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 and the brain. In each of the many spinal cords, segments live a pair of roots formed by nerve fibers.
These roots are the dorsal roots (which are toward the back) and ventral (far from the back).
We depend on the spine for the direct 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 repair the damage completely.
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 body segments 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. Still, in reality, many factors must be appropriately coordinated to allow this movement to occur.
These generators of star 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 protection of the spine encloses the general structure of the spinal cord. The spinal nerves are found in the spaces between the arches of the vertebrae.
The spinal nerves are divided into these separate regions:
- White matter and gray matter
The spinal cord is divided into gray matter (a butterfly shape) and white matter (the material surrounding 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. In contrast, 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 provides the cardiac and smooth muscle.
The spinal nerves allow the spinal cord and the rest of the body to communicate.
A nerve is a small card-shaped organ made up of several joined axons. There are 31 pairs of spinal nerves:
- Eight are cervical nerves located in the neck.
- Twelve are thoracic nerves situated in the thorax.
- Five are lumbar nerves located in the abdomen.
- Five are sacral nerves located in the pelvis.
- One is the coccygeal nerve situated in the coccyx.
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 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 reaction 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 a significant effect on the body’s sensory, motor, and reflex abilities 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 for paralysis have never been more promising.
The technology is proving to help communicate between the brain and limbs that have suffered nerve damage.
Research is advancing rapidly, and in just five years, we may have the means to reverse the most severe spinal cord injuries.
Cells of the central nervous system
Neurons connect to send and receive messages in the brain and spinal cord. Many neurons that work together are responsible for every decision, emotion, sensation, and action.
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 in sending and receiving 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 surrounding the lesion site, creating a glial scar that is a barrier to the regeneration of axons.
Microglia are immune cells in the brain. After a lesion, they migrate to the lesion site 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.
This cleansing process likely plays a vital role in recovering 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 100 meters per second. In contrast, threads without myelin can only take notes 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 the 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 impact the neighboring regions of the body.
Paralysis occurs when communication between the brain and the spinal cord fails. This may result from injury to brain neurons (a stroke) or the 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 with 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 similarly or to the same degree.
Each spinal cord segment receives sensory information from a particular body region.
Scientists have mapped these areas and determined each spinal cord level’s “receptive” fields. 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 brain region 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 from 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.
This sensory information is transmitted directly to the lower motor neurons before it reaches the brain, leading to involuntary movements or reflexes; the remaining 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 loss of sensation. No two injuries are the same.
How do the spinal cord and internal organs work together?
In addition to controlling voluntary movement, the central nervous system contains the sympathetic and parasympathetic pathways that control the “fight or flight” response to danger and regulate 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 immune system cells migrate to the injury site, 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 significantly hinders the conduction of the action potential, and 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, many basic circuits to control movement and process information may remain intact.