Hypothalamus · Grey Matter

The hypothalamus is a small region at the base of the brain, about the size of an almond, that keeps the body's internal conditions in balance: it measures temperature, water, salt, energy and time of day, and corrects them through behaviour, the autonomic nervous system and hormones.

Hypothalamus. The hypothalamus is a small region at the base of the brain, about the size of an almond, that keeps the body's internal conditions in balance: it measures temperature, water, salt, energy and time of day, and corrects them through behaviour, the autonomic nervous system and hormones.

It lies below the thalamus and forms the floor of the third ventricle, with the optic chiasm in front of it and the pituitary gland hanging below on a stalk. It is made of nuclei with specific jobs. The suprachiasmatic nucleus is the master circadian clock, set by light from the retina. The paraventricular and supraoptic nuclei make oxytocin and vasopressin and release the hormones that command the anterior pituitary. Other nuclei handle hunger and satiety, thirst, temperature, sexual behaviour and sleep. Through its links to the brainstem it sets heart rate and blood pressure, and the amygdala uses it to turn fear into a bodily alarm.

Lesions reveal the jobs one by one. In classic rat experiments, destroying the lateral hypothalamus stopped eating and drinking, destroying the ventromedial nucleus caused overeating and obesity, and damage to the sleep-promoting ventrolateral preoptic nucleus produced lasting insomnia; damage to the hormone-producing nuclei disturbs the whole endocrine system.

It generates states, and thoughts take their colour from them. Hunger, thirst, fatigue and stress are hypothalamic states, and they bias attention and decisions without being thoughts themselves.

Its plasticity is real and specialised: its circuits change with fasting, lactation, stress and the seasons, which is why the notes' label of limited plasticity is too strong.

Questions: Which areas decide what you eat when you are hungry and a pastry is on offer? The hypothalamus supplies the need: it senses low energy through hormones and nutrients in the blood and makes food more wanted. The orbitofrontal and ventromedial prefrontal cortex give each option its current value, which depends on hunger (the same pastry is worth less after a meal), and the striatum adds the pull of habit and reward. When a long-term goal argues against the pastry, the dorsolateral prefrontal cortex is engaged in weighing health against taste, and how much it does so tracks how much self-control the choice shows. How does a fright become a racing heart and sweaty hands? The central nucleus of the amygdala sends its output to the hypothalamus and the brainstem. The lateral hypothalamus and brainstem centres switch on the sympathetic nervous system within a second or two (faster heart, higher blood pressure, sweating, wide pupils, adrenaline from the adrenal medulla), while the paraventricular nucleus starts the slower hormonal arm, the HPA axis, that brings cortisol minutes later. The hypothalamus generates none of the thought about the threat; it translates the amygdala's verdict into the state of the body. Why are oxytocin and vasopressin said to come from the pituitary when the hypothalamus makes them? Both hormones are made in the cell bodies of large neurons in the supraoptic and paraventricular nuclei of the hypothalamus. The neurons pack them into vesicles that travel down their axons, through the pituitary stalk, to endings in the posterior pituitary, where they are stored. When the neurons fire, the endings release the hormones into the capillaries of the posterior pituitary, so the gland is the place of release and the hypothalamus is the place of manufacture. How does a stressful thought end up raising cortisol in the blood? A situation judged threatening by the cortex and the amygdala excites the paraventricular nucleus of the hypothalamus, which releases CRH into the portal vessels of the pituitary. The anterior pituitary responds with ACTH into the general circulation, and ACTH stimulates the adrenal cortex to make cortisol, which peaks in the blood about 20 to 30 minutes after the stressor. The chain explains why a purely psychological event (an exam, an argument) produces the same hormonal response as a physical one. How does the hypothalamus give orders to the pituitary? It uses two channels, one for each half of the gland. For the anterior pituitary, hypothalamic neurons release releasing and inhibiting hormones (CRH, TRH, GnRH, GHRH, somatostatin, dopamine) into a small portal system of blood vessels that carries them straight down the stalk, and the pituitary cells answer with their own hormones. For the posterior pituitary there is no messenger at all: the gland is made of the axon endings of hypothalamic neurons, which release oxytocin and vasopressin directly into the blood when those neurons fire. Why did destroying the septal area make rats furious, while stimulating it made them seek more? The lateral septum sends largely inhibitory projections to the hypothalamus and other regions that generate defensive and aggressive behaviour, so it acts as a brake on how strongly an animal reacts. Brady and Nauta found in 1953 that rats with septal lesions attacked or startled at harmless handling, the brake gone, an effect that fades over weeks. In 1954 Olds and Milner found that rats would press a lever repeatedly to stimulate electrodes in or near the septal area, the first evidence of reward sites in the brain, later traced mostly to the medial forebrain bundle that runs nearby toward the hypothalamus. How does daylight set the brain's clock, and why does the clock sit on top of the optic chiasm? The master circadian clock is the suprachiasmatic nucleus, a small pair of nuclei in the front of the hypothalamus resting just above the optic chiasm, as its name says. Its neurons keep a rhythm of about 24 hours on their own, and it is reset each day by light through the retinohypothalamic tract, fibres that leave the optic nerves near the chiasm and come from a special class of light-sensitive retinal ganglion cells. The clock then times sleep, body temperature, cortisol and melatonin across the day, which is why light in the evening shifts sleep later.