Brain Facts and Figures

These data were obtained from several textbooks. Note that all data are estimates and averages. Check literature for appropriate references. All numbers are for humans unless otherwise indicated.

Table of Contents
BRAIN

NEURON

SPINAL CORD


SENSORY APPARATUS

BLOOD SUPPLY



Brain

Average Brain Weights (in grams)
Species Weight (g)
Species Weight (g)
adult human 1,300 - 1,400
newborn human 350 - 400
sperm whale 7,800
fin whale 6,930
elephant 6,000
humpback whale 4,675
gray whale 4,317
killer whale 5,620
bowhead whale 2,738
pilot whale 2,670
bottle-nosed dolphin 1,500 - 1,600
walrus 1,020 - 1,126
Pithecanthropus Man 850 - 1,000
camel 762
giraffe 680
hippopotamus 582
leopard seal 542
horse 532
polar bear 498
gorilla 465 - 540
cow 425-458
chimpanzee 420
orangutan 370
California sea lion 363
manatee 360
tiger 263.5
lion 240
grizzly bear 234
sheep 140
baboon 137
adult rhesus monkey 90-97
dog (beagle) 72
aardvark 72
beaver 45
shark (great white) 34
shark (nurse) 32
cat 30
porcupine 25
squirrel monkey 22
marmot 17
rabbit 10-13
platypus 9
alligator 8.4
squirrel 7.6
opossum 6
flying lemur 6
fairy anteater 4.4
guinea pig 4
hedgehog 3.35
tree shrew 3
fairy armadillo 2.5
owl 2.2
rat (400 g body weight) 2
hamster 1.4
elephant shrew 1.3
turtle 0.3-0.7
bull frog 0.24
viper 0.1
goldfish 0.097
green lizard 0.08
Reference for many of these brain weights:
1. Blinkov, S.M. and Glezer, I.I. The Human Brain in Figures and Tables. A Quantitative Handbook, New York: Plenum Press, 1968.
2. Demski, L.S. and Northcutt, R.G. The brain and cranial nerves of the white shark: an evolutionary perspective. In Great White Sharks. The Biology of Carcharodon carcharias, San Diego: Academic Press, 1996.
3. Nieuwenhuys, R., Ten Donkelaar, H.J. and Nicholson, C. The Central Nervous System of Vertebrates. Vol. 3, Berlin: Springer, 1998.
4. Berta, A., et al. Marine Mammals. Evolutionary Biology, San Diego: Academic Press, 1999.
5. Mink, J.W., Blumenschine, R.J. and Adams, D.B. Ratio of central nervous system to body metabolism in vertebrates: its constancy and functional basis. Am. J. Physiology, 241:R203-R212, 1981.
6. Ridgway, S.H. and Harrison, S., Handbook of Marine Mammals, Vol. 3, London: Academic Press, 1985.

% brain of total body weight (150 pound human) = 2%
Average brain width = 140 mm
Average brain length = 167 mm
Average brain height = 93 mm
Average number of neurons in the brain = 100 billion
Number of neurons in brain (octopus) = 300 million (from How Animals See, S. Sinclair, 1985)
Number of neurons in Aplysia nervous system = 18,000-20,000
Number of neurons in each segmental ganglia in the leech = 350
Volume of the brain of a locust = 6mm3 (from The Neurobiology of the Insect Brain, Burrows, M., 1996)
% of brain that is white matter = 60%
% of cerebral oxygen consumption by white matter = 6%
% of brain that is gray matter = 40%
% of cerebral oxygen consumption by gray matter = 94%
Average number of glial cells in brain = 10-50 times the number of neurons
(For more on the number of neurons in the brain, see R.W. Williams and K. Herrup, Ann. Review Neuroscience, 11:423-453, 1988)
Number of neocortical neurons (females) = 19.3 billion (Pakkenberg, B., Pelvig, D., Marner,L., Bundgaard, M.J., Gundersen, H.J.G., Nyengaard, J.R. and Regeur, L. Aging and the human neocortex. Exp. Gerontology, 38:95-99, 2003 and Pakkenberg, B. and Gundersen, H.J.G. Neocortcial neuron number in humans: effect of sex and age. J. Comp. Neurology, 384:312-320, 1997.)
Number of neocortical neurons (males) = 22.8 billion (Pakkenberg et al., 1997; 2003)
Average loss of neocortical neurons = 85,000 per day (~31 million per year) (Pakkenberg et al., 1997; 2003)
Average loss of neocortical neurons = 1 per second (Pakkenberg et al., 1997; 2003)
Average number of neocortical glial cells (young adults ) = 39 billion (Pakkenberg et al., 1997; 2003)
Average number of neocortical glial cells (older adults) =36 billion (Pakkenberg et al., 1997; 2003)
Length of myelinated nerve fibers in brain = 150,000-180,000 km (Pakkenberg et al., 1997; 2003)
Number of synapses in cortex = 0.15 quadrillion (Pakkenberg et al., 1997; 2003)
Difference number of neurons in the right and left hemispheres = 186 million MORE neurons on left side than right side (Pakkenberg et al., 1997; 2003)

