Hypothalamus, limbic system and olfactory system


In order to survive, there must be continual biochemical and physiological adjustments to preserve the internal environment of the body in a balanced and stable state (homeostasis). Interoceptor signals from the internal organs and body fluids initiate homeostatic responses to achieve this end and the hypothalamus is the structure responsible for orchestrating the task.

Exteroceptive information concerning the outside world strongly influences behaviour. This is relatively simple and stereotyped in lower animals and is directed towards satisfying the drives of thirst, hunger, sex and defence, in instinctive repertoires. These also depend upon the hypothalamus. The limbic system, which is strongly connected to the hypothalamus, is essential for adaptive behaviour, meaning the ability to learn new responses based on previous experience (memory). The complex and non-stereotyped behaviour of humans manages not only to preserve the individual within the physical landscape but also within a changing social environment (‘individual homeostasis’). The association areas of the neocortex are capable of analysing exteroceptive information from the environment and from other individuals, enabling adaptive personal and social responses. These phylogenetically more recent structures are also partly connected to the limbic system.

As a result, the hypothalamus, limbic system and association neocortices act as interfaces in a hierarchical fashion between the internal structure of the individual and the environment. A reminder of this evolutionary ascent in humans is the olfactory system: vital for sensing the environment in lower animals, overwhelmed by visuospatial dominance in humans and intimately related to the limbic system.

Hypothalamus

Topographical anatomy of the hypothalamus

The hypothalamus is the most ventral part of the diencephalon, lying beneath the thalamus and ventromedial to the subthalamus ( Fig. 16.1 ; see also Fig. 12.1, Fig. 12.2, Fig. 12.3 ). It forms the floor and the lower part of the lateral wall of the third ventricle, below the hypothalamic sulcus (see Fig. 12.2 ). On the base of the brain, parts of the hypothalamus can be seen occupying the small area circumscribed by the crura cerebri, optic chiasm and optic tracts (see Fig. 12.1 ). Between the rostral limits of the two crura cerebri, on either side of the midline, lie two distinct, rounded eminences, the mammillary bodies , which contain the hypothalamic mammillary nuclei . In the midline, immediately caudal to the optic chiasm, lies a small elevated area known as the tuber cinereum , from the apex of which extends the thin infundibulum (infundibular process), or pituitary stalk . This is attached to the pituitary gland ( hypophysis ), a pea-sized structure which lies within the hypophyseal fossa (sella turcica) of the sphenoid bone (see Figs 5.1 , 5.4 ). The pituitary gland consists of two major, cytologically distinct, parts: the posterior pituitary or neurohypophysis and the anterior pituitary or adenohypophysis ( Figs 16.2 , 16.3 ). The posterior pituitary is a neuronal structure, being an expansion of the distal part of the infundibulum. The anterior pituitary is not neural in origin. The two parts are, however, closely linked by the pituitary (hypophyseal) portal system of vessels ( Fig. 16.3 ), which are derived from the superior hypophyseal artery. Releasing factors, which are synthesised in the hypothalamus, pass to the adenohypophysis through these vessels to control the release of anterior pituitary hormones.

Fig. 16.1, Schematic drawing of a mid-sagittal section of the diencephalon.

Fig. 16.2, Supraoptic and paraventricular nuclei projecting to the posterior pituitary via the hypothalamohypophyseal tract.

Fig. 16.3, Pituitary portal system linking anterior and posterior parts of the pituitary gland.

The hypothalamus is able to integrate interoceptive signals from internal organs and fluid-filled cavities and make appropriate adjustments to the internal environment by virtue of its input and output systems.

Input to the hypothalamus is both circulatory and neural in origin ( Fig. 16.4 ). The circulating blood provides physical (temperature, osmolality), chemical (blood glucose, acid–base state) and hormonal signals of the state of the body, its growth and development and its readiness for action. (e.g. sex, suckling, defence, escape, etc.). Neural signals come from a number of sources. The largest input originates from limbic structures, the hippocampus and the amygdala. Fibres of hippocampal origin constitute the fornix, a large component of which terminates in the medial mammillary nucleus within the mammillary body ( Figs 16.5–16.7 ). Fibres from the amygdala to the hypothalamus run in the stria terminalis (see Fig. 12.3 ). The nucleus solitarius of the medulla projects to the hypothalamus, conveying information collected by the autonomic nervous system concerning the pressure within the smooth-muscled walls of organs (baroreceptors) and the chemical constituents of the fluid-filled cavities (chemoreceptors). The state of arousal is communicated by connections that originate in the brainstem. Monoaminergic projections ascend in the medial forebrain bundle (see Fig. 9.14 ) and the reticular formation provides input both directly and indirectly via the thalamus.

