The brainstem, positioned at the base of the posterior region of the brain, functions as the critical point of connection between the central and peripheral nervous systems.
There are twelve pairs of nerves that stem directly from the brainstem to control all senses that can be detected from the organs in the cranial region. It is divided into three subsections: the midbrain, pons, and – everyone’s favorite – the medulla oblongata.
Its functions extend far past the head, however, in that it is a critical control point for many autonomous actions of the body, including the heartbeat, respiration, regulation of blood pressure, and reflexes.
Additionally, all efferent (impulses directed away from the brain) and afferent (impulses directed toward the brain) communications pass through the brainstem to reach the cerebrum and cerebellum.
The significance of the brainstem to the human nervous system cannot be overstated. It is the point of action between many motor functions, all of our senses, our regulatory functions, and so much more.
Anatomy of the Brainstem
As previously mentioned, the brainstem is split into three different subdivisions: the midbrain, pons, and medulla oblongata. Around the base of the brainstem, extending up into the cerebrum, are the vertebral arteries that transition into the basilar artery.
Also known as the mesencephalon, the midbrain is specifically associated with the eyes, hearing, motor control, the Circadian rhythm, alertness, and temperature regulation.
The general areas of the midbrain are known as the tegmentum, the ventral (underside) region, and the tectum, the dorsal (topside) region. Other structures within the midbrain are the cerebral peduncles, corpora quadrigemina, and the cerebral aqueduct.
The midbrain connects the hindbrain and the forebrain and, with the nerve pathways that run through it, connects the cerebrum, cerebellum, and other hindbrain structures. More specifically, the midbrain controls the physiological responses to sight, movement of the eyes and pupil dilation, and auditory sensation.
In the tegmentum are three distinct regions: the red nucleus, the periaqueductal gray, and the substantia nigra. Each of these regions, both individually and collectively are associated with motor capabilities.
The red nucleus is involved in sensorimotor coordination, being informed by the major neural output tract of the cerebellum, the superior cerebellar peduncle. There are more neural fibers that surround and terminate or originate from the red nucleus that go to the spinal cord and play a role in the functioning of the motor cortex.
The periaqueductal gray region is comprised of gray matter and surrounds one of the ventricles, the cerebral aqueduct. The primary function of the periaqueductal gray region is the suppression of pain. It is capable of doing this due to its high concentration of endorphins.
Lastly, the substantia nigra (notice a pattern yet? These regions of the tegmentum are identified by their color: red, gray, and nigra or black). This region is made up of a large, highly pigmented gathering of neurons in two distinct parts, the pars reticulata, and the pars compacta.
The pars compacta is primarily meant to synthesize dopamine and transmit to structures in the basal ganglia for the purpose of mediating movement and coordination.
Made up of two bulbous structures known as the superior colliculus and the inferior colliculus, the tectum is primarily associated with the senses of hearing and vision.
The superior colliculus receives neural transmissions from the retina and visual cortex and is specifically associated with reflex movements of the eyes and the ability to visually track objects. The inferior colliculus, on the other hand, receives auditory signals and projects them to the thalamus.
The cerebral peduncles are a pair of cylindrical bodies in the ventrolateral region of the midbrain. They come up from the ventral (upper) portion of the pons on either side of the midline of the brain and extend up into the cerebral hemispheres. These are involved in the movement of the eyes as it is connected to the oculomotor sulcus (from where the oculomotor nerves arise).
These are four rounded structures located on the dorsal portion of the midbrain. They are arranged in pairs and are the structural basis of the superior and inferior colliculi. Composed of both gray and white matter, the corpora quadrigemina participate in the reception and processing of visual sensory information.
Part of the ventricular system, the cerebral aqueduct is a canal of sorts through which cerebrospinal fluid flows. It is located between the corpora quadrigemina and the tegmentum and connects with the third and fourth ventricles.
The pons, positioned between the medulla oblongata and the midbrain, contains nuclei that transmit signals between the cerebrum, medulla oblongata, cerebellum, and thalamus.
This structure is a dense assemblage of nerve fibers that arises from the embryonic brain region, the metencephalon (a subregion of the rhombencephalon, or hindbrain).
There are a few different regions of the pons as well: the basal (or basilar) pons and the dorsal pons, also called the pontine tegmentum. The pons gets its name from the Latin word for “bridge,” as its most prominent, recognizable part appears to be a bridge between the two hemispheres of the cerebrum.