Proportion by Volume (%)

Rat Human
Cerebral Cortex 31 77
Diencephalon 7 4
Midbrain 6 4
Hindbrain 7 2
Cerebellum 10 10
Spinal Cord 35 2
(Reference: Trends in Neuroscience, November 1995)
Composition of Brain and Muscle

Skeletal Muscle (%) Whole Brain (%)
Water 75 77 to 78
Lipids 5 10 to 12
Protein 18 to 20 8
Carbohydrate 1 1
Soluble organic substances 3 to 5 2
Inorganic salts 1 1
(Reference: McIlwain, H. and Bachelard, H.S., Biochemistry and the Central Nervous System, Edinburgh: Churchill Livingstone, 1985)
Total surface area of the cerebral cortex = 2,500 sq. cm (2.5 sq.ft.; A. Peters, and E.G. Jones, Cerebral Cortex, 1984)
Total surface area of the cerebral cortex (lesser shrew) = 0.8 sq. cm
Total surface area of the cerebral cortex (rat) = 6 sq. cm
Total surface area of the cerebral cortex (cat) = 83 sq. cm
Total surface area of the cerebral cortex (African elephant) = 6,300 sq.
Total surface area of the cerebral cortex (Bottlenosed dolphin) = 3,745 sq. cm (S.H. Ridgway, The Cetacean Central Nervous System, p. 221)
Total surface area of the cerebral cortex (pilot whale) = 5,800 sq. cm
Total surface area of the cerebral cortex (false killer whale) = 7,400 sq. cm
(Reference for surface area figures: Nieuwenhuys, R., Ten Donkelaar, H.J. and Nicholson, C., The Central nervous System of Vertebrates, Vol. 3, Berlin: Springer, 1998)
Total number of neurons in cerebral cortex = 10 billion (from G.M. Shepherd, The Synaptic Organization of the Brain, 1998, p. 6). However, C. Koch lists the total number of neurons in the cerebral cortex at 20 billion (Biophysics of Computation. Information Processing in Single Neurons, New York: Oxford Univ. Press, 1999, page 87).
Total number of synapses in cerebral cortex = 60 trillion (yes, trillion) (from G.M. Shepherd, The Synaptic Organization of the Brain, 1998, p. 6). However, C. Koch lists the total synapses in the cerebral cortex at 240 trillion (Biophysics of Computation. Information Processing in Single Neurons, New York: Oxford Univ. Press, 1999, page 87).
Percentage of total cerebral cortex volume (human): frontal lobe = 41%; temporal lobe = 22%; parietal lobe = 19%; occipital lobe = 18%. (Caviness Jr., et al. Cerebral Cortex, 8:372-384, 1998.)
Number of cortical layers = 6
Thickness of cerebral cortex = 1.5-4.5 mm
Thickness of cerebral cortex (Bottlenosed dolphin) = 1.3-1.8 mm (S.H. Ridgway, The Cetacean Central Nervous System, p. 221)
EEG - beta wave frequency = 13 to 30 Hz
EEG - alpha wave frequency = 8 to 13 Hz
EEG - theta wave frequency = 4 to 7 Hz
EEG - delta wave frequency = 0.5 to 4 Hz
World record, time without sleep = 264 hours (11 days) by Randy Gardner in 1965. Note: In Biopsychology (by J.P.J. Pinel, Boston: Allyn and Bacon, 2000, p. 322), the record for time awake is attributed to Mrs. Maureen Weston. She apparently spent 449 hours [18 days, 17 hours] awake in a rocking chair. The Guinness Book of World Records [1990] has the record belonging to Robert McDonald who spent 453 hours, 40 min in a rocking chair.
Time until unconsciousness after loss of blood supply to brain = 8-10 sec
Time until reflex loss after loss of blood supply to brain = 40-110 sec
Rate of neuron growth (early pregnancy) = 250,000 neurons/minute
Length of spiny terminals of a Purkinje cell = 40,700 micron
Number spines on a Purkinje cell dendritic branchlet = 61,000
Weight of adult human cerebellum = 150 g
Surface area of cerebellar cortex = 50,000 sq. cm (from G.M. Shepherd, The Synaptic Organization of the Brain, 1998, p. 255)
Weight of newborn human cerebellum = 21 g
Number of Purkinje cells = 15-26 million
Number of synapses made on a Purkinje cell = up to 200,000
Weight of hypothalamus = 4 g
Volume of suprachiasmatic nucleus = 0.3 cubic mm
Number of fibers in pyramidal tract above decussation = 1,100,000
Number of fibers in corpus callosum = 250,000,000
Area of the corpus callosum (midsagittal section) = 6.2 sq. cm
Total volume of cerebrospinal fluid = 125-150 ml
Half life of cerebrospinal fluid = 3 hours
Daily production of CSF = 400 to 500 ml
Specific gravity of cerebrospinal fluid = 1.007
Color of normal CSF = clear and colorless
White Blood cell count in CSF = 0-3 per cubic mm.
Red Blood cell count in CSF = 0-5 per cubic mm.
Normal intracranial pressure = 150 - 180 mm of water
Composition of Serum and Cerebrospinal Fluid (CSF)

