Thirst caused by blood loss (hypovolaemic thirst) differs from osmotic thirst chiefly in that it:

Is triggered by angiotensin II and also produces an appetite for salt

Is triggered by a rise in extracellular salt concentration

Is abolished by drinking pure water alone

Depends on ghrelin released by the stomach

Is triggered by angiotensin II and also produces an appetite for salt

Assertion (A): A heritability of 0.60 for intelligence means that 60 per cent of each individual's intelligence is inherited. Reason (R): Heritability is the proportion of variation in a trait within a population that is associated with genetic differences. Choose the correct option.

Both A and R are true, and R is the correct explanation of A

Both A and R are true, but R is not the correct explanation of A

The circadian "master clock" of the brain is the:

Lateral geniculate nucleus of the thalamus

Biological Basis of Behavior II: Muscles and Glands; the Biology of Hunger, Thirst, Sleep and Sex; the Limbic System and Hormonal Regulation of Emotion; and Genetics — Chromosomal Anomalies, Twin and Adoption Studies

The second half of Unit 4 reads: "Muscular and Glandular system: Types and functions. Biological basis of Motivation: Hunger, Thirst, Sleep and Sex. Biological basis of emotion: The Limbic system, Hormonal regulation of behavior. Genetics and behavior: Chromosomal anomalies; Nature-Nurture controversy [Twin studies and adoption studies]." This chapter connects each motive and emotion to the structures and hormones that regulate it — the lateral and ventromedial hypothalamus, leptin and ghrelin, osmotic and hypovolaemic thirst, the stages and circuits of sleep, the Papez circuit and the amygdala — then names the chromosomal anomalies with their karyotypes and works the heritability estimate that twin studies yield.

1. The muscular and glandular systems

There are three types of muscle. Skeletal (striated) muscle is voluntary and moves the skeleton, contracting when motor neurons release acetylcholine; its muscle spindles signal stretch and its Golgi tendon organs signal tension. Smooth muscle lines the gut, blood vessels and other viscera and is involuntary. Cardiac muscle is striated but involuntary. Glands are exocrine (secreting through ducts — sweat, salivary, tear glands) or endocrine (ductless, releasing hormones into the blood).

The endocrine glands
Endocrine gland	Hormones and behavioural relevance
Pituitary	The "master gland", controlled by the hypothalamus. Anterior lobe: growth hormone, TSH, ACTH, FSH, LH, prolactin. Posterior lobe releases oxytocin and vasopressin (ADH), which are made in the hypothalamus.
Pineal	Melatonin, released in darkness; regulates circadian rhythm.
Thyroid	Thyroxine regulates metabolic rate; deficiency brings lethargy and depressed mood (and cretinism in infancy), excess brings restlessness and anxiety.
Adrenal cortex and medulla	The cortex secretes cortisol (the stress hormone) and aldosterone; the medulla secretes epinephrine and norepinephrine for the sympathetic emergency response.
Pancreas	Insulin (lowers blood glucose) and glucagon (raises it) — central to hunger.
Gonads	Testosterone, oestrogen and progesterone — sexual development and behaviour.

🧠 The HPA axis

Stress runs down a chain: the hypothalamus releases CRH, the anterior pituitary releases ACTH, and the adrenal cortex releases cortisol, which feeds back to switch the chain off. Chronic stress keeps this axis active and wears down the hippocampus and immune function.

2. Hunger and thirst

Early theory located hunger in the stomach: Cannon and Washburn (1912) had Washburn swallow a balloon and found that stomach contractions coincided with reported hunger pangs — yet people whose stomachs have been removed still feel hungry. Mayer's glucostatic theory tied hunger to glucose available to cells; Kennedy's lipostatic theory to stored fat and a body-weight set point. The classic dual-centre theory placed a feeding centre in the lateral hypothalamus (lesions cause aphagia, refusal to eat) and a satiety centre in the ventromedial hypothalamus (lesions cause hyperphagia and obesity).

