1. Anatomy and Physiology of Neurons
AP Biology
Chapter 48

2. Objectives Describe the different types of neurons
Describe the structure and function of dendrites, axons, a synapse, types of ion channels, and neurotransmitters.
Describe resting potential and the sequence of events that occur during an action potential.

3. Overview
The human brain contains an estimated 1011 (100 billion) neurons.
Each neuron may communicate with thousands of other neurons in complex information-processing circuits.
Results of brain imaging and other research methods show that groups of neurons function in specialized circuits dedicated to different tasks.

4. Structural Organization of the Nervous System
Central nervous system (CNS) – brain and spinal cord
Responsible for integration of sensory input and associating stimuli with appropriate motor output
Peripheral nervous system (PNS) – network of nerves extending into different parts of the body that carry sensory input to the CNS and motor output away from the CNA

6. Nervous systems consist of circuits of neurons and supporting cells
All animals except sponges (Phylum Porifera) have a nervous system
What distinguishes nervous systems of different animal groups is how neurons are organized into circuits

7. Organization of Nervous Systems: Cnidarians
The simplest animals with nervous systems, the cnidarians, have neurons arranged in nerve nets

8. Echinoderms
Sea stars have a nerve net in each arm connected by radial nerves to a central nerve ring
What is the difference?

9. Platyhelmenthyes
Relatively simple cephalized animals, such as flatworms, have a central nervous system (CNS)
What changed?

10. Annelids and Arthropods
Annelids and arthropods have segmentally arranged clusters of neurons called ganglia
These ganglia connect to the CNS and make up a peripheral nervous system (PNS)
Change?

11. Mollusks Nervous systems in mollusks correlate with lifestyles
Sessile mollusks have simple systems, whereas more complex mollusks have more sophisticated systems
Why?

12. Vertebrates
In vertebrates, the central nervous system consists of a brain and dorsal spinal cord
The PNS connects to the CNS
What has been selected for?

13. Information Processing
Nervous systems process information in three stages:

14. Information Processing
Sensory neurons pick up and transmit information from sensors that detect external stimuli (light, heat, touch) and internal conditions (blood pressure, muscle tension).
Interneurons, in the CNS, integrate the sensory input
Motor output leaves the CNS via motor neurons, which communicate with effector cells (muscle or endocrine cells).
Effector cells carry out the body’s response to a stimulus.

15. Reflexes
The stages of sensory input, integration, and motor output are easy to study in the simple nerve circuits that produce reflexes, the body’s automatic responses to stimuli.

17. Neuron Structure 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
Many axons are covered with a myelin sheath
Which speeds up transmission

19. Neurons have a wide variety of shapes that reflect input and output interactions

20. LE 48-6
Neurons have a wide variety of shapes that reflect input and output interactions
Dendrites
Axon
Cell
body
Sensory neuron
Interneurons
Motor neuron

21. Neuron Structure POGIL

22. Ion pumps and ion channels maintain the resting potential of a neuron
Across its plasma membrane, every cell has a voltage difference called a membrane potential
The cell’s inside is negative relative to the outside

23. Animation: Resting Potential
The Resting Potential
Resting potential is the membrane potential of a neuron that is not transmitting signals
Resting potential depends on ionic gradients across the plasma membrane
Concentration of Na+ is higher in the extracellular fluid than in the cytosol
The opposite is true for K+
Animation: Resting Potential

24. LE 48-10
CYTOSOL
EXTRACELLULAR
FLUID
Plasma
membrane

25. Neuron Function POGIL
#s 1-6

26. Gated Ion Channels
Gated ion channels open or close in response to one of three stimuli:
Stretch-gated ion channels open when the membrane is mechanically deformed
Ligand-gated ion channels open or close when a specific chemical binds to the channel
Voltage-gated ion channels respond to a change in membrane potential

27. Neuron Function POGIL
#s 7-9

28. Action potentials are the signals conducted by axons
If a cell has gated ion channels, its membrane potential may change in response to stimuli that open or close those channels
Action potential: rapid change in the membrane potential of an excitable cell, caused by stimulus-triggered selective opening and closing of gated ion channels.
Once generated, the impulse travels rapidly down the axon away from the cell body and toward the axon terminals.

