Brainstem -Neuroanatomy-

Brainstem:

The Brainstem occupies the posterior cranial fossa of the skull and comprise of three parts:

1. Medulla (Myelincephalon)
2. The Pons (Metincephalon)
3. The Midbrain (Mesencephalon)

The pons and medulla together form Rhombincephalon. The brainstem connects the spinal cord inferiorly to the brain (diencephalon specifically) superiorly.  It has three functions:

1. it channels the ascending and descending tracts between the spinal cord and brain.
2. it houses the reflex centers e.g. those associated with respiratory, cardiovascular system and with control of consciousness
3. it contains the nuclei of all the cranial nerves except the first 2.

Medulla connects the pons superiorly with the spinal cord. On its anterior surface is a fissure called Anterior Median Fissure which continues with the fissure of spinal cord. Lateral to the fissure, on either side are large elevations called Pyramids that houses bundles of descending nerve fibers called Corticospinal Fibers. Lateral to pyramids are Olives which marks the location of underlying Olivary Nuclei.

 The groove between the pyramids and the olives bear the rootlets of Hypoglossal (12^th cranial) Nerve.  Posterior to olives, the inferior cerebellar peduncles connect the medulla to the cerebellum. The grove between the inferior cerebellar peduncle and the olives are rootlets for the glossopharyngeal (9^th), Vegas (10^th) and cranial roots of accessory nerve (11^th). Posterior Surface of superior half medulla forms the inferior part of the floor of the 4^th ventricle. The posterior surface possesses the posterior median sulcus, lateral to which are Gracile and Cuneate tubercle respectively.

Pons connects the Midbrain to the Medulla. Its anterior surface is convex and bears many transverse fibers the form the Middle Cerebellar Peduncle. There is a shallow grove in the midline, the basilar grove for the basilar artery. On the anterolateral surface of the pons, the sensory and motor roots of trigeminal nerve emerge. At the pontomedullary junction, from medial to lateral emerges the Abducent (6^th), Facial (7^th) and Vestibulocochlear (8^th) nerve. Posterior surface of the pons form the superior part of the floor of the 4^th ventricle. Posterior median sulcus divides the posterior surface into symmetric halves, limited by the superior cerebellar peduncles. Lateral to the sulcus are elevations called medial eminence that are bounded laterally by Sulcus limitans. Inferior end of median eminence is slightly expanded to form the Facial Colliculus.

Midbrain connects the pons and medulla to the cerebrum. It is traversed by narrow channel, the cerebral aqueduct which is filled with CSF. On the posterior surface are four Collicili (corpora quadrigemina). The superior colliculi are centers for visual reflex while the inferior colliculi are lower auditory centers. In the midline, below the inferior Colliculus, the trochlear (4^th) nerve emerges. On the anterior surface of midbrain there is a deep depression called the interpeduncular fossa which is bounded on either side by crus cerebri. The Occulomotor nerve emerges for from the groove on the medial side of crus cerebri and passes forward in the lateral wall of cavernous sinus.  

Histology Of Neuroanatomy.

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Base of the Skull

Base of the Skull:

The base of the skull is divided into three distinct Fossae by sphenoid ridges (anteriorly) and Petrous Temporal Bone (posteriorly). These are the Anterior, Middle and Posterior Cranial Fossae.

Anterior Cranial Fossa:

Anterior cranial fossa lies above the nasal cavity and the orbits and houses the frontal lobe of the two cerebral hemispheres. Bones that form the anterior cranial fossa are:

·         The Ethmoid bone (in the midline)

·         The orbital part of frontal bone (anterolateral to Ethmoid bone)

·         The Sphenoid Bone (posteriorly)

The Ethmoid bone found in the middle of the anterior cranial fossa has a small prominent bony ridge called the Cristal Galli. This along with the Frontal Crest on the anteriorly provide attachment site for Falx Cerebri –vertical extension of Dura matter, partially separating the two cerebral hemispheres-. On either side of the Crista Galli are cribriform plates –for the olfactory bundle- that form the narrow roof of the nasal cavity. Anteriorly between the Crista Galli and frontal crest is Foramen Cecum.

Lateral to the Ethmoid bone are the orbital part of the frontal bone, forming the floor of the anterior cranial fossa and roof of the orbits. Posterior to these two bones is the Ethmoid bone, the body of which extends posteriorly into the middle cranial fossa. Laterally projecting from the body are the lesser wings of the sphenoid bone that extends as far as the junction between the temporal and greater wing of sphenoid bone on the lateral wall of the skull. The lesser wing of sphenoid bone also forms boundary between the anterior and middle cranial fossa. Medially the lesser wing of sphenoid fossa form the Anterior Clinoid Process that provide attachment site for the Tentorium Cerebri –sheet of dura matter separating parts of cerebral hemisphere from cerebellum. Anterior to it is an opening –optic canal- for the ophthalmic artery and optic nerve.

