1. Neurons, Synapses, and Signaling
Constructed by Fred Brown, Science Education Consultant, West Hartford, CT

2. Overview: Lines of Communication
Neurons are nerve cells that transfer information within the body
Neurons use two types of signals to communicate: electrical signals (long-distance) and chemical signals (short-distance)

3. Overview: Lines of Communication
The transmission of information depends on the path of neurons along which a signal travels
Processing of information takes place in simple clusters of neurons called ganglia or a more complex organization of neurons called a brain

4. Introduction to Information Processing
Nervous systems process information in three stages:
Sensory input
Integration
Motor output

5. Introduction to Information Processing
Sensors detect external stimuli and internal conditions and transmit information along sensory neurons
Sensory information is sent to the brain or ganglia, where interneurons integrate the information
Motor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity

6. Introduction to Information Processing
Many animals have a complex nervous system which consists of:
A central nervous system (CNS) where integration takes place; this includes the brain and a nerve cord
A peripheral nervous system (PNS), which brings information into and out of the CNS

7. Introduction to Information Processing
Sensor
Sensory Input
Peripheral Nervous
System (PNS)

8. Introduction to Information Processing
Sensor
Sensory Input
Integration
Peripheral Nervous
System (PNS)
Central Nervous
System (CNS)

9. Introduction to Information Processing
Sensor
Sensory Input
Integration
Effector
Motor Output
Peripheral Nervous
System (PNS)
Central Nervous
System (CNS)

10. Neuron Structure and Function
Most of a neuron’s organelles are in the cell body
Most neurons have dendrites, highly branched extensions that receive signals from other neurons
The axon is typically a much longer extension that transmits signals to other cells at synapses

11. Neuron Structure and Function
Dendrites
Stimulus
Nucleus
Cell
body
Axon
hillock

12. Neuron Structure and Function
A synapse is a junction between an axon and another cell
The synaptic terminal of one axon information across the synapse in the form of chemical messengers called neurotransmitters

13. Neuron Structure and Function
Dendrites
Stimulus
Nucleus
Cell
body
Axon
hillock
Synaptic terminals
Synapse
Neurotransmitter

14. Neuron Structure and Function
Information is transmitted from a presynaptic cell (a neuron) to a postsynaptic cell (a neuron, muscle, or gland cell)
Most neurons are nourished or insulated by cells called glia

15. Neuron Structure and Function
Dendrites
Stimulus
Nucleus
Cell
body
Axon
hillock
Presynaptic
cell
Synaptic terminals
Synapse
Postsynaptic cell
Neurotransmitter

16. Neuron Structure and Function
Dendrites
Axon
Cell
body
Sensory Neuron

17. Neuron Structure and Function
Dendrites
Axon
Cell
body
Sensory Neuron
Motor Neuron

18. Messages are transmitted as changes in membrane potential
Concept Ion Pumps and Ion Channels Maintain the Resting Potential of a Neuron
Every cell has a voltage (difference in electrical charge) across its plasma membrane called a membrane potential
Messages are transmitted as changes in membrane potential
The resting potential is the membrane potential of a neuron not sending signals

19. Formation of the Resting Potential
In a mammalian neuron at resting potential, the concentration of K+ is greater inside the cell, while the concentration of Na+ is greater outside the cell
Sodium-potassium pumps use the energy of ATP to maintain these K+ and Na+ gradients across the plasma membrane
These concentration gradients represent chemical potential energy

20. Production of Action Potentials
Rising
phase
Threshold (–55)
Time (msec)
Depolarization
–70
–100
–50
+50
Membrane potential (mV)
Resting
potential
Falling
Undershoot

21. Production of Action Potentials
Neurons contain gated ion channels that open or close in response to stimuli
Membrane potential changes in response to opening or closing of these channels

22. Production of Action Potentials
Stimuli trigger a depolarization, a reduction in the magnitude of the membrane potential
For example, depolarization occurs if gated Na+ channels open and Na+ diffuses into the cell
Graded potentials are changes in polarization where the magnitude of the change varies with the strength of the stimulus

23. Membrane potential (mV)
Stimuli
+50
Membrane potential (mV)
–50
Threshold
Resting
potential
Depolarizations
–100 2 3 4
Time (msec)
(b) Graded depolarizations 1 5
Figure 48.9b Graded potentials and an action potential in a neuron

24. Production of Action Potentials
Voltage-gated Na+ and K+ channels respond to a change in membrane potential
When a stimulus depolarizes the membrane, Na+ channels open, allowing Na+ to diffuse into the cell
The movement of Na+ into the cell increases the depolarization and causes even more Na+ channels to open
A strong stimulus results in a massive change in membrane voltage called an action potential

25. Production of Action Potentials
An action potential occurs if a stimulus causes the membrane voltage to cross a particular threshold
An action potential is a brief all-or-none depolarization of a neuron’s plasma membrane
Action potentials are signals that carry information along axons

26. Production of Action Potentials
Stimuli
+50
Membrane potential (mV)
–50
Threshold
Resting
potential
Hyperpolarizations
–100 1 2 3 4 5
Time (msec)
(a) Graded hyperpolarizations
(b) Graded depolarizations
Depolarizations
Strong depolarizing stimulus 6
(c) Action potential
Action

27. Generation of Action Potentials: A Closer Look
When an action potential is generated
Voltage-gated Na+ channels open first and Na+ flows into the cell

28. Generation of Action Potentials: A Closer Look
When an action potential is generated
Voltage-gated Na+ channels open first and Na+ flows into the cell
During the rising phase, the threshold is crossed, and the membrane potential increases
During the falling phase, voltage-gated Na+ channels become inactivated; voltage-gated K+ channels open, and K+ flows out of the cell

