64 The Brain Stem

Functions of the Brain Stem

The brainstem regulates vital cardiac and respiratory functions and acts as a vehicle for sensory information.

Location and Basic Physiology

In vertebrate anatomy, the brainstem is the most inferior portion of the brain, adjoining and structurally continuous with the brain and spinal cord. The brainstem gives rise to cranial nerves 3 through 12 and provides the main motor and sensory innervation to the face and neck via the cranial nerves. Though small, it is an extremely important part of the brain, as the nerve connections of the motor and sensory systems from the main part of the brain that communicate with the peripheral nervous system pass through the brainstem. This includes the corticospinal tract (motor), the posterior column-medial lemniscus pathway (fine touch, vibration sensation, and proprioception ) and the spinothalamic tract ( pain, temperature, itch, and crude touch). The brain stem also plays an important role in the regulation of cardiac and respiratory function. It regulates the central nervous system (CNS) and is pivotal in maintaining consciousness and regulating the sleep cycle.

Components of the Brainstem

The three components of the brainstem are the medulla oblongata, midbrain, and pons.

The medulla oblongata (myelencephalon) is the lower half of the brainstem continuous with the spinal cord. Its upper part is continuous with the pons. The medulla contains the cardiac, respiratory, vomiting, and vasomotor centers regulating heart rate, breathing, and blood pressure.

The midbrain (mesencephalon) is associated with vision, hearing, motor control, sleep and wake cycles, alertness, and temperature regulation.

The pons (part of metencephalon) lies between the medulla oblongata and the midbrain. It contains tracts that carry signals from the cerebrum to the medulla and to the cerebellum. It also has tracts that carry sensory signals to the thalamus.

Brainstem Function

The brainstem has many basic functions, including regulation of heart rate, breathing, sleeping, and eating. It also plays a role in conduction. All information relayed from the body to the cerebrum and cerebellum and vice versa must traverse the brainstem. The ascending pathways from the body to the brain are the sensory pathways, including the spinothalamic tract for pain and temperature sensation and the dorsal column, fasciculus gracilis, and cuneatus for touch, proprioception, and pressure sensation. The facial sensations have similar pathways and also travel in the spinothalamic tract and the medial lemniscus.

Descending tracts are upper motor neurons destined to synapse on lower motor neurons in the ventral horn and intermediate horn of the spinal cord. In addition, upper motor neurons originate in the brain stem’s vestibular, red, tectal, and reticular nuclei, which also descend and synapse in the spinal cord. The brainstem also has integrative functions, including cardiovascular system control, respiratory control, pain sensitivity control, alertness, awareness, and consciousness.

Medulla Oblongata

The medulla oblongata controls autonomic functions and connects the higher levels of the brain to the spinal cord.

The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla. The medulla contains the cardiac, respiratory, vomiting, and vasomotor centers and regulates autonomic, involuntary functions such as breathing, heart rate, and blood pressure.

The medulla is often divided into two parts:

1. An open or superior part where the dorsal surface of the medulla is formed by the fourth ventricle.
2. A closed or inferior part where the metacoel (caudal part of fourth ventricle) lies within the medulla oblongata.

Structure of the Medulla Oblongata

The region between the anterior median and anterolateral sulci is occupied by an elevation on either side known as the pyramid of medulla oblongata. This elevation is caused by the corticospinal tract. In the lower part of the medulla, some of these fibers cross each other, thus obliterating the anterior median fissure. This is known as the decussation of the pyramids. Other fibers that originate from the anterior median fissure above the decussation of the pyramids and run laterally across the surface of the pons are known as the external arcuate fibers.

The region between the anterolateral and posterolateral sulcus in the upper part of the medulla is marked by a swelling known as the olivary body, caused by a large mass of gray matter known as the inferior olivary nucleus.

The posterior part of the medulla between the posterior median and posterolateral sulci contains tracts that enter it from the posterior funiculus of the spinal cord. These are the fasciculus gracilis, lying medially next to the midline, and the fasciculus cuneatus, lying laterally.

The fasciculi end in rounded elevations known as the gracile and cuneate tubercles. They are caused by masses of gray matter known as the nucleus gracilis and the nucleus cuneatus. Just above the tubercles, the posterior aspect of the medulla is occupied by a triangular fossa, which forms the lower part of the floor of the fourth ventricle. The fossa is bounded on either side by the inferior cerebellar peduncle, which connects the medulla to the cerebellum.

The lower part of the medulla, immediately lateral to the fasciculus cuneatus, is marked by another longitudinal elevation known as the tuberculum cinereum. It is caused by an underlying collection of gray matter known as the spinal nucleus of the trigeminal nerve. The gray matter of this nucleus is covered by a layer of nerve fibers that form the spinal tract of the trigeminal nerve.

The base of the medulla is defined by the commissural fibers, crossing over from the ipsilateral side in the spinal cord to the contralateral side in the brain stem; below this is the spinal cord.

