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Anatomy of the Brain: Diencephalon, Medulla Oblongata, and Cranial Nerves, Study notes of Embryology

Umpqua Community CollegeEmbryology

An in-depth exploration of the diencephalon, medulla oblongata, and cranial nerves. It covers the functions, structures, and connections of these vital parts of the brain. The diencephalon is divided into the thalamus and hypothalamus, with the thalamus relaying sensory information and the hypothalamus controlling hormone production and autonomic functions. The medulla oblongata regulates autonomic functions and includes the reticular formation and various sensory and motor nuclei. Twelve pairs of cranial nerves are discussed, each with its primary function, origin, pathway, and destination.

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14: The Brain and Cranial Nerves
I. An Introduction to the Organization of the Brain, p. 452
Objectives
1. Name the major regions of the brain and describe their functions
2. Name the three primary brain vesicles and indicate which adult structures each
gives rise to.
3. Name the ventricles of the brain and describe their locations and the connections
between them.
• The human brain ranges in size from 750 cc to 2100 cc and contains 98% of the
body’s neural tissue. The average brain weighs about 1.4 kg (3 lb.).
A Preview of Major Regions and Landmarks, p. 452
Figure 14-1
• The largest part of the brain is the cerebrum, which controls the higher mental
functions such as thought, memory and conscious movement. The cerebrum is
divided into left and right cerebral hemispheres, and covered by a surface layer of
gray matter or neural cortex (cerebral cortex). The surface is folded to increase
surface area, forming elevated ridges (gyri), shallow depressions, (sulci), and deep
grooves (fissures).
• The second largest part of the brain is the cerebellum, which coordinates
repetitive body movements. The cerebellum also has 2 hemispheres and is
covered in cerebellar cortex.
• Underneath the cerebrum and cerebellum is the diencephalon, which links the
cerebrum with the brain stem. The diencephalon is divided into the left thalamus
and right thalamus, which relay and process sensory information, and the
hypothalamus, which is involved in hormone production, emotion and autonomic
function.
• The hypothalamus is connected to the pituitary gland (a major endocrine gland)
via a small stalk called the infundibulum. The hypothalamus and pituitary gland
are the interface between the nervous system and the endocrine system.
• The brain stem, which processes information between the spinal cord and the
cerebrum or cerebellum, includes the mesencephalon, the pons, and the medulla
oblongata:
- The mesencephalon (midbrain) processes sight and sound (and their
associated reflexes) and maintains consciousness.
- The pons connects the cerebellum to the brain stem, and is involved in
somatic and visceral motor control.
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14: The Brain and Cranial Nerves

