The human brain is a superb, complex organ. The brain consists of the cerebrum, cerebellum, and brainstem. Each region deals with different functions, but all work together to promote functionality within the organism. The spinal cord connects to the cerebrum via the brainstem, and this is where we find the Anterior Horn, but how much do we know about this essential part of the spinal cord?
The spinal cord's anterior horn is essential for striated skeletal muscles' movement. The primary purpose of the anterior horn is to send impulses through the motor neurons located along its length to the appropriate muscles. There are several diseases related to the anterior horn.
Although we'll focus on the anterior horn's function, location, operation, and complications, it is important to remember that the brain and spinal cord do not act in isolation but as a communication and information processing network.
The anterior horn of the spinal cord, also called the anterior gray column, anterior horn, anterior cornu, and ventral horn, is an essential part of the human brain and central nervous system.
To fully appreciate what the anterior horn is responsible for in the brain, we need to investigate where it’s located, what it connects to, and how it works.
If you were to make a cross-section of the spinal cord before it enters the brain, you would find the anterior horn is the "front" facing a column of gray brain matter. Together, the anterior and posterior horns make an "H-shape," with the anterior horns forming the bottom half of the “H.”
A simple way to identify where the anterior horn of the spinal cord (anterior horn) is, is to break up the name.
We know it’s part of the spinal cord (due to that part of the name). Anterior means nearer to the front side of the body (also towards the head).
A “horn,” when dealing with the brain," is when something branches off to make a point from the main body. Similar to how the horns of an antelope start on the head and then "branch off," making an almost "C-shape."
The anterior horn runs the length of the spinal cord and varies in size according to the central cavity that houses the spinal cord.
The anterior horn connects to the cerebral cortex via the pyramidal tract.
The spinal cord's anterior (and posterior) horn is composed of gray matter. “Gray” matter has a gray coloration due to the presence of neuron cell bodies. The anterior horn houses motor neuron cell bodies (specifically alpha motor neurons) along its length.
Cells located within this area of the spinal cord are also called anterior horn cells. Although they occur throughout the anterior horn, the largest concentrations of these cells are in the cervical and lumbosacral regions (where the anterior horn expands).
This “enlargement” of the anterior horn of the spinal cord correlates with the increased innervation (supplying nerves to the area in question) of skeletal muscles. I.e., the "thicker" areas of the anterior horn are those areas that need to supply a greater amount of nerves to the adjacent muscles.
Scientists group the anterior horn cells into medial and lateral cell divisions. Those cells in the medial group are further divided into ventromedial and dorsomedial, while the lateral cell split into the ventrolateral and ventrodorsal groupings.
The anterior horn connects to the ventral roots, which then connect to the surrounding muscles.
The primary responsibility of the anterior horn of the spinal cord is to supply the skeletal (striated) muscles with nerves (innervate), which send signals to the muscles to begin moving or to cease movement.
Cells within the ventromedial clusters are responsible for innervating the large superficial muscles, while the smaller deep muscles (close to the spine) are innervated through the dorsomedial cells.
The ventrolateral cells innervate extensor muscles (those muscles which straighten limbs and increase the angle between bones around a joint), and the ventrodorsal cells innervate the flexor muscles (those muscles which cause limbs to bend, reducing the angle of the bones around a joint).
However, there are additional neurons within the anterior horn. These neurons facilitate the local circuit (a connection of various neurons which function together to send messages through the tissue). Other neurons send sensory information regarding pain and temperature to the brain.
The anterior horn is also responsible for regulating muscle spindle sensitivity through the presence of gamma motor neuron cell bodies that are embedded in the horn.
The spinal cord is a highway for information (chemical and electrical messages) to pass to and from the brain to the skeletal muscles along the body. Continuing with the highway analogy, the posterior horn "drives" sensory impulses, and information to the brain.
Once the information arrives at the cerebral cortex, the brain processes the information and then sends a response signal down the anterior horn (traffic moving in the opposite direction).
