The Forebrain: Telencephalon & Diencephalon

The forebrain is split into 2 sections: The telencephalon and the diencephalon.

Parts of the telencephalon

The Telencephalon & Limbic System

  • Cerebral cortex – The cerebral cortex is the “gray matter” of your brain, and is comprised of the fissures (valleys) and gyri (hills). Most information processing occurs in the cerebral cortex. Each of its 6 layers has different composition in terms of neurons and connectivity. However, there are 2 types of basic neurons: Star-shaped cells (small interneurons with no tail) and triangular cells (large multipolar neurons). There are 4 lobes in the cerebral cortex:
    4 Lobes of the Brain

    • Frontal lobe – The frontal lobe is associated with personality, conscience (right/wrong/consequences), planning and is the source of inhibitions. Moniz won a Nobel Prize for developing the prefrontal lobotomy. It was later replaced by Walter Freeman’s transorbital lobotomy. They were both later replaced with safer alternatives (drugs like thorazine).
    • Parietal lobe – The parietal  lobe is in charge of somatosensory processing (touch). See Oliver Sack’s case study about the man who fell out of bed.
    • Occipital lobe – The occipital lobe processes visual memory, and is associated with migraine headaches.
    • Temporal lobe – Auditory and language processing occurs in the temporal lobe; about 90% on the left side.
  • Corpus callosum – The white matter in the brain that connects the left and right hemispheres. Split brain occurs when the connection in the corpus callosum is severed.
  • Limbic system – The limbic system is the collective name for the parts of the brain that control emotion, motivation, and emotional association with memory, and includes the hippocampus, cingulate cortex, mammillary bodies, amygdala, fornex and septum.
    • Hippocampus – It’s easy to remember where the hippocampus is because it’s shaped like a seahorse. The hippocampus is associated with short and long term memory indexing (moves memories in and out), and is one of the first parts of the brain affected in Alzheimer’s. Damage to the hippocampus can cause amnesia, preventing the formation of new memories (anterograde amnesia), as well as recollection of old ones (retrograde amnesia). Elderly people with shrinkage of the hippocampus tend to have memory problems (episodic and working memory). Abnormalities in development of the hippocampus are associated with schizophrenia.
    • Anterior cingulate cortex (ACC) – In PTSD, there is lower activity and fewer neurons in the the anterior cingulate cortex.
    • Amygdala -The amygdala is shaped like an almond, and located on the fatter end of the hippocampus. It is responsible for emotional processing, and associated with conditioned learning, especially fear/anger/rage. Dysfunction of the amygdala is linked to anxiety, autism, depression, post-traumatic stress disorder, phobias and binge drinking. PTSD is now being treated with small doses of esctasy (plus counseling). Also see case study below of Little Albert by John Watson.
    • Olfactory bulb - Sense of smell, connected to the amygdala, which is why smells are strong sources of memories.

Parts of the diencephalon

The Forebrain: Parts of the Diencephalon

  • Thalamus - The thalamus has 2 lobes, and is responsible for sensory relay in your brain. Essentially, it is the “traffic cop” that directs information. It does NOT help with recognition.
  • Hypothalamus – The hypothalamus controls motivated behavior by regulating the release of hormones from the pituitary gland. It is responsible for the 4 F’s: Fighting, fleeing, feeding and sex.
  • Pituitary gland – Small pea-sized gland of the endocrine system, often called the “Master Gland.” The pituitary gland hangs from the hypothalamus.
  • Pineal gland – Small gland of the endocrine system that controls melatonin (a hormone that affects the modulation of wake/sleep patterns and seasonal functions) production that’s sometimes referred to as the “third eye.”