The Brain-Machine Interface: A Technical and Strategic Analysis for the Next Decade of Neurotechnology

0.0 Numbers to Know

86 billion neurons: The approximate total number of nerve cells in the human brain.
16 billion neurons: The estimated number of neurons located specifically in the human cerebral cortex（大脳皮質）.
10,000 synapses: The number of unique connections a single neuron can make with other neurons.
20% to 25% of energy: The proportion of the body’s total energy budget consumed by the brain, despite it making up only about 2% of total body mass.
70 mV: The typical resting membrane potential of a neuron.
1 ms: The approximate duration of an action potential (spike).

333 Regions of Interest (ROIs): The number of distinct brain structures identified in the NextBrain probabilistic histological atlas.
16,000–24,000 neurons: The number of cells typically found in a single 1 mm³ fMRI voxel (along with 32,000–48,000 glia).
100 μm: The resolution provided by ex vivo MRI, which allows for much finer detail than standard 1 mm in vivo scans.
1 to 5 mm: The millimeter-scale spatial resolution achieved by transcranial focused ultrasound (tFUS).

25,000–30,000 muscle spindles: The total number of biological sensory receptors in the human body used for proprioception.
4,000 vs. 7,000 spindles: Each human arm contains about 4,000 spindles, while each leg contains roughly 7,000.
3,000 inner hair cells: The approximate number of cells lining the basilar membrane of the cochlea responsible for resolving sound frequencies.