Book summary “The Human Brain”: summary

Introduction

You often wonder how a collection of cells can coordinate actions, feel emotions, and contemplate its own existence. The human body operates through a vast network of chemical messengers and electrical signals that together maintain life and generate consciousness. You might struggle to understand how these microscopic biological processes produce complex behavior, memory, and reasoning.

In The Human Body: Its Structure and Operation, Isaac Asimov guides you through the intricate architecture of human biology. The central idea of the book is that the body relies on the dual coordination of the chemical endocrine system and the electrical nervous system. These systems work in constant harmony to regulate metabolism, transmit sensory data, and process thought. Asimov demonstrates how simple chemical molecules and ionic charges scale up to form the advanced awareness of the cerebral cortex.

The book follows a logical path from fundamental chemistry to the highest levels of thought. You begin by exploring hormones, glands, and the metabolic control of blood sugar and salt balance. You then examine how individual nerve cells conduct electricity and communicate across synapses. The text moves upward through the evolutionary stages of brain development, from simple nerve cords to the convoluted human cerebrum. Finally, you investigate the mechanics of sensory perception, reflexes, conditioned learning, and the physical basis of reason.

As you read this summary, you will find answers to several fundamental questions about your own biology:

• How do hormones and enzymes regulate chemical reactions across different organs?
• What physical mechanisms allow nerves to transmit electrical impulses along your limbs?
• Why did the brain evolve specialized regions for movement, sensation, and speech?
• How do sensory organs translate light and sound waves into neural signals?
• What distinguishes human reason and memory from simple animal instincts?

1. Our Hormones and Chemical Messengers

The organization of the human body operates on two complementary levels: the electrical system of the nervous system and the chemical system of hormones.

While the electrical system utilizes the brain, spinal cord, and nerves for fast signaling, the chemical system relies on hormones and chemical messengers carried via the bloodstream. Chemical organization is older and less specialized, as even plants and unicellular organisms react to chemical stimuli without a nervous system.

Discovery of Secretin

William Maddock Bayliss and Ernest Henry Starling cut all the nerves leading to the pancreas of experimental animals and found that the pancreas still secreted digestive juice the moment acid touched the intestinal lining.

In 1902, physiologists demonstrated that introducing hydrochloric acid into the denervated small intestine of an animal caused the pancreas to secrete digestive juices. They isolated the chemical messenger produced by the intestinal lining and named it secretin. Bayliss proposed the term hormone in 1905 to describe substances that arouse another organ to activity.

Our Pancreas and Insulin

Frederick Grant Banting and Charles Herbert Best tied off the pancreatic duct in a living animal, waited seven weeks, and extracted the active hormone from the remaining islets of Langerhans.

Insulin is the pancreatic hormone responsible for lowering blood glucose levels, and its discovery resolved the primary cause of diabetes mellitus. Researchers began their research in Toronto in 1921, and Banting received the Nobel Prize in Medicine and Physiology in 1923.

The normal human pancreas weighs 85 grams. The islets of Langerhans make up 1 to 2 percent of the organ. Insulin facilitates the entry of glucose into cells and helps the liver convert excess glucose into stored glycogen.

2. Insulin Structure and Glucose Regulation

Frederick Sanger determined the exact amino acid sequence of ox insulin, while glucagon and epinephrine act in opposition to insulin to raise blood glucose levels.

Insulin Structure

An insulin molecule has an approximate molecular weight of 6000 and contains roughly 50 amino acid units. Frederick Sanger and his team worked for eight years to map the exact order of amino acids in the insulin molecule, earning Sanger the Nobel Prize in Chemistry for 1958.

Using partial breakdown methods and paper chromatography, Sanger's group spent eight years piecing together the jigsaw puzzle of insulin's two amino acid chains. These polypeptide units are held together by cystine linkages. The complete structure of insulin was published in 1955, marking the first time the complete molecular structure of a natural protein was mapped.

