The three levels of the brainstem from superior to inferior are the midbrain, pons, and medulla. The midbrain is just inferior to the bilateral thalami, and the medulla transitions inferiorly into the cervical spinal cord. Most simply, the brainstem can be thought of as a “spinal cord for the head and neck”: Just as the spinal cord has sensory information coming in and motor information going out for the extremities and torso, the brainstem has sensory information coming in and motor information going out for the head and neck. In addition to somatic sensory information, however, the brainstem also receives vestibular, auditory, taste, and visceral sensory information. Motor functions of the brainstem include control of ocular, pupillary, facial, laryngeal, pharyngeal, and visceral musculature.

Understanding the brainstem requires a general framework for what is there and where it is. As far as what is in the brainstem, there are five general categories of structures:

1. 
   

   The descending motor pathways for the extremities and torso (corticospinal tracts; see Ch. 4)

   
   
2. 
   

   The ascending somatosensory pathways from the extremities and torso (dorsal columns and spinothalamic tracts; see Ch. 4)

   
   
3. 
   

   The cranial nerve nuclei and associated structures

   
   
4. 
   

   Connections with the cerebellum (the cerebellar peduncles; see Ch. 8)

   
   
5. 
   

   The reticular activating system and ascending neurotransmitter-specific projection pathways: substantia nigra (dopamine), locus coeruleus (norepinephrine), median raphe nuclei (serotonin), pedunculopontine nuclei (acetylcholine)

As points of orientation for where structures are in the brainstem, the following principles apply at all three levels of the brainstem (Fig. 9–1):

• 
  

  The corticospinal tracts run in the anterior (ventral) aspect of the brainstem.

  
  
• 
  

  The somatosensory pathways for the extremities and torso move a bit over the course of their ascent, but are most often posterior (dorsal) within the brainstem (with the exception of the mid-medulla, where the medial lemnisci are medial and extend anteriorly; see Ch. 4).

  
  
• 
  

  The cranial nerve nuclei are all posterior (dorsal).

  
  
  
  
  • 
    

    In general, the motor cranial nerve nuclei are closest to the midline, and their cranial nerves emerge medially/anteriorly (CN 4 is an exception in that it exits posteriorly).

    
    
    
    
    • 
      

      The motor cranial nerve nuclei innervating skeletal muscle are at the midline: CNs 3, 4, and 6 (innervating extraocular muscles) and CN 12 (innervating tongue muscles).

      
      
    • 
      

      The motor cranial nerve nuclei innervating branchial muscles are more lateral: CN 7 (facial muscles), CN 5 (jaw muscles), and CNs 9 and 10 (muscles of the larynx/pharynx)

    
    
  • 
    

    The sensory and special sensory cranial nerve nuclei are all more lateral than the motor cranial nerve nuclei: sensory nuclei of CN 5, vestibular and cochlear nuclei (CN 8), and nucleus solitarius (for taste and visceral sensation)

  
  
• 
  

  The cerebellar peduncles all arise from the posterior/dorsal brainstem (logically, since the cerebellum is posterior to the brainstem)

  
  
• 
  

  The ascending neurotransmitter-specific projection pathways are found throughout the brainstem, but the reticular-activating system involved in maintaining arousal and consciousness reside in the upper pons and midbrain.

The Cranial Nerve Nuclei

The 12 cranial nerves can be divided into three groups of four that mostly correspond to the three brainstem levels with a few exceptions denoted by asterisks:

• 
  

  Midbrain: 1*-2-3-4

  
  
• 
  

  Pons: 5*-6-7-8*

  
  
• 
  

  Medulla: 9-10-11*-12

The exceptions are as follows:

• 
  

  CN 1 and CN 11 do not connect with the brainstem.

  
  
• 
  

  CN 5 has nuclei at all three levels of the brainstem (although its fibers enter at the level of the pons as would be expected from the pons = 5*-6-7-8* schema; see Ch. 13).

  
  
• 
  

  For CN 8, the vestibular nuclei are in the medulla and the cochlear nuclei are at the pontomedullary junction.

  
  
• 
  

  CN 2 can be considered a partial exception: Although CN 2’s main projections are the visual pathways, which do not project to the brainstem (see Ch. 6), the afferent limb of the pupillary light reflex is communicated by CN 2 to the midbrain as would be expected by the midbrain = 1*-2-3-4 schema.

The Cerebellar Peduncles

The three cerebellar peduncles nearly correspond to the three levels of the brainstem, with one exception (point 3 below; see also Fig. 8–3).

1. 
   

   The inferior cerebellar peduncles connect the medulla to the cerebellum.

   
   
2. 
   

   The middle cerebellar peduncles connect the pons to the cerebellum.

   
   
3. 
   

   Although things would have been simpler if the superior cerebellar peduncles connected to the midbrain to give one pair of cerebellar peduncles per brainstem level, the superior cerebellar peduncles connect the cerebellum to the upper pons (en route to connections with the midbrain and thalamus).

The Arterial Supply of the Brainstem

The vascular supply of the brainstem corresponds to its three levels, with one pair of circumferential arteries per level of the brainstem (Fig. 9–2):

1. 
   

   Superior cerebellar arteries (SCAs) for the midbrain (though the superior midbrain is supplied by the PCAs)

   
   
2. 
   

   Anterior inferior cerebellar arteries (AICAs) for the pons

   
   
3. 
   

   Posterior inferior cerebellar arteries (PICAs) for the medulla

These arteries all have “cerebellar” in their names since they not only supply the brainstem, but also the cerebellum posterior to it. Most commonly, the SCAs and AICAs arise from the basilar artery and the PICAs from the vertebral arteries. The anterior spinal artery arises from the vertebral arteries and supplies the medial medulla and anterior spinal cord. At the level of the pons and midbrain, the midline basilar artery supplies the medial brainstem through penetrating branches.