Spinal Cord: Description, Functions, Structure, Injuries and Relationship with Other Organs

What is the Spinal Cord?

It is a complex cylinder of nerves that begins at the base of the brain and runs down the spinal canal to the spinal column.

It is part of the collection of nerves in the body, called the central nervous system, together with the brain. In each of the many segments of the spinal cord lives a pair of roots that are formed by nerve fibers.

These roots are known as the dorsal roots (which is toward the back) and ventral (which is far from the back).

We depend on the spine for the main support of our body. It allows us to stand, bend and turn, while protecting the spinal cord from injuries.

If the spinal cord is injured, it often causes permanent changes in strength, sensation and a handful of other body functions due to its connection to the brain.

Because the spinal cord is the center of the body’s functions, a person’s life can change drastically when an injury is severe enough.

Much research is being done to treat spinal cord injuries and scientists are optimistic that the advances they are finding will be enough to completely repair the damage.

To understand how a spinal cord injury can affect a person’s life, you will need a good understanding of the multiple functions that the spinal cord provides.

Main functions of the spinal cord

Electrical communication: Electric currents travel up and down the spinal cord, sending signals that allow different segments of the body to communicate with the brain.

Walking: While a person walks, a collection of muscle groups in the legs contracts constantly.

The step-by-step action may seem incredibly simple, since we have been doing it all our lives, but in reality there are many factors that must be properly coordinated to allow this movement to occur.

These generators of central patterns in the spinal cord are formed by neurons that send signals to the muscles of the legs, causing them to extend or contract, and produce the alternate movements that occur when a person walks.

Reflexes : Reflexes are involuntary responses that result from stimuli that involve the brain, spinal cord, and nerves of the peripheral nervous system.

Structure of the spinal cord

The general structure of the spinal cord is enclosed by the protection of the spine. The spinal nerves are found in the spaces between the arches of the vertebrae.

The spinal nerves are divided into these separate regions:

  • Cervical
  • Thoracic
  • Lumbar
  • Sacred
  • Coccígeo
  • White matter and gray matter

The spinal cord is divided into gray matter (which has a butterfly shape) and white matter (which is the material that surrounds gray).

The white substance is formed by nerve fibers, called axons, that extend up and down.

Each group of axons carries a specific type of information it needs to communicate, the ascending tracts of the axons communicate with the brain, while the descending ones carry signals from the brain to various muscles and glands throughout the body.

Gray matter is also organized according to its function. It could be said that each half has a dorsal horn, a ventral horn and a lateral horn.

The dorsal and ventral horns supply skeletal muscle, while the lateral horn supplies cardiac and smooth muscle.

Spinal nerves

The spinal nerves are what allow the spinal cord and the rest of the body to communicate.

A nerve is a small cord-shaped organ that is made up of several joined axons. There are 31 pairs of spinal nerves:

  • 8 are cervical nerves located in the neck.
  • 12 are thoracic nerves located in the thorax.
  • 5 are lumbar nerves located in the abdomen.
  • 5 are sacral nerves located in the pelvis.
  • 1 is the coccygeal nerve located in the coccyx.

Reflexes

A reflex can be a simple and uncontrolled response or a learned response, the simple ones are integrated into our nervous system, such as moving the hand away from something hot. A reflex that is acquired comes from practice, like playing the piano.

A reflex consists of 5 components:

  • Receiver: The receiver responds to an electrical signal.
  • Via afferent: This route sends the action to the integrating center.
  • Integration center: This is typically the nervous system and is where all the action potentials are processed.
  • Once the information is processed, the integrating center determines how the body should respond.
  • Efferent route: The response travels through this path to the effector organ.
  • Effector organ: This body carries out the response to all the above. The body that responds is usually a muscle or gland in the body.

Spinal cord injury

A spinal cord injury occurs when a part of the cord or nerves located at the base of the spine are damaged.

This can have an important effect on the sensory, motor and reflex abilities of the body if the brain can not send information beyond the location of the injury.

The closer the brain injury is, the more expansive the damage. As you can probably imagine, a spinal cord injury can alter a person’s life forever.

However, there are many options for treatment available and the research results for a cure of paralysis have never been more promising.

The technology is proving to be able to help in communication between the brain and limbs that have suffered nerve damage.

Research is advancing rapidly, and in just 5 years we may have the means to reverse the most severe spinal cord injuries.

Cells of the central nervous system

Neurons connect with each other to send and receive messages in the brain and spinal cord. Many neurons that work together are responsible for every decision made, every emotion or sensation felt, and every action taken.

The complexity of the central nervous system is surprising, there are approximately 100 billion neurons in the brain and spinal cord combined.

Up to 10,000 different subtypes of neurons have been identified, each specialized to send and receive certain types of information.

Each neuron is composed of a cell body, which houses the nucleus. Axons and dendrites form extensions of the cell body.