CSF Serum
Water (%) 99 93
Protein (mg/dl) 35 7000
Glucose (mg/dl) 60 90
Osmolarity (mOsm/l) 295 295
Na (meq/l) 138 138
K (meq/l) 2.8 4.5
Ca (meq/l) 2.1 4.8
Mg (meq/l) 0.3 1.7
Cl (meq/l) 119 102
pH 7.33 7.41
(Reference: Fishman, R.A. Cerebrospinal Fluid in Disease of the Nervous System. Philadelphia: Saunders, 1980)
Number of cranial nerves = 12
I- olfactory
II- optic
Number of fibers in human optic nerve = 1,200,000
Number of fibers in cat optic nerve = 119,000
Number of fibers in albino rat optic nerve = 74,800
Length of optic nerve = 50 mm
III- oculomotor
Number of fibers in oculomotor nerve = 25,000-35,000
IV- trochlear
Number of fibers in trochlear nerve = 2,000-3,500
Number of neurons in nucleus of the trochlear nerve = 2,000-3,500
V- trigeminal
Number of fibers in motor root of trigeminal nerve = 8,100
Number of fibers in sensory root of trigeminal nerve = 140,000
VI- abducens
Number of fibers in abducens nerve (at exit from brain stem) = 3,700
VII- facial
Number of fibers in facial nerve (at exit from brain stem) = 9,000-10,000
Length of nucleus of the facial nerve = 2 to 5.6 mm
Number of neurons in nucleus of the facial nerve = 7,000
VIII-vestibulocochlear
IX- glossopharyngeal
X- vagus
Length of dorsal motor nucleus of the vagus nerve = 10 mm
XI- spinal accessory
XII- hypoglossal
Number of neurons in nucleus of the hypoglossal nerve = 4,500-7,500
Length of nucleus of the hypoglossal nerve = 10 mm

