Hypothalamus

You can consider the thalamus to be one of the major relay stations in the brain. Much of the input it takes is from outside the brain. The only sense it does not filter or relay is olfaction or the sense of smell. All other senses are perceived by the thalamus and sent to cortical areas to be further interpreted. There is a lateral geniculate nucleus that takes in visual sensory information. There is a medial geniculate nucleus that takes in auditory sensory information. There is a ventral posterior nucleus that takes in sensations from the spinothalamic tract, which is where peripheral pain, touch, and temperature come from. The ventral posteromedial nucleus is another one that takes up information from the trigeminal nerve. There is also a ventral intermediate nucleus that seems to be causative of tremors if it is damaged in some way.

In the most ventral part of the thalamus is the reticular nucleus. It helps to form a capsule around the thalamus and, unlike the other nuclei, it processes information from within the other thalamic nuclei rather than information outside the thalamus. It also helps to initiate voluntary movement by taking on input from the globus pallidus.

In summary, the thalamus has five major functions, which are sensory regulation (except for the sense of smell), the regulation of pain and arousal, the modulation of motor language skills, the modulation of motivation and mood, and the ability to regulate cognitive functioning as it relates to the inputs received by the different thalamic nuclei.

HYPOTHALAMUS

The hypothalamus is also a deeper brain structure made of different nuclei divided into three separate zones that surround the mammillary bodies and the third ventricle. There is a periventricular zone, which is endocrine in nature. There are two medial and lateral nuclei that instead help to regulate somatic and autonomic systems. Figure 14 shows what the hypothalamus looks like:

Figure 14.

The hypothalamus has many areas of the brain it connects to. It connects to the brainstem through the dorsal longitudinal fasciculus. It connects to the hippocampus through the fornix. It connects to the amygdala through the stria terminalis. It connects to the cerebral cortex through the medial forebrain bundle. It connects to the thalamus through the mammillothalamic tract. It connects to the retina through the retinohypothalamic tract. Finally, it connects to the pituitary gland through the median eminence.

Based on the number of connections, you can imagine that the hypothalamus is very important. It is a major organ for motor output, sensory integration, and the maintenance of body homeostasis when it comes to endocrine function, the autonomic nervous system, and somatic behavior.

The hypothalamus is always taking in information from circulating hormones in the body. It participates in several endocrine feedback loops, particularly from the adrenal and thyroid glands. It also senses the osmolarity of the bloodstream so it can regulate the output of antidiuretic hormone or ADH. ADH keeps the water and salt balance in the body.

The hypothalamus also receives pain input from the spinothalamic tract. It is hugely involved in the ways the limbic system integrates sensory information it receives. It also takes in sensory information already processed in the cerebral cortex and picks up light signals from the retinohypothalamic tract to its suprachiasmatic nucleus. By doing this, it helps to regulate the fact that hormone levels rise and fall with light exposure from day to night. In total, the hypothalamus has 11 nuclei. Among the different hormones produced by the hypothalamus are oxytocin, antidiuretic hormone or ADH, CRH or corticotropin releasing hormone, TRH or thyroid releasing hormone, and GnRH or gonadotropin releasing hormone. Temperature in the body is also regulated by the hypothalamus because of its effects on the sympathetic tone to skin and muscles.

The suprachiasmatic nucleus in the hypothalamus is responsive to retinal input of light and helps to regulate the differences in cortisol release according to a circadian rhythm. It helps to alter a person’s locomotor activity during the day.

The ventromedial nucleus of the hypothalamus regulated a person’s feeding behavior. If this nucleus is not functional or is damaged, severe overeating occurs, which can be seen in patients who have Prader-Willi syndrome. It is where a person perceives satiety and where the sensation to decrease eating comes from. The dorsomedial nucleus, on the other hand, controls rage. If a person is not sated through eating enough food, there are hypothalamic interactions that cause aggression and rage. The lateral hypothalamus is related to the sensation of hunger. Without this nucleus, a person will have anorexia.

You can also consider the hypothalamus to be intricately connected to the ability to respond to psychological and physiological stress. Cortisol, which is one of the major stress hormones in the body, is intricately connected to the arcuate and suprachiasmatic nuclei in the hypothalamus so it takes a normal function of the hypothalamus to allow cortisol to have its own circadian rhythm and for it to respond to stress properly.