The Pons

The pons is a structure of the vertebrate nervous system that can be found in the hindbrain, the lowest region of the brain. It is an especially important structure of the human nervous system as it is a portion of the brainstem – an area through which all information that travels throughout the nervous system must pass at some point.

In Latin pons means “bridge,” referring to the function and anatomical location of this structure – connecting the two hemispheres of the cerebrum. The pons also serves to connect the cerebral cortex to the medulla oblongata via the cerebral peduncles.

The pons is involved in many autonomic and sensory functions including arousal, respiratory processes, fine motor control, equilibrium, muscle tone, and the Circadian cycle (specifically regulating sleep).

It is also a vital portion of the brainstem considering that many cranial nerves arise from the pons, including nerves that collect sensory information and control motor functions in the face.

Anatomy and Functions of the Pons

The pons is one of the smallest parts of the brain at only 2.5cm in length, but, especially since it’s a part of the brainstem, it is involved in a great many processes that run the central and peripheral nervous systems.

Cranial Nerves

The pons is vital to the central and peripheral nervous system – a major reason for this being its connections to several cranial nerves including the trigeminal, abducens, facial, and vestibulocochlear nerves.

In the center of the pons is an indentation, or line, called the basilar groove (also where the basilar artery is located). All of the cranial nerves originate from the same side of the basilar groove, with the exception of the trigeminal nerve.

Because of its abundance of nerve connections, the pons is involved in many nervous system functions ranging from sensory to motor functions. The trigeminal nerve is the largest cranial nerve and earns its name from its three branches: the ophthalmic, maxillary, and mandibular nerves.

This collection of nerves controls sensory information gathered from organs of the face and the motor control of chewing.

The abducens controls the movement of the eye, facial nerve controls the expressions of the face (therefore controls the relay of neuronal signals from the brain to all of the fine muscles in the face – that is a lot to manage!) and the sense of taste and the vestibulocochlear regulates the equilibrium and auditory sensations.

All of the cranial nerves associated with this structure emerge from the ventral surface of the pons.

Even if we were to stop here, it is clear that the functionality of the pons is wide-reaching. But, it doesn’t stop there!

Other External Anatomical Features of the Pons

The same area from which these cranial nerves emerge is distinguished by a bulge formed by a structure called the transverse pontocerebellar fibers. This bundle of nerves connects to the cerebellum and constitutes the main afferent source of neuronal information to the cerebellum.

A large portion of the information sent via the pontocerebellar fibers concerns the planning and execution of fine movements, specifically, movements of the arm, forearm, and hand. These fibers wrap around the remainder of the brainstem.

An important landmark to take note of is the pontomedullary junction: this is marked by the angle between the inferior region of the pons and the superior boundary of the medulla oblongata.

The floor of the fourth ventricle constitutes the dorsal surface of the pons, along with that of the medulla oblongata. A few more structures can be identified in this region of the pons including the medial eminence, which marks the midline of the dorsal surface, the facial colliculus, a bulge formed by the fibers of the facial nerves that loop around the nucleus of the abducens, and the stria medullaris, a bundle of nerves belonging to the fourth ventricle.

Another landmark to help you identify the pons is called the cerebellopontine angle, where the cerebellar flocculus (a small part of the cerebellum involved in motor control), the ventricular choroid plexus, and the facial and vestibulocochlear nerves surround the foramen of Luschka (structures that link the fourth ventricle to the cerebellopontine cistern, another space into which cerebrospinal fluid can pool).

Internal Anatomy of the Pons

The pons is recognized as having two major divisions: the ventral pons and the tegmentum (different from the tegmentum of the midbrain.) The ventral pons is home to the pontine nuclei, structures that are responsible for the coordination of movement. These nuclei travel from the pons across the midline and form the middle cerebellar peduncles as they make their way to the cerebellum.

The tegmentum of the pons is considered to be the older region of the pons evolutionarily (meaning that this structure was present in the ancestors of humans and other vertebrates that have a pons as a part of their nervous system).

The tegmentum forms a part of the reticular formation, a network of nerves that extend from the medulla oblongata and connect to the spinal cord and thalamus.

Pontine Nuclei

These nuclei are a part of the pons that is concerned with motor activity. They are among the largest nuclei informing the cerebellum and provide some of the most important neuronal transmissions. The pontine nuclei are informed primarily by the cerebral cortex and project to the cerebellar hemispheres.

Reticular Formation

The reticular formation is a complex collection of nerve fibers and cell bodies that are comprised of both ascending and descending nerve tracts. Nuclei within the reticular formation are involved with the production of neurotransmitters, and associated with several cranial nerves, controlling both sensory and motor functions with the descending tracts and arousal and consciousness with the ascending tracts.

The neurotransmitters produced by the reticular formation are connected with many parts of the central nervous system and regulate many types of activity in several different areas of the brain. This reticular formation is related to the production of dopamine, the release of serotonin, production of acetylcholine, and more. All of these hormones and neurotransmitters are related to sensory perception, motor control, and behavioral responses to various stimuli.

Nerve Tracts That Pass Through the Pons

There are four major nerve tracts that pass through the pons to control the sensory, autonomic, and voluntary functions of the body.

Corticospinal Tract

The corticospinal tract (CST), also known as the pyramidal tract, comprises part of the descending nerve tracts that emerge from the pons into the spinal cord and into the peripheral nervous system. There are about 1 million nerve fibers that make up the CST, each of the transmitting neuronal information at speeds of 60m/s!

The CST travels through the corona radiata (a sheet of white matter) and posterior limb of the internal capsule to terminate in the brainstem. Once it reaches the brainstem, one of the structures it passes through, of course, is the pons. The CST controls many motor functions including spinal reflexes, and the most notable of the voluntary movements are the voluntary distal movements.

Corticobulbar Tract

This is a descending neuronal pathway responsible for innervating several of the cranial nerves, controlling the muscles in the face, tongue, jaw, and pharynx.

The cranial nerves that are supplied by the corticobulbar tract include the trigeminal nerve (controls the process of chewing), facial nerve (controls muscles of the face), accessory nerve (specifically control the sternocleidomastoid and trapezius muscles), and the hypoglossal nerve (controls tongue muscles).

Medial Lemniscus Tract

The nerve tract is part of a greater pathway called the dorsal column-medial lemniscal pathway, which is responsible for the transmission of sensory information related to fine tactile sensation, detection of vibrations, and proprioception (awareness of the position of certain body parts).

Spinothalamic Tract

The spinothalamic tract works along with the medial lemniscus tract to create one of the most important pathways of the nervous system, responsible for transmitting information regarding sensation.

Neuronal signals related to pain, temperature, and touch are relayed to the somatosensory region of the hypothalamus through this nerve tract. In total, the spinothalamic tract is composed of four sub-tracts: the anterior spinothalamic tract, lateral spinothalamic tract, spinoreticular tract, and spinotectal tract.

Significance of the Pons

The pons is a structure of the brain that is vital to the autonomic, sensory, and motor functions of the human body. Along with the medulla oblongata and midbrain, it comprises one of the most important nervous system structures, the midbrain – the point through which all transmissions of the central and peripheral nervous systems pass.

Damage to this structure can result in the loss of sensation in the face, loss of the corneal reflex (the blink reflex, specifically induced by a stimulus of the cornea, such as being touched or an eyelash getting into your eye), and loss of the ability to gaze. Additional symptoms related to the damage of the pons are very similar to those of a stroke like a lack of control of the facial muscles which would lead to the face appearing to droop.

As the bridge of the central nervous system, the pons is an integral part of the functionality of the human nervous system and overall survival.

References

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