1. Nervous System
Chapter 34

2. 34.1
Neurons and Glia

3. Nervous System – 2 types of cells
Neurons – Nerve Cells
Glia – Glial cells

4. Neurons
4 main parts
Cell Body
Dendrite
Axon
Axon terminal

5. Cell body Contains the nucleus and the organelles
Has dendrites extending from it

6. Dendrites Extends from the cell body Shrublike
Bring information from other neurons or sensory cells to the cell body
Different degree of branching depending on the type of neuron

7. Axon Long projection off cell body
Can extend for example from your spinal cord to your toe!
“telephone lines” of the nervous system
Act on information received by the dendrites
Generates action potentials (nerve impulse) down the axon toward a target cell
A bundle of axons is called a nerve

8. Axon Terminal Swelling of nerve endings
Is very close to the membrane of the target cell to form a synapse
Tiny gap across which 2 neurons communicate either with electical signals or with chemical signals
Neuron sending the information is the presynaptic neuron
Neuron receiving the information is the postsynaptic neuron

10. Glial Cells More numerous than neurons They release neurotransmitters
Chemicals associated with nerve impulses
Provide homeostasis for neurons by clearing the synapse of neurotransmitters
Repair neurons and remove dead neurons
3 different kinds of glial cells
Astrocytes, microglia, and Shwann cells

11. Astrocytes
Surround the smallest, most permeable blood vessels in the brain
Contributes to the blood-brain barrier that prevents toxic chemicals from reaching the brain (it’s not perfect)
This barrier usually prevents antibodies from entering the brain – so the brain gets its immune defenses by another glial cell, called microglia

12. Microglia
Glial cell that acts as macrophages and mediators of inflammatory responses

13. Schwann Cells
Forms a multilayered wrap on axons, which forms a lipid-rich sheath called myelin.
Myelinated axons have a white appearance, giving rise to white matter
Areas of the brain that do not appear white (areas rich in cell bodies) area gray and are called gray matter
Nodes of Ranvier are between the schwann cells
ALD (Lorenzo) and Multiple Sclerosis are diseases where the myelin is affected

15. 3 Functional Categories of Neurons
Afferent neurons
Carry sensory information into the nervous system (coming from sensory cells) that transduce sensory stimuli into action potentials (nerve impulses)
Efferent neurons
Carry commands to physiological and behavioral effectors such as muscles and glands (example – motor neurons carry commands to muscle cells)
Interneurons
Integrate and store information and communicate between afferent and efferent neurons (most neurons in brain are interneurons)

16. Neural Networks

17. Neural Networks

18. Neural Networks

19. 34.2 Electrical Signals

20. Action Potentials Nerve impulses Carry information along neurons
Sudden and large changes in membrane potential (difference in electrical charge across the plasma membrane) that travel along axons and cause the release of chemical signals at the axon terminal

21. Voltage
Measure of the difference in electrical charge between 2 points
Represents potential energy because opposite charges will move together if given a chance
In wires, electrical current is carried by electrons
In solutions and across cell membranes, electric current is carried by ions

22. Ions Major ions involved are
Sodium (Na+)
Potassium (K+)
Calcium (Ca(2+)
Chloride (Cl-)
In cells these ions are kept at different concentrations inside and outside the cell
The result of differing these concentrations is the voltage across the cell membrane, known as a membrane potential

23. Resting Membrane Potential
Occurs in an inactive neuron (not sending or receiving a signal)
Typically between -60 and -70 millivolts (mV)
The minus sign refers to a electrically negative cell compared to the outside of the cell
Action Potentials (nerve impulses) are generated when there is sudden change in this voltage to where it is more positive inside than outside
How is this done?
Sodium Potassium Pump!

24. Sodium Potassium Pump Review
Form of active transport, uses ATP
Na+ ions are pumped out of the cell and exchanged for potassium ions from the outside of the cell
Remember this exchange is uneven. The sodium potassium pump is constantly pumping Na+ out and K+, but the concentration of Na+ is higher outside than inside and the concentration of K+ is higher inside than outside.
These concentration gradients will be used to generate the resting potential and changes in the resting potential
How does the resting potential change?

27. A stimulus occurs (light, pinch, etc.)
That triggers a voltage gated Na+ channel to open which brings Na+ ions into the cell
Sodium is going into the cell because it moves from H  L and there is more sodium outside the cell than inside the cell
So now the inside of the cell becomes less negative (70mV  50mV) – this is called depolarization
When the inside of the neuron becomes less negative (more positive)

28. What happens next?
Additional voltage gated Na+ channels open, causing a rapid spike of depolarization – an action potential.
The action potential is traveling down the axon.
The depolarization triggers voltage gated K+ ions to open, which allows K+ to flow out of the cell (from H  L) – this is called hyperpolarized
Membrane is becoming even more negative

30. Action Potential Chapter 49 Section 1 Neurons and Nerve Impulses
Click below to watch the Visual Concept.
Visual Concept

31. Action Potentials Signal strength does not change during travel
All or nothing
Positive feedback mechanism to ensure that action potentials always rise to their maximum value
Self regenerating – 1 action potential stimulates another etc.
Cannot go in reverse due to the refractory period (time during which membrane is returning to resting potential)
Travel faster in myelinated axons and in larger-diameter axons
Squid axons are big, so their response time is rapid!

32. Communication between neurons
Once an action potential reaches the axon terminal, it releases neurotransmitters into the synaptic cleft. These neurotransmitters bind to receptors proteins and open the ion channels of the new neuron cell.
If enough ion channels are opened, the action potential will continue through the new neuron. If not, the nervous signal will be terminated.
After the neurotransmitters have opened the ion channels, they will be cleared out of the synaptic cleft by being reabsorbed by the neuron that released them or broken down by enzymes.

34. Release of Neurotransmitter
Section 1 Neurons and Nerve Impulses
Chapter 49
Release of Neurotransmitter
Click below to watch the Visual Concept.
Visual Concept

35. Chemical Synapse
Neurotransmitters released from a presynaptic cell bind to receptors in the membrane of a postsynaptic cell
The neurotransmitter used by all vertebrate neuromuscular synapses is acetylcholine (ACh)
7 steps

36. 7 steps to a chemical synapse
Action potential arrives at axon terminal.
Na+ channels open; depolarization causes voltage gated Ca2+ channels to open
Ca2+ enters the cell and triggers fusion of acetylcholine vesicles with the presynaptic membrane
Acetylcholine molecules diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane
When binding occurs, they open up their channels and depolarize the postsynaptic membrane
The spreading depolarization fires an action potential in the postsynaptic membrane
Acetylcholine is broken down

38. Neurotransmitter Action – how does it stop?
They must be cleared from synapse
Enzymes may destroy the neurotransmitters
Or neurotransmitters might simply diffuse away from the cleft

39. Types of Neurotransmitters
More than 50 recognized
Acetylcholine is used in the brain with motor neurons
Others are GABA, dopamine, norepinephrine, serotonin, endorphins

40. Drugs interfere Drugs can interfere with neurotransmitter release
Toxins from Clostridium destroy proteins necessary for the binding of vesicles to the presynaptic membrane.
These toxins cause botulism and tetanus – fatal diseases that involve muscle impairment due to loss of neurotransmitter release