Spinal Cord
Number of neurons in human spinal cord = 1 billion (from Kalat, J.W., Biological Psychology, 6th Edition, 1998, page 24)
Length of human spinal cord = 45 cm (male); 43 cm (female)
Length of human vertebral column = 70 cm
Length of cat spinal cord = 34 cm
Length of rabbit spinal cord = 18 cm
Weight of human spinal cord = 35 g
Weight of rabbit spinal cord = 4 g
Weight of rat spinal cord (400 g body weight) = 0.7 g
Maximal Circumference of cervical enlargement = 38 mm
Maximal Circumference of lumbar enlargement = 35 mm
Pairs of Spinal Nerves = 31
Number of Spinal Cord segments = 31
8 cervical segments
12 thoracic segments
5 lumbar segments
5 sacral segments
1 coccygeal segment

Sensory Apparatus
Audition
Surface area of the tympanic membrane = 85mm2 (Statistics from "Hearing. Its Physiology and Pathophysiology" by A.R. Moller, San Diego, Academic Press, 2000.)
Length of the eustachian tube = 3.5 to 3.9 cm (Statistics from "Hearing. Its Physiology and Pathophysiology" by A.R. Moller, San Diego, Academic Press, 2000.)
Number of hair cells in cochlea = 10,000 inner hair cells; 30,000 outer hair cells (Statistics from "Hearing. Its Physiology and Pathophysiology" by A.R. Moller, San Diego, Academic Press, 2000. However, in the same book, Moller states that there are only 3,500 inner hair cells and 12,000 outer hair cells.)
Number of fibers in auditory nerve = 28,000-30,000
Length of auditory nerve = 2.5 cm
Number of neurons in cochlear nuclei = 8,800 (Northern, J.L. and Downs, M.P., Hearing in Children, 5th edition, Philadelphia: Lippincott Williams & Wilkins, 2002.)
Number of neurons in inferior colliculus = 392,000 (Northern, J.L. and Downs, M.P., Hearing in Children, 5th edition, Philadelphia: Lippincott Williams & Wilkins, 2002.)
Number of neurons in medial geniculate body = 570,000
Number of neurons in auditory cortex = 100,000,000
Hearing Range (young adult human) = 20 to 20,000 Hz
Hearing Range (elderly human) = 50 to 8,000 Hz (Guyton, A.C., Textbook of Medical Physiology, 1986)
Hearing Range (rat) = 1,000 to 50,000 Hz
Hearing Range (cat) = 100 to 60,000 Hz
Most sensitive range of human hearing = 1,000-4,000 Hz
Length of external auditory meatus = 2.7 cm
Diameter of external auditory meatus = 0.7 cm
Weight of malleus = 23 mg
Weight of incus = 25 mg
Weight of stapes = 2-4 mg
Length of cochlea = 35 mm
Width of cochlea = 10 mm
Number of turns in the cochlea = 2.2-2.9
Length of basilar membrane = 25-35 mm
Width of basilar membrane = 150 microns (at base of cochlea) (Statistic from "Hearing. Its Physiology and Pathophysiology" by A.R. Moller, San Diego, Academic Press, 2000.)
Auditory Pain Threshold = 130 db
Threshold for hearing damage = 90 db for an extended period of time
Decibel Sound Scale
Decibels Sound
160 Jet plane engine
130 Pain threshold
120 Discomfort
100 Subway
80 Heavy motor vehicle traffic
60 Normal conversation
20 Whisper conversation
0 Threshold of hearing
Taste
Total number of human taste buds (tongue, palate, cheeks) = 10,000
Number of taste buds on the tongue = 9,000
Height of taste bud = 50-100 microns (From: Farbman, A.I., Taste Bud, in G. Adelman, eds., Encyclopedia of Neuroscience, 1987)
Diameter of taste bud = 30-60 microns (From: Farbman, A.I.)
Number of receptors on each taste bud = 50-150 (Boron, W.F. and Boulpaep, E.L., Medical Physiology. A Cellular and Molecular Approach, Philadelphia: Saunders, 2003)
Diameter of taste receptor = 10 micron
Diameter of taste fiber = less than 4 micron
Taste threshold for quinine sulfate = 3.376 mg/liter water
