The Brainstem and Reticular Formation

Brainstem

A stalk-like lower part of the brain that connects the cerebrum and cerebellum to the spinal cord, which controls critical involuntary life functions, consisting of the medulla oblongata, pons, and midbrain

Medulla Oblongata

The lowest part of the brainstem that connects the upper brain to the spinal cord and manages vital involuntary survival functions

Medulla Oblongata: White Matter

The medulla’s white matter contains all sensory (ascending) tracts and motor (descending) tracts that extend between the spinal cord and other parts of the brain

Medulla Oblongata: Pyramids

However, some white matter forms protrusions from the large corticospinal tracts that pass from the cerebrum to the spinal cord and carry nerve impulses for voluntary movement of the limbs and trunk.

Medulla Oblongata: Pyramids: Corticospinal Tracts

Large bundles of axons that travel from the cerebrum to the spinal cord and control voluntary movements of the limbs and trunk.

Medulla Oblongata: Decussation of Pyramids

The crossing of corticospinal axons from one side of the medulla to the opposite side just superior to the junction of the medulla and spinal cord, which explains why each side of the brain controls the opposite side of the body

Medulla Oblongata: Nuclei

Collections of neuronal cell bodies within the CNS that carry out vital body functions such as heartbeat, blood vessel diameter, and breathing.

Medulla Oblongata: Medullary Respiratory Center

The medullary respiratory center adjusts the basic rhythm of breathing.

Medulla Oblongata: Cardiovascular Center

The medulla helps automatically regulate essential functions such as heart rate, blood vessel diameter, and breathing.

Medulla Oblongata: Reflexes

The medulla controls reflexes involved in vomiting, deglutition (swallowing), sneezing, coughing, and hiccupping.

• Vomiting Center

• Deglutition Center

• Sneezing Center

• Coughing Center

• Hiccupping Center

Medulla Oblongata: Olives

Oval-shaped swellings located just lateral to each pyramid on the anterior medulla.

Medulla Oblongata: Inferior Olivary Nucleus

The nucleus is located within the olive of the medulla, receives information from the cerebral cortex, the red nucleus of the midbrain, and the spinal cord, and sends axons to the cerebellum

Medulla Oblongata: Inferior Olivary Nucleus: Function

The inferior olivary nucleus influences cerebellar neurons and helps the cerebellum adjust muscle activity during the learning of new motor skills.

Medulla Oblongata: Gracile Nucleus and Cuneate Nucleus

Nuclei in the posterior medulla process and carry sensory information about touch, pressure, vibration, and conscious proprioception toward the medulla.

Medulla Oblongata: Gracile and Cuneate Nuclei: Sensory Input

Ascending axons from the gracile and cuneate fasciculi synapse in the gracile and cuneate nuclei of the medulla.

Medulla Oblongata: Medial Lemniscus

A band of myelinated nerve fibers in the brainstem that carries sensory information (touch, pressure, vibration, and conscious proprioception) through the medulla, pons, and midbrain to the thalamus on the opposite side of the brain

Medulla Oblongata: Posterior Column–Medial Lemniscus Pathway

The tracts of the posterior funiculi and the axons of the medial lemniscus carry touch, pressure, vibration, and conscious proprioception from the body to the thalamus and brain are known as the posterior column–medial lemniscus pathway

Medulla Oblongata: Gracile vs. Cuneate Nucleus

The gracile and cuneate nuclei both process touch, pressure, vibration, and conscious proprioception; the gracile pathway mainly serves lower body regions, while the cuneate pathway mainly serves upper body regions.

Medulla Oblongata: Sensory Nuclei

The medulla also contains nuclei that are part of sensory pathways for taste, hearing, and equilibrium (balance).

• Gustatory Nucleus

• Cochlear Nuclei

• Vestibular Nuclei

Medulla Oblongata: Cranial Nerve Nuclei

The medulla contains nuclei associated with five pairs of cranial nerves: VIII, IX, X, XI, and XII.

Medulla Oblongata: Vestibulocochlear Nerve (VIII)

Nuclei in the medulla receive sensory input from and provide motor output to the cochlea through the vestibulocochlear nerves, which carry impulses related to hearing.

Medulla Oblongata: Glossopharyngeal Nerve (IX)

Medullary nuclei relay sensory and motor impulses related to taste, swallowing, and salivation through the glossopharyngeal nerves.

Medulla Oblongata: Vagus Nerve (X)

Medullary nuclei receive sensory impulses from and provide motor impulses to the pharynx, larynx, and many thoracic and abdominal organs through the vagus nerves.

Medulla Oblongata: Accessory Nerve (XI)

The cranial portion of the accessory nerve is actually part of the vagus nerve; medullary nuclei provide impulses that help control swallowing through these fibers.

Medulla Oblongata: Hypoglossal Nerve (XII)

Medullary nuclei generate motor impulses that control tongue movements during speech and swallowing through the hypoglossal nerves.

Pons

A bulb-shaped part of the brainstem that lies superior to the medulla and anterior to the cerebellum, acting as a main communication bridge between different brain regions via bundles of axons

Pons: Axons Bundle Example

Some axons of the pons connect the right and left sides of the cerebellum, while others are part of ascending sensory tracts and descending motor tracts.

