Structure of the spinal cord
Definition 1
The spinal cord (SC) is phylogenetically the oldest section of the central nervous system of vertebrates, located in the bony vertebral (spinal) canal from the first cervical vertebra to the first - second lumbar vertebrae and ends in a medullary cone, from which a thin thread stretches down, surrounded by long roots of the lower segments brain (the so-called “horse tail” is formed).
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The spinal cord is a cylindrical cord of segmental structure, 40-45 cm long, 1-1.5 cm in diameter and weighing about 30 g. It is slightly compressed in the anterior-posterior direction.
In the center of the spinal cord there is a reduced canal, inside of which there is cerebrospinal fluid.
In the central part of the spinal cord there is gray matter, in a cross section resembling butterfly wings or the letter H. It consists of the bodies of neurons united by processes and glia.
The gray matter of the spinal cord forms paired anterior and posterior horns. There are lateral horns in the thoracic region and upper segments of the lumbar region.
Note 1
Functionally, the anterior horns are motor, the lateral are vegetative, and the posterior are sensitive.
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In humans, the spinal cord consists of the following segments:
- 8 cervical C (I – VIII);
- 12 thoracic Th (I – XII);
- 5 lumbar L (I – V);
- 5 pelvic S (I – V);
- 1 - 3 coccygeal Co (I - III).
Note 2
According to the Bell-Magendie law, the posterior (dorsal) roots of the spinal cord are responsible for sensory function, and the ventral (anterior) roots are responsible for motor function.
The ratio of fibers that enter through the dorsal and exit through the ventral roots is also characteristic (for mammals - about 2: 1). It is known that the common final path for impulses coming from different receptors and competing for the common final path is the same motor neuron.
Risk of organ damage
Due to the characteristic structure of the brain, it is connected to most systems in the body. The integrity of its structure is extremely important for the correct functioning of the musculoskeletal system and the health of internal organs. Any injury, regardless of severity, can lead to disability. Sprains, dislocations, disc damage, vertebral fractures with or without displacement can cause spinal shock and paralysis of the legs, and disrupt the normal functioning of the cords.
Severe injuries result in shock lasting from several hours to several months. In this case, the pathological condition is accompanied by a number of neurological symptoms. These include numbness, sensory disturbances, pelvic organ dysfunction, and inability to control the process of urination and bowel movements.
Treatment of minor spinal injuries is carried out on an outpatient basis, using medications, therapeutic exercises and massage. Severe injuries require surgical intervention, especially if compression of the spinal cord is detected. Cells are quickly damaged and die, so any delay can cost a person’s health. The recovery period after such an intervention is up to two years. Various physiotherapeutic procedures help with this, for example, reflexology, ergotherapy, electrophoresis, magnetic therapy, etc.
The spinal cord is a key element of the human central nervous system, which is connected in one way or another with almost all internal organs and human muscle tissue. The specific structure allows you to transmit impulses and signals, ensure full motor activity, and perform a number of other functions.
Spinal cord membranes
The spinal cord is covered by three membranes:
- soft,
- arachnoid,
- solid.
These membranes pass into similar membranes of the brain.
The inner shell is soft, tightly covers the spinal cord, penetrating into all the cracks and lining them.
The arachnoid membrane is thin, transparent, covering all convexities. Unlike the soft shell, it does not extend into the fissures and grooves of the brain, resulting in the formation of a subarachnoid space filled with cerebrospinal fluid.
The dura mater is the outer shell, forms a dense fibrous sac around the brain, which extends to the roots and ganglia. Below, the hard shell surrounds the cauda equina and, together with the filum terminale, grows to the periosteum of the coccyx.
Connective tissue fibers (22 - 23 pairs) extend from the hard shell, forming jagged ligaments between the hard and soft shells. Thus, it strengthens the spinal cord.
Between the dura mater and the arachnoid membrane, a subdural space is formed, which is filled with cerebrospinal fluid.
The space between the periosteum of the spinal column and the dura mater of the spinal cord is called the epidural. It contains fatty tissue and venous plexuses.
Neural structure of the spinal cord
Sensory cell bodies are located outside the spinal cord. Some of them are located in special ganglia (somatic afferent skeletal muscles), others - in extra- and intramular nodes of the autonomic nervous system. Thanks to them, only the internal organs are sensitive.
Sensitive cells have a process that divides directly near the cell body into two branches: along one, excitation passes from the receptors to the cell body, and along the other, from the neuron body to the nerve cells of the brain or spinal cord. Excitation from one branch to another sometimes occurs without the participation of the neuron body.
In total, the human spinal cord contains about 13 million neurons. Of this total, only about 3% are efferent, motor, or motor neurons, while the remaining 97% are interneurons. Motor neurons are the original cells of the spinal cord. Among them are alpha and gamma motor neurons, as well as preganglionic neurons of the autonomic nervous system.
Reflex function of the spinal cord (SC)
Note 3
In the study of reflex activity of the brain, an important role is played by the discoveries and generalizations of the British physiologist, Nobel Prize winner Charles Sherrington.
The scope of functions that the SM performs is extremely large. The spinal cord contains centers:
- motor reflexes (except for the muscles of the head),
- reflexes of the rectum and genitourinary system,
- reflexes that provide thermoregulation,
- reflexes that regulate tissue metabolism,
- center of contraction of the diaphragm
- centers of most vascular reflexes, etc.