Glucagon and Epinephrine

Glucagon is produced by the alpha cells of the islets of Langerhans and maintains blood glucose during normal fluctuations. It acts in opposition to insulin to raise blood glucose levels by breaking down stored glycogen.

Epinephrine is produced by the adrenal medulla and mobilizes glycogen reserves in both the liver and muscles during fight-or-flight situations. An adult adrenal gland has an approximate weight of 10 grams.

Key historical milestones in hormone research include:

• 1895: the year adrenal gland extracts were first known to raise blood pressure;
• 1901: the year Jokichi Takamine isolated pure epinephrine.

Jokichi Takamine isolated epinephrine in pure form in 1901, demonstrating that tiny quantities could markedly raise blood pressure.

3. Our Thyroid and Parathyroid Glands

The thyroid gland requires trace amounts of iodine to manufacture thyroxine and tri-iodothyronine, which control the basal metabolic rate of the human body.

The thyroid gland stores a maximum iodine content of 8 milligrams. E. Baumann located iodine in the thyroid gland in 1896. Edward Calvin Kendall isolated thyroxine in 1915. Rosalind Pitt-Rates isolated tri-iodothyronine in 1951.

Thyroxine contains four heavy iodine atoms. The thyroid-pituitary axis regulates the production of thyroid hormone through the anterior pituitary's secretion of TSH, balancing metabolism against the body's energy requirements.

Swiss surgeons who completely removed goitrous thyroids from 46 patients in 1883 observed the subsequent onset of severe dull and listless hypothyroid symptoms.

Parathyroid and Posterior Pituitary

Parathyroid hormone regulates blood calcium levels by eroding bone storage, while posterior pituitary hormones control water reabsorption and uterine contractions.

The parathyroid glands maintain calcium ion balance similarly to how glucagon maintains glucose balance. You find that the normal concentration of calcium ion in blood is 9 to 11 milligrams per cent.

The parathyroid hormone isolated in pure form in 1960 has a molecular weight of 9500.

• Vasopressin controls kidney water reabsorption and prevents diabetes insipidus;
• Oxytocin stimulates muscular contractions during childbirth.

Severe cases of diabetes insipidus produce a daily volume of urine reaching 20 to 30 quarts.

Vincent du Vigneaud determined the structure of oxytocin and vasopressin as eight-amino-acid peptides and synthesized them artificially, winning the 1955 Nobel Prize in Chemistry.

4. Steroid Hormones and the Adrenal Cortex

Certain key hormones are steroids built upon a four-ring carbon nucleus derived from cholesterol, with the adrenal cortex producing corticoids for metabolism and salt balance.

Cholesterol serves as the parent substance for bile acids, vitamin D, and various steroidal compounds. Because of their fat-soluble nature, steroids tend to concentrate at biological interfaces such as cell membranes, altering membrane permeability and physiological behavior.

In 1814, a white substance with a fatty consistency was first obtained from gallstones and later named cholesterol. A cholesterol molecule contains 27 carbon atoms. An average 70-kilogram man carries an approximate total amount of cholesterol of 230 grams.

Adrenal Cortex Corticoids

The adrenal cortex produces steroid hormones called corticoids, which are divided into glycocorticoids for carbohydrate metabolism and mineralocorticoids for salt and water balance. Adrenal cortex extracts yielded numerous crystalline compounds synthesized from cholesterol. Those with an oxygen atom at carbon-11 are glycocorticoids like cortisone that promote glycogen storage. Those lacking it are mineralocorticoids like DOC that maintain mineral and water balance.

Corticoids contain a total of 21 carbon atoms, compared to the 27 carbon atoms in cholesterol.

Thomas Addison first described adrenal cortical insufficiency in 1855, characterized by bronze skin discoloration and fatal wasting, now known as Addison's disease. Edward Kendall and Tadeus Reichstein shared the Nobel Prize in 1950 for their work on adrenal hormones. Philip Showalter Hench discovered in 1948 that cortisone and ACTH provided dramatic relief for rheumatoid arthritis sufferers.