Pons: Major Structural Components

The pons divides internally into a ventral pons and a dorsal pons

Pons: Ventral Region

The ventral region contains the pontine nuclei and numerous white matter tracts that

relay voluntary motor information from the cerebral cortex to the opposite side of the cerebellum

Pons: Ventral Region: Pontine Nuclei

The pontine nuclei are scattered groups of gray matter in the ventral pons that act as major synaptic relay stations that relay voluntary motor information from the cerebral cortex to the opposite side of the cerebellum

Pons: Dorsal Pons

The dorsal region resembles other brainstem regions and contains ascending and descending tracts as well as cranial nerve nuclei that control arousal, sleep, cranial nerve reflexes, and autonomic functions, including breathing.

Pons: Dorsal Region: Ascending Tracts

The dorsal pons contains ascending tracts that carry sensory information toward higher brain regions.

Pons: Dorsal Region: Descending Tracts

The dorsal pons contains descending tracts that carry motor information toward lower regions of the nervous system.

Pons: Pontine Respiratory Group

A network of neurons in the pons of the brainstem that helps control the rate, depth, and rhythm of breathing

Pons: Trigeminal Nerve (V)

Nuclei in the pons receive sensory impulses for somatic sensations from the head and face and provide motor impulses that control chewing.

Pons: Abducens Nerve (VI)

Nuclei in the pons provide motor impulses that control eyeball movement through the abducens nerves.

Pons: Facial Nerve (VII)

Nuclei in the pons receive sensory impulses for taste and provide motor impulses that carry taste information and control salivation, tear production, and facial expressions.

Pons: Vestibulocochlear Nerve (VIII)

Nuclei in the pons receive sensory impulses and carry information related to balance and equilibrium to the vestibular apparatus through the vestibulocochlear nerves

Midbrain

The topmost region of the brainstem that extends from the pons to the diencephalon and controls vital functions like vision, hearing, and motor movement

Midbrain: Cerebral Peduncles

The anterior part of the midbrain contains paired bundles of axons of the corticospinal, corticobulbar, and corticopontine tracts, carrying motor impulses from the cerebral cortex to the spinal cord, medulla, and pons, respectively.

Midbrain: Tectum

The posterior part of the midbrain contains four rounded elevations

Midbrain: Tectum: Superior Colliculi

The two superior elevations serve as reflex centers for certain visual activities, controlling reflexive eye movements for tracking moving objects and scanning stationary images, as well as reflex movements of the head, eyes, and trunk in response to visual stimuli.

Midbrain: Tectum: Inferior Colliculi

The two inferior elevations serve as the auditory pathway, relaying information from receptors in the inner ear to the brain and serving as reflex centers for the startle response to sudden loud sounds.

Midbrain: Substantia Nigra

An example of the several nuclei within the midbrain that is a dark-colored structure that plays a vital role in controlling movement, reward, and coordination via dopamine

Midbrain: Red Nuclei

A structure in the upper midbrain that helps coordinate body movements also appears reddish because of its rich blood supply and iron-containing pigments.

Midbrain: Oculomotor Nerve (III)

The oculomotor nerve controls eyeball movement and, through accessory oculomotor nuclei, controls pupil constriction and changes in lens shape.

Midbrain: Trochlear Nerve (IV)

Nuclei in the midbrain provide motor impulses that control movements of the eyeball through the trochlear nerves.

Reticular Formation

A broad region of the brainstem where small clusters of gray matter are interspersed among small bundles of white matter, creating a netlike arrangement, where the nucleus has both ascending sensory functions and descending motor functions.

Reticular Formation: Location

The reticular formation extends from the superior part of the spinal cord through the brainstem and into the inferior part of the diencephalon.

Reticular Formation: Reticular Activating System (RAS)

The ascending portion of the reticular formation sends sensory information to the cerebral cortex directly and through the thalamus

Reticular Formation: RAS: Sensory Inputs

The RAS can be activated by visual and auditory stimuli, mental activities, pain, touch, pressure, and information about body position.

Reticular Formation: RAS: Function

The RAS helps maintain consciousness, arousal, attention, and alertness, while preventing sensory overload by filtering out insignificant visual and auditory information from reaching consciousness.

Reticular Formation: RAS: Sleep

Inactivation of the RAS produces sleep, a state of partial consciousness from which a person can be awakened; however, damage to the RAS can cause coma

Reticular Formation: Coma: Effects

In lighter stages of coma, brainstem and spinal cord reflexes may remain present; in deeper stages, these reflexes can be lost; however, if respiratory and cardiovascular controls are lost during deep coma, death can occur.

Reticular Formation: Descending Portion

The descending portion of the reticular formation has connections to the cerebellum and spinal cord and helps regulate motor functions, such as muscle tone, heart rate, blood pressure, and respiratory rate

Reticular Formation: Ascending vs. Descending

The ascending portion (RAS) is mainly involved in consciousness, arousal, attention, and sensory filtering, while the descending portion helps regulate muscle tone and autonomic functions.