Brainstem

Diencephalon

The diencephalon makes up the caudal part of the forebrain between the telencephalon and midbrain. It consists of the hypothalamus, thalamus, epithalamus, and the subthalamus.

Hypothalamus

The hypothalamus functions as the central control center for the autonomic nervous system and endocrine function. Its nuclei produce releasing hormones for the anterior pituitary and the hormones that are released by the posterior pituitary.

Structure Function Associated pathology
Lateral nucleus
Ventromedial nucleus
  • Satiety: stimulated by leptin
  • Hyperphagia
  • Obesity
  • Disinhibition
Anterior nucleus (hypothalamus)
  • Hyperthermia
Posterior nucleus
Suprachiasmatic nucleus

Supraoptic nucleus

Paraventricular nucleus

  • Regulate water balance
  • Produces ADH and oxytocin
    • Carried to posterior pituitary (for storage) by neurophysins in axonal connections
Preoptic nucleus
  • Thermoregulation
  • Sexual dysmorphic nucleus → sexual function
    • Regulates secretion of gonadotropic hormones
Arcuate nucleus (hypothalamus)
  • Projects to the portal vessels of the infundibulum
  • Regulates secretion of neuroendocrine hormones from the pituitary gland (via hypothalamic releasing factors)
  • Contain dopaminergic neurons that inhibit prolactin and GnRH release → play a role in stimulating lactation
  • Neuroendocrine dysregulation

Thalamus

The thalamus consists of several nuclei that are tightly connected to the cerebral cortex and functions as the main conductor of sensory information.

Structure Input Projects to Function
Ventral posterolateral nucleus (VPL)
  • Somatosensory cortex
Ventral posteromedial nucleus (VPM)
  • Somatosensory cortex
  • Taste
  • Facial sensation
Lateral geniculate nucleus
  • Vision
Medial geniculate nucleus
  • Hearing
Ventral lateral nucleus
  • Motor
Anterior nucleus
  • Mamillary nucleus (via the mammillothalamic tract)
  • Hippocampus (via the fornix)
Dorsomedial nucleus
  • Part of the limbic system
    • Expression of affect
    • Behavior
    • Emotions
Intralaminar nuclei
  • Reticular formation (brainstem)
  • Thalamic nuclei
  • Ascending reticular system
  • Trigeminal tract
  • Spinothalamic tract
  • Divided into anterior and posterior nuclei

Thalamic stroke causes contralateral hemiparesis and hemisensory loss, miotic and unreactive pupils, upgaze palsy with gaze deviation away from the side of the lesion, a phenomenon known as “wrong-way eyes”!

Limbic system

The limbic system is a collection of neuronal pathways from different anatomical parts of the brain that are responsible for feeding, emotions, memories, attention, and sexual behavior. It is composed of the hippocampal formation (includes the hippocampus, dentate gyrus, and the entorhinal cortex) mammillary bodies, amygdalae, cingulate gyrus, and the anterior thalamic nuclei.

Structure Location Characteristics
Hippocampal formation Hippocampus
  • Contain pyramidal cells
    • Projects to the fornix and the hypothalamus
  • Center of memory formation
  • Bilateral injury causes anterograde amnesia
  • Affected in early stages of Alzheimer disease
Dentate gyrus
Entorhinal cortex
Mammillary bodies
  • On the distal and anterior end of the fornix bilaterally
Amygdala
  • Receives signals from the hypothalamus, cerebral cortex, and the olfactory cortex
  • Relays emotional reactions, especially to fear, aggression, and anxiety
  • Recognition of emotional facial expressions
  • Involved in decision-making
Cingulate gyrus
  • Above the corpus callosum bilaterally
  • On the medial surface of each hemisphere
Anterior thalamic nuclei
  • In the thalamus

Papez circuit

  • Description:
    • An anatomical circuit within the limbic system that starts and ends in the hippocampus.
    • It helps modulate learning and emotional expressions such as sexual desire, pleasure, fear, and anxiety.
  • Circuit: Pyramidal cells of the hippocampus → fornix → mammillary bodies (hypothalamus) → via the mammillothalamic tract → anterior thalamic nucleus → anterior limb of the internal capsulecingulate gyrusentorhinal cortex and cingulum → hippocampus
    • The hippocampus receives input from cortical associations such as the cingulum, cingulate gyrus, and parahippocampal gyrus
    • The hippocampus receives input from and sends output to the amygdala, which plays an important role in the limbic system by:
      • Creating emotional significance to feelings (positive or negative)
      • Helps to store that emotion in the memory
      • For example: If smelling coffee in the morning makes a person feel good, the amygdala helps to store that information to feel good again the next time the individual smells coffee.

Midbrain

The midbrain mediates visual and auditory reflexes. It contains the nuclei of the oculomotor nerve (CN III) and the trochlear nerve (CN IV).

Structure Location Characteristics
Tectum
Tegmentum
  • Contains sensory pathways and nuclei of cranial nerves
  • Involved in modulation of extrapyramidal motor function
Crus cerebri
Cerebral peduncles
  • Arise at the lateral ventral aspect of the pons

Pretectal area
(pretectum)

  • Modulates behavior in reaction to light, e.g. pupillary reflex, accommodation
Cerebral aqueduct
  • Connects the 3rd and 4th ventricles
Periaqueductal gray matter
  • Contains the nuclei of
    • Locus coeruleus
    • Dorsal tegmental nucleus
    • Dorsal raphe nucleus
    • Mesencephalic nucleus and tract
Medial longitudinal fasciculus
Substantia nigra
  • Formed by two parts
    • Pars compacta (contains melanin) → dopamine↑ motor function
    • Pars reticularis → GABA↓ motor function
  • Aids in movement initiation and coordination
  • Receives inhibitory input (GABA) from the caudate nucleus and the putamen
  • Projects dopaminergic fibers to the caudate nucleus and the putamen
  • Projects nondopaminergic fibers to the mediodorsal, ventral lateral, and ventral anterior nuclei of the thalamus
  • For more information on disorders associated with dopamine, see Parkinson disease and Parkinson-plus syndromes.
Central tegmental tract
  • Contain reticulothalamic and rubro-olivary fibers
  • Contain taste fibers
Red nucleus

Dopaminergic pathways

These pathways are collections of neurons that release dopamine. They are not anatomically limited to the midbrain but involve some midbrain nuclei. Their dysfunction is implicated in some psychiatric diseases and they play a role as the target of antipsychotic drugs.

