The diencephalon is a complex of structures within the brain, whose major divisions are the thalamus and hypothalamus. It functions as a relay system between sensory input neurons and other parts of the brain, as an interactive site for the central nervous and endocrine systems, and works in tandem with the limbic system.
The diencephalon is composed of several structures, the whole about the size of an apricot, situated near the core center of the brain, just above the brainstem. It is made up of the medulla oblongata, pons, and midbrain, below the telencephalon, the most basal part of the cerebrum. The two major components of the diencephalon are the thalamus and the hypothalamus. Other important structures within the diencephalon complex are the epithalamus, subthalamus, third ventricle, mammillary bodies, posterior pituitary gland, and the pineal body. The diencephalon interconnects with a larger, surrounding array of structures called the limbic system, which is the seat of emotions and memory.
The diencephalon functions in the following ways:
- As a junction and relay system that receives and filters afferent (incoming) sensory information, then relays it on to other parts of the brain, mainly the cerebral cortex, but also to the cerebellum and brainstem.
- As an interactive site between the central nervous system and the endocrine system.
- As an interactive complementary to the limbic system.
The upper part of the diencephalon, making up about 80% of its mass, is the thalamus, a small pillow of neural gray matter divided into two egg-shaped lobes. The lobes' long axes run toward the front and back of the head, and are connected to each other by a small stalk, the intermediate mass. The two thalamic lobes are filled with numerous pairs of nuclei, which are concentrations of synapsing afferent, or incoming, and efferent, or outgoing, neurons. Numerous such nuclei are situated throughout the brain.
The thalamic nuclei are named and classified according to their positions within the thalamus (medial, lateral, central, etc.), by their neural connections, and by their functions. In terms of function, there are three types of thalamic nuclei: sensory, motor, and arousal.
Layered sheets of myelinated axons, the internal thalamic medullary laminae, run vertically through the lobes of the thalamus. These laminae are full of neurons that interconnect various thalamic nuclei. The edges of the internal lamina reach the surfaces of the lobes. They show as narrow, whitish, cable-like bands, running across either lobe from its posterior underside, across the top, and forward, bifurcating into two bands (two vertical layers) toward the front. The main lamina divide the lobes of the thalamus into portions containing the medial and lateral geniculate nuclei, while the anterior bifurcations enclose the anterior nuclei.
The thalamus, the basal ganglia, and the cerebellum, which is the main movement coordination center of the brain, are neurally linked to the cerebral motor cortex in reciprocal, or feedback, fashion. Together, they regulate and fine-tune motor functions. The basal ganglia, which are part of the telencephalon, are groupings of gray matter within the white matter of the cerebral hemispheres. The basal ganglia function directly with the cerebellum to modify and fine-tune body movements.
A small part of the diencephalon, the epithalamus, extends rearward from, and slightly higher than, the thalamus. It holds the habenular nuclei, the stria medullaris thalami nerve tracts, and the pineal body, or epiphysis. The habenular nuclei play a role in emotional responses to odors. They receive afferent nerves from the septum, a complex of structures within the telencephalon and limbic system, and from the lateral preoptic nuclei of the basal forebrain, which is the lowermost region of the cerebrum; the stria medullaris tracts and the basal ganglia are the conduits. The habenular nuclei send efferents to the interpeduncular nucleus of the midbrain via the habenulo-interpeduncular nerve tract.
The pea-sized, conically shaped pineal body, on a short stalk, projects rearward and downward from the epithalamus. The pineal is a gland-like organ whose functions are still only poorly understood. It is a functional, light-sensitive remnant of an ancient and much more complex system of visually oriented organs, the pineal complex. The pineal is neurally connected with the suprachiasmatic nuclei of the hypothalamus, which hold the circadian internal clock. This is located just above the optic chiasma, the point at which the optic nerves from both eyes cross. The human pineal secretes melatonin, a hormone that seems to have a calming effect on the nervous system. The pineal, in response to the level of daylight, may induce sleepiness by increasing the output of melatonin.
All sensory input, except the olfactory (smell), passes through the thalamus, where it is filtered, integrated, and passed on to proper sites in the brain, most of them within the cerebral cortex. The route is as follows:
- Impulses from the auditory organs synapse in the medial geniculate thalamic nuclei, where they are sent to the auditory centers of the cerebral cortex.
