1. 3: Neurons and Synapses Brain and Behavior David Eagleman
Jonathan Downar

2. Chapter Outline The Cells of the Brain
Synaptic Transmission: Chemical Signaling in the Brain
Spikes: Electrical Signaling in the Brain
What Do Spikes Mean? The Neural Code
Individuals and Populations

3. The Cells of the Brain Neurons: A Close-Up View
Many Different Types of Neurons
Glial Cells

4. 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.

5. Neurons: A Close-Up View
FIGURE 3.3 A typical neuron in the cortex.

6. Neurons: A Close-Up View
Neurons have four important regions.
Dendrites: Branching projections that collect information
FIGURE 3.4 Dendrites. The integrators of thousands of tiny chemical signals come in a variety of shapes.

7. Neurons: A Close-Up View
Neurons have four important regions.
Soma (Cell Body): Contains the nucleus and integrates information
FIGURE 3.5 The cell body, or soma, is the central command center of a neuron. The dendrites and a single axon grow from the soma, the former for collecting incoming signals and the latter for transmitting outgoing signals over long distances.

8. Neurons: A Close-Up View
Neurons have four important regions.
Axon: Conducts the neural signal across a long distance
FIGURE 3.6 An axon is a single, slender extension from the soma. It is essentially a cable to conduct signals rapidly across long distances.

9. 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.

10. Neurons: A Close-Up View
FIGURE 3.7 Axon terminals are the end points of the axon, where chemical signals are released.

11. 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.

12. Many Different Types of Neurons
FIGURE 3.8 Different types of neurons. Examples of (a) sensory neurons, (b) motor neurons, and (c) interneurons. Interneurons can be of two types: those with long projections to other regions are termed projection interneurons, whereas those that stay within a region are termed local interneurons.

13. 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.

14. Many Different Types of Neurons
FIGURE 3.9 Classifying neurons by their shape. Examples of (a) multipolar neurons, (b) bipolar neurons, and (c) monopolar neurons.

15. Glial Cells Glia play many roles within the nervous system:
Speeding up the neuronal signaling
Regulating extracellular chemicals
Enabling neurons to modify their connections

16. 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.

17. Glial Cells
FIGURE 3.11 Some glial cells myelinate axons. (a) In the central nervous system, a single oligodendrocyte will wrap up to 50 different axons with myelin sheaths. (b) In the peripheral nervous system, myelination is accomplished by Schwann cells, which wrap around a single axon. Note that the layer of insulation is not continuous, but exists in small sections. (c) Transmission electron micrograph of a myelin sheath.

18. Glial Cells
Astrocytes regulate extracellular chemicals and regulate local blood flow.
Microglia provide immune system functions for the central nervous system.

19. Synaptic Transmission: Chemical Signaling in the Brain
Release of Neurotransmitter at the Synapse
Types of Neurotransmitters
Receptors
Postsynaptic Potentials

20. 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.

21. Release of Neurotransmitter at the Synapse
FIGURE 3.14 Vesicles carrying neurotransmitter molecules dock with the presynaptic membrane, releasing the signaling molecules into the synaptic cleft. The neurotransmitters diffuse across the cleft and interact with receptors on the postsynaptic target.

22. 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.

23. Types of Neurotransmitters

24. 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.

25. Receptors
FIGURE 3.15 Two types of channels allow neurotransmitters to effect target cells. (a) Ionotropic receptors are opened—or gated—allowing ions to move through a passage in the membrane. (b) Metabotropic receptors relay signals to proteins inside the cell.

26. 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.

27. Receptors
FIGURE 3.16 There are three ways by which neurotransmitters are cleared from the cleft: (a) degradation, (b) diffusion, and (c) reuptake.

28. 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.

29. 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).

30. Postsynaptic Potentials
FIGURE 3.17 Postsynaptic potentials. (a) An excitatory postsynaptic potential (EPSP) occurs when positive ions flow through an ionotropic receptor into the cell, causing depolarization. (b) An inhibitory postsynaptic potential (IPSP) occurs when positive ions flow out of the cell, or negative ions flow in. This causes the difference in voltage between the inside and outside of the cell to grow larger, known as hyperpolarization.

31. Spikes: Electrical Signaling in the Brain
Adding up the Signals
How an Action Potential Travels
Myelinating Axons to Make the Action Potential Travel Faster
Action Potentials Reach the Terminals and Cause Neurotransmitter Release

32. 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.

33. Adding up the Signals
FIGURE 3.19 Temporal and spatial summation. (a) No summation occurs when EPSPs arrive with a delay between them; they, individually, cannot drive the membrane voltage to the threshold for a spike. (b) Temporal summation occurs when EPSPs arrive close in time and their contributions add up at the soma, leading to an action potential. (c) Spatial summation occurs when signals arrive on different branches of the dendrites, converging at the soma. (d) If an EPSP and an IPSP arrive at different locations at the same time, they will cancel each other’s effect at the soma.

34. 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.

35. 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.

36. How an Action Potential Travels
FIGURE The sequence of a voltage spike. (a) At rest, there are more Na+ ions outside the cell than inside and more K+ ions inside the cell than outside. (b) When voltage-gated Na+ channels open, Na+ ions rush from the outside to the inside—both because of the concentration differences and because of the electrical field. (c) The depolarization caused by Na+ influx triggers the opening of K+ channels, which cause K+ ions to rush out, thus making the outside more positive again (repolarization).

37. 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.

38. 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.

39. Myelinating Axons to Make the Action Potential Travel Faster
FIGURE 3.22 The nodes of Ranvier. (a) Diagram. (b) Microscopic view.

40. 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.

41. Action Potentials Cause Neurotransmitter Release
FIGURE 3.23 Action potentials lead to neurotransmitter release.

42. What Do Spikes Mean? The Neural Code
Encoding Stimuli in Spikes
Decoding Spikes

43. 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.

44. Encoding Stimuli in Spikes
FIGURE 3.24 A selective neuron responds with greater activity to one particular type of stimulus than to other types. The blue dashes represent individual action potentials; different rows represent individual trials. The red histograms summarize the response of the neuron over many trials.

45. 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.

46. Encoding Stimuli in Spikes
FIGURE 3.26 A palette of coding possibilities for carrying information about a stimulus (Bullock, 1968).

47. Decoding Spikes A typical neuron receives 10,000 incoming synapses.
Neurons may be responding not to individual input but to the average input.

48. Decoding Spikes
FIGURE 3.27 Are neurons integrators or coincidence detectors? (a) In the “assembly line” view of neurons, neurons pass messages to one another: the cell on the left is the sender, and the cell on the right integrates those signals as the receiver (Konig, Engel, & Singer, 1996). (b) Because neurons receive thousands of inputs, they may be better thought of as coincidence detectors. The cell body of the postsynaptic cell is unable to determine which presynaptic neuron sent which signal—instead, a postsynaptic spike will only signal the coincidence of many excitatory inputs arriving simultaneously.

49. Individuals and Populations
Populations of Neurons
Forming a Coalition: What Constitutes a Group?
Open Questions for Future Investigation

50. 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.

51. 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.

52. Forming a Coalition: What Constitutes a Group?
FIGURE 3.29 Neurons that excite each other can form coalitions. (a) Two neurons that mutually excite one another. (b) A larger coalition of excitatory neurons.

53. 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?