Medulla, Pons and Midbrain

Instructors

  • Thomas J. Perrault Jr., PhD

    • Position: Assistant Professor of General Surgery

    • Institution: Wake Forest School of Medicine

    • Email: Thomas.Perrault@wfusm.edu

    • Office: 4430

  • Lauren Fowler, PhD

    • Position: Professor of Neuroscience

    • Department: Department of Physiology and Pharmacology

    • Institution: Wake Forest School of Medicine

    • Email: lafowler@wakehealth.edu

Objectives

  • Gross and Internal Anatomy of Medulla:

    • Discuss the anatomy including:

    • Ascending pathways: Such as the Medial Lemniscus (carrying discriminative touch, vibration, proprioception) and Spinothalamic tracts (carrying pain and temperature sensation).

    • Descending pathways: Primarily the Pyramidal (corticospinal) tracts for voluntary motor control.

    • Identify key nuclei and pathways in transverse sections of the medulla, including the inferior olivary nucleus, solitary nucleus, nucleus ambiguus, and cranial nerve nuclei (IX, X, XII).

    • Correlate abnormalities such as:

    • Lateral medullary syndrome: Which involves deficits in pain/temperature, swallowing, and balance due to specific lesions.

    • Medial medullary syndrome: Characterized by motor and specific sensory losses due to damage to medial structures.

    • Relate the basic structure and function of the medulla to clinical presentations.

  • Organization of the Pons:

    • Discuss the structural organization, including the ventral pons (containing pontine nuclei which relay motor information to the cerebellum, and transverse pontine fibers, corticospinal, corticopontine, and corticonuclear tracts) and the dorsal pons (tegmentum, containing cranial nerve nuclei V-VIII, medial lemniscus, spinothalamic tracts, and reticular formation).

    • Discuss lesions affecting the pons, such as those causing Locked-In Syndrome, often due to basilar artery occlusion.

  • Comparison of Colliculi:

    • Superior colliculi: Involved in visual reflexes and controlling eye movements (e.g., saccades, visual tracking) in response to visual stimuli.

    • Inferior colliculi: A major relay center for auditory pathways, involved in sound localization and the auditory startle reflex.

  • Neurotransmitter Synthesis:

    • Describe regions of the brainstem involved in neurotransmitter synthesis and related pathways:

    • Raphe nuclei for serotonin.

    • Locus Coeruleus for norepinephrine.

    • Substantia Nigra and Ventral Tegmental Area (VTA) for dopamine.

    • Discuss the functional implications of these neurotransmitter systems in mood, arousal, reward, and motor control.

Clinical Correlations and Syndromes

  • Medial Medullary Syndrome (Dejerine): Clinical features and implications related to damage of specific medial brainstem structures.

  • Lateral Medullary Syndrome (Wallenberg): Clinical features and implications stemming from ischemia in the lateral medulla.

  • Locked-In Syndrome: Mechanism (ventral pontine lesion) and clinical presentation (quadriplegia and anarthria with preserved consciousness and vertical eye movement).

  • Weber’s Syndrome: Description (midbrain lesion affecting oculomotor nerve and cerebral peduncle) and clinical implications.

  • Pyramidal and Extrapyramidal Tract Deficits: Overview and effects on motor function and coordination.

Brain Structure Overview

  • Components of the Brainstem:

    • Medulla Oblongata: Regulates vital autonomic functions like heart rate, breathing, and blood pressure.

    • Pons: Acts as a bridge connecting various parts of the brain, involved in sleep, respiration, swallowing, bladder control, and facial sensation/expression.

    • Midbrain: Involved in motor control, auditory and visual reflexes.

  • Connections: Connects the spinal cord inferiorly with the forebrain (diencephalon and cerebrum) superiorly.

Brainstem Composition

  • White and Gray Matter:

    • White matter contains:

    • Ascending fibers (e.g., Medial Lemniscus, Spinothalamic Tracts, Spinocerebellar Tracts) carrying sensory information to higher centers.

