Brainstem Anatomy: Structure and Function

Objectives

• Identify locations, describe functions, and understand the organization of brainstem local nuclei and tracts.
• Identify locations, describe functions, and understand organization of brainstem motor and sensory cranial nerve nuclei.
• Motor Nuclei: Somatic, Parasympathetic, and Branchial
• Sensory Nuclei: General, Visceral, and Special
• Describe, understand, and identify the blood supply to the midbrain.
• Describe and understand the patterns of deficits characteristic of occlusion of internal vascularization of medial and central midbrain (midbrain syndromes) and the corresponding structures damaged in each.
• Describe, understand, and identify the blood supply to the pons.
• Describe and understand the patterns of deficits characteristic of occlusion of internal vascularization of basilar and tegmental regions (pontine syndromes) and the corresponding structures damaged in each.
• Describe, understand, and identify the blood supply to the medulla.
• Describe and understand the patterns of deficits characteristic of occlusion of medial and lateral internal vascularization (medullary syndromes) and the corresponding structures damaged in each.

Somatic Motor Nuclei

• Innervate the tongue and extraocular muscles

Somatic Motor Nuclei Locations

• Midbrain houses the oculomotor (CN III) and trochlear (CN IV) nuclei
• The pons contains the abducens nucleus (CN VI)
• The medulla contains the hypoglossal nucleus (CN XII)

Oculomotor Nucleus (CN III)

• Located in the rostral midbrain tegmentum, ventromedial to the periaqueductal gray (PAG).
• Lower motor neuron fibers originate and innervate four extraocular muscles (medial, inferior and superior recti, inferior oblique) and the levator palpebrae muscle.
• Fibers pass through the red nucleus before exiting as CN III.

Trochlear Nucleus (CN IV)

• Found in the caudal midbrain tegmentum, ventromedial to the PAG.
• Lower motor neuron fibers encircle the PAG dorsally, decussate near the anterior medullary velum, and exit the midbrain dorsally as CN IV.
• Axons innervate the superior oblique muscle.
• Damage results in diplopia (double vision) on downward gaze

Abducens Nucleus (CN VI)

• Situated in the caudal pons tegmentum, dorsomedial, close to the floor of the 4th ventricle
• Underlies the facial colliculus of the rhomboid fossa.
• Gives rise to lower motor neurons that travel in the abducens nerve (CN VI) and innervate the lateral rectus muscle.
• Projects exiting fibers through structures of the basilar pons (lateral to the corticospinal tract).
• Functions as the pontine center for lateral conjugate gaze, aiding coordinated movement of the eyes in the lateral plane.
• Fibers project via the medial longitudinal fasciculus (MLF) to the contralateral medial rectus subnucleus of the oculomotor nucleus (CN III) in the rostral midbrain.

Hypoglossal Nucleus (CN XII)

• Located in the caudal medullary tegmentum, dorsomedially, near the floor of the 4th ventricle and the central canal
• Underlies the hypoglossal trigone of the rhomboid fossa.
• Gives rise to lower motor neuron fibers that travel in the hypoglossal nerve (CN XII) and supply the intrinsic and extrinsic muscles of the tongue.

Parasympathetic Motor Nuclei

• Provide parasympathetic innervation to the head and abdominal viscera

Parasympathetic Motor Nuclei Locations

• Midbrain contains the Edinger-Westphal nucleus
• Pons contains the superior salivatory nucleus
• Medulla contains the inferior salivatory and dorsal vagal nuclei

Edinger-Westphal Nucleus

• Location: rostral midbrain tegmentum, medially, between the PAG and the oculomotor nucleus.
• Gives rise to preganglionic parasympathetic fibers that exit through CN III and synapse in the ciliary ganglion.
• Controls activation of the ciliary body (lens accommodation) and the sphincter muscle of the iris (pupillary light reflex, pupillary constriction)

Superior Salivatory Nucleus

• Location: caudal pontine tegmentum, dorsally and lateral to the abducens nucleus.
• Gives rise to preganglionic parasympathetic fibers that travel in CN VII and synapse in the pterygopalatine and submandibular ganglia.
• Mediates lacrimation, salivation, nasal mucous secretion, and vasodilation.

