Each protein family is smaller in flies or worms than in mice or people, but its functions are quite similar. Perhaps the most remarkable finding is that most of these proteins are common to many organisms-worms, insects, and mammals, including humans. In most cases, these are families of related molecules for example, researchers have identified at least fifteen semaphorins and at least nine ephrins. These signaling molecules include proteins with names such as netrin, semaphorin, and ephrin. The binding of particular signals with receptors tells the growth cone whether to move forward, stop, recoil, or change direction. The growth cones, in turn, bear molecules that serve as receptors for the environmental cues. Some molecules lie on the cells that growth cones contact, whereas others are released from sources found near the growth cone. Researchers have discovered many special molecules that help guide growth cones. Growth cones, enlargements on the axon’s tip, actively explore the environment as they seek out their precise destination. In the case of motor neurons, the axon may travel from the spinal cord all the way down to a foot muscle. Axons enable connections between neurons at considerable distances, sometimes at the opposite side of the brain, to develop. Neurons become interconnected through (1) the growth of dendrites-extensions of the cell body that receive signals from other neurons and (2) the growth of axons-extensions from the neuron that can carry signals to other neurons. After birth and beyond, such activities as listening to a voice, responding to a toy, and even the reaction evoked by the temperature in the room lead to more connections among neurons. Unlike induction, proliferation, and migration, which occur internally during fetal development, the next phases of brain development are increasingly dependent on interactions with the environment. The dendrites are covered with synapses formed by the ends of axons from other neurons. Synapses are the contact points where one neuron communicates with another. Dendrites extend from the neuron cell body and receive messages from other neurons. The axon extends from the cell body and often gives rise to many smaller branches before ending at nerve terminals. The cell body contains the nucleus and cytoplasm. Most neurons have a cell body, an axon, and dendrites. Neurons are cells within the nervous system that transmit information to other nerve cells, muscle, or gland cells. Once the neurons reach their final location, they must make the proper connections so that a particular function, such as vision or hearing, can emerge.
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