General Features

The brain stem is located in the posterior cranial fossa on the dorsal aspect of the clivus. There should be CSF space between the clivus and the brain stem in normal patients, and the 4th ventricle is posterior to the brain stem between it and the cerebellum Cerebellum The cerebellum, Latin for "little brain," is located in the posterior cranial fossa, dorsal to the pons and midbrain, and its principal role is in the coordination of movements. The cerebellum consists of 3 lobes on either side of its 2 hemispheres and is connected in the middle by the vermis. Cerebellum. The midbrain is most superior, followed by the pons, and the medulla oblongata, which is most inferior. 

Divisions (from rostral to caudal)

• Midbrain
• Pons
• Medulla oblongata

Midbrain

• Connects to the diencephalon above and the pons below
• Divided into the ventral tegmentum and the dorsal tectum
• Contains many vital structures:
  • Cranial nerves (CNs) III and IV nuclei
  • Reticular formation
  • Extrapyramidal structures
  • Cerebral aqueduct
• Blood supply: branches of posterior cerebral artery (PCA)

Pons

• Connects the midbrain above to the medulla oblongata below
• Cranial nerves: site of CNs V, VI, VII, and VIII nuclei
• Blood supply: branches of the basilar artery and anterior inferior cerebellar artery

Medulla oblongata

• Connects brain stem with the spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord, with the transition occurring at the level of the foramen magnum
• 3 vital centers located in the medulla:
  • Cardiac center
  • Medullary rhythmicity center
  • Vasomotor center
• Cranial nerves: site of CNs IX, X, XI, and XII nuclei
• Blood supply: branches of anterior spinal artery, vertebral artery, and posterior inferior cerebellar artery

Internal structure of the brain stem

3 laminae:

• Tectum:
  • Consists of quadrigeminal plate and medullary velum 
  • Responsible for auditory and visual reflexes
• Tegmentum:
  • Middle layer
  • Site of cranial nerve nuclei somatotopic organization 
• Basis:
  • Most anterior part
  • Route of motor pathways

Cranial nerve origins

• Midbrain:
  • Oculomotor (III)
  • Trochlear (IV)
• Pons:
  • Trigeminal (V)
  • Abducens (VI)
  • Facial (VII)
  • Vestibulocochlear (VIII)
• Medulla oblongata:
  • Glossopharyngeal (IX)
  • Vagus (X)
  • Spinal accessory (XI)
  • Hypoglossal (XII)

Midbrain

Overview

• Connects diencephalon to the pons
• Origin of 2 cranial nerves Cranial nerves There are 12 pairs of cranial nerves (CNs), which run from the brain to various parts of the head, neck, and trunk. The CNs can be sensory or motor or both. The CNs are named and numbered in Roman numerals according to their location, from the front to the back of the brain. Overview of the Cranial Nerves 
  • III (oculomotor) 
  • IV (trochlear)

Internal organization

• Crus cerebri:
  • Major pathway of motor output from the cortex
  • Anterior portion of cerebral peduncle
• Substantia nigra:
  • Member of the basal ganglia Basal Ganglia Basal ganglia are a group of subcortical nuclear agglomerations involved in movement, and are located deep to the cerebral hemispheres. Basal ganglia include the striatum (caudate nucleus and putamen), globus pallidus, substantia nigra, and subthalamic nucleus. Basal Ganglia
  • Conveys dopaminergic fibers to the basal ganglia Basal Ganglia Basal ganglia are a group of subcortical nuclear agglomerations involved in movement, and are located deep to the cerebral hemispheres. Basal ganglia include the striatum (caudate nucleus and putamen), globus pallidus, substantia nigra, and subthalamic nucleus. Basal Ganglia
  • Involved in regulation of movement
  • Site of dopamine production
• Mesencephalic tegmentum:
  • Red nucleus
  • Oculomotor nucleus
  • Trochlear nucleus
  • Medial longitudinal fasciculus
• Tectum:
  • Inferior colliculus
  • Superior colliculus
  • Pretectal nucleus

Additional features

• Cerebral aqueducts
• Periaqueductal gray

Arterial supply of the midbrain

• Various branches of the posterior cerebral artery
• Thalamoperforate arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries
• Medial posterior choroidal artery

Pons

Overview

• Connects the midbrain above to the medulla oblongata below
• Origin of 4 cranial nerves Cranial nerves There are 12 pairs of cranial nerves (CNs), which run from the brain to various parts of the head, neck, and trunk. The CNs can be sensory or motor or both. The CNs are named and numbered in Roman numerals according to their location, from the front to the back of the brain. Overview of the Cranial Nerves: 
  • V (trigeminal)
  • VI (abducens) 
  • VII (facial)
  • VIII (vestibulocochlear)

Internal organization

• Basilar pons: motor fibers of the corticopontine, corticospinal, and corticobulbar descending via the crus cerebri
• Pontine tegmentum: site of all cranial nerve nuclei associated with the pons

Arterial supply

• Blood supply to the pons is primarily derived from branches of the basilar artery.
• Additional supply comes from branches of anterior inferior cerebellar artery.