Middle Cranial Fossa:

Middle cranial fossa is bounded anteriorly by the chiasmatic sulcus and the lesser wing of the sphenoid bone. While the posterior boundary is formed by the anterior surface of the superior part, of the petrous part of the temporal bone. The bones that form the floor of this fossa are parts of sphenoid and temporal bone.

The floor at the midline of the middle cranial fossa is formed by the body of the sphenoid bone. On either side of it are large depressions formed by the greater wing of the sphenoid and squamous part of temporal bone. These depressions contain the temporal lobe.

Sella Tursica is a complex bony structure formed by the sphenoid bone. The anterior wall of the Sella Tursica is formed called the Tuberculum Sellae. The posterior wall – the Dorsum Sellae- of Sella Tursica have prominent bony projection on the lateral edges which form the Posterior Clinoid Process, which like its anterior counterpart, provide attachment site for the Tentorium Cerebri. Between the two walls is the Hypophysial Fossa containing the pituitary gland.

On the petrous part of temporal bone, medially there is a slight depression called the trigeminal impression for the sensory ganglia of trigeminal nerve. Lateral to that is a round bony protuberance – the Arcuate eminence –produced by the underlying semicircular canal (internal ear). Anterolateral to that is Tegmen Tempani which is a slight depression that marks the roof of the middle ear cavity.

Foramen and fissures that are found in the middle cranial fossa are:

·         The Superior Orbital Fissure – Occulomotor, Trochlear, Opthalmic (nerve and vein) and Abducent  Nerve

·         Foramen Rotundum – maxillary branch of trigeminal nerve

·         Foramen Ovale – Mandibular nerve, Petrosal Nerve and the accessory middle meningeal artery

·         Foramen Spinosum – The Middle meningeal artery and vein.

Posterior Cranial Fossa:

It is formed mainly by the occipital bone and temporal bone. It is the largest and deepest cranial fossa and houses the brainstem (medulla, pons, and midbrain) and cerebellum. The anterior boundary of the posterior cranial fossa is the Dorsum Sellae and Clivus. The Clivus slopes from the foramen magnum and is partly formed by the sphenoid and partly by the occipital bone. It also forms the base of the occipital bone. Laterally the anterior boundary of the posterior cranial fossa is formed by part of the petrous part of temporal bone. Posteriorly it is bounded by the internal surface of the squamous part of the occipital bone.

There is a large foramen in the deepest part of the posterior cranial fossa, the Foramen magnum, allows the spinal cord to pass into the skull and connect to the brainstem.

Foramen:

·         Internal acoustic meatus –Facial nerve, Vestibulocochlear nerve

·         Jugular foramen –Glossopharyngeal, Vegas and Accessory nerve

·         Hypoglossal foramen –Hypoglossal nerve  

Inferior View Of The Skull

Inferior View Of The Skull:

In this section we’ll look the skull inferiorly. The bones that form this part of the skull are the Maxilla, Palatine, Sphenoid, Occipital and Temporal Bone. Inferiorly, the skull is divided into 3 parts: 

1. An anterior part –that includes the teeth and the hard palate
2. A middle part –that extends from the base of the hard palate to the anterior margin of the Foramen Magnum
3. A posterior part –that extends from the anterior margin of Foramen Magnum, all the way to the superior nuchal line posteriorly.

Now let’s examine the individual parts and the bones they house, in detail.

Anterior Part:

This part mainly forms the teeth and the hard palate.

The teeth are planted in the Alveolar Arch of the maxilla.

The Hard palate is formed anteriorly by the paired Palatine process of the maxilla and posteriorly by the Horizontal Plates of the Palatine Bone. The palatine process of the maxilla meets at the midline to form the intermaxillary suture, posteriorly with the palatine bone at the palatomaxillary suture. The paired horizontal plate of Palatine bone meets at the midline at the interpalatine suture. Together all the three sutures form the crucifix suture.

More of the bony feature include;

• The incisive fossa –present behind the incisors. 
• The Greater Palatine Foramen leading to Greater palatine canal –present on the posterolateral border of the hard palate. 
• The Lesser Palatine Foramen Leading to Lesser Palatine Canal –present behind the Greater palatine foramen, on the pyramidal process of palatine bone. 
• And the Posterior Nasal Spine –at the posterior free border of the hard palate

Middle Part:

For sake of description, the middle part if further divided into 2 parts: 

• Anterior Half –the sphenoid and vomer bones.
• Posterior Half –the occipital and paired temporal bones.

