1. The Nervous System Chapter 8 – Overview and Neural Tissue

2. The Nervous system has three major functions:
Sensory – monitors internal & external environment through presence of __________
Integration – interpretation of sensory information (information processing); complex (higher order) functions
Motor – response to information processed through stimulation of ____________
_________________
_______________

3. General Organization of the nervous system
Two Anatomical Divisions
Central nervous system (CNS)
__________
Peripheral nervous system (PNS)
All neural tissue outside CNS – includes cranial nerves and spinal nerves
__________ division (sensory input)
__________ division (motor output)
Somatic nervous system
Autonomic nervous system

4. General Organization of the nervous system
Brain & spinal cord

5. Histology of neural tissue
Two types of neural cells in the nervous system:
Neurons - For processing, transfer, and storage of information
Neuroglia – For support, regulation & protection of neurons

6. Neuroglia (glial cells)
CNS neuroglia:
astrocytes
oligodendrocytes
microglia
ependymal cells
PNS neuroglia:
Schwann cells (neurolemmocytes)

7. Astrocytes
create supportive framework for neurons
create “blood-brain barrier”
monitor & regulate interstitial fluid surrounding neurons
secrete chemicals for embryological neuron formation
stimulate the formation of scar tissue secondary to CNS injury

8. Oligodendrocytes
create myelin sheath around axons of neurons in the CNS. Myelinated axons transmit impulses faster than unmyelinated axons
Microglia
“brain macrophages”
phagocytize cellular wastes & pathogens

9. Ependymal cells
line ventricles of brain & central canal of spinal cord
produce, monitor & help circulate __________

10. Schwann cells
surround all axons of neurons in the PNS creating a neurilemma around them. Neurilemma allows for potential regeneration of damaged axons
creates myelin sheath around most axons of PNS

11. Neuron structure

12. Most axons of the nervous system are surrounded by a myelin sheath (myelinated axons)
The presence of myelin speeds up the transmission of action potentials along the axon
Myelin will get laid down in segments (internodes) along the axon, leaving unmyelinated gaps known as “__________________”
Regions of the nervous system containing groupings of myelinated axons make up the “_____________”
“gray matter” is mainly comprised of groups of neuron cell bodies, dendrites & synapses (connections between neurons)
of Ranvier

13. Anatomical organization of neurons
Neurons of the nervous system tend to group together into organized bundles
The axons of neurons are bundled together to form nerves in the PNS & tracts, columns & pathways in the CNS.
The cell bodies of neurons are clustered together into ganglia in the PNS & nuclei/centers in the CNS.

14. Classification of neurons
Structural classification based on number of processes coming off of the cell body:
Multipolar neuron
multiple dendrites & single axon
most common type

15. Bipolar neuron
two processes coming off cell body – one dendrite & one axon
only found in eye, ear & nose
Unipolar neuron
single process coming off cell body, giving rise to dendrites (at one end) & axon (making up rest of process)

16. Classification of neurons
Functional classification based on type of information & direction of information transmission:
Sensory (afferent) neurons –
transmit: ________________________________________
most sensory neurons are unipolar, a few are bipolar
Motor (efferent) neurons –
(muscles/glands/adipose tissue) in the periphery of the body
all are multipolar
Association (interneurons) –
transmit information between neurons, located entirely within the CNS; analyze inputs, store information, coordinate outputs
are the most common type of neuron (20 billion)
are all multipolar

17. Reflex arc (p )
Reflex – a quick, unconscious, automatic response to a stimulus to protect or maintain homeostasis.
e.g. stretch reflex, withdrawal reflex, pupillary light reflex, etc.
Reflex arc – neural pathway involved in the production of a reflex. Structures include:
receptor
sensory neuron
integrating center (brain or spinal cord; may or may not involve association neurons (interneurons))
motor neuron
effector

18. Stretch reflex - simplest type of reflex
- no association neuron involved
Figure 8-29

19. Simplified Withdrawal reflex
Figure 8-28

20. Neuron Function
Neurons at rest have an unequal distribution of charged ions inside/outside the cell, which are kept separate by the plasma membrane
more Na+ ions outside
more K+ ions inside
large negatively charged proteins & phosphate ions inside
The sum of charges makes the outside of the membrane positive, & the inside of the membrane negative

