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Nucleus. Dendrites Collect electrical signals. Cell body Integrates incoming signals and generates outgoing signal to axon. Axon Passes electrical signals to dendrites of another cell or to an effector cell.

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Presentation Transcript
slide1

Nucleus

Dendrites

Collect

electrical

signals

Cell body

Integrates incoming signals

and generates outgoing

signal to axon

Axon

Passes electrical signals

to dendrites of another

cell or to an effector cell

slide2

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

Na+

440 mM

OUTSIDE

K+

20 mM

INSIDE -70mV

Na+

50 mM

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0

-40

-80

K+

400 mM

slide3

1.Depolarization

phase

2.Repolarization

phase

Threshold potential

Resting potential

3.Hyperpolarization phase

Figure 45.6

slide4

Action potentials propagate by positive feedback.

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

slide5

Action potentials jump down axon.

Axon

Nodes of Ranvier

Schwann cells (glia)

wrap around axon,

forming myelin sheath

Schwann cell membrane

wrapped around axon

WHY ACTION POTENTIALS JUMP DOWN MYELINATED AXONS

Schwann cell

1. As charge spreads down

an axon, myelination (via

Schwann cells) prevents

ions from leaking out across

the plasma membrane.

Node of

Ranvier

2. Charge spreads

unimpeded until it reaches

an unmyelinated section of

the axon, called the node

of Ranvier, which is packed

with Na+ channels.

3. In this way, electrical

signals continue to jump

down the axon much faster

than they can move down

an unmyelinated cell.

slide6

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?

slide7

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.

slide9

Neurotransmitters lead to either

Excitatory or Inhibitory Postsynaptic Potentials: EPSPs and IPSPs

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0

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slide10

presynaptic membrane

Neurotransmitter

Enzyme recycler

Receptor

Myasthenia gravis

Acetylcholine (Ach) binds to receptors

Positive ions flow in – depolarizing postsynaptic cell

Acetylcholinesterase breaks Ach into acetate + choline

These are transported back into cell

Very fast (~25,000/sec)!

slide12

Temporal summation

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 ?

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slide13

Worksheet

Neurotransmitter

Enzyme recycler

Receptor

slide14

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
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!
slide18

From stimulus to action potential: metabotropic example

G protein

Na+

Adenylate cyclase

ATP cAMP

GTP GDP

receptor

cAMP activates many channels

Amplification:

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

slide19

Vision: also metabotropic

transducin

Phosphodiesterase

cGMP 5’cGMP

Rhodopsin

GTP GDP

Disk membrane

5’ cGMP changes many ion channels

Amplification:

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

how does a single sensory neuron encode stimuli
How does a single sensory neuron encode stimuli?

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

00000001100010000000001110000100000001000000000

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

stimulus intensity is encoded by spike rate
Stimulus intensity is encoded by spike rate

Spike rate

Intensity

(brightness, concentration, loudness, pressure, temperature)

slide22

Different neurons respond best to different frequencies

Louder

Threshold (dB SPL)

Quieter

Frequency

Shape of curve = selectivity for frequency

slide23
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
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.

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