Anterior Half of the Middle part of the Base of the skull is formed by Vomer and Sphenoid Bone.

Vomer bone rests on the body of the sphenoid bone at the midline. It forms the bony nasal septum, separating the two Choanae.

Sphenoid bone is composed of 4 parts: 

1. The Body
2. The Pterygoid Processes.
3. The Lesser Wing
4. The Greater Wing

However only 3 parts (excluding the lesser wing) of the sphenoid bone are visible from the Inferior view of the skull. The Body of the Sphenoid Bone is place at the midline and comprises of 2 sphenoid sinuses divided by a septum. It articulates anteriorly with the Vomer, Ethmoid, Palatine bones, posteriorly with the base of occipital bone, and laterally with the temporal bone.

The two Pterygoid Processes project inferiorly from the junction of body and greater wing of the sphenoid bone. These processes comprise of the medial and lateral plate separated by the Pterygoid fossa. The bony features i.e the Pterygoid hamulus, pteregoid fossa and the scaphoid fossa are labeled on the models.

The Greater Wing of the Sphenoid bone not only forms the base of the skull but also contributes to the lateral wall as well. It articulates laterally and posteriorly with parts of the temporal bone.

Posterior Half of the middle part of the base of the skull is composed of occipital and temporal bones. The occipital bone or more specifically the basilar part of the occipital bone is bounded anteriorly by the body of the sphenoid bone, laterally by the temporal bone and posteriorly it is limited by the foramen magnum. Prominent feature on the basilar part of the occipital bone is the Pharyngeal Tubercle which provides attachment for part of the pharynx to the base of the skull. The occipital bone is discussed in detail later in the ‘posterior part’ of the middle part of the base of the skull.

Temporal bone is found immediately lateral to the sphenoid and basilar part of occipital bone. The petrous part of temporal bone is bounded anteriorly by the greater wing of sphenoid bone and posteriorly by the occipital bone. Lateral to the junction of the basilar part of occipital bone and body of sphenoid, is the irregular shaped foramen Lacerum. Posterolateral to the foramen Lacerum is the carotid canal for the internal carotid artery. Lateral to the greater wing of the sphenoid bone is the squamous part of the temporal bone that houses the Mandibular Fossa for the temporomandibular joint. Anterior to that is a prominent feature, the Articulate Tubercle.

Posterior Part:

Posterior part of the base of the skull extends from the anterior border of the foramen magnum all the way to the external occipital protuberance posteriorly. It consists of occipital bone medially and temporal bone laterally.

Occipital Bone is the major bony element in this part of the base of the skull. It comprise of 4 parts: 

• The squamous part -posterior to the foramen magnum
• The lateral parts –lateral to the foramen magnum
• The basilar part –anterior to the foramen magnum

The squamous and lateral parts form the posterior part of the base of the skull.

The most prominent feature of the squamous part of the occipital bone is the Occipital Crest, extending from the external occipital protuberance to foramen magnum posteriorly. On each anterolateral border of the foramen magnum are occipital condyles for the articulation of the skull with the first (C-1 atlas) Vertebra. Posterior to each condyle is a depression –the condylar fossa- containing a condylar canal. Anterior and superior to each condyle is the large hypoglossal canal. Lateral to that is the large jugular foramen, formed by the jugular notch of occipital and petrous part of temporal bone.

Temporal Bone is found lateral to the posterior part of the base of the skull. The parts of temporal boon seen here are the mastoid process (and mastoid notch) and the styloid process. Both of which provide attachment for muscles. Between the two is Stylomastoid Foramen for the facial nerve.

Superior View of the Skull

Superior View of the Skull:

In this section we will look at the parts of Frontal, Parietal and Occipital bones as seen from superiorly. These bones make up the Calva of the skull (superior part of calvaria).

At the front, the frontal bone articulates with the paired parietal bone (at the coronal suture). The parietal bones articulate with one another at the midline via sagittal suture and posteriorly with the occipital bone at the lambdoid suture.

The paired parietal foramen on each side of the parietal bone can be seen just lateral to the sagittal suture. The junction of sagittal and coronal suture is called Bregma.

Posterior View Of Skull

Posterior view of the Skull:

Bones that are visible from the posterior view of the skull are the Occipital bone and Paired Parietal and Temporal bone.

