Step 1: The Neuron

Neurons are the simplest functional unit of the nervous system. Understanding how these cells signal one another is the basis of understanding the nervous system as a whole. Shown below is a picture of a generic neuron. However, neurons are one of the most morphologically diverse cells in an organism. Click here to view a few of the many morphologies of neurons.

Neurons are polarized, one end is functionally and morphologically distinct from the other end. Input (arriving from either the environment or other cells) is received through the dendrites. The input (inhibitory or excitatory) is changed into electrochemical signals which travel toward the cell body. Out of the cell body extends a long process called an axon. At the junction of the axon and the cell body is the axon hillock. This is where all input signals are summed and if threshold is reached, an action potential (AP) is generated. The AP travels down the axon very quickly until it reaches the axon terminal, where the neuron sends output to neighboring cells, i.e. other neurons, muscle cells, blood vessels, etc. The output is usually chemical, i.e. via neurotransmitters. The interface between two neurons where one is sending info and the other is receiving this info is called a synapse. In most nervous systems, electrochemical signals travel in only one direction-from dendrites to axon terminal. Which group of organisms is the exception to this rule? Note APs only occur in axons, the electrochemical signals traveling in the dendrites to the cell body are known as electrotonic potentials.

Myelin is a feature of most vertebrate neurons and some unique invertebrate neurons (e.g., in the Earthworm) that allows the axon to conduct APs (notice the axon is the only portion of the neuron that has myelin) Myelin is an insulatory sheath made primarily of cell membranes (membranes from a special cell, the Schwann cell, found in nervous systems that wrap around axons and form myelin sheaths). The Schwann cell wraps around and around small sections of the axon forming short (~1 mm long) segments of the axon that are covered in a myelin sheath. These myelin sheaths are found at regular intervals along the entire length of the axon, leaving only short sections bare. These bare, non-myelinated sections are called Nodes of Ranvier. Myelin dramatically increases the resistance of the membrane (one of the three passive properties of the membrane) so that the electrochemical signal: the AP does not degrade significantly as it travels similar to the plastic coating on wires that prevents electricity from "escaping", myelin insulates the axon. Most invertebrates do not have myelin. What adaptation has evolved in invertebrates instead of increasing the resistance of the axon membrane with myelin?

Figure modified from "Essentials of Neural Science and Behavior", Kandel, Schwartz, and Jessell. Appleton and Lange, 1995.