Hunger and satiety signals
Signal	Source	Effect
Ghrelin	Empty stomach	Increases hunger (rises before meals)
Leptin	Fat cells	Long-term satiety signal proportional to fat stores
CCK and PYY	Intestine	Short-term satiety after a meal
Insulin	Pancreas	Promotes glucose uptake; long-term satiety signal to the brain
NPY and AgRP versus POMC and CART	Arcuate nucleus of the hypothalamus	NPY/AgRP neurons stimulate eating; POMC/CART neurons inhibit it

Thirst has two separate triggers. Osmotic thirst follows a rise in the salt concentration of extracellular fluid, which draws water out of cells; osmoreceptors in the lamina terminalis (around the third ventricle) detect it and the posterior pituitary releases vasopressin (ADH) so the kidneys conserve water. Hypovolaemic thirst follows a loss of fluid volume, as in bleeding or diarrhoea; the kidneys release renin, producing angiotensin II, which acts on the subfornical organ, and aldosterone makes the kidneys retain sodium — which is why hypovolaemic thirst creates an appetite for salt as well as water.

3. Sleep and sex

Sleep stages — a cycle lasts about 90 minutes and repeats four or five times a night
Stage	Features
N1	Transition from waking; theta waves; hypnic jerks.
N2	Sleep spindles and K-complexes; about half of total sleep time.
N3 (slow-wave sleep)	High-amplitude delta waves; deepest sleep, hardest to wake; growth-hormone release; sleepwalking and night terrors arise here; concentrated in the first half of the night.
REM (paradoxical sleep)	Rapid eye movements, a waking-like EEG, vivid dreaming and paralysis of skeletal muscles (atonia); triggered from the pons; periods lengthen toward morning.

The suprachiasmatic nucleus of the hypothalamus is the circadian clock, entrained by light and signalling the pineal gland to release melatonin. The ventrolateral preoptic area promotes sleep; orexin (hypocretin) neurons in the lateral hypothalamus stabilise wakefulness, and their loss causes narcolepsy. Adenosine accumulating during waking builds sleep pressure, which is why caffeine, an adenosine antagonist, keeps people awake. Sleep is explained as restoration, as an adaptive conservation of energy and safety, and as a period of memory consolidation.

Sex hormones have organisational effects (prenatal, shaping brain and body permanently) and activational effects (in adulthood, switching behaviour on). Testosterone contributes to sexual desire in both sexes. In the hypothalamus the medial preoptic area is central to male sexual behaviour and the ventromedial nucleus to female receptivity in rodents. Masters and Johnson described a four-phase sexual response cycle — excitement, plateau, orgasm and resolution.

4. The limbic system and hormonal regulation of emotion

James Papez (1937) proposed a circuit for emotion: hippocampus → fornix → mammillary bodies → anterior thalamus → cingulate cortex → back to the hippocampus. Paul MacLean (1952) expanded it and named it the limbic system, adding the amygdala and septal area. Today the hippocampus is seen mainly as a memory structure and the amygdala as the key emotional one. Philip Bard showed that cats with the cortex removed but the hypothalamus intact showed "sham rage", implicating the hypothalamus in the expression of emotion.

The emotional brain
Structure	Role in emotion
Amygdala	Detecting threat, fear conditioning, emotional memory. LeDoux's "low road" (thalamus → amygdala, fast and crude) and "high road" (thalamus → cortex → amygdala, slow and detailed). Bilateral temporal-lobe removal in monkeys produces the Klüver–Bucy syndrome: tameness and loss of fear, hyperorality, hypersexuality and "psychic blindness".
Hypothalamus	Autonomic and endocrine expression of emotion; rage and defence (sham rage).
Prefrontal cortex	Appraisal and regulation of emotion; damage (Phineas Gage, 1848) brings impulsivity and poor social judgement.
Cingulate cortex and insula	Conflict monitoring and the affective component of pain (cingulate); disgust and awareness of bodily states (insula).