Where bundles of fibers connect one region of the pons to another, the middle cerebral peduncles are formed. The peduncles are major pathways of communication between the cerebrum and the rest of the central nervous system.
The Medulla Oblongata
Alas, we have arrived at the medulla oblongata! Besides being a fun term to say, the medulla oblongata is known for its critical role in autonomic functions that keep us alive including respiration, digestion and swallowing, control of the heart rate, constriction and relaxation of blood vessels, and sneezing.
Quite literally, the medulla oblongata regulates all the functions we need to live.
This structure is also home to all of the tracts that transmit neuronal impulses between the brain and the spinal cord and contains several nuclei that are central to the maintenance of homeostasis.
These nuclei are the cardiovascular center, respiratory rhythmicity center, nucleus gracilis and nucleus cuneatus, olivary nucleus, the reticular formation of the medulla oblongata, and the sensory and motor nuclei of the cranial nerves VIII, IX, X, XI, and XII.
Each of these nuclei performs one of the functions listed above, plus a few more:
- Transmission of afferent sensory information to the thalamus (done by the nucleus gracilis)
- Transmission of information between the cerebral cortex, diencephalon, brainstem, and the cerebellum (olivary nucleus)
Because of its critical role in keeping us alive, the medulla oblongata has been referred to as the most important part of the vertebrate brain.
Cranial Nerves Connected to the Brainstem
There are ten pairs of cranial nerves that either terminate or originate (depending on which direction the information is flowing, afferent or efferent) at the brainstem:
- Oculomotor nerve: controls constriction of pupils.
- Trochlear nerve: controls eye movement.
- Trigeminal nerve: the largest cranial nerve, involved in facial sensation, chewing, and corneal reflex.
- Ophthalmic nerve: transmits sensory information from the eyes, scalp, nasal cavity, and parts of the face
- Maxillary nerve: transmits sensory information from the teeth, gums, nasal cavity, and parts of the face.
- Mandibular nerve: controls the movement of chewing and transmits sensory information from the mouth and lower parts of the face
- Abducent nerve: controls eye movement.
- Facial nerve: controls facial expressions and plays a part in the sensation of taste
- Vestibulocochlear nerve: involved in hearing.
- Glossopharyngeal nerve: plays a role in swallowing, taste, and the secretion of saliva.
- Vagus nerve: involved in smooth muscle sensory and motor control of the digestive system (including the esophagus), lungs, and heart.
- Accessory nerve: controls neck and shoulder movement.
- Hypoglossal nerve: controls tongue movement, swallowing, and speech.
Cranial nerves are either efferent, afferent, or both. Only one of those that stem from the brainstem is afferent, the vestibulocochlear nerve; five are efferent, the oculomotor nerve, trochlear nerve, abducent nerve, accessory nerve, and hypoglossal nerve; and the rest contain both afferent and efferent fibers, the trigeminal nerve, facial nerve, glossopharyngeal nerve, and vagus nerve.
Efferent fibers of the cranial nerve originate from a type of nerve cell called motor nuclei, and afferent fibers, from clusters of nerve cells within a sensory ganglion, typically.
They terminate inside nerve cells of the brain called sensory nuclei. Further, there are many different kinds of afferent and efferent fibers, making up two distinct groups of cranial nerves, the “general” and the “special” cranial nerves.
General Cranial Nerves
The general cranial nerves are somatic afferent, visceral afferent, visceral efferent, and somatic efferent.
Special Cranial Nerves
This group is made up of afferent fibers for visual, auditory, olfactory, and taste. Two more functions of these afferent fibers are control of visceral reflexes, which are the dilation of the pupils, defecating and vomiting, and maintenance of the equilibrium.
The visual, auditory, and equilibrium maintenance regulatory processes are usually referred to as somatic, and the control of olfactory, taste, and visceral reflexes are regarded as visceral.
Special efferent fibers
Regarded as visceral – are responsible for regulating skeletal muscles involved in the digestion process (including the muscles required for chewing, swallowing, and facial muscles).
The brainstem is a critical structure of the nervous system, functioning as a central point of communication between the central nervous system and the peripheral nervous system. It is the base from which the majority of the cranial nerves arise, providing integral sensory information and is critical to our survival in its regulatory role of autonomous bodily functions such as respiration and heart rate.
Make sure to feed your curiosity about the nerves that stem from this structure (pun intended) and more by continuing your educational journey deeper into the human brain.