Section 9.2 Page 418 The Electrochemical Impulse

Background Messages are sent throughout the nervous system by electrochemical impulses Electrical impulses transmitted by movement of ions Hence the “electro” and the “chemical”

Recall: a long, long time ago... Passive transport: Movement of a substance along its concentration gradient Charged entities cannot pass freely through the cell membrane Can move by passive transport through ion channels

Recall: a long, long time ago... Ion channels are not constantly open: Voltage-gated – Open when a change in voltage across the membrane is detected Ligand-gated – Open when something binds to the channel

Nerve membrane potential Electrical potential – A difference in charge Resting potential The normal “resting” potential of a cell membrane is -70 mV The membrane has more negative charges inside than it does outside The charge difference between the outer and inner surfaces is 70 mV

Nerve membrane potential When a nerve is excited, the potential becomes +40 mV Reversal of potential (+110 mV) Called an action potential

Outline 1. The resting potential 2. The action potential Generation The all-or-nothing response Propagation along an axon Synaptic transmission 3. Diseases and disorders (if time)

Establishing the resting potential At rest, The concentration of sodium ions (Na + ) outside the membrane is higher than the concentration of potassium ions (K + ) inside. Uneven concentrations cause the inside to be more negative Resting potential is actually an accumulation of a lesser amount of positive charges

Maintaining the resting potential The cell membrane is “leaky”: Some ions diffuse across Resting potential is maintained by the sodium- potassium pump (active transport – antiport) 3 Na + out, 2 K + in

The Action Potential

Generating the action potential Na + ions can also diffuse in through ion channels Recall that channels can be voltage-gated they can be stimulated to open by disrupting membrane potential

Generating the action potential During excitation, A stimulus disrupts the resting potential of the cell The Na + channels are opened Na + rushes in through voltage-gated ion channels The inner surface of the membrane becomes positive relative to the outer The process of Na + rushing in is called depolarization.

Repolarization Once the membrane is depolarized, Na + gates close K + channels open, allowing K + to exit the cell The original polarity of the membrane is restored to -70 mV The membrane is said to be repolarized Then... The Na + / K + pump restores the balance of ions

Repolarization Depolarization

Refractory period The membrane must be repolarized before it can generate another nerve impulse Refractory period: The time required for repolarization 1 to 10 ms

The all-or-nothing response What can stimulate initial depolarization of the membrane? Pressure Changes in pH Electrical shock The intensity of the stimulus must reach a threshold level in order to stimulate an action potential.

The all-or-nothing response What is the stimulus threshold? What happens when the stimulus is beyond the threshold?

The all-or-nothing response Any stimulus below the threshold will not elicit a response. Additionally, increasing the stimulus intensity beyond the threshold will not change the degree of response.

The all-or-nothing response: Neurons either fire maximally, or they do not fire at all.

All action potentials travel with the same speed and intensity. Differences in stimulus intensity are detected by changes in the number and frequency of action potentials. More intense stimulus = More + More frequent action potentials

Propagation of the action potential Nerve impulses are transmitted: a. along the length of neurons b. from neuron to neuron (synaptic transmission) The action potential travels down axons, like a wave

Propagation along a neuron

Animation hill.com/olc/dl/120107/bio_d.swf

Propagation along a neuron How? The positive charges inside the cytoplasm are attracted to the negatively-charged areas adjacent to them Movement of the positive charges depolarizes the area "downstream" Upstream area is still experiencing a refractory period, so will be unaffected Na + channels in this area now open, causing sodium to rush in The cycle repeats itself in order to propagate the action potential

Result Each successive region becomes depolarized, passing the impulse along the length of the axon One direction only The wave of depolarization is followed by a wave of repolarization

Synaptic transmission Neurons are separated by a gap (the synapse) Nerve impulses must somehow get across the gap from the axon of one neuron to the dendrites of another

Synaptic transmission Synaptic transmission occurs due to neurotransmitters Chemicals contained within vesicles, at the ends of axons The action potential reaches the end of the axon, which stimulates the release of neurotransmitters

Synaptic transmission Neurotransmitters diffuse across the synapse and bind to the dendrites post-synaptic neuron Ligand-gated Na + channels Binding can either: Stimulate another action potential (excitatory effect); or Make another action potential less likely (inhibitory effect) Animation Featuring the excitatory effect of neurotransmitters:

Excitatory effect Binding depolarizes the membrane of the dendrites Na + channels open, and the action potential is re- intiated Inhibitory effect Opens K + channels in the membrane, so K + diffuses out The inner surface becomes even more negative in relation to the outer the membrane is hyperpolarized makes it harder to depolarize the membrane in order to initiate an action potential

Acetylcholine and cholinesterase Two neurotransmitters: Acetylcholine (ACh) acts has an excitatory effect on most neurons Causes depolarization Cholinesterase is then released by the postsynaptic neuron It destroys Ach so that the sodium channels close, and the cell can be repolarized

Summation Often the axons of many neurons will synapse with the dendrites of another. Some of the presynaptic neurons may release excitatory neurotransmitters, while some may release inhibitory ones.

Summation The effect on the postsynaptic neuron will be determined by the sum of all the effects of the neurotransmitters released. This principle is called summation.

Diseases and disorders Parkinson’s disease Linked to decreased production of the neurotransmitter dopamine. Dopamine is the messenger between parts of the brain that control smooth muscle movement.

Diseases and disorders Depression Linked to imbalances in neurotransmitters serotonin, norepinephrine, and dopamine. The nature of the relationship is currently unclear. Anti-depressant drugs act on pre- and post-synaptic neurons to alter rate of neurotransmitter breakdown.

Diseases and disorders Addiction Dopamine is associated with feelings of pleasure. Many drugs mimic, or stimulate the release of, dopamine. Massive stimulation of dopamine receptors gives the “high”

Overstimulation of dopamine receptors on postsynaptic neurons causes the neuron to decrease the number of dopamine receptors. This is responsible for drug tolerance. Progressively increasing amounts of drug are required to achieve the same effect. Body also decreases the amount of dopamine it makes. This is responsible for withdrawal once drug use is discontinued.

Homework Pg. 426 #3-10