21. Because of the difference of ionic charges inside/outside the cell, the membrane of the resting neuron is “polarized”
The difference in charges creates a potential electrical current across the membrane known as the “membrane potential (transmembrane potential)”

22. At rest, the transmembrane potential can also be referred to as the “_______________________” (RMP)
The RMP of a neuron = -70mV

23. For ions to cross a cell membrane, they must go through transmembrane channels
“leakage channels” – open all the time, allow for diffusion
“gated channels” – open & close under specific circumstances (e.g. voltage changes)
When a stimulus is applied to a resting neuron, gated ion channels can open

24. If a stimulus opens gated K+ channels in a resting neuron, positive charges leave cell  membrane potential becomes more negative (-70mV  -90mV)
This change from the cell’s resting membrane potential is known as hyperpolarization

25. When a stimulus causes Na+ gates open, Na+ diffuses into the cell
This changes the electrical charge inside the cell membrane, bringing it away from its RMP of -70mV toward 0mV
This change in membrane potential is known as depolarization

26. If a stimulus only affects a few Na+ gates at a specific site of the axon, the depolarization is small & localized only to that region of the cell. This is known as a graded potential
But if the stimulus reaches a certain level (threshold level), voltage controlled Na+ gates will begin to open in sequence along the length of the axon. The depolarization will propagate (spread) along the entire surface of the cell membrane
The propagated change in the membrane potential is known as an action potential (nerve impulse)

27. Action potentials
Propagated change in transmembrane potential with two phases:
Depolarization - movement of Na+ ions into the cell through voltage controlled Na+ gates; followed immediately by
Repolarization - movement of K+ ions out of the cell through voltage controlled K+ gates
Only nerve cells & muscle cells are excitable, i.e. can generate APs.
Once an AP begins, it will propagate down the entire cell at a constant & maximum rate. This is known as the “__________” principle

28. Transmembrane potential (mV)
Action Potential Conduction
Activation of voltage-
regulated sodium channels
and rapid depolarization
Depolarization to threshold
Sodium ions
Local
current
Potassium ions
Inactivation of sodium
channels and activation of
voltage-regulated
potassium channels
+30
DEPOLARIZATION
REPOLARIZATION 3 2
Transmembrane potential (mV)
The return to normal
permeability and resting state _
Threshold 60 _ 1 70 4
Resting
potential
REFRACTORY PERIOD 1 2 3
Time (msec)
Figure 8-8
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Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

29. Transmembrane potential (mV)
Depolarization to threshold
Nerve cell at rest (RMP= -70mV)
Stimulus applied to cell
Na+ gates at axon hillock cause localized depolarization (graded potential)
If stimulus is strong enough, flow of Na+ ions into cell reach threshold level triggering opening of voltage gated Na+ channels & formation of an action potential (nerve impulse)
Sodium ions
Local
current
+30
DEPOLARIZATION
Transmembrane potential (mV) _
Threshold 60 _ 1 70
Resting
potential 1 2 3
Time (msec)
Figure 8-8
2 of 5
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

30. Transmembrane potential (mV)
Activation of voltage-
regulated sodium channels
and rapid depolarization
Depolarization to threshold
Sodium ions
Local
current
Potassium ions
Once threshold is reached, Na+ will quickly diffuse into the cell causing a rapid depolarization of the membrane (- 70 mV  0 mV  +30 mV)
this depolarization will spread to adjacent parts of the membrane, activating more voltage controlled Na+ gates in succession
+30
DEPOLARIZATION 2
Transmembrane potential (mV) _
Threshold 60 _ 1 70
Resting
potential 1 2 3
Time (msec)
Figure 8-8
3 of 5
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

31. Transmembrane potential (mV)
Activation of voltage-
regulated sodium channels
and rapid depolarization
Depolarization to threshold
Sodium ions
Local
current
Potassium ions
Inactivation of sodium
channels and activation of
voltage-regulated
potassium channels
+30
DEPOLARIZATION
REPOLARIZATION 3 2
When the transmembrane potential reaches +30mV, Na+ gates will close & K+ gates will open
Transmembrane potential (mV) _
Threshold 60 _ 1 70
Resting
potential
K+ will quickly exit cell resulting in repolarization of membrane & return to resting state
Figure 8-8
4 of 5 1 2 3
Time (msec)
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