    • Descending fibers (e.g., Corticospinal Tracts, Rubrospinal Tract, Reticulospinal Tract) conveying motor commands from the cortex and brainstem.

    • Numerous nuclei associated with cranial nerves (III-XII) for sensory, motor, and parasympathetic functions.

    • Functions:

      • Involvement in breathing and heartbeat regulation (e.g., medullary respiratory and cardiovascular centers).

      • Coordination of states of alertness or arousal (primarily via the Ascending Reticular Activating System - ARAS).

      • Mediation of motor activities (via descending tracts and brainstem nuclei) and relay of sensory information to the thalamus and cerebellum.

The Hindbrain

  • Components:

    • Medulla: Vital autonomic control.

    • Pons: Respiratory regulation, cerebellar relay.

    • Cerebellum: Fine motor control, coordination, balance, motor learning.

    • Reticular Formation: A diffuse network of neurons involved in arousal, sleep/wake cycles, pain modulation, and motor control.

    • Raphe System: Produces serotonin, influencing mood, sleep, appetite, and pain perception.

    • Locus Coeruleus: Primary source of norepinephrine in the brain, playing a role in attention, arousal, and stress response.

    • Area Postrema (CTZ): Chemoreceptor trigger zone, involved in inducing vomiting in response to toxins in the blood.

Neurotransmitter Systems

  • Serotonin System:

    • Related areas include: Neocortex, Hypothalamus, Temporal lobe, Basal ganglia, Thalamus, Raphe nuclei, Cerebellum.

    • Involved in regulating mood, sleep-wake cycles, appetite, pain perception, and executive function.

  • Norepinephrine System:

    • Involves: Neocortex, Hypothalamus, Temporal lobe, Locus coeruleus, Thalamus, Cerebellum.

    • Primarily involved in arousal, attention, vigilance, and the body's 'fight-or-flight' response to stress.

Midbrain Anatomy

  • Key Structures:

    • Tegmentum: The ventral part of the midbrain, containing the red nucleus (motor coordination), substantia nigra, VTA, and nuclei for cranial nerves III (oculomotor) and IV (trochlear).

    • Substantia nigra: Characterized by melanin-pigmented neurons, a major source of dopamine projection to the basal ganglia (nigrostriatal pathway), crucial for motor control. Degeneration leads to Parkinson's disease.

    • Ventral Tegmental Area (VTA): Another major source of dopamine, projecting to the limbic system and prefrontal cortex (mesolimbic and mesocortical pathways), involved in reward, motivation, and cognition.

    • Pontomesencephalotegmental Area: Contains various reticular nuclei and projections, including cholinergic and monoaminergic pathways influencing sleep and arousal.

    • Tectum: Consists of the Superior and Inferior Colliculus (Corpora Quadrigemina) posteriorly.

    • Reticular Formation: The midbrain portion of the ascending reticular activating system.

    • Periaqueductal Gray (PAG): Involved in pain modulation (descending pain control system) and defensive behaviors.

Dopamine Systems

  • Key pathways:

    • Nigrostriatal pathway: Originates from the substantia nigra pars compacta and projects to the striatum; primarily involved in motor control. Impairment causes Parkinsonian symptoms.

    • Mesolimbic pathway: Originates from the VTA and projects to limbic structures (e.g., nucleus accumbens, amygdala); associated with reward circuitry, motivation, and addiction. Overactivity is implicated in positive symptoms of schizophrenia.

    • Mesocortical pathway: Originates from the VTA and projects to the prefrontal cortex; involved in cognition, executive function, and emotional regulation. Dysfunction is linked to negative and cognitive symptoms of schizophrenia.

    • Tuberoinfundibular pathway: Originates from the arcuate nucleus of the hypothalamus and projects to the anterior pituitary; regulates prolactin release from the anterior pituitary (dopamine inhibits prolactin secretion).

  • Major Dopamine Production Areas:

    • Substantia Nigra (pars compacta).