29. Generation of Action Potentials: A Closer Look
During the refractory period after an action potential, a second action potential cannot be initiated
The refractory period is a result of a temporary inactivation of the Na+ channels
BioFlix: How Neurons Work

30. Conduction Speed
The speed of an action potential increases with the axon’s diameter
In vertebrates, axons are insulated by a myelin sheath, which causes an action potential’s speed to increase
Myelin sheaths are made by glia — oligodendrocytes — in the CNS and Schwann cells in the PNS

31. Conduction Speed Node of Ranvier Layers of myelin Schwann cell
Axon
Myelin sheath
Schwann
cell
Nodes of
Ranvier
Nucleus of
Schwann cell
Node of Ranvier
Layers of myelin

32. Conduction Speed
Myelinated axon (cross section)
0.1 µm

33. Conduction Speed
Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage-gated Na+ channels are found
Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction

34. Conduction Speed Schwann cell Depolarized region (node of Ranvier)
Cell body
Schwann cell
Depolarized region
(node of Ranvier)
Myelin
sheath
Axon

35. Concept 48.4 - Neurons Communicate With Other Cells at Synapses
At electrical synapses, the electrical current flows from one neuron to another across the gap junctions
At chemical synapses, a chemical neurotransmitter carries information across the synaptic cleft
Most synapses are chemical synapses

36. Neurons Communicate With Other Cells at Synapses
Postsynaptic
neuron
Synaptic
terminals
of pre-
synaptic
neurons
5 µm
Synaptic terminals on the cell body of a postsynaptic neuron

37. Neurons Communicate With Other Cells at Synapses
The presynaptic neuron synthesizes and packages the neurotransmitter in synaptic vesicles located in the synaptic terminal
The action potential causes the release of the neurotransmitter
The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell

38. Neurons Communicate With Other Cells at Synapses
When an action potential depolarizes the plasma membrane of the presynaptic terminal, it
Opens voltage-gated calcium ion channels, triggering an influx of Ca2+.

39. Neurons Communicate With Other Cells at Synapses
Voltage-gated
Ca2+ channel
Ca2+ 1 2
Action potential
Presynaptic
membrane

40. Neurons Communicate With Other Cells at Synapses
The elevated calcium ion (Ca2+) concentration in the terminal causes synaptic vesicles to fuse with the presynaptic membrane.
The vesicles release neurotransmitter into the synaptic cleft.

41. Neurons Communicate With Other Cells at Synapses
Voltage-gated
Ca2+ channel
Ca2+ 1 2 3 4
Synaptic
cleft
Presynaptic
membrane
Synaptic vesicles
containing
neurotransmitter
Action potential

42. Neurons Communicate With Other Cells at Synapses
The neurotransmitter binds to the receptor portion of ligand-gated ion channels in the postsynaptic membrane, opening the channels. (In the synapse illustrated next, both Na+ and K+ can diffuse through the channels.)

43. Neurons Communicate With Other Cells at Synapses
Voltage-gated
Ca2+ channel
Ca2+ 1 2 3 4
Synaptic
cleft
Ligand-gated
ion channels
Postsynaptic
membrane
Presynaptic
Synaptic vesicles
containing
neurotransmitter 5 K+
Na+
Action potential

44. Neurons Communicate With Other Cells at Synapses
The neurotransmitter is released from the receptors, and the channels close.

45. Neurons Communicate With Other Cells at Synapses
Voltage-gated
Ca2+ channel
Ca2+ 1 2 3 4
Synaptic
cleft
Ligand-gated
ion channels
Postsynaptic
membrane
Presynaptic
Synaptic vesicles
containing
neurotransmitter 5 K+
Na+
Action potential

46. Neurons Communicate With Other Cells at Synapses
Synaptic transmission ends when the neurotransmitter
diffuses out of the synaptic cleft,
is taken up by surrounding cells, or
is degraded by an enzyme.

47. Generation of Postsynaptic Potentials
Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels in the postsynaptic cell
Neurotransmitter binding causes ion channels to open, generating a postsynaptic potential

48. Generation of Postsynaptic Potentials
Postsynaptic potentials fall into two categories:
Excitatory postsynaptic potentials (EPSPs) are depolarizations that bring the membrane potential toward threshold
Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizations that move the membrane potential farther from threshold

49. Acetylcholine
Acetylcholine is a common neurotransmitter in vertebrates and invertebrates
In vertebrates it is usually an excitatory transmitter
Found in CNS, PNS, vertebrate neuromuscular junction

50. Biogenic Amines
Biogenic amines include epinephrine, norepinephrine, dopamine, and serotonin
Epinephrine (CNS & PNS) can be excitatory or inhibitory
Norepinephrine (CNS & PNS) can be excitatory or inhibitory
Dopamine (CNS & PNS) is generally excitatory
Serotonin (CNS) is generally inhibitory

51. Amino Acids
Two amino acids are known to function as major neurotransmitters in the CNS: gamma-aminobutyric acid (GABA) and glutamate
GABA (CNS) is inhibitory
Glutamate (CNS) is excitatory
Glycine (CNS) is inhibitory

52. Neuropeptides
Several neuropeptides, relatively short chains of amino acids, also function as neurotransmitters
Neuropeptides include substance P (CNS & PNS) and endorphins, (CNS) which both affect our perception of pain
Opiates bind to the same receptors as endorphins and can be used as painkillers

53. Gases
Gases such as nitric oxide and carbon monoxide are local regulators in the PNS