The anterior pituitary gland controls adrenal cortex activity through the release of ACTH, mediating the body's response to environmental stress. Swine corticotropin contains 39 amino acids, and the molecular weight of ACTH was determined to be 20,000 by the early 1950s. Removal of the pituitary causes the adrenal cortex to shrivel and lose its stress response. Under physical stress, pituitary ACTH production increases, prompting the adrenal cortex to rapidly consume cholesterol to manufacture corticoids.

5. Growth Hormone and Sex Hormones

The anterior pituitary secretes growth hormone directly to stimulate tissue growth, while gonads produce sex hormones that govern reproductive systems.

Plant Hormones and Auxins

Plants utilize specialized growth-accelerating hormones called auxins, which direct cell elongation and plant movement in response to stimuli. Auxins formed in growing plant tips move downward to cause cell elongation. They concentrate on the shaded side of a stem, causing uneven growth that bends the plant toward the sun.

In 1935, plant growth-accelerating auxins were first isolated in pure form. Scientists identified indolyl-3-acetic acid, commonly known as IAA. Another plant growth compound, 2,4-D, acts as a selective weed killer.

Japanese farmers observed rice shoots extending dangerously high due to fungus infestation, leading to the isolation of gibberellins. In 1938, growth-stimulating compounds were isolated from the Gibberella fungus.

Growth Hormone and Gigantism

The anterior pituitary secretes growth hormone directly to stimulate body tissue growth, and its over- or under-production leads to marked physical abnormalities. Unlike other pituitary hormones, growth hormone acts directly on tissues rather than intermediary glands. Deficiencies during youth produce proportional midgets, while excesses result in giantism or acromegaly.

P.T. Barnum made Charles Sherwood Stratton famous as General Tom Thumb, a perfectly proportioned midget who stood about 3 feet tall. Robert Wadlow reached a final height of 8 feet 11.1 inches before his death due to pituitary overactivity.

Biochemical analysis reveals distinct variations across species:

• Growth hormone obtained from cattle has a molecular weight of 45,000.
• Primate growth hormone has a molecular weight of 25,000.

The anterior pituitary produces somatotrophic hormone, abbreviated as STH, which dictates the overall stature you attain during your developmental years.

Sex Hormones and Androgens

Gonads produce sex hormones that govern reproductive systems, directing secondary sexual characteristics across your lifespan. These chemical messengers coordinate with pituitary outputs to regulate maturation.

6. Nerve Conduction and Ions

Nerves transmit signals electrically through rapid depolarization waves mediated by the active transport of sodium and potassium ions across cell membranes.

Living cells maintain an unequal distribution of ions using energy via a sodium pump, concentrating sodium outside and potassium inside. When stimulated, the membrane depolarizes as sodium floods inward, creating an electrical impulse.

The neuron receives signals through dendrites and transmits them along an axon. This electrical potential across a living cell membrane is 0.07 volts, which is equivalent to 70 millivolts. Hermann von Helmholtz first measured the speed of the nerve impulse in 1852.

Nerve Conduction and Ion Pumps

Polarization and depolarization govern the passage of every signal. After an impulse fires, a brief refractory period occurs while the cell resets its ion balance.

Electrophorus electricus, the electric eel of South American fresh waters, can develop potentials up to 700 volts using modified muscle series. A fully rested electric eel develops an electrical potential between 600 to 700 volts.

7. Myelin Sheath and Synaptic Transmission

Vertebrates evolved the myelin sheath to accelerate nerve impulse speeds while maintaining thin nerve fibers.

Unlike invertebrates such as the squid which rely on giant axons up to a 1 millimeter thick, vertebrates surround nerve fibers with satellite cells called Schwann's cells. These cells wrap around the axon in a tight spiral to form a fatty myelin sheath interrupted by unmyelinated nodes of Ranvier, allowing impulses to travel much faster.

