Neurons and Action Potentials - AP Psychology

Myelin is a white, fatty compound that surrounds the axons of white matter neurons. Its purpose is to increase the speed of an action potential.

At rest, there is a high concentration of sodium (Na+) outside the neuron and a high concentration of potassium (K+) inside the neuron. During an action potential, the gated channels for sodium open and, because there is such a difference in concentration, the sodium rushes into the axon. This makes the axon much more positive in charge. This positivity propagates along the axon until it reaches the end of the axon, where it triggers release of neurotransmitters into the synapse.

The depolarization of the neural axon during an action potential is driven by an influx of sodium ions, entering through voltage-gated sodium channels. Following this stage, voltage-gated potassium channels are stimulated, allowing potassium ions to exit the axon and causing hyperpolarization. The sodium-potassium pump then restores the ions to their original positions in preparation for the next action potential, known as repolarization.

Glial cells are located in the nervous system, and serve as support and protection for the neurons. Schwann cells, oligodendrocytes, astrocytes, and ependymal cells are all examples of neuroglia.

Neurotransmitters are chemical substances that travel across synapses in the nervous system. Acetylcholine, dopamine, epinephrine, and GABA are all widely-studied neurotransmitters. Glucose, however, is a monosaccharide used for energy in the body. It serves no purpose as a neurotransmitter.

After crossing the synapse, neurotransmitter molecules bind to receptors on the postsynaptic neuron, initiating an excitatory signal (EPSP). The signal then travels through the dendrites to the cell body, where it becomes an action potential based on the degree of stimulation from other EPSP signals. After traveling through the cell body and down the axon hillock, the signal is sent out by the axon to the axon terminal, or synaptic terminal. There, synaptic vesicles containing neurotransmitters are released into the synaptic cleft (the space between two neurons). Synaptic vesicles fuse with the membrane at the axon and release neurotransmitter into the synaptic cleft. The neurotransmitters diffuse toward the postsynaptic neuron and bind to receptors to begin the process again. Once the signal reaches an effector organ, the neurotransmitters can elicit their ultimate effect.

Agonists are chemicals that mimic the action of a neurotransmitter. Agonists bind to the same receptor sites as neurotransmitters, but cause their own unique biological responses. Agonists activate the receptors to which they bind.

Afferent neurons, also known as sensory neurons, are the neurons that relay information to the central nervous system from the sensory organs. Efferent neurons are the motor neurons that carry the nerve impulses away from the central nervous system to the effectors. Interneurons are the neurons that transmit impulses between other neurons.

Efferent neurons are responsible for relaying information from the central nervous system to the muscles or glands. These signals allow for movement.

Cell wall is a cell organelle found in plants, bacteria and archea; it is not found in neurons. The soma is the cells body of the neuron, this is where the nucleus contained but the dendrites and axon are not part of the soma. Dendrites are short branched extensions of a neuron where impulses are received in the synapses and transmitted to the soma. Axons are the long threadlike part of the neuron that conduct the impulses from the soma to other cells.

Glial cells are specialized cells found in the central nervous system. Myelin sheaths are made up of glial cells that insulate the axon of a neuron.

The Myelin Sheath is a fatty substance that covers the axon and helps speed up impulses. A neuron is a nerve cell. Dendrites are the message receiving part of a neuron, and axons are the message sending part. A synapse is the gap between the terminal buttons of the axon of one neuron and the dendrites of another neuron. Soma is a term synonymous with cell body.

Neurons fire according to the All-or-Nothing Principle, meaning that they will either fire completely or not at all. There is no in-between or half-fire.

The time frame when a neuron cannot fire because it has just fired is called the refractory period.

Action potentials travel down the axon to the axon terminal. When an action potential arrives in the axon terminal it signals the synaptic vesicles to move toward the cell membrane. The synaptic vesicle fuses with the cell membrane and releases neurotransmitter.

Reuptake is a term used to describe the process of a neuron absorbing the remaining neurotransmitter back into the axon terminal for release. SSRIs used to treat depression, they function by inhibiting this reuptake process for serotonin.

Acetylcholine affects movement, learning, memory and REM sleep. It has an excitatory affect on skeletal muscle fiber and an inhibitory affect on heart muscle fibers.

Serotonin affects mood, sleep, appetite, impulsivity and aggression. If a person's level of serotonin is too high or too low it could affect mood and aggression. Depression has been directly linked to serotonin levels, and the most regularly prescribed Anti-Depressant drugs (SSRIs) regulat the serotonin re-uptake process, thus elevating the serotonin levels in patients.

The axons of one neuron release neurotransmitters which are received by the dendrites of the adjacent neuron.