1. Chapter 2 Nerve Cells and Nerve Impulses

2. The Cells of the Nervous System
The human nervous system is comprised of two kinds of cells:
Neurons
Glia
The human brain contains approximately 100 billion individual neurons.
Behavior depends upon the communication between neurons.

3. Fig. 2-1, p. 30 Figure 2.1: Estimated numbers of neurons in humans.
Because of the small size of many neurons and the variation in cell density from one spot to another, obtaining an accurate count is difficult. (Source: R. W. Williams & Herrup, 1988)
Fig. 2-1, p. 30

4. Fig. 2-4, p. 32 Figure 2.4: Neurons, stained to appear dark.
Note the small fuzzy-looking spines on the dendrites.
Fig. 2-4, p. 32

5. The Cells of the Nervous System
Spaniard Santiago Ramon y Cajal ( ) was the first to demonstrate that the individual cells comprising the nervous system remained separate.
He showed that they did not grow into each other as previously believed.

6. The Cells of the Nervous System
Like other cells in the body, neurons contain the following structures:
Membrane
Nucleus
Mitochondria
Ribosomes
Endoplasmic reticulum

7. Figure 2.2: An electron micrograph of parts of a neuron from the cerebellum of a mouse.
The nucleus, membrane, and other structures are characteristic of most animal cells. The plasma membrane is the border of the neuron. Magnification approximately x 20,000. (Source: Micrograph courtesy of Dennis M. D. Landis)
Fig. 2-2, p. 31

8. The Cells of the Nervous System
The membrane refers to the structure that separates the inside of the cell from the outside environment.
The nucleus refers to the structure that contains the chromosomes.
The mitochondria are the strucures that perform metabolic activities and provides energy that the cells requires.
Ribosomes are the sites at which the cell synthesizes new protein molecules

9. Fig. 2-3, p. 32 Figure 2.3: The membrane of a neuron.
Embedded in the membrane are protein channels that permit certain ions to cross through the membrane at a controlled rate.
Fig. 2-3, p. 32

10. The Cells of the Nervous System
Neuron cells are similar to other cells of the body but have a distinctive shape.
A motor neuron has its soma in the spinal cord and receives excitation from other neurons and conducts impulses along it axon to a muscle.
A sensory neuron is specialized at one end to be highly sensitive to a particular type of stimulation (touch, temperature, odor etc.)

11. Figure 2.5: The components of a vertebrate motor neuron.
The cell body of a motor neuron is located in the spinal cord. The various parts are not drawn to scale; in particular, a real axon is much longer in proportion to the soma.
Fig. 2-5, p. 32

12. Fig. 2-6, p. 33 Figure 2.6: A vertebrate sensory neuron.
Note that the soma is located on a stalk off the main trunk of the axon. (As in Figure 2.5, the various structures are not drawn to scale.)
Fig. 2-6, p. 33

13. The Cells of the Nervous System
All neurons have the following major components:
Dendrites.
Soma/ cell body.
Axon.
Presynaptic terminals.

14. The Cells of the Nervous System
Dendrites- branching fibers with a surface lined with synaptic receptors responsible for bringing in information from other neurons.
Some dendrites also contain dendritic spines that further branch out and increase the surface area of the dendrite.

15. Fig. 2-7, p. 33 Figure 2.7: Dendritic spines.
The dendrites of certain neurons are lined with spines, short outgrowths that receive specialized incoming information. That information apparently plays a key role in long-term changes in the neuron that mediate learning and memory. (Source: From K. M. Harris and J. K. Stevens, Society for Neuroscience, “Dendritic spines of CA1 pyramidal cells in the rat hippocampus: Serial electron microscopy with reference to their biophysical characteristics.” Journal of Neuroscience, 9, 1989, 2982–2997. Copyright © 1989 Society for Neuroscience. Reprinted by permission.)
Fig. 2-7, p. 33

16. The Cells of the Nervous System
Soma - contains the nucleus, mitochondria, ribosomes, and other structures found in other cells.
Also responsible for the metabolic work of the neuron.

17. The Cells of the Nervous System
Axon - thin fiber of a neuron responsible for transmitting nerve impulses away to other neurons, glands, or muscles.
Some neurons are covered with an insulating material called the myelin sheath with interruptions in the sheath known as nodes of Ranvier.

18. The Cells of the Nervous System
Presynaptic terminals refer to the end points of an axon responsible for releasing chemicals to communicate with other neurons.

19. The Cells of the Nervous System
Terms used to describe the neuron include the following:
Afferent axon - refers to bringing information into a structure.
Efferent axon - refers to carrying information away from a structure.
Interneurons or Intrinsic neurons are those whose dendrites and axons are completely contained within a structure.

