1. Nervous System
Chapter 9
Pages

2. Chapter 9 Wordbytes af- = toward 11. -ferrent = carried
arachn- = spider gangli- = swelling
astro- = star glia = glue
auto- = self mening- =membrane
dendro- = tree micro- = small
di- = 2, through 16. neuro- = nerve
ef- = away from 17. –oid = similar to
encephalo- = brain 18. oligo- = few
enter- = intestines peri- = around
epen- = above somat- = body

3. Nervous System Overview
Master controller and communicator for the body
Responsible for all behavior
3 functions:
Sensory input monitors changes inside/outside of body
Integration processes and interprets, then decides what should be done
Motor output causes a response in effector organs

4. Organization—2 main parts:
Central Nervous System (CNS) = brain and spinal cord
Interprets incoming sensory info. and dictates motor responses
Peripheral Nervous System (PNS) = nerves from brain & in spinal cord
INPUT-Afferent or Sensory division
OUTPUT- Efferent or Motor division
Subdivided: Somatic (SNS—from CNS to skeletal muscles=voluntary) & Autonomic (ANS—regulate smooth & cardiac muscle, glands=involuntary)

5. Major structures:

6. Histology Highly cellular—densely packed & tightly intertwined
2 types of cells:
Neuron= nerve cell
Specialized for signal carrying & information processing
Neuroglia cells support, nourish & protect neurons
Neuroglia critical for homeostasis of interstitial fluid around neurons

7. Supporting cells (Neuroglia)
~ half the volume of CNS
Cells smaller than neurons
Can multiply and divide and fill in brain areas
Do not conduct nerve impulses

8. Supporting Cells in CNS
Astrocytes most abundant and most versatile; blood-brain barrier
Oligodendrocytes have fewer branches; produce insulating myelin sheath in CNS
Microglia ovoid cells with thorny processes; provide defense (because immunity cells not allowed in CNS)
Ependymal cells squamous/columnar cells with cilia; produce cerebrospinal fluid (CSF)

9. Supporting Cells in PNS
Schwann cells PNS cell support; produce & maintain myelin sheath, regenerate PNS axons
Satellite cells in PNS ganglia; support neurons in ganglia, regulate exchange of materials between neurons and interstitial fluid

12. Neuron Characteristics
They conduct nerve impulses from one part of the body to another
They have extreme longevity live/function for a lifetime
They are amitotic lose their ability to divide
They have a high metabolic rate = need O2 and glucose

13. Neuronal Structure
Cell body nucleus, cytoplasm with typical organelles; most within CNS = protected by cranial bones & vertebrae
Dendrites short, highly branched input structures emerging from cell body = high surface area to receive signals
Axon may be short or long, only one per neuron; conducts away from cell body toward another neuron or effector
Emerges at cone-shaped axon hillock
Axon terminals at end of axon with synaptic bulbs

14. Figure 9.3
(Neurilemma)
= impulse direction
Pg. 391

15. Myelination Axons covered with a myelin sheath
Many layered lipid & protein creating insulations
Increases speed of nerve conduction.
Formed by:
Schwann cells in PNS (pg. 393 fig. 11.5)
Oligodendrocytes in CNS
Nodes of Ranvier= gaps in the myelin
Nodes are important for signal conduction
Some diseases destroy myelin multiple sclerosis & Tay-Sachs

16. Gray and White Matter White matter- primarily myelinated axons
Gray matter- nerve cell bodies, dendrites, unmyelinated axons, axon terminals & neuroglia
Spinal cord gray matter is centrally located

17. Classification of Neurons
Structural according to # of processes:
Multipolar 3 or more; most common, especially in CNS
Bipolar 2 processes (an axon and a dendrite) that extend from opposite sides; found in special sense organs
Unipolar 1 process that divides like a T; found in ganglia in PNS

18. Functional according to the direction impulse travels (Table 11.1)
Sensory (afferent) neurons transmit impulses from sensory receptors toward or into the CNS; mostly unipolar, with cell bodies in ganglia outside CNS
Motor (efferent) neurons carry impulses away from CNS to muscles and glands; multipolar, usually with cell bodies in CNS
Interneurons (association neurons) between motor & sensory neurons; most in CNS; 99% of neurons in body; mostly multipolar

