Human brain: Difference between revisions

Brain evolution, from the earliest [[shrew]]like mammals through [[primate]]s to [[hominid]]s, is marked by a steady increase in [[encephalization]], or the ratio of brain to body size. The human brain has been estimated to contain 50–100 billion (10¹¹) [[neuron]]s, of which about 10 billion (10¹⁰) are [[pyramidal cell|cortical pyramidal cells]]. These cells pass signals to each other via as many as 1000 trillion (10¹⁵) synaptic connections.<ref>{{Cite pmid|8527499}}</ref>

 

 

In spite of the fact that it is protected by the thick bones of the skull, suspended in [[cerebrospinal fluid]], and isolated from the bloodstream by the [[blood-brain barrier]], the delicate nature of the human brain makes it susceptible to many types of damage and disease. The most common forms of physical damage are [[closed head injuries]] such as a blow to the head, a [[stroke]], or poisoning by a wide variety of chemicals that can act as [[neurotoxin]]s. Infection of the brain is rare because of the barriers that protect it, but is very serious when it occurs. The human brain is also susceptible to degenerative disorders, such as [[Parkinson's disease]], [[multiple sclerosis]], and [[Alzheimer's disease]]. A number of psychiatric conditions, such as [[schizophrenia]] and [[major depressive disorder|depression]], are widely thought to be caused at least partially by brain dysfunctions, although the nature of such brain anomalies is not well understood.