The Nervous SystemPart II

The electrochemical impulse

• In 1939, by placing tiny electrodes across the membrane of the giant axon of a squid the researchers Cole and Curtis measured an electrical potential across the membrane every time the nerve was stimulated.

• At rest, the potential was about -70 mV (this is called the resting potential); however, when the nerve was stimulated, the potential reversed to + 40 mV.

• This reversal of potential is called the action potential, which lasted only a few milliseconds before the potential returned to - 70 mV.

No, not a giant squid, a giant axon from a squid…

20, 000 Leagues Under The Sea (Disney, 1954)

The electrochemical impulse

The electrochemical impulse

• Under resting conditions, the membrane of a neuron is charged and is called a polarized membrane.

• Why? The polarization of the membrane is the result of the uneven concentration of positive ions across the neuronal membrane.

• There are more sodium ions on the outside of the membrane, and less potassium ions on the inside of the membrane. As we will see later, this is mostly due to the action of sodium-potassium pumps located in the membrane.

• At rest, the overall charge difference is more positive outside, and less positive inside.

The electrochemical impulse

Na+

K+Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

K+ K+K+

K+K+K+

Under resting conditions, the neuronal membrane is polarized because of the unequal distribution of positively charged ions inside and outside the neuron.

inside

outside

Na+Na+Na+

K+K+

The electrical basis of the action potential

• Upon excitation, the neuronal membrane becomes highly permeable to sodium.

• The influx of sodium is what leads to the reversal in potential (also referred to as depolarization). This event occurs very rapidly (less than a few milliseconds) and in a localized area on the neuronal membrane.

The electrical basis of the action potential

• After depolarization occurs, sodium-potassium pumps begin work to restore the resting membrane condition by transporting three sodium ions out of the neuron while transporting two potassium ions out, using ATP as fuel.

• This process is called repolarization.

The refractory period

• Nerves conducting an impulse cannot be activated until the condition of the resting membrane is restored – the nerve must repolarize before the next action potential can be conducted.

• The time required for the neuron to become repolarized is called the refractory period, and it lasts about 1 to 10 milliseconds.

Movement of the action potential

• A nerve impulse can be thought of as wave of depolarization that moves in one direction along the neuronal membrane of the axon.

• The initiation point of the action potential enters a refractory period, so the impulse cannot be conducted “backwards”.

• The wave of depolarization is followed by a wave of repolarization as the resting membrane potential is restored.

Movement of the action potential

direction of nerve impulse

inside

outside

refractory area

action potential

resting membrane

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

repolarized area

depolarized area area at rest

Movement of the action potential

http://highered.mcgraw-hill.com/sites/0072943696/student_view0/chapter8/animation__action_potential_propagation_in_an_unmyelinated_axon__quiz_2_.html

Threshold levels and theall-or-none response

• A potential stimulus must be above a critical value to produce a response. This is known as the threshold level.

• Increasing the intensity of the stimulus above the threshold does not produce an increased response – the intensity of the nerve impulse and the speed of transmission remains the same.

• Neurons either fire maximally or do not fire at all. This is referred to as an all-or-none reponse.

Threshold levels and theall-or-none response

• How is possible to detect the intensity of stimuli if neurons respond maximally or not at all?

• (i) Information is encoded in the frequency of neuronal firing.

• (ii) Because neurons have different threshold levels, a particular stimulus may only cause a single neuron to fire, whereas a more intense stimulus can recruit more neurons to fire at nearly the same time.

• Your brain can sort all of this out!!

Synaptic transmission

• Small gaps between neurons (or between neurons and effectors) are called synapses.

• Vesicles containing chemicals called neurotransmitters are located at axon terminals.

• The electrical signal (the action potential) in the presynaptic neuron leads to the rapid release of neurotransmitter. The neurotransmitter diffuses across the synaptic cleft, causing localized depolarization in the dendrites of the postsynaptic neuron.

Synaptic transmission

Synaptic transmission

• Localized depolarization in the dendrites may or may not lead to the propagation of an action potential in the postsynaptic neuron (more about this later).

• Synaptic transmission can be excitatory (just as described above) or inhibitory, where the postsynaptic becomes hyperpolarized in response to the neurotransmitter.

A very brief introduction to neural networks

Input Output (D)

A No response

B No response

C No response

A + B RESPONSE

A + B + C No response

Excitatory

Inhibitory

The production of an action potential in neuron D requires the sum of two excitatory inputs. This principle is called summation.

THE END