Brain image adapted, with permission, from the Yakovlev-Haleem collection from the National Museum of Health and Medicine.
Brainstem
Brainstem Overview
Summary
major nuclear groups
- Supplementary motor nuclei (eg, the substantia nigra, pontine nuclei, and inferior olive).
- Cranial nerve nuclei
major tracts
- Major motor tracts (eg, corticospinal tract)
- Major sensory tracts (eg, medial lemniscus)
- Supplementary tracts (eg, rubrospinal tract)
Key brainstem features
Midbrain
See: Midbrain
- Posteriorly, the posterior commissure, which is a white matter tract involved in the pupillary light reflex, and the paired colliculi (superior and inferior):
- The superior colliculi are involved in visual function; the inferior colliculi are involved in auditory function.
- Anteriorly, the cerebral peduncles, which carry descending motor tracts.
- The anterior location of the cerebral peduncles helps us remember that throughout the brainstem, major motor tracts localize anteriorly (whereas sensory pathways mostly localize posteriorly).
Major Midbrain Syndromes:
Pons
See: Pons
- Pontine basis notably houses large pontine nuclei and pontocerebellar fibers, which help modulate movement.
Major Pontine Syndromes:
Medulla
See: Medullary pyramids
- Carry descending motor fibers into the spinal cord.
Major Medullary Syndromes:
Key regional anatomy
- Posteriorly - the cerebellum the major balance center.
- Superiorly - the thalamus the sensory relay center, and the hypothalamus, the central autonomic nervous system command center.
- Laterally - the temporal lobe can compress the midbrain during brain herniation.
General construct
- The following is a general construct for 5 of the major groups of nuclei and tracts to help remember their general positions.
From anterior to posterior:
Major Motor Tracts
- Corticobulbar tracts
- which innervate the cranial nerves
- Corticospinal tracts
- which innervate the spinal motor neurons
Supplementary Motor Nuclei
- Red nuclei,
- Substantia nigra,
- Pontine nuclei
- Inferior olivary nuclei
We'll learn these in detail elsewhere.
Major Sensory Tracts
- Medial lemniscus and spinothalamic tracts (from the body),
- Trigeminothalamic tract (from the face)
Supplementary Tracts
- Central tegmental, rubrospinal, and reticulospinal tracts (which we'll learn these in detail elsewhere).
Cranial Nerve Nuclei
- CNs 3 through 12 lie in the brainstem (11 lies in the cervical spinal cord).
Anterior/Posterior regions
Basis
- Descending white matter tracts and certain supplementary motor nuclei.
Tegmentum
- Majority of remaining nuclei and tracts
Tectum
- Roof of the brainstem; it's only functionally relevant in the midbrain.
- Definition of tectum:
- In the midbrain, the tectum is the quadrigeminal plate, whereas in the pons and medulla, by at least one commonly held definition, the tectum is limited to the nonfunctioning medullary velum.
- Note, however, that select authors include the superior cerebellar peduncles as part of the pontine tectum, as well.
functional groups
Tracts
From anterior to posterior:
- Major motor tracts
- Major sensory tracts
- Supplementary tracts
Nuclei
- Supplementary motor nuclei
- Cranial nerve nuclei (posterior tegmentum)
- Neurobehavioral cells (interspersed in the tegmentum):
- periaqueductal gray area, locus coeruleus in the pons, and the raphe nuclei.
- Reticular formation (throughout the tegmentum)
- this histologically indistinct area comprises highly specific nuclei and tracts.
- Major sensory nuclei (the gracile and cuneate nuclei):
- lie only within the medulla.
Full-Length Text
Overview
- Here, we will generate an overview of the brainstem, which we can apply to its specific levels.
- If we learn the basic layout of the 5 major nuclei and tracts, we'll be able to learn the details of the different brainstem levels, later, much easier.
- To begin start a table to list the major nuclear and tract groupings of the brainstem.
Major Nuclear Groups
Let's list 2 major nuclear groups in the brainstem from anterior to posterior:
- Supplementary motor nuclei (eg, the substantia nigra, pontine nuclei, and inferior olive).
- Cranial nerve nuclei.
Major Tracts
The three major tracts from anterior to posterior:
- Major motor tracts (eg, corticospinal tract)
- Major sensory tracts (eg, medial lemniscus)
- Supplementary tracts (eg, rubrospinal tract)
Anatomical Context
To begin, let's put the brainstem in within its general anatomic context.
- Start a mid-sagittal section.
- Label the anterior/posterior and superior/inferior orientational planes.
- Now, draw the brainstem from superior to inferior:
- The midbrain.