    • Ventral Tegmental Area (VTA).

Pathways within Hindbrain and Midbrain

  • Reticular Formation:

    • Ascending Reticular Activating System (ARAS): A network of neuronal pathways originating in the brainstem reticular formation, projecting to the thalamus and cerebral cortex. Functions in sleep/wake cycles, alertness, and attention.

    • Descending Reticulospinal Tract: Originates from pontine and medullary reticular formation, involved in gross locomotion, posture, and modulating muscle tone.

  • Motor Pathways:

    • Corticospinal tract (pyramidal): Originates mainly from the primary motor cortex, travels through the brainstem, and decussates in the medulla. Responsible for voluntary, fine motor control of the contralateral limbs.

    • Tectospinal tract (extrapyramidal): Originates in the contralateral superior colliculus and mediates head and eye movements in response to visual and auditory stimuli.

    • Rubrospinal tract: Originates in the red nucleus of the midbrain, contributes to motor coordination, especially of the upper limbs.

    • Vestibulospinal tract: Originates in the vestibular nuclei, helps maintain balance and posture.

Clinical Considerations: Medulla

  • First Pass Details:

    • At the level of decussation of pyramids: Marks the major cross-over of motor fibers of the corticospinal tracts, where approximately 80-85% of fibers cross to the contralateral side. This is why a lesion above this point causes contralateral motor deficits, while a lesion below causes ipsilateral deficits.

    • Abnormalities in the medulla can lead to distinct clinical syndromes affecting motor function, sensation, and vital autonomic processes due to the compact arrangement of ascending, descending, and cranial nerve pathways.

Medulla, pons and midbrain: Second Pass

The Brain Stem - Medulla Oblongata

  • Olive

    • Proprioceptive info from SC to cerebellum is processed through the olive, which plays a crucial role in motor coordination and balance.

  • Inferior Cerebellar Peduncle

    • Medulla to cerebellum

    • Mostly proprioception and balance

  • Middle Cerebellar Peduncle

    • Connects the pons to the cerebellum

    • Primarily transmits information related to motor control and coordination from the cerebral cortex to the cerebellum, facilitating smooth voluntary movements and maintaining posture.

      • Damage produces deficits ipsilateral to damage

  • Superior Cerebellar Peduncle

    • From midbrain to cerebellum, the superior cerebellar peduncle also carries sensory information about the position of the body and limbs, further enhancing the precision of motor commands.

Pons and Sleep

  • The pons play a crucial role in regulating sleep cycles, particularly in the transition between wakefulness and REM sleep.

  • Activates REM

  • Uses ARAS to increase brain activity

  • Help suppress corticospinal activity during REM

Internal Medulla Structure Analysis

  • Transverse Sections: Study at four levels:

    • Decussation of pyramids (most caudal): Here, the corticospinal tracts cross the midline, situated ventrally.

    • Decussation of medial lemnisci: Internal arcuate fibers, transmitting sensory information from the nucleus gracilis and cuneatus, cross the midline to form the medial lemniscus dorsally to the pyramids.

    • At the level of the olives: Features the prominent inferior olivary nucleus (involved in motor learning and coordination with the cerebellum), as well as nuclei for cranial nerves IX, X, and XII.

    • Just inferior to the pons (most rostral): Contains upper medullary structures like the vestibular nuclei, cochlear nuclei, solitary nucleus, and nucleus ambiguus, alongside ascending and descending tracts.

Pathways in Medulla

  • Sensory Information:

    • Spinal trigeminal tract and nucleus: Located laterally in the medulla, receives pain and temperature sensation from the ipsilateral face and projects to the thalamus.

    • Dorsal column-medial lemniscus system: Carries discriminative touch, vibration, and proprioception from the body. Fasciculus gracilis (lower body) and fasciculus cuneatus (upper body) synapse in the nucleus gracilis and cuneatus, whose axons decussate as internal arcuate fibers, forming the contralateral medial lemniscus.