I. An Introduction to the Organization of the Brain, p. 452 Objectives

  1. Name the major regions of the brain and describe their functions
  2. Name the three primary brain vesicles and indicate which adult structures each gives rise to.
  3. Name the ventricles of the brain and describe their locations and the connections between them.
  • The human brain ranges in size from 750 cc to 2100 cc and contains 98% of the body’s neural tissue. The average brain weighs about 1.4 kg (3 lb.). A Preview of Major Regions and Landmarks, p. 452 Figure 14-
  • The largest part of the brain is the cerebrum, which controls the higher mental functions such as thought, memory and conscious movement. The cerebrum is divided into left and right cerebral hemispheres, and covered by a surface layer of gray matter or neural cortex (cerebral cortex). The surface is folded to increase surface area, forming elevated ridges (gyri), shallow depressions, (sulci), and deep grooves (fissures).
  • The second largest part of the brain is the cerebellum, which coordinates repetitive body movements. The cerebellum also has 2 hemispheres and is covered in cerebellar cortex.
  • Underneath the cerebrum and cerebellum is the diencephalon, which links the cerebrum with the brain stem. The diencephalon is divided into the left thalamus and right thalamus, which relay and process sensory information, and the hypothalamus, which is involved in hormone production, emotion and autonomic function.
  • The hypothalamus is connected to the pituitary gland (a major endocrine gland) via a small stalk called the infundibulum. The hypothalamus and pituitary gland are the interface between the nervous system and the endocrine system.
  • The brain stem, which processes information between the spinal cord and the cerebrum or cerebellum, includes the mesencephalon, the pons, and the medulla oblongata:
  • The mesencephalon (midbrain) processes sight and sound (and their associated reflexes) and maintains consciousness.
  • The pons connects the cerebellum to the brain stem, and is involved in somatic and visceral motor control.
  • The medulla oblongata connects the brain to the spinal cord. In addition to relaying information, the medulla oblongata regulates autonomic functions such as heart rate, blood pressure and digestion. Embryology of the Brain, p. 452 Table 14-
  • The organization of brain structures is determined by their embryological development.
  • The origin of the brain is the neural tube, which enlarges into 3 areas called primary brain vesicles (the prosencephalon, mesencephalon and rhombencephalon).
  • The prosencephalon and rhombencephalon subdivide to form 5 secondary brain vesicles. The telencephalon becomes the cerebrum. The metencephalon forms the cerebellum and pons. The myelencephalon becomes the medulla oblongata. Ventricles of the Brain, p. 453 Figure 14-
  • The neural tube encloses a fluid-filled cavity called the neurocoel. During development, the neurocoel expands to form chambers called ventricles, which are lined with cells of the ependyma.
  • The cerebral hemispheres each contain a large lateral ventricle, separated from each other by a thin medial partition called the septum pellucidum. The ventricle of the diencephalon is called the third ventricle. The lateral ventricles communicate with the third ventricle via the interventricular foramen (foramen of Monro).
  • The mesencephalon contains a narrow canal called the mesencephalic aqueduct (cerebral aqueduct), which connects the third ventricle with the fourth ventricle. The fourth ventricle extends into the medulla oblongata and becomes continuous with the central canal of the spinal cord. Key
  • The brain is a large, delicate mass of neural tissue containing internal passageways and chambers filled with cerebrospinal fluid.
  • Each of the five major regions of the brain has specific functions.
  • As you ascend from the medulla oblongata (which connects to the spinal cord) to the cerebrum, those functions become more complex and variable.
  • Conscious thought and intelligence are provided by the neural cortex of the cerebral hemispheres. II. Protection and Support of the Brain, p. 455

Cerebrospinal Fluid, p. 456

  • Cerebrospinal fluid (CSF) surrounds all exposed surfaces of the CNS and interchanges with the interstitial fluid of the brain. The major functions of CSF are: 1. Cushioning delicate neural structures. 2. Supporting the brain. 3. Transporting nutrients, chemical messengers, and waste products. Figure 14- The Formation of CSF
  • The choroid plexus is a combination of specialized ependymal cells and capillaries that produce cerebrospinal fluid. The ependymal cells secrete CSF into the ventricles, remove waste products from the CSF, and adjust the composition of CSF over time. Circulation of CSF
  • The choroid plexus produces about 500 ml of CSF a day, replacing the entire volume of CSF about every 8 hours.
  • CSF circulates from the choroid plexus thorough the ventricles to the central canal of the spinal cord.
  • CSF enters the subarachnoid space through 2 lateral apertures and 1 median aperture to circulate around the brain, spinal cord and cauda equina.
  • Extensions of the subarachnoid space (arachnoid villi) extend through the dura mater to the superior sagittal sinus.
  • Large clusters of villi form arachnoid granulations which absorb CSF into the venous circulation. The Blood Supply to the Brain, p. 458
  • The brain has a continuous, high demand for nutrients and oxygen which must be supplied by blood circulation. Blood is supplied to the brain by the internal carotid arteries and vertebral arteries, and drained from the dural sinuses by the internal jugular veins.
  • Disorders that interfere with normal blood circulation to the brain are called cerebrovascular diseases. A stroke or cerebrovascular accident (CVA) occurs when the blood supply to a portion of the brain is shut off, and neurons die. The Blood-Brain Barrier
  • Neural tissue in the CNS is isolated from general circulation by the blood-brain barrier (BBB) formed by an extensive network of tight junctions between endothelial cells lining the capillaries of the CNS.
  • Only lipid-soluble compounds (e.g. O2, CO2, steroids, and prostaglandins) can diffuse into the interstitial fluid of the brain and spinal cord. Astrocytes release

chemicals that control the permeability of the endothelium to other substances, effectively controlling the blood-brain barrier.

