Pons

The pons is a critical part of the brain located in the brainstem, sitting superior to (above) the medulla oblongata and inferior to (below) the midbrain. On the transverse plane, the pons is situated anterior to (in front of) the cerebellum. Its name derives from the Latin word for "bridge," reflecting its function as a connection between different parts of the brain.

As the pons sits between the medulla oblongata and the thalamus, it is involved in sending messages between different areas of the brain, particularly those involved in sleep, respiration, swallowing, bladder control, hearing, equilibrium, taste, eye movement, facial expressions, and posture^[2].

These are some of the functions of the pons.

1. Relaying information: The pons helps relay messages between the cortex and the cerebellum. The pons and the cerebellum work together to serve many functions, including the management of sleep, respiration, swallowing, bladder control, hearing, equilibrium, taste, eye movement, facial expressions, facial sensation, and posture^[3].

2. Sleep regulation: The pons has been implicated in sleep regulation, particularly REM sleep. During REM sleep, your brain is more active, producing the vivid dreams we often remember. The pons sends signals to inhibit motor neurons during this phase, causing temporary paralysis and preventing us from physically acting out our dreams^[4].

3. Breathing and circulation: While the medulla oblongata is the primary brain structure that directly controls breathing and heart rate, the pons assists by helping to control the body's autonomic functions and overall level of arousal and alertness^[5].

4. Sensory roles: The pons also plays a role in relaying sensory information. It contains several cranial nerve nuclei and is implicated in the sensation and control of the face and jaw via these nerves^[6].

The pons can be divided into two main parts: the ventral pons (also known as basilar part of the pons) and the dorsal pons (also known as tegmentum).

Almost all of the signals between the brain and the rest of the body, including the medulla oblongata, have to pass through the pons. You can see in the image below how it is home to many important cranial nerve nuclei and cerebral tracts.

The ventral pons, also referred to as the basilar part of the pons, is located anteriorly or ventrally (meaning towards the front of the pons). It forms a bridge between the midbrain (above) and medulla oblongata (below)^[1].

Structurally, the ventral pons is marked by its broad, convex shape, which presents a bulge on the anterior surface of the brainstem. It is bisected by the basilar sulcus, where the basilar artery runs to supply blood to the brainstem and posterior cerebral areas^[9].

The ventral pons is primarily made up of pontine nuclei and fibers. The pontine nuclei receive inputs from the cerebral cortex and send information to the cerebellum via transverse pontine fibers, which form the middle cerebellar peduncles^[10].

Functionally, the ventral pons is involved in several important pathways:

1. Corticospinal Tract: It carries motor signals from the primary motor cortex to the spinal cord. Decussation of these fibers mainly happens in the medulla, but a number of fibers relay through the pons^[1].

2. Corticobulbar Tract: These fibers run with the corticospinal tract but they synapse with cranial nerve nuclei^[11].

3. Pontocerebellar Fibers: These fibers carry information from the pontine nuclei in the ventral pons to the contralateral cerebellar hemisphere, contributing to the coordination of voluntary movements^[12].

In addition to the aforementioned, the ventral pons also houses several cranial nerve nuclei including the abducens nerve (CN VI), facial nerve (CN VII), and the vestibulocochlear nerve (CN VIII). It also contains parts of the reticular formation, a group of neurons responsible for maintaining consciousness and regulating the sleep-wake cycle^[13].

Damage to the ventral pons can lead to various neurological conditions, such as locked-in syndrome, where an individual is aware and awake but cannot move or communicate verbally due to the disruption of the corticospinal and corticobulbar pathways^[14].

The dorsal pons, or pontine tegmentum, is brain located in the hindbrain, specifically in the metencephalon, which also includes the cerebellum, and contains numerous important structures and pathways.