Smell
Number of human olfactory receptor cells = 40 million
Number of rabbit olfactory receptor cells = 100 million
Number of dog olfactory receptor cells = 1 billion
Surface area of olfactory epithelium (contains olfactory receptor cells) in humans = 10 cm2 (Bear, M.F., Connors, B.W. and Pradiso, M.A., Neuroscience: Exploring the Brain, 2nd edition, Baltimore: Lippincott Williams and Wilkins, 2001, p. 269)
Area of olfactory epithelium in some dogs = 170 cm2 (Bear, M.F., Connors, B.W. and Pradiso, M.A., Neuroscience: Exploring the Brain, 2nd edition, Baltimore: Lippincott Williams and Wilkins, 2001, p. 269)
Area of olfactory epithelium in cats = 21 sq. cm (Bradshaw, J., Behavioral biology, in The Waltham Book of Dog and Cat Behaviour, ed. C. Thorne, Oxford: Pergamon Press, 1992)
Thickness of olfactory epithelium mucous layer = 20-50 microns. (Boron and Boulpaep, 2003)
Diameter of olfactory receptor axons = 0.1-0.2 micron
Diameter of distal end olfactory receptor cell = 1 micron
Diameter of olfactory receptor cell = 40-50 micron
Number of cilia per olfactory receptor cell = 10-30
Length of cilia on olfactory receptor cell = 100-150 micron
Concentration for detection threshold of musk = 0.00004 mg/liter air
Vision
Length of eyeball = 24.5 mm (from Spaide, R.F., 1999)
Volume of eyeball = 5.5 cm3
Weight of eyeball = 7.5 g
Average time between blinks = 2.8 seconds
Average duration of a single blink = 0.3 seconds
Thickness of cornea = 0.54 mm in center; 0.65 in periphery
Diameter of cornea = 11.5 mm
Thickness of lens = 4 mm
Diameter of lens = 9 mm
Composition of lens = 65% water; 35% protein
Number of retinal receptor cells = 5-6 million cones; 120-140 million rods
Number of retinal ganglion cells = 800 thousand to 1 million
Density of cones in fovea = 200,000 per square mm
Peak density of rods = 175,000 per square mm
Peak density of rods (cat) = 400,000 per square mm
Number of fibers in optic nerve = 1,200,000
Number of neurons in lateral geniculate body = 570,000
Number of cells in visual cortex (area 17) = 538,000,000
Wavelength of visible light (human) = 400-700 nm
Amount of light necessary to excite a rod = 1 photon
Amount of light necessary to excite a cone = 100 photons
Location of the greatest density of rods = 20 degrees from fovea
Highest density of rods = 160,000 per mm2
Density of cones in fovea = 200,000 per mm2
Diameter of fovea = 1.5 mm
Intraocular pressure = 10-20 mm Hg
Volume of orbit = 30 ml
Area of retina = 2,500 square mm
Thickness of retina = 120 microns (ranges from 100 to 230 microns)
Production rate of aqueous humor = 2 microliters/min
Turnover of aqueous humor = 15 times/day
% volume of eye occupied by the vitreous = 80%
Maximal sensitivity of red cones = 570 nm
Maximal sensitivity of green cones = 540 nm
Maximal sensitivity of blue cones = 440 nm
More Facts and Figures about the Human Retina from WebVision.
Touch
Weight of skin (adult human) = 6 lb.(2.7 kg)
Area of skin (adult human) = 20 sq. ft.
Number of tactile receptors in the hand = 17,000
Number of nerve endings in hand = 1,300 per sq. in.
von Frey threshold (Face) = 5 mg
2 point threshold (Finger) = 2-3 mm
Length of Meissner corpuscle = 90 - 120 micron
Density of receptors on finger tips = 2,500 per sq.cm
Density of Meissner's corpuscles on finger tips = 1,500 per sq. cm
Density of Merkel's cells on finger tips = 750 per sq. cm
Density of Pacinian corpuscles on finger tips = 75 per sq. cm
Density of Ruffini's corpuscles on finger tips = 75 per sq. cm
Thermal pain threshold = 45 degrees centigrade