A myelinated nerve impulse travels at about 100 meters per second, allowing the nerve impulse in a 100-foot blue whale to travel from nose to tail in 3/10 of a second. In a six-foot man, a nerve impulse covers head to toe in 1/50 of a second. Mammalian axons can be 50 times thinner than squid axons, occupying 2500 times less cross-sectional area.

Synaptic Transmission

Nerve impulses cross the microscopic synaptic gap between neurons through chemical mechanisms rather than electrical sparks.

Electrical potentials are generally not high enough to jump the synaptic gap. The arrival of a stimulus prompts the reaction of acetic acid and choline to form acetylcholine. This substance alters the sodium pump to initiate depolarization, and is subsequently broken down by the enzyme cholinesterase to allow repolarization.

German chemists in the 1940s developed powerful cholinesterase inhibitors as deadly nerve gases during research on insecticides. Inhibition of cholinesterase causes death by paralysis in 2 to 10 minutes. Epilepsy affects 1 in 200 people.

All-or-None Law

A nerve fiber responds to stimuli according to the all-or-none law, propagating either a maximum strength impulse or none at all.

Weak stimuli below the threshold level fail to initiate an effective impulse, while a threshold stimulus produces the fiber's maximum response. Finer gradations of muscle contraction are achieved because organs are fed by multiple nerve fibers with varying thresholds and firing frequencies.

The refractory period for small nerve fibers is 1/250 of a second, allowing up to 250 impulses per second.

8. Cephalization and the Cerebral Cortex

Bilateral symmetry led to cephalization, and the expansion of the neopallium and convolutions in mammals enabled complex behavior and intelligence.

Cephalization and Chordate Nervous System

As animals developed bilateral symmetry and moved in a preferred direction, sense organs and mouths concentrated in the head region. This process formed a primitive brain.

Unlike invertebrates with solid ventral nerve cords, chordates developed a single hollow cylinder. This nerve cord is located dorsally along the back surface and is protected by vertebrae.

Early vertebrates first developed these features half a billion years ago. The primitive brain soon differentiated into three main regions:

• forebrain
• midbrain
• hindbrain

These regions further specialized into the cerebrum, cerebellum, and medulla oblongata. Reptiles underwent changes to become mammals 100 million years ago.

Neopallium and Mammalian Intelligence

Reptiles developed the neopallium for non-smell sensations to coordinate movement against gravity. In mammals, this area expanded to cover the top half of the cerebral cortex. This expansion allowed a greater variety of responses to stimuli.

Larger mammals developed folded cerebral surfaces with convolutions. These folds increase gray matter area without outgrowing the skull.

The stegosaur weighed over 10 tons with a brain no larger than a kitten. It required an enlarged spinal brain near the base of the spinal cord to coordinate its rear half.

Primates first developed from Insectivora 70 million years ago. Later fossils such as Zinjanthropus were dated to 1,750,000 years old through potassium radioactive decay.

Meninges and Cerebrospinal Fluid

The brain and spinal cord are enclosed within three protective membranes:

• dura mater
• arachnoid membrane
• pia mater

The subarachnoid space and internal brain ventricles contain cerebrospinal fluid. An adult contains 100 to 150 milliliters of this fluid.

This fluid acts as a shock absorber, counters inertia, and neutralizes gravity. It prevents delicate brain tissue from distorting.

The brain's outermost portions have an 85 per cent water content. Glia cells outnumber nerve cells in the brain by a 10 to 1 ratio.

The cerebrum contains 10 billion nerve cells and 100 billion glia cells. The resting brain consumes one-fifth of total body oxygen despite having a mass ratio of one-fiftieth to total body mass.

9. Cerebral Cortex and Subcortical Structures

Specific regions of the cerebral cortex control movement and sensation, while subcortical structures regulate motor coordination and basic body drives.

Cerebral Cortex Functional Mapping

The cerebral cortex is divided into lobes mapped by sulci and gyri. The frontal lobe contains the motor area with large Betz cells that control voluntary movement via the pyramidal tract. Each hemisphere's motor area contains 30,000 of these unusually large pyramidal cells.