Embryonic Development

During development, the medulla oblongata forms from the myelencephalon. The final neuroblasts from the alar plate of the neural tube produce the sensory nuclei of the medulla. The basal plate neuroblasts give rise to the motor nuclei.

Function of the Medulla Oblongata

The medulla oblongata controls autonomic functions and connects the higher levels of the brain to the spinal cord. It is also responsible for regulating several basic functions of the autonomic nervous system, including:

• Respiration: chemoreceptors
• Cardiac center: sympathetic system, parasympathetic system
• Vasomotor center: baroreceptors
• Reflex centers of vomiting, coughing, sneezing, and swallowing

Pons

The pons is a relay station between the forebrain and cerebellum that passes sensory information from the periphery to the thalamus.

The pons is a structure located on the brainstem, named after the Latin word for “bridge.” It is above the medulla, below the midbrain, and anterior to the cerebellum. The white matter of the pons includes tracts that conduct signals from the cerebrum down to the cerebellum and medulla, and tracts that carry the sensory signals up into the thalamus.

Structure

The pons measures about 2.5 cm in length in adults. Most of it appears as a broad anterior bulge rostral to the medulla. Posteriorly, it consists mainly of two pairs of thick stalks called cerebellar peduncles. These connect the cerebellum to the pons and midbrain.

The pons contains nuclei that relay signals from the forebrain to the cerebellum, along with nuclei that regulate sleep, respiration, swallowing, bladder control, hearing, equilibrium, taste, eye movement, facial expressions, facial sensation, and posture. Within the pons is the pneumotaxic center, a nucleus that regulates the change from inspiration to expiration. The pons also contains the sleep paralysis center of the brain and also plays a role in generating dreams.

Development

During embryonic development, the metencephalon develops from the rhombencephalon and gives rise to two structures: the pons and the cerebellum. The alar plate produces sensory neuroblasts, which will give rise to the solitary nucleus and its special visceral afferent column, the cochlear and vestibular nuclei (which form the special somatic afferent fibers of the vestibulocochlear nerve), the spinal and principal trigeminal nerve nuclei (which form the general somatic afferent column of the trigeminal nerve), and the pontine nuclei, which is involved in motor activity. Basal plate neuroblasts give rise to the abducens nucleus (forms the general somatic efferent fibers), the facial and motor trigeminal nuclei (form the special visceral efferent column), and the superior salivatory nucleus, which forms the general visceral efferent fibers of the facial nerve.

Cranial Nerves of the Pons

A number of cranial nerve nuclei are present in the pons:

• The chief or pontine nucleus of the trigeminal nerve sensory nucleus (V)- mid-pons
• The motor nucleus for the trigeminal nerve (V)-mid-pons
• Abducens nucleus (VI)-lower pons
• Facial nerve nucleus (VII)-lower pons
• Vestibulocochlear nuclei (VIII)-lower pons

Functional Characteristics

The functions of the four nerves of the pons include sensory roles in hearing, equilibrium, taste, and facial sensations such as touch and pain. They also have motor roles in eye movement, facial expressions, chewing, swallowing, urination, and the secretion of saliva and tears. Central pontine myelinosis is a demyelination disease that causes difficulty with sense of balance, walking, sense of touch, swallowing, and speaking. If it is not diagnosed and treated, it can lead to death or locked-in syndrome (a condition in which a person is conscious but cannot move or communicate).

Midbrain

The midbrain plays a major role in both wakefulness and regulation of homeostasis.

The midbrain or mesencephalon (from the Greek mesos, middle, and enkephalos, brain ) is a portion of the central nervous system (CNS) associated with vision, hearing, motor control, sleep and wake cycles, arousal (alertness), and temperature regulation. Anatomically, it comprises the tectum (or corpora quadrigemina), tegmentum, ventricular mesocoelia (or “iter”), and the cerebral peduncles, as well as several nuclei and fasciculi. Caudally (posteriorly) the mesencephalon adjoins the pons (metencephalon), and rostrally it adjoins the diencephalon (eg., thalamus, hypothalamus). The midbrain is located below the cerebral cortex and above the hindbrain placing it near the center of the brain.

Primary Midbrain Components

The tectum (Latin for “roof”) is formed by the superior and inferior colliculi and comprises the rear portion of the midbrain. The superior colliculus regulates preliminary visual processing and eye movement, while the inferior colliculus is involved in auditory processing. Collectively, the colliculi is referred to as the corpora quadrigemina.

The tegmentum is involved in many unconscious homeostatic and reflexive pathways, and is the motor center that relays inhibitory signals to the thalamus and basal nuclei to prevent unwanted body movement. It extends from the substantia nigra to the cerebral aqueduct (also called the ventricular mesocoeli). The nuclei of cranial nerves III and IV are located in the tegmentum portion of the midbrain.