  • A blood-CSF barrier is formed by specialized ependymal cells surrounding the capillaries of the choroid plexus, which is not a part of the neural tissue of the brain.
  • Transport across the blood-brain and blood-CSF barriers is selective and directional, limiting the movement of many compounds. As a result, the chemical composition, pH and concentrations of major ions in blood and CSF are different.
  • The blood-brain barrier is continuous except in 4 specific cases:
    1. In portions of the hypothalamus, where hypothalamic hormones enter the systemic circulation.
    2. In the posterior lobe of the pituitary gland, where the hormones ADH and oxytocin are released into the circulation.
    3. In the pineal glands, where pineal secretions enter the circulation.
    4. At the choroid plexus, where specialized ependymal cells maintain the blood-CSF barrier. Key
  • The meninges stabilize the position of the brain within the cranial cavity.
  • Cerebrospinal fluid provides protection against sudden jolts and shocks.
  • CSF also provides nutrients and removes wastes generated by active neural tissues.
  • The blood-brain barrier and the blood-CSF barrier selectively isolate the brain from chemicals in blood that might disrupt neural function. III. The Medulla Oblongata, p. 459 Objectives
  1. Describe the anatomical differences between the medulla oblongata and the spinal cord.
  2. List the major components of the medulla oblongata and specify their functions. Figure 14-
  • The medulla oblongata is continuous with the spinal cord. It is here that the central canal opens into the fourth ventricle. Figure 14-
  • The medulla oblongata contains all of the ascending and descending tracts that allow the brain and spinal cord to communicate, coordinates complex autonomic reflexes, and controls visceral functions.
  • The medulla oblongata includes 3 groups of nuclei:
  1. List the main components of the cerebellum and specify their functions.
  • The cerebellum is an autonomic processing center. It has 2 primary functions:
    1. Adjusting the postural muscles of the body.
    2. Programming and fine tuning movements controlled at the conscious and subconscious levels. Table 14-3 shows the components of the cerebellum and their functions. Figure 14-
  • The surface of the cerebellum is composed of highly folded neural cortex (the folia).
  • The anterior and posterior lobes are separated by the primary fissure.
  • At the midline, a narrow band of cortex (the vermis) separates the cerebellar hemispheres.
  • The flocculonodular lobe lies below the fourth ventricle.
  • The cerebellar cortex contains large, branched Purkinje cells that receive input from up to 200,000 synapses.
  • The internal white matter of the cerebellum is highly branched, in a formation called the arbor vitae.
  • Cerebellar nuclei embedded in the arbor vitae relay information to the Purkinje cells.
  • The superior cerebellar peduncles, middle cerebellar peduncles, and inferior cerebellar peduncles are tracts that link the cerebellum with the brain stem, cerebrum and spinal cord.
  • Damage to the cerebellum, or alcohol intoxication, can disturb muscle coordination (ataxia). VI. The Mesencephalon, p. 464 Objective
  1. List the main components of the mesencephalon and specify their functions. Table 14-4 summarizes the components and functions of the mesencephalon or midbrain. Figure 14-
  • Structures of the tectum (the roof of the mesencephalon posterior to the mesencephalic aqueduct) include 2 pairs of sensory nuclei called the corpora quadrigemina:
  1. the superior colliculus receives visual input
  2. the inferior colliculus receives auditory input
  • Structures of the tegmentum (anterior to the mesencephalic aqueduct) include:
  • the red nucleus (many blood vessels)
  • the substantia nigra (pigmented gray matter)
  • The cerebral peduncles (nerve fiber bundles on the ventrolateral surfaces of the mesencephalon) contain descending fibers to the cerebellum, and fibers carrying motor commands. VII. The Diencephalon, p. 465 Objective
  1. List the main components of the diencephalon and specify their functions. Figure 14-
  • The diencephalon integrates conscious and unconscious sensory information and motor commands. It contains the epithalamus, the thalamus, and the hypothalamus.
  • The posterior part of the epithalamus contains the pineal gland, which secretes the hormone melatonin. The Thalamus, p. 466
  • The thalamus filters ascending sensory information for the primary sensory cortex, and relays information between the basal nuclei and cerebral cortex.
  • The left thalamus and right thalamus are separated by the third ventricle. A projection of gray matter called the intermediate mass extends into the ventricle from each side. Functions of the Thalamic Nuclei Figure 14-
  • Each thalamus consists of a rounded mass of thalamic nuclei which relay sensory information to the basal nuclei and cerebral cortex.
  • There are 5 major groups of thalamic nuclei: anterior, medial, ventral, posterior and lateral
  1. the anterior group includes the anterior nuclei, part of the limbic system involved with emotion.
  2. the medial group provides awareness of emotional states
  3. the ventral group relays sensory information
  4. the posterior group includes the pulvinar and genicular nuclei
  • the pulvinar integrates sensory information
  • the lateral geniculate nucleus receives visual information
  • the medial geniculate nucleus relays auditory information
  1. the lateral group affects emotional states and integrates sensory information Table 14-5 lists the 5 major groups of thalamic nuclei and their functions.

