The brainstem isn’t the most impressive-looking part of the complex organ. However, without it, our bodies would be much like having an incredible robot that can’t access its software. But the brainstem isn’t as simple as an electrical cord and plug. It is divided into three parts, including the midbrain. Within the midbrain is the tectum. But what does this tiny section of our minds do?
The tectum (“roof” in Latin) is the rear section of the midbrain, composed of the superior and inferior colliculi. The first takes in information from the retina and visual cortex. The latter takes in input from auditory fibers. In short: the tectum is essential for visual and auditory processing.
The occipital lobe of the brain deals with vision. Whereas the auditory cortex are the areas that handle auditory information. But first, these signals pass through the brainstem. But the brainstem is more than a tunnel or post office for information. It potentially saves our lives in situations where we don’t have time to wait for the central brain to process the situation.
Tectum: A Small Part Of The Brainstem
The tectum is located in the brainstem, which connects the brain to the spinal cord. The average brainstem is three inches long and serves as an information highway, transferring signals from the brain to its body and back again. But the brainstem also plays an essential role in other areas, including:
- Facial movements
- Regulating heartbeat
- Regulating blood pressure
The brainstem is divided into three primary sections:
- Medulla oblongata (bottom section, connected to the spinal cord)
- Pons (the “bridge” and largest section)
- Midbrain (mesencephalon, top section, only 2cm long)
The tectum is found within the midbrain.
Tectum: The Roof Of The Midbrain
The midbrain (mesencephalon) is the top 2cm section of the brainstem and is divided into three parts:
- Cerebral peduncles (front of the midbrain, connecting brainstem to the thalami)
- Corpora quadrigemina (visual and auditory reflexes)
- Cerebral aqueduct (connects the fourth ventricle to the third)
The tectum sits behind the cerebral aqueduct, forming part of the “roof” to the midbrain. In the simplest terms, the tectum’s function is auditory and visual processing. Whereas the area in front of the cerebral aqueduct is the tegmentum, and its prominent roles are pain processing, alertness, and coordination.
Tectum: The Superior And Inferior Colliculi
The tectum is primarily distinguished by four bumps made up of clusters of neurons. These “bumps” are the superior and inferior colliculi.
The Superior Colliculi: Eye And Neck Reflexes
The superior colliculi are the top two of the “bumps,” sitting below the thalamus and above the inferior colliculi (the lower bumps.) These top two clusters of neurons are responsible for reflexes in the eyes and neck.
We use these reflexes all the time without consciously having to think about it. For example, if you are at an American football game and somebody throws a “Hail Mary,” you can follow the ball’s trajectory without conscious effort. You will track it as it leaves the quarterback and follows it until it is caught or hits the ground.
You also use this when reading a book or a webpage. Your eyes will effortlessly move along the text to take in the information. Your focus is on taking in the information and interpreting it, not on the physical action of moving your eyes word by word along the page or screen.
These reflexes can also help keep us safe. A flash of light to the side or a touch to the shoulder draws our attention to the surprise. The neck and eyes work together without us having to consciously consider it. These reflexive actions save us time and give us a chance to save ourselves by running away or fighting, if necessary.
The superior colliculi’s role in survival might be why it makes up a significantly large brain region in certain animals such as birds or fish, as appose to being a much smaller region in the human brain. The fast reflexes in reaction to input are not just necessary to stay out of harm but to successfully hunt or reach the food source the fastest.
However, this is a rudimentary explanation of the superior colliculi. Nor is its role fully understood. The superior colliculus comprises many layers that receive information from various sources.
These layers have been divided into two primary groups:
- Deep layers
The superficial layers primarily relate to what was discussed above, as the majority of information they receive is from the retina and visual cortex.
The deep layers are more complex and less understood. In addition to visual input, these deeper layers receive information from the auditory and somatosensory systems. The latter involves touch, such as pressure, temperature, pain, texture, and movement.
The Inferior Colliculi: Auditory Processing
The inferior colliculi are the other two “bumps” below the ones making up the superior colliculi. It plays a crucial role in hearing, gathering information from all the auditory pathways in the body and sending it on to the cerebral cortex. It’s like the “all roads lead to Rome,” but for hearing. Once the inferior colliculi have the information, it passes it to places such as the superior colliculus or thalamus.
The inferior colliculi are divided into the areas:
- Central nucleus
- Dorsal cortex
- External cortex
The knowledge of the dorsal and external cortex is somewhat lacking. Scientists know damaging them can impact our ability to process sound, but precisely how and why is unclear. However, there is a better grasp of the central nucleus. It is the receiver for the cochlea, the superior olivary nuclei, and other areas.
The central nucleus also contains neuronal fibers that link up with the thalamus. Within the thalamus is the medial geniculate nucleus, and it takes the information from the central nucleus and passes it on to the auditory cortex. It’s like having a postal system for your hearing.
But the inferior colliculus has a more significant role than being a sound post office, collecting and distributing auditory input. Neurons within it can determine the intensity of a sound and even differentiate the time it took the sound to enter one ear from the other.
At first, it might not seem a big deal that you can determine which ear picked up a sound first and by how much. But this information helps tell you where a sound is coming from. In addition, your eyes and neck respond to this input, which can be vital in a dangerous situation.
