1. David Sadava H. Craig Heller Gordon H. Orians William K. Purves David M. Hillis Biologia.blu C – Il corpo umano Neurons and Nervous Tissue

2. What cells are unique to the nervous system?
Neurons and Nervous Tissue
What cells are unique to the nervous system?
How do neurons generate and conduct signals?
How do neurons communicate with other cells?

3. Nervous systems have two categories of cells:
Neurons and Nervous Tissue - What cells are unique to the nervous system?
Nervous systems have two categories of cells:
neurons generate and propagate electrical signals, called action potentials;
glial cells provide support and maintain extracellular environment.

4. Neurons are organized into networks.
Neurons and Nervous Tissue - What cells are unique to the nervous system?
Neurons are organized into networks.
Afferent neurons carry information into the system.
Sensory neurons convert input into action potentials.
Efferent neurons carry commands to effectors.
Interneurons store information and help with communication in the system.

5. Networks vary in complexity. Nerve net: simple network of neurons.
Neurons and Nervous Tissue - What cells are unique to the nervous system?
Networks vary in complexity.
Nerve net: simple network of neurons.
Ganglia: neurons organized into clusters, sometimes in pairs.
Brain: the largest pair of ganglia.

6. Neurons and Nervous Tissue - What cells are unique to the nervous system?
Central nervous system (CNS) – consists of cells found in brain and spinal cord.
Peripheral nervous system (PNS) – neurons and support cells found outside the CNS.

7. Neurons pass information at synapses:
Neurons and Nervous Tissue - What cells are unique to the nervous system?
Neurons pass information at synapses:
the presynaptic neuron sends the message;
the postsynaptic neuron receives the message.

8. Most neurons have four regions:
Neurons and Nervous Tissue - What cells are unique to the nervous system?
Most neurons have four regions:
cell body contains the nucleus and organelles;
dendrites bring information to the cell body;
axon carries information away from the cell body;
axon terminal forms synapse at tip of axon.

9. Neurons and Nervous Tissue - What cells are unique to the nervous system?

10. Glial cells, or glia, outnumber neurons in the human brain.
Neurons and Nervous Tissue - What cells are unique to the nervous system?
Glial cells, or glia, outnumber neurons in the human brain.
Glia do not transmit electrical signals but have several functions:
support during development;
supply nutrients;
maintain extracellular environment;
insulate axons.

11. Oligodendrocytes produce myelin and insulate axons in the CNS.
Neurons and Nervous Tissue - What cells are unique to the nervous system?
Oligodendrocytes produce myelin and insulate axons in the CNS.
Schwann cells insulate axons in the PNS.
Astrocytes contribute to the blood– brain barrier, which protects the brain.

12. Neurons and Nervous Tissue - What cells are unique to the nervous system?
Wrapping up an axon

13. Oligodendrocytes produce myelin and insulate axons in the CNS.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Oligodendrocytes produce myelin and insulate axons in the CNS.
Schwann cells insulate axons in the PNS.
Astrocytes contribute to the blood– brain barrier, which protects the brain.

14. Membrane potential is the electric potential across the membrane.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Action potentials are the result of ions moving across the plasma membrane.
Ions move according to differences in concentration gradients and electrical charge.
Membrane potential is the electric potential across the membrane.
Resting potential is the membrane potential of a resting neuron.

15. Voltage causes electric current as ions to move across cell membranes.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Voltage causes electric current as ions to move across cell membranes.
Major ions in neurons:
Sodium (Na+)
Potassium (K+)
Calcium (Ca2+)
Chloride (Cl–)

16. Membrane potentials are measured with electrodes.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Membrane potentials are measured with electrodes.
The resting potential of an axon is –60 to –70 millivolts (mV).
The inside of the cell is negative at rest. An action potential allows positive ions to flow in briefly, making the inside of the cell more positive.

17. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Measuring the resting potential

18. This establishes concentration gradients for Na+ and K+.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
The plasma membrane contains ion channels and ion pumps that create the resting and action potentials.
The sodium–potassium pump uses ATP to move Na+ ions from inside the cell and exchanges them for K+ from outside the cell.
This establishes concentration gradients for Na+ and K+.

19. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Ion pumps and channels (part 1)

20. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Ion pumps and channels (part 2)

21. These two forces acting on an ion are its electrochemical gradient.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Ion channels in the membrane are selective and allow some ions to pass more easily.
The direction and size of the movement of ions depends on the concentration gradient and the voltage difference of the membrane.
These two forces acting on an ion are its electrochemical gradient.

22. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Potassium channels are open in the resting membrane and are highly permeable to K+ ions.
K+ ions diffuse out of the cell along the concentration gradient and leave behind negative charges within the cell.
K+ ions diffuse back into the cell because of the negative electrical potential.

23. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Membranes can be depolarized or hyperpolarized (part 1)

24. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Membranes can be depolarized or hyperpolarized (part 2)

25. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Membranes can be depolarized or hyperpolarized (part 3)

26. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Membranes can be depolarized or hyperpolarized (part 4)

27. Action potentials are sudden, large changes in membrane potential.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Action potentials are sudden, large changes in membrane potential.
Voltage-gated Na+ and K+ channels are responsible for action potentials.
If a cell body is depolarized, voltage- gated Na+ channels open and Na+ rushes into the axon. The influx of positive ions causes more depolarization.

28. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
A threshold is reached at 5–10 mV above resting potential.The influx of Na+ is not offset by the outward movement of K+.
Many voltage-gated Na+ channels then open, the membrane potential becomes positive, and an action potential occurs.
The axon returns to resting potential as voltage-gated Na+ channels close and voltage-gated K+ channels open.

29. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
The course of an action potential (part 1)

30. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
The course of an action potential (part 2)

31. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
The course of an action potential (part 3)

32. Na+ channels have two gates:
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Voltage-gated Na+ channels have a refractory period during which they cannot open.
Na+ channels have two gates:
an activation gate is closed at rest but opens quickly at threshold;
an inactivation gate is open at rest and closes at threshold but responds more slowly, the gate reopens 1–2 milliseconds later than the activation gate closes.

33. The dip after an action potential is called hyperpolarization.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Voltage-gated K+ channels contribute to the refractory period by remaining open.
The efflux of K+ ions makes the membrane potential less negative than the resting potential for a brief period.
The dip after an action potential is called hyperpolarization.

34. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
An action potential is an all-or-none event because voltage-gated Na+ channels have a positive feedback mechanism that ensures the maximum value of the action potential.
An action potential is self- regenerating because it spreads to adjacent membrane regions.

35. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Action potentials travel along axons (part 1)

36. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Action potentials travel along axons (part 2)

37. Myelination by glial cells increases the conduction velocity of axons.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Myelination by glial cells increases the conduction velocity of axons.
The nodes of Ranvier are regularly spaced gaps where the axon is not covered by myelin.
Action potentials are generated at the nodes and the positive current flows down the inside of the axon.

38. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
When the positive current reaches the next node, the membrane is depolarized and another axon potential is generated.
Action potentials appear to jump from node to node, a form of propagation called saltatory conduction.

39. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Saltatory action potentials

40. Neurons communicate with other neurons or target cells at synapses.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Neurons communicate with other neurons or target cells at synapses.
In a chemical synapse chemicals from a presynaptic cell induce changes in a postsynaptic cell.
In an electrical synapse the action potential spreads directly to the postsynaptic cell.

41. The motor neuron releases acetylcholine (ACh) from its axon terminals.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
The neuromuscular junction is a chemical synapse between motor neurons and skeletal muscle cells.
The motor neuron releases acetylcholine (ACh) from its axon terminals.
The postsynaptic membrane of the muscle cell is the motor end plate.

42. Vesicles release ACh into the synaptic cleft.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
The synaptic cleft is the space between the presynaptic and postsynaptic membranes.
An action potential causes release of the neurotransmitter ACh when voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal.
Vesicles release ACh into the synaptic cleft.

43. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
Chemical synaptic transmission begins with the arrival of an action potential

44. The postsynaptic membrane responds to ACh.
Neurons and Nervous Tissue - How do neurons generate and conduct signals?
The postsynaptic membrane responds to ACh.
ACh diffuses across the cleft and binds to ACh receptors on the motor end plate.
These receptors allow Na+ and K+ to flow through and the increase in Na+ depolarizes the membrane.

45. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
The acetylcholine receptor is a chemically gated channel

46. Neurons and Nervous Tissue - How do neurons communicate with other cells?
The synapses between motor neurons and muscle cells are excitatory. ACh always causes depolarization.
Other synapses can be inhibitory, if the postsynaptic response is hyperpolarization.
A neuron has many synapses and may receive many different chemical messages.

47. The postsynaptic cell must sum the excitatory and inhibitory input.
Neurons and Nervous Tissue - How do neurons communicate with other cells?
The postsynaptic cell must sum the excitatory and inhibitory input.
Summation takes place at the part of the cell body at the base of the axon.
Spatial summation adds up messages at different synaptic sites.
Temporal summation adds up potentials generated at the same site, over time.

48. Neurons and Nervous Tissue - How do neurons generate and conduct signals?
The postsynaptic neuron sums information