29. Production of Action Potentials
Depolarizations are usually graded only up to a certain membrane voltage, called the threshold
A stimulus strong enough to produce depolarization that reaches the threshold triggers a response called an action potential

30. Action Potential
An action potential is a brief all-or- none depolarization of a neuron’s plasma membrane
It carries information along axons

31. Action Potential
Voltage-gated Na+ and K+ channels are involved in producing an action potential
When a stimulus depolarizes the membrane, Na+ channels open, allowing Na+ to diffuse into the cell
As the action potential subsides, K+ channels open, and K+ flows out of the cell

32. Conduction of Action Potentials
An action potential can travel long distances by regenerating itself along the axon
At the site where the action potential is generated, an electrical current depolarizes the neighboring region of the axon membrane

34. LE 48-14c Axon Action potential
An action potential is generated as Na+ flows inward across the membrane at one location.
Action
potential K+
Na+ K+
The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward.
Action
potential K+
Na+ K+
The depolarization-repolarization process is repeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon.

35. Conduction Speed
The speed of an action potential increases with the axon’s diameter
In vertebrates, axons are myelinated, also causing an action potential’s speed to increase

36. LE 48-15 Schwann cell Depolarized region (node of Ranvier) Cell body
Myelin
sheath
Axon

37. Neuron Function POGIL
#s 10-15

38. Four Phases of an Action Potential
Resting state: no channels are open
Depolarizing phase: membrane briefly reverses polarity
Cell interior becomes positive to the exterior
Repolarizing phase: returns membrane to its resting level
Hyperpolarized phase: refractory period

39. Depolarization phase
Na+ activation gates open allowing an influx of Na+
Potassium gates remain closed
Interior of the cell becomes more positive charged than the exterior

40. Repolarization phase Returns membrane to its resting level
Gates close sodium channels and opens potassium channels
The inside of the cell becomes more negative compared to the outside of the cell

41. Hyperpolarization phase
Membrane potential is temporarily more negative than the resting state
Sodium channels remain closed by potassium channels remain open
Refractory period occurs during this phase
Neuron is insensitive to depolarizing stimuli
This limits the maximum rate at which action potentials can be stimulated in a neuron.

42. LE 48-13_5 Na+ Na+ Na+ Na+ K+ Rising phase of the action potential K+
Falling phase of the action potential
+50
Action
potential
Na+
Na+
Membrane potential
(mV)
–50
Threshold K+
Resting potential
–100
Depolarization
Time
Na+
Na+
Extracellular fluid
Potassium
channel
Activation
gates
Na+ K+
Plasma membrane
Undershoot
Cytosol
Sodium
channel K+
Inactivation
gate
Resting state

43. Neuron Function POGIL
#s 16-20

44. Neurons communicate with other cells at synapses
In an electrical synapse, current flows directly from one cell to another via a gap junction
The vast majority of synapses are chemical synapses
In a chemical synapse, a presynaptic neuron releases chemical neurotransmitters stored in the synaptic terminal

45. LE 48-17
Postsynaptic cell
Presynaptic
cell
Na+
Neuro-
transmitter
Synaptic vesicles
containing
neurotransmitter K+
Presynaptic
membrane
Postsynaptic
membrane
Ligand-
gated
ion channel
Voltage-gated
Ca2+ channel
Postsynaptic
membrane
Ca2+
Synaptic cleft
Ligand-gated
ion channels
When an action potential reaches a terminal, the final result is release of neurotransmitters into the synaptic cleft

46. Synaptic Transmission
Synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels
Neurotransmitter binding causes ion channels to open, generating a new postsynaptic potential
After release, the neurotransmitter diffuses out of the synaptic cleft
It may be taken up by surrounding cells (reuptake) and/or degraded by enzymes

47. Neurotransmitters
The same neurotransmitter can produce different effects in different types of cells