The parietal lobe contains the sensory area for touch, temperature, and muscle position, while the temporal and occipital lobes handle hearing and sight. Electroencephalography taps a maximum number of 24 skull electrode positions. Alpha waves show a potential variation of 20 microvolts in a 10 times per second cycle.

Pierre Paul Broca showed in 1861 that patients unable to speak had physical damage to the third left frontal gyrus.

Basal Ganglia and Hypothalamus

Subcortical structures including the basal ganglia, thalamus, and hypothalamus regulate motor coordination, sensory filtering, and basic body drives. The basal ganglia modulate motor control from the cortex, and their damage results in conditions like Parkinson's disease or chorea. The thalamus filters and relays somesthetic sensations, while the hypothalamus manages automated responses, water metabolism, temperature regulation, and contains pleasure centers.

James Parkinson clinically described paralysis agitans in 1817, a condition marked by resting tremors and masklike facial expressions due to basal ganglia dysfunction.

Surgeons use a temperature of -50 degrees Celsius with a thin probe to cool abnormal basal ganglia regions to relieve tremors.

You find that these regions combine to process every physical sensation and voluntary movement you execute.

10. Brain Stem, Regulation, and Senses

The hypothalamus regulates temperature and appetite, the reticular activating system controls wakefulness, and specialized receptors detect cutaneous sensations.

Hypothalamus Thermostat and Appestat

Appropriate cells in the hypothalamus sense blood temperature. It controls muscle shivering and perspiration to keep the core temperature stable within a narrow range. Foreign proteins or toxins can reset this thermostat upward to cause a fever, which helps destroy invading bacteria and viruses.

Normal human body temperature is 98.6° F. A body temperature of 108° F places a person in serious danger of death.

Human beings trapped in snowdrifts have recovered from body temperatures as low as 60° F, where the metabolic rate drops to 15 per cent of normal.

When fasting causes blood glucose to drop below a key level, the appestat turns on the appetite. Once glucose is restored, the appetite is turned off. Overeating can result from an appestat that is set too high or from a malfunctioning satiety center.

Animals began to eat voraciously and became grotesquely obese after the corresponding area of the hypothalamus was destroyed.

Reticular Activating System and Cerebellum

The wake-sleep cycle is governed by the hypothalamus and the reticular activating system, which filter sensory inputs and maintain consciousness. Extended wakefulness disrupts the coordination between the hypothalamus and the cortex, leading to hallucinations and mental disturbance. The reticular activating system controls arousal by deciding whether to pass sensory stimuli along to the cortex.

Encephalitis lethargica or African sleeping sickness, spread by the tsetse fly, can produce long periods of coma.

The cerebellum uses sensory feedback and prediction to coordinate smooth muscle movements and maintain equilibrium. When performing actions like reaching for an object, the cerebellum looks ahead to predict future positions and continuously corrects motion. Damage to this area causes wild oscillations and jerky movements known as hunting or ataxia.

A person with cerebellar damage reaching for a pencil will overshoot the mark, come back, overshoot in reverse, and continue in a series of oscillations. Four per cent of cerebral palsy cases show ataxia.

Cutaneous Senses and Weber-Fechner Law

Cutaneous sensations of touch, pressure, heat, cold, and pain are detected by specialized nerve endings distributed across the skin. Different specialized structures receive specific sensations:

• Meissner's corpuscles handle touch
• Krause's end bulbs handle cold
• Ruffini's end organs handle heat
• Pacinian corpuscles handle pressure
• bare nerve endings handle pain

The skin contains 200,000 nerve endings for temperature, 500,000 nerve endings for touch or pressure, and 3,000,000 nerve endings for pain.

Cats use vibrissae or whiskers as long hairs that transmit touch from objects at a distance to touch-receptors near their roots.

The human body detects differences in sensory stimulus intensity according to a logarithmic scale known as the Weber-Fechner Law. The ability to distinguish between two intensities of a stimulus depends on the percentage difference rather than the absolute difference. This logarithmic scaling allows sense organs to operate across immense ranges of intensity.

