The Neuron & Action Potential The basic building block of our nervous system and how it sends messages.
What do you think of the brain series we are watching? • It is interesting • I learned something new • It is ok • I haven’t been paying attention
What part of the brain is most responsible for fear? • Frontal Lobes • Cerebellum • Medulla • Amygdala
The Cell Body Contains the cell’s nucleus • round, centrally located structure • contains DNA • no role in neural signaling (Not involved in sending or receiving messages)
Dendrites • Information collectors or receivers • Receive inputs or signals from neighboring neurons • Inputs may number in thousands • If enough inputs the cell’s AXON may generate an electrical output
Dendritic Growth • Dendritic growth allows for more connections to other neurons • New connections are basis for learning • Studies show people with higher education have more dendritic connections than someone that is a high school dropout.
Which of the following states is considered the biggest “couch potato” state in the country this year? • Mississippi • Oklahoma • Georgia • Tennessee
Axon • 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
Which of the following receives messages from other neurons? • Axon • Cell Body • Dendrite • Nucleus
Myelin Sheath • White fatty casing on axon • Acts as an electrical insulator • When present, increases the speed of neural signals - like a paved highway • If this degenerates (dirt road), you have multiple sclerosis and can’t control your muscles.
Axon Terminal or Buttons Axon Terminals
Axon Terminal or Buttons • This is where the electrical impulse triggers synaptic transmission (release of neurotransmitters) to the dendrites of a receiving neuron. • Neurotransmitters are chemicals that enable neurons to “talk” to one another
Synapse or Synaptic Gap • The Synapse is the tiny space between neurons • The dendrites of one neuron do not touch the axon buttons of another
Which of the following is true of neurons? • Neurons generate electrical impulses. • The axon isn’t involved in sending messages • Neurons do touch one another • None of the above
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
Step 1: Threshold is Reached • Axon at Resting Potential - fluid inside the axon is mostly negatively (protein) charged with positive on the outside (sodium) • An impulse is triggered in the neuron’s dendrite when stimulated by pressure, heat, light or a chemical messenger from another neuron (stimulus threshold). • This 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 giving it a brief positive electrical charge the axon • The brief positive charge is action potential.
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. • This step is called the refractory period and the axon cannot fire again until it returns to resting 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.
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.