How do neurons conduct impulses?

Answer 1

axons

Action Potential and Axon Conduction

- Resting membrane potential provides and immediate source of power (it can cause a rapid change)

- Hyperpolarize - makes membrane potential more negative

- Depolarize - makes membrane potential less negative

o Depolarization reaches a threshold, at this threshold you cause a massive electrical change (Action Potential aka Impulse aka Spike)

- Threshold - generally 15mV above resting membrane potential

o Threshold for a neuron is around -70 less 15 = -55mV

- All-or-none law

o Size of action potential for a given neuron is always the same regardless of the size of the stimulus that initiated it

- Information about Magnitude

o Conveyed by frequency of action potentials (#/sec [Hz])

- Alternative to Action Potential:

o Graded Potential - passive change occurs

o Signal gets smaller and smaller as it moves on - such as some neurons found in the eyes

Molecular Basis of Action Potential

- Depolarization

o Results in sodium membrane "channels" or "gates" begin to open

- At threshold, the number of open channels overcomes the sodium-potassium pump

o The channels have a time-limit and once open will automatically close after ½ msec.

- Sodium current makes membrane potential positive, and at peak, sodium channels close and potassium channels open

- Now, potassium channels open and potassium ions rush out (triggered by threshold, but have a delay to open so open after the sodium channels)

- Brief hyperpolarization (voltage surpasses -70mV) while sodium potassium pump restores ion distribution

- These voltage-dependant (activated) channels define the action potential

- Sustained activity leads to an increase in extra-cellular potassium (typically picked up by astrocytes)

- Glial cell (astrocytes) transports excess potassium to nearby arteries causing a dilation of the artery wall

o More active areas need more oxygen and glucose, so the movement of the potassium to the arteries, cause them to dilate and thus allow more volume of blood (and nutrients) to the area

- Refractory period - period when the cell is resistant to the generation of further action potentials

o Restricts the firing rate of the cell

- Two phases of the refractory period:

o Absolute - no firing regardless of the size of the stimulus

o Relative - threshold is higher than normal

Conduction of the Nerve Impulse

- Action potential is regenerated at each adjacent patch of the membrane (because of diffusing sodium from generation of action potential)

- Cannot move backwards - seen as a wave rather than distinct action potentials because the patches are so small, and it moves so fast

- Called the propagation of the action potential

- Slower than conduction of electricity down a copper wire (1-10m/sec vs. 300million m/sec)

- Axons with myelin sheaths are faster (120m/sec)

o Myelin sheath insulates the axon, so that sodium ions cannot pass into or out of the cell

o Sodium can cross at Nodes of Ranvier to generate a new action potential

o From one Node to the next, a graded potential regenerates a new action potential at the next node

o Called "salutatory conduction"

Answer 2

When a nerve cell undergoes its function of secreting a hormone, changes occur in its outer cell membrane that allow electrically-charged ions to move in and out of the cell in a step-wise fashion along the full extent of the nerve.

Answer 3

Neurons conduct impulses through electrical and chemical signals. When a neuron is activated, an electrical signal called an action potential travels down the length of the cell's axon. At the end of the axon, neurotransmitters are released into the synapse to signal the next neuron in the pathway.

Answer 4

Dendrites.

Answer 5

Neurones