The Electrochemical Impulse Action Potential Threshold Level All-or-None Response

What is the difference between electrical and neural transmission? Current speed  Faster in electrical wire because cytoplasm resists flow of ions Current diminishes on electrical wire with distance but not on a neuron Neurons rely on cellular energy while electrical current must be pushed by some other force.

How are nerve impulses created? See website

How do nerve cell membranes become charged? Neurons have a rich supply o f positive and negative ions both inside and outside the cell Negative ions do little to create a charged membrane  Mainly large ions that cannot cross the membrane and so stay inside the cell Unequal concentration of positive ions across the nerve cell membrane

Resting State Highly concentrated potassium ions inside the nerve cells have a tendency to diffuse outside the nerve cells Similarly, high concentrated sodium ions outside the nerve cell tend to diffuse into the cell Positively charged ions move both in and out  Unequal  Resting membrane is 50x more permeable to potassium than to sodium  Therefore, more potassium ions diffuse out than sodium diffuse in (2 to 1)

Movement of Action Potential Threshold = minimum electrical stimulus to cause neuron to “fire” Threshold stimulus produces nerve impulses of identical speed and intensity Threshold level may vary for individual neurons Intensity of stimulus does not vary intensity or speed of impulse along axon

All-of-None Response Increasing the intensity of a stimulus does not change the intensity or speed of response  Neurons either fire at the maximum or not at all The frequency of impulses affects the brains interpretation and response  The greater the frequency of stimulus the brain receives, the more intense the response  Ex) the more frequent the stimulus the brain receives the hotter the object “feels” (more intense than warm) and the faster the reaction response

SYNAPSE Small spaces between the end of one neuron (end brush) and the beginning of another (dendrite) Rarely involves just 2 neurons (one neuron branches out and can “touch” many others)

Neurotransmitters Small vesicles in end plate of axon contain neurotransmitters (chemicals) Neurotransmitters are released when electrical stimulus arrives through axon Nerve transmission is slower here due to diffusion across gap (synapse) The greater the number of synapses the slower the speed over a specified distance Thus, reflex arcs are quicker than thought processes (problem solving) since fewer synapses must be crossed

Role of the Neurotransmitter Ex) Acetylcholine  can excite postsynaptic neuron by opening sodium channels (Na+)  neuron depolarizes Problem – How do the neurons keep from firing constantly?

Not all transmissions are excitatory Other neurotransmitters inhibit chemicals: Ex) Making the postsynaptic membrane more permeable to K+ (open more K+ gates) hyperpolarization – making neurons resting membrane even more negative (-) This prevents the postsynaptic membrane from being activated Ex) Cholinesterase an enzyme that follows release of acetylcholine and destroys it sodium channels close Neuron begins to recover (repolarize)

Summation More than one neuron fires at the same time to produce action potential in a subsequent neuron Ex) A 3rd neuron requires the sum of the first two excitatory neurons to activate