Scientists constantly marvel at the intricacy of the human brain, “the cathedral of complexity” (Coveney & Highfield, 1995, p. 279). Here, I share some facts about the brain to make clear its complexity and, in turn, the difficulty that neuroscientists face in understanding it.
Size and Weight
The average human brain is a small, lightweight object, measuring about 73 in3 (1,200 cm3; Cosgrove, Masure, & Staley, 2007) and weighing about 3.3 lb (1.5 kg; Herculano-Houzel, 2009). But these summary statistics belie the significant variability in brain size and weight across individual people. For example, brain size differs by demographic attributes like age, race, sex, and social class (Rushton & Ankney, 1996).
The brain’s cortex is shaped in gyri (ridges) and sulci (fissures). The organization of gyri and sulci show significant variability between brains (Ono, Kubik, & Abernathey, 1990) and even between hemispheres in the same brain (Brett, Johnsrude, & Owen, 2002). Moreover, not all cortical regions show the same extent and type of variability (Thompson, Schwartz, Lin, Khan, & Toga, 1996).
The brain is composed of approximately 170 billion cells, about half of which are neurons; the rest are glial cells (Azevedo et al., 2009). Neurons communicate information with electrical and chemical signals (Purves et al., 2001). Glial cells serve many functions, from the production of neurotransmitters to maintenance of the blood-brain barrier (Oberheim, Wang, Goldman, & Nedergaard, 2006).
The brain’s neurons form a densely-connected network with connections in the order of 1 quadrillion (1015; Murry & Sturde, 1995). The strength and existence of many of these connections are not fixed (Sporns, Tononi, & Kötter, 2005); they change across development as a function of normal maturation and specific experiences (Holtmaat & Svoboda, 2009).
Neurotransmitters, of which more than 100 have been identified, help neurons communicate information (Purves et al., 2001). The amount and behavior of these chemicals varies across individual people, including systematic differences as a function of age (Rosene & Nicholson, 1999), sex (Zaidi et al., 2010), and mental health (Charney, Buxbaum, Sklar, & Nestler, 2013).