Occipital Bone forms most of the posterior and inferior (base) part of the skull. The squamous part of the occipital bone articulates superiorly with the paired Parietal bones at the Lambdoid suture and laterally with the temporal bone (mastoid part of temporal bone) at the occipitomastoid suture.

Posteriorly there is a midline projection called the external occipital protuberance which gives attachment to muscles and the ligamentum nuchae.  Superior and inferior to it on either side are the Superior Nuchal line and Inferior Nuchal line respectively. Most prominent feature of external occipital protuberance is the inion.

Point where the saggital suture meets the lambdoid suture is called the Lambda.

Lateral View of Skull

Lateral view of the skull:

This section discusses the osteology of the lateral part of the calvaria, facial skeleton and the mandible.

The lateral portion of Calvaria begins anteriorly with the frontal bone, the paired parietal and temporal bone and ending posteriorly at the occipital bone. The frontal bone articulates with the parietal bone at the Coronal Suture and parietal bone in turn articulates with the occipital bone posteriorly at the Lambdoid Suture. Coming below, the Greater wing of the sphenoid bone articulates anteriorly with the frontal bone, superiorly with the parietal bone (sphenoparietal suture), posteriorly with the squamous part of the temporal bone (sphenosquamous suture). This area is also referred to as the Pterion and is site of numerous clinically important facts.

The squamous part of the temporal bone is attached superiorly to the parietal bone via squamous suture and posteriorly with the occipital bone via occipitomastoid suture. The mastoid process is a visible protuberance of the temporal bone, behind the external acoustic meatus and provides attachment site for several muscle.

Laterally the visible part of the Facial skeleton is formed by the nasal bones, the maxilla (discussed in previous section) and the zygomatic bone. The zygomatic bone is an irregular shaped bone that contributes –medially- in the formation of the inferio-lateral part of the orbital rim. Laterally the temporal process of the zygomatic bone arches out and meets posteriorly with the zygomatic process of the temporal bone to for the zygomatic arch. The zygomatic arch encloses the temporal fossa and below the infra-temporal fossa. The zygomaticofacial foramen can be usually seen on the body of the zygomatic bone along with the zygomaticotemporal foramen.

Now let’s briefly review some of the important bones i.e. the temporal bone and the mandible.

The Temporal bone:

  

It is and irregular shaped bone with many parts and forms most of the lateral wall of the Calvaria. Briefly it has squamous, tympanic, petro-mastoid parts.

·         The squamous part of the temporal bone is the large flat surface above the zygomatic arch that is visible in the lateral view of the skull. It is attached anteriorly to the greater wing of the sphenoid bone through sphenosquamous suture. And superiorly with the parietal bone through squamous suture.

·         The tympanic part can be seen below the zygomatic arch. It houses the external acoustic meatus. The styloid process can be seen behind the tympanic part of the temporal bone as small bony projections.

·         The petro-mastoid part is usually separated into two, the petrous part and the mastoid part. The mastoid part is the posterior most region of the temporal bone. Superiorly it articulates with the parietal bone (parietomastoid suture) and posteriorly with the occipital bone (occipitomastoid suture). Point where these two sutures meet the lambdoid suture is called Asterion. Inferiorly, the mastoid process is a large bony projection that provides attachment site for many muscles. The Petrous part of the temporal bone will be discussed in detail when we will look at the skull inferiorly. On the underside of the temporal bone –adjacent to styloid process- are numerous foramen for facial and hypoglossal nerve, external carotid artery and internal jugular vein.

·         The zygomatic process of the temporal bone forms most of the zygomatic arch. On its underside is a curved depression for the articulation of the condyle of the mandible at the temporomandibular joint. The zygomatic arch encloses the temporal fossa and the infra-temporal fossa.

Anterior View of Skull

Anterior View of the Skull:

This section includes the anatomy of the Forehead and the bones around the nasal region that forms the Facial skeleton.  In total the viscerocranium is composed of 14 bones.  Let us start with them main bones that are:

• The Frontal Bone
• Zygomatic Bone and Nasal Bone
• Sphenoid Bone
• Ethmoid Bone

The Frontal bone is a large bone that forms the forehead, the temporal fossa, superior part of the orbit, and the roof of the orbital cavity.

Inferiorly, it forms the superior rim of the Orbital Cavity. On the medial side of this part of the orbit are present, the Supra Orbital Foramen.  Just above the Superior rim of the orbit is large, visible protuberance –The Supercillary Arches- which dips down, medially into a depression called Glabella.

Laterally the Zygomatic process of the frontal bone articulates with the Frontal process of the zygomatic bone. This also forms the superiolateral part of the orbit. Similarly the frontal process also forms part of the superior medial process of the orbit.