Consider a ball whizzing towards your head while half asleep in the outfield. The sound helps your eyes locate the ball you didn’t notice being hit by the batter. Since you didn’t see the hit, your eyes can’t track the ball. Thus, your auditory calculations help narrow down the possibilities of where the ball might be. Ideally, you’ll spot it and get the glove up in time to protect your face before contact.
It is this calculation that also saves people when they are around traffic. For example, you might not see the oncoming car when stepping off the curb, but those that can hear will be alerted to its presence so you can jump back.
The inferior colliculus also acts as a filter, prioritizing sounds. For example, when having a conversation with others at a dinner party, it can heighten the voices of the speakers while helping filter out sounds of breathing and chewing.
Damage to the inferior colliculus can result in deafness, even if all ear mechanisms work perfectly. This rare occurrence is a result of bilateral damage. But if the damage is unilateral, a person can still hear but will have difficulty processing where the sound is coming from.
However, damage to the tectum without damage to the rest of the brainstem, or at least the midbrain, is uncommon. Thus, the precise results of the tectum being harmed or having unusual development are better understood within a broader context.
Tegmentum: The Floor Of The Midbrain
The tegmentum is thought of as the floor of the midbrain. In Latin, it means “hood” or “cover.” Its prominent roles are in alertness, coordination, and pain processing. Unlike the tectum, the tegmentum extends beyond the midbrain, running down the brainstem. However, the portion of the tegmentum within the midbrain that gets the most attention is:
- Red nucleus
- Periaqueductal grey matter
- Reticular formation
Tegmentum: Red Nucleus
The red nucleus of the tegmentum gets its name from its pale pink color. Why they couldn’t name it the “pink nucleus” is a good question without a definite answer. It is believed the color comes from iron in the area, primarily hemoglobin and ferritin.
The red nucleus is divided into two main sections:
- Parvocellular red nucleus (medium-sized neurons)
- Magnocellular red nucleus (Large-sized neurons)
Parvocellular Red Nucleus: Two-Legged Mammals
Between the parvocellular red nucleus and the magnocellular red nucleus, the former is the dominant structure in the human brain. However, it is less understood than the magnocellular red nucleus as it is easier to study animal brains and neuro functions.
But scientists know that the parvocellular is connected to motor movement. Thus, damage to the area can cause tremors and is being investigated for its potential role in Parkinson’s disease.
Magnocellular Red Nucleus: Four-Legged Mammals
In four-legged animals, the magnocellular red nucleus is dominant, helping with walking and postural stability. It is also believed to be crucial in coordinating the four legs and avoiding obstacles. To do this, the magnocellular red nucleus produces neurons that travel out of the nucleus to the rubrospinal tract, which plays an important role in involuntary movement and fine motor control. From there, we reach the spinal cord.
The magnocellular red nucleus is an area in the human brain that plays a more significant role in newborns but regresses as we learn to walk on two legs. Yet, there does seem to be a continued role in how humans control their hands.
Red Nucleus: Pain And Analgesia
The red nucleus isn’t only essential for coordination. For example, it is believed to have a role in how we feel pain or don’t (analgesia). The precise role of the red nucleus in pain management isn’t understood, but it is thought to be due to its connection to the periaqueductal gray and raphe nuclei, the former of which is part of the tegmentum.
Tegmentum: Periaqueductal Gray Matter
The periaqueductal gray matter is often called the “analgesia center” for its role in inhibiting pain, but it is far from its sole purpose. It is located within a column of the brainstem, surrounding the cerebral aqueduct in the midbrain. It has neurons that connect it with the medulla oblongata, including the serotonin-producing neurons known as the raphe nuclei.
However, the periaqueductal gray matter has other roles still being explored. Some of these include:
- Regulation of heart rate
- Regulation of blood pressure
- Control and contraction of the bladder
The periaqueductal gray matter is also of significant interest to those studying PTSD. The midbrain contains many neurochemicals:
- Amino acid
- Opioid neurotransmitters
If the regulation of these changes, it can alter the functional connectivity of the midbrain, including the periaqueductal gray matter’s connectivity with the amygdala. The amygdala is one of the three primary brain regions that are impacted by PTSD suffers, along with the hippocampus and the medial prefrontal cortex.
Tegmentum: Reticular Formation
The reticular formation is found in the center of the tegmentum. Its net-like structure is the source of its name and is a highly intricate and complex section. It is crucial to many functions, but it is best known for its roles in consciousness (alertness) and arousal. These two functions are brought about by the reticular activating system (ascending arousal system).
The reticular activating system has circuits from various areas of the brainstem. Its neurotransmitters contain acetylcholine and norepinephrine, which are crucial in arousal and wakefulness. If pathways of this system are damaged through lesions, consciousness is impaired. The greater the damage, the more serve the consequences, including coma or a vegetative state.
However, this is a simplistic explanation of the reticular formation. As stated above, it is a highly complex area with many functions. Moreover, its interaction with other areas of the brain is also various and intricate.
The tectum is a part of the midbrain that makes up the top portion of the brainstem. The tegmentum makes up the “floor.” The full roles these parts of the midbrain serve is still being explored. However, we know the tectum is crucial to our auditory and visual processing; the tegmentum is necessary for coordination, pain processing, and alertness.