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Brain Stem: Definition, Structures and Functions

Brain Stem Definition

The midbrain, pons, and medulla oblongata comprise the brain stem (or brainstem), which connects the brain to the spinal cord. It has ascending channels for sensory information to be processed by the brain and descending pathways for motor instructions to be transmitted back to the body via the autonomic nervous system. Additionally, 10 of the 12 cranial nerves originate here. In terms of these functions, the brain stem is particularly noticeable in the cranial areas for circulatory and respiration control, pain and temperature regulation, sleep patterns, muscular activity, and sensory management.

Brain Stem Background

Consider the link between the brain stem and the nervous system before we look at each of the sub-structures and their activities within it.

The Nervous System, Neurons, and Brain

The nervous system is also a large network that runs throughout the whole body and is essential for life and control. It’s in charge of conveying sensory data from the body to the brain, which subsequently returns acceptable responses to the body.These reactions might be motor, physiological, or storage-related.

The nervous system is made up of single nerve cells (or neurons) that detect messages from the body and its surroundings. Electrical signalling allows the neurons to send these messages to their appropriate destinations in the brain practically immediately. A synapse occurs when one neuron sends a signal to the next nerve. In the gaps between two neurons, electrical impulses turn chemical before becoming electrical again at the next neuron.

The nervous system is physically separated into two subsystems: the central and peripheral nervous systems.Neurons make up the complete nervous system. The brain and spinal cord are considered to be part of the central nervous system, while the rest of the body’s neurons are considered to be part of the peripheral nervous system. The cerebrum, cerebellum, diencephalon, and brain stem are the four areas that make up the brain. While each area has its own distinct traits and functions in accordance with the rest of the body, multiple connecting routes and neurological connections run through different tissues.

The brain stem

The brain stem is a structure within the central nervous system that connects the brain to the spinal cord. As with the other components of the brain and nervous system, the brain stem is completely devoid of neurons. Depending on which component of the neuron is being emphasised, these neurons are classified as fibres, axons, or nuclei.

The whole brain stem is not a single structure. The midbrain, pons, and medulla oblongata are the three primary brain stem structures. Each of these areas has notable sub-structures and jobs that are centred within each region and overlap between them. These areas are also where the beginnings of various cranial nerves are found.

The autonomic nervous system is highly dependent on the brain stem and its associated components. The brain stem, in particular, is involved in the circulatory, pulmonary, and digestive systems, as well as other bodily functions that occur involuntarily.

The brain stem is crucial for life, and loss of neuronal attachment is exceedingly dangerous. In medical terms, death of the brain stem is defined as an “irreversible loss” of consciousness and the capacity to breathe. The brain stem ceases to function after brain death, but projections in the cortex may still exist. The organism, on the other hand, dies biologically when both the cortical and brain stem projections are eliminated. When cardiopulmonary activity stops, true death occurs. Following brain stem death, ventilators may be used to keep the heart pumping and oxygen flowing, but there is no genuine cure.

The autonomic nervous system

The somatic nervous system and the autonomic nervous system are the two primary functioning nerve systems in the body. The somatic nervous system is responsible for the regulation and execution of voluntary responses throughout the body. These are the reactions to which the awareness is especially sensitive (for example, lifting an arm to hold a drink or kicking your legs to practise a dance routine).As a consequence, skeletal muscles are often targeted by the somatic nervous system.

Voluntary movement, on the other hand, is not the sole activity that takes place in the body. The body also goes through a lot of undirectional movements, which are movements that you aren’t aware of. Heartbeat, which is regulated by cardiac muscle, and digestion, which is managed by smooth muscle, are examples of such movements. Glands and glandular functions are included in this category. The autonomic nervous system is in charge of all of these processes.