It is easy to distinguish a 1-ounce weight from a 2-ounce weight, but difficult to distinguish a 90-ounce weight from a 91-ounce weight.

11. Hearing, Vestibular Sense, and Vision

Mammalian hearing utilizes middle ear ossicles and cochlear hair cells, while the eye achieves visual acuity using a retinal mosaic of rods and cones.

Evolution of Mammalian Ossicles

Mammalian hearing is made exceptionally keen through a unique three-bone ossicle system derived from reptilian jawbones and fish gill bars. During vertebrate evolution from water to land, gill bars and unneeded reptilian jawbones were co-opted into the middle ear. The malleus, incus, and stapes form a lever system that concentrates sound wave energies, amplifies sound up to fiftyfold at the oval window, and utilizes tiny muscles to damp out excessively loud sounds.

Sound travels through the air at 1090 feet per second at freezing temperature. Middle C corresponds to a frequency of 264 cycles per second.

Cochlea Pitch Perception and Echolocation

Pitch is perceived through wavelike displacements of the basilar membrane within the cochlea, which stimulate specific hair cells depending on frequency. Sound waves entering through the oval window travel through the cochlear fluid and set up wavelike displacements in the basilar membrane. Human hearing covers a frequency range from 16 to 25,000 cycles per second, utilizing 24,000 parallel fibers stretching across the basilar membrane and 15,000 hairs in each human cochlea.

Physicist Georg von Bekesy conducted careful experiments with an artificial system designed to possess all the essentials of the cochlea. Georg von Bekesy found that lower frequencies cause peak displacements more distant from the oval window, and the nerve network identifies the position of this peak to distinguish pitch.

Animals such as bats utilize high-frequency ultrasonic squeaks and echoes to navigate and locate small objects in total darkness, a principle mimicked by human technology. By emitting continuous series of high-frequency ultrasonic squeaks ranging from 40,000 to 80,000 cycles per second, with resulting wavelengths of 1/3 to 1/6 of an inch, bats determine the exact location of prey.

Deafening bats destroys their ability to avoid obstacles in flight and catch insects on the wing at night, even after having been blinded. This biological echolocation inspired human inventions like sonar and radar.

Vestibular Sense and Vertebrate Eye

The internal ear contains the utricle for sensing gravity and static body posture, and semicircular canals for detecting dynamic changes in motion and acceleration. The utricle contains a bit of calcium carbonate resting on hair cells that signals the direction of gravity and helps maintain posture. Meanwhile, the three mutually perpendicular semicircular canals contain fluid that moves by inertia during head acceleration or deceleration, stimulating specific hair cells to judge motion.

A researcher replaced the sand in a crustacean's statocysts with iron filings, causing the animal to stand on its head when a magnet was held above it. A vertebrate upside-down experiences a 180-degree displacement removal of otoconia.

Vertebrates achieve high visual acuity by using a lens to concentrate light onto a fine, cell-sized mosaic of photoreceptors on the retina. Unlike insect compound eyes which use large facets, vertebrate eyes concentrate light via the cornea and lens onto microscopic cells. Humans can squeeze 10,000 dots into the space a honeybee covers with one dot.

Front-facing eyes with overlapping fields of vision provide stereoscopic depth perception through the phenomenon of parallax. By positioning two eyes forward so their fields of vision overlap, the brain fuses slightly different images received by each eye.

Holding a pencil a foot before your eyes and viewing it while closing first one eye then the other demonstrates the positional shift. The human neck turn is limited to 90 degrees, supplemented by movable eyeballs.

Rods, Cones, and Rhodopsin Cycle

Visual sensitivity and color detection are divided between two types of photoreceptors, with rod vision relying on a light-bleached pigment cycle involving vitamin A. Cones operate in bright light to detect color and fine detail, concentrated heavily in the macula lutea. The human retina contains 125 million rods and 7 million cones, operating within a visible light wavelength range of 3800 A to 7600 A.