Pathway Location Characteristics Defect
Tuberoinfundibular
  • From the arcuate nucleus (infundibular nucleus) to the median eminence
Mesolimbic
  • From the ventral tegmental area (VTA) to the nucleus accumbens and olfactory tubercle
  • Controls the reward system of the brain (e.g., pleasure, positive reinforcement)
Mesocortical
  • Controls executive functions
Nigrostriatal
  • Controls movement

Pons

The pons is located between the midbrain and the medulla, within the superior and inferior pontine sulcus. It is connected to the cerebellum via the middle cerebellar peduncles.

Structure Location Function
Cochlear nuclei
  • Receive auditory signals from the cochlear branch of the vestibulocochlear nerve (CN IIX)
    • Via special somatic afferent fibers (SSA)
  • Fibers project to the ipsilateral and contralateral lateral lemniscus
Trapezoid body
  • Formed by decussation of the ventral cochlear fibers
Superior olivary nucleus
Corticopontine tract
  • Located along the pons
Pontine nuclei
  • Located in the base of the pons
Locus coeruleus
Medial lemniscus
Lateral lemniscus
  • Relays cochlear input
Spinal lemniscus

Pontine stroke can cause ipsilateral loss of function of the facial and abducent nerves with contralateral hemiparesis (Millard-Gubler syndrome or Foville syndrome)!

Medulla oblongata

The medulla is an important structure located between the pons and the spinal cord. It contains autonomic centers that regulate autonomic functions such as circulation, respiration, and gastrointestinal activity. It contains centers for swallowing, sneezing, coughing, and vomiting. It is connected to the cerebellum via the inferior cerebellar peduncle.

Structure Characteristics
Nucleus gracilis and Nucleus cuneatus
Pyramids
  • Form the base of the medulla
  • Fibers in this region are uncrossed → cross at the pyramidal decussation
Lateral cuneate nucleus
(accessory)
Inferior olivary nucleus
Lateral reticular nucleus
Arcuate nucleus (medulla)
  • Located anterior to the pyramids
  • Forms the arcuatocerebellar tract of the striae medullares (in the rhomboid fossa)

Solitary nucleus and tract

(nucleus tractus solitarius)

  • Nucleus
    • Receives general visceral afferent (GVA) signals from the glossopharyngeal and vagus nerves
    • Receives special visceral afferent (SVA) signals from the facial, glossopharyngeal, and vagus nerves
  • Tract

Area postrema

(vomiting center)

  • Located anterior to the 4th ventricle on the floor of the medulla
  • This area lacks the blood-brain barrier
  • Contains the vomiting center
    • Via the chemoreceptor trigger zone (CTZ) and the nucleus tractus solitarius
    • Receives input from the following receptors

Ventricular system

Overview

There are four ventricles that are interconnected to each other. They contain cerebrospinal fluid (CSF), which serves as a fluid cushion and immunological reservoir.

Ventricle Anatomy
Lateral ventricles

Third ventricle

Fourth ventricle

Cerebrospinal fluid

  • Description: A clear and colorless fluid derived from blood plasma that provides mechanical support to the CNS and transports biochemical compounds (e.g., neuromodulators).
  • Pathway: produced by ependymal cells of the choroid plexus in the lateral, third, and fourth ventricles → subarachnoid space (via foramina of Luschka and Magendie) → reabsorbed in the arachnoid granulations → drains into the dural venous sinuses.

Vasculature

Territory Main Branches Features of infarction

Basilar artery

Formed by the union of the two vertebral arteries

  • Kaplan. USMLE Step 1 Anatomy Lecture Notes 2016. Kaplan Publishing; 2015.
  • Felten DL, O'Banion MK, Maida ME. Netter's Atlas of Neuroscience. Amsterdam, The Netherlands: Elsevier Health Sciences; 2015.
  • Le T, Bhushan V,‎ Sochat M, Chavda Y, Zureick A. First Aid for the USMLE Step 1 2018. New York, NY: McGraw-Hill Medical; 2017.
  • Chung KW, Chung HM. Gross Anatomy. Philadelphia, PA: Lippincott Williams & Wilkins; 2012.
  • Gould DJ. BRS Neuroanatomy (Board Review Series). Lippincott Williams & Wilkins; 2013.
  • Mu Y, Gage FH. Adult hippocampal neurogenesis and its role in Alzheimer's disease. Mol Neurodegener. 2011; 6: p. 85. doi: 10.1186/1750-1326-6-85.
  • Goodwin J. Third Nerve Palsy (Oculomotor Nerve Palsy). In: Lee AG. Third Nerve Palsy (Oculomotor Nerve Palsy). New York, NY: WebMD. https://emedicine.medscape.com/article/1198462. Updated October 8, 2018. Accessed December 7, 2018.
  • Sharma R, Rezaee A. Millard-Gubler Syndrome. https://radiopaedia.org/articles/millard-gubler-syndrome-2. Updated January 1, 2018. Accessed December 7, 2018.
last updated 12/13/2018
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