The sympathetic and parasympathetic nervous systems are the two sub-categories of the autonomic nervous system. These two sub-systems, which are portions of the autonomic nervous system, also govern areas of the body that move involuntarily. The sympathetic nervous system (sometimes known as the “fight or flight reaction”) helps the body adapt to challenging conditions. This includes an elevated heart rate, higher glucose release into the blood, and slower digestion. The parasympathetic system (referred to as the “rest and digest response”) enables the body to produce and store energy. This can be accomplished in two ways: by lowering the heart rate and by enhancing digestion.

The brain stem controls the majority of the autonomic nervous system, which comprises both the sympathetic and parasympathetic nervous systems.

Brain Stem Structures and Functions

There are ascending and descending routes in the brain stem. Ascending pathways process sensory information, and descending pathways elicit motor responses in accordance with sensory information.

As previously stated, the brain stem is made up of three key structures: the midbrain, pons, and medulla oblongata. The tectum, tegmentum, and basis are the three areas that these structures are divided into (from posterior to anterior). In general, the tectum has specific sensory and movement functions. In the meantime, the tegmentum houses the cranial nuclei, reticular formation, and connections between the brain stem and other regions of the brain. Finally, fibres from the cerebral cortex’s descending pathways are seen in the basis. The next parts of this essay will go over particular structures found in each of these three areas in further depth.

Midbrain Overview

The midbrain is a small component of the brain stem and is located at the top. It is placed between the cerebellum, cerebrum, diencephalon, and pons. The general role of the midbrain, which is part of the brain stem, is to govern sensory and motor pathways. It is necessary for the transmission of nerve impulses between the spinal cord and the rest of the brain, and vice versa. While the midbrain governs or aids in a variety of physical activities, among its most prominent functions are visual and auditory perception, the internal reward system, and a variety of muscle movements. The roots of cranial nerves III and VI are also found in the midbrain.

The cerebral aqueduct is located in the middle of the midbrain and serves as a conduit for cerebrospinal fluid to flow between the third and fourth ventricles. The cerebral aqueduct divides the anterior and posterior tectum and tegmentum sectors of the midbrain, accordingly.

Midbrain Regions and Functions

The corpora quadrigemina, a pair of protrusions on the midbrain’s posterior (the tectum), is located in the midbrain’s posterior (the tectum). The superior colliculi, which house the visual reflex centre, and the inferior colliculi, which house the auditory relay centre, make up this pair.

The anterior region of the midbrain (known as the tegmentum) is responsible for a variety of functions and components.

The Reticular Formation: The reticular formation’s most significant function is to filter sensory information given to the brain, enabling the conscious to focus on the most critical sensations. As a consequence, this area is critical for sustaining general awareness and concentration. The reticular formation also plays a role in circulatory systemic control, respiratory regulation, and awareness throughout the waking and sleeping cycles. It also includes networks for regulating mood and pain. All of these networks originate in the brain stem and branch out across the brain, with ascending routes leading to the thalamus and cortex and descending pathways leading to the spinal cord.

The Red Nucleus: Located in the midbrain, the red nucleus is a sub-part of the reticular formation. It’s crucial for motor control, especially when it comes to synchronising autonomic action between swinging arms and walking legs. This trait is critical for maintaining equilibrium. 

The Periaqueductal Gray Regions (or PAG): The periaqueductal grey areas (or PAG) are another structure in the midbrain that regulates pain. Neurotransmitters produced by neurons in the PAG, such as dynorphin and serotonin, help to control pain. In addition to avoiding unpleasant circumstances and limiting risky actions, this area plays a vital part in the overall autonomic system for survival.

Substantia Nigra: Dopamine-producing neurons in the substantia nigra help with motor control (alongside the basal ganglia, another brain structure). The thalamic motor activity is inhibited by sections of this region. In Parkinson’s disease sufferers, this area declines early on.

In the ventral segmental region, dopamine-producing neurons may also be found in the ventral segmental area. Unlike the substantia nigra, this region is in charge of the brain’s reward system. All of these factors are rewarded, including motivational salience, associative learning, and positive feelings. Additionally, this is the place of orgasms. The ventral segmental area’s neurons project to areas of the brain associated with awareness and sleep.