12. Reflexes, Learning, and Reason

Reflex arcs enable rapid motor responses, conditioning creates learned behaviors, and speech acts as a phase change that separates human thought from animal instinct.

Reflex Arcs and Innate Behavior

A stimulus travels along a sensory nerve to the central nervous system, crosses a reflex center via connector neurons, and moves down a motor nerve to an effector. This automatic mechanism executes fast actions before conscious awareness occurs.

A person being tested for the patellar reflex sits with a crossed leg, and when the tendon below the patella is tapped, the thigh muscle contracts sharply to kick the lower leg up.

Innate behavior does not require learning. Embryonic development reveals these movements in sequence:

• head-bending reflex in a chick embryo at 70 hours
• head-turning reflex in a chick embryo at 90 hours
• beak-movement reflexes at 5 days
• swallowing reflex at 8 days
• reflex movement of the head and neck away from touch around the mouth and nose in an 8-week human embryo

Conditioning and Sensory Testing

Ivan Pavlov experimented with artificial associations using dogs. He severed a dog's gullet so food dropped out, yet tasting food still caused gastric juices to flow, demonstrating an unconditioned reflex.

Ringing a bell 20 to 40 times with food creation led to salivation at the bell alone. Conditioning lets scientists test animal perception. Bees were tested by placing sugar solution on blue cards versus gray cards. They successfully conditioned to blue and avoided gray, proving they can see blue. When tested with red and gray cards, they flew to both indiscriminately, proving they cannot see red.

Association Areas and Speech

Sensory association areas converge into a gnostic area. Antonio Egas Moniz performed prefrontal lobotomy in 1935 by severing prefrontal lobes to disconnect undesirable associations in psychiatric patients, winning a Nobel Prize in 1949, though it caused undesirable personality changes.

Complex thought requires the convergence of sensory interpretation, symbolic representation, and physical manipulation.

When reason becomes complex enough to allow abstraction and symbolic representation through speech, a phase change occurs. Dolphins may possess complex speech, but living in sea water confines them to flippers and sound, whereas humans combine mind, eye, and hand.

Psychobiochemistry and Memory

Mental illnesses often have organic foundations. Pellagra causes dementia and is cured simply by adding milk or nicotinic acid to the diet, whereas phenylpyruvic oligophrenia causes mental deficiency because the body cannot convert phenylalanine to tyrosine.

Memory storage relies on vast neural capacity and molecular alterations within nerve cells.

A brain stores up to 1,000,000,000,000,000 bits of information over a lifetime. Nerve cells are rich in ribonucleic acid, or RNA. Each new bit of information may introduce a change in an RNA molecule, creating a unique protein match that constitutes a memory.

10 Key Ideas

1. The endocrine system utilizes hormones carried via the bloodstream to regulate metabolism, while the nervous system employs electrical signals for fast signaling.
2. Nerves transmit signals electrically through rapid depolarization waves mediated by the active transport of sodium and potassium ions across cell membranes.
3. Vertebrates evolved the myelin sheath to accelerate nerve impulse speeds while maintaining thin nerve fibers.
4. Bilateral symmetry led to cephalization, and the expansion of the neopallium and convolutions in mammals enabled complex behavior and intelligence.
5. Specific regions of the cerebral cortex control movement and sensation, while subcortical structures regulate motor coordination and basic body drives.
6. The hypothalamus regulates body temperature and appetite through specific neural mechanisms.
7. The human body detects differences in sensory stimulus intensity according to a logarithmic scale known as the Weber-Fechner Law.
8. Mammalian hearing utilizes middle ear ossicles and cochlear hair cells to perceive sound waves and pitch.
9. Visual sensitivity and color detection are divided between two types of photoreceptors, with rod vision relying on a light-bleached pigment cycle involving vitamin A.
10. Speech acts as a phase change that separates human thought and abstraction from simple animal instinct.