1. What Does the Brainstem Do?

The brainstem serves as a communication point between the body, the spinal cord, and the brain through ascending and descending sensory and motor pathways [1].

Ascending pathways are groups of nerve fibers that bring sensory and motor information from the body (peripheral nervous system) to the brain through the spinal cord, while descending pathways work in the opposite way.

Ascending and descending pathways are also known as ascending and descending tracts of the spinal cord.

The brainstem also contains most cranial nerve nuclei (group of neurons), and therefore, is involved in the processing of efferent (coming from) information of the head.  

As the brainstem is made of the midbrain, pons, and medulla, it performs its functions through these structures.

2. What Does the Midbrain Do?

Midbrain or mesencephalon is the most rostral part (top) of the brainstem, found just below the cerebral cortex and above the pons. It is involved in vision, hearing, sleep, wakefulness, temperature regulation, alertness (arousal), and motor control [2]. 

It is composed of the tectum, the tegmentum, and cerebral peduncles, and receives blood supply from the basilar artery, superior cerebellar artery, and posterior cerebellar artery.

The tectum is divided into the superior colliculi that contain nuclei (group of neurons) involved in visual (vision) reflexes and the inferior colliculi involved in auditory (hearing) reflexes.

The tegmentum is divided into the substantia Negra where dopamine is released to the basal ganglia, and the red nucleus, which is involved, together with the cerebellum, in movement coordination.

The cerebral peduncles are paired structures separated by the interpeduncular cistern (Filled with cerebrospinal fluid) that connects the brainstem to the thalami (plural of the thalamus).

Lesions affecting the midbrain such as infarct, tumor, hemorrhage, or infections, can result in neurological disorders, including Benedikt syndrome, Claude syndrome, Weber syndrome, or Nothnagel syndrome.

These neurological disorders are characterized by palsies (paralysis) and ataxia (movement incoordination).

3. What Does the Pons Do?

The pons or pons Varolii is the structure of the brainstem located between the midbrain and the medulla (Medulla oblongata) and serves as a relay between the cerebral hemispheres and the cerebellum [3].

It is divided into the basilar part of the pons (Basis pontis) and the pontine tegmentum and receives blood supply from branches of the superior cerebellar artery and the pontine branches of the basilar artery.

The basilar part of the pons contains the pontine nuclei (cluster of neurons) that relay motor information coming from the motor cortex in the frontal lobe to the cerebellum. This activity is mediated by the corticofugal fibers.

The pontine tegmentum contains nuclei involved in sleep and wakefulness, breathing, and alertness.

It also contains nuclei of the cranial nerves, trigeminal (V), abducens (VI), facial (VII), and vestibulocochlear (VIII), and therefore, is also involved in the functions of these trigeminal nerves.

Lesions affecting the pons such as infarct, hemorrhage, tumors, or infections, can lead to the following neurological disorders:

• Gasperini Syndrome
• Facial Colliculus Syndrome
• Millard-Gubler Syndrome
• Brissaud-Sicard Syndrome
• Raymond Syndrome
• Foville Syndrome
• Marie-Foix Syndrome
• Locked-In Syndrome
• Tuberculosis Rhombencephalis

These neurological disorders are characterized by palsies (paralysis), ataxia, and symptoms associated with alterations in the function of some cranial nerves, such as loss of taste, loss of pain, temperature sensations on the face, or hearing loss.

4. What Does the Medulla Do?

The medulla (Medulla oblongata) is the part of the brainstem located between the pons and the spinal cord. It is responsible for the body’s involuntary functions (autonomous nervous system) such as cardiovascular function, heart rate, breathing, sleep-wake cycle, vomiting, sneezing, and swallowing [4].

The medulla is composed of the ventral medulla and the medulla tegmentum and receives its blood supply from the posterior inferior cerebellar artery (PICA), the anterior spinal artery, and branches of the distal vertebral artery.

The ventral medulla contains the olive, the pyramidal tracts, and the rootlets (nerve fibers) of the cranial nerve IX (Glossopharyngeal nerve) and XII (Hypoglossal nerve).

The olive is involved in movement coordination and in sound perception, while the pyramidal tracts are involved in the movement of muscles, and the rootlets of cranial nerve IX and XII carry away information from the central nervous system (brain and spinal cord) to the tongue, pharynx, larynx, parotid gland, and sinus.

The medulla tegmentum contains the inferior olivary nuclei and the nuclei of the cranial nerves IX to XII.