Inferior Salivatory Nucleus

• Location: rostral medullary tegmentum, dorsally.
• Gives rise to preganglionic parasympathetic fibers that travel in CN IX and synapse in the otic ganglion.
• Serves to mediate salivation and vasodilation

Dorsal Motor Nucleus of Vagus

• Location: caudal medullary tegmentum, dorsally near the central canal.
• Gives rise to preganglionic parasympathetic fibers that travel in CN X and synapse in ganglia of the thoracic and abdominal viscera.
• Underlies the vagal trigone of the rhomboid fossa.

Branchial Motor Nuclei

• Innervate muscles for mastication, facial expression, as well as pharynx and larynx

Branchial Motor Nuclei Locations

• Pons contains the motor trigeminal (masticatory) and motor facial nuclei
• Medulla contains the nucleus ambiguus and the accessory nucleus

Motor Nucleus of CN V (Trigeminal)

• Location: rostral to midpontine tegmentum
• Gives rise to lower motor neuron fibers that innervate muscles of mastication and exit the brainstem via the motor root of CN V.
• Receives bilateral upper motor neuron input.

Facial Motor Nucleus

• Lies ventrolaterally within the caudal pontine tegmentum between the sulcus limitans and the cerebellopontine angle.
• It contains neurons that project axons dorsomedially and encircle the abducens nucleus (internal genu of the facial nerve).
• These axons pass ventrolaterally between the facial nucleus and the spinal trigeminal nucleus
• The axons then exit the brainstem through the cerebellopontine angle of the pontomedullary junction.
• Axons innervate ipsilateral muscles of facial expression, traveling through the facial nerve (CN VII).

Facial Motor Nucleus Input

• Receives bilateral upper motor neuron input for upper facial muscles and contralateral input for lower facial muscles

Nucleus Ambiguus

• Characterized as a column of lower motor neuron cell bodies that are located in the medulla
• The main location is in the lateral, medullary tegmentum dorsal to the inferior olivary nucleus
• More specifically it is medial to a line drawn from the sulcus limitans to the postolivary sulcus
• Axons innervate pharyngeal arch muscles of the pharynx and larynx and travel through cranial nerves IX & X.

Spinal Accessory Nucleus

• Located at the transition between the spinal cord and medulla, at the level of the pyramidal decussation.
• Contains lower motor neurons of CN XI

General Sensory

• General Sensory Nuclei Locations:
• Midbrain: Mesencephalic nucleus of V
• Pons: Mesencephalic nucleus of V, chief or principal sensory nucleus, spinal trigeminal nucleus
• Medulla: Spinal trigeminal nucleus

Mesencephalic Nucleus of CN V

• Primary location extends from the rostral midbrain to rostralmost pons
• Location: dorsolateral or lateral aspect of the PAG
• Contains primary afferent pseudounipolar neurons.
• Receives input from muscle spindles and Golgi tendon organs of the muscles of mastication.
• Projects fibers bilaterally to the motor nucleus of CN V and to the reticular formation

Principal or Chief Sensory Nucleus of CN V

• Found rostral to mid pontine tegmentum, lateral to the motor nucleus of CN V
• Receives input from primary afferent fibers, originating in the trigeminal (semilunar) ganglion of CN V
• This carries information about fine touch, vibration, and pressure sensations from the face
• Projecting axons terminate in the VPM of the thalamus bilaterally, through two tracts
• Contralateral Ventral Trigeminothalamic Tract
• Ipsilateral Dorsal Trigeminothalamic Tract

Spinal Nucleus of CN V

• A columnar nucleus present throughout the caudal pons and the entire length of the medulla, located in the lateral aspect of the tegmentum.
• Continuous longitudinally with the substantia gelatinosa of the spinal cord, and the principal or chief sensory nucleus of the pons.
• Receives 1ary afferent fibers originating of CN V, VII, IX and X and mediates crude touch, pain, and temperature sensations form the face
• Tract formed by axons of incoming 1ary afferent fibers is located lateral to the nucleus
• It projects mainly to contralateral VPM of the thalamus through the Ventral Trigemino-thalamic tract.

