The neuron is a specialized electrically excitable brain cell. We are born with most of our 100 billion neurons . This cell is able to create, send, and receive messages. There are many different kinds of neurons. Sensory neurons are special structures that detect changes on or inside the body. Motor neurons carry messages to muscles. Interneurons sum up information from sensory neurons before they communicate messages to motor neurons.
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The neuron is enclosed in a cell membrane and is composed of three main parts; a body or soma, axon and dendrites. Cell growth occurs from increasing the size of dendrites and forming new connections in response to stimulation.
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These connections are affected by increased and decreased activity. Pathways that are used regularly are "hard wired" while less used pathways are lost or "pruned". Practice makes perfect; use it or lose it.
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A cell can have tens of thousands of dendrites branching off the soma like branched on a tree, but a neuron has only one axon that branches out at its end. Dendrites receive electrical impulses from other neurons and transmit the impulse through the axon to the next cell.
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Sometimes, however, messages are not sent. A neuron is often bombarded by thousands of inputs; both excitatory and inhibitory actions. The neuron is able to integrate these actions, sum them up and respond to the greater of the actions. If the dendrite receives enough messages that are excitatory in nature, it fires an impulse, called an action potential through the axon. A fatty substance called the myelin sheath wraps around and insulates the axon. The thickness of the axon and its myelin wrapping determines the speed with which information travels in it - the thicker the nerve fiber, the faster information travels in it. The myelin sheath is absent at regular intervals along the axon. These exposed parts of the axon are called, Nodes of Ranvier. The journey is enhanced by electrical boosts produced along the axon by the nodes. Electrical impulses move down the axon, often at great speeds. Some axons conduct action potentials at 100 meters per second, while others conduct at less than a meter per second. The electrically charged journey propels the neurotransmitters to their final destination, the axon terminal, where they can be released into the synapse.
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Synapse
The synapse is the tiny space between the delivering neuron and the receiving neuron. This is the space that the neurotransmitters cross. Neurotransmitters are chemical molecules that allow signals to pass from neuron to neuron. Neurotransmitter molecules are made in the soma. These molecules are transported along the axon by a tube like conveyor belt and packaged into tiny little balloon like packages called synaptic vesicles. When these little synaptic packages come to the end of the axon, they fuse to membrane. When an action potential arrives, they release their contents, the neurotransmitter molecule, into the synapse. After the neurotransmitters are released into the synapse, or tiny space between the two neurons, they bind to the receptors of the receiving neuron. When this binding occurs, it creates a change in potential in the receiving neuron that may allow for it to pass on the message to the next neuron. This can be quite complicating because of the many connections involved within a single neuron, but the action is determined by the stronger of the forces at the time- inhibitory or excitatory.
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Neural pathway form and change because of increased or decreased need for these electrical connections and modifications. The adaptability to change structurally is called brain plasticity. At birth, each neuron in the cerebral cortex has approximately 2,500 synapses. By the time, an infant is two or three years old, the number of synapses is approximately 15,000 synapses per neuron.
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This amount is about twice that of the average adult brain. As we develop, unused connections are deleted through a process called synaptic pruning. When neuronal synapses activate often, they make more connections and they become stronger. The ability of the brain to change in this way is called plasticity.
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