    • Loss of pain and temperature sensation: Typically related to specific lesion locations affecting the spinothalamic tracts (body) or the spinal trigeminal tract/nucleus (face).

Pathways in the Brainstem

   Descending

  • Corticospinal tract: A major pathway that carries motor signals from the cerebral cortex to the spinal cord, responsible for voluntary movement.

  • Reticulospinal tract: A crucial pathway that helps regulate automatic movements and postural adjustments by transmitting motor information from the brainstem to the spinal cord.

  • Rubrospinal tract: A pathway that facilitates motor control and coordination by transmitting signals from the red nucleus in the midbrain to the spinal cord, primarily influencing flexor muscles.

  • Tectospinal tract: A pathway involved in coordinating head and eye movements in response to visual stimuli, transmitting signals from the superior colliculus in the midbrain to the spinal cord.

  • Medial-Longitudinal Fasciculus: A fiber tract that plays a key role in coordinating eye movements and head position by connecting the vestibular system with the cranial nerve nuclei responsible for eye movement.

Ascending

  • Spinothalamic tract: A major sensory pathway that transmits pain and temperature sensations from the spinal cord to the thalamus in the brain, helping to process and relay these sensations to the somatosensory cortex.

  • Spinal Trigeminal tract: A sensory pathway that carries information about pain, temperature, and touch from the face, providing input to the brain regarding facial sensations by connecting with the spinal trigeminal nucleus.

  • Dorsal column-medial lemniscus: The dorsal column-medial lemniscus pathway is crucial for the proprioceptive and discriminative touch sensations, transmitting sensory information from the body to the brain, ultimately providing detailed spatial awareness and tactile recognition.

  • Medical-Longitudinal Fasciculus: A white matter tract in the brainstem that plays a key role in coordinating eye movements and integrating auditory and vestibular information, significantly contributing to maintaining balance and spatial orientation.

    • Responsible for “yolking” the eyes together so they more together from side to side

Clinical Correlations

  • Medial Medullary Syndrome (Dejerine):

    • Caused by occlusion of the vertebral artery or anterior spinal artery. Symptoms include:

    • Contralateral hemiparesis/hemiplegia (due to pyramidal tract damage).

    • Contralateral loss of vibratory sense, proprioception, and discriminative touch (due to medial lemniscus damage).

    • Ipsilateral tongue weakness and atrophy, with deviation of the tongue towards the side of the lesion (due to hypoglossal nerve (XII) nucleus/nerve damage).

  • Lateral Medullary Syndrome (Wallenberg):

    • Most commonly caused by occlusion of the posterior inferior cerebellar artery (PICA) or vertebral artery. Symptoms include:

    • Ipsilateral facial pain and temperature loss, and contralateral body pain and temperature loss (due to damage to the spinal trigeminal nucleus/tract and the spinothalamic tract, respectively).

    • Dysphagia (difficulty swallowing), hoarseness, and dysarthria (speech difficulty) due to involvement of the nucleus ambiguus (motor innervation for IX, X, XI).

    • Vertigo, nystagmus, nausea, vomiting, and ataxia (due to vestibular nuclei and inferior cerebellar peduncle damage).

    • Ipsilateral Horner's syndrome (ptosis, miosis, anhidrosis) due to disruption of descending sympathetic fibers.

  • Locked-In Syndrome:

    • Caused by bilateral lesions of the ventral pons, typically due to basilar artery occlusion, which disrupts all descending motor pathways while sparing ascending sensory pathways and the reticular activating system.

    • Results in complete paralysis of voluntary muscles (quadriplegia and anarthria) except for vertical eye movements and blinking, which are controlled by midbrain structures.

    • Patients are fully conscious and cognitively intact.

  • Weber’s Syndrome:

    • A midbrain lesion, specifically affecting the cerebral peduncle and the exiting oculomotor nerve (III).

    • Presents with ipsilateral oculomotor palsy (ptosis, dilated pupil, and loss of accommodation) and contralateral hemiparesis due to disruption of motor pathways.