physiology

Spinal cord

Generality

The spinal cord constitutes, along with the brain, the central nervous system (CNS).

Extremely complex structure, it has inside two zones rich in neurons, called gray substance and white substance.

Contrary to what happens in the brain, in the spinal cord the gray substance is surrounded by the white substance.

The spinal cord covers several functions. In fact, it presents neurons with sensory properties and neurons with motor properties. Furthermore, from the gray substance, pairs of mixed nerves originate, known as spinal nerves.

There are 31 pairs (or pairs) of spinal nerves, such as the number of segments that ideally divide the spinal cord.

To protect this fundamental organ there are vertebrae of the spine 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 to elaborate the most appropriate answers (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 informative data captured inside and outside the organism and spreads all processing with origin in the periphery. central nervous system.

What is the spinal cord?

The spinal cord is, together with the brain, one of the two nervous 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 capable of processing an autonomous motor response, better known as the spinal reflex .

Like the brain, the spinal cord has two areas rich in neurons called gray matter and white substance ; otherwise, however, from the case of the encephalon, these two areas are located in exactly the opposite way: in the spinal cord the gray substance is found internally and the white substance is found externally.

NEURONS AND NERVES: SOME IMPORTANT DEFINITIONS

Before going on to describe the spinal cord, it is wise to review what neurons and nerves are.

Neurons are the cells of nervous tissue. Their task is to generate, exchange and transmit all those (nervous) signals that allow muscle movement, sensory perceptions, reflex responses and so on.

Typically, one 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 diffusers of the nervous signal).

A bundle of axons forms 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, therefore the motor sphere.
  • From the periphery to the CNS . Nerves with this capacity are called afferent. The afferent nerves signal to the CNS what they detected in the periphery, therefore they play a sensitive role.
  • From the SNC to the periphery and vice versa . Nerves with this double property are defined as mixed. At the same time, mixed nerves cover motor functions and sensory functions.

Note: nerve and nerve fiber are not exactly the same thing. For nerve fiber, it means an axon covered by its covering sheath.

A set of nerve fibers can be a nerve .

Anatomy and physiology

The spinal cord is a cylindrical-shaped nerve structure housed within a vertebral column canal and can be subdivided ideally 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 millimeters in the cervical region and the lumbosacral region (the so-called "swellings") to 6.4 millimeters in the thoracic region.

Proceeding from top to bottom, the spinal cord starts from an area called foramen magnum (or occipital hole ) and ends at the level of the second lumbar vertebra (although it has some extensions that reach the sacro-coccygeal region). In correspondence of where it originates - that is in the foramen magno - it is closely connected to the brainstem or, better, to the portion of the latter better known as medulla oblongata .

In terms of nerve composition, the spinal cord is a decidedly very complex element. Here is why gray matter and white substance will be analyzed separately, in their most important details. Here we will only describe what emerges from a cross section of the spinal cord:

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

difference between neurons of the gray matter and neurons of the white matter

Readers are reminded that the difference between gray and white matter lies essentially in the type of neurons present within one and the other: the gray substance, unlike the white, contains only neurons without myelin .

Myelin is a whitish insulating substance, composed mainly of lipids and proteins, which increases the conduction of the nervous 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 glia cells ): 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, nerve pairs (exactly 31 pairs) are born from the spinal cord, called spinal nerves . This topic also deserves to be explored in one of the next sub-chapters.

Vertebral column and meninges

As mentioned above, the spinal cord runs inside a vertebral column canal.

Supporting axis of the human body, the vertebral column is a bone structure of about 70 centimeters, composed of 33-34 vertebrae stacked one on top of the other.

Its function in relation to the spinal cord is essentially to protect it from traumatic insults that could jeopardize its good health.

The sections of the spine:

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

Other elements with a protective function towards the spinal cord (and the entire central nervous system) are the meninges .

In number of three, the meninges are in fact membranes that stand between the spinal cord and the vertebral bone lining (NB: in the case of the brain, they are between this and the skull).

Proceeding from the outside to the inside, the names of the meninges are:

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

SEGMENT ORGANIZATION OF THE SPINAL CORD

In addition to organization in 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. Therefore, there is not an 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 substance, three regions populated by neurons can be recognized:

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

If we observe the marrow from top to bottom (longitudinal section), these three regions form elements that are called by the term of columns .

In the three horns mentioned (6 in all, if we consider both wings), the cellular bodies of different types of neurons - including motoneurons, interneurons and neuroglial cells - and a fair number of demyelinated axons (ie, myelin-free axons ) take place. ).