cerebrum and the sensory systems.

  • the limbic lobe of the cerebral hemisphere, including:
    • the cingulate gyrus
    • the dentate gyrus
    • the parahippocampal gyrus
    • the hippocampus (important in long-term memory)
  • the fornix, a tract of white matter connecting the hippocampus with the hypothalamus
  • the anterior nucleus of the thalamus relays information from the mamillary body of the hypothalamus to the cingulate gyrus.
  • stimulation or inhibition of the reticular formation affects a number of emotions (rage, fear, pain, sexual arousal, pleasure). Table 14-7 summarizes the structures and functions of the limbic system. IX. The Cerebrum, p. 470 Objectives
  1. Identify the major anatomical subdivisions of the cerebrum.
  2. Locate the motor, sensory, and association areas of the cerebral cortex, and discuss their functions.
  3. Discuss the origin and significance of the major categories of brain waves seen in an electroencephalogram.
  • The cerebrum is the largest part of the brain. It controls all conscious thoughts and intellectual functions, as well as processing somatic sensory and motor information.
  • Gray matter is located in the cerebral cortex and basal nuclei. White matter is found deep to the basal cortex and around the basal nuclei. The Cerebral Cortex, p. 470 Figure 14-
  • The gyri of the neural cortex increase its surface area, which increases the number of cortical neurons.
  • Some important structural features of the cerebrum include:
  • the longitudinal fissure which separates the two cerebral hemispheres
  • each hemisphere is divided into lobes:
  • the central sulcus divides the anterior frontal lobe from the posterior parietal lobe
  • the lateral sulcus separates the frontal lobe from the temporal lobe
  • the parieto-occipital sulcus separates the parietal lobe from the occipital lobe
  • the insula (island) of cortex, medial to the lateral sulcus
  • Each lobe contains functional regions that do not have clear structural boundaries.
  • There are 3 important guidelines concerning the function of the cerebral lobes:
  1. Each cerebral hemisphere receives sensory information from, and sends motor commands to, the opposite side of the body.
  2. The 2 hemispheres have different functions (although their structures are the same).
  3. The correspondence between a specific function and a specific region of the cerebral cortex is not precise. The White Matter of the Cerebrum, p. 472 Figure 14-
  • The axons (white matter) inside the cerebrum may be classified as association fibers, commissural fibers or projection fibers.
  1. Association fibers interconnect areas of neural cortex within the same hemisphere:
  • arcuate fibers are short, from one gyrus to another
  • longitudinal fasciculi are longer bundles, connecting the frontal lobe with other lobes of the same hemisphere
  1. Commissural fibers are bands of fibers connecting the 2 cerebral hemispheres:
  • corpus callosum
  • anterior commissure
  1. Projection fibers pass through the diencephalon to link the cerebral cortex with the diencephalon, brain stem, cerebellum and spinal cord.
  • the internal capsule is made up of all ascending and descending projection fibers The Basal Nuclei, p. 472 Figure 14-
  • Basal nuclei (cerebral nuclei) are masses of gray matter embedded in white matter of the cerebrum, deep to the floor of the lateral ventricle within each hemisphere. Basal nuclei direct subconscious activities such as processing sensory information and subconscious motor commands.
  • The major structures of the basal nuclei include:
  • the caudate nucleus (with a slender tail)
  • the lentiform nucleus (consisting of the globus pallidus and the putamen)
  • The amygdaloid body (limbic system) lies anterior to the tail of the caudate nucleus and inferior to the lentiform nucleus.

as predicting the consequences of different actions.

