Download Eagleman Ch 3. Neurons and Synapses

3: Neurons 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
 What Do Spikes Mean? The Neural Code
 Individuals and Populations

2
The Cells of the Brain
Neurons: A Close-Up View
 Many Different Types of Neurons
 Glial Cells

3
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.

4
Neurons: A Close-Up View
5
Neurons: A Close-Up View

Neurons have four important regions.
 Dendrites:
Branching projections that collect
information
6
Neurons: A Close-Up View

Neurons have four important regions.
 Soma
(Cell Body): Contains the nucleus and
integrates information
7
Neurons: A Close-Up View

Neurons have four important regions.
 Axon:
Conducts the neural signal across a
long distance
8
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.

9
Neurons: A Close-Up View
10
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.
11
Many Different Types of Neurons
12
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.
13
Many Different Types of Neurons
14
Glial Cells

Glia play many roles within the nervous
system:
 Speeding
up the neuronal signaling
 Regulating extracellular chemicals
 Enabling neurons to modify their connections
15
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.

16
Glial Cells
17
Glial Cells
Astrocytes regulate extracellular chemicals
and regulate local blood flow.
 Microglia provide immune system
functions for the central nervous system.

18
Synaptic Transmission: Chemical
Signaling in the Brain
Release of Neurotransmitter at the
Synapse
 Types of Neurotransmitters
 Receptors
 Postsynaptic Potentials

19
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.

20
Release of Neurotransmitter at
the Synapse
21
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.

22
Types of Neurotransmitters
23
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.

24
Receptors
25
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.
26
Receptors
27
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.

28
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).

29
Postsynaptic Potentials
30
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

31
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.

32
Adding up the Signals
33
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.

34
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.

35
How an Action Potential Travels
36
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.

37
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.

38
Myelinating Axons to Make the
Action Potential Travel Faster
39
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.

40
Action Potentials Cause
Neurotransmitter Release
41
What Do Spikes Mean? The
Neural Code
Encoding Stimuli in Spikes
 Decoding Spikes

42
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.

43
Encoding Stimuli in Spikes
44
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.

45
Encoding Stimuli in Spikes
46
Decoding Spikes
A typical neuron receives 10,000 incoming
synapses.
 Neurons may be responding not to
individual input but to the average input.

47
Decoding Spikes
48
Individuals and Populations
Populations of Neurons
 Forming a Coalition: What Constitutes a
Group?
 Open Questions for Future Investigation

49
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.

50
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.

51
Forming a Coalition: What
Constitutes a Group?
52
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