Anatomical localization

• Anterior: prepontine cistern ( cranial nerves Cranial nerves There are 12 pairs of cranial nerves (CNs), which run from the brain to various parts of the head, neck, and trunk. The CNs can be sensory or motor or both. The CNs are named and numbered in Roman numerals according to their location, from the front to the back of the brain. Overview of the Cranial Nerves (CNs) V and VI)
• Posterior: 4th ventricle
• Lateral: cerebellopontine angle (CNs VII and VIII)
• Superior: continuous with midbrain
• Inferior: continuous with medulla oblongata

Mnemonic: pons rule of 4

• Origin to 4 of the cranial nerves Cranial nerves There are 12 pairs of cranial nerves (CNs), which run from the brain to various parts of the head, neck, and trunk. The CNs can be sensory or motor or both. The CNs are named and numbered in Roman numerals according to their location, from the front to the back of the brain. Overview of the Cranial Nerves: 
  • V (trigeminal)
  • VI (abducens)
  • VII (facial) 
  • VIII (vestibulocochlear)
• 4 medial structures with “M”: 
  • Motor pathway (corticospinal tract)
  • Motor nucleus (abducens)
  • Medial lemniscus (somatosensory fibers)
  • Medial longitudinal fasciculus (coordinating CNs III, VI, and VIII functions with the spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord)
• 4 lateral structures with “S”: 
  • Sensory nuclei (trigeminal)
  • Spinothalamic tract ( pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain and temperature)
  • Spinocerebellar pathway (unconscious proprioception) 
  • Sympathetic pathway

Medulla Oblongata

Overview

• Extends from the 1st part of the cervical spinal nerves (at the foramen magnum) → lower border of the pons
• Origin for the lower cranial nerves Cranial nerves There are 12 pairs of cranial nerves (CNs), which run from the brain to various parts of the head, neck, and trunk. The CNs can be sensory or motor or both. The CNs are named and numbered in Roman numerals according to their location, from the front to the back of the brain. Overview of the Cranial Nerves:
  • IX (glossopharyngeal)
  • X (vagus)
  • XI (accessory)
  • XII (hypoglossal)
• 3 vital centers in the medulla: 
  • Cardiac center: sets the rate of cardiac contractions
  • Medullary rhythmicity center: synchronizes with the pontine respiratory centers to regulate the rhythm of breathing
  • Vasomotor center: controls vascular tone

Internal organization

• Corticospinal tract:
  • Motor pathway beginning in motor cortex → lower motor neurons in the spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord 
  • Controls motor movements of limbs and trunk
• Sensory nucleus of the trigeminal nerve:
  • Carries pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain and temperature sensation through Aδ and C nerve fibers from the ipsilateral face 
  • Project to the ventral posteromedial nucleus (VPM) of the thalamus Thalamus The thalamus is a large, ovoid structure in the dorsal part of the diencephalon that is located between the cerebral cortex and midbrain. It consists of several interconnected nuclei of grey matter separated by the laminae of white matter. The thalamus is the main conductor of information that passes between the cerebral cortex and the periphery, spinal cord, or brain stem. Thalamus
• Dorsal column nuclei:
  • Carries sensation of pressure, vibration, fine touch, and proprioception from the ipsilateral body
  • Aɑ and Aß somatosensory fibers from gracile and cuneate nuclei of ipsilateral body
  • Decussate in medulla and continue contralaterally as medial lemniscus
  • Project to the ventral posterolateral nucleus (VPL) of the thalamus Thalamus The thalamus is a large, ovoid structure in the dorsal part of the diencephalon that is located between the cerebral cortex and midbrain. It consists of several interconnected nuclei of grey matter separated by the laminae of white matter. The thalamus is the main conductor of information that passes between the cerebral cortex and the periphery, spinal cord, or brain stem. Thalamus
• Spinothalamic tract: 
  • Carries pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain, temperature, and crude touch via Aδ and C nerve fiber from the contralateral body
  • Projects to VPL of the thalamus Thalamus The thalamus is a large, ovoid structure in the dorsal part of the diencephalon that is located between the cerebral cortex and midbrain. It consists of several interconnected nuclei of grey matter separated by the laminae of white matter. The thalamus is the main conductor of information that passes between the cerebral cortex and the periphery, spinal cord, or brain stem. Thalamus
• Inferior olivary nucleus:
  • Major source of input to the cerebellum Cerebellum The cerebellum, Latin for "little brain," is located in the posterior cranial fossa, dorsal to the pons and midbrain, and its principal role is in the coordination of movements. The cerebellum consists of 3 lobes on either side of its 2 hemispheres and is connected in the middle by the vermis. Cerebellum
  • Involved in motor control
• Nucleus solitarius: gustatory information
• Nucleus ambiguus: associated with speech and swallowing