Under natural conditions, these reflexes are constantly influenced by parts of the brain.
The reflex arcs of spinal reflexes are mono- and polysynaptic.
A detailed analysis of peripheral influences and control of the reflex activity of the spinal cord is carried out thanks to long multi-level reflex arcs, the centers of which are localized in the cerebral cortex and subcortical areas.
The degree of manifestation of the reflex depends on whether the connection between the structures of the spinal cord and the brain is maintained.
Example 1
After decerebralization (removal of the brain) or spinalization (removal of the spinal cord), many complex forms of activity that are provided by the spinal cord disappear. This is explained by a violation of the reflex arcs responsible for carrying out certain reactions. In this case, periodic discharges of the respiratory muscles that provide respiratory movements and tonic discharges of sympathetic neurons that maintain vascular tone and, accordingly, blood pressure may disappear.
SC reflexes include:
- protective,
- vegetative,
- visceromotor (caused by irritation of receptors of internal organs, or changes in their functions),
- stretch reflexes of antagonist muscles.
Note 4
This classification is quite arbitrary, since even in a spinal animal it is difficult to determine reflexes that belong only to one of the named groups.
Functions
Functional anatomy implies that, being part of the central nervous system, the spinal cord performs a reflex and conductive function. In the first case, the body controls the implementation of the simplest actions at the level of reactions contained in the subconscious. A striking example is the initiation of a motor function by withdrawing the hand if the surface is too hot. The limb does this before the person himself understands what happened. The second task of the organ is to transmit nerve impulses to the head section of the central nervous system, along the ascending and descending pathways.
Briefly about the main functions of the spinal cord
Reflex function
This main function of the organ is a response to external irritation. For example, the appearance of a reflex cough due to foreign objects and particles entering the respiratory tract, removing the hand from cactus spines or a source of danger. The impulse enters the spinal canal through motor neurons, which also trigger muscle contraction. This process does not require the involvement of the brain, and the motor reaction occurs without its participation. That is, a person does not even think about his action, often does not realize it.
Children's innate reflexes are checked after birth. They usually include the ability to suck milk, breathe, and jerk their legs. During development, acquired reflexes also appear, which help doctors identify the correct functioning of the arch elements and individual segments of the spinal cord. The test is carried out during a neurological examination. The main emphasis is on the plantar reflex, knee and abdominal. They allow you to check how healthy a person is at one time or another.
Conductor function
Treatment of spinal cord myelitis
Another important function of the spinal cord is conduction. It ensures the transmission of impulses from the skin, mucosal surface, internal organ to the brain and in the opposite direction. The white matter acts as a “conductor”. It is this that carries information about incoming impulses from the outside. Thanks to this ability, a person can characterize any object that surrounds him.
Cognition of the world is carried out through the transfer of information after touch to the brain. It is thanks to this function that a person understands that an object is slippery, smooth, rough or soft. With loss of sensitivity, the patient ceases to understand what is in front of him when touching an object. In addition, the brain receives data about the position of the body in space, tension in muscle tissue or irritation of pain receptors.
Conducting function of the spinal cord
Along with reflex activity, an important function of the SC is conduction - the conduction of nerve impulses by white matter (nerve spinal fibers).
Pathways are groups of nerve fibers with a common structure and function that connect the spinal cord and brain, as well as parts of the spinal cord.
Nerve fibers are distinguished according to their functional characteristics:
- associative,
- commissural,
- projection (afferent and efferent).
Thanks to associative fibers, one-way communication is carried out between individual parts of the SM, combining different segments and forming their own bundles.
Commissural fibers unite opposite sections of various parts of the SC, homogeneous in their functions.
Projection fibers connect the SC with the sections located above. With their help, the main pathways are formed, consisting of ascending (centripetal, afferent, sensory) and descending (centrifugal, efferent, motor) pathways.
The ascending pathways of the spinal cord transmit information from receptors that perceive information from the environment and internal environment.
Depending on the type of sensitivity carried out, the pathways are divided into:
- extero-sensitivity (external),
- propriosensitivity (articular-tendon-muscular, tissue mechanoreception),
- interosensitivity (external).
Along the descending pathways of the spinal cord, impulses are transmitted from the structures of the brain to the motor nuclei, and thus appropriate reactions to external and internal stimuli are carried out, that is, the higher parts of the central nervous system communicate with the effector neurons of the spinal cord.
general information
The anatomy of the spinal cord differs from the brain in its oblong structure. In Latin the organ is called medulla spinalis. It is a thickened tube with a small channel inside, slightly flattened in front and behind. It is this structure that ensures the normal transportation of nerve impulses from the main organ located in the cranium to the peripheral structures of the nervous system.
Locally, the organ is located in the spinal canal, where soft and bone tissues and nerve endings responsible for many functions of the human body are concentrated. Without a normally functioning spinal cord, natural breathing, digestion, heartbeat, reproductive activity, and any motor activity are not possible.
In humans, it begins to form at about 4 weeks of development inside the mother’s womb. But in what form it is observed in an adult, it appears much later; at first it is a neural tube, gradually developing into a full-fledged organ. It completes its formation within 2 years after birth.