1. Nervous System
AP Biology Chap 48

2. Neuron
The basic structural unit of the nervous system

3. The job of the neurons
Neurons transfer long-distance information via electrical signals and usually communicate between cells using short-distance chemical signals.

4. The higher order processing of nervous signals may involve clusters of neurons called ganglia or most structured groups of neurons organized into a brain.

5. Types of neurons Sensory (afferent) – receive stimulus
Motor (efferent) stimulate effectors which are target cells, muscles, sweat glands, stomach, etc.
Association (interneurons) located in spinal cord or grain integrate or evaluate impulses for appropriate responses.

7. The transmitting cell is called the presynaptic cells
The receiving cell is the postsynaptic cell

8. Neuron Structure
Cell body which contains the nucleus and organelles and numerous extensions
Dendrites receive signals
Axon longer, transmits signals
Ends of axons end in synaptic terminals which release neurotransmitters across a synapse
Glial cells nourish and support the neurons

9. Direction of impulse Dendrites Stimulus Presynaptic Nucleus cell Axon
Fig. 48-4
Dendrites
Stimulus
Nucleus
Presynaptic
cell
Axon
hillock
Cell
body
Direction of impulse
Axon
Synapse
Synaptic terminals
Postsynaptic cell
Neurotransmitter

10. Glial Cells Nourish neurons Insulate axons
Regulate the extracellular fluid around the neuron

11. Nerve conduction
In order to conduct an electrical nerve impulse, a voltage or membrane potential, exists across the plasma membrane of all cells.
For a typical non-transmitting neuron, this is called the resting potential and is between -60 and -80 mV.

12. Membrane Potential Inside is NEGATIVE!
Principal cation inside of cell K
Principle anion inside of cell:
negatively-charged proteins, amino
acids, PO4 and SO4. Symbol is A-.
Inside is NEGATIVE!

13. Outside of the cell
Principal ion is Na+
Outside is positive!

16. Measuring membrane potential

17. How is the membrane potential established?
Ion channels
Concentration of ions
Size of particles (proteins too large – semipermeable nature of membrane)
Na-K pump maintains Na outside and K inside

18. Key Na+ Sodium- potassium Potassium Sodium pump channel channel K+
Fig. 48-6b
Key
Na+
Sodium-
potassium
pump
Potassium
channel
Sodium
channel K+
OUTSIDE
CELL
INSIDE
CELL
(b)

20. What causes the generation of a nerve signal?
Neurons and muscle cells are excitable cells – they can change their membrane potentials due to gated ion channels* – can be chemically gated which respond to neurotransmitters or voltage-gated which respond to a change in membrane potential.
* Found only in nerve cells

21. Upon receiving a stimulus, Na+  channels open and Na+ flows into the cells and thus they become more positive inside and
more negative outside and the charge on the membrane becomes depolarized.
The stronger the stimulus, the more Na
gated Ion channels open.

22. Production of an Action Potential
Once depolarization reaches a certain membrane voltage called the threshold level (-50 mv), more Na gates open and an action potential is triggered that results in complete depolarization.
This stimulates neighboring Na gates, further down the neuron, to open. The action potential is an all or none event, always creating the same voltage spike once the threshold is reached.

23. Notice, gates are closed!
Fig
Key
Na+ K+
+50
Action
potential 3
Membrane potential
(mV) 2 4
Threshold
–50 1 1 5
Resting potential
Depolarization
–100
Time
Extracellular fluid
Sodium
channel
Potassium
channel
Plasma
membrane
Notice, gates are closed!
Cytosol
Inactivation loop
Undershoot 1
Resting state

24. Notice, gates are closed!
Fig
Key
Na+ K+
Some Na+ gates open!
+50
Action
potential 3
Membrane potential
(mV) 2 4
Threshold
–50 1 1 5
Resting potential 2
Depolarization
–100
Time
Extracellular fluid
Sodium
channel
Potassium
channel
Notice, gates are closed!
Plasma
membrane
Cytosol
Inactivation loop
Undershoot 1
Resting state

25. A lot of Na+ gates open! Fig. 48-10-3 Key Na+ Na+ gates open! K+
Rising phase of the action potential
+50
Action
potential 3
Membrane potential
(mV) 2 4
Threshold
–50 1 1 5
Resting potential 2
Depolarization
–100
Time
Extracellular fluid
Sodium
channel
Potassium
channel
Plasma
membrane
Cytosol
Inactivation loop
Undershoot 1
Resting state

26. In response to the inflow of Na, the gated K channels begin to open, allowing K to rush to the outside of the cell. Na gates close. This creates a reverse charge polarization, (neg outside, positive inside) called repolarization.

27. Na closes, K opens Fig. 48-10-4 Key Na+ K+3
Rising phase of the action potential 4
Falling phase of the action potential
+50
Na closes, K opens
Action
potential 3
Membrane potential
(mV) 2 4
Threshold
–50 1 1 5
Resting potential 2
Depolarization
–100
Time
Extracellular fluid
Sodium
channel
Potassium
channel
Plasma
membrane
Cytosol
Inactivation loop
Undershoot 1
Resting state

28. In fact more K ions go out than is actually needed to return to threshold,
resulting in an increased negative charge inside called a hyperpolarization or undershoot.
This keeps the direction of the nerve impulse going one way and not backing up.

