Neuroanatomy lab navigation
Parts of the brainstem
Midbrain
Mesencephalon
Pons
Metencephalon
Medulla
Myelencephalon
See Figure 1 and Review 2.
- The basal subdivision of the brainstem is the most anterior and contains mostly the descending fiber tracts.
- The tegmentum is the location of the cranial nerve nuclei, ascending fiber tracts, and the reticular formation. In the medulla, it also contains important neurons that are involved in cardiovascular and breathing reflexes.
- The tectum, which is posterior to the tegmentum, is really only prominent in the midbrain and is grossly represented by the superior and inferior colliculi. It contains the nuclei that are involved in vision, hearing, and reflexive control of pupillary constriction. Tectospinal tracts originating in the tectum supply the postural muscles that directly initiate reflexive movement to auditory and visual stimuli.
- Brain stem vascular lesions lead to multiple signs that may seem unrelated but can often be localized to a specific brain stem region. Knowledge of the brain stem anatomy will help with the localization. Examples of this will be presented in the functional neuroanatomy and circulation lecture.
Figure 1. Subdivisions of the brainstem; mid-sagittal plane. Three subdivisions of the brainstem:
-
- Tectum

- Tegmentum

- Basal region 

Figure from Neuroanatomy: 
A Laboratory Guide (2e); Jansen and Lampa (2018).
Identify the portions and subdivisions of the brain stem on a mid-sagittal brain.
Basal region of midbrain (cerebral peduncle)
Tegmentum of midbrain
Tectum of midbrain
Basal region of pons (basis points)
Tegmentum of pons
Tectum of pons
Basal region of medulla
Tegmentum of medulla
Review 2.
Subdivisions of the brainstem; mid-sagittal plane. 
 (Tap the right arrow for labels)
Specimen from Neuroanatomy Collection, Washington State University College of Veterinary Medicine.
Check yourself
On a sagittal image of a brainstem, locate the tegmentum and tectum (Figure 1 and Review 2).
Identify the exernal features of the brain stem.
The relevant portions of the brain stem and their specific important nuclear groups and tracts contained within will be discussed in the large-group sessions.
Utilize the external topography of the brain stem to locate internal structures as you a beginning to learn the cross sectional and and the related significance to lesion localization.
Examine the external topography of the brainstem and cerebellum and identify the following structures (Figure 2 and Figure 3):
Midbrain
Superior and inferior colliculi
Cerebral peduncles (aka crus cerebri)
Axons of cranial nerves III and IV
Pons
Basal pons (aka basis pontis or base of pons)
Middle cerebellar peduncle
Axons of cranial nerve V
Pons/medulla junction
Axons of cranial nerves VI, VII, and VIIII
MedullaÂ
Pyramids
Olives (inferior olivary nucleus)
Axons of cranial nerves IX, X, XI, XII
Cerebellum (pictured in Figure 3)
Vermis: Separates two cerebellar hemispheres
Cerebellar tonsil
Note the exit points of the cranial nerve axons from medial to lateral on the brainstem (Figure 2 and Figure 4).
The cerebellopontine angle is located laterally at the junction of the caudal pons and cerebellum. A patient with a tumor in this area would have what signs or symptoms related to cranial nerve dysfunction?
 
Thought question
Describe a common benign neoplasm that could affect structures in this region.
The cerebellar tonsil may herniate into the brainstem, leading to significant mortality with increases in intracranial pressure.
Table 1. Cranial nerves: Numbers, names, and basic functions
|
Number |
Name |
Basic function (afferent or efferent) |
|
I |
Olfactory |
Transmits smell to CNS |
|
II |
Optic |
Transmits visual information to CNS |
|
III |
Oculomotor |
Innervates inferior rectus, superior rectus, levator palpebrae superioris, medial rectus and inferior oblique; visceral motor (parasympathetic [PS] to sphincter pupillae and ciliary muscle) |
|
IV |
Trochlear |
Innervates the superior oblique muscle |
|
V |
Trigeminal
|
Transmits somatic afferent sensory information from the face, cornea, and anterior portion of the tongue; innervates muscles of mastication, palate, and middle ear (Mandibular n.) |
|
VI |
Abducens |
Innervates lateral rectus muscle |
|
VII |
Facial |
Transmits taste from anterior portion of the tongue; provides motor innervation to muscles of the face; supplies visceral motor (PS) information to mucous glands in nasal and oral cavities, salivary glands, and lacrimal gland |
|
VIII |
Vestibulocochlear |
Transmits hearing and balance information from the inner ear |
|
IX |
Glossopharyngeal |
Transmits taste and general sensations from posterior portion of the tongue and pharynx, innervates pharyngeal muscles, supplies visceral motor information to the parotid salivary gland |
|
x |
Vagus |
Transmits sensations from larynx and pharynx, supplies visceral motor information to thoracic and abdominal viscera to descending colon; supplies innervation to laryngeal and pharyngeal muscles |
|
XI |
Accessory |
Innervates trapezius and sternocleidomastoid; innervation to laryngeal muscles via vagus |
|
XII |
Hypoglossal |
Innervates muscles of the tongue |
Overview of the brain stem
Image credits
Unless otherwise noted, images are from Adobe Stock.