32. Transmembrane potential (mV)
Activation of voltage-
regulated sodium channels
and rapid depolarization
Depolarization to threshold
Sodium ions
Local
current
Potassium ions
Inactivation of sodium
channels and activation of
voltage-regulated
potassium channels
+30
DEPOLARIZATION
REPOLARIZATION 3 2
Transmembrane potential (mV)
The return to normal
permeability and resting state _
Threshold 60 _ 1 70 4
Resting
potential
REFRACTORY PERIOD 1 2 3
Time (msec)
Figure 8-8
5 of 5
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

33. Propagation of an Action Potential
Continuous propagation
(continuous conduction)
Involves entire membrane surface
Proceeds in series of small steps (slower)
Occurs in unmyelinated axons (& in muscle cells)
Figure 8-9(a)

34. Propagation of an Action Potential
Saltatory propagation
(saltatory conduction)
Involves patches of membrane exposed at nodes of Ranvier
Proceeds in series of large steps (faster)
Occurs in myelinated axons
Figure 8-9(b)

35. “Information” travels within the nervous system primarily in the form of propagated electrical signals known as action potentials.
An action potential occurs due to a rapid change in membrane polarity (depolarization followed by repolarization)
Depolarization is due to: ____________________; repolarization is due to: ________________________

36. Conduction across synapses
In order for neural control to occur, “information” must not only be conducted along nerve cells (via action potentials), but must also be transferred from one nerve cell to another across a synapse
Most synapses within the nervous system are chemical synapses, & involve the release of a neurotransmitter

37. The Structure of a Typical Synapse
Figure 8-10

38. Events at a Typical Synapse
An action potential arrives & depolarizes the synaptic knob (end bulb)
Before repolarization can occur, _____ channels open & diffuses _______ end bulb
Repolarization occurs
An action potential arrives and
depolarizes the synaptic knob
PRESYNAPTIC
NEURON
Action potential
Synaptic vesicles
EXTRACELLULAR
FLUID ER
Synaptic
knob
AChE
POSTSYNAPTIC
NEURON
CYTOSOL
Figure 8-11
2 of 5
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

39. An action potential arrives and
depolarizes the synaptic knob
Extracellular Ca2+ enters the synaptic
cleft triggering the exocytosis of ACh
PRESYNAPTIC
NEURON
Action potential
Synaptic vesicles
EXTRACELLULAR
FLUID
ACh ER
Synaptic
knob
Synaptic
cleft
Ca2+
Ca2+
AChE
Chemically regulated
sodium channels
POSTSYNAPTIC
NEURON
CYTOSOL
Ca+2 causes the synaptic vessicles to fuse with the end bulb membrane causing ___________ of the neurotransmitter
Figure 8-11
3 of 5
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

40. An action potential arrives and
depolarizes the synaptic knob
Extracellular Ca2+ enters the synaptic
cleft triggering the exocytosis of ACh
PRESYNAPTIC
NEURON
Action potential
Synaptic vesicles
EXTRACELLULAR
FLUID
ACh ER
Synaptic
knob
Synaptic
cleft
Ca2+
Ca2+
AChE
Chemically regulated
sodium channels
POSTSYNAPTIC
NEURON
CYTOSOL
The neurotransmitter diffuses across the synaptic cleft & binds to its receptors on the post synaptic membrane, causing an effect on the post synaptic cell
ACh binds to receptors and depolarizes
the postsynaptic membrane
Initiation of
action potential
if threshold
is reached
Na2+
Receptor
Na2+
Na2+
Na2+
Na2+
Figure 8-11
4 of 5

41. Excitatory (e.g. Ach, norepinephrine (NE))
The effect on the post synaptic neuron will depend on whether the neurotransmitter released is
Excitatory (e.g. Ach, norepinephrine (NE))
Inhibitory (e.g. seratonin, dopamine, GABA)
Excitatory neurotransmitters cause Na+ gates to open in the post synaptic membrane  ________________  impulse conduction
Inhibitory neurotransmitters cause K+ or Cl- gates to open in the post synaptic cell  _______________  no impulse conduction

42. The effects of neurotransmitters on the post synaptic neurons are usually short lived because most neurotransmitters are rapidly removed from the synaptic cleft by enzymes or reuptake