Once the message reaches the intended muscle group, it travels from the anterior horn through the ventral roots along nerves connecting to the muscles (taking an off-ramp from the highway and driving into your neighborhood and eventually home).
For example, if you place your trip and fall, the message moves from the sensory nerves, along the posterior horn, to your brain. The cerebral cortex instructs your body to put your hands up to brace for the impact by sending a message down the anterior horn of the spinal cord.
The anterior horn cells (also known as the Alpha motor neurons) bodies are located within the anterior horn. The brain activates the upper motor neurons, which relay the message to interneurons and lower motor neurons.
Once the lower motor neurons receive the message, the impulse moves from their bodies through their axons (which comprise the white matter of the spinal cord) and to the muscles. The longest motor neurons stretch from the lower spinal cord (in the lumbar and sacral region) to the toes.
Like most regions of our bodies, the anterior horn of the spinal cord is not barred from injury or diseases. Below we’ll investigate some of the prominent complications the anterior horn experiences.
Many brain diseases affect the motor neurons in the anterior horn. Some are genetic, while an infection or virus causes others.
There are 16 types of senile plaques that affect the brain. Senile plaque build-up in the gray matter leads to various diseases. The plaque also inhibits the individual's gross and fine motor skills.
The plaque reduces the functionality of the gray matter as it replaces the gray matter.
The reduced functionality of the neurons leads to a lack of dopamine secretions, which is the cause of loss of fine motor skills.
Although it is a hypothesis, researchers believe that one of the causes of Alzheimer's Disease is plaque build-up. One of the plaques that develop in the gray matter is amyloid-beta. Scientists believe that the glycoprotein, amyloid precursor protein, breaks down.
The amyloid precursor protein is essential for activities including intracellular transport, signaling, and overall neural homeostasis.
Once broken down, the free peptides bind to the gray matter, causing plaque build-up.
As this plaque builds, Alzheimer’s disease develops. The most common form of dementia.
Symptoms of Alzheimer’s Disease include:
- A loss of cognitive function (reasoning and thinking).
- Memory loss.
- Behavioral changes (depression, mood swings, delusions, and wandering).
- In later stages, motor coordination is impaired.
As the disease progresses, brain cells die (through atrophy), and the patient deteriorates until simple functions are no longer possible.
Many of us know this disease as polio. In acute poliomyelitis, the poliovirus attacks the motor neurons in the spinal cord and brainstem. This attack results in asymmetric weakening of the muscles and muscular atrophy.
As the motor neurons die, the muscles they connect to are no longer innervated, which causes their degradation.
The polio virus resides in the throat and intestinal tract of the infected individual.
Symptoms of polio infection are rare. Of the 25% that show symptoms, those include:
- Stomach pain
- Sore throat
Symptoms usually present between 2 and 5 days.
Serious symptoms may also develop in 0.5% of cases, including:
Those who develop paralysis are in danger of permanent damage or death (up to 10% of paralysis cases result in death).
Amyotrophic Lateral Sclerosis, ALS, is a degenerative disease that attacks the brain and spinal cord's motor neurons (and other) cells. The patient loses control over their muscles as the disease progresses.
Over time the respiratory muscles no longer function correctly, and the individual dies.
Symptoms of ALS include:
- Muscle weakness
- Muscle twitches in limbs
- Slurred speech
- Loss of mobility
- Muscle cramps
- Behavioral changes
- Cognitive changes
There is no cure for ALS, and management of the symptoms is difficult. However, no pain is usually associated with ALS (especially in the early stages).
ALS is one of the most commonly associated motor neuron diseases.
Spinal and bulbar muscular atrophy (SBMA), another name for Kennedy’s Disease, is a disease that affects the lower motor neurons. As the disease progresses, the affected neurons degenerate, causing muscle weakness and atrophy.
This disease is gender-specific to males and often results in:
- Atrophy of the testicles.
- Gynecomastia (an increase in the breast gland tissues).
- Reduced fertility.
Some symptoms include:
- Muscle cramps.
- Frequent falls.
- Muscle weakness and some tremors.
- Some speech and swallowing issues.