- Impulses from the eyes, via the optic nerves, synapse in the lateral geniculate thalamic nuclei, and are sent on to the calcarine cerebral cortex.
- Other sensory input synapses in the ventral posterome-dial thalamic nuclei, which receive, process, and pass on somatosensory input from the head, while the ventral posterolateral thalamic nuclei do likewise with input from the rest of the body.
- The thalamic nuclei also receive input from subcortical sources and feedback from the cortical areas. These operate in tandem to filter and control input to the cortex.
The ventral anterior and ventral lateral thalamic nuclei, involved with motor function, receive sensory input relayed through the basal ganglia and through the superior cerebellar peduncle, the main neural tract connecting the cerebellum and the red nuclei. The ventral anterior and ventral lateral thalamic nuclei project to the premotor and motor cerebral cortex. In addition, the ventral anterior thalamic nuclei are the main relay nuclei between the thalamus and the limbic system, receiving the mammillothalamic nerve tract from the mammillary bodies in the hypothalamus and projecting to the cingulate gyrus.
The cingulate gyrus, which is not a part of the diencephalon, is the part of the cerebrum closest to the limbic system, and serves to neurally connect the thalamus and hippocampus. The cingulate gyrus associates memories and emotional responses with smells, sights, and pain , and allows movement of attention among objects or ideas.
The medial dorsal thalamic nuclei receive nerve tracts from the amygdala of the limbic system and send efferents to the prefrontal cerebral cortex (not part of the diencephalon), which has numerous feedback connections with the thalamus, amygdala, and other subcortical structures.
The anterior thalamic nuclei connect with the mammillary bodies of the hypothalamus, and through them, via a nerve tract, the fornix, with the hippocampus and the cingulate gyrus.
The centromedian thalamic nuclei regulate excitability levels within the cerebral cortex and thus play a major role in arousal and alertness. The centromedian thalamic nuclei receive motor-related input from the basal ganglia, cerebellum, and the reticular formation of the brainstem and midbrain, and send efferent nerves to the cerebral cortex. The reticular formation is a network of nerves running through the brainstem and hindbrain, and containing the reticular activating system, which plays a key role in inducing arousal and alertness in tune with the circadian rhythm (sleeping and waking cycles). The reticular thalamic nuclei, which receive neural input from the reticular formation, regulate general thalamic output in accordance with the circadian rhythm.
The dorsomedial thalamic nuclei are involved with emotional arousal and the expression of emotionally based behavior, as well as memory, foresight, and feelings of pleasure. These nuclei receive input from many sites and interconnect with the prefrontal cerebral cortex.
That part of the diencephalon immediately below the two lobes of the thalamus is the subthalamus. It contains several nerve tracts and the subthalamic nuclei. Small portions of the red nuclei and the substantia nigra of the mid-brain reach into the subthalamus. The subthalamic nuclei are interconnected with the basal ganglia and are involved in controlling motor functions.
The hypothalamus is the lowermost structure of the diencephalon. The thalamus, epithalamus, and hypothalamus surround and define most of the third ventricle of the brain, which, like all the ventricles, is filled with cerebrospinal fluid. The third ventricle communicates with the lateral ventricles and, via the cerebral aqueduct, with the fourth ventricle.
The hypothalamus contains several nuclei, nerve tracts, and the pituitary gland. It is the regulatory seat of the autonomic nervous system, while the hypothalamus and the pituitary are the major sites in which the two regulatory systems of the body, the central nervous system and the endocrine system, interact. The hypothalamus regulates the production of pituitary hormones, influencing and being influenced by emotional states, physical appetites, autonomic functions, temperature control, and diurnal rhythms. It is thus the main control center for homeostasis, or keeping physiological maintenance systems functioning at optimal states.
Efferent nerves from the hypothalamus extend into the brainstem and the spinal cord, where they synapse with neurons of the autonomic nervous system, which regulates a number of involuntary functions, among them the rate of heartbeat, urine release, and peristalsis. The hypothalamus responds to sensations of temperature extremes, the posterior hypothalamus stimulating muscle shivering to deal with cold, via efferent neurons to motor neurons within the spinal cord, and the anterior hypothalamus producing sweating as a reaction to overheating.
The pair of globular mammillary bodies are partially embedded in the underside of the hypothalamus. They are involved in olfactory reflexes and emotional responses to odors. Also on the underside of the hypothalamus, and toward the front, is the optic chiasma, where the two optic nerve cables of the eyes cross.
From the floor of the hypothalamus, the posterior pituitary gland, or neurohypophysis, extends forward and downward at the end of a long peduncle or stalk, the infundibulum. Efferent hypothalamic nerves extend through the infundibulum to the posterior portion of the pituitary gland, others extend to the trigeminal and facial nerve nuclei, to help control the head muscles involved in swallowing.
The posterior pituitary is an extension of the hypothalamus, but the anterior part of the pituitary is glandular tissue with an embryonic origin separate from that of the posterior pituitary. During embryonic development, the anterior and posterior lobes of the pituitary eventually meet and fuse.
The hypothalamus plays a pivotal role in regulating the endocrine system via its control of the pituitary gland's production of several hormones, while the hypothalamus is influenced in turn by hormones in the bloodstream and by nerve input. A partial list of hormones secreted by the pituitary includes cortisol, prolactin, antidiuretic hormone (ADH), oxytocin, growth hormone (GH), thyroid stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), lipotropins, beta-endorphins, melanocyte stimulating hormone, luteinizing hormone, and follicle stimulating hormone.
Hormones influence functions as diverse as metabolism, growth and maturation, reproduction, dealing with stress, urine production, ion balance, sexual development, and sexual function. The hypothalamus regulates physical appetites for food, water, and sex. Afferent fibers synapsing in the hypothalamus carry input from the internal organs, the taste receptors of the tongue, the limbic system, the nipples, and the external genitalia. The hypothalamus responds to and accords with emotional states, and thus plays a major role in affecting emotions and moods, among them sexual pleasure, tranquility, rage, and fear.
The hypothalamus contributes to the regulation of the circadian rhythm via an internal clock within the suprachiasmatic nuclei. This internal clock communicates with the reticular formation of the midbrain. The reticular formation contains the reticular activating system, which plays a key role in inducing arousal and alertness, in tandem with the circadian rhythm.
The diencephalon is interconnected with a surrounding complex of brain structures, the limbic system, which functions as the center of emotional states and responses, and of memory. Besides the various structures within the diencephalon, the limbic system includes the olfactory cortex, hippocampus, amygdala, cingulate gyrus, septal nuclei, the dorsomedial nuclei of the thalamus, and the anterior nuclear complex of the thalamus.
Memories of vividly emotional experiences are recorded and kept within easy reach of consciousness within the limbic system. Connections between, and functions of, the hypothalamus and limbic system are intimately intertwined. The ventral anterior thalamic nuclei are the main relay nuclei connecting the thalamus and the limbic system, receiving the mammillothalamic tract and projecting to the cingulate gyrus.
The olfactory sense is the only one whose neurons directly connect with a processing center within the limbic system and outside the thalamus. Within the hypothalamus, relayed olfactory impulses are used to regulate appetite and sexual behavior, and to regulate autonomic reactions initiated by odors. Since the limbic system processes memory and stores important memories, the direct connection of the olfactory neurons to the limbic system helps explain why odors serve as alarms (e.g., the odor of smoke) and can trigger strong emotional responses and vivid, detailed memories of events and emotional states.
The hippocampus, the main processor of memory, is a paired structure looping over the tops of the thalamic lobes and rearwards, curving downward and forward and ending at the paired, globular, cherry-sized amygdala, below and in front of the hypothalamus. The amygdala connect with the hippocampus, the septal nuclei, the prefrontal area of the cerebrum, and the medial dorsal nucleus of the thalamus. The amygdala also send nerves to the hypothalamus via the ventral amygdalofugal pathway.
The amygdala are centers for associating strong emotions, good or bad, with memories of the experiences that triggered those emotions. Fear responses and fear-charged memories are centered in the amygdala, which can retain vivid memories of traumatic experiences, and initiate the survival fight-or-flight response.
The hippocampus sends efferents, via a cable of nerves, the fornix, to the mammillary bodies within the hypothalamus. The mammillary bodies send efferents to the anterior nuclei of the thalamus via the mammillothalamic tract.
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