Lesions affecting the medulla oblongata such as infarct, hemorrhage, tumors, or degeneration, lead to the following neurological disorders:

• Babinski-Nageotte Syndrome
• Opalski Syndrome
• Wallenberg Syndrome
• Déjerine syndrome
• Wallerian Degeneration
• Demyelinating lesions
• Hypertrophic Olivary Degeneration
• Brainstem Glioma
• Syringobulbia

These neurological disorders manifest with symptoms such hemiparesis/hemiplegia, paralysis of parts of the body (e.g., tongue, soft palate, the pupil of the eye), ataxia, sensory deficits, and slurred speech.

5. What Are the Developmental Anomalies of the Brainstem?

Developmental anomalies of the brainstem are disorders that are originated during embryonic development of the brainstem [4].

The main known developmental disorders of the brainstem are Möbius syndrome (congenital facial diplegia syndrome) and axonal guidance disorders.

5.1. What Is Möbius syndrome?

Möbius syndrome or congenital facial diplegia syndrome is due to the underdevelopment or absence of the nuclei of the abducens cranial nerve (VI) and the facial cranial nerve (VII) resulting in paralysis of one side (unilateral) or both sides (bilateral) of the face muscles [5].

5.2. What Are Axonal Guidance Disorders of the Brainstem?

This group of disorders is associated with abnormal white matter tracts in the brainstem such as ectopic white matter tracts or anomalies in their location [6].

These disorders result in disorders such as horizontal gaze palsy, Joubert syndrome, progressive scoliosis, and pontine tegmental cap dysplasia [4].

Conclusion

The brainstem is made of the midbrain, pons, and the medulla oblongata and plays an essential role in communicating information between the body, the spinal cord, the cerebellum, and the brain.

This information includes instruction related to vision, hearing, sleep, wakefulness, temperature regulation, alertness (arousal), motor control, and the autonomous nervous system.

Lesions such as those caused by trauma, hemorrhage, tumors, or infections can affect the different parts of the brainstem (midbrain, pons, and the medulla oblongata) resulting in paralysis of parts of the body, ataxia, and sensory deficits.

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The auditory cortex is located in the temporal lobe of the brain.

The transmission of the auditory information through neurons to the auditory cortex is known as neurotransmission.

Sounds are that are received by the ear are transmitted by the vestibulocochlear nerve (8^th cranial nerve) and processed by the auditory sensory system.

The auditory sensory system includes the cochlear nuclei, the superior olivary complex, the inferior colliculus, the medial geniculate nucleus, and the auditory cortex

1. What Are the Steps of Hearing?

Sound processing is ensured by the auditory system that includes the ears and the sensory system and follows the following steps:

• Step 1: The sound enters the ear, travels through the ear canal, and reaches the eardrum.

• Step 2: The sound waves are amplified by the eardrum and the ossicles.

• Step 3: The amplified sound waves is received by the cochlea where it is transformed into nerve impulses by the hair cells (stereocilia) found in the organ of Corti.

Stereocilia are connected to sensory neurons through synapses where the auditory information is transferred through neurotransmission.

• Step 4: The nerves impulses are transmitted to the sensory cortex for processing by the auditory cortex.

1.1. What Is Ear and Its Function?

The ear is the first organ of hearing and is composed of the outer ear, middle ear, and inner ear [1].

• What Is Outer Ear?

The outer ear is composed of the visible part of the ear (Pinna or auricle), and the ear canal.

• What Is Middle Ear?

The middle of the ear contains the eardrum and 3 small bones known as ossicles and which are also involved in amplifying the sound waves before reaching the cochlea in the inner ear.

The eardrum is the membrane involved in the amplification of the sound waves that travel through the ear canal. It separates between the outer ear and the inner ear.

• What Is Inner Ear?

The inner ear contains the utricle, saccule, and the bony labyrinth which includes the semicircular canals, and the cochlea.

The utricle and saccule are involved in displacement and linear accelerations such as tilting the head and orientation.

The semicircular canals are filled with fluids and are involved in maintaining balance and coordination.

The cochlea is a cavity that is shaped like a spiral filled with fluids. It contains the organ of Corti where the hair cells (stereocilia) are found. The hair cells are responsible for transforming sound waves into impulses in coordination with sensory neurons in proximity.