20. Fig. 2-8, p. 34 Figure 2.8: Cell structures and axons.
It all depends on the point of view. An axon from A to B is an efferent axon from A and an afferent axon to B, just as a train from Washington to New York is exiting Washington and approaching New York.
Fig. 2-8, p. 34

21. The Cells of the Nervous System
Neurons vary in size, shape, and function.
The shape of a neuron determines it connection with other neurons and its connections with other neurons.
The function is closely related to the shape of a neuron.
Example: Pukinje cells of the cerebellum branch extremely widely within a single plane

22. Fig. 2-9, p. 34 Figure 2.9: The diverse shapes of neurons.
(a) Purkinje cell, a cell type found only in the cerebellum; (b) sensory neurons from skin to spinal cord; (c) pyramidal cell of the motor area of the cerebral cortex; (d) bipolar cell of retina of the eye; (e) Kenyon cell, from a honeybee. (Source: Part e, from R. G. Coss, Brain Research, October Reprinted by permission of R. G. Coss.)
Fig. 2-9, p. 34

23. The Cells of the Nervous System
Glia are the other major component of the nervous system and include the following:
Astrocytes helps synchronize the activity of the axon by wrapping around the presynaptic terminal and taking up chemicals released by the axon.
Microglia - remove waste material and other microorganisms that could prove harmful to the neuron.

24. Fig. 2-10, p. 35 Figure 2.10: Shapes of some glia cells.
Oligodendrocytes produce myelin sheaths that insulate certain vertebrate axons in the central nervous system; Schwann cells have a similar function in the periphery. The oligodendrocyte is shown here forming a segment of myelin sheath for two axons; in fact, each oligodendrocyte forms such segments for 30 to 50 axons. Astrocytes pass chemicals back and forth between neurons and blood and among neighboring neurons. Microglia proliferate in areas of brain damage and remove toxic materials. Radial glia (not shown here) guide the migration of neurons during embryological development. Glia have other functions as well.
Fig. 2-10, p. 35

25. Figure 2.11: How an astrocyte synchronizes associated axons.
Branches of the astrocyte (in the center) surround the presynaptic terminals of related axons. If a few of them are active at once, the astrocyte absorbs some of the chemicals they release. It then temporarily inhibits all the axons to which it is connected. When the inhibition ceases, all of the axons are primed to respond again in synchrony. (Source: Based on Antanitus, 1998)
Fig. 2-11, p. 36

26. The Cells of the Nervous System
(Types of glia continued)
Oligdendrocytes & Schwann cells- build the myelin sheath that surrounds the axon of some neurons.
Radial glia- guide the migration of neurons and the growth of their axons and dendrites during embryonic development.

27. The Cells of the Nervous System
The blood-brain barrier is a mechanism that surrounds the brain and blocks most chemicals from entering.
Our immune system destroys damaged or infected cells throughout the body.
Because neurons in the brain generally do not regenerate, it is vitally important for the blood brain barrier to block incoming viruses, bacteria or other harmful material from entering.

28. Fig. 2-12, p. 37 Figure 2.12: The blood-brain barrier.
Most large molecules and electrically charged molecules cannot cross from the blood to the brain. A few small, uncharged molecules such as O2 and CO2 cross easily; so can certain fat-soluble molecules. Active transport systems pump glucose and amino acids across the membrane.
Fig. 2-12, p. 37

29. The Cells of the Nervous System
Active transport is the protein mediated process by which useful chemicals are brought into the brain.
Glucose, hormones, amino acids, and vitamins are brought into the brain via active transport.
Glucose is a simple sugar that is the primary source of nutrition for neurons.
Thiamine is a chemical that is necessary for the use of glucose.

30. The Nerve Impulse
A nerve impulse is the electrical message that is transmitted down the axon of a neuron.
The impulse does not travel directly down the axon but is regenerated at points along the axon.
The speed of nerve impulses ranges from approximately 1 m/s to 100 m/s.

31. The Nerve Impulse
The resting potential of a neuron refers to the state of the neuron prior to the sending of a nerve impulse.
The membrane of a neuron maintains an electrical gradient which is a difference in the electrical charge inside and outside of the cell.

32. Figure 2.13: Methods for recording activity of a neuron.
(a) Diagram of the apparatus and a sample recording. (b) A microelectrode and stained neurons magnified hundreds of times by a light microscope.
Fig. 2-13, p. 40

33. The Nerve Impulse
At rest, the membrane maintains an electrical polarization or a difference in the electrical charge of two locations.
the inside of the membrane is slightly negative with respect to the outside. (approximately -70 millivolts)

34. The Nerve Impulse
The membrane is selectively permeable, allowing some chemicals to pass more freely than others.
Sodium, potassium, calcium, and chloride pass through channels in the membrane.
When the membrane is at rest:
Sodium channels are closed.
Potassium channels are partially closed allowing the slow passage of sodium.