Special Sensory Nuclei

• Located in the Pons: Vestibular (S, L, M)
• Located in the Medulla: both vestibular (L,M, I or Sp) and cochlear.

Vestibular Nuclei (Superior, Medial, Lateral, Inferior or Spinal)

• Location: in the caudal pontine and along the full extent of the medullary tegmentum, dorsolaterally.
• Receive 1st order proprioception fibers from semicircular ducts, utricle and saccule, through CN VIII.
• Receives input from the cerebellum and sends projections to the cerebellum, MLF and spinal cord.

Cochlear Nuclei (Dorsal and Ventral)

• Location: rostral medullary tegmentum, lateral to the inferior cerebellar peduncle
• Receives first order auditory fibers from CN VIII

Visceral Sensory Nuclei

• Medulla contains the solitary nucleus

Solitary Nucleus & Tract

• Location: medullary tegmentum
• Receives afferent input on taste from CN VII, IX, and X in its rostral portion
• Caudal portion receives afferent input on sensations from the viscera from CN IX and X

Midbrain Nuclei

• Includes the Superior Colliculus, Inferior Colliculus, Periaqueductal Gray, Substantia Nigra, and Red Nucleus.

Superior Colliculus (SC)

• Located in the rostral tectum.
• Is concerned with detection and tracking of movement of objects in visual fields (horizontal conjugate gaze).
• Receives visual input from the retina and cortex through the brachium of the superior colliculus.
• Receives auditory input from the inferior colliculus to mediate audiovisual reflexes.
• Both sides are interconnected through a commissure.
• Projects to the cervical spinal cord via the tectospinal tract to mediate reflex movements of the head/neck in response to visual/auditory inputs.

Inferior Colliculus

• Located in the caudal tectum, towards the pons-midbrain junction.
• Relays auditory information.
• Receives ascending bilateral input from cochlear nuclei and the superior olivary nucleus through the lateral lemniscus (LL).
• Projects to the medial geniculate nucleus (MGN) of the thalamus, through the brachium of the inferior colliculus.
• Both sides are interconnected through a commissure.

Periaqueductal Gray (PAG)

• Central gray matter surrounding the cerebral aqueduct.
• Plays roles in regulation of autonomic functions, affective/emotional processes, and modulation of pain impulses.
• Contains various nuclei formed by enkephalinergic neurons involved in endogenous pain control.
• Drugs such as morphine act on opioid receptors in regions including the PAG.

Substantia Nigra

• Located between the crus cerebri and the midbrain tegmentum.
• Divided into a dorsomedial pars compacta and a ventrolateral pars reticularis.
• Projects dopaminergic fibers to the neostriatum which regulate voluntary movements.
• Damage to dopaminergic neurons leads to Parkinson's disease.

Red Nucleus

• Located in the rostral midbrain tegmentum.
• Receives bilateral input from the cerebral cortex.
• Receives contralateral input from cerebellar nuclei.
• Gives rise to the rubrospinal tract which reaches all spinal levels, facilitating activation of flexor motor neurons.

Pons Nuclei

• Include the Superior Olive, Deep Pontine Nuclei, Locus Ceruleus, Reticular Nuclei, and Parabrachial Nuclei.

Superior Olive

• Located in the ventrolateral aspect of the rostral pontine tegmentum.
• Receives auditory inputs from the medullary cochlear nuclei.
• Projects auditory information bilaterally to higher brainstem centers, such as the inferior colliculus, through the lateral lemniscus.

Deep Pontine Nuclei

• Located in the basilar pons.
• Receive input from cortical areas.
• Origin of the transverse pontine fibers that project to the cerebellum via the medial cerebellar peduncle.
• Serves as a pathway between the cerebral and cerebellar cortices.

Locus Ceruleus

• Located in the dorsolateral aspect of the rostral pontine tegmentum, at the level of the motor nucleus of CN V and above.
• It's a nucleus of the monoamine system, projecting norepinephrine (NE) axons to various regions of the forebrain, brainstem, and cerebellum.
• It also contains melanin.