Respiratory complications (aspiration pneumonia) may develop in some individuals, but this is usually rare. Once these issues develop, they could prove fatal.
Charcot-Marie-Tooth Disease (CMT) are diseases that attack (particularly) the periphery nerves. As the disease progresses, the individual loses muscle strength in the extremities. Loss of sensation, muscle contractions, and mobility issues are also common.
This disease often leads to foot abnormalities, like high arches and hammertoes.
Other symptoms include:
- Loss of muscle mass in the legs, ankles, and feet, which become weak.
- Frequent tripping and falling.
CMT stems from genetic mutations, and it is usually an inherited genetic condition. The mutations often damage nerve cells or the myelin sheath around the nerves. Once damaged, the nerves’ efficacy in relaying messages reduces.
In some cases, the muscles controlling breathing, speaking, and swallowing are affected by CMT.
GBS is a rare disease where your body’s immune system attacks the nerve cells (including the myelin sheath). As the nerve cells die, the efficacy of communication between the brain and other peripheral nerves is hindered.
- Weakness in your extremities.
- A tingling sensation in your hands.
As the symptoms spread, your body goes into a paralyzed state. In severe cases, hospitalization is required.
Although the causes of this disease are unknown, many scientists associate the preceding infection with being the catalyst for the disease.
Aside from disease, some external forces impact the effectiveness of the anterior horn of the spinal cord. The most prevalent force is trauma.
Gray matter in the brain requires sufficient oxygen for optimal functioning. When the oxygen supply is interrupted, motor neurons begin to die. In severe cases, this loss of motor neurons leads to irreparable brain damage.
Although the severity of brain damage varies, the consequence is usually a loss of motor function.
Trauma (blunt force) often causes inflammation, which restricts blood flow to the gray matter, depriving it of oxygen.
Trauma also has a direct impact on the anterior horn cells. When a blunt force strikes the anterior horn cells, it often leads to intracerebral hemorrhaging and destruction of the cells.
Cells within the lateral cell division are relatively closely grouped, and injuries to the lateral cell divisions of the anterior horn cause widespread damage (impacting multiple types of cells), resulting in extensive weakness.
Although many of the diseases associated with the anterior horn are incurable, there are various methods to treat and manage the symptoms of these diseases, including:
The anterior horn's chemical treatment usually depends on the disease and symptoms.
- For ALS, doctors prescribe riluzole to try and slow down the disease.
- Doctors often prescribe medication with a spasmolytic agent to treat muscle spasms.
- In traumatic situations where your spinal cord might be injured and you develop neurogenic shock, doctors might use Norepinephrine.
Many hereditary genetic diseases and viruses do not have a direct treatment, so managing the symptoms and maintaining the patient's health and quality of life are the only options.
The focus behind surgeries is to restore movement to damaged limbs.
In a 2019 study, researchers attempted to prove how effective nerve transposition was in restoring reinnervation to damaged areas.
For example, in a traumatic injury, the median nerve and peripheral nerves leading to a limb are often damaged beyond repair. “Replacing” the nerve with an artificial nerve or another nerve from the body might not work when severe damage occurs.
In these cases, doctors suggest nerve translocation repair. The surgeon selects a suitable, healthy nerve nearby. This nerve needs to have a solid connection to the central nervous system (CNS). Once selected, the surgeon connects the nerve to the affected area.
Results show that the nerve innervates the area, and connection with the CNS is restored, allowing for (sometimes limited) functioning of the injured limb. During these procedures, surgeons also noted a reduced risk of complications developing after surgery.
Although this method does not directly affect the anterior horn, the periphery nerves branch from the horn, and the motor signals that the anterior horn carries to the muscles can now complete their course, enabling movement of the injured area.
The anterior horn of the spinal cord is a vital highway of information that connects the brain to the peripheral nervous system. Motor neurons' bodies lie along the length of the anterior horn, giving it its "gray" appearance. These cells innervate the striated muscles of the skeleton to move as required. Conditions affecting the anterior horn are often genetic, virus-based, or traumatic and are treated chemically or surgically.