Overview and Functions of Pons

The pons, which is positioned between the midbrain and the medulla oblongata, is the brain stem’s central structure. It has longitudinal fibres that link to higher centres throughout the brain and spinal cord, as well as transversal and dorsal fibres that transmit information between the cortex and the cerebellum’s motor working areas. The pons also houses the origins of cranial nerves V, VI, VII, and VIII. 

Functions associated with the various cranial nerves originating from this component are among the pons’ most notable functions. Recognizing feelings in the head and facial regions, as well as movement of the face, eyes, ears, and lips, is part of this. In addition to maintaining homeostasis, the pons is vital for autonomic activities such as salvia generation. The pons, like the midbrain, includes a portion of the reticular formation, which is involved in cardiovascular regulation and breathing rhythm.

Gliomas (glial tumours) may develop anywhere in the brain stem, which is why they’re called “brain stem gliomas.” Gliomas in the midbrain and medulla oblongata, on the other hand, are usually low-grade. Gliomas in the pons develop quickly for unclear reasons and are considered high-grade tumours.

Medulla Oblongata Overview

The medulla oblongata (also known as the medulla) is the brain stem’s bottommost part. There is no distinction between the brain stem and the spinal cord since the medulla joins them directly. A portion of this tissue forms the bottom of the fourth ventricle. It is vital for conveying descending motor control and ascending sensory input, much like the other brain stem areas.

The medulla, on the other hand, is best known for its control of the body’s important functions, such as maintaining optimal circulatory and respiratory regulation. It’s also known for triggering a variety of responses, including vomiting, coughing, and sneezing. Many of the medulla’s tasks overlap with those of the hypothalamus, and the medulla is often the tissue via which the hypothalamus sends instructions. The medulla also houses the origins of cranial nerves IX, X, XI, and XII.

The tectum, tegmentum, and basis divide the medulla into three portions, much like the other brain stem structures. The inferior olivary nucleus is located in the tectum, while the medulla cranial nerves are located in the tegmentum. The pyramid’s decussation is found in the foundation.

Medulla Oblongata: Regions and Functions

The medulla, like the midbrain and pons, includes a portion of the reticular formation. Within this midbrain structure, the cardiovascular and respiratory systems are linked as a unified system. At the medulla, afferent cardiorespiratory signals synapse and act to control respiration. The medulla also houses the ventral respiratory columnist, which regulates breathing rhythm and oscillations. The medulla also serves as a vasomotor centre. This is due to the fact that neurons present may trigger changes in blood vessel width while monitoring baseline arterial pressure.

The medulla’s solitary tract nucleus is arranged by the kind of information being communicated as well as active pathways in response to the information. It primarily coordinates afferent data. This nucleus houses the medulla’s most critical activities, including input from baroreceptors and chemoreceptors. To modify heart rate and blood flow, blood vessel baroreceptors provide information to the nucleus of the solitary tract.

Chemoreceptors in blood vessels detect oxygen and carbon dioxide levels, helping the solitary tract’s nucleus to maintain normal breathing. Taste synapses are also processed initially in this nucleus before being transported to the thalamus and brain for sensory processing.

The Area Postrema: The area postrema is found on the dorsal surface of the medulla. The cells in this area, which serve as the vomiting centre, lack a blood-brain barrier. As a result, big and polar molecules may pass through. Innervation in this area might make a person feel sick. According to research, the human chorionic gonadotropic (hCG) hormone, which is a pregnancy hormone, has receptors in this location. This might point to a reason for pregnant women’s greater sensitivity to morning sickness.

The spinal trigeminal nucleus is involved in the perception of temperature discomfort and deep ipsilateral (or same-side) facial touches. It is the initial site of synapse formation for orofacial pain (a broad term for head and neck pain) nerves.

The inferior olivary nuclei are involved in the processing of proprioception (knowledge of one’s own body’s location and motion), muscle and joint tension, and motor intention. The nuclei in this region form direct connections with the cerebellum, which controls skeletal mobility and balance. Additionally, these nuclei are responsible for swallowing, sneezing, and coughing.

The Pyramidal Decussation: The pyramidal decussation includes the majority of motor fibres from the motor cortex. It creates the lateral corticospinal tract in the spinal cord. At this stage, the pyramidal tracts cross over one another to connect to the opposing sides of the body. The benefit of this crossing over as a common characteristic in vertebrates is still being debated by evolutionary scientists.

The Cuneate and Gracious Nuclei: The cuneate nucleus of the medulla receives information from the upper extremities, whereas the gracious nucleus receives information from the lower extremities. The medial lemniscus is a pathway that connects the thalamus to proprioception, vibration, and information about delicate touch.

The Spinothalamic Tract: The spinothalamic tract is an ascending channel that runs from the spinal cord to the brain, carrying pain, temperature, and rudimentary touch. The fibre eventually comes to a stop at the thalamus’s ventral posterior lateral nucleus. The anterior tract carries pain and temperature, whereas the lateral tract carries crude touch.

Cranial Nerves of the Brain Stem

Ten of the twelve cranial nerves have their roots in the brain stem, as discussed earlier in this article. Nerves III and IV come from the midbrain, while nerves V, VI, VII, and VIII come from the pons, and nerves IX, X, XI, and XII come from the medulla. These nerves may have sensory, motor, or a combination of these functions. The next sections will discuss the overall function of each brain stem cranial nerve.

Only the olfactory (I) and visual (II) nerves originate in the brain stem. Rather than that, these nerves originate in the cerebrum.

Nerves Originating in the Midbrain:

The oculomotor motor nerve (III) controls pupils and ocular movements.

The trochlear nerve (IV): The trochlear nerve controls the superior oblique muscle. This muscle is located in the upper and medial portions of the obit (the muscle that surrounds the eye). It permits the eye to abduct, depress, and internally spin.

Nerves Originating in the Pons

The trigeminal nerve (V) supplies sensory and motor functions to the jaw and adjacent mastication muscles. Numerous regions of the face, including the orbital structures, nasal cavity, forehead skin, brows, eye lids, nostrils, lips, gums, teeth, skeet, palate, and pharynx, all have sensory functions.

The abducens nerve (VI) controls the lateral rectus muscle, one of six muscles that mediate eye movement.

On the first two-thirds of the tongue, the facial nerve (VII) is responsible for sensory processes such as taste. Additionally, it houses the motor activities of the muscles that control facial expressions, the lacrimal glands (which convey fluids to lubricate the eye), and the submandibular and sublingual salivary glands (which are two of the three major glands that aid in saliva production within the mouth).

Vestibulocochlear Nerve (VIII): The vestibulocochlear nerve transmits sensory information to the cochlea for hearing and to the vestibule for motion and balance.

Nerves Originating in the Medulla Oblongata

The glossopharyngeal nerve (IX) transmits sensory information to the back third of the tongue, the pharynx, and palate, as well as blood pressure, pH, oxygen, and carbon dioxide levels. Additionally, it regulates the motor activity of the pharyngeal muscles and parotid salivary gland (the final remaining major gland for saliva production in the mouth).

The auricle and external acoustic canal, as well as the diaphragm and visceral organs of the thoracic cavity, receive sensory input from the vagus nerve (X). It also controls the palate and pharynx, as well as the thoracic visceral organs.

The accessory nerve (XI) controls the skeletal muscles of the palate, throat, and larynx as well as the sternocleidomastoid and trapezius muscles in the neck and spine [in collaboration with the vagus nerve (X)].

The hypoglossal nerve (XII) has motor activities for controlling tongue musculature.

Conclusion

The brain stem is the structure that connects the brain to the spinal cord in the central nervous system. The midbrain, pons, and medulla oblongata comprise the brain stem, which plays a range of tasks in the autonomic nervous system, including circulatory and respiratory regulation, as well as sensory and motor activity. Furthermore, the brain stem contains the precursors to ten of the twelve cranial nerves, including nerves III–XII.

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