- Pons.
- And medulla.
- Then, include the cervical spinal cord.
Now, let's walk through these levels and point out some key features
Midbrain
In the midbrain,
Posteriorly
- Posteriorly, draw the posterior commissure, which is a white matter tract involved in the pupillary light reflex.
- Then, draw the paired colliculi (superior and inferior): the superior are involved in visual function; the inferior colliculi are involved in auditory function.
Anteriorly
- Anteriorly, draw a cerebral peduncle, which carry descending motor tracts.
- The anterior location of the cerebral peduncles helps us remember that throughout the brainstem, major motor tracts localize anteriorly (whereas sensory pathways mostly localize posteriorly).
pons
Pontine Basis
- Now, in the pons, label the pontine basis, which notably houses large pontine nuclei and pontocerebellar fibers, which help modulate movement.
Medulla
- In the medulla, label the medullar pyramids, which carry descending motor fibers into the spinal cord.
CSF
- Next, shade the CSF flow that tracts through the posterior aspect of the brainstem.
- In the midbrain, label the cerebral aqueduct (of Sylvius).
- In the pons and medulla, label the 4th ventricle.
Outside of the Brainstem
Surrounding the brainstem:
- Posteriorly draw the cerebellum, the major balance center.
- Superiorly, draw the thalamus, the sensory relay center.
- Then, draw the hypothalamus, the central autonomic nervous system command center.
- From anterior to posterior, specify the pituitary body, a key hormone center; one of the paired mammillary bodies, involved in memory, and the pineal gland, the site of melatonin production and release, which helps settle us into sleep.
- Now, laterally, draw the temporal lobe, which can compress the midbrain during brain herniation.
General Construct
- Now, let's create a general construct for 5 of the major groups of nuclei and tracts and show where the typically lie from anterior to posterior. As we go, keep a table of these categories and specify key examples of them.
From anterior to posterior, along the brainstem, lie the:
Major motor tracts
- the corticobulbar tracts (which innervate the cranial nerves, corticospinal tracts (which innervate the spinal motor neurons)
Supplementary motor nuclei
- red nuclei, substantia nigra pontine nuclei, inferior olivary nuclei – we'll learn these in detail elsewhere.
Major sensory tracts
- Medial lemniscus and spinothalamic tracts (from the body), trigeminothalamic tract (from the face)
Supplementary tracts:
- Central tegmental, rubrospinal, and reticulospinal tracts (which we'll learn these in detail elsewhere).
Cranial nerve nuclei
- Cranial nerve nuclei 3 through 12 (11 lies in the cervical spinal cord).
Axial Composite
- Now, let's switch gears and draw an axial composite of the brainstem: a consolidation of the different brainstem levels into a single general organizational pattern.
- Let's use the radiographic (ie, MRI and CT) perspective, since it's what's used clinically to evaluate patients.
- Show the anterior/posterior orientational plane.
- Then, draw an ovoid outline of the brainstem.
- Label the cerebrospinal fluid (CSF) space.
- Divide the brainstem in two halves: one for the Nuclei, the other for the Tracts.
- Now, separate the brainstem from anterior to posterior into its basis, tegmentum, and tectum.
Basis
Based on our prior diagram, try to guess what the basis comprises:
- Descending white matter tracts and certain supplementary motor nuclei.
Tegmentum
And the tegmentum?
- The rest.
Tectum
- The tectum is the roof of the brainstem; it's only functionally relevant in the midbrain.
- In the midbrain, the tectum is the quadrigeminal plate, whereas in the pons and medulla, by at least one commonly held definition, the tectum is limited to the nonfunctioning medullary velum.
- Note, however, that select authors include the superior cerebellar peduncles as part of the pontine tectum, as well.
Functional Groups
Now, let's label the functional groups from anterior to posterior.
- On the tracts side, add a cerebral peduncle (of the midbrain), and label the major motor tracts.
- Then, the major sensory tracts.
- Followed by the supplementary tracts.
- Include the cerebellar peduncles off the posterolateral aspect of the brainstem.
- On the nuclei side, label the supplementary motor nuclei.
- Then, in the posterior tegmentum, label the cranial nerve nuclei.
- Next, interspersed in the tegmentum, label the neurobehavioral cells, which include the periaqueductal gray area, locus coeruleus in the pons, and the raphe nuclei; they have diffuse behavioral functions.
- Next, label the broad reticular formation across the tegmentum: this histologically indistinct area comprises highly specific nuclei and tracts.
- Lastly, in the very posterior midbrain label the major sensory nuclei (the gracile and cuneate nuclei), which lie only within the medulla.