Nervous System

The dendrites an cell body act as the receptive area of the neurn. In this region, receptor proteins for neurotransmitters and gated channels are located. EPSPs (excitatory postsynaptic potentials) will stimulate the production of action potentials. If depolarization is at or above threshold when it reaches the initial segment of the axon, EPSP will produce action potentials. Gradation in strength of EPSP, above threshold, determine frequency at which action potentials will be produced and where the impulse will be conducted. No loss of amplitude occurs as it travels toward the axon terminals. The axon hillock is an elevation on the cell body. This is where the axon arises.
The first voltage - regulated gated channel is located at the axon hillock.

Dendrites and cell bodies: Opens ligand gated channels. Inward diffusion of Na+ causes EPSP. Localized conduction of EPSP.
Axon hillock: Opens voltage gated Na+ and then K+ channels.
Axon: Conduction of action potentials.

Action Potentials

axonhillock.jpg (Source for picture)

Within the presynaptic neuron ending are neurotransmitters molecules. They are contained in synaptic vesicles. The vesicle membrane fuses with the axon membrane in the process called exocytosis. A pore forms and the molecules diffuse into the synaptic cleft. Then the neurotransmitter is released in multiples of an amount contained in one vesicle. The number of vesicles that undergo exocytosis will depend on the number frequency of action potentials. These action potentials are produced at the presynaptic axon endings. The release of neurotransmitter occurs quite rapidly due to the action potentials that arrive at the end of the axon. The arrival of action potentials at the axon terminal opens voltage regulated calcium channels. Inward diffusion of Ca+ triggers diffusion of the synaptic vesicle with the axon membrane. It also triggers release of neurotransmitter through exocytosis. The diffusion of Ca+ activates regulatory proteins and the release of neurotransmitters. Cells have numerous Na+ and K+ pumps that are constantly active.
Axon terminals: Action potentials conducted b axon. Opens voltage gated Ca2+ channels. Release of excitatory neurotransmitter.


180px-Synapse_diag1.png (Source for picture)

350px-Action_potential_vert.png (Source for picture)

This picture illustrates the different threshold levels and membrane voltage.

Some neuroransmitters like glycine and GABA hyperpolarize the postsynaptic membrane. This means they make the inside of the membrane more negative. This drives the membrane poential away from the threshold depolarization required for stimulation of action potentials.This inhibits the activity. For example, resting threshold of -70 m V might become -85 m V. Depolarizations must be stronger to reach the threshold required to produce action potentials at the axon hillock.

Neurons in the adult brain generally cannot undergo cell division and usually cannot be replaced after being lost. Sometimes through the use of stem cell research these cells can sometimes be reproduced. In the nursing profession, it is very to understand the nervous system and how it works. A nurse works with many patients who are diagnosed with neurodegenerative diseases such as Alzheimer's, depression or Parkinson's disease.

Fox, Stuart Ira. Human Physiology, Tenth Edition