Neuroscience and Behavior Most information in this presentation is taken directly from UCCP content, unless otherwise noted.

Diagram of a Neuron Diagram taken from:

Definitions Neuron – Highly specialized cell that communicates information in electrical and chemical form; a nerve cell. Cell Body – Processes nutrients and provides energy for the neuron to function; contains the cell’s nucleus; also called the soma. Dendrites – Multiple short fibers that extend from the neuron’s cell body and receive information from other neurons or from sensory receptor cells. Axon – The long, fluid-filled tube that carries a neuron’s messages to other body areas. Myelin Sheath – A white, fatty covering wrapped around the axons of some neurons that increases their communication speed. Synapse – The point of communication between two neurons. Synaptic Gap – The tiny space between the axon terminal of one neuron and the dendrite of an adjoining neuron. (All definitions in this power point are from the Hockenbury text)

Definitions cont … Action Potential – A brief electrical impulse by which information is transmitted along the axon of a neuron. Stimulus Threshold – The minimum level of stimulation required to activate a particular neuron. Resting Potential – State in which a neuron is prepared to activate and communicate its message if it receives sufficient stimulation. All-Or-None Law – The principle that either a neuron is sufficiently stimulated and an action potential occurs or a neuron is not sufficiently stimulated and an action potential does not occur. Neurotransmitters – Chemical messengers manufactured by a neuron.

Communication Between Neurons While a nerve impulse is primarily electrical, communication between neurons is chemical. When a neuron fires, chemicals called neurotransmitters are released from the axon terminal buttons into the synaptic cleft, the space between two neurons over which messages pass. Neurotransmitters are chemicals that alter activity in neurons.

Communication Between Neurons After the presynaptic neuron releases the neurotransmitters into the synaptic cleft, they cross the cleft and attach themselves to receptor sites on the postsynaptic neuron, or receiving neuron. Receptors sites are areas on the cell membrane that are sensitive to neurotransmitters. Neurotransmitters and receptor sites work sort of like a lock and key. Each receptor site (lock) is designed to receive only one type of neurotransmitter (key). Once released, not all molecules of neurotransmitters find their way into receptor sites of other neurons. Neurotransmitter molecules that do not attach to receptor sites are either broken down or reabsorbed by the presynaptic neuron in a process called reuptake.

Communication Between Neurons Does the release of a neurotransmittor always trigger an action potential in the next neuron? Not necessarily. Some neurotransmittors act to excite receiving neurons, causing them to fire: Other neurotransmittors act to inhibit receiving neurons, preventing them from firing. If a receiving neuron receives several “exciting” messages close in time, the neuron will fire. However, if the receiving neuron also receives “inhibiting” messages, it may or may not fire. The sum of these “exciting” and “inhibiting” messages determine whether or not the receiving neuron will fire.

How Neurons Transmit Information Neural impulses are messages that travel along neurons. They travel somewhere between 2 (in non- myelinated neurons) and 225 miles per hour (in myelinated neurons). Messages can travel from your toe to your brain in about 1/50 of a second. Neurons carry information in one direction only: From dendrites to cell body to axon to terminal buttons. Messages are then transmitted from the terminals buttons to the dendrites or cell body of another neuron.

What makes a Neuron “fire”? Neural impulses travel by using an electrochemical process. Chemical changes take place within a neuron that cause an electric charge to be transmitted along the neuron. The conduction of the neural impulse along the length of a neuron is what is meant by “firing.” Think of a neuron as a tiny biological battery. Ions, electrically charged chemical molecules, are located in and around nerve cells. Some of the ions have a positive charge; others have a negative charge. The ions involved in firing a neuron are Sodium (Na +), Chloride (Cl-) and Potassium (K+). Different numbers of these ions exist inside and outside of the neuron. More chloride ions inside the neuron create an overall negative electrical charge of 70 millivolts (mv) inside the neuron relative to the outside. This state is called the resting potential and the neuron is polarized.

What makes a Neuron “fire”? Neurons, however, rarely get rest. They constantly receive messages from sensory sources or other neurons that alter its electrical charge. When these messages stimulate the neuron, the permeability of the cell membrane changes allowing Sodium ions (Na+) to enter the cell. The inside of the cell becomes more positive, or depolarized. If the electrical charge inside the cell changes enough, it reaches a threshold, or trigger point. A neuron’s threshold is around -50 mv. When the neuron reaches this threshold, the neuron fires a nerve impulse that sweeps down the axon and causes the terminal buttons to release a chemical called neurotransmittors. The firing of a neuron is called an action potential.

What makes a Neuron “fire”? An action potential is an all-or-nothing event; the neuron fires completely or doesn’t fire at all and each time it fires, the impulse is of the same strength. This is known as the all-or-none principle. To help illustrate this point, think of a row of dominoes that are set on end. Once you tip the first domino, all of the other dominos topple in turn until the last domino falls. When an action potential is triggered, the nerve impulse travels the complete length of the axon until it reaches the terminal buttons. Once the neuron fires, it enters a refractory period, when the neuron cannot fire. The refractory period can be thought of as the time it takes to place the dominoes upright again before they are again “ready” to be toppled. During the refractory period, potassium ions (K+) flow out of the axon restoring the neuron’s negative charge and preparing the neuron to fire again. The sodium ions (Na+) that entered the axon during the action potential are pumped back out of the axon at a slower rate, while the Potassiuim ions (K+) that exited are pumped back in. This results in restoring the original resting potential.