerves.

Motor functions of the spinal nerves.

Spinal nerves (or segments) Motor functions C1-C6 They innervate the flexor muscles of the neck. C1-T1 They innervate the extensor muscles of the neck. C3, C4, C5 They innervate the muscles of the diaphragm. C5, C6 They allow the movement of the shoulders, the lifting of the arms (deltoid) and the flexion of the elbows (biceps); C6 in particular allows the outward rotation of the arms. C6, C7 They allow the extension of the elbows and wrists (triceps and wrist extensors); they also allow the pronation of the wrists. C7, T1 They allow flexion of the wrists and innervate the small muscles of the hands. T1-T6 They innervate the intercostal muscles for breathing and the trunk muscles. T7-L1 They innervate the abdominal muscles. L1-L4 They allow the flexion of the thighs. L2, L3, L4 They allow the adduction of the thighs and the extension of the legs to the knees. L4, L5, S1 They allow the abduction of the thighs, the flexion of the legs up to the knees, the dorsiflexion of the foot (or "pull" the foot towards oneself) and the extension of the toes. L5, S1, S2 They allow the extension of the legs starting from the hips, the plantar flexion of the feet and the flexion of the toes.

Spinal reflexes

The spinal reflexes are very particular responses of the spinal cord, which make the latter an organ independent of the brain.
Their generation is the result of a direct connection between some afferent (therefore sensitive) and some efferent (therefore motor) pathways.
When the cutaneous receptors of one of these afferent pathways pick up a certain signal of change, they communicate it to the associated sensory neurons; the sensory neurons carry the information captured in the periphery up to the spinal cord, where they are in direct contact with some motor neurons, or motor nerve cells. The transmission of information from sensory neurons to motor neurons (innervating specific muscles) causes movement to be produced ad hoc, that is, based on what is perceived by the skin receptors.

In simpler words, there are receptors on the skin connected to certain sensitive nerve fibers which, when they reach the spinal cord, are directly connected to specific motor nerve fibers. The passage of information between these nerve pathways causes a rapid and adequate response to be generated to what the skin receptors have sensed.
The figure can be of great help in understanding what happens during a spinal reflex.
According to Sherrington's classification, there are several types of spinal reflexes:

• The proprioceptive spinal reflexes, starting from the cutaneous receptors present in the muscles, joints and vestibular apparatus.
• The exteroceptive spinal reflexes, originating from the cutaneous receptors concerning tactile sensitivity.
• The nociceptive spinal reflexes, starting from the skin receptors related to pain (the flexor reflexes are an example).
• The exteroceptive spinal reflexes, starting from the receptors present at the visceral level.
• The teleceptive spinal reflexes, coming from the visual, acoustic and olfactory teleceptors (N.B: a telecector is a particular receptor, which is activated by energy signals emanating from a distance from the organism).

Blood circulation

Like any organ in the human body, the spinal cord also needs to receive blood to survive, so it is vascularized.
The system of arterial and venous blood vessels is very complicated; for this reason, only the main points will be outlined:

• Originating from the descending aorta and vertebral arteries, the arterial vessels that supply the spinal cord are: the anterior spinal artery (which nourishes the anterior 2/3 of the spinal cord), the two posterior spinal arteries (which nourish about 1 / 3 of the posterior part of the spinal cord) and, finally, the arterial anastomoses constituting the so-called vasocorona of the spinal cord (which nourish the remaining part of the cord).
  N.B: an "anastomosis is a fusion of blood vessels.
• The outflow of oxygen-poor blood (ie venous drainage) occurs through a venous system that first affects the anterior spinal vein, posterior spinal veins, anterior radicular veins and posterior radicular veins and, then, the so-called plexus internal vertebral venous and the so-called external vertebral venous plexus.
  From here, therefore, the blood that fed the spinal cord passes into the vertebral, intercostal, lumbar and lateral sacral veins.