1.2. What Is the Auditory Sensory System Function?

The auditory sensory system includes the vestibulocochlear nerve (8^th cranial nerve), the cochlear nuclei, the superior olivary complex, the inferior colliculus, the medial geniculate nucleus, and the auditory cortex.

• What Does the 8^th Cranial Nerve Do?

The 8^th cranial nerve is known as the vestibulocochlear nerve (cranial nerve VIII) or auditory nerve. It has two nerve branches, the cochlear nerve, and the vestibular nerve [2].

The cochlear nerve is involved in transmitting auditory information to the cochlear nucleus in the medulla oblongata found in the brainstem.

The vestibular nerve transmits information to the brain for the processing of body balance.

• What Is the Cochlear Nucleus?

The cochlear nucleus or cochlear nuclei are located in the brainstem and receive auditory information from the cochlear nerve. They work like a distribution center through the processing of different acoustic waves.

• What Does the Superior Olivary Nuclei do?

The superior olivary nuclei are also known as superior olive or superior olivary complex. It is located in the pons of the brain where it is involved in measuring the difference in sound intensity and azimuth.

The superior olivary complex receives auditory information from the cochlear nuclei.

• What Does the Inferior Colliculus in the Brain?

The inferior colliculus is located in the midbrain and plays a role as a relay for auditory information coming from the two ears and its integration. It is also involved in distinguishing between a pitch and a rhythm [2].

The inferior colliculus receives auditory information from the superior olivary complex and the auditory cortex.

• What is the medial geniculate nucleus?

The medial geniculate nucleus is located in the part of the brain known as the thalamus and is involved in the detection of sound intensity and duration.

The median geniculate nucleus receives auditory information from the medial geniculate nucleus.

• What is the Role of the Auditory Cortex?

The auditory cortex is part of the temporal lobe involved in transforming acoustics into perceptual representation such as recognizing the sound and its identification. It is also involved in language switching [3].

2. What Are Hearing Disorders?

Hearing disorders can affect both the ear and the auditory sensory system.

2.1. What Are the Most Common Ear Disorders?

What Are disorders of the Outer Ear?

There are several disorders of the outer ear that are due to the following causes:

• Absence of the outer ear (Anotia)
• Malformation of the outer ear (Microtia)
• Infection (Otitis externa)
• Wax build-up
• Bony tumor (Osteoma)
• Absence of ear canal (Atresia)
• Narrowing of the ear canal (stenosis)
• Fungal infection (Otomycosis)
• Disorders of the eardrum (perforation, thickening (tympanosclerosis), and inflammation (Myringitis).

What Are disorders of the Middle Ear?

• Otitis media (infection of the middle ear)
• Otosclerosis (bony growth in the middle ear)
• Eustachian tube dysfunction
• Ossicular chain discontinuity (loss of connectivity between the ossicles)
• Cholesteatoma (Tumor)

What Are disorders of the Inner Ear?

• Ménière’s disease (increased build-up of fluids in the inner ear) characterized by symptoms such as vertigo, ringing in the ear, and loss of hearing.
• Nose-induced hearing loss (NIHL)
• Presbycusis (age related degeneration of the cochlea)
• Perilymph fistula (fistula causing a leak of the inner ear fluid into the middle ear)

2.2. What Are the Most Common Disorders of the Auditory Sensory System?

What Are the Disorders of the Auditory Nerve?

• Acoustic Neuroma (tumor growing on the auditory nerve) causing hearing loss, dizziness, and ringing in the ear (tinnitus)
• Auditory Neuropathy Spectrum Disorder (auditory neuropathy) is due to alterations in the transmission of auditory information between the cochlea and the cochlear nuclei in the brainstem.

What Are the Disorders of the Cochlear Nuclei?

• These disorders are due to alterations in the transmission of the auditory information within the cochlear nuclei that can be caused by diseases (e.g., syphilis, multiple sclerosis, congenital malformation), stroke, or aging.

What Are the Disorders of the Higher Auditory Pathways?

• Minimal Auditory Deficiency Syndrome is due to language deprivation in children which may be caused by undiagnosed otitis media
• Central Deafness is rare and can be caused by a vascular lesion in the brain hemispheres.
• Auditory Processing Disorder is cause by a deficit of transmission of auditory information between the ear and the brain.

Conclusion

The auditory system is very complex and fascinating and involves both the ears and the brain processing sounds that are extremely diverse. Meanwhile, I am certainly going to enjoy listening to a nice piece of music while I still can.

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