1. Nervous Tissue overview of the nervous system properties of neurons
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overview of the nervous system
properties of neurons
supportive cells (neuroglia)
electrophysiology of neurons
synapses
neural integration
Neurofibrils
Axon
(d)
Figure 12.4d

2. Overview of Nervous System
nervous system carries out its task in three basic steps:
Sense organs receive information
They transmit coded messages to the spinal cord and the brain
The brain and spinal cord processes this information, relates it to past experiences, and determine what response is appropriate to the circumstances and issues commands to muscles and gland cells to carry out such a response

3. Two Major Anatomical Subdivisions of Nervous System
central nervous system (CNS)
brain and spinal cord enclosed in bony coverings
enclosed by cranium and vertebral column
peripheral nervous system (PNS)
all the nervous system except the brain and spinal cord
composed of nerves and ganglia
nerve – a bundle of nerve fibers (axons) wrapped in fibrous connective tissue
ganglion – a knot-like swelling in a nerve where neuron cell bodies are concentrated

4. Subdivisions of Nervous System
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Central nervous
system (CNS)
Peripheral nervous
system (PNS)
Brain
Spinal
cord
Nerves
Ganglia
Figure 12.1

5. Subdivisions of Nervous System
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Central nervous system
Peripheral nervous system
Spinal
cord
Sensory
division
Motor
division
Brain
Visceral
sensory
division
Somatic
sensory
division
Visceral
motor
division
Somatic
motor
division
Figure 12.2
Sympathetic
division
Parasympathetic
division

6. Functional Classes of Neurons
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Peripheral nervous system
Central nervous system 1
Sensory (afferent)
neurons conduct
signals from
receptors to the CNS. 2
Interneurons
(association
neurons) are
confined to
the CNS. 3
Motor (efferent)
neurons conduct
signals from the CNS
to effectors such as
muscles and glands.
Figure 12.3

7. Functional Types of Neurons
sensory (afferent) neurons
specialized to detect stimuli
transmit information about them to the CNS
begin in almost every organ in the body and end in CNS
afferent – conducting signals toward CNS
interneurons (association) neurons
lie entirely within the CNS
process, store, and retrieve information and ‘make decisions’ that determine how the body will respond to stimuli
99% of all neurons are interneurons
motor (efferent) neuron
send signals out to muscles and gland cells (the effectors)
motor because most of them lead to muscles
efferent neurons conduct signals away from the CNS

8. Structure of a Neuron Figure 12.4a Figure 12.4a
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soma – the control center of the neuron
also called neurosoma, cell body, or perikaryon
dendrites – vast number of branches coming from a few thick branches from the soma
primary site for receiving signals from other neurons
axon (nerve fiber) – originates from a mound on one side of the soma called the axon hillock
cylindrical, relatively unbranched for most of its length
distal end of the axon has the Telodendria– extensive complex of fine branches
Dendrites
Soma
Nucleus
Nucleolus
Trigger zone:
Axon hillock
Initial segment
Axon collateral
Axon
Direction of
signal transmission
Internodes
Node of Ranvier
Myelin sheath
Schwann cell
Figure 12.4a
Synaptic knobs
12-8
Figure 12.4a
(a)

9. Universal Properties of Neurons
excitability (irritability)
respond to environmental changes called stimuli
conductivity
neurons respond to stimuli by producing electrical signals that are quickly conducted to other cells at distant locations
secretion
when electrical signal reaches end of nerve fiber, a chemical neurotransmitter is secreted that crosses the gap and stimulates the next cell

10. Variation in Neuron Structure
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multipolar neuron
one axon and multiple dendrites
most common
most neurons in the brain and spinal cord
bipolar neuron
one axon and one dendrite
olfactory cells, retina, inner ear
unipolar neuron
single process leading away from the soma
sensory from skin and organs