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.

Brain

Average adult brain weight = 1,300 -1,400 gm (about 3 pounds)
Average number of neurons in the brain = 10 billion to 100 billion
Average number of glial cells in brain = 10-50 times the number of neurons
Number of synapses for a "typical" neuron = 1,000 to 10,000
% brain of total body weight (150 pound human) = 2%
Total surface area of the cerebral cortex = 2200 sq. cm (2.5 sq.ft.)
Total number of neurons in cerebral cortex = 2.6 billion
Thickness of cerebral cortex = 1.5-4.5 mm
% brain utilization of total resting oxygen = 20%
% blood flow from heart to brain = 20%
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
Combined synaptic surface area of dendritic branchlet = 222,000 sq. micron
Weight of hypothalamus = 4 gm
Area of the corpus callosum (midsaggital section) = 6.2 sq. cm
Total volume of cerebrospinal fluid = 125 ml
Half life of cerebrospinal fluid = 3 hours
Number of cranial nerves = 12

I- olfactory
II- optic
III- oculomotor
IV- trochlear
V- trigeminal
VI- abducens
VII- facial
VIII-vestibulocochlear
IX- glossopharyngeal
X- vagus
XI- spinal accessory
XII- hypoglossal

Spinal Cord

Length of average spinal cord = 45 cm (male); 43 cm (female)
Length of vertebral column = 70 cm
Weight of spinal cord = 35 gm
Pairs of Spinal Nerves = 31
Number of Spinal Cord segements = 31

8 cervical segments
12 thoracic segments
5 lumbar segments
5 sacral segments
1 coccygeal segment

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Sensory Apparatus

Audition

Number of hair cells in cochlea = 3500 inner hair cells; 12-20,000 outer hair cells
Number of fibers in auditory nerve = 28,000
Hearing Range (young adult human) = 20 Hz to 20,000 Hz
Length of external auditory meatus = 2.7 cm

Taste

Number of human taste buds = 10,000
Number of receptors on each taste bud = 100
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
Area of olfactory mucosa per nostril = 2.5 sq. cm
Diameter of olfactory receptor axons = 0.2 micron
Concentration for detection threshold of musk = 0.00004 mg/liter air

Vision

Number of retinal receptor cells = 6 million cones; 120 million rods
Number of retinal ganglion cells = 800 thousand to 1 million
Wavelength of visible light (human) = 380-760 nm
Amount of light necessary to excite a rod = 1 photon
Location of the greatest density of rods = 20 degrees from fovea

Touch

Number of tactile receptors in the hand = 17,000
von Frey threshold (Face) = 5 mg
2 point threshold (Finger) = 2-3 mm

Neurons

Size of neuron = 4 micron (granule cell) to 100 micron (motor neuron in cord)
Size of neuron nucleus = 3 to 18 micron
Length of Giraffe primary afferent axon (from toe to neck) = 15 feet
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 = 100-200 pumps/square mm of membrane surface
Total number of sodium pumps for a small neuron = 1 million
Typical synaptic cleft distance = 10-20 nanometers across
% 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 = 10,000-100,000
Diameter of synaptic vesicle = 50 nanometer (small); 70-200 nanometer (large)
% composition of myelin = 70-80% lipid; 20-30% protein