All these neurons organize themselves into two large groups of cells; groups that the experts have named with the terms of nuclei and laminae . There are different types of nuclei, each with its own specific function, and 10 laminae, also with a specific task. As a matter of complexity of the subject, nuclei and plates will not be treated further.

  • The posterior or dorsal horns (NB: the back of the spinal cord looks towards our back) contain sensitive nerve fibers, which process the information coming from the periphery (proprioceptive sensitivity, exoceptive sensitivity, etc.).
  • In the lateral horns, they house the neurons that control the pelvic and visceral organs. The lateral horns are present only in the bone marrow tract from the eighth cervical segment (C8) to the second lumbar segment (L2).
  • Finally, the anterior or ventral horns (NB: the belly of the spinal cord looks towards our abdomen) host nuclei of motoneurons, which are the neurons that innervate the skeletal musculature.

Finally, to complete the anatomical-functional picture of the gray substance, there is 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.

Cervical swelling (or intumescentia cervicalis ) contains neurons that innervate the upper limbs of the body; it resides approximately at the height of the brachial plexus nerves, exactly between the IV cervical segment (C4) and the thoracic segment I (T1).

The lumbosacral swelling (or intumescentia lumbalis ), on the other hand, contains the neurons innervated in the lower limbs; this is found approximately at the nerves of the lumbosacral plexus, between the II lumbar segment (L2) and the III sacral segment (S3).

White substance

In the white substance, around a wing of the central butterfly, three 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, the posterior cord (or dorsal note) resides; in an intermediate position, the side cord takes place; finally, in a ventral position, houses the anterior cord (or ventral note).

Inside the various cords, there are three different types of nerves:

  • The so-called ascending bundles or traits .

    These nerve elements carry sensory information from the periphery to the central nervous system, exactly to the nuclei of the brainstem, the cerebellum and the dorsal part of the thalamus.

    In the dorsal cords, we find the bundles (or fascicles) known as gracile and cuneato; in the lateral cords, the neospinatalamic and spinocerebellar features (distinct in anterior and posterior) take place; finally, in the ventral cords, they house the paleospinothalamic bundles, the spino-olivary bundles, the spinoreticular tracts and the spino-tectal traits.

  • The so-called bundles or descending traits .

    These nerve elements transmit motor information, 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.

  • Nerve fibers responsible for coordinating flexor reflexes .

    We speak of a flexor reflex when, after a painful stimulus, 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 pick up a burning coal: the answer consists, respectively, in withdrawing the affected limb and opening your hand to leave the hot object.

Function of the main ascending beams (or sections)

Cord of white substanceStreet (or bundle)Function
Dorsal cordGracile and cuneatoThey deal with conscious proprioceptive sensitivity and tactile perception, transporting information regarding the sense of pressure, vibration, position and movement.
Side cordNeospinothalamic (or lateral spinothalamic) pathwayIt mainly carries thermal information (concerning temperature) and pain (or nociceptive) information.

Anterior and posterior spinocerebellar tractsThey carry information concerning unconscious proprioceptive sensitivity and some aspects of skin sensitivity.
Ventral cordPaleospinothalamic wayIt carries tactile information regarding temperature and pain to the nuclei of the brainstem and the diencephalon.
Spino-olivar bundleIt deals with the transport of proprioceptive and tactile information.
Spinoreticular beamIt deals with the transmission of deep tactile information concerning pain.
Street spino-tectaleIt mainly carries thermal, nociceptive and itching and tickling information.

Function of the main descending beams (or sections)

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

Spinal nerves

As anticipated, each spinal cord segment corresponds to a pair of spinal nerves .

Spinal nerves are mixed nerves, so they have both motor and sensory functions.

The nerve cells that make up the spinal nerves are somehow related to the gray substance . To be precise, the motor component of the spinal nerves is connected to the ventral horn, while the sensitive component derives from the dorsal horn.

The emergence points of the nerve fibers coming from the ventral horn and from 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 encloses the axons innervating the skeletal and visceral muscles and a branch that includes axons in itself of sensitive nerve cells (NB: visceral cells are present only in the spinal cord section between segments C8 and L2).

It is important to point out that there is a notable difference between the two roots: unlike the ventral root, the dorsal root has a small bulge, called the ganglion, inside which all the bodies of the sensory neurons of the resulting spinal nerve are contained.

The ventral root does not have this peculiarity, since the bodies of the motor neurons reside within the gray substance.

