The Brain Stem and Cerebellum: Part 1, the Medulla Brad Cole, MD
Introduction The brain stem is a complex structure because of the multitude of various nuclei, pathways, and cranial nerves which are all in this relatively small area. It is an important structure clinically since there are a number of specific neurological syndromes which occur due to lesions in the brain stem. Because the cerebellum has so many connections with the brain stem, this will be covered in this section as well. In these lectures we will combine the normal anatomy with an introduction to the more common lesions within the brain stem.
First some “big picture” points:
Review books sometimes refer to the “rule of 4’s”. These are not entirely accurate (see footnotes) but I’ll share them here as a starting point: • There are 4 CNs in each section: 1 o Cranial nerves 3 and 4: Midbrain 2 o Cranial nerves 5-8: Pons o Cranial nerves 9-12: Medulla • CNs that divide evenly into 12 are found in the midline (3,4,6, and 12) • CNs that do not divide evenly into 12 are found more laterally (5,7,8,9,10 and 11)3 • 4 Midline pathways/structures: o Medial Longitudinal Fasciculus 4 o Motor tract of the Corticospinal Tract o Medial Lemniscus o Motor nuclei of CN 3,4,6, and 12 • 4 Sensory pathways are lateral: o Spinothalamic tract o Spinocerebellar tracts o Sympathetic descending first order pathway o Sensory CN nuclei
Special thanks to Stephanie Hynes and Heidi Spady (both LLU class of 2020) for their artistic contributions to the brain stem handouts!
1 CNs 1-2 are CNS pathways and should not be included in this 2 CN 5 is also found in the medulla! 3 CN 7 is midline when it wraps around CN 6 in the pons 4 This pathway is lateral in the midbrain
As we move from the medulla to the midbrain using the slides from the DeArmond atlas, you will need to be able to identify the structures that are labeled with *asterisks*. Those in bold indicate that this is a new structure at this particular level. Pay close attention to the lesions and clinical vignettes since all of your work to understand the normal brain stem anatomy is of little use if it cannot be applied to real patients.
Medulla – level of motor decussation
Pathways: • Fasciculus Gracilis* – The rostral continuation of the vibration and proprioception pathways from the posterior columns of the spinal cord, below T6. o Lesion: ipsilateral loss of vibration and proprioception below T6 – essentially this means the leg, since vibration and proprioception are not assessed in the thoracic area. • Fasciculus Cuneatus* – The rostral continuation of the vibration and proprioception pathways from the posterior columns of the spinal cord, above T6. o Lesion: ipsilateral loss of vibration and proprioception above T6 – essentially this means the arm. • Decussation (crossing) of the Corticospinal tract (CST)* – It is important to remember that the lower medulla is the crossing of this major descending motor pathway. o Lesion: Above the level of the decussation (brain and brain stem), the patient will have contralateral UMN weakness of the arm and leg. Below the level of the decussation (spinal cord), the patient will have ipsilateral UMN weakness of the arm and leg. • Dorsal and ventral spinocerebellar tract (DSCT and VSCT)* – The continuation of the ascending cerebellar pathways from the spinal cord which are destined for the cerebellum. • Spinothalamic tract (STT)* – The ascending continuation of the spinal cord STT, which contains the pain and temperature pathways. This pathway is often difficult to identify as a distinctive pathway until we arrive at the midbrain. Thus, we will stop pointing out the STT until the midbrain, but it is important to remember that it is located in the lateral medulla and pons. o Lesion: Since this pathway already crossed in the spinal cord, a lesion here results in contralateral loss of pain and temperature in the arm and leg. • Spinal Trigeminal Tract* – The function of this pathway is pain and temperature sensation for the ipsilateral face. (The function of the other components of the trigeminal system will be covered when they can be visualized in the pons) o Lesion: Decreased pain and temperature sensation for the ipsilateral face.
