1. Neurons
Chapter 7

2. Learning Objectives
Identify and describe the functions of the two main divisions of the nervous system.
Differentiate between a neuron and neuroglial cells in terms of structure and function.
Describe the features of the three major categories of neurons.
Label a generalized neuron diagram, and explain the function of each of the structures.
Explain the role of the myelin sheath.
Describe how a nerve cell maintains a resting potential using the sodium-potassium pump and what changes occur in the membrane upon stimulation.
Explain how a nerve impulse is a bioelectrical signal, including how ions move across the membrane.
Describe the events of an action potential.
Compare and contrast what happens during resting and action potentials.
Draw and label a synapse diagramming the movement of neurotransmitter
Differentiate between excitatory and inhibitory synapses.
Describe how neurotransmitters can be removed from the synapse.
Exemplify the specific role of common neurotransmitters in the maintenance of stable behavioral conditions and in disease situations.

3. Cells of the Nervous System
Neurons (nerve cells)
Excitable cells that generate and transmit messages
Neuroglial cells (glial cells)
Outnumber neurons by 10 to 1
Several types, each with a specific function
Provide structural support, growth factors, and insulating sheaths around axons

4. Cells of the Nervous System
Three categories of neurons
Sensory (or afferent) neurons
Motor (or efferent) neurons
Interneurons (or association) neurons

5. Cells of the Nervous System
Sensory neurons
Carry information toward the CNS from sensory receptors
Motor neurons
Carry information away from the CNS to an effector (muscle or gland)

6. Cells of the Nervous System
Interneurons
Found only in the brain and spinal cord (CNS)
Between sensory and motor neurons
Integrate and interpret sensory signals
Account for more than 99% of the body’s neurons

7. Figure 7.1 Neurons may be sensory neurons, interneurons, or motor neurons.

8. Structure of Neurons
The shape of a typical neuron is specialized for communicating with other cells
Parts of a neuron
Dendrites  many short, branching projections
Axon  a single long extension
Cell body

9. Axons and Dendrites Dendrites Receive signals from other cells
Carry information toward the cell body of a neuron
Axon
Carries information away from the cell body to either another neuron or an effector
Cell body
Contains nucleus and other organelles
Functions to maintain the neuron

10. Figure 7.2 The structure of a neuron.10

11. Axons and Dendrites Nerves
Consist of parallel axons, dendrites, or both from many neurons
Covered with tough connective tissue
Classified as sensory, motor, or mixed (sensory and motor together) depending on the type of neurons they contain

12. Myelin Sheath
Found on most axons outside the CNS and some of those within the CNS
Provides electrical insulation that increases the rate of conduction of a nerve impulse
Composed of the plasma membranes of glial cells
In the PNS, Schwann cells (a type of glial cell) form the myelin sheath
Gaps between adjacent Schwann cells are called nodes of Ranvier: messages travel faster as they jump from one node of Ranvier to the next in a type of transmission called saltatory conduction

13. Figure 7.3 The myelin sheath.13

14. Myelin Sheath Multiple Sclerosis (MS)
A disease in which the myelin sheaths in the brain and spinal cord are progressively destroyed
Results from the destruction of the myelin sheath that surrounds axons in the CNS
The resulting scars (scleroses) interfere with the transmission of nerve impulses
Can result in paralysis and loss of sensation, including loss of vision

15. Myelin Sheath
Web Activity: Myelinated Neurons and Saltatory Conduction

16. Nerve Impulses
A nerve impulse, or action potential, is an electrochemical signal involving sodium and potassium ions (Na and K) that cross the cell membrane through ion channels
Each ion channel is designed to allow only certain ions to pass through it
Ion channels may be permanently open or regulated by a “gate,” which is a protein that changes shape and opens or closes a channel. The transport does not require any energy as the ions follow a gradient of concentration

17. Nerve Impulses
Ions also are transported across the membrane by the sodium-potassium pump
Special proteins in the cell membrane that actively transport sodium and potassium ions across the membrane against their concentration gradients
These pumps use cellular energy to eject sodium ions from within the cell and to bring potassium ions into the cell

