Brainstem III: tracts and reticular formation

Functions of the Brainstem

  • Provides ascending and descending conduits for signals traveling between the spinal cord, forebrain, and cerebellum.

  • Houses the nuclei of many cranial nerves (III-X, XII) for communication between the sensory organs of the head, brain, and the rest of the body.

  • Maintains neuronal circuits for the regulation of many critical bodily functions essential for homeostasis and survival, including:

    • Breathing

    • Swallowing

    • Locomotion

    • Cranial nerve reflexes (e.g., in babies with hydranencephaly).

  • Alters behavioral states (e.g., highly alert vs. drowsy) through ascending and descending modulations to forebrain and spinal circuits.

Baroreceptor Reflex

  • The baroreceptor reflex regulates blood pressure by balancing parasympathetic and sympathetic outflows.

  • Key Components:

    • Carotid Sinus

    • Aortic Arch

Chemoreceptor Reflex

  • The chemoreceptor reflex ensures a sufficient blood oxygen level by regulating ventilation.

Cranial Nerve Reflexes

Blink Reflex Assessment

  • Traditionally used to test both the trigeminal nerve (CN V) and the facial nerve (CN VII).

  • Response Mechanism:

    • Bilateral responses of orbicularis oculi around the orbit, elicited by touching the cornea.

    • Ipsilateral connection through the main sensory nucleus.

    • Bilateral responses through the spinal trigeminal nucleus (rostral to obex).

    • Ipsilateral connections from the main sensory nucleus to the facial motor nucleus.

    • Bilateral connections from the spinal trigeminal nucleus to the facial motor nucleus.

    • Reference: www.youtube.com/watch?v=49nnrBwCoYs

Gag Reflex Assessment

  • Traditionally used to test both the glossopharyngeal nerve (CN IX) and the vagus nerve (CN X).

  • Response Mechanism:

    • Elicited by touching the posterior pharyngeal wall, tonsillar area, or base of the tongue.

    • Involves complex bilateral motor responses primarily involving the posterior pharyngeal muscles, uvula, tongue, and jaw muscles.

    • The neural circuits involved are incompletely understood.

    • Reference: www.youtube.com/watch?v=uq8cRzi1nkU

Summary of Cranial Nerve Nuclei and Tracts in Brainstem Sections

  • Accessory Nucleus (CN XI): Extends through the upper 5 cervical spinal segments, with rootlets exiting more laterally than spinal ventral roots.

  • Spinomedullary Junction: Important for understanding the accessory nucleus (CN XI) and spinal trigeminal nucleus & tract (CN V).

  • Major structures and nuclei listed include:

    • Pyramidal decussation

    • Anterolateral system

    • Spinocerebellar tracts

    • Fasciculus cuneatus

    • Nucleus gracilis

    • Nucleus cuneatus

Neural Connections in Caudal Medulla

  • Caudal to obex, primary sensory fibers in the spinal trigeminal tract:

    • Synapse with neurons in the spinal trigeminal nucleus, which then course rostrally to cross the midline at more rostral levels and join the anterolateral system (in rostral medulla to caudal pons).

  • Medial Longitudinal Fasciculus (MLF): Interconnects neck muscles, cranial nerve nuclei for eye muscles, and vestibular nuclei for head-eye coordination, leading to:

    • Medial vestibulospinal tract

    • Pontine reticulospinal tract.

Additional Nuclei and Structures in Caudal Medulla

  • Nucleus of Solitary Tract: CN VII, IX, X processing.

  • Dorsal Motor Nucleus of Vagus: CN X functions.

  • Hypoglossal Nucleus: CN XII functions.

  • Includes neural circuits involved in cranial reflexes and sensory processing.

Structures in the Pons and Midbrain

Caudal Pons

  • Key Components:

    • Internal genu of facial nerve

    • Trapezoid body

    • Vestibular nuclei (CN VIII)

    • CN VI, VII, VIII structures.

Midpons

  • Key Structures:

    • Trigeminal motor nucleus (CN V)

    • Main trigeminal sensory nucleus (CN V)

    • Mesencephalic nucleus of V.

Rostral Midbrain

  • Oculomotor Nucleus (CN III): Functions in eye movement.

  • Edinger-Westphal Nucleus (CN III): Controls pupil constriction.

Avatar Analysis of Cranial Nerve Damage

  • Signs of Lesions Involving Cranial Nerves and Their Nuclei:

    • Problems with pupil constriction, eye movements, corneal reflex, swallowing, voice, and articulation.

    • The affected cranial nerves can include CN III, IV, V, VII, IX, X, XI, and XII.

Example: Wallenberg’s Syndrome

  • Symptoms include:

    • Loss of pain & temperature sensation on the ipsilateral face and contralateral limbs & trunk (affecting CN V).

    • Loss of corneal reflex (involving CN V and/or CN VII).

    • Dysphagia and dysphonia (involving nucleus ambiguus and/or CN XII).

    • Deviation of soft palate (involving the nucleus ambiguus).

    • Pathological nystagmus and vertigo (involving CN VIII).

  • Determining lesion levels requires knowledge of affected nuclei and tracts.

Functions of the Reticular Formation

  1. Motor Control: Involved in balance, locomotion, or voluntary movement through:

    • Pontine and medullary reticulospinal tracts

    • Reticulocerebellar tracts.

  2. Sensory Modulation: Modulates sensory information (especially pain) from the spinal cord via raphespinal tract for pain control.

  3. Autonomic Modulation: Controls autonomic activities regulating:

    • Inspiratory and expiratory centers,

    • Heart rate,

    • Blood pressure.

  4. Control of Arousal and Consciousness: The Ascending Reticular Activating System (ARAS) provides diffuse and non-specific projections to thalamus and cortex, highlighting the cortex's dependency on ARAS to maintain consciousness.

Monoaminergic Neurons and Their Importance

  • Monoaminergic neurons in nuclei rostral to midpons are crucial for arousal and consciousness:

    • Locus coeruleus (norepinephrine): Influences attention and vigilance.

    • Raphe nuclei (serotonin): Modulates overall arousal and has a descending pain control system.

    • Substantia nigra and ventral tegmental area (dopamine): Involved in movement initiation and reinforcement signals for pleasurable experiences.

Conceptual Essence of Brainstem Organization

  • Despite its complexity, the organization of the brainstem largely follows a basic plan similar to that of the spinal cord. However, this organization is enriched by structures associated with the cerebellum and sensory organs in the head, as noted by WH Nauta.

Practice Tools

  • Acknowledgment of practice resources is emphasized for comprehension and retention of the material.

  • Sections of the brainstem without labels are provided for additional practice.

Anatomical References

  • Caudal and rostral sections of various parts of the brainstem including:

    • Caudal Medulla

    • Rostral Medulla

    • Caudal Pons

    • Midpons

    • Rostral Pons

    • Caudal Midbrain

    • Rostral Midbrain

References and Sources

  • Kandel, Schwartz, Jessell. Principles of Neural Science, 6th edition.

  • Kwong WH, class notes for CUHK.

  • Nauta WJH, Feirtag M. Fundamental Neuroanatomy.

  • Schneider GE. Brain Structure and Its Origin.

  • Vanderah TW, Nolte. The Human Brain in Photographs and Diagrams, 5th edition.

  • Vanderah TW, Gould DJ. The Human Brain: An Introduction to its Functional Anatomy, 8th edition.