Reticular Nuclei

• Located in the central pontine tegmentum.
  • Raphe nuclei: near the midline, formed by serotonin neurons, and project widely to the brain and spinal cord.
  • Parvocellular: located laterally within the central tegmental area, and receive information from afferent fibers.
  • Magnocellular: located medially within the central tegmental area, and project fibers from the reticular formation to other CNS areas.

Parabrachial Nuclei

• Located in the dorsolateral aspect of the rostral pontine tegmentum, surrounding the superior cerebellar peduncles.
• Relay sensory information from the solitary nucleus to the forebrain, aiding in the regulation of behavioral responses to taste and other visceral sensations.

Medulla Nuclei

• Includes the Nucleus Gracilis, Nucleus Cuneatus, Reticular Nuclei, and Inferior Olive.

Nucleus Gracilis & Nucleus Cuneatus

• Located dorsally in the caudal medulla.

Medullary Reticular Nuclei

• The rostral ventrolateral medulla (RVLM) contains neurons that are involved in the control of blood pressure, regulating vasoconstriction, and cardiac output.
• RVLM neurons receive input from peripheral baroreceptors, chemoreceptors, and various brainstem and brain areas, including the periaqueductal gray (PAG) nuclei and the hypothalamus.
• Its axons descend to sympathetic preganglionic neurons in the spinal cord.
• Some RVLM neurons are tonically active.

Inferior Olive

• Is a cerebellar relay nucleus that projects to the contralateral cerebellum via the inferior cerebellar peduncle.
• Receives input from the red nucleus and the spinal cord.

Tracts

• Include Ascending, Descending, and Local Tracts.

Diffusion Tensor Imaging (DTI)

• Generates MRI images using diffusion of water molecules to create contrast.
• Is used to map white matter tractography in the brain.

Ascending Tracts

• Includes the Anterolateral System, Medial Lemniscus, Spinocerebellar, and Spinal Trigeminal tracts.

Anterolateral System

• Carries pain and temperature sensations from the body.
• Retains its same general position it occupies in the lateral and ventral columns of the spinal cord.
• Exhibits somatotopic organization.
  • Lower limb: lateral
  • Upper limb: medial

Medial Lemniscus (ML)

• Carries touch, vibration, and proprioception sensations from the body.
• Originates at nucleus gracilis (1) and nucleus cuneatus (2).
• Internal arcuate fibers (IAF) arise from the nucleus gracilis and nucleus cuneatus, decussating to form the contralateral medial lemniscus.
• Is medial in the Medulla.
• Is Somatotopically organized
  • lower limb - lateral
  • upper limb - medial

Spinocerebellar Tracts

• Coordination of movement.
• Retains same general position they occupy in the lateral and ventral columns of the spinal cord.
• Dorsal Spinocerebellar tract becomes the inferior cerebellar peduncle of the medulla.

Spinal Trigeminal Tract

• Contains 1st order afferent fibers that mediate crude touch, pain, and temperature sensations from the face, from CN V, VII, IX, and X.
• Projects to the Spinal Nucleus of V.
• Occupies a lateral position, lateral to the Spinal Trigeminal Nucleus in the pons and Medulla.
• Is somatotopically organized
  • Afferent fibers from CN VII, IX and X travel more dorsally.
  • Afferent fibers from CN V travel more ventrally.
• Is continuous longitudinally with the Tract of Lissauer of the spinal cord.

Descending Tracts

• Includes the Corticospinal, Corticobulbar, Tectospinal, Rubrospinal, and Reticulospinal tracts.

Crus Cerebri (Cerebral Peduncles)

• Contains bundles of axons from cerebral cortex which terminate in the brainstem (corticobulbar) or spinal cord (corticospinal).
• Exhibits somatotopic organization.
• Corticobulbar fibers are located in the lateral and medialmost edges.
  • Lateral edge: Fibers from cortex of parietal, occipital and temporal lobes.
  • Medial edge: Fibers from cortex of frontal lobes.
• Corticospinal fibers are located in the central 3/5 of the crus cerebri.
  • Head: Medial
  • Limbs and trunk: Lateral

Corticospinal Tracts

• In the Medulla, they are located in a ventromedial position, in the pyramids, above the level of the decussation.
• 90% of fibers of the pyramids cross the midline (pyramidal decussation) in the caudal medulla.
• Exhibits somatotopic organization
  • lower limb information located laterally
  • upper limb information located medially

Local Tracts

• Includes the Medial Longitudinal Fasciculus (MLF), Lateral Lemniscus, and Transverse Pontine Fibers.