Hormonal regulation of behaviour: epinephrine and norepinephrine produce the arousal of fear and anger; cortisol sustains the stress response; oxytocin promotes bonding, maternal behaviour and trust; testosterone is modestly associated with dominance and aggression; and low serotonin activity is linked to impulsive aggression. Hormones act slowly and diffusely compared with neurotransmitters, and they both shape behaviour and respond to it — winning a contest raises testosterone.

5. Genetics and behaviour: chromosomal anomalies and the nature–nurture controversy

Chromosomal anomalies (phenylketonuria and Huntington's disease, by contrast, are single-gene disorders)
Condition	Karyotype or cause	Psychological features
Down syndrome	Trisomy 21 (47 chromosomes); risk rises with maternal age	Intellectual disability of varying degree, characteristic facial features, early-onset Alzheimer-type changes
Turner syndrome	45,X — a female with one X chromosome	Short stature, infertility; typically average verbal ability with weaker spatial and mathematical skills
Klinefelter syndrome	47,XXY — a male with an extra X	Tall stature, small testes, infertility; language and learning difficulties
XYY syndrome	47,XYY	Tall; early claims of criminality were not borne out; some learning difficulties
Fragile X syndrome	Expanded CGG repeat in the FMR1 gene on the X chromosome	The most common inherited cause of intellectual disability; strong association with autism features
Cri-du-chat syndrome	Deletion on the short arm of chromosome 5	Cat-like cry in infancy, intellectual disability

The nature–nurture controversy is studied with designs that pull genes and environment apart. Twin studies compare identical (monozygotic, MZ — sharing essentially all their genes) with fraternal (dizygotic, DZ — about half) twins; greater MZ similarity implies genetic influence. Falconer's formula estimates heritability as h² = 2(r_MZ − r_DZ): if MZ twins correlate 0.80 and DZ twins 0.50 on a trait, h² = 2 × 0.30 = 0.60. Adoption studies compare adopted children with their biological and adoptive parents; twins reared apart (the Minnesota study under Bouchard, 1990) combine both designs and found IQ correlations of about 0.70 for separated MZ twins.

⚠️ What heritability does not mean

Heritability describes variation in a population, not an individual: h² = 0.60 does not mean 60 per cent of a person's IQ is genetic. It can change when environments change, and a highly heritable trait can still respond strongly to environment (PKU is genetic and is treated by diet). Genes and environment also correlate — passively, evocatively and actively (Scarr and McCartney) — and interact.

Key takeaways

Skeletal muscle is voluntary and striated, smooth muscle involuntary, cardiac striated but involuntary; the pituitary, under the hypothalamus, directs the thyroid, adrenals and gonads, and the HPA axis ends in cortisol.
Lateral hypothalamic lesions cause aphagia and ventromedial lesions hyperphagia; ghrelin signals hunger and leptin long-term satiety; osmotic thirst works through vasopressin and hypovolaemic thirst through angiotensin II.
Sleep cycles through N1–N3 and REM about every 90 minutes; the suprachiasmatic nucleus sets the circadian clock and orexin loss causes narcolepsy.
The Papez circuit and MacLean's limbic system frame the emotional brain; the amygdala detects threat (Klüver–Bucy syndrome when removed) and the prefrontal cortex regulates emotion.
Down (trisomy 21), Turner (45,X), Klinefelter (47,XXY) and Fragile X are the key anomalies; twin, adoption and reared-apart studies estimate heritability, with Falconer's h² = 2(r_MZ − r_DZ).

