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Area of the human body surface innervated by each spinal nerve. Even mammals, including humans, show the segmented bilaterian body plan at the level of the nervous system. The spinal cord contains a series of segmental ganglia, each giving rise to motor and sensory nerves that innervate a portion of the body surface and underlying musculature ...
[1] [2] In Ancient Greece , interest in the brain began with the work of Alcmaeon , who appeared to have dissected the eye and related the brain to vision. He also suggested that the brain, not the heart, was the organ that ruled the body (what Stoics would call the hegemonikon ) and that the senses were dependent on the brain.
LD 350-1 is now considered the earliest known specimen of the genus Homo, dating to 2.75–2.8 Ma, found in the Ledi-Geraru site in the Afar Region of Ethiopia. It is currently unassigned to a species, and it is unclear if it represents the ancestor to H. habilis and H. rudolfensis , which are estimated to have evolved around 2.4 Ma.
Many neurons migrating along the anterior-posterior axis of the body use existing axon tracts to migrate along in a process called axophilic migration. [18] An example of this mode of migration is in GnRH-expressing neurons, which make a long journey from their birthplace in the nose, through the forebrain, and into the hypothalamus. [19]
Progressing along the human ancestral timeline, brain size continues to steadily increase (see Homininae) when moving into the era of Homo. For example, Homo habilis, living 2.4 million to 1.4 million years ago and argued to be the first Homo species based on a host of characteristics, had a cranial capacity of around 600 cm 3. [57]
The grain-of-sand-sized placozoan has oceans of information
Many neurons migrating along the anterior-posterior axis of the body use existing axon tracts to migrate along; this is called axophilic migration. An example of this mode of migration is in GnRH-expressing neurons , which make a long journey from their birthplace in the nose, through the forebrain, and into the hypothalamus. [ 29 ]
In a human, there are an estimated 10–20 billion neurons in the cerebral cortex and 55–70 billion neurons in the cerebellum. [64] By contrast, the nematode worm Caenorhabditis elegans has just 302 neurons, making it an ideal model organism as scientists have been able to map all of its neurons.