19. Neurophysiology Neurons are highly irritable = responsive to stimuli
When stimulated, an electrical impulse (action potential) is conducted along its axon
Action potential underlies all functional activities of the nervous system

20. Action Potentials Action potentials = nerve impulses
Require a membrane potential
electrical charge difference across cell membrane – like a battery
Ion Channels allow ions to move by diffusion = current
If no action potential then resting cell has resting membrane potential

21. Ion Channels Allow specific ions to diffuse across membrane
Move from high concentration to low
or toward area of opposite charge
Leakage channels
Gated channels- require trigger to open
Voltage- Gated channels respond to a change in membrane potential

22. Resting Membrane Potential
Leakage channels
Cytosol high in K+ & interstitial fluid high in Na+ (sodium –potassium pumps)
Leakage lets K+ through easily and Na+ poorly
inside is negative relative to outside
actual value depends on the relative leakage channel numbers

23. Figure 9.4

24. Graded Potentials Short-lived, local changes to membrane potential
Cause current flows that decrease with distance
Magnitude varies with strength of stimulus

25. Action Potential (AP) Generated by neurons and muscle cells
Series of active events
Channels actively open & close
Some initial event is required to reach a voltage threshold (~ = - 55 mv)
Stimulus = any event bringing membrane to threshold

26. Action Potential Resting state Depolarizing phase- Repolarizing phase-
voltage-gated channels closed
Depolarizing phase-
membrane potential rises and becomes positive
Repolarizing phase-
potential restored to resting value ( PNa,  PK)
Undershoot
Potassium permeability continues

27. Figure 9.5

28. Active Events Stimulus to reach threshold Na+ channel opens=>
Na+ ions enter=>
positive potential=>
Causes K+ channel opening =>
repolarization

29. All- or –None Phenomenon
This sequence is always the same
If threshold then the same size of changes occur no larger or smaller APs
Stimulus must reach threshold to start
After one AP there is a short period before next can be triggered= absolute refractory period each AP is a separate, all-or-none event; enforces one-way transmission of AP

30. Conduction of Nerve Impulses
Each section triggers next locally
Refractory period keeps it going the right direction
unmyelinated fiber- continuous conduction
With myelin- saltatory conduction
Can only be triggered at nodes of Ranvier
Myelinated fibers faster & larger neurons faster

31. Figure 9.6a

32. Figure 9.6b

33. The Syanpse
Synapse (to clasp or join)- junction that mediates information transfer from 1 neuron to another or from a neuron to an effector cell
Axodendritic or axosomatic synapses – most synapses occur between the axonal ending of a neuron and the dendrites or cell body of other neurons

34. Synaptic Transmission – Electrical synapse
Sequence of events at synapse
Triggered by voltage change of the Action Potential
Sending neuron = presynaptic
Receiving neuron = postsynaptic
Space between = synaptic cleft
Neurotransmitter carries signal across cleft

35. Events at Synapse – Chemical synapse
AP arrives at presynaptic end bulb=>
Opens voltage gated Ca2+ channels=>
Ca2+ flows into cell
increased Ca2+ concentration =>
exocytosis of synaptic vesicles=>
Neurotransmitter released into cleft
Diffuse across and bind to receptors in postsynaptic cell membrane

36. Synaptic Transmission
Binding at receptors
Chemical trigger of ion channels
May depolarize or hyperpolarize postsynaptic cell membrane
If threshold reached at axon hillock then postsynaptic cell action potential results

37. Synaptic Transmission
Finally the neurotransmitter must be removed from the cleft-
Diffusion away
Destroyed by enzymes in cleft
Transport back into presynaptic cell
Neuroglia destruction

38. Figure 9.7

39. Neurotransmitters AcetylCholine (Ach)- common in PNS
Biogenic amines - Norepinephrine (NE), Dopamine (DA), serotonin, Histamine
Amino Acids-
Glutamate, Aspartate, gamma aminobutyric acid (GABA), glycine
Neuropeptides – endorphins
Novel Messengers - ATP/ Nitric oxide (NO)/ Carbon monoxide (CO)

40. Development of Neurons
Neuroblasts
Growth cone
Programmed cell death

41. Web sites: http://www.sciencecases.org/chin/chin.asp