Neurons
Mass of a large sensory neuron = 10-6gram (from Groves and Rebec, Introduction to Biological Psychology, 3rd edition, Dubuque: Wm.C. Brown Publ., 1988)
Number of synapses for a "typical" neuron = 1,000 to 10,000
Diameter of neuron = 4 micron (granule cell) to 100 micron (motor neuron in cord)
Diameter of neuron nucleus = 3 to 18 micron
Length of Giraffe primary afferent axon (from toe to neck) = 15 feet
Resting potential of squid giant axon = -70 mV
Conduction velocity of action potential = 0.6-120 m/s (1.2-250 miles/hr)
Single sodium pump maximum transport rate = 200 Na ions/sec; 130 K ions/sec
Typical number of sodium pumps = 1000 pumps/square micron of membrane surface (from Willis and Grossman, Medical Neurobiology, Mosby, St. Louis, 1981, p. 36)
Total number of sodium pumps for a small neuron = 1 million
Density of sodium channels (squid giant axon) = 300 per sq. micron (from Hille, B., Ionic Channels of Excitable Membranes, Sinauer, Sunderland, 1984, p. 210.)
Number of voltage-gated sodium channels at each node = 1,000 to 2,000 per sq. micron (from Nolte, J., The Human Brain, Mosby, 1999, p. 163.)
Number of voltage-gated sodium channels between nodes = 25 per sq. micron (from Nolte, J., The Human Brain, Mosby, 1999, p. 163.)
Number of voltage-gated sodium channels in unmyelinated axon = 100 to 200 per sq. micron (from Nolte, J., The Human Brain, Mosby, 1999, p. 163.)
Diameter of microtubule = 20 nanometer
Diameter of microfilament = 5 nanometer
Diameter of neurofilament = 10 nanometer
Thickness of neuronal membrane = 5 nanometer
Thickness of squid giant axon membrane = 50-100 A
Membrane surface area of a typical neuron = 250,000 um2 (Bear et al., 2001)
Membrane surface area of 100 billion neurons = 25,000 m2, the size of four soccer fields (Bear, M.F., Connors, B.W. and Pradiso, M.A., Neuroscience: Exploring the Brain, 2nd edition, Baltimore: Lippincott Williams and Wilkins, 2001, p. 97)
Typical synaptic cleft distance = 20-40 nanometers across (from Kandel et al., 2000, p. 176)
% neurons stained by Golgi method = 5%
Slow axoplasmic transport rate = .2-4 mm/day (actin, tubulin)
Intermediate axoplasmic transport rate = 15-50 mm/day (mitochondrial protein)
Fast axoplasmic transport rate = 200-400 mm/day (peptides, glyolipids)
Number of molecules of neurotransmitter in one synaptic vesicle = 5,000 (from Kandel et al., 2000, p. 277)
Diameter of synaptic vesicle = 50 nanometer (small); 70-200 nanometer (large)
Diameter of neurofilament = 7 - 10 nm
Diameter of microtubule = 25 nm
Internodal Length = 150 - 1500 microns (depends on fiber diameter
% composition of myelin = 70-80% lipid; 20-30% protein
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Ion Concentration (mM) - SQUID NEURON

Intracellular Extracellular
Potassium 400 20
Sodium 50 440
Chloride 40-150 560
Calcium 0.0001 10


Ion Concentration (mM) - MAMMALIAN NEURON

Intracellular Extracellular
Potassium 140 5
Sodium 5-15 145
Chloride 4-30 110
Calcium 0.0001 1-2
Data from Purves et al., Neuroscience, Sunderland: Sinauer Associates, 1997.
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Neurotoxins

Blood Supply
% brain utilization of total resting oxygen = 20%
% blood flow from heart to brain = 15-20%
Blood flow through whole brain (adult) = 750 ml/min
Blood flow through whole brain (adult) = 54 ml/100 g/min
Blood flow through whole brain (child) = 105 ml/100 g/min
Oxygen consumption whole brain = 46 cubic cm/min
Oxygen consumption whole brain = 3.3 ml/100 g/min
Blood flow rate through each internal carotid artery = 180 ml/min
Blood flow rate through basilar artery = 380 ml/min
Diameter of vertebral artery = 2-3 mm
Diameter of common carotid artery (adult) = 6 mm
Diameter of common carotid artery (newborn) = 2.5 mm