Table 2.

Functional anatomy of the brainstem

Brainstem structures	Functions	Centers	Symptoms
Midbrain (rostral to the pons and caudal to the thalamus and the basal ganglia)	Eye movements	Cranial nerve nuclei: III oculomotor nerve (mainly motor) IV trochlear nerve (motor)	Oculomotor signs: Ptosis (III) Ophthalmoplegia (III, IV)
	Pupillary size: sphincter pupillae and muscles of the ciliary body, pupil light reflex	Cranial nerve nuclei: III oculomotor nerve	Pupillary anomalies: Myosis (sympathetic lesion) Mydriasis (parasympathetic lesion) Anisocoria
	Movement control	Substantia nigra	Parkinsonian syndrome and movement disorders (hemichorea, hemiballism, dystonia, tremor, asterixis, pseudo-athetosis, non-epileptic myoclonus)
	Posture tone	Red nucleus	Postural tone impairment
	Posture/auditory and visual integration	Accessory optic tractus	Balance disorder
	Posture and movement integration	Tectum (dorsal part)	Balance disorder
	Posture and inhibitor motor centers	Tegmentum (ventral portion) (basal ganglia and thalamus connections)	Involuntary movements
	Sleep/wake cycles, alertness, and arousal	ARAS: composed of almost 100 nuclei, including locus coeruleus-raphe nuclei with neocortex connections	Sleep disturbance Consciousness disorders
	Central thermic regulation	ARAS-hypothalamus connections	Hypo/hyperthermia
Pons (between the medulla and the midbrain)	Facial sensitivity, muscles of mastication	Cranial nerve nuclei: V trigeminal nerve (sensory and motor)	Facial symptoms: Facial dysesthesia Oculomotor signs: Corneal/ciliary reflex impairment
	Facial muscles and taste from the anterior 2/3 of the tongue (VII)	Cranial nerve nuclei: VII facial nerve (sensory and motor)	Facial symptoms: Peripheral facial palsy
	Eye movement (abduction)	Cranial nerve nuclei: VI abducens nerve (motor)	Oculomotor signs: Ophthalmoplegia
	Posture, sensation of rotation, gravity, and sound	Cranial nerve nuclei: VIII vestibulocochlear nerve (mostly sensory) Cerebellum tract	Altered audition (VIII) Balance disorders (VIII and cerebellum tract)
	Posture Posture and inhibitor motor center	Spinocerebellar tracts Tegmentum (thalamus and basal nuclei connections)	Cerebellar ataxia Involuntary movement
	Motor efference integration Sensory efference integration	Tracts carrying signals to the thalamus	Motor deficit Sensory deficit
	Consciousness, alertness, and sleep regulation	Tracts carrying signals to the thalamus	Sleep disturbance Consciousness disorders
	Sleep/wake cycles, alertness, and arousal	ARAS: composed of almost 100 nuclei, including raphe nuclei and locus coeruleus-raphe nuclei-neocortex connections	Sleep disturbance Consciousness disorders
	Emotion	ARAS: locus coeruleus and amygdala connections	Anxiety and post-traumatic stress disorder (PTSD)
	Central thermic regulation	ARAS-hypothalamus connections	Hypo/hyperthermia
	Respiratory drive: respiratory rate and tidal volume control	Pedunculopontine tegmentum, locus coeruleus, lateral parabrachial respiratory group, and Kölliker-Fuse nuclei	Respiratory drive dysfunction: Kölliker-Fuse and parabrachial nuclear: increase tidal volume, decrease respiratory rate Lower part/ponto-peduncular injury: respiratory asynchronism
Medulla (lower half of the brainstem, connects the higher levels of the brain to the spinal cord)	Taste from the posterior 1/3 of the tongue	Cranial nerve nuclei: IX glossopharyngeal (sensory and motor)	Tongue sensory impairment
	Pharyngo-laryngeal reflex	Cranial nerve nuclei: IX glossopharyngeal nerve X vagus nerve (sensory and motor) XI spinal nerve (motor)	Oro-pharyngo-laryngeal anomalies: Dysphagia (swallowing impairment) Dysphonia Velo-pharyngo-laryngeal impairment Absence of pharyngeal/gag reflex
	Glossal muscles	XII hypoglossal (mainly motor)	Tongue motor impairment (fasciculation, motor deficit)
	Cough	IX glossopharyngeal nerve X vagus nerve	Absence of cough reflex (IX, X)
	Posture	Spinocerebellar tracts	Cerebellar ataxia
	Regulation of autonomic nervous system:	Sympathetic nuclei Parasympathetic nuclei: vagus nerve (X) control of the heart, lung, digestive tracts	Autonomic dysfunction
	Cardiac regulation	Sympathetic nuclei Parasympathetic nuclei: vagus nerve (X) control of the heart, lung, digestive tracts	Oculocardiac reflex impairment (X) Dysautonomia: tachycardia (parasympathetic impairment), bradycardia (sympathetic impairment), sudden death
	Vasomotor regulation		Hemodynamic failure: Dysautonomia with hypertension (parasympathetic impairment), hypotension (sympathetic impairment)
	Gastrointestinal motility		Gastrointestinal motility anomalies
	Respiratory drive: respiratory rate and tidal volume control	Respiratory centers: dorsal respiratory complex	Respiratory drive dysfunction: respiratory rate irregularities and ataxic breathing, hyperventilation, respiratory-ventilator asynchronism, central apnea
	Microbiota gut-brain axis, senses and peripheral inflammation modulation	Vagus nerve (X)	Maladaptive immune response, gut-brain axis impairment
Tracts all along the brainstem	Connection of the oculomotor nerves (see Fig. 1)	Medial longitudinal fasciculus	Internuclear ophthalmoplegia
	Motor information from the periphery to supratentorial structures	Corticospinal tract Pyramidal and extrapyramidal tracts	Motor deficit, locked-in syndrome Tetrapyramidal and extrapyramidal syndromes with movement disorders (tremor) Non-epileptic myoclonus
	Sensory information from the periphery to supratentorial structures	Posterior column-medial lemniscus pathway and spinothalamic tracts	Sensory deficit
	Oculosympathetic control	Centers control of the ciliary nerve, superior tarsal muscle, pupillary sphincter/dilator	Horner’s syndrome (ptosis, myosis, enophtalamos, anhidrosis)