29. K just keeps flowing out. Hyperpolarization Fig. 48-10-5 Key Na+ K+3
Rising phase of the action potential 4
Falling phase of the action potential
+50
Action
potential 3
Membrane potential
(mV) 2 4
Threshold
–50
K just keeps
flowing out. 1 1 5
Resting potential 2
Depolarization
–100
Time
Extracellular fluid
Sodium
channel
Potassium
channel
Plasma
membrane
Cytosol
Inactivation loop 5
Undershoot 1
Resting state
Hyperpolarization

30. Refractory Period
After the impulse, the Na channels remain inactivated
Since the neuron cannot respond to another stimulus with the reversal of charges, the Na-K pump has to restore the original charge location. This is called the refractory period.
Action Potentials Video | DnaTube.com - Scientific Video Site

31. Requires the Na-K pump

34. Fig. 48-11-3 Axon Plasma membrane Action potential Na+ Cytosol ActionK+
Na+ K+
Action
potential K+
Na+ K+

36. Properties of an Action Potential
Are all or none depolarization – once threshold is reached (-50 mV) – always creates the same voltage spike regardless of intensity of the stimulus.
The frequency of the action potentials increases with intensity of stimulus.
Action potentials travel in only ONE direction!
The greater the axon diameter, the faster action potentials are propagated.

38. Importance of myelin Acts as insulators.
Gaps in the myelin are called nodes of Ranvier and serve as points along which the action potential is propagated, increasing the speed.
This is called saltatory conduction.

39. The myelin sheath is composed of Schwann cells (PNS) or oligodendrocytes (CNS) that encircle the axon in vertebrates.

40. Saltatory Conduction
Voltage channels concentrated at the nodes of Ranvier - jumping action potentials

41. Multiple Sclerosis

42. The Synapse
Area between two neurons, between sensory receptors and neurons or between neurons and muscle cells or gland cells

43. What happens at the synapse?
Fig
What happens at the synapse? 5
Na+ K+
Synaptic vesicles
containing
neurotransmitter
Presynaptic
membrane
Voltage-gated
Ca2+ channel
Postsynaptic
membrane 1
Ca2+ 4 2 6
Synaptic
cleft 3
Figure A chemical synapse
Ligand-gated
ion channels

44. Types of synapses
Electrical – via gap junctions such as in giant axons of crustaceans
**Chemical – electrical impulses changed into chemical signals
Arrival of action potential opens Ca+ channels (membrane signaling cAMP), causes synaptic vesicles full of NT’s to fuse with membrane and pop open

45. Post-synaptic Responses
EPSP - excitatory post-synaptic potential     --> open Na channels --> inside +
May generate an AP
IPSP - inhibitory post-synaptic potential
opens Cl channels -  Cl-in  -> 
more neg  >  no AP                                                   
-->  opens K channels - K-out ->  more  neg  >  no AP   

46. EPSP and IPSP

47. Integration of impulses

48. summation Through summation, an IPSP can counter the effect of an EPSP
The summed effect of EPSPs and IPSPs determines whether an axon hillock will reach threshold and generate an action potential

49. Summation of impulses

50. Temporal and Spatial Summation

51. Temporal summation occurs with repeated release of nt’s from one or more synaptic terminals before RP
Spatial summation occurs when several different presynaptic terminals release NT’s simultaneously

52. Assume a single IPSP has a negative magnitude of -0
Assume a single IPSP has a negative magnitude of -0.5 mV at the axon hillock and that a single EPSP has a positive magnitude of +0.5 mV, for a neuron with initial membrane potential of -70 mV, the net effect of 5 IPSP’s and 2 EPSPs spatially would be to move the membrane potential to? Would the impulse continue?
-85 mV

53. Neurotransmitters Affect ion channels
Affect signal transduction pathways
How? Involve cAMP, cAMP protein kinases, GTP, GTP binding proteins

54. After release, the neurotransmitter
May diffuse out of the synaptic cleft
May be taken up by surrounding cells
May be degraded by enzymes

55. Neurotransmitters
The same neurotransmitter can produce different effects in different types of cells
There are five major classes of neurotransmitters: acetylcholine, biogenic amines, amino acids, neuropeptides, and gases

56. a. ACETYLCHOLINE Found in vertebrate neuromuscular junctions
- excitatory at skeletal muscles
- inhibitory at heart

57. b) Biogenic Amines (derived from amino acids)
epinephrine, norepinephrine (fight or flight),
dopamine, serotonin (involved in sleep, mood, attention, and learning).

58. Blocking epinephrine

59. c) Amino Acids Types: GABA – most common inhibitor
Glutamate - excitatory

60. d) Neuropeptides (short chains of amino acids)
Types
Endorphins – inhibitory, relieves pain
Opiates – mimic endorphins

61. e) Gaseous signals
Gases such as nitric oxide and carbon monoxide are local regulators in the PNS

62. How do drugs work?
Agonists – mimic drugs such as in nicotine mimicking acetycholine
Antagonists – block action of NT’s such as atropine and curare (poisons) – block acetylcholine and thus prevent nerve firing in muscles – leads to paralysis and death
Cocaine and amphetamines block the reuptake of NT’s at adrenergic synapses
Many antidepressants block reuptake of serotonin so serotonin lingers longer in synaptic cleft.