  • A procedure called prefrontal lobotomy was once used to treat mental illnesses by causing patients to lose all concerns about the consequences of their actions.
  • Brodmann’s areas describe patterns of cellular organization within the cerebral cortex, some of which correspond with functional areas such as the speech center. Hemispheric Lateralization Figure 14-
  • Each cerebral hemisphere is responsible for certain functions that are not normally performed by the opposite hemisphere (hemispheric lateralization).
  • In most people, the left brain (the dominant hemisphere) controls the reading, writing, mathematical, decision-making, speech and language skills.
  • The right cerebral hemisphere relates to the senses of touch, smell, sight, taste and feel, such as recognizing faces and inflections of voice. Monitoring Brain Activity: The Electroencephalogram
  • The electrical activity of the brain can be monitored to assess brain activity by placing electrodes on the skull and printing out a pattern of the electrical activity (or brain waves) called the electroencephalogram (EEG). Figure 4-
  • Typical brain waves are divided into 4 categories:
  1. alpha waves are found in healthy, awake adults at rest with their eyes closed
  2. beta waves, of higher frequency, are seen when the person is concentrating or mentally stressed
  3. theta waves are found in children or intensely frustrated adults, or may indicate a brain disorder in adults
  4. delta waves are seen during sleep, or in adults with brain damage
  • A pacemaker mechanism synchronizes electrical activity between the 2 hemispheres. Brain damage can cause the hemispheres to become unsynchronized.
  • During a temporary cerebral disorder called a seizure, the electroencephalogram changes significantly. Other symptoms may occur, depending upon the regions of the brain affected. X. Focus: Cranial Nerves, p. 480 Figure 14-
  • Twelve pairs of cranial nerves (Roman numerals I-XII) connect directly to the

brain, near associated sensory or motor nuclei.

  • Cranial nerves are classified by their primary function:
    1. Sensory nerves carrying somatic sensory information (touch, pressure, vibration, temperature, pain)
    2. Special sensory nerves carrying sensations of smell, sight, hearing or balance.
    3. Motor nerves: axons of somatic motor neurons.
    4. Mixed nerves: a mixture of motor and sensory fibers.
  • Most cranial nerves also have important secondary functions, such as distributing autonomic fibers to peripheral ganglia.
  • For each of the 12 cranial nerves, we will summarize its primary function, origin, pathway and destination. The Olfactory Nerves (I) Figure 14- Primary function: special sensory (smell) Origin: receptors of olfactory epithelium Pathway: olfactory foramina in cribriform plate of ethmoid Destination: olfactory bulbs
  • The olfactory nerves are made up of about 20 bundles of sensory neurons in epithelium covering the roof of the nasal cavity. The bundles enter the olfactory bulbs on either side of the crista galli. The olfactory tracts are axons of postsynaptic neurons leading to the cerebrum. The Optic Nerves (II) Figure 14- Primary function: special sensory (vision) Origin: retina of eye Pathway: optic canals of sphenoid Destination: diencephalon via the optic chiasm
  • The optic nerves contain about 1 million sensory nerve fibers that converge at the optic chiasm where fibers cross over to the opposite side of the brain. The optic tracts are reorganized axons leading toward the lateral geniculate nuclei. The Oculomotor Nerves (III) Figure 14- Primary function: motor (eye movements) Origin: mesencephalon Pathway: superior orbital fissures of sphenoid Destination:
  • somatic motor: superior, inferior and medial rectus muscles; inferior

semilunar ganglion contains the cell bodies of the sensory neurons. Tic douloureux is a painful disorder of the maxillary and mandibular branches. The Facial Nerves (VII) Figure 14- Primary function: mixed (sensory and motor) to face Origin:

  • sensory: taste receptors on anterior 2/3 of tongue
  • motor: motor nuclei of pons Pathway: internal acoustic canals to facial canals (stylomastoid foramina) Destination:
  • sensory: sensory nuclei of pons
  • somatic motor: muscles of facial expression
  • visceral motor: tear and nasal mucous glands; submandibular and sublingual salivary glands
  • The facial nerves split to form the temporal, zygomatic, buccal, mandibular and cervical branches. The sensory neurons monitor parts of the face and the anterior 1/3 of the tongue. The cell bodies of the sensory neurons are located in the geniculate ganglia, and the motor nuclei are in the pons. Postganglionic fibers from the pterygopalatine ganglia innervate lacrimal, nasal cavity, and pharynx glands. The submandibular ganglia innervate the salivary glands. An inflammation of the facial nerve causes Bell’s palsy. The Vestibulocochlear Nerves (VIII) Figure 14- Primary function: special sensory
  • vestibular branch: balance and equilibrium
  • cochlear branch: hearing Origin: receptors of the inner ear Pathway: internal acoustic canals of temporal bones Destination: vestibular and cochlear nuclei of pons and medulla oblongata
  • The vestibulocochlear nerves (auditory nerves) have 2 distinct bundles of sensory fibers:
  • the vestibular branch originates at receptors of the vestibule (balance), and connects to the vestibular nuclei of the pons and medulla oblongata.
  • the cochlear branch originates at sensors of the cochlea (hearing), and connects with the cochlear nuclei of the pons and medulla oblongata. The Glossopharyngeal Nerves (IX) Figure 14- Primary function: mixed (sensory and motor) to head and neck Origin:
  • sensory: posterior 1/3 of tongue, part of the pharynx and palate, carotid arteries
  • motor: motor nuclei of medulla oblongata Pathway: jugular foramina between the occipital bone and the temporal bones Destination:
  • sensory: sensory nuclei of medulla oblongata
  • somatic motor: nerves involved in swallowing
  • visceral motor: parotid salivary gland
  • The glossopharyngeal nerves innervate the tongue and pharynx. The sensory neurons are in the superior ganglion and inferior ganglion. Visceral motor fibers synapse in the otic ganglion. The Vagus Nerves (X) Figure 14- Primary function: mixed (sensory and motor): thorax and abdomen Origin:
  • sensory: part of pharynx, auricle and external acoustic canal, diaphragm, visceral organs of thoracic and abdominopelvic cavities
  • motor: motor nuclei in medulla oblongata Pathway: jugular foramina between the occipital bone and temporal bones Destination:
  • sensory: sensory nuclei and autonomic centers of medulla oblongata
  • visceral motor: palate, pharynx, digestive, respiratory and cardiovascular systems in thoracic and abdominal cavities
  • The vagus nerves branch and radiate extensively. Sensory neurons are located in the jugular ganglion and the inferior nodose ganglion. Preganglionic autonomic motor fibers affect the heart, smooth muscle and glands. The Accessory Nerves (XI) Figure 14- Primary function: motor to muscles of neck and upper back Origin: motor nuclei of spinal cord and medulla oblongata Pathway: jugular foramina between occipital bone and temporal bones Destination:
  • internal branch: voluntary muscles of palate, pharynx, larynx
  • external branch: sternocleidomastoid and trapezius muscles
  • The accessory nerves (spinal accessory nerves) each have some motor fibers that originate in the anterior gray horns of the first 5 cervical segments of the spinal cord, forming the spinal root. Motor fibers of the cranial root originate in the medulla oblongata. The 2 roots combine in the cranium, then divide into 2 branches as they leave the cranium: The internal branch joins the vagus nerve; the external branch controls muscles of the neck and back. The Hypoglossal Nerves (XII) Figure 14-
  • the mesencephalon
  • the diencephalon
    • epithalamus, thalamus, and hypothalamus
  • the limbic system (gyri and fornix)
  • the cerebrum
    • the cerebral cortex
    • the white matter
    • the basal nuclei
    • motor and sensory cortex
    • association areas
    • functional centers
    • the electroencephalogram
  • the 12 pairs of cranial nerves (sensory, motor and mixed)
  • their primary functions, origins, pathways and destinations
  • cranial reflexes