Each pair of spinal nerves owes its name to the corresponding spinal cord segment . Thus, the cervical spinal nerves are indicated by the letter C and the numbers from 1 to 8, depending on the segment of membership; thoracic spinal nerves with the letter T and the numbers 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, readers should be reminded that the name of the spinal cord segments is closely linked to the vertebrae from which the spinal nerves come out and not to the vertebrae located nearby. To better understand this concept, it is useful to report some examples: the lumbar spinal nerves originate at the level of the thoracic vertebrae T11 and T12 (the sacral segment of the medulla lies here), but emerge from the spine only at the lumbar level; in a similar way, the sacral spinal nerves arise at the first lumbar vertebra, but come out of the column only starting from the sacral portion.

  • The sensory nerve cells of the spinal nerves convey to the spinal cord information regarding tactile perception, proprioceptive sensitivity, skin temperature and pain. Once in the spinal cord, this information is sent to the brain and, here, processed.

    On the surface of the body, the signals to be transmitted first to the marrow and then to the brain are the dermatomers. Dermatomers are skin regions innervated by nerve fibers of a specific spinal nerve. In fact, if a given spinal nerve is cut, the sensory ability of the area of ​​skin that it controls is lost.

    This particular property is useful in the diagnostic field, because the loss of sensitivity of a certain dermatome indicates a problem with a given spinal nerve.

  • Motor nerve cells of the spinal nerves reach and stimulate 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 muscles and the intercostal ones for breathing; the 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 shows in detail the various actions of the spinal motor nerves.

Motor functions of spinal nerves.

Spinal nerves (or segments)Motor functions
C1-C6Innervate the neck flexor muscles.
C1-T1Innervate the extensor muscles of the neck.
C3, C4, C5They strengthen the diaphragm muscles.
C5, C6They allow the movement of the shoulders, the raising of the arms (deltoid) and the flexion of the elbows (biceps); C6 in particular allows the rotation towards the outside of the arms.
C6, C7Allow the extension of the elbows and wrists (triceps and wrist extensors); they also allow the pronation of the wrists.
C7, T1They allow the flexion of the wrists and innervate the small muscles of the hands.
T1-T6Innervate the intercostal muscles for breathing and the trunk muscles.
T7-L1Innervate the abdominal muscles.
L1-L4Allow the thighs to flex.
L2, L3, L4They allow the adduction of the thighs and the extension of the legs up to the knees.
L4, L5, S1They 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 themselves) and the extension of the toes.
L5, S1, S2They allow the extension of the legs starting from the hips, plantar flexion of the feet and flexion of the toes.

Spinal reflexes

Spinal reflexes are very specific 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 (and therefore motor) pathways .

When the cutaneous receptors of one of these afferent pathways capture a certain signal of change, they communicate it to the associated sensory neurons; sensory neurons carry information collected in the periphery to the spinal cord, where they are in direct contact with certain motor neurons, or motor nerve cells. The transmission of information from sensory neurons to motor neurons (innervating specific muscles) causes an ad hoc movement to be produced, that is based on what is perceived by the skin receptors.

Said in simpler words, there are receptors on the skin, connected to certain sensitive nerve fibers, which, when they reach the level of 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 for what the skin receptors have felt.

The figure can be of great help in understanding what happens during a spinal reflex.

According to the Sherrington classification (1906), there are different types of spinal reflexes:

  • The proprioceptive spinal reflexes, starting from the cutaneous receptors present at the level of muscles, joints and vestibular apparatus.
  • Exteroceptive spinal reflexes, coming from cutaneous receptors concerning tactile sensitivity.
  • Nociceptive spinal reflexes, starting from cutaneous receptors linked to pain (flexor reflexes are an example).
  • Exteroceptive spinal reflexes, starting from receptors present at visceral level.
  • The spinal reflexes of telecommunication, coming from the visual, acoustic and olfactory teleconectors (NB: a telecector is a particular receptor, which is activated by energy signals emanated at a distance from the body).

Blood spraying

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 supplying the spinal cord are: the anterior spinal artery (which nourishes the anterior 2/3 of the spinal cord), the two posterior spinal arteries (which feed about 1 / 3 of the posterior spinal cord) and, finally, the arterial anastomoses constituting the so-called vasocorona of the spinal cord (which feed the remaining part of the marrow).

    NB: an anastomosis is a fusion of blood vessels.

  • The outflow of oxygen-poor blood ( venous drainage ) occurs through a venous system that first involves 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 veins and lateral sacral veins.