11.2 Electrochemical Impulse December 17, 2013

Tuesday, 17 December, 13

• at REST, the neuron has an imbalance of positive (+) and negative (-) charges on each side of the membrane• resting membrane electrical potential is normally -70 mV

Nerve Impulses

outside

inside

+ + + + + + + + + + + + + + +

- - - - - - - - - - - - - - - REST

Tuesday, 17 December, 13

• the charged membrane is said to be polarized (imbalanced positive and negative charges)• this imbalance is caused by positive charges being pumped out of the neuron membrane by a sodium-potassium pump

Nerve Impulses

• neurons have many pumps that pump 3 Na+ out and 2 K+ ions in causing a slight positive charge on the outside

Tuesday, 17 December, 13

Resting membrane: more positive charges are pumped out leaving

the inside negative

Tuesday, 17 December, 13

Electrical disturbance opens Na+ gates and sodium ions rush into

the cell: depolarization.

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Na+ gates to close again and K+ gates to open. K+ moves out.

Tuesday, 17 December, 13

Electrical disturbance moves along

membrane. Na+/K+ pump restores and maintains resting

potential.

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The amount of depolarization needed to send

the signal

Tuesday, 17 December, 13

• as soon as the depolarized wave has gone by, the Na+/K+ pump works to restore resting potential in that area, in a wave of repolarization

Movement of Action Potential+ + + + + + + + + + + + + + +

- - - - - - - - - - - - - - -

- - - - - - + + + + + + + + +

+ + + + + + - - - - - - - - - depolarized

+ + + - - - - - - + + + + + +

- - - + + + + + + - - - - - - depolarized

+ + + + + + + + + - - - - - -

- - - - - - - - - + + + + + +depolarized

action potential

Tuesday, 17 December, 13

• neurons cannot send another impulse until the membrane is polarized; this is called the refractory period (1-10 ms)

Nerve Impulses

Tuesday, 17 December, 13

• nerve cells respond to change in pH, pressure, chemicals and electric shock

• there is a minimum amount needed to trigger a response (threshold level)

• the all-or-nothing characteristic of the action potential means that the intensity of a stimulus is reflected in the frequency of action potentials rather than in their magnitudes

•also different neurons in the same nerve have different threshold levels (so while one may trigger at 40oC, 2 at 50oC and all at 70oC - allows for sensation of warm vs. hot)

Threshold Levels and the All-or-None Response

Tuesday, 17 December, 13

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/

animation__the_nerve_impulse.html

Tuesday, 17 December, 13

Synaptic Transmission

• synapses are the spaces between neighbouring neurons

• each neuron connects to many others

• end of the axon contains many small vesicles containing neurotransmitters which are released into the synapse by the electrical impulse

Tuesday, 17 December, 13

• neurotransmitters are released from the pre-synaptic neuron and diffuse across the synaptic cleft, depolarize the dendrites of the post-synaptic neuron and therefore create another action potential

synaptic cleft

pre-synaptic neuron

post-synaptic neuron

Synaptic Transmission

Tuesday, 17 December, 13

• diffusion slows transmission, so while reflex arcs (few synapses) are fast, our higher functions (problem-solving) have many synapses and are slower

Synaptic Transmission

Tuesday, 17 December, 13

• acetylcholine is the most common neurotransmitter: opens Na+ channels on post-synaptic neuron causing depolarization

• enzyme called cholinesterase is released by the post-synaptic neuron and destroys acetylcholine, allowing Na+ gates to close and repolarization to occur

• neurotransmitters (like acetylcholine) can also be inhibitory, increasing the negative resting potential (by opening K+ gates = hyperpolarized)...now more Na+ gates will have to be opened for depolarization to occur (need greater threshold)

Synaptic Transmission

Tuesday, 17 December, 13

• in many cases, for a neuron to fire it must receive excitatory neurotransmitters from at least 2 different neurons (process called summation)

• inhibitory neurotransmitters can also be released by other neurons

• there are many different neurotransmitters and they can have either excitatory or inhibitory effects, depending on location

• the interaction of excitatory/inhibitory neurotransmitters control the muscles involved in complex motions like throwing a ball

Neural Pathways

Tuesday, 17 December, 13

• inhibitory impulses in the CNS allow you to prioritize info and “tune out” less important info

• low levels of neurotransmitters are associated with Parkinson’s disease and Alzheimer’s

Neural Pathways

Tuesday, 17 December, 13