The nervous system contains billions of neurons. Neurons transmit messages to other neurons by means of chemical substances called neurotransmitters. Neurons have a cell body or soma: Dendrites, which receive transmissions; and axons, which extend trunk like from the cell body. Chemicals called neurotransmitters travel across synapses to transmit messages to other neurons.
Many neurons have a fatty myelin sheath. These sheaths are missing at the nodes of Ranvier Neural impulses travel more rapidly along myelinated axons, where they can jump from node to node.
Sensory or afferent neurons transmit sensory messages to the central nervous system. Motor or efferent neurons conduct messages from the central nervous system that stimulate glands or cause muscles to contract.
Neural transmission is an electrochemical process. An electric charge is conducted along an axon through a process that allows sodium ions into the cell and then pumps them out. The neuron has a resting potential of -70 mill volts in relation to the body fluid outside the cell membrane and an action potential of +110 mill volts. The conduction of the neural impulse along the length of the neuron is what is meant by firing.
Neurons fire according to an all-or-none principle. Neurons may fire hundreds of times per second. Firing is first followed by an absolute refractory period. During which neurons do not fire in response to further stimulation. Then they undergo a relative refractory period, during which they will fire, but only in response to stronger-than-usual messages.
A synapse consists of an axon terminal from the transmitting neuron, a dendrite cleft of a receiving neuron, and a small, fluid-filled gap between them that is called the synaptic cleft. Excitatory synapses stimulate neurons to fire. Inhibitory neurons influence neurons in the direction of not firing. `
Neurotransmitters are contained within synaptic vesicles. These vesicles are found in the knobs at the... [continues]
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