Neurons Structure and Communication

Neurons – What are they?  The basic building block of the nervous system -- a nerve cell  Neurons perform three basic tasks ReceiveReceive CarryCarry Pass on to the next neuronPass on to the next neuron The brain is made up of approximately ________ neurons. 100 billion electrochemical information

Kinds of Neurons Sensory Neurons (a.k.a. Afferent Neurons) carry incoming information from the sense receptors to the CNS. Motor Neurons (a.k.a. Efferent Neurons) carry outgoing information from the CNS to muscles and glands. Interneurons connect the two neurons (connects nerves, over 90%). Sensory NeuronInterneuron Neuron Motor Neuron

Sensory neurons take info. from the senses to the brain. Interneurons take messages from sensory neurons to other parts of the brain or to motor neurons. Motor neurons take info. from the brain to the rest of the body.

Structure of a Neuron

Structure of a Neuron – Dendrites

Structure of a Neuron – Soma

Structure of a Neuron – Axon

Structure of a Neuron – Neural Impulse

Structure of a Neuron – Myelin Sheath

Structure of a Neuron – Terminals

 Neurons come in a variety of shapes and sizes, but they all have the same basic structure

Neurons – magnified view

Neuron – another view

4 Make your own neuron! For more information and to test yourself, see h289/Biotutorials/1/part1.shtml h289/Biotutorials/1/part1.shtml

Neurons – How do they work?  Neurons send messages to other neurons – this is what keeps every part of our body in communication with every other part.  Neurons “fire” – send an impulse (message) down their length – or they don’t “fire”

Action Potentials  This “firing” of impulse messages is called the action potential.  An action potential is a brief electrical charge that travels down the axon of the neuron. Play Animation Watch “The Action Potential” movie at home: /action_potential_cartoon.swf /action_potential_cartoon.swf

What causes an impulse to fire or not fire?  When a neuron is at rest and capable of generating an action potential, it is called the resting potential  There are fluids inside and outside of the neuron, filled with electrically charged particles (ions)  When the neuron is at rest, there is a negative charge on the inside of the neuron compared to the outside.  At rest, the inside of the cell is at -70 millivolts

Neuron Communication Resting Potential

Action Potentials, cont.  Stimulation from inputs to dendrites causes the cell membrane to open briefly  Positively charged sodium ions flow in through the cell membrane  If resting potential rises above threshold, an action potential starts to travel from the cell body down the axon Threshold - Each neuron receives excitatory and inhibitory signals from many neurons. When the excitatory signals minus the inhibitory signals exceed a minimum intensity (threshold) the neuron fires an action potential.Threshold - Each neuron receives excitatory and inhibitory signals from many neurons. When the excitatory signals minus the inhibitory signals exceed a minimum intensity (threshold) the neuron fires an action potential.

Figure 2.3 Action potential Myers: Psychology, Ninth Edition Copyright © 2010 by Worth Publishers

Action Potential, cont.  The shift in electrical charge travels along the neuron  The intensity of an action potential remains the same throughout the length of the axon  Refractory period - The “recharging phase” when a neuron, after firing, cannot generate another action potential

For more information on action potentials, see er/ap.html er/ap.html For an interactive game/demo to help you learn about action potentials, see nimations/actionpotential_short.swf nimations/actionpotential_short.swf To help a mad scientist make a “mad, mad, mad neuron” in a cartoon game, see utah.edu/content/add iction/madneuron/ utah.edu/content/add iction/madneuron/

Neuron Communication All-or-None Principle  The principle that if a neuron fires it will always fire at the same intensity  A strong stimulus can trigger more neurons to fire, and to fire more often, but a ll action potentials are of the same strength and speed.  A neuron does NOT fire at 30%, 45% or 90% but at 100% each time it fires. Just like a gun, there is no “part-way” firing

 Consider the following concepts: DepolarizationDepolarization All-or-none principleAll-or-none principle Direction of impulseDirection of impulse Refractory periodRefractory period ThresholdThreshold Resting potentialResting potential Action potentialAction potential How is a neuron firing similar to a toilet flushing? How about a school bus? See “The School Bus Story” ppt. here: 1st.grandblanc.high.schoolf usion.us/modules/locker/fil es/group_files.phtml?gid= &parent= &sessionid=8b85e917de89 43dc49c835c286f82aff 1st.grandblanc.high.schoolf usion.us/modules/locker/fil es/group_files.phtml?gid= &parent= &sessionid=8b85e917de89 43dc49c835c286f82aff

What happens when an action potential reaches the end of the axon and enters the terminal buttons?

Action Potential Instructor’s Notes Copyright © Houghton Mifflin Company. All rights reserved. PowerPoint® 2000 or better with Flash® plug-in required to view animations. Right-click on animation for playback controls.