35. Figure 2.14: Ion channels in the membrane of a neuron.
When a channel opens, it permits one kind of ion to cross the membrane. When it closes, it prevents passage of that ion.
Fig. 2-14, p. 40

36. The Nerve Impulse
The sodium-potassium pump is a protein complex that continually pumps three sodium ions out of the cells while drawing two potassium ions into the cell.
helps to maintain the electrical gradient.
The electrical gradient and the concentration gradient work to pull sodium ions into the cell.
The electrical gradient tends to pull potassium ions into the cells.

37. Figure 2.15: The sodium and potassium gradients for a resting membrane.
Sodium ions are more concentrated outside the neuron; potassium ions are more concentrated inside. Protein and chloride ions (not shown) bear negative charges inside the cell. At rest, very few sodium ions cross the membrane except by the sodium-potassium pump. Potassium tends to flow into the cell because of an electrical gradient but tends to flow out because of the concentration gradient.
Fig. 2-15, p. 41

38. The Nerve Impulse
The resting potential remains stable until the neuron is stimulated.
Hyperpolarization refers to increasing the polarization or the difference between the electrical charge of two places.
Depolarization refers to decreasing the polarization towards zero.
The threshold of excitement refers any stimulation beyond a certain level and results in a massive depolarization.

39. The Nerve Impulse
An action potential is a rapid depolarization of the neuron.
Stimulation of the neuron past the threshold of excitation triggers a nerve impulse or action potential.

40. The Nerve Impulse
Voltage-activated channels are membrane channels whose permeabililty depends upon the voltage difference across the membrane.
Sodium channels are voltage activated channels.
When sodium channels are opened, positively charged sodium ions rush in and a subsequent nerve impulse occurs.

41. Figure 2.16: The movement of sodium and potassium ions during an action potential.
Sodium ions cross during the peak of the action potential and potassium ions cross later in the opposite direction, returning the membrane to its original polarization.
Fig. 2-16, p. 43

42. The Nerve Impulse
After an action potential occurs, sodium channels are quickly closed.
The neuron is returned to its resting state by the opening of potassium channels.
potassium ions flow out due to the concentration gradient and take with them their positive charge.
The sodium-potassium pump later restores the original distribution of ions.

43. The Nerve Impulse
Local anesthetic drugs block sodium channels and therefore prevent action potentials from occurring.
Example: Novocain

44. The Nerve Impulse
The all-or-none law states that the amplitude and velocity of an action potential are independent of the intensity of the stimulus that initiated it.
Action potentials are equal in intensity and speed within a given neuron.

45. The Nerve Impulse
After an action potential, a neuron has a refractory period during which time the neuron resists another action potential.
The absolute refractory period is the first part of the period in which the membrane can not produce an action potential.
The relative refractory period is the second part in which it take a stronger than usual stimulus to trigger an action potential.

46. The Nerve Impulse
In a motor neuron, the action potential begins at the axon hillock (a swelling where the axon exits the soma).
Propagation of the action potential is the term used to describe the transmission of the action potential down the axon.
the action potential does not directly travel down the axon.

47. Figure 2.17
Current that enters an axon during the action potential flows down the axon, depolarizing adjacent areas of the membrane. The current flows more easily through thicker axons. Behind the area of sodium entry, potassium ions exit.
Fig. 2-17, p. 45

48. The Nerve Impulse
The myelin sheath of axons are interrupted by short unmyelinated sections called nodes of Ranvier.
At each node of Ranvier, the action potential is regenerated by a chain of positively charged ion pushed along by the previous segment.

49. Figure 2.18: An axon surrounded by a myelin sheath and interrupted by nodes of Ranvier.
The inset shows a cross-section through both the axon and the myelin sheath. Magnification approximately x 30,000. The anatomy is distorted here to show several nodes; in fact, the distance between nodes is generally about 100 times as large as the nodes themselves.
Fig. 2-18, p. 46

50. The Nerve Impulse
Saltatory conduction is the word used to describe this “jumping” of the action potential from node to node.
Provides rapid conduction of impulses
Conserves energy for the cell
Multiple sclerosis is disease in which the myelin sheath is destroyed and associated with poor muscle coordination.

51. Figure 2.19: Saltatory conduction in a myelinated axon.
An action potential at the node triggers flow of current to the next node, where the membrane regenerates the action potential.
Fig. 2-19, p. 46

52. The Nerve Impulse Not all neurons have lengthy axons.
Local neurons have short axons, exchange information with only close neighbors, and do not produce action potentials.
When stimulated, local neurons produce graded potentials which are membrane potentials that vary in magnitude and do not follow the all-or-none law,.
A local neuron depolarizes or hyperpolarizes in proportion to the stimulation.