Within the frontal bone -just above the orbital margins- are two hollow spaces lined by mucous membranes called the Frontal Air Sinus.

The Zygomatic bone articulates superiorly with the frontal bone via frontal process of the zygomatic bone. It forms mainly the inferiolateral and part of the inferior rim of the orbit. Medially the zygomatic process of the maxilla articulates with the body of the zygomatic bone. It extends laterally and posteriorly to articulate with the zygomatic process of the temporal bone to form the Zygomatic Arch. We will look at that in detail in lateral view of the skull later. It also forms the lateral wall of the orbital cavity.

The Nasal bones form the bony part of the nasal cavity. They articulate with one another medially, and with the frontal bones superiorly at the Frontonasal Suture. This region is also called the Nasion. Laterally they articulate with the frontal process of the maxilla. The periform aperture is the largest cavity in the nasal region and also forms the anterior opening of nasal cavity. The periform aperture is separated into right and left nasal cavity by the nasal septum. The bony nasal septum is formed by the two fused nasal crest that also gives rise, anteriorly to the Nasal Spine. Behind the nasal bones are the paired lacrimal bones, which house the lacrimal duct.

The Maxilla is the part of the facial bone between the inferior rim of the orbit and the upper teeth. Superiorly the maxilla gives rise to the inferior rim of the orbit and most of the floor of the orbital aperture. There, it also articulates with the frontal bone via frontal process of the maxilla superiorly and with the nasal bones medially. Laterally the zygomatic process of maxilla articulates with the zugomatic bone. It forms the major part of the lateral and inferior border of nasal cavity. Between the nasal cavity and the alveolar process (that holds the teeth) is the body of the maxilla.

Inferior to the inferior rim of the orbit, on the body of the maxilla is the infra-orbital foramen.  Within each maxilla is a large pyramidal shaped cavity lined with mucous membrane called the Maxillary Sinus.

Sphenoid Bone has two major parts, the greater wing and the lesser wing. The greater wing forms most of the lateral wall of orbital cavity. The greater and the lesser wing are separated by medial triangular gap called the sub orbital fissure.

Ethmoid Bone forms the medial wall of the orbital cavity. These two bones will be discussed in detail in later sections.

The Mandible, although not part of the facial skeleton is discussed next. The mandible has two parts:

1.       The body

2.       And the ramus

These two meet posteriorly at the angle of the mandible. The body of the mandible is further divided into two parts, namely:

1.       The base of the mandible

2.       And the alveolar process of the mandible.

The base of the mandible has median swelling called the symphysis menti, below which is a visible protuberance, the Mental Protuberance. On either side of the mental protuberance –laterally- are the tubercle of the mandible. Laterally, at the oblique line of the mandible, are openings called external mental foramen.

The mandibular condyles articulate with the temporal bone at the temporomandibular joint. Inferior to the condyle is a sharp projection called the coronoid process. Between the two is mandibular notch.

Skull Overview

Skull:

The skull comprise of 22 bones (binded by sutures) and houses the most important organs of the body.  The brain gives rise to 12 pair of cranial nerves which passes through foramen and fissure into the skull. All the nerves supply structures in head and neck except the 10^th (Vegas nerve) which supply the thorax and the abdomen. The skull is composed of two parts, the “Cranium” and the “Facial Skeleton” or the Viscerocranium.

The Cranium is further divided into:

1.       The Calvaria: that covers the cranium superiorly. Bones forming the calvaria are the paired perital and temporal bone, the unpaired frontal and occipital bone, and the sphenoid bone.

2.       And the Base where the brain rests in the anatomical position. It comprises of the paired temporal bone and the unpaired occipital and sphenoid bones.

Anteriorly, the facial skeleton is brought together by the frontal bone, maxilla, nasal and the zygomatic bone. They are discussed in detail in later sections.

The mandible is neither part of the Cranium nor the Facial Skeleton. We will review the osteology of the skull through different angles in the next sections.

Here is a list of bones that forms the cranium and the viscerocranium:

The Cranium consists of the following bones, two of which are paired.

·         Frontal bone: 1

·         Parietal bone: 2

·         Occipital bone: 1

·         Temporal bone: 2

·         Sphenoid bone: 1

·         Ethmoid bone: 1

The Facial Bones are (two of which are paired)

·         Zygomatic bone: 2

·         Maxilla: 2

·         Nasal bone: 2

·         Lacrimal bone: 2

·         Vomer: 1

·         Palatine bone: 2

·         Inferior nasal concae: 2

·         Mandible: 1