Lecture 7: The Forebrain Flashcards
How is the telencephalon connected at either side of the midbrain in development?
The telencephalic vesicles - one on each side - are connected by the lamina terminalis. This is even at the level of the embryonic tube. The telencephalon massively grows to fold over the diencephalon. The Lamina terminalis goes on to become the corpus callosum and the anterior commissure in the adult brain.
How does the diencephalon develop?
As the cells lining the lumen proliferate the give rise to the thalamus, an upper swelling and a hypothalamus. The lumen is going to narrow down and adopt the shape of the third ventricle. The third ventricle is found in the space between the diencephalon as each side of the brain. The hypothalamic sulcus divides the two and appears even in early embryonic development.
How is the diencephalon organised?
Formed by 4 subdivisions. It sits on top of the brainstem. The divisions are:
- Thalamus - made of several nuclei and acts as a rely station between parts of the CNS - Hypothalamus (below the thalamus) - composed of several nuclei - its function is autonomic and neuroendocrine functions - Subthalamus - Functionally part of the basal ganglia. Involved in motor control (not visible in midsagittal view) - Epithalamus (upon the thalamus) includes the pineal gland and secretes melatonin
What structure is found between the two thalami? What are they connected by?
III Ventricle - The two thalami are connected by the inter thalamic adhesion (massa intermedia).
How is the thalamus organised?
The thalamus is organised into different motor groups: anterior nuclear group, medial nuclear group and a bigger lateral nuclear group.
What are the key nuclei of the thalamus?
The key nuclei are:
- Ventral Posterior Lateral (VPL)
- Ventral Posterior Medial (VPM)
These both receive information from our somatic sensory system and will project this information to the primary sensory cortex
- Lateral Geniculate (LG) - receives visual system and projects to the visual cortex
- Medial Geniculate (MG) - connects to the auditory apparatus and project to the auditory cortex
What separates the thalamus from the hypothalamus?
Hypothalamic sulcus
What are the borders of the hypothalamus?
Anterior commissure, lamina terminalis remnants anteriorly and the optic chasm.
Where is the limbic lobe located?
The limbic lobe sits on the medial surface and spans all of the lobes. It is part of the limbic system important for memory and emotions.
What is the corpus striatum? How is it organised?
These are internal structures of grey matter (nuclei) that sit in each hemispheres. They are functionally part of they basal ganglia involved in motor control. Made of the:
- Globus pallidus - made of an internus and externus
- Putamen
These two are grouped together to form the lentiform nucleus
- Caudate nucleus - “nucleus with a tail”
The Globus pallidus is medial to the putamen. The thalamus is more medially.
Why is the corpus striatum named as such?
“Striped body”
When we make sections of these structures we see a straited appearance between them. Even though the caudate nucleus and the putamen are separate, there are cellular bridges connecting them. Between the cellular bridges, there are bundles of white matter fibres connecting them.
The fibres that seperate the lentiform nucleus from the caudate nucleus form apart of the internal capsule. These are white matter fibres.
How is the caudate nucleus organised? How does this relate to the lateral ventricles?
The head of the caudate is lateral to the lateral ventricles forming the lateral wall. The tail of the caudate, all the way down into the temporal lobe, forms the roof of the tail of the lateral ventricle. The globus pallidus is more internal.
Which inter hemispheric structures form apart of the limbic system?
- Hippocampus (look like a big white sausage in dissection) - connected to the fornix which arches over the thalamus
- The amygdala - tiny nucleus
- The Fornix has a C shape over the thalamus. It is made of the fimbria of the fornix which brings white matter fibres from the hippocampus. These converge to form the crus of the fornix which goes onto become the body anteriorly and lastly the columns of the fornix ending in the mammillary bodies.
We therefore have a connection between memory and emotions (limbic system) with the hypothalamus.
How does the hippocampus relate to the ventricular system?
The hippocampus forms the floor of the inferior horns of the lateral ventricle.
What are the 3 classes of white matter subcortical fibres? Give examples of each.
- Association fibres - interconnect areas within a hemispheres and adjacent gyri e.g. between the precentral and postcentral gyrus.
- Commissural fibres - These interconnect between hemispheres. The biggest example is the corpus callosum also referred as the Great cerebral commissure. It is C shaped. It is made up of a Rostrum, Genu (bend), the Body and the Splenium (the most dorsal part).
- Projection fibres - These interconnect the cerebrum with the rest of the CNS. An example is the corona radiata. This funnels down as the internal capsule. They are continuous with each other. They containing both ascending and descending fibres.