Practice questions (10)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

Rats with lesions of the ventromedial hypothalamus typically show:
1. Refusal to eat (aphagia)
2. Excessive drinking
3. Loss of REM sleep
4. Overeating and obesity
Show answer
Answer: D — Overeating and obesity
In the classic dual-centre account the ventromedial hypothalamus is a satiety centre, so its destruction produces hyperphagia and obesity. Lateral hypothalamic lesions produce aphagia.
Which hormone, secreted by fat cells, acts as a long-term satiety signal to the brain?
1. Glucagon
2. Melatonin
3. Ghrelin
4. Leptin
Show answer
Answer: D — Leptin
Leptin is released in proportion to fat stores and reduces appetite over the long term. Ghrelin from the stomach increases hunger, glucagon raises blood glucose, and melatonin regulates circadian rhythm.
Sleep spindles and K-complexes are characteristic of which stage of sleep?
1. N1
2. N3
3. N2
4. REM
Show answer
Answer: C — N2
Spindles (bursts of 12–14 Hz activity) and K-complexes define stage N2. N1 shows theta, N3 high-amplitude delta, and REM a waking-like EEG with rapid eye movements.
On a trait, identical (MZ) twins correlate 0.80 and fraternal (DZ) twins correlate 0.50. Using Falconer's formula h² = 2(r_MZ − r_DZ), estimate the heritability. Type a number.
Numerical answer — type the value.
Show answer
Answer: 0.6
h² = 2(0.80 − 0.50) = 2 × 0.30 = 0.60. About 60 per cent of the variation in this trait in this population is attributed to genetic differences.
A person with the karyotype 47,XXY has:
1. Turner syndrome
2. Klinefelter syndrome
3. Fragile X syndrome
4. Down syndrome
Show answer
Answer: B — Klinefelter syndrome
An extra X in a male (47,XXY) is Klinefelter syndrome. Turner syndrome is 45,X, Down syndrome is trisomy 21, and Fragile X is a gene mutation rather than an extra chromosome.
Monkeys given bilateral removal of the temporal lobes, including the amygdala, became tame, lost their fear and examined objects with their mouths. This pattern is called the:
1. Klüver–Bucy syndrome
2. Korsakoff syndrome
3. General adaptation syndrome
4. Capgras syndrome
Show answer
Answer: A — Klüver–Bucy syndrome
Klüver and Bucy described tameness, loss of fear, hyperorality, hypersexuality and psychic blindness after temporal lobectomy, pointing to the amygdala's role in emotion. Korsakoff is an alcohol-related amnesia, the general adaptation syndrome is Selye's stress model, and Capgras is a delusion about impostors.
Which structures were part of the circuit for emotion proposed by Papez (1937)? Select all that apply.
1. Cingulate cortex
2. Mammillary bodies
3. Hippocampus
4. Cerebellum
Show answer
Answer: A — Cingulate cortex; B — Mammillary bodies; C — Hippocampus
The Papez circuit runs hippocampus → fornix → mammillary bodies → anterior thalamus → cingulate cortex → hippocampus. The cerebellum is concerned with coordination, not with the Papez circuit.
Thirst caused by blood loss (hypovolaemic thirst) differs from osmotic thirst chiefly in that it:
1. Is triggered by a rise in extracellular salt concentration
2. Is abolished by drinking pure water alone
3. Depends on ghrelin released by the stomach
4. Is triggered by angiotensin II and also produces an appetite for salt
Show answer
Answer: D — Is triggered by angiotensin II and also produces an appetite for salt
Volume loss triggers the renin–angiotensin system; angiotensin II drives drinking and aldosterone salt retention, and animals seek salty water to restore volume. A rise in salt concentration is the trigger for osmotic thirst, and ghrelin concerns hunger.
Assertion (A): A heritability of 0.60 for intelligence means that 60 per cent of each individual's intelligence is inherited. Reason (R): Heritability is the proportion of variation in a trait within a population that is associated with genetic differences. Choose the correct option.
1. Both A and R are true, and R is the correct explanation of A
2. Both A and R are true, but R is not the correct explanation of A
3. A is true, but R is false
4. A is false, but R is true
Show answer
Answer: D — A is false, but R is true
A is false and R is true. Because heritability is defined for variation across a population (R), it cannot be applied to a single person's score, which is exactly what A wrongly does.
The circadian "master clock" of the brain is the:
1. Basal ganglia
2. Suprachiasmatic nucleus of the hypothalamus
3. Medulla oblongata
4. Lateral geniculate nucleus of the thalamus
Show answer
Answer: B — Suprachiasmatic nucleus of the hypothalamus
The suprachiasmatic nucleus receives light information from the retina and synchronises bodily rhythms, signalling the pineal gland to release melatonin at night. The lateral geniculate nucleus relays vision.