Medial Longitudinal Fasciculus (MLF)

• Located dorsal to the tectospinal tract.
• Contains vestibular fibers that coordinate eye movements.
• Interconnects motor nuclei of CN III, IV, and VI to coordinate the function of extraocular muscles.
• Also projects to the spinal cord.
• Provides information about the position of the head in space to the spinal accessory nucleus and cervical motor nuclei.

Lateral Lemniscus

• An auditory pathway extending from the contralateral ventral cochlear nucleus, and bilateral superior olives, to the nucleus of the inferior colliculus of the midbrain.

Transverse Pontine Fibers or Pontocerebellar Tract

• Massive bundles of fibers that run transversely in the basilar pons, and form the middle cerebellar peduncles.
• Originate at the deep pontine nuclei.
• Project to the cerebellar cortex.

Rule of 4

• A method developed to help students of neurology remember brainstem anatomy and various brainstem vascular syndromes.
• There are 4 rules total

Four Midline Structures beginning with M

• Motor (corticospinal) pathway: contralateral weakness of arm/leg
• Medial Lemniscus: contralateral loss of vibration/proprioception of arm/leg
• MLF: ipsilateral internuclear ophthalmoplegia
• Motor nucleus and nerve: ipsilateral loss of affected CN (3, 4, 6, or 12)
• Paralysis of CN III would affect eye movements and pupil dilation

Four Lateral Structures beginning with S

• Spinocerebellar pathway: ipsilateral ataxia of arm/leg
• Spinothalamic pathway: contralateral loss of pain/temperature of arm/leg/trunk
• Sensory nucleus of V: ipsilateral loss of pain/temperature on face
• Sympathetic pathway: ipsilateral Horner's syndrome

Cranial Nerves and Motor Nuclei Rules

• There are 4 cranial nerves in the medulla, 4 in the pons, and 4 above the pons (2 in the midbrain).
• 4 motor nuclei that are in the midline divide equally into 12 (3, 4, 6, and 12) (the others are either not motor or not in the midline).
• Medial motor nerves are:
• Medulla: Hypoglossal (12)
• Pons: Abducens (6)
• Midbrain: Trochlear & Oculomotor (4, 3)
• A medial brainstem syndrome consists of the 4 M's and the relevant motor cranial nerve (3rd-4th in the midbrain, 6th in the pons, 12th in the medulla).
• A lateral brainstem syndrome consists of the 4 S's and the relevant cranial nerves (9-11th in the medulla, or the 5th, 7th and 8th in the pons).

Brainstem Lesions

• Commonly a result of vascular lesions

Midbrain Vascular Supply

• Principle artery that supplies the midbrain is the posterior cerebral artery

Medial Midbrain Basis Syndrome (Basis Infarct or Weber's Syndrome)

• Occlusion of paramedian branches of the posterior cerebral artery (PCA)
• Damaged structures include the crus cerebri (corticospinal tract) and the oculomotor nerve.
• Resulting deficits include contralateral hemiparesis or hemiplegia and ipsilateral CN III palsy or paralysis (weakness or paralysis of eye movements & pupil dilation).