Synaptic Connections Elimination and creationElimination and creation Synaptic pruningSynaptic pruning Thought occurs through the firing of millions of neurons in unison. Our perceptions, thoughts, and actions depend on patterns of neural activity in interconnected neurons that fire together or sequentially – neural networks. The links in these networks are constantly changing, with synaptic pruning or the elimination of old or unused synapses playing a larger role than the creation of new synapses in the sculpting of neural networks. For example, the number of synapses in the human visual cortex begins to decline after the age of 1 year.

Synaptic Transmission – Lock and Key Mechanism Play Animation

Neurotransmitters Locks and Keys  Neurotransmitter molecules have specific shapes.  When NT binds to receptor, ions enter.  Receptor molecules have binding sites. Agonist – mimics neurotransmitter action Antagonist – opposes action of a neurotransmitter

Figure 2.6 Agonists and antagonists Myers: Psychology, Ninth Edition Copyright © 2010 by Worth Publishers Novocaine (the shot you get at the dentist) is also an antagonist – half of your face is numb because the novocaine is sitting in the receptor sites, so nothing happens to the post-synaptic neuron.

When a Neurotransmitter Binds: The Postsynaptic Potential  Voltage change at receptor site – postsynaptic potential (PSP) Changes the probability of the postsynaptic neuron firingChanges the probability of the postsynaptic neuron firing

Combining Within & Between Cell Communication  Excitatory Postsynaptic Potentials: when neurotransmitter binds at receptor and makes the neuron more positive because positive ions are allowed in (makes the neuron more likely to fire)  Inhibitory Postsynaptic Potentials: when neurotransmitter binds at receptor and makes the neuron more negative because negative ions are allowed in (makes the neuron less likely to fire)

So what, exactly, are neurotransmitters and what do they do?

Neurotransmitters A chemical messenger that travels across the synapse from one neuron to the next Can influence whether the second neuron will generate an action potential or not Each neuron has the capability to make 3-5 different neurotransmitters. Each branch of terminal buttons can usually make only one NT – it will depend on which dendrite it is connected to. 15 – 20 neurotransmitters known at present Most aspects of behavior are probably regulated by many neurotransmitters regulated by many neurotransmitters interacting interacting

NeurotransmitterFunctionExamples Acetylcholine (ACh) Released by motor neurons controlling skeletal muscles. Enables muscle action, attention, arousal, and memory With Alzheimer’s disease, ACh-producing neurons deteriorate. Some ACh receptors stimulated by nicotine. Dopamine (DA) Influences voluntary movement, learning, attention, and pleasurable emotions Excess dopamine receptor activity linked to schizophrenia. Decreased levels produce the decreased mobility and tremors of Parkinson’s disease. Cocaine and amphetamines elevate activity at DA synapses. Serotonin Affects mood, hunger, sleep, and arousal Undersupply linked to depression and obsessive- compulsive disorder. Prozac and some other antidepressant drugs raise serotonin levels. Select Neurotransmitters

How Neurotransmitters Influence Us Serotonin pathways are involved with mood regulation. Dopamine pathways are involved with diseases such as schizophrenia (too much DA) and Parkinson’s disease (not enough DA).

Select Neurotransmitters (cont.) NeurotransmitterFunctionExamples Norepinephrine (NE) Helps control alertness, mood, and arousal Undersupply can depress mood. Cocaine and amphetamines elevate activity at NE synapses GABA (gamma- aminobutyric acid) A major inhibitory neurotransmitter Undersupply linked to seizures, tremors, and insomnia. Valium and similar anti-anxiety drugs work at GABA synapses Endorphins Contribute to pain relief and perhaps to some pleasurable emotions (“Runner’s High”) Resemble opiate drugs in structure and effects (morphine is an agonist – mimics endorphins) Glutamate A major excitatory neurotransmitter; involved in memory Oversupply can overstimulate brain, producing migraines or seizures (which is why some people avoid MSG, monosodium glutamate, in food) For more information on neurotransmitters, see: euro/Default.aspx?pageid= euro/Default.aspx?pageid=1912

Neurons in the news Neurogenesis The production of new neurons from immature stem cells Stem cells Immature cells that renew themselves and have the potential to develop into mature cells 4

Stem-cell research Embryonic stem cells appear most useful yet federal funding faces resistance from some advocates. In 2001, President Bush signed executive order preventing creation of new cell lines. In March of 2009, President Obama lifted the ban against using federal money to fund stem cell research. Some scientists advocate for ban to be lifted because stem cell research shows promise in helping those suffering from diseases of the brain (Alzheimer’s, stroke, Parkinson’s, paralysis, etc.). 4