The substantia nigra is closely associated with motor system pathways of the basal ganglia. The human mesencephalon is archipallian in origin, sharing its general architecture with the most ancient of vertebrates. Dopamine produced in the substantia nigra plays a role in motivation and habituation of species from humans to the most elementary animals such as insects. The midbrain is the smallest region in the brain and helps to relay information for vision and hearing.

The cerebral peduncles are located on either side of the midbrain and are its most anterior part, acting as the connectors between the rest of the midbrain and the thalamic nuclei. The cerebral peduncles assist in motor movement refinement, motor skill learning, and converting proprioceptive information into balance and posture maintenance.

Embryonic Development

During embryonic development, the midbrain arises from the second vesicle, also known as the mesencephalon, of the neural tube. Unlike the other two vesicles (the prosencephalon and rhombencephalon), the mesencephalon remains undivided for the remainder of neural development. It does not split into other brain areas while the prosencephalon, for example, divides into the telencephalon and the diencephalon. Throughout embryonic development, the cells within the midbrain continually multiply and compress the still-forming aqueduct of sylvius or cerebral aqueduct. Partial or total obstruction of the cerebral aqueduct during development can lead to congenital hydrocephalus.

Reticular Formation

The reticular formation assists in regulation of the sleep cycle and detecting sensory salience.

The reticular formation is a region in the pons involved in regulating the sleep-wake cycle and filtering incoming stimuli to discriminate irrelevant background stimuli. It is essential for governing some of the basic functions of higher organisms, and is one of the phylogenetically oldest portions of the brain.

Divisions of the Reticular Formation

Traditionally, the nuclei are divided into three columns:

1. Raphe nuclei (medium column)
2. Magnocellular red nucleus (medial zone)
3. Parvocellular reticular nucleus (lateral zone)

Sagittal division reveals more morphological distinctions. The raphe nuclei form a ridge in the middle of the reticular formation, and directly to its periphery, there is a division called the medial reticular formation. The medial reticular formation is large, has long ascending and descending fibers, and is surrounded by the lateral reticular formation. The lateral reticular formation is close to the motor nuclei of the cranial nerves and mostly mediates their function. The raphe nuclei is the place of synthesis of the neurotransmitter serotonin, which plays an important role in mood regulation.

The medial reticular formation and lateral reticular formation are two columns of neuronal nuclei with ill-defined boundaries that send projections through the medulla and into the mesencephalon (midbrain). The nuclei can be differentiated by function, cell type, and projections of efferent or afferent nerves. The magnocellular red nucleus is involved in motor coordination, and the parvocellular nucleus regulates exhalation.

The original functional differentiation was a division of caudal and rostral, based on the observation that damage to the rostral reticular formation induces a hypersomnia in the cat brain. In contrast, damage to the more caudal portion of the reticular formation produces insomnia in cats. This study led to the idea that the caudal portion inhibits the rostral portion of the reticular formation.

Functions

The reticular formation consists of more than 100 small neural networks, with varied functions including:

1. Somatic motor control: Some motor neurons send their axons to the reticular formation nuclei, giving rise to the reticulospinal tracts of the spinal cord. These tracts play a large role in maintaining tone, balance, and posture, especially during movement. The reticular formation also relays eye and ear signals to the cerebellum so that visual, auditory, and vestibular stimuli can be integrated in motor coordination. Other motor nuclei include gaze centers, which enable the eyes to track and fixate objects, and central pattern generators, which produce rhythmic signals to the muscles of breathing and swallowing.
2. Cardiovascular control: The reticular formation includes the cardiac and vasomotor centers of the medulla oblongata.
3. Pain modulation: The reticular formation is one means by which pain signals from the lower body reach the cerebral cortex. It is also the origin of the descending analgesic pathways. The nerve fibers in these pathways act in the spinal cord to block the transmission of some pain signals to the brain.
4. Sleep and consciousness: The reticular formation has projections to the thalamus and cerebral cortex that allow it to exert some control over which sensory signals reach the cerebrum and come to our conscious attention. It plays a central role in states of consciousness like alertness and sleep. Injury to the reticular formation can result in irreversible coma.
5. Habituation: This is a process in which the brain learns to ignore repetitive, meaningless stimuli while remaining sensitive to others. A good example of this is when a person can sleep through loud traffic in a large city, but is awakened promptly by the sound of an alarm or crying baby. Reticular formation nuclei that modulate activity of the cerebral cortex are part of the reticular activating system.

Effects of Damage

Mass lesions in the brainstem cause severe alterations in the level of consciousness (such as coma) because of their effects on the reticular formation. Lesions in the reticular formation have been found in the brains of people who have post-polio syndrome. Some imaging studies have shown abnormal activity in this area in people with chronic fatigue syndrome, indicating a high likelihood that damage to the reticular formation is responsible for the fatigue associated with these syndromes.