Neurons

The building blocks of our body ' s neural information system are neurons. Each neuron is composed of dendrites, a cell body, axon with myelin sheath, and vesicles with neurotransmitters. The neuron receives information from sense receptors or from chemical messages from neighboring neurons via its dendrites, which pass the information to the cell body. The cell body fires an impulse, called the action potential, down the axon. The impulse is protected and sped along its way by the myelin sheath, which is a layer of fatty tissue that surrounds the axon. Each neuron receives signals from many other neurons. Some signals are excitatory, and some are inhibitory. If the excitatory signals minus the inhibitory signals exceed a certain level, called the threshold, then the signals trigger an action potential. When the action potential reaches the axon terminals, called vesicles, it triggers the release of chemical messengers called neurotransmitters. The neurotransmitters cross the gap between the neurons, called the synapse, and bind to receptor sites on receiving neurons. The neurotransmitters unlock tiny channels in the receiving neuron for just an instant, which allows electrically charged atom to enter it thereby exciting or inhibiting

readiness to fire. The sending neuron normally reabsorbs excess neurotransmitters in

process called reuptake. There are various drugs that affect the communications of neurons, by either exciting or inhibiting the neurons ' firing. Agonists excite by mimicking a neurotransmitter or blocking its reuptake. Antagonists inhibit by blocking a neurotransmitter ' s release or its place in a receptor site. The nervous system is comprised of the central nervous system and the peripheral nervous system. The central nervous system is comprised of the brain and the spinal cord. The peripheral nervous system links the central nervous system with the body ' s sense receptors, muscles, and glands. Information travels in the nervous system via three types of neurons. Sensory neurons send information from the senses to the brain and spinal cord, which process the information. Interneurons enable the nervous systems internal communications. Motor neurons enable the central nervous system to send instructions to the tissues. Glial cells support these neurons by guiding neural connections, providing nutrients, and mopping

debris. The peripheral nervous system has two components