Arterial supply

• Lateral: posterior inferior cerebellar artery ( PICA Pica Pica is an eating disorder characterized by a desire or recurrent compulsion to eat substances that are nonnutritive and not food. These compulsions and ingested substances are inappropriate for age or culture. Pica)
• Medial: anterior spinal artery/vertebral artery (ASA/VA)

Brain Stem Reticular Formation

Overview

• Set of interconnected nuclei:
  • Midbrain: dorsal tegmental nuclei
  • Pons: central tegmental nuclei
  • Medulla: inferior nuclei 
• Composed of 2 components: 
  • Ascending reticular formation (reticular activating system) 
  • Descending reticular formation

Internal organization

• Median column of reticular nuclei:
  • Extends through medulla, pons, and midbrain
  • Collectively, these constitute raphe nuclei
• Medial column of reticular nuclei:
  • Consists of medium and very large neurons, gigantocellular nuclei
  • Innervates CN XII nuclei and responses to glutamatergic stimuli
• Lateral column of reticular nuclei:
  • Consists of 6 nuclear groups known as the parvocellular nuclei
  • Involved in inspiratory portion of respiration

Clinical Relevance

• Chiari malformation: Arnold- Chiari malformations Chiari Malformations Chiari malformations (CMs) are a group of central nervous system (CNS) conditions characterized by the underdevelopment of the posterior cranial fossa with subsequent protrusion of neural structures through the foramen magnum. Chiari Malformations are a group of congenital abnormalities that are associated with a bony base of the cranium causing limitation of space in the posterior fossa affecting the cerebellum Cerebellum The cerebellum, Latin for "little brain," is located in the posterior cranial fossa, dorsal to the pons and midbrain, and its principal role is in the coordination of movements. The cerebellum consists of 3 lobes on either side of its 2 hemispheres and is connected in the middle by the vermis. Cerebellum, brain stem, and upper spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord.
• Cranial nerves: emerge directly via the brain and brain stem through the bony skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull. Cranial nerves supply cranial structures and fulfill specific functions. There are 12 cranial nerves Cranial nerves There are 12 pairs of cranial nerves (CNs), which run from the brain to various parts of the head, neck, and trunk. The CNs can be sensory or motor or both. The CNs are named and numbered in Roman numerals according to their location, from the front to the back of the brain. Overview of the Cranial Nerves that transmit sensory, motor, and autonomic information to and from the intracranial and pericranial structures, with some interconnection to the rest of the body. 
• Embryologic development: before the brain reaches the phase at which it looks like the organ, there are several complex processes that occur to achieve this goal. Beginning with neurulation, the neural tube and neural crest cells allow the developing embryo to form a central and peripheral nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. General Structure of the Nervous System. 
• Diencephalon: The diencephalon of the brain consists of 4 components: the thalamus Thalamus The thalamus is a large, ovoid structure in the dorsal part of the diencephalon that is located between the cerebral cortex and midbrain. It consists of several interconnected nuclei of grey matter separated by the laminae of white matter. The thalamus is the main conductor of information that passes between the cerebral cortex and the periphery, spinal cord, or brain stem. Thalamus, the epithalamus, the hypothalamus Hypothalamus The hypothalamus is a collection of various nuclei within the diencephalon in the center of the brain. The hypothalamus plays a vital role in endocrine regulation as the primary regulator of the pituitary gland, and it is the major point of integration between the central nervous and endocrine systems. Hypothalamus, and the subthalamus. Overall, the diencephalon coordinates unconscious vegetative and sensorimotor functions.
• Hydrocephalus: disorder of CSF disequilibrium of formation, flow, and/or absorption, resulting in an increase in the volume of fluid within the cranial cavity. 
• Parkinson disease: a movement disorder that is characterized by degeneration of dopaminergic neurons of the substantia nigra, which is a part of the basal ganglia Basal Ganglia Basal ganglia are a group of subcortical nuclear agglomerations involved in movement, and are located deep to the cerebral hemispheres. Basal ganglia include the striatum (caudate nucleus and putamen), globus pallidus, substantia nigra, and subthalamic nucleus. Basal Ganglia.