Brainstem
Truncus encephali
Also known as: brain stem
The stalk of the brain (midbrain, pons and medulla oblongata) connecting the forebrain to the spinal cord; it carries every ascending and descending tract, contains the nuclei of cranial nerves III–XII and the vital cardiorespiratory centres, and sustains consciousness.
Overview
The brainstem is ≈7–8 cm long. It sits on the clivus in the posterior cranial fossa, in front of the cerebellum, to which it is joined by three pairs of cerebellar peduncles. The fourth ventricle lies between the two. It is divided into the midbrain (≈2 cm), the pons (≈2.5 cm) and the medulla oblongata (≈3 cm). In cross-section each level has three zones:
• a ventral basal part with descending motor fibres: cerebral peduncles, basilar pons, pyramids;
• a central tegmentum with the cranial nerve nuclei, the ascending lemnisci, the medial longitudinal fasciculus and the reticular formation;
• a dorsal tectum (midbrain) or roof (the cerebellum and medullary vela of the fourth ventricle).
The brainstem's functions are packed into a very small volume:
• it is the conduit for the corticospinal, corticobulbar, sensory and cerebellar pathways;
• it holds the cranial nerve nuclei III–XII, mediating the brainstem reflexes (pupillary, corneal, vestibulo-ocular, gag and cough);
• it houses the cardiovascular and respiratory centres of the medulla and pons;
• it contains the reticular formation and the monoaminergic nuclei, which sustain arousal.
Lesions therefore produce characteristic crossed syndromes: an ipsilateral cranial-nerve palsy with contralateral hemiplegia or hemisensory loss. The level of the cranial nerve involved localises the lesion precisely.
- Location
Posterior cranial fossa, on the clivus; from the diencephalon above to the foramen magnum (level of the C1 rootlets) below.
Parts & subdivisions
- Cerebral peduncles, tegmentum (red nucleus, substantia nigra, III and IV nuclei), tectum (colliculi).
- Basilar pons (pontine nuclei) and tegmentum (V, VI, VII, VIII nuclei).
- Pyramids, olives, dorsal column nuclei, IX–XII nuclei, vital centres.
- Core network for arousal, posture, autonomic and respiratory control.
- Arranged in longitudinal columns: motor (basal plate) medially, sensory (alar plate) laterally, separated by the sulcus limitans in the floor of the fourth ventricle.
- Superior (to the midbrain; mainly efferent), middle (from the pons; afferent), inferior (from the medulla and cord; mainly afferent).
- Medial longitudinal fasciculus (MLF)Links the III, IV and VI nuclei with the vestibular nuclei to yoke the eyes (conjugate gaze, vestibulo-ocular reflex). A lesion causes internuclear ophthalmoplegia.
Anatomy
- Arterial supply
Vertebrobasilar system: anterior spinal and vertebral arteries and PICA (medulla); basilar artery paramedian, short and long circumferential branches, AICA (pons); PCA and superior cerebellar artery branches (midbrain).
Function
- Conducts all tracts between the forebrain, cerebellum and spinal cord
- Origin of cranial nerves III–XII
- Vital control of breathing, heart rate and blood pressure
- Consciousness and sleep–wake regulation
- Reflexes of swallowing, coughing, vomiting, gaze and posture
- Descending pain modulation (periaqueductal grey, raphe)
Clinical significance
Classic crossed brainstem syndromes
• Weber (ventral midbrain, PCA perforators): ipsilateral CN III palsy + contralateral hemiplegia.
• Millard–Gubler (ventral caudal pons): ipsilateral VI and VII palsies + contralateral hemiplegia.
• Lateral medullary (Wallenberg) (vertebral artery or PICA): ipsilateral facial pain and temperature loss, Horner syndrome, ataxia, dysphagia and hoarseness (nucleus ambiguus), vertigo and nystagmus, with contralateral loss of body pain and temperature.
• Medial medullary (Dejerine) (anterior spinal artery): ipsilateral tongue weakness + contralateral hemiplegia and loss of vibration and proprioception.
Locked-in syndrome
A ventral pontine infarct from basilar artery occlusion destroys the corticospinal and corticobulbar fibres but spares the tegmentum. The patient is conscious and aware, with quadriplegia and anarthria, but can still move the eyes vertically and blink.
Mnemonics
The "rule of 4" of the brainstem
• 4 midline (medial) structures, each starting with M: Motor pathway (corticospinal), Medial lemniscus, Medial longitudinal fasciculus, Motor nuclei of the cranial nerves that divide evenly into 12 (III, IV, VI, XII).
• 4 lateral structures, each starting with S: Spinocerebellar tracts, Spinothalamic tract, Sensory nucleus of V, Sympathetic pathway.
• 4 cranial nerves in the medulla (IX–XII), 4 in the pons (V–VIII) and 4 above the pons (I–IV; III and IV in the midbrain).
Medial syndromes therefore involve M structures, and lateral syndromes involve S structures.
Test yourself
Which cranial nerve is the only one to emerge from the dorsal surface of the brainstem?
- Oculomotor (III)
- Hypoglossal (XII)
- Abducens (VI)
- Trochlear (IV)
Answer: Trochlear (IV)
The trochlear nerve emerges just below the inferior colliculi, decussates, and supplies the contralateral superior oblique. It is also the thinnest cranial nerve and has the longest intracranial course.
Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.