The Neuron & Action Potential Module 9: Biological Psychology & Neurotransmission The basic building block of our nervous system and how it sends messages.

Cell Body & Nucleus Key words: Neuron; structures of neurons Graphic From Image and lecture gallery

The Cell Body Contains the cell’s nucleus round, centrally located structure contains DNA controls protein manufacturing directs metabolism no role in neural signaling Graphic from Hockenbury slides Key words: Cell body; soma; cell nucleus Interesting facts: The DNA in the nucleus of the cell has lost its ability to divide. therefore, when a neuron dies,for the most part, the adult brain cannot simply grow new neurons. (Note there are a few exceptions to this rule.) The relative inability to grow new neurons leads to two interesting questions: Q1: How do brain tumors (cancer) occur? A: Unlike neurons, glial cells can divide and grow new cells throughout one's lifetime. Most brain tumors are limited to glial cells, not neurons. Q2: If a person cannot grow new neurons, how does the brain change in order to accommodate new learning? A: One mechanism by which the brain adapts to help you learn new information involves the structure on the next slide: the dendrites.

Dendrites Key words: Neuron; structures of neurons Graphic From Image and lecture gallery

Dendrites RECEIVE info/signals from other neurons Inputs/receptor sites may number in thousands If enough inputs received it may cause the cell’s axon to generate an electrical impulse (action potential)

Dendritic Growth As you learn, new dendrites can grow, creating more connections to other neurons New connections are basis for learning People with higher education have more dendritic connections than a high school dropout.

Axon Axon Key words: Neuron; structures of neurons Graphic From Image and lecture gallery

Axon SENDS an electrical message when signaled by the dendrites Where all the action is Action Potential takes place electrical charge is sent down the axon. One axon per cell, 2 distinct parts tube-like structure branches at end (axon terminals) that branch out to dendrites of other cells Hockenbury slides (Schulman) Key words: axon; action potentials Interesting facts: - The diameter of an axon may vary from approximately 1mm-20mm. - An axon may travel long distances to reach it's destination (longest axon is approximately 3 feet in humans and 10 feet in giraffes).

Myelin Sheath & Nodes of Ranvier Key words: Neuron; structures of neurons Graphic From Image and lecture gallery

Myelin Sheath White fatty casing on axon Acts as an electrical insulator Not present on all cells When present, increases the speed of neural signals down the axon allowing the action potential to “jump” to each Node of Ranvier - like a paved highway (see video below to compare mylenated axons vs. non-mylenated axons If this degenerates (dirt road), you have multiple sclerosis and can’t control your muscles. Graphic from Hockenbury slides Key words: myelin sheath; action potentials; axon Interesting facts: - The myelin sheath is NOT a part of the axon. The myelin sheath is actually formed of glial cells (oligodendricytes and Schwann cells) that wrap around the axon. - You may have often heard the brain referred to as either white matter or gray matter. The myelin sheath appears white in nature. Hence, the term white matter refers to areas of the brain that are myelinated. Gray matter refers to areas of the brain that are unmyelinated. - When you accidentally cut yourself, you often visually notice that you've cut yourself before you actually feel any pain from the cut. The reason for this is that visual information uses myelinated axons; whereas, pain information uses unmyelinated axons. - The loss of myelin is a significant factor in the disease multiple sclerosis (MS). When myelin is lost, the high-speed transmission of information is slowed down or blocked completely, which could lead the person with the inability to walk, write or speak.

Axon Terminal or Buttons Axon Terminals Key words: Neuron; structures of neurons Graphic From Image and lecture gallery

Axon Terminal or Buttons End of Axon where the electrical impulse triggers synaptic transmission sending message to the dendrites of a receiving neuron. Let’s Review with this Quick Video.

One Direction Communication

Glial Cells They are the janitors of the neuron. Support cells that provide neurons with structural support and nutrition. They also remove cell wastes and enhance the speed of the neuron

How neurons send an electrical message Action Potential How neurons send an electrical message

How Neurons Communicate Neurons communicate by means of an electrical signal called the Action Potential Action Potentials are based on movements of ions between the outside and inside of the axon When an Action Potential occurs, a molecular message is sent to neighboring neurons Action Potential is an All or Nothing Process (like a gun firing)

Threshold: Triggering Action Potential When a neuron is resting = balance of excitatory & inhibitory signals. If one of these exceeds the other stimulus threshold is reached Triggering the neuron to transmit an electrical impulse down its axon (action potential) How do you feel something that is intense? More neurons fire, the intensity of their electric impulse always stays the same. Lou Gehrig’s Disease - too many inhibitory stimuli cause the muscles to freeze up. Parkinson’s Disease - too many excitatory stimuli cause the muscles to move without control.

Steps to Action Potential

Resting Potential Axon at Resting Potential - fluid inside the axon is mostly negatively charged with positive on the outside (polarized) Graphic, Hockenbury slides At rest, the inside of the cell is at -70 microvolts With inputs to dendrites inside becomes more positive If resting potential rises above threshold, an action potential starts to travel from cell body down the axon Figure shows resting axon being approached by an AP

Step 1: Threshold is Reached An impulse is triggered in the neuron’s dendrite when stimulated by: Pressure Heat Light NT - chemical messenger from another neuron (synaptic transmission) Minimal level of stimulation that causes the axon to fire is called Stimulus Threshold

Step 2: Action Potential Begins When neuron fires, its axon membrane is selectively permeable. Gates in the axon called ion channels open allowing positive sodium ions to enter the axon while potassium ions leave causing a brief positive electrical charge in the axon (depolarized). The brief positive charge is action potential.

Depolarization Ahead of AP Graphic from Hockenbury slides AP opens cell membrane to allow sodium (Na+) in Inside of cell rapidly becomes more positive than outside This depolarization travels down the axon

Step 3: Refractory Period As the next gates open allowing positive sodium ions in, the previous gates close and begin to pump the positively charged sodium ions out of the axon and potassium ions back inside. (repolarized). This step is called the refractory period and the axon cannot fire again until it returns to resting potential (negative polarized state). The entire process is like falling dominoes all the way down the axon except these dominoes can set themselves back up as soon as they fall over. Why do you think the axon has to set itself back to a resting state so quickly (3 milliseconds)? So the neuron can fire again and send another message immediately after the last one.

Repolarization follows Graphic from Hockenbury slides After depolarization potassium (K+) moves out restoring the inside to a negative voltage This is called repolarization The rapid depolarization and repolarization produce a pattern called a spike discharge

Finally, Hyperpolarization Graphic from Hockenbury slides Repolarization leads to a voltage below the resting potential, called hyperpolarization Now neuron cannot produce a new action potential It must return a resting state This is the refractory period

Action Potential Within a Neuron

Threshold is reached +Na ions enter beginning of axon this triggers the next Na gates to open. As they open & allow in Na+, previous gates begin pumping the Na+ out. Before the action potential has reached the end, the beginning of the axon is back at resting potential & ready for another firing.

A Review Action Potential

DAILY DOUBLE

How can a toilet represent Action Potential? Full Toilet – Resting Potential Push Flush Lever – Threshold Stimulus triggering Action Potential. Toilet Refilling/Can’t Flush – Repolarization/Refractory Period Sewer Pipes – One-way communication like action potential only goes from dendrite end to axon terminal end.