The following are some key features of the dorsal pons:

Cranial nerve nuclei: There are several cranial nerve nuclei located in the dorsal pons, including the main and accessory sensory nuclei of the trigeminal nerve (CN V), the abducens nucleus (CN VI), the facial nerve nucleus (CN VII), and part of the vestibulocochlear nuclei (CN VIII)^[15].

Reticular formation: This is a complex network of neurons running through the core of the brainstem that plays a vital role in maintaining consciousness and regulating the sleep-wake cycle. In the pons, the reticular formation is involved in controlling autonomic functions and modulating pain^[1].

Ascending and descending tracts: The dorsal pons carries sensory information to the brain via ascending tracts and motor information from the brain via descending tracts. Notably, it contains part of the medial lemniscus (for somatosensory information from the body), the spinothalamic tracts (for pain and temperature sensation), and the corticospinal and corticobulbar tracts (for voluntary motor control)^[10].

Pontine nuclei: These are clusters of neurons that connect the cerebral cortex with the cerebellum, playing an essential role in motor control and learning. The fibers from these nuclei form the middle cerebellar peduncles^[13].

Parabrachial area and locus coeruleus: These are critical for processing sensory information and modulating pain, and they play a role in autonomic functions like respiration. The locus coeruleus is also the brain's principal site for synthesis of norepinephrine, a neurotransmitter involved in arousal and alertness^[16].

Pontine respiratory group. The pontine respiratory group, also known as the pneumotaxic center or the pontine respiratory center, is a collection of neurons located in the upper pons. This center plays a key role in the regulation of respiration and is mainly involved in modulating the output of the medullary respiratory centers, which directly control the rhythm of breathing^[17].

The pontine respiratory group consists of two sub-regions: the parabrachial complex and the Kölliker-Fuse nucleus. Both these regions communicate with the ventral and dorsal respiratory groups in the medulla oblongata, providing modulatory input^[18].

1. Pace and pattern of respiration: The pontine respiratory group influences the length of the inspiration and expiration phases of breathing, and therefore, the overall respiratory rate. It provides an "off-switch" to the inspiratory phase, transitioning from inspiration to expiration^[19].

2. Adaptive responses: The pontine respiratory group helps regulate adaptive responses to various physiological needs, including changes in oxygen and carbon dioxide levels, pH balance, and physical activity levels^[20].

3. Control of upper airway muscles: The pontine respiratory group contributes to the coordination of the upper airway and laryngeal muscles during breathing and swallowing, preventing aspiration^[21].

4. Integration of non-respiratory stimuli: It is also involved in the integration of non-respiratory stimuli into the respiratory pattern, such as emotional and sleep-wake state inputs^[22].

The pons is supplied by several arteries, which are branches of the vertebrobasilar system^[1]. These include:

1. Basilar artery: This artery, formed by the union of the two vertebral arteries, runs along the anterior surface of the pons. It gives off several small pontine branches (pontine arteries) that penetrate the pons.

2. Anterior inferior cerebellar artery (AICA): This artery usually arises from the basilar artery. It supplies parts of the pons, the middle cerebellar peduncle, and the anterior part of the cerebellum. It may contribute to the blood supply of the lateral part of the lower pons.

3. Superior cerebellar artery (SCA): This artery, also a branch of the basilar artery, supplies the superior part of the cerebellum and may also supply a small part of the upper pons.

4. Posterior inferior cerebellar artery (PICA): While this artery primarily supplies the posterior part of the cerebellum and the medulla oblongata, it can also contribute to the blood supply of the lower part of the pons.

The arteries supply the pons in a topographical manner, with each artery supplying a specific region. Notably, the 'pontine arteries' derived from the basilar artery supply the bulk of the pons. These arteries form an extensive network that ensures the continued supply of blood, even if one of the arteries becomes occluded.

The venous drainage of the pons is via small veins that empty into the superior, inferior, and great cerebral veins, and eventually into the dural venous sinuses. These structures are responsible for carrying deoxygenated blood and cerebrospinal fluid away from the brain.