2. The Nervous System
Chapter 12

3. Overview Regulates and maintains homeostasis
Communicate electrochemically
Messages are rapid and specific
Almost always an immediate response

4. Function Monitor changes inside and outside body
Process input and make decisions
Effect a response

5. Organization
Nervous system is composed of nerve cells that form incoming (afferent) and outgoing (efferent) pathways
Organized and divided:

6. CNS – brain and spinal cord; command center
PNS – cranial, spinal and peripheral nerves; 2 divisions:
Sensory - incoming

7. Motor - outgoing
Somatic – carry information to somatic effectors (skeletal muscles)

8. Autonomic – carry information to the visceral or autonomic effectors (cardiac, smooth muscles and glands); further divided into:

9. Sympathetic –”fight or flight” response; “on switch”
Parasympathetic – “resting & digesting”; “off switch”

11. Histology
Composed of glial (support) cells and neurons (nerve cells)

12. Astrocytes – anchors neurons to capillaries controls ions; ½ of neural tissue are these
(think Phenomena)

13. Microglia – protect CNS; engulf micro-
organisms;
Dedicated
immune-type cell

14. Ependymal - circulate cerebral spinal fluid with their cilia

15. Oligodendrocytes – insulate
Neurons;
Found
Only in
the
CNS

16. Schwann cells – found in PNS; act as oligo-dendrocytes and microglia

17. Satellite cells – control chemical environment of neurons in the PNS

19. Neurons 3 basic characteristics: Longevity Amitotic – lack centrioles
High metabolic rate (need O2 and glucose)

20. Structure – pg. 348 – composed of a cell body & one or more slender “processes” extending out from it; plasma membrane is the site of electrical stimulation

21. Cell body – typical cell structures
Dendrites – process that receive information
Axon – process that sends information
Synaptic knobs – secrete neurotransmitters

22. Myelin sheath – insulates axon
Synapse – gap between axon of one neuron and dentrite of next neuron
Nodes of Ranvier – gaps in myelin

23. Synaptic knobs

24. Classified by structure
Multipolar – most common of the nerve structure; one axon and several dendrites
Bipolar – retina, cochlear, olfactory sensory receptors; one axon and one highly branched dendrite

25. Unipolar – spinal and cranial nerves; single process that splits into one axon (to the CNS) and one dendrite (away from CNS)

26. Classified by function – by the direction in which they conduct impulses.
Afferent (sensory) – incoming information; cell body is in the PNS, axon is in the CNS

27. Efferent (motor) – outgoing information; cell body is in the CNS, axon is in the PNS
Interneuron – connects the two; cell body and axon is in the CNS

28. Reflex Arc
Patterned arrangement of neurons through which flow information. Usually receptor → afferent → interneuron → efferent → effector

30. Synapse
Space between the synaptic bulbs of an axon and the dendrites of the next neuron.

31. Ipsilateral – synapses between multiple neurons are on the same side of the body

32. Contralateral – synapses between multiple neurons are on opposite sides of the body

33. Multisynaptic arcs

34. Nerves and Tracts
Nerve – bundles of nerve fibers (neurons) surrounded by connective tissue
Endoneurium – membrane surrounding each neuron; bundled together into a fascicle

35. Perineurium – membrane surrounding each fascicle
Epineurium – membrane surrounding bundles of fascicles.
Tract – bundles of neurons in the CNS

37. Repair of Neurons Regeneration of damaged neurons
Close to cell body = death
PNS repair
Requires Schwann cells to produce myelin “tunnel” through which the axon grows
Effector neuron may require “relearning”

39. CNS repair
Never regrow due to the type of support cells
Oligodendrocytes die; replaced by “scars” which block regeneration
At the center of much research

40. Neurophysiology
How neurons are excited, inhibited and/or how they communicate with each other

41. Difference in ion numbers on either side of the neuron’s plasma membrane
Sets up situation in which there is potential energy

42. Measured in volts; the greater the ion difference = higher voltage
Flow of charges (ions) across the membrane = current
(“action potential”) _ _

43. Resting Membrane Potential
Communication of neurons depends on 2 properties of their plasma membrane:
Electrical voltage across membrane – “resting membrane potential” (think mousetrap)

44. Ion channels which open and close in response to changes in permeability of plasma membrane

45. Ions
flow down a concentration gradient (from high to low concentration)
UNLESS the cell uses active transport to keep the ions concentrated on one or the other side of the membrane

48. A nerve conducts an impulse when:
active transport is turned off
ions are allowed to flow down gradients
This flow:
changes the voltage
causes impulse to travel down axon

50. When a neuron is at rest (“polarized”) the concentration of Na+ outside the cell is greater than the K+ inside the cell.

51. Therefore, the inside of the cell is more (-) compared to the outside.
The neuron is “charged” and ready to receive an impulse

52. When cell body is stimulated (usually by a neurotransmitter), ion gates open up – in sequence – along the axon allowing Na+ and K+ ions to flow down concentration gradients (Na+ in, K+ out)

53. The flip in electrical charges is an “impulse”
The neuron is now “depolarized”

54. Once impulse has passed, and ions have flowed into/out of the cell, active transport is reimposed and Na+/K+ ions are returned to their original positions

55. The cell is said to be “repolarized” and is now ready to receive another impulse

57. Saltatory Conduction
In order to speed the impulse along and axon, ion gates only open in areas of the axon not covered by myelin (at the Nodes of Ranvier).
In this way, the impulse “jumps” from node to node speeding the impulse along

59. Synaptic Transmission
Neuron synapses – specialized for the release of chemicals (“neurotransmitters”) when the end of the axon is depolarized
Nerve impulse triggers the opening of Ca+ channels

60. Ca+ is released into the vesicles of the axon terminals and cause them to release their neurotransmitters into the synapse
Neurotransmitters diffuse across the synapse and bind to the dendrites of the next neuron causing it to depolarize.

63. Types of Neurotransmitters
Excitatory – neurotransmitters released to send a message
Inhibitory – neurotransmitters released to prevent a message from being sent

65. Examples of Neurotransmitters
Acetylcholine (Ach) found in PNS nerves, causes skeletal muscles to contract (excitatory) but causes heart muscle to slow (inhibitory) – lacking in Alzheimer’s patients

66. Dopamine – found in CNS (inhibitory in skeletal muscles) – lacking in Parkinson’s patients
Epinephrine and norepinephrine – provides fight or flight (excitatory in sympathetic system)

67. Enzymes are used to remove or begin uptake of neurotransmitters from the synapse.
Ex: cholinesterase breaks down Ach and thus stops the stimulation of a muscle.

68. New medications can increase the amount of neurotransmitter secreted into the synapse OR inhibit the activity of “uptake” enzymes so to allow the neurotransmitter to stay in the synapse longer

69. Any questions?