The human brain contains around ten billion neurons. Each cell forms connections with thousands of others, allowing the brain to store and transmit information by passing electrical signals within the cell and chemical signals (neurotransmitters) between cells in a complex network extending throughout the body. Millions of sensory neurons have receptors that convert stimuli from the environment (light, touch, sound, smell) into electrical signals that feed back to the brain. Other motor neurons send information from the brain to the muscles and hormonesecreting glands. Interneurons mediate the information between the sensory and motor neurons. Neurons have specialized extensions called dendrites and axons. Dendrites bring information to the cell body and axons take information away from the cell body. Extended axons run large distances and are surrounded by a specialized structure called a myelin sheath, made up of cells that wrap the axon in multiple layers of membrane. The myelin sheath acts as insulation to promote the passage of nerve impulses and isolate the nerve axon from external interference. Synapses are the junctions between the cells where chemical or electrical signals are transferred. Neurons are our longest and most long-lived cells. Corticospinal neurons (which connect the motor cortex to the spinal cord) and primary afferent neurons (which extend from the organs such as skin and gut into the spinal cord and up to the brain stem) can be several feet long. Neurons last a lifetime but numbers do decrease with age. Glia cells are closely associated with all neurons, having a protective and nurturing role. We cannot understand the complex functions of vision, consciousness, and memory at the single cell level but only in terms of interconnections between many billions of cells, and perhaps the most extreme emergent property in biology. Important insights have been provided by the continuing study of the nervous system in nematodes, fruit flies, and other simple organisms which will, in time, shed light on how our own brain functions.
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