Neurons and Nervous System

Study Notes

Most synapses occur between an axon terminal of one neuron and a dendrite or the cell body of a second neuron.
A neuron that conducts a signal toward a synapse is called a presynaptic neuron.
A neuron conducting signals away from a synapse is a postsynaptic neuron.
A postsynaptic neuron may have thousands of synaptic junctions on the surface of its dendrites and cell body.

Neurons account for only about 10% of the cells in the central nervous system, the remainder are glial cells.
Glial cells surround the soma, axon, and dendrites of neurons and provide them with physical and metabolic support.
One type of glial cell, the oligodendrocyte, forms the myelin covering of CNS axons.
A second type of glial cell, the astrocyte, helps regulate the composition of the extracellular fluid in the central nervous system.

Afferent neurons have both the cell body and the long axon outside the central nervous system, and only a part of the central process enters the brain or spinal cord.
Efferent neurons have the cell bodies and dendrites within the central nervous system, and the axons extend out to the periphery.
Interneurons lie entirely within the central nervous system and account for over 99% of all neurons.

The anatomically specialized junction between two neurons where one neuron alters the electrical and chemical activity of another is called a synapse.
At most synapses, the signal is transmitted from one neuron to another by neurotransmitters.
Neurotransmitters released from one neuron alter the receiving neuron by binding with specific protein receptors on the membrane.

Neurons operate by generating electrical signals that move from one part of the cell to another part of the same cell or to neighboring cells.
In most neurons, the electrical signal causes the release of chemical messengers—neurotransmitters—to communicate with other cells.
Neurons serve as integrators because their output reflects the balance of inputs they receive from thousands or hundreds of other neurons.

Neurons occur in a wide variety of sizes and shapes, but all share features that allow cell-to-cell communication.
Long extensions, or processes, connect neurons to each other and perform the neuron’s input and output functions.
Most neurons contain a cell body and two types of processes—dendrites and axons.
The cell body (or soma) contains the nucleus and ribosomes and thus has the genetic information and machinery necessary for protein synthesis.
Dendrites are a series of highly branched outgrowths of the cell body that receive most of the inputs from other neurons.
The branching dendrites increase the cell’s surface area, allowing neurons to receive signals from many other neurons.