Nuclei: • Gracile nucleus* – The synapse for the ascending gracile fasciculus. After this synapse the pathway crosses in the internal arcuate pathway. o Lesion: Decreased vibration and proprioception of the ipsilateral leg (below T6) • Cuneate nucleus* – The synapse for the ascending cuneate fasciculus. After this synapse the pathway crosses in the internal arcuate pathway. o Lesion: Decreased vibration and proprioception of the ipsilateral arm (above T6) • Spinal Trigeminal Nucleus (V)* – Pain and temperature sensation for the ipsilateral face. This is essentially the rostral continuation of the substantia gelatinosa in the spinal cord o Lesion: Decreased pain and temperature sensation of the ipsilateral face • Spinal Accessory nucleus* – the motor nucleus for CN 11 is found in the upper cervical cord and lower medulla
Now, try to identify the nuclei and pathways on your own:
Medulla – level of sensory decussation
Pathways: • Gracile fasciculus* • Cuneate fasciculus* • DSCT • VSCT • Spinal Trigeminal Tract • Tectospinal tract* – traveling from the quadrigeminal plate to the cervical cord AHCs to control head turning relative to visual and auditory input • Corticospinal Tract (Medullary pyramid)* – a lesion here would result in contralateral UMN weakness • Internal Arcuate fibers* – after synapsing in the nucleus gracilus and cuneatus, the vibration and proprioception fibers cross as this named pathway to eventually form the medial lemniscus. Remember: pain and temperature sensation crosses in the spinal cord as the anterior white commissure; vibration and proprioception cross in the lower medulla as the internal arcuate fibers. • Medial lemniscus* – The continuation of the internal arcuate fibers which contain vibration and proprioception pathways (arm and leg) o Lesion: Decreased vibration and proprioception in the contralateral arm and leg
Nuclei: • Gracile nucleus* • Cuneate nucleus* • Spinal Trigeminal nucleus* • Hypoglossal nucleus* o Lesion: Deviation of the tongue toward the side of the lesion. Atrophy of that half of the tongue • Solitary nucleus and tract* – This structure has an unusual appearance because the nucleus surrounds the tract. It extends from the medulla and into the caudal pons. The solitary nucleus and tract is a sensory structure which receives input from cranial nerves 7, 9 and 10. o Sensory information from the viscera in the head, throat, thoracic, and abdominal cavities are processed in the solitary complex: . The solitary complex receives input from stretch receptors in the lungs innervated by cranial nerve 10. This is involved in respiratory reflexes. . Baroreceptor reflex: Pressure receptors in the arterial system innervated by cranial nerve 9 also project to the solitary complex. This is involved in blood pressure regulation. The solitary complex responds to elevated blood pressure and pulse rate (tachycardia) by exciting the dorsal motor nucleus of the vagus which results in a reduced pulse and blood pressure. . Lesion: blood pressure instability, loss of taste and tachycardia
• Dorsal efferent (motor) nucleus of the vagus (X) – (better identified on next section)
Cushing’s reflex: Acute increased intracranial pressure (ICP) activates hypothalamic sympathetics with subsequent increased blood pressure. This increase in pressure is needed in order to better perfuse the brain with oxygen. The hypertension activates carotid sinus baroreceptors with resulting parasympathetic activation and bradycardia. The mechanism is debated, but it is likely that brainstem compression due to increased ICP impairs the normal baroreceptor reflex to also reduce the blood pressure and also contributes to irregular respirations. This results in what is known as the Cushing’s triad and is a neurologic emergency since it suggests impending brain herniation:
o Hypertension (high systolic, low diastolic) o Bradycardia o Irregular respirations including periods of apnea (“Cheyne-Stokes” respiration)
In addition to this, patients with increased ICP also typically exhibit headache, nausea and vomiting, encephalopathy
• The solitary system also processes information from the taste receptors innervated by cranial nerves 7, 9, and 10. This ascends to the VPM of the thalamus as the solitariothalamic tract and then to the primary gustatory cortex which is a part of the insular lobe and inferior frontal gyrus. A lesion of this pathway will result in ipsilateral loss of taste.
Nucleus ambiguus – This nucleus cannot be visualized, but you need to know that its location is in the lateral medulla. The nucleus ambiguus is the motor nucleus for cranial nerves 9, 10, and 11 for swallowing (pharyngeal muscles) and phonation (laryngeal muscles). Remember Solitary nucleus = Sensory; nucleus aMbiguus = Motor. o Lesion – ipsilateral paralysis of the voluntary muscles in the pharynx and larynx supplied by cranial nerves 9 and 10. Clinically, there is inability to raise the soft palate, which will cause dysphagia (difficulty swallowing). Loss of function of the ipsilateral vocal cord results in dysphonia (hoarseness of the voice) dysarthria and hiccups.
