Chapter 9.2: Electrochemical Impulse Pages 418-426

Recap

•The Importance of the Nervous System•CNS (Brain and Spinal Cord)•PNS (Somatic and Autonomic Nerves)•Anatomy of a Nerve Cells (Dendrites,

Axon, Cell Body, Nodes of Ranvier etc)•Types of Neurons (SIM)•Reflex Arc (5 steps)

The Impulse18th century: • Galvani realized that

the leg muscle of a dead frog could be made to twitch electrical stimulation

• Lead to scientists questioning how electric current is generated in the body

• Electrical vs. Neural transmission

Current in Wire, Current in Nerves

There are differences between current in wire and those that travel through nerves:

1.Current in a wire travels much faster

1.The cytoplasmic core of a nerve cell offers great resistance to the movement of electric current

1.Electric currents diminish as they move through a wire

Current in Wire, Current in Nerves

1. Nerve impulses remain as strong at the end as they were at the beginning of the impulse

1. Nerves cellular energy to generate currentElectric conductor An external force to push the current through the conductor

Figure 2. page 418

- 70 mV + 40 mV

Placed an electrode inside a large nerve cell

•The resting membrane normally had a potential near – 70 mV and registered + 40 mV when the nerve became excited

•The voltage difference across a nerve cell membrane during the resting stage is called the resting potential

•The reversal of potential is described as an action potential – the voltage difference across a nerve cell membrane when the nerve is excited

How do nerve cells become charged?• Molecular level nerve cells have a rich supple

of + and – ions both inside and outside of the cell• negative ions stay inside the cell• positive ions move and cause an unequal

concentration

50x more permeable to

potassium

•The membranes become charged due to the movement of positive ions

•Potassium is high inside, and sodium is high outside

•As potassium diffuses out, sodium diffuses in

The Resting MembraneRapid diffusion of K+ ions

nerve cells loses a greater number of positive ions than it gains

Exterior membrane is + compared to interior

The resting membrane: • Charged• Called a polarized membrane

unequal distribution of + ions

• Difference between + ions inside nerve membrane relative to outside = -70 mV

When the nerve gets excited…

•Nerve cell membrane more permeable to Na+ than K+

•Na+ ions rush into nerve cell through diffusion and charge attraction

•Sodium inflow causes charge reversal depolarization

• A sodium-potassium pump restores the original polarity of the nerve membrane repolarization

When the nerve gets excited…

Resting membrane

Depolarization

Repolarization

Sodium-Potassium Pump (Repolarization)

Active Transport

•Nerve cells conducting an impulse cannot be activated until resting membrane condition is restored

•Once depolarization is complete, nerves must repolarize before next action potential takes place

•Time required to become repolarized refractory period (1-10 ms)

Movement of the Action Potential

•For the impulse to be conducted on the axon, the impulse moves from zone of depolarization to adjacent areas

Action Potential• The flow of positively charged ions from the

area of the action potential toward the adjacent regions of the resting membrane causes a depolarization in the adjoining area

• This creates an electric disturbance, which causes adjacent sodium channels to open

• The result is a wave of action potential that moves along the cell membrane

• The wave of action potential and depolarization is followed by a wave of repolarization

Action Potential

Threshold Levels: All or None• Minimum level of a stimulus required to produce a

response

Figure 8. page 422

Threshold Levels: All or None

•Increasing the intensity of the stimulus above critical threshold does NOT produce an increased response

•Neurons fire maximally or none at all all-or-none response

Detecting Intensity1. The more intense

the stimulus the greater the frequency of impulsesWarm rod: lower frequency of impulse sent to brainHot rod: higher frequency

2. Different threshold levels of neurons

Synaptic Transmisson•Synapse: regions between neurons, or

between neurons and effectors•A single neuron may branch off and join

with many different neurons•Involves neurotransmitters: chemicals

release from vesicles to synapses•Presynaptic neuron: neuron that carries

impulses to the synapse•Postsynaptic neuron: neuron that carries

impulses away from the synapse

Synaptic Transmisson• Neurotransmitters released from presynaptic

neuron, diffuse across the synaptic cleft, create a depolarization of dendrites of the postsynaptic neuron

Acetylcholine• An example of an excitatory neurotransmitter • Acts on many postsynaptic neurons by opening the

sodium ion channels

Causes depolarization

•An enzyme that breaks down acetylcholine to reverse depolarization

•Once acetylcholine is destroyed, Na+ close neuron begins recovery phase

Cholinesterase

Any Questions?