In this article we will discuss about:- 1. Nuclei in Hypothalamus 2. Connections of Hypothalamus 3. Functional Aspect.
Nuclei in Hypothalamus:
Some of important nuclei in hypothalamus (Fig. 9.51):
1. Preoptic nuclei
2. Anterior hypothalamic nuclei
3. Posterior hypothalamic nuclei
4. Ventromedial nuclei of hypothalamus
5. Lateral hypothalamic nuclei
6. Supraoptic nuclei
7. Paraventricular nuclei
8. Mammillary body
9. Suprachiasmatic nuclei, etc.
Connections of Hypothalamus:
Some important afferent and efferent connections of hypothalamus are given in Fig. 9.52 and Table 9.5.
Afferent Connections:
These various nuclei of hypothalamus bring a lot of afferent input/connections which are:
i. Visceral and somatic inputs through ascending tracts
ii. Olfactory pathway or olfactory tract (medial fore brain bundle)
iii. Corticohypothalamic (from neocortex, i.e. frontal lobe) tract
iv. Hippocampus (via fornix)
v. Amygdalo-hypothalamic (via stria terminalis)
vi. Thalamohypothalamic (from midline nucleus of thalamus)
vii. From tegmentum of midbrain
Efferent Connections:
i. Hypothalamohypophyseal tract
ii. Mammillothalamic tract (connects mammillary body to anterior thalamic nuclei)
iii. Mammillotegmental tract (connects mammillary body to tegmentum present in midbrain)
iv. Connected to vital centers in brainstem motor neurons in spinal cord.
The above mentioned are neural tracts—but hypothalamus is connected to anterior pituitary gland through hypothalamohypophyseal portal system (vascular connection).
Functional Aspects of Hypothalamus:
1. Regulation of Body Temperature:
Preoptic nucleus of hypothalamus acts as biologic thermostat. This thermostat is normally set to maintain a temperature of 37°C. In addition to this, anterior hypothalamus acts as heat loss center and posterior hypothalamus as heat gain center. Concerted activity of all these centers will help in regulation of body temperature.
Information about state of temperature in different parts of body reaches the nuclear areas through lateral spinothalamic tract which carry pain and temperature information from peripheral thermoreceptors. In addition to this, in hypothalamus itself there are thermoreceptors which are termed as central thermoreceptors. These receptors sense temperature of blood flowing through hypothalamus.
Depending on temperature of body (fever) or if body is exposed to altered temperature, hypothalamus tries to influence restoration of body temperature by bringing about alterations in autonomic, endocrine, sweat gland activity and behavioral changes. These will ultimately ensure maintenance of body temperature.
Resetting of biological thermostat to a high temperature is very detrimental to body. Hence when person is having severe temperature (as during high fever) applying cold packs on the forehead is done in order to prevent resetting of thermostat.
2. Regulation of Food Intake:
Ventromedial and lateral hypothalamic nuclei regulate food intake. Ventromedial nucleus acts as satiety center and lateral hypothalamic nucleus as hunger center. These two nuclear regions have role to play in glucose homeostasis in body.
In whole of brain for cells to utilize glucose insulin is not necessary. But for cells of ventromedial nucleus for their glucose utilization, insulin is necessary. Hence in diabetes mellitus when insulin is unable to exert action, there is impairment of glucose homeostasis.
The lateral hypothalamic nucleus area which acts as hunger center is a tonically active center. Normally this center activity is inhibited by impulses coming from ventromedial nucleus. In diabetes mellitus, when ventromedial nucleus activity is decreased or lost, inhibition on hunger center is lost (disinhibition). This leads to polyphagia.
If hunger center is destroyed, person suffers from aphagia: don’t eat at all.
If hunger center is over stimulated, person suffers from hyperphagia/polyphagia, person keeps eating more and more.
If satiety center is destroyed, it results in hyperpolyphagia and leads to hypothalamic obesity (Fig. 9.53).
3. Regulation of Body Water Content:
The lateral hypothalamic nuclear area also acts as thirst center. In addition to this, in hypothalamus there is presence of osmoreceptors which sense osmolality of plasma flowing through hypothalamus. When body water content is decreased, thirst center gets stimulated and person drinks water and thereby there would be replenishment of body water.
When osmolality of plasma is increased, it brings about stimulation of osmoreceptors due to the process of exosmosis. Osmoreceptors stimulation in turn leads to stimulation of supraoptic nuclear regions of hypothalamus and bring about an increase in amount of secretion of ADH.
ADH in turn brings about conservation of body water by enhancing water reabsorption from distal convoluted tubule and collecting ducts in kidney. This leads to decreased volume of urinary output, increased retention of water and correction of crystalloid osmotic pressure/osmolality.
4. Highest Center for Autonomic Nervous System:
Hypothalamus is known to act as head ganglia of autonomic nervous system. Both sympathetic and parasympathetic activity would be regulated by hypothalamus. Stimulation of various parts of hypothalamus is known to alter heart rate, blood pressure, respiration, etc.
Stimulation of posterior hypothalamus increases heart rate and blood pressure and stimulation of anterior hypothalamus does the converse. Also influences GI secretion and motility and contraction of urinary bladder.
5. Endocrine Control:
Almost all hormonal secretions in body are regulated by hypothalamus. Anterior pituitary gland which secretes many of the tropic hormones is constantly under influence of releasing factors or release- inhibiting factors secreted by hypothalamus.
These factors are secreted from median eminence region of hypothalamus. These factors reach anterior pituitary gland through hypothalamohypophyseal portal system to exert their actions on anterior pituitary.
In addition to above, posterior pituitary hormones are synthesized in supraoptic and paraventricular nuclear regions of hypothalamus. The hormones reach posterior pituitary gland through hypothalamohypophyseal tract and get released from here to circulation.
Since hypothalamus controls most of hormonal secretions in body, it has role to play in growth, metabolism of various substances and hence controls homeostasis.
6. Role in Reproduction:
Hypothalamus also controls secretion of gonadotropic hormone secretions from anterior pituitary gland which in turn control growth and function of gonads.
Gonads are responsible for secretion of sex hormones and gametogenesis. Because of these reasons, hypothalamus is essential for normal reproductive/sexual activity.
7. Influence on Sleep, Arousal and Alert State:
The impulses from ascending reticular activating system during their course to reach cerebral cortex do relay in various nuclei of hypothalamus. From hypothalamus, the onward transmission of impulses is regulated by a center which regulates sleep, arousal state of individual. Destruction of some of these neurons in hypothalamus induces permanent sleep suggesting existence of a sleep center in hypothalamus.
8. Acts as a Biologic Clock:
The suprachiasmatic nucleus region of hypothalamus is believed to act as biologic clock. This nucleus keeps track of time inside the body.
Tracking time is essential as many of activities in body do get altered based on time with respect to:
a. 24 hours in a day which is known as circadian rhythm/diurnal variation.
b. Number of days of a month and this is known as circalunar rhythm.
c. Number of years in life of an individual and this rhythm is known as circannual rhythm.
9. Role in Emotional State and Learning:
Many of changes that occur in the body during various emotional states, like anger, fear, sorrow, anxiety, etc., are being controlled by neurons present in hypothalamus. In addition to this, hypothalamic neurons also form part of limbic system and thereby have role in learning, motivation, etc.
In nutshell, functions of hypothalamus can be remembered with following pneumonic—SEAT2
S: Sleep, sexual activity
E: Emotion, endocirne
A: Autonomic nervous system, appetite
T: Thermoregulation, thirst.