Things get more complex as we continue up from here. Below is an overview of these sections:
Medulla – Rostral sensory decussation
Pathways: • Gracile fasciculus* • Cuneate fasciculus* • DSCT • VSCT • CST (pyramid)* • STT • Spinal Trigeminal Tract • Internal Arcuate fibers* • Tectospinal tract (TST)* • Medial lemniscus* Nuclei: • Gracile nucleus* • Cuneate nucleus* • Spinal Trigeminal Nucleus (V)* • Hypoglossal nucleus (XII)* • Solitary nucleus and tract* • Dorsal motor nucleus of the vagus (X)* – supplies parasympathetic preganglionic fibers that supply the heart, lung and GI tract up to the splenic flexure. Since this is parasympathetic, the effect is to slow the heart and to stimulate the GI tract (in contrast to sympathetic fibers which stimulate the heart and slow the GI tract). o Lesion: increased heart rate (tachycardia); medications such as atropine and tricyclic antidepressants (amitriptyline) block vagus nerve acetylcholine muscarinic receptors (anti-muscarinic) and can result in constipation, urinary retention and tachycardia. • Nucleus ambiguus
Medial Medullary Syndrome: This stroke syndrome is due to occlusion of the anterior spinal artery involving one-half of the medial medulla. It presents with:
- Contralateral arm and leg weakness (CST – pyramidal tract) - Contralateral decreased arm and leg proprioception and vibration (medial lemniscus) - Tongue deviation to the side of the lesion (hypoglossal nucleus)
Below is an MRI scan of a patient with an acute medial medullary stroke:
Medulla – Level of cranial nerves 10 and 12
Pathways: • VSCT • CST (pyramid)* • STT • Spinal Trigeminal Tract. • Tectospinal tract* • Medial lemniscus* • Inferior Cerebellar Peduncle (ICP)* – The ICP contains 3 of the 4 ascending spinal cord cerebellar pathways. The VSCT is the only spinal cord cerebellar pathway which does not travel through the ICP. Notice that the DSCT is not identified at this level since this pathway has now entered the ICP. • Fascicles of the hypoglossal nerve (XII)* • Fascicles of the vagus nerve (X)* • Lateral vestibulospinal tract* – this is a descending upper motor neuron pathway from the lateral vestibular nucleus to the spinal cord and is the strongest pathway for muscle extension movements. It is best seen as it travels through the inferior vestibular nucleus and has a “salt and pepper” appearance. o A lesion above the level of the lateral vestibular nucleus, typically a lesion in the pons, results in decerebrate posturing. Typically, the patient is comatose and has profound extensor posturing in both the arms and legs. Other: • 4th Ventricle* • Choroid Plexus* Nuclei: • Spinal Trigeminal Nucleus (V) • Hypoglossal nucleus (XII)* – most medial • Dorsal motor nucleus of the vagus (X)* – in between XII and the solitary nucleus and tract • Solitary nucleus and tract* – lateral to X • Inferior Olivary Nucleus (ION)* – The ION is an important relay nucleus for the cerebellum. • Medial and inferior vestibular nucleus – These nuclei are important for vestibular function which will be discussed next year. In particular, the medial VST provides input from the semicircular canals to the cervical AHCs as discussed in the spinal cord lecture. o Lesion: Vertigo and nystagmus • Nucleus Ambiguus
Lateral Medullary Syndrome: A common stroke syndrome (on boards) that is due to occlusion of the vertebral artery or the posterior inferior cerebellar artery (PICA), resulting in a stroke in the distribution of the PICA. When related to trauma, LMS is due to dissection of the vertebral artery:
o Vertigo and nystagmus (vestibular nuclei) st o Ipsilateral Horner’s syndrome (descending 1 order sympathetic pathway) o Dysphagia, dysphonia, hiccups, and ipsilateral decreased gag reflex (nucleus ambiguus) o Ipsilateral ataxia and inferior cerebellum (inferior cerebellar peduncle) o Ipsilateral facial numbness (spinal trigeminal nucleus and tract) o Contralateral arm and leg numbness (spinothalamic tract - STT)
Below is an MRI of a patient with an acute stroke in the lateral medulla. Notice also that, since the PICA supplies the undersurface of the cerebellum, that there is also some cerebellar involvement: