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3: Neurons 3: Neurons

3: Neurons - PowerPoint Presentation

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3: Neurons - PPT Presentation

and Synapses Brain and Behavior David Eagleman Jonathan Downar Chapter Outline The Cells of the Brain Synaptic Transmission Chemical Signaling in the Brain Spikes Electrical Signaling in the Brain ID: 576764

action neurons potentials cell neurons action cell potentials voltage potential brain spikes cells neurotransmitter neurotransmitters close release membrane ions

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Slide1

3: Neurons and Synapses

Brain and Behavior

David Eagleman

Jonathan

DownarSlide2

Chapter Outline

The Cells of the Brain

Synaptic Transmission: Chemical Signaling in the Brain

Spikes: Electrical Signaling in the BrainWhat Do Spikes Mean? The Neural CodeIndividuals and Populations

2Slide3

The Cells of the Brain

Neurons: A Close-Up View

Many Different Types of Neurons

Glial Cells

3Slide4

Neurons: A Close-Up View

Ramon y

Cajal

established the Neuron Doctrine, which states that the brain is made of many small, discrete cells.There are almost 100 billion neurons in the human brain.These neurons are like any other cell in the body, with a membrane, a nucleus, and specialized organelles.

4Slide5

Neurons: A Close-Up View

5Slide6

Neurons: A Close-Up View

Neurons have four important regions.

Dendrites: Branching projections that collect information

6Slide7

Neurons: A Close-Up View

Neurons have four important regions.

Soma (Cell Body): Contains the nucleus and integrates information

7Slide8

Neurons: A Close-Up View

Neurons have four important regions.

Axon: Conducts the neural signal across a long distance

8Slide9

Neurons: A Close-Up View

Neurons have four important regions.

Axon terminals: Small swellings that release signals to affect other neurons

Chemical signals, known as neurotransmitters, cross small gaps, known as synapses.It is estimated that there are about 500 trillion synapses in the adult brain.

9Slide10

Neurons: A Close-Up View

10Slide11

Many Different Types of Neurons

Neurons can be classified by their function:

Sensory neurons carry information to the brain.

Motor neurons carry information from the brain to the muscles.Interneurons convey the signals around the nervous system.

11Slide12

Many Different Types of Neurons

12Slide13

Many Different Types of Neurons

Neurons can be classified by their shape:

Multipolar neurons have many dendrites.

Bipolar neurons have one dendrite and one axon.Monopolar neurons have only one projection from the soma, which branches to form the axon and the dendrite.

13Slide14

Many Different Types of Neurons

14Slide15

Glial Cells

Glia play many roles within the nervous system:

S

peeding up the neuronal signalingRegulating extracellular chemicalsEnabling neurons to modify their connections

15Slide16

Glial Cells

Oligodendrocytes, in the central nervous system, and Schwann cells, in the peripheral nervous system, wrap myelin around axons to speed up signals.

Nodes of Ranvier are small gaps in the myelin sheath.

16Slide17

Glial Cells

17Slide18

Glial Cells

Astrocytes regulate extracellular chemicals and regulate local blood flow.

Microglia provide immune system functions for the central nervous system.

18Slide19

Synaptic Transmission: Chemical Signaling in the Brain

Release of Neurotransmitter at the Synapse

Types of Neurotransmitters

ReceptorsPostsynaptic Potentials

19Slide20

Release of Neurotransmitter at the Synapse

Neurotransmitters are chemicals released by the presynaptic cell to affect the postsynaptic cell.

The synaptic cleft is the 20- to 30-nm space between the cells.

The small size of the synaptic cleft allows the concentration of the neurotransmitter to change rapidly.

20Slide21

Release of Neurotransmitter at the Synapse

21Slide22

Types of Neurotransmitters

There are small-molecular-weight neurotransmitters, such as monoamines and amino acids, soluble gases, such as NO and CO, and large-molecular-weight neurotransmitters, which are peptides.

Most neurons release one or two small transmitters as well as a peptide.

22Slide23

Types of Neurotransmitters

23Slide24

Receptors

Specialized proteins in the cell membrane

Neurotransmitters interact with receptors to affect the postsynaptic cell.

Ionotropic receptors allow ions to flow across the membrane, changing the charge of the cell membrane.Metabotropic receptors relay information into the cell using a series of proteins.

24Slide25

Receptors

25Slide26

Receptors

Neurotransmitters only bind to receptors for a short time and need a way to be removed.

Degradation: The neurotransmitter is broken apart.

Diffusion: The neurotransmitter moves down the concentration gradient and out of the synapse.Reuptake: Neurotransmitter is transported back into the original cell.

26Slide27

Receptors

27Slide28

Postsynaptic Potentials

When at rest, there is a voltage difference between the inside and the outside of the cell.

The inside of the cell is more negative than the outside, about -70 mV.

28Slide29

Postsynaptic Potentials

Excitatory postsynaptic potentials alter the membrane voltage, moving the voltage closer to 0.

Inhibitory postsynaptic potentials move the voltage further from 0.

Postsynaptic potentials are small (about 1 mV) and fast (a few milliseconds).

29Slide30

Postsynaptic Potentials

30Slide31

Spikes: Electrical Signaling in the Brain

Adding up the Signals

How an Action Potential Travels

Myelinating Axons to Make the Action Potential Travel FasterAction Potentials Reach the Terminals and Cause Neurotransmitter Release

31Slide32

Adding up the Signals

Action potentials are all or none.

EPSPs and IPSPs combine to affect the membrane voltage.

In temporal summation, PSPs arriving at the soma at close to the same time are combined.In spatial summation, PSPs arriving at different locations on the soma are combined.

32Slide33

Adding up the Signals

33Slide34

Adding up the Signals

The soma receives 100s or 1000s of PSPs at a time.

EPSPs sum together to depolarize the cell (move the voltage closer to 0).

If the membrane voltage reaches threshold (approximately -60 mV), an action potential is generated at the axon hillock.

34Slide35

How an Action Potential Travels

In neurons at rest, there are more Na+ ions outside the cell and more K+ ions inside the cell.

At threshold, voltage-gated Na+ channels open, allowing Na+ ions to flow into the cell, down the chemical concentration and electrical gradients.

Voltage-gated K+ channels open, allowing K+ ions to flow out of the cell.

35Slide36

How an Action Potential Travels

36Slide37

How an Action Potential Travels

The current formed by the Na+ ions flows down the neuron, depolarizing the next part of the neuron.

There is a refractory period after the action potential, when the voltage-gated Na+ ion channels are less likely to open.

Calcium and chloride ions also contribute to the action potential.

37Slide38

Myelinating Axons to Make the Action Potential Travel Faster

Myelin is interrupted by gaps, known as nodes of Ranvier, where the action potential is regenerated.

The action potential jumps from node to node, greatly speeding up transmission.

Myelination decreases the amount of energy used by the neuron.

38Slide39

Myelinating Axons to Make the Action Potential Travel Faster

39Slide40

Action Potentials Cause Neurotransmitter

Release

Action potentials cause voltage changes in the axon terminals, causing voltage-gated calcium channels to open.

Calcium ions cause vesicles with neurotransmitters to bind to the presynaptic membrane.Neurotransmitters are released and cross the synapse.

40Slide41

Action Potentials Cause Neurotransmitter

Release

41Slide42

What Do Spikes Mean? The Neural Code

Encoding Stimuli in Spikes

Decoding Spikes

42Slide43

Encoding Stimuli in Spikes

In the brain, there are approximately 100 billion neurons, each sending up to a few hundred action potentials per second.

The number of spikes per second is used to describe the neuron’s response to a stimulus.

43Slide44

Encoding Stimuli in Spikes

44Slide45

Encoding Stimuli in Spikes

Neurons have a baseline level of activity, so the neuron can either increase or decrease the firing rate.

Research suggests that there may be other coding methods.

45Slide46

Encoding Stimuli in Spikes

46Slide47

Decoding Spikes

A typical neuron receives 10,000 incoming synapses.

Neurons may be responding not to individual input but to the average input.

47Slide48

Decoding Spikes

48Slide49

Individuals and Populations

Populations of Neurons

Forming a Coalition: What Constitutes a Group?

Open Questions for Future Investigation

49Slide50

Populations of Neurons

Local coding is the idea that stimuli in the outside world are encoded by different neurons.

Population coding is the idea that each stimulus is represented by a collection of neurons.

Each individual neuron many participate in multiple collections of neurons.

50Slide51

Forming a Coalition: What Constitutes a Group?

Neurons can be mutually excitatory or a coalition of neurons can support the high firing rate of the population.

Neurons may form a coalition by firing in synchrony.

51Slide52

Forming a Coalition: What Constitutes a Group?

52Slide53

Open Questions for Future Investigation

At present, the neural code is not understood.

Why do neurons have random changes in membrane voltage?

What is the role of the non-spiking neurons in the brain?What is the role of glia in information processing?

53