Midbrain - from gemini

Midbrain - General Position

Most cranial of the 3 components of the brainstem, and the smallest (approx. 1.5 cm).

Midbrain - Boundaries

Delimited superiorly by the thalamus and inferiorly by the pons.

Tectum (Dorsal Midbrain) - Structures

Contains the corpora quadrigemina (superior and inferior colliculi), the trochlear nerve (IV), and the cerebral aqueduct (of Sylvius).

Superior Colliculi - Function

Processing of visual signals before passing them on to the occipital lobe.

Inferior Colliculi - Function

Processing of auditory signals before passing them through the thalamus to the main auditory processing area in the cortex.

Tegmentum (Ventral Midbrain) - Structures

Contains the red nucleus, substantia nigra, cerebral peduncles, and the oculomotor nerve (III).

Red Nucleus - Function

Coordination of movements, specifically targeting limb flexors.

Periaqueductal Grey (PAG) - Function

Primary control center for descending pain modulation; plays a pivotal role in autonomic function and behavioral responses to threat.

Reticular Formation - Columns & Functions

Divided into raphe nuclei (medial: mood/pain), gigantocellular reticular nuclei (medial: motor coordination), and parvocellular reticular nuclei (lateral: respiratory regulation/exhalation).

Locus Coeruleus - Function

Acts alongside raphe nuclei as an activator of the cortex related to alert situations and attention.

Cerebral Peduncles - Structure & Function

Pair of large nerve fiber bundles connecting the brainstem to the forebrain; important relay for signals from the cortex to the CNS and for body coordination.

Substantia Nigra - Structure & Function

Darkly pigmented cluster of neurons containing melanin that produce dopamine; key structure affected in Parkinson's Disease.

Cranial Nerve III (Oculomotor Nerve) - Exit & Innervation

Passes through the tegmentum to exit between the peduncles; innervates medial, superior, and inferior rectus, inferior oblique, and levator palpebrae superioris muscles.

Edinger-Westphal Nucleus - Function

Origin of the autonomic (parasympathetic) motor component of CN III; innervates ciliary muscle and constrictor pupillae muscle.

Cranial Nerve IV (Trochlear Nerve) - Unique Features

Innervates the superior oblique muscle; only cranial nerve to exit the posterior surface of the brainstem and completely decussate.

Medial Lemniscus (Midbrain Tract) - Function

Carries sensory information for proprioception, vibration, and fine touch.

Lateral Lemniscus (Midbrain Tract) - Function

Carries auditory signals and runs in close proximity to the inferior colliculus.

Spinothalamic Tract - Function

Ascending sensory tract found in the midbrain cross-sections.

Parinaud Syndrome (Dorsal Midbrain Syndrome) - Symptoms

Decreased/absent upward gaze, convergence retraction nystagmus, pupils unreactive to light but reactive to near-focusing, and eyelid retraction (Collier's sign).

Weber Syndrome (Ventromedial Midbrain Syndrome) - Symptoms & Cause

Caused by PCA/paramedian branch ischemia; results in ipsilateral oculomotor nerve palsy and contralateral body weakness.

Geniculate Nuclei Rule of Thumb

Medial geniculate = Music/Melody (Auditory); Lateral geniculate = Light (Visual).