Astrocytes, a type of glial cells, are the primary support cells of the brain and spinal cord, secreting proteins called neurotrophic factors.

They also degrade and eliminate proteins or chemicals that can be harmful to neurons (for example, glutamate, a neurotransmitter that excessively causes cells to be overexcited and die by a process called excitotoxicity).

Astrocytes are not always beneficial; After an injury, they divide to form new cells that surround the site of the lesion, forming a glial scar that is a barrier to the regeneration of axons.

Microglia are immune cells to the brain. After a lesion, they migrate to the site of the lesion to help eliminate dead and dying cells.

They can also produce small molecules called cytokines that activate the cells of the immune system to respond to the site of injury.

It is likely that this cleansing process plays an important role in the recovery of function after a spinal injury.

Oligodendrocytes are glial cells that produce a fatty substance called myelin that envelops axons in layers.

The axon fibers isolated by myelin can carry electrical messages (also called action potentials) at a speed of 100 meters per second, while fibers without myelin can only carry messages at a rate of one meter per second.

Synapse and neurotransmission

The messages are transmitted from the neurons through the synapses, small gaps between the cells, with the help of chemicals called neurotransmitters.

To transmit a message of action potential through a synapse, the neurotransmitter molecules are released from one neuron (the “presynaptic” neuron) through space to the next neuron (the “postsynaptic” neuron). The process continues until the message reaches its destination.

There are millions and millions of connections between neurons within the spinal cord. These connections are made during development, using positive signals (neurotrophic factors) and negative signals (inhibitory proteins) to adjust them.

Surprisingly, a single axon can synapse with up to 1000 other neurons.

What causes paralysis?

There is a logical and physical topographical organization for the anatomy of the central nervous system, which is an elaborate network of closely connected neuronal pathways.

This orderly relationship means that different segmental levels of the cord control different things, and the injury to a particular part of the cord will have an impact on the neighboring parts of the body.

Paralysis occurs when communication between the brain and the spinal cord fails. This may be the result of injury to brain neurons (a stroke) or spinal cord.

Trauma to the spinal cord affects only the areas below the level of injury. However, polio (a viral infection) or Lou Gehrig’s disease can affect neurons throughout the spinal cord.

The information channels

Specialized neurons carry messages from the skin, muscles, joints and internal organs to the spinal cord about pain, temperature, touch, vibration and proprioception.

These messages are transmitted to the brain along one of two pathways: the spinothalamic tract and the lemniscal pathway.

These pathways are located in different places in the spinal cord, so a lesion may not affect them in the same way or to the same degree.

Each segment of the spinal cord receives sensory information from a particular region of the body.

Scientists have mapped these areas and determined the “receptive” fields for each level of the spinal cord. The neighboring fields overlap each other, so the lines in the diagram are approximate.

Voluntary and involuntary movement

More than a million axons travel through the spinal cord, including the longer axons of the central nervous system.

Neurons in the motor cortex, the region of the brain that controls voluntary movement, send their axons through the corticospinal tract to connect with the motor neurons of the spinal cord.

Spinal motor neurons project out of the cord to the right muscles through the ventral root, these connections control conscious movements, such as writing and running.

The information also flows in the opposite direction resulting in an involuntary movement. Sensory neurons provide feedback to the brain through the dorsal root.

Some of this sensory information is transmitted directly to the lower motor neurons before it reaches the brain, leading to involuntary movements or reflexes, the remaining sensory information travels back to the cortex.

How do the spinal cord and muscles work together?

The spinal cord is divided into five sections: the cervical, thoracic, lumbar, sacral and coccygeal regions. The level of injury determines the degree of paralysis and / or loss of sensation. No two injuries are the same.

How do the spinal cord and internal organs work together?

In addition to the control of voluntary movement, the central nervous system contains the sympathetic and parasympathetic pathways that control the “fight or flight” response to danger and the regulation of bodily functions.

These include the release of hormones, the movement of food through the stomach and intestines, and the sensations and muscular control of all internal organs.

What happens after a spinal cord injury?

The cells of the immune system migrate to the site of injury, causing additional damage to some neurons and death to others who survived the initial trauma.

The death of the oligodendrocytes causes the axons to lose their myelination, which greatly hinders the conduction of the action potential, messages or makes the remaining connections useless.

The path of neuronal information is further affected because many axons are cut, cutting the lines of communication between the brain and muscles and between the sensory systems of the body and the brain.

Within several weeks of the initial injury, the area of ​​tissue damage has been removed by microglia, and a fluid-filled cavity surrounded by a glial scar is left behind.

Although the spinal cord injury causes complex damage, a surprising amount of basic circuits to control movement and process information may remain intact.

This is because the spinal cord is arranged in layers of circuits. Many of the connections and neuronal cell bodies that form this circuit above and below the site of the injury survive the trauma.