18. Resting Potential
When a neuron is not conducting a nerve impulse, it is in a resting state called the resting potential
The inner surface of the membrane is about 70 mV more negative than the outer surface
There are more sodium ions outside the membrane than inside
There are more potassium ions inside the membrane than outside
This state is maintained by the sodium-potassium pump

19. Action Potential
When the neuron is stimulated, there is a sudden reversal of charge across the membrane because the sodium gates open and sodium ions enter the cell. Next, the potassium gates open and potassium ions rush out of the cell.
Threshold: Minimum charge that causes the sodium gates to open
Depolarization: Reduction of the charge difference across the membrane
Repolarization: Restoration of the charge difference across the membrane
These changes occur in a wave along the axon

20. Figure 7.4 The resting state and the propagation of an action potential along an axon.20

21. Figure 7.5 A graphic representation of an action potential.21

22. Action Potential Does not diminish, once started
Does not vary in intensity with the strength of the stimulus that triggered it
Is “all-or-nothing”
For a very brief period following an action potential, the neuron cannot be stimulated again
This is called the refractory period
It occurs because the sodium channels are closed and cannot be reopened

23. Action Potential
Web Activity: Nerve Impulse

24. Synaptic Transmission
Communication between a neuron and an adjacent cell occurs by neurotransmitters
Synapse
Junction between a neuron and another cell

25. Synaptic Transmission
Synaptic cleft
Gap between two cells
Neurotransmitters diffuse across the gap
In the case of two neurons, the presynaptic neuron sends a message to the postsynaptic neuron
Synaptic knob: swelling at the end of the axon of the presynaptic neuron

26. Figure 7.6 Structure of a synapse.26

27. Release of the Neurotransmitter and the Opening of Ion Transmitters
The nerve impulse reaches the synaptic knob of the presynaptic neuron
Calcium ions move into the synaptic knob which releases the neurotransmitter into the synaptic cleft
The neurotransmitter diffuses across the synaptic cleft and binds with receptors on the membrane of the postsynaptic neuron causing an ion channel to open

28. Figure 7.7 Transmission across an excitatory synapse.28

29. Release of the Neurotransmitter and the Opening of Ion Transmitters
At an excitatory synapse, binding of the neurotransmitter to the receptor causes sodium channels to open increasing the likelihood that an action potential will begin
At an inhibitory synapse, binding of the neurotransmitter to the receptor opens different ion channels
The postsynaptic cell’s interior becomes more negatively charged than usual, reducing the likelihood that an action potential will begin

30. Summation of Input from Excitatory and Inhibitory Synapses
A neuron may have as many as 10,000 synapses with other neurons at the same time
Some synapses have excitatory effects and some have inhibitory effects
Summation
Combined effects of excitatory and inhibitory effects at any given moment
Determines whether an action potential is generated
This level of integration provides fine control over neuronal responses

31. Figure 7.8 A neuron may have as many as 10,000 synapses.31

32. Removal of Neurotransmitters
Temporary effects
Once released into a synapse, neurotransmitters are quickly removed
Some are deactivated by enzymes
The enzyme acetylcholinesterase removes acetylcholine from synapses
Others are pumped back into the synaptic knob of the presynaptic axon

33. Removal of Neurotransmitters
Web Activity: The Synapse

34. Roles of Different Neurotransmitters
There are dozens of neurotransmitters
Some neurotransmitters produce different effects on different types of cells
Example: acetylcholine
Acts in both the PNS and the CNS
Released at every neuromuscular junction
Myasthenia gravis: autoimmune disease that attacks the acetylcholine receptors at neuromuscular junctions, resulting in little muscle strength

35. Roles of Different Neurotransmitters
In the CNS, different neurotransmitters are associated with different behavioral systems
Norepinephrine regulates mood, hunger, thirst, and sex drive
Serotonin promotes a feeling of well-being
Dopamine regulates emotions and complex movements

36. Health Issue: Neurotransmitters and Disease
Changes in the levels of neurotransmitters cause disorders
Alzheimer’s disease
Associated with decreased levels of acetylcholine
Clinical depression
Associated with decreased levels of serotonin, dopamine, and norepinephrine
Parkinson’s disease
Associated with decreased levels of dopamine

37. Health Issue: Neurotransmitters and Disease
| Attention Deficit Disorder
(controversial RX patch)