Brain Structure And Imaging Notes

Brain Structure and Imaging

The forebrain (prosencephalon)

The forebrain is composed of the paired cerebral hemispheres (the telencephalon) and the median diencephalon. The olfactory bulb is also located within the forebrain, as is the limbic system.

1. The diencephalon

The diencephalon forms the most rostral part of the brainstem. Only the most ventral part, the hypothalamus, is visible on an intact brain. It contains the slit-like third ventricle. Fibres pass from the diencephalon to the cerebral hemispheres via the internal capsule. The diencephalon is composed of three parts – the epithalamus, the hypothalamus and the thalamus.

The main functions of the diencephalon are to transfer and modulate information to the cerebral cortex from sensory organs and other parts of the brain, and to maintain homeostasis.

1. The thalamus

The thalamus is the largest component of the diencephalon, and is within the lateral walls of the diencephalon. The right and left thalami bulge into the third ventricle, forming a bridge between themselves. This bridge is known as the interthalamic adhesion, and reduces the third ventricle to a ring like cavity.

It is composed of a large number of nuclei arranged in groups, which together form one of the most important relay and integration centres in the brain. These nuclei groups include …

• Ventral group – this receives input from the medial lemniscus, and receives the most afferent systems. (General sensory information).

• Geniculate nuclei – receives input from visual and auditory pathways. (Special sensory information).

• Lateral group – receives input from the ventral group.

• Central (intralaminar) group – receives input from the ascending reticular formation.

Impulses that pass through these nuclei then pass via the internal capsules to reach appropriate cortical sensory areas. All sensory pathways terminating in the cerebral cortex (except the olfactory nerve tracts) must pass through the thalamus.

2. The hypothalamus

The hypothalamus forms the ventral part of the diencephalon, from the more ventral parts of the lateral walls. It is exposed on the ventral surface of the brain in an intact specimen. It lies rostral to the optic chiasm. The tuber cinereum lies on its surface, and provides attachment to the infundibulum of the pituitary gland. Mammillary bodies are also present on the ventral surface of the hypothalamus. These receive input from the hippocampus via the fornix, which they relay to the thalamus via the mammillothalamic tract. Their function is unknown.

The hypothalamus contains many nuclei, including the supraoptic and paraventricular nuclei. These are found above the optic chiasm and beside the third ventricle respectively. Axons from both nuclei descend via the infundibulum to the neurohypophysis, where they release ADH and oxytocin.

It is classed as part of the limbic system, and is the principle regulating centre of the ANS. It provides a link between nervous and endocrine systems. It receives and outputs both neural and humoral information.

The pituitary gland is attached to the hypothalamus by the pituitary stalk. It consists of an anterior pituitary gland (adenohypophysis), intermediate lobe and posterior pituitary gland (neurohypophysis).

The posterior pituitary is a neuroectoderm derivative, is part of the nervous system and does not produce hormones itself. It releases ADH, oxytocin and peptide hormones, but hormone synthesis occurs in the hypothalamus in nerve cell bodies in the supraoptic and paraventricular nuclei. These prohormones are then transported via axons to the posterior lobe by neurophysins, and stored in nerve endings until an action potential causes them to be released into the blood by exocytosis.

The anterior pituitary lobe is a portal system, and produces many hormones, including …

• Thyroid stimulating hormone (TSH).

• Adrenocorticotrophic hormone (ACTH).

• Follicle stimulating hormone (FSH).

• Luteinizing hormone (LH).

• Growth hormone (GH).

• Prolactin

3. The epithalamus

The epithalamus is the most dorsal part of the diencephalon (its roof). It contains the pineal gland, an endocrine gland that projects dorsally on the roof of the third ventricle. It is important in regulation of circadian and seasonal rhythms, and its activity is inhibited by light. It produces melatonin, which inhibits the gonads and other endocrine functions.

2. The telencephalon

The telencephalon consists of the paired cerebral hemispheres. These are semi ovoid structures and the largest part of the brain. They are connected by a mass of white matter called the corpus callosum. Each hemisphere is split into four lobes – frontal, parietal, occipital and temporal. The cerebrum extends caudally over the brainstem and is in short distance of the cerebellum – the two are separated by the longitudinal fissure.

The surface of each hemisphere is made up of grey matter (neurons and their unmyelinated fibres) known as cerebral cortex. Inside the cortex is white matter (myelinated nerve fibres), within which are number of nuclei (grey matter) known as the basal nuclei. The basal nuclei receive information from the cortex to regulate skeletal movement and other higher motor functions.

In mammals, the cerebral cortex is responsible for the conscious experience of sensory input and conscious direction of body movements, as well as other cognitive abilities. The areas of the brain that stimulate relatively simple reactions are called primary cortical areas. Depending on the reaction, these areas can be divided into visual, auditory, somatosensory or motor cortex. The function of association cortex areas are less well clarified.

3. The olfactory bulb

The olfactory bulb is responsible for olfaction, and is located in the rostral forebrain area, supported by the cribiform plate and the ethmoid bone.

4. The limbic system

The limbic system is made up of parts of the brain bordering the corpus collosum, including areas of cerebral cortex, the cingulate gyrus, the parahippocampus...

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