Nucleus

DendritesCollectelectricalsignals

Cell bodyIntegrates incoming signalsand generates outgoingsignal to axon

AxonPasses electrical signalsto dendrites of anothercell or to an effector cell

The membrane potential drives the responsiveness to stimulation. How are signals conducted along the length of a neuron?

OUTSIDE

INSIDE -70mV

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Na+

440 mM

K+

400 mM

Na+

50 mM

K+

20 mM

1. Depolarization phase

2. Repolarization phase

3. Hyperpolarization phase

Resting potentialThreshold potential

Figure 45.6

Action potentials propagate by positive feedback.

Speed is critical: (1) large diameter and (2) myelination

Action potentials jump down axon.

Nodes of Ranvier Schwann cells (glia)wrap around axon,forming myelin sheath

WHY ACTION POTENTIALS JUMP DOWN MYELINATED AXONS

Schwann cell

Node ofRanvier

Axon

Schwann cell membranewrapped around axon

1. As charge spreads downan axon, myelination (viaSchwann cells) preventsions from leaking out acrossthe plasma membrane.

2. Charge spreadsunimpeded until it reachesan unmyelinated section ofthe axon, called the nodeof Ranvier, which is packedwith Na+ channels.

3. In this way, electricalsignals continue to jumpdown the axon much fasterthan they can move downan unmyelinated cell.

Sample problem.The distance from your toe to your spinal column is about 1m. If your sensory axon is 5 um in diameter, how much time elapses before your CNS receives the signal? How much time would elapse if your nerve was not myelinated?

What you should understand

How the generation of an action potential represents an example of positive feedback.

How voltage gated channels generate and keep brief the action potential.

The flows of major ions during resting, depolarization, repolarization, and hyperpolarization.

How myelination leads to rapid propagation velocities.

Synapses: Calcium mediates synaptic vesicle fusion with SNARE, SNAP AND SYNAPTOTAGMIN

Neurotransmitters lead to eitherExcitatory or Inhibitory Postsynaptic Potentials: EPSPs and IPSPs

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-120

Neurotransmitter

Enzyme recycler

Receptor

presynaptic membrane

Acetylcholine (Ach) binds to receptorsPositive ions flow in – depolarizing postsynaptic cellAcetylcholinesterase breaks Ach into acetate + cholineThese are transported back into cellVery fast (~25,000/sec)!

Myasthenia gravis

Summation: EPSPs and IPSPs from multiple inputs sum at postsynaptic cells

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-120

Temporal summation

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-120

A neuron in your spinal column receives input from a sensor in your leg. Under resting conditions, that sensor sends a signal every 10 seconds. Under extreme stretch of your leg, it sends signals every second. Why would our spinal nerve only respond to the more frequent stimulus ?

Neurotransmitter

Enzyme recycler

Receptor

Worksheet

What you should understand

The roles of neurotransmitters, postsynaptic receptor molecules and enzyme recycling components of synapses.

Summation of IPSPs and EPSPs by postsynaptic cells (temporal and spatial)

The consequences of up- and down-regulation of postsynaptic receptor molecules.

Sensory systems

• Stimuli are transduced into changes in membrane potential by ionotropic and metabotropic mechanisms

• Four characteristics of the stimulus are encoded – Intensity: spike rate– Frequency: tuning curves– Location: receptive fields– Modality: labeled line

• Sensory systems are diverse and adapted for their specific tasks…and amazing!

From stimulus to action potential: ionotropic example

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receptor GTP GDPATP cAMP

cAMP activates many channelsAmplification:

1 active receptor ~10 GTP conversions each GTP powers ~10 cAMP about 1:100

Na+ G proteinAdenylate cyclase

From stimulus to action potential: metabotropic example

Vision: also metabotropic

Rhodopsin

transducinPhosphodiesterase

cGMP 5’cGMP5’ cGMP changes many ion channelsAmplification:

1 active receptor ~500 transducin activations -> each one converts 103 GMPs

Disk membrane

GTP GDP

How does a single sensory neuron encode stimuli?

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Characteristics of the stimulus: intensity, frequency, location, modality

Ways the nervous system encodes these: spike rate, tuning curves, receptive fields, labeled line

Stimulus intensity is encoded by spike rate

Intensity

(brightness, concentration, loudness, pressure, temperature)

Spi

ke ra

te

Quieter

Louder

Different neurons respond best to different frequencies

Frequency

Thre

shol

d (d

B S

PL)

Shape of curve = selectivity for frequency

Receptive fields: area of space in which the presence of a stimulus will alter the firing of a sensory neuron

These receptive fields form spatiotopic maps of the world on the sensory organ… and these maps usually translate to areas of cortex as well

If all neurons communicate using action potentials, how can we keep the modalities apart?

Specific sensory cells with specific receptors project to specific parts of the thamalus…which project to specific parts of cortex. LABELED LINE.