Medial Midbrain Tegmentum Syndrome (Tegmentum Infarct or Claude's Syndrome)

• Caused by occlusion of the paramedian branches of the posterior cerebral artery (PCA)
• Damaged structures include the oculomotor nerve and the red nucleus
• Resulting deficits include ipsilateral CN III palsy or paralysis, and contralateral ataxia

Medial Midbrain Syndrome (Basis + Tegmentum Infarct or Benedikt's Syndrome)

• Caused by a wide range occlusion of the PCA
• Structures damaged include the crus cerebri (corticospinal tract), oculomotor Nerve, and red Nucleus
• Resulting deficits include contralateral hemiparesis, ipsilateral CN III palsy , and ataxia

Lesions of the Midbrain

• Other common sources include tumors in the pineal region

Parinaud's Syndrome

• Caused by a tumor in the pineal region
• Damaged structures include the superior colliculus, cerebral aqueduct and oculomotor and trochlear nuclei/nerve
• Resulting deficits include paralysis or weakness of directed gaze, hydrocephalus and paralysis of eye muscles

Pons Vascular Supply

• Supply is by the superior cerebellar artery (1), basilar artery (2) and anterior inferior cerebellar artery (AICA) (3)

Medial Pontine Syndromes

• Caused by occlusion of branches of the basilar artery
• Occlusion in the base causes paramedian deficits
• Occlusion in the base and tegmentum cause paramedian and circumferential deficits

(Dorso) Lateral Rostral Pontine Syndrome

• Caused by occlusion of the superior cerebellar artery
• Damaged structures include the superior cerebellar peduncle/cerebellum, the mesencephalic nucleus of V, as well as the motor nucleus of V
• Resulting deficits include ipsilateral ataxia, loss of coordination of the muscles of mastication and weakness of the muscles of mastication

Medial Pontine Basis Infarct

• Damaged structures include the corticospinal tract, the pontine nuclei and pontocerebellar fibers
• Resulting deficits include contralateral hemiparesis or hemiplegia and Contralateral ataxia

Medial Pontine Basis + Tegmentum Infarct

• Damaged structures include the corticospinal tract, pontine nuclei and pontocerebellar fibers, medial lemniscus, MLF, abducens nucleus/nerve fibers AND facial nerve fibers at facial colliculus
• Resulting deficits include contralateral hemiparesis or hemiplegia, ataxia, a decrease of body touch/vibration/position sense, an impairment of lateral conjugate gaze, ipsilateral horizontal gaze palsy and weakness of face muscles

Lateral Caudal Pontine Syndrome

• Caused by occlusion of the anterior inferior cerebellar artery (AICA)
• Damaged structures include middle cerebellar peduncle, vestibular nuclei, spinal trigeminal tract and nucleus, anterolateral system, facial nucleus and nerve fibers AND descending hypothalamic and ventrolateral tegmentum fibers
• Resulting deficits include ipsilateral ataxia, vertigo/nausea/nystagmus, a decrease of pain and temperature sensations of the face, a contralateral decrease of pain and temperature sensations of the body, ipsilateral facial muscles weakness and horner's Syndrome

Medulla Vascular Supply

• The arteries that supply the medulla primarily include the anterior spinal artery (1), posterior spinal artery (2) and vertebral artery (3), as well as PICA (4)

Lateral Medullary (Wallenberg) Syndrome

• Caused by posterior inferior cerebellar artery (PICA) and/or vertebral artery occlusion
• Damaged structures include the inferior cerebellar peduncle & spinocerebellar fibers, spinal trigeminal tract and nucleus, the anterolateral system, descending hypothalamic and ventrolateral tegmental fibers, the nucleus ambiguous, vestibular nuclei, solitary tract and nucleus AND dorsal motor nucleus of vagus
• Resulting Deficits include include ataxia to the ipsilateral side, ipsilateral loss of pain and temperature sensation from the face, contralateral loss of pain and temperature sensation from the body, ipsilateral Horner’s Syndrome, dysphagia, nausea/diplopia/tendency to fall to ipsilateral side/nystagmus/vertigo, a loss of taste from ipsilateral side of tongue AND dyspnea & tachycardia

Medial Medullary (Dejerine) Syndrome

• Caused by anterior spinal artery occlusion
• Damaged structures include the pyramid / Corticospinal Tract, medial Lemniscus, MLF and hypoglossal Nerve and/or Nucleus
• As a result there is contralateral hemiplegia or hemiparesis, a contralateral loss of discriminative touch/ vibratory and position sense, an impairment of conjugate lateral gaze AND a deviation of tongue to ipsilateral side when protruded (muscle atrophy with fasciculations)