Fig. 16.4, Neural and non-neural inputs of the hypothalamus.

Fig. 16.5, The interconnection of limbic structures that constitute the Papez circuit.

Fig. 16.6, Dissection of the medial aspect of the diencephalon to show the relationships of the fornix, mammillary body and mammillothalamic tract.

Fig. 16.7, The Papez circuit projected onto the medial aspect of the cerebral hemisphere.

The hypothalamus generates responses to these varied stimuli by, again, both circulatory and neural means ( Fig. 16.8 ). An intimate relationship with the pituitary gland and privileged access to its circulation (portal system) confers the role of ‘orchestrator of the endocrine system’ on the hypothalamus, as it directs hormonal synthesis and release. The neural output of the hypothalamus is directed to widespread regions of the neuraxis. Descending fibres pass to the brainstem and some reach the spinal cord. In this way, connection is made with various brainstem nuclei, including the reticular formation, influencing wakefulness and sleep, and control is exerted over preganglionic sympathetic and parasympathetic neurones of the autonomic nervous system. Ascending connections pass to the limbic system, both directly and, via the thalamus, to the orbital frontal cerebral cortex. The hypothalamus can initiate appropriate motor behavioural repertoires of an instinctive kind through its connections with the limbic system and limbic part of the corpus striatum (the nucleus accumbens). Furthermore, it can influence, or even override, more complex adaptive behaviour because of its close links with the limbic system and the orbital frontal association cortex.

Fig. 16.8, Neural and non-neural outputs of the hypothalamus.

Hypothalamic nuclei

The hypothalamus consists of many nuclear divisions, only some of which will be described here ( Fig. 16.1 ). The region lying medial and ventral to the structures of the subthalamus is known as the lateral hypothalamus . It is traversed longitudinally by many fibres, including the medial forebrain bundle. The lateral hypothalamic area is important in the control of food and water intake and is, in part, equivalent to the physiologically defined ‘feeding centre’. Lateral hypothalamic lesions cause aphagia and adipsia.

The medial region of the hypothalamus contains various nuclei, only some of which have well-defined functions. Anteriorly lie the supraoptic, paraventricular and suprachiasmatic nuclei. The supraoptic and paraventricular nuclei both produce systemically acting hormones, which are released from the posterior pituitary into the general circulation. The supraoptic nucleus produces vasopressin (antidiuretic hormone; ADH). The supraoptic nucleus contains osmosensitive neurones that are activated by changes in the osmolality of circulating blood. An increase in osmolality causes release of vasopressin. This acts upon the tubules of the kidney to increase water reabsorption, thus maintaining water homeostasis. The paraventricular nucleus synthesises oxytocin. In the female, activation of the paraventricular nucleus, and release of hormone, is induced by suckling. This stimulates milk production by the mammary gland and causes contraction of uterine muscle. The axons of cells in the supraoptic and paraventricular nuclei pass to the neurohypophysis in the hypothalamohypophyseal tract ( Fig. 16.2 ). The neuroendocrine products are transported in this tract to the neurohypophysis, where they are released into the capillary bed and, thus, reach the general circulation. The suprachiasmatic nucleus is concerned with the control of diurnal rhythms and the sleep/waking cycle. It receives some afferent fibres directly from the retina.

More caudally, dorsomedial and ventromedial nuclei lie deep to the lateral wall of the third ventricle. The dorsomedial nucleus has connections with the suprachiasmatic nucleus and is involved in the control of circadian rhythms. The ventromedial nucleus , like the lateral hypothalamus, is concerned with the control of food and fluid intake. The ventromedial nucleus is equated with the physiologically defined ‘satiety centre’ and lesions of this region cause abnormally increased food intake. In the most caudal part of the hypothalamus lie the posterior nucleus and the medial mammillary nucleus , the latter being located within the mammillary body. The mammillary body is part of the limbic system, receiving afferents from the hippocampus via the fornix and projecting to the anterior nucleus of the thalamus and the brainstem.

The hypothalamus is the brain centre for regulation of the autonomic nervous system. Generally, activation of the posterior hypothalamic domain is associated with sympathetic responses, whereas activation of the anterior hypothalamus is associated with parasympathetic activity.

The hypothalamus also synthesises releasing factors and release-inhibiting factors, which control the release of hormones by the adenohypophysis. The adenohypophysis produces adrenocorticotropic hormone (ACTH), luteinising hormone (LH), follicle-stimulating hormone (FSH), thyroid-stimulating hormone (TSH), growth hormone and prolactin, which are released into the general circulation. The factors that control them are released from the terminals of hypothalamic neurones into the capillary bed of the pituitary portal system ( Fig. 16.3 ). These vessels, which are intrinsic to the hypophyseal stalk, convey the released agents to the adenohypophysis, where they act upon the hormone-secreting cells. Within this system, the neurotransmitter dopamine is synthesised by neurones of the hypothalamic arcuate nucleus and is released within the neurohypophysis by axons travelling in the hypothalamohypophyseal tract (see Fig. 9.14 ). Dopamine acts to inhibit the release of prolactin by the adenohypophysis. The synthesis of hypothalamic releasing factors is under feedback regulation by hormones produced by target organs.

Hypothalamus

  • The hypothalamus is part of the diencephalon; it is connected to the pituitary gland via the infundibulum.

  • The hypothalamus has autonomic, neuroendocrine and limbic functions and is involved in the coordination of homeostatic mechanisms.

  • The hypothalamus produces hormones that are released from the posterior pituitary and also releasing factors that control the release of hormones from the anterior pituitary.

  • The supraoptic and paraventricular nuclei of the hypothalamus produce vasopressin and oxytocin, respectively.

  • Vasopressin and oxytocin are transported to the posterior pituitary in the hypothalamohypophyseal tract.

  • The anterior pituitary produces: adrenocorticotropic hormone, luteinising hormone, follicle-stimulating hormone, thyroid-stimulating hormone, growth hormone and prolactin. Factors that control their secretion are released into the pituitary portal system of the pituitary stalk and carried to the anterior pituitary.

  • The lateral hypothalamus and the ventromedial nucleus regulate eating and drinking.

  • The suprachiasmatic nucleus controls circadian rhythms.

Tumours of the hypothalamus and pituitary gland

Tumours and other diseases of the hypothalamus and associated pituitary gland lead to under- or over-production of circulating hormones. These, in turn, produce disorders of growth ( dwarfism , gigantism and acromegaly ), sexual function ( precocious puberty, hypogonadism ), body water control ( diabetes insipidus and pathological drinking ), eating ( obesity and bulimia ) and adrenal cortical control ( Cushing's disease and adrenal insufficiency ). Since the pituitary gland is closely adjacent to the optic chiasm, tumours of the gland ( pituitary adenomas ) may also lead to bitemporal visual field loss (see Fig. 15.8 ).

Limbic system

The limbic system consists of a number of phylogenetically ancient cortical and subcortical structures, with complex and widespread connections that provide the fundamental neural basis for instinctive and emotional aspects of behaviour and for memory. It has rich interconnections with the hypothalamus ( Figs 16.4 , 16.8 ), through which emotional states are influenced by, and mediate, changes in physiological and biochemical conditions. The limbic system earns its title from the location of some of its major cortical components on the medial rim of the cerebral hemisphere ( le grand lobe limbique ). It consists of a number of structures with complex interconnections and several major fibre pathways that project to the hypothalamus ( Fig. 16.9 ). The powerful input to the limbic system from the neocortical association areas links complex ‘goal-directed’ behaviour to more primitive, instinctive behaviour and internal homeostasis in a cascade of neural connections ( Figs 16.10 , 16.11 ). In a simplified way, we may conceive of information from the outside world being collected in modality-specific ways (e.g. vision, hearing, touch) and refined in the parieto-occipital association areas (perceptuospatial function). This information is then conveyed to the frontal association areas involved in planned behaviour (regulation) and also to the inferior temporal association areas, where information can reach supramodal status and meaning (semantic processing). Entry of information into the limbic system is either directly to the amygdala or indirectly to the hippocampal formation, via the entorhinal cortex (see Fig. 16.14 ). The amygdala is vital to the motivational and emotional connotations of experience. The information flow into the hippocampal formation permits a link to previous experience, since the hippocampal formation is essential to remembering and learning (episodic memory).

Fig. 16.9, The major components of the limbic system and their relationship with the hypothalamus.

Fig. 16.10, The basic link between associative areas of neocortex, the limbic system, the hypothalamus and their major output pathways.

Fig. 16.11, The interconnections between associative neocortical regions and the component parts of the limbic system.

The limbic system is able to influence motor responses appropriate to its informational analyses, through projections to the nucleus accumbens, which forms part of the basal ganglia, and autonomic responses through projections to the hypothalamus.

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