Diencephalon and brainstem

The diencephalon is the caudal part of the forebrain (prosencephalon) located between the midbrain (mesencephalon) and the cerebrum (telencephalon). It consists of the thalamus, hypothalamus, epithalamus, and subthalamus. The thalamus is the relay center for sensory information. The hypothalamus regulates autonomic function and the endocrine system. The epithalamus, which consists of the pineal gland, habenula, habenular commissure, and stria medullaris, regulates the sleep-wake cycle. The subthalamus, which contains the subthalamic nuclei, is part of the indirect basal ganglia circuit and is involved in the inhibition of unnecessary movements. The brain stem is the caudal part of the brain and consists of the midbrain, pons, and medulla oblongata. The brain stem regulates autonomic function (respiration, circulation, lacrimation, salivation), controls visual and auditory reflexes, and maintains vigilance. It is also a hub through which run all ascending sensory pathways, descending motor pathways, and other local pathways of the central nervous system. The limbic system is a group of brain pathways that consists of the amygdala, hypothalamus, thalamus, hippocampus, and limbic cortex. These are involved in memory formation, regulation of appetite and satiety, attention, emotional responses, and sexual arousal. The ventricular system consists of four interconnected ventricles (a pair of lateral ventricles, third ventricle, and fourth ventricle). It is involved in the production, transport, and drainage of cerebrospinal fluid, which plays a role in waste removal and cushioning the brain.

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.

Thalamus

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

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”!

References:^[1][2]

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.

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 capsule → cingulate gyrus → entorhinal 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.

References:^[3]

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

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.

References:^[1]

• Location
  • Rostral to the medulla, caudal to the midbrain
  • Connected to the cerebellum via the middle cerebellar peduncles
• Embryogenesis: rhombencephalon → metencephalon → pons (+ cerebellum)
• CN nuclei: trigeminal (CN V), abducens (CN VI), facialis (CN VII), and vestibulocochlear (CN VIII) nerves
• Composition: divided into 2 parts (dorsal to ventral)
  • Pontine tegmentum - contains CN nuclei
  • Basilar pons - contains fibers and tracts

References:^[1][4]

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.

References:^[1][4][5]

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.

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.

• Arterial supply: basilar artery (see table below)
• Venous drainage: great cerebral vein of Galen, the basal veins (of Rosenthal), and into the straight sinus

References:^[6][7][8]