THE ADVANCED GUIDE TO BRAINSTEM

The brainstem is the lower part of the brain that connects the spinal cord to the rest of the brain. It is made up of three main parts: the medulla oblongata, the pons, and the midbrain. The gross features of the brainstem include:

• Medulla Oblongata: It is the most inferior part of the brainstem, located just above the spinal cord. It controls vital functions such as breathing, heart rate, and blood pressure.
• Pons: It lies above the medulla and acts as a bridge between the medulla and the rest of the brain. It is involved in regulating breathing, sleep, and arousal.
• Midbrain: It is the uppermost part of the brainstem and is responsible for relaying sensory and motor information between the brain and spinal cord. It also plays a role in controlling eye movements and coordination.

Other notable features of the brainstem include the reticular formation, which is a network of neurons that controls arousal and attention, and the cranial nerve nuclei, which are clusters of neurons that control various functions such as facial expression, eye movement, and hearing.

Internal structure of the brainstems

The brainstem is a crucial region of the brain that connects the spinal cord to the brain and is involved in several important functions such as breathing, heart rate, consciousness, and movement.

1. Ascending pathways: These are the sensory pathways that carry information from the body to the brain. The main ascending pathway in the brainstem is the spinothalamic tract, which transmits pain, temperature, and crude touch sensations.
2. Descending pathways: These are the motor pathways that carry information from the brain to the body. The main descending pathways in the brainstem are the corticospinal tracts, which control voluntary movement.
3. Sensory and motor cranial nuclei: These are clusters of nerve cells in the brainstem that are responsible for processing sensory and motor information from the head and neck. The sensory nuclei include the trigeminal nucleus (which receives information from the face), the vestibular nuclei (which receive information from the inner ear), and the cochlear nuclei (which receive information from the ear). The motor nuclei include the oculomotor, trochlear, and abducens nuclei (which control eye movement) and the facial, glossopharyngeal, vagus, and accessory nuclei (which control facial expression, swallowing, and speaking).
4. Substantia nigra: This is a small structure located in the midbrain that is involved in the production of dopamine, a neurotransmitter that is important for movement control. Damage to the substantia nigra is associated with Parkinson’s disease.
5. Red nucleus: This is a structure located in the midbrain that is involved in motor control, particularly the control of arm and leg movements.
6. Olivary nucleus: This is a structure located in the medulla oblongata that is involved in the control of balance and coordination.
7. Reticular formation: This is a diffuse network of nerve cells that runs through the brainstem and is involved in a variety of functions, including arousal, attention, and sleep. It also plays a role in the regulation of vital functions such as breathing and heart rate.

The sensory cranial nuclei are a group of nuclei located in the brainstem that receive and process sensory information from various regions of the head and neck. There are four main sensory cranial nerves: the trigeminal nerve (CN V), the facial nerve (CN VII), the glossopharyngeal nerve (CN IX), and the vagus nerve (CN X). These nerves are responsible for carrying sensory information related to touch, pain, temperature, taste, and smell.

The main connections of the sensory cranial nuclei are as follows:

• The trigeminal nerve (CN V) connects to the trigeminal sensory nucleus, which is located in the pons. This nucleus receives sensory information from the face, mouth, and nasal cavity. The nucleus is divided into three parts, each corresponding to one of the three divisions of the trigeminal nerve: the ophthalmic (V1), maxillary (V2), and mandibular (V3) divisions.
• The facial nerve (CN VII) connects to the solitary nucleus, which is located in the medulla. This nucleus receives sensory information from the taste buds on the anterior two-thirds of the tongue, as well as from the soft palate and the pharynx.
• The glossopharyngeal nerve (CN IX) connects to the solitary nucleus, which also receives sensory information from the posterior one-third of the tongue, as well as from the tonsils, the pharynx, and the carotid body.
• The vagus nerve (CN X) connects to the solitary nucleus, as well as to the spinal trigeminal nucleus, which is also located in the medulla. The vagus nerve carries sensory information from the larynx, trachea, esophagus, and thoracic and abdominal viscera.

Overall, the sensory cranial nuclei are essential for processing sensory information from the head and neck, and their connections to other brain regions allow for the integration of this information with other sensory and motor functions.

The motor cranial nuclei are a collection of 12 nuclei located in the brainstem that control the voluntary and reflex movements of the head and neck. These nuclei are connected to various parts of the central nervous system, including the cerebral cortex, basal ganglia, cerebellum, and spinal cord, through a network of white matter tracts.

The main connections of the motor cranial nuclei can be summarized as follows:

• Cerebral cortex: The cortical motor areas in the frontal lobe project to the motor cranial nuclei via the corticobulbar tract. These connections are responsible for initiating and controlling voluntary movements of the head and neck.
• Basal ganglia: The motor cranial nuclei receive input from the basal ganglia, which are involved in motor planning and control. The basal ganglia project to the motor nuclei via the striatonigral and striatopallidal pathways.
• Cerebellum: The cerebellum plays a crucial role in motor coordination and balance. The motor cranial nuclei receive input from the cerebellum via the cerebellopontine tract, which helps to adjust and fine-tune movements of the head and neck.
• Spinal cord: The motor cranial nuclei are also connected to the spinal cord through the corticospinal tract, which allows for the execution of voluntary movements, and the vestibulospinal tract, which controls reflex movements of the head and neck in response to changes in body position and balance.

Overall, the motor cranial nuclei receive input from various parts of the central nervous system and are responsible for coordinating voluntary and reflex movements of the head and neck.

The brainstem receives its blood supply from several arteries that arise from the base of the brain.

1. Vertebral arteries: The vertebral arteries are the first arteries to supply the brainstem. They arise from the subclavian arteries and enter the skull through the foramen magnum. They then merge to form the basilar artery, which supplies the pons and midbrain.
2. Basilar artery: The basilar artery is formed by the merging of the vertebral arteries. It travels along the midline of the brainstem, supplying blood to the pons and midbrain.
3. Posterior cerebral arteries: The posterior cerebral arteries arise from the basilar artery and supply the midbrain and parts of the pons.
4. Anterior inferior cerebellar arteries: The anterior inferior cerebellar arteries arise from the basilar artery and supply the lateral part of the pons.
5. Superior cerebellar arteries: The superior cerebellar arteries arise from the basilar artery and supply the midbrain and the superior cerebellum.
6. Pontine arteries: The pontine arteries arise from the basilar artery and supply the pons.

The blood supply to the brainstem is crucial, and any interruption of blood flow can lead to severe consequences such as brainstem stroke, which can result in death or significant disability.

Lesions in the brainstem can cause a wide range of neurological symptoms, depending on the location and severity of the damage. The brainstem is responsible for many vital functions, including regulating breathing, heart rate, blood pressure, and consciousness, as well as relaying sensory and motor signals between the brain and the rest of the body.

Medial medullary syndrome and lateral medullary syndrome are two distinct types of brainstem lesions that can produce different clinical presentations.

Medial medullary syndrome, also known as Dejerine syndrome, is caused by a lesion in the medial part of the medulla oblongata, which is the lower part of the brainstem. This area contains the corticospinal tracts, which are responsible for motor function, and the medial lemniscus, which transmits sensory information from the body to the brain. The most common cause of this syndrome is occlusion of the anterior spinal artery, which supplies blood to this region.

The classic triad of symptoms in medial medullary syndrome includes contralateral hemiplegia (paralysis of one side of the body), contralateral loss of proprioception (the ability to sense the position and movement of body parts), and ipsilateral tongue deviation (the tongue points towards the side of the lesion). Other possible symptoms may include dysarthria (difficulty speaking), dysphagia (difficulty swallowing), and dysphonia (hoarse voice).

Lateral medullary syndrome, also known as Wallenberg syndrome, is caused by a lesion in the lateral part of the medulla oblongata, typically involving the posterior inferior cerebellar artery (PICA). This area contains several important structures, including the vestibular nuclei (which control balance and eye movements), the nucleus ambiguus (which controls swallowing and speaking), and the spinothalamic tracts (which transmit pain and temperature sensations).

The most common symptoms of lateral medullary syndrome include vertigo (a spinning sensation), nystagmus (involuntary eye movements), ataxia (uncoordinated movements), ipsilateral facial pain and temperature loss, and contralateral body pain and temperature loss. Other possible symptoms may include dysphagia, hoarseness, and ipsilateral Horner syndrome (a combination of ptosis, miosis, and anhidrosis).

Overall, brainstem lesions such as medial medullary syndrome and lateral medullary syndrome can produce a wide range of neurological symptoms that can have a significant impact on a person’s quality of life. Prompt diagnosis and treatment are important to prevent further damage and to manage symptoms effectively.

The substantia nigra and the red nucleus are two structures located in the midbrain that are involved in the control of movement. The substantia nigra is a region of the brain that produces dopamine, a neurotransmitter that plays a key role in regulating movement, while the red nucleus is involved in the coordination of limb movements.

The main connections between the substantia nigra and the red nucleus are through a group of nerve fibers known as the nigro-rubral pathway. This pathway originates in the substantia nigra and terminates in the red nucleus. The nigro-rubral pathway is a part of the basal ganglia circuit, which is responsible for controlling movement.

The neurons in the substantia nigra that project to the red nucleus are known as dopaminergic neurons. These neurons release dopamine, which acts on receptors in the red nucleus to modulate its activity. The exact nature of the connection between the substantia nigra and the red nucleus is complex and involves multiple neurotransmitters, including glutamate and GABA.

Overall, the connection between the substantia nigra and the red nucleus is essential for the proper control of movement. Dysfunction of this circuitry can lead to movement disorders such as Parkinson’s disease, which is characterized by the degeneration of dopaminergic neurons in the substantia nigra.

The reticular formation (RF) is a complex network of neurons located in the brainstem, consisting of several nuclei and pathways that are involved in a variety of functions, including sleep, arousal, attention, and motor control.

The RF is connected to many other areas of the brain, including the thalamus, cortex, basal ganglia, and cerebellum. Some of the main connections of the RF in the brainstem include:

• Thalamus: The RF has direct connections with the thalamus, which is involved in relaying sensory information to the cortex. The RF plays a key role in regulating the level of arousal and attention, which can influence the processing of sensory information.
• Cortex: The RF has extensive connections with the cortex, particularly the prefrontal cortex, which is involved in executive functions such as decision-making, planning, and working memory. The RF also plays a role in regulating the level of arousal and attention, which can impact cognitive processes.
• Basal Ganglia: The RF has connections with the basal ganglia, which are involved in motor control and habit formation. The RF plays a role in modulating the activity of the basal ganglia, which can influence movement and behavior.
• Cerebellum: The RF has connections with the cerebellum, which is involved in motor coordination and balance. The RF plays a role in modulating the activity of the cerebellum, which can impact motor control.

Overall, the main functions of the RF are to regulate the level of arousal and attention, modulate sensory processing, and control motor behavior. The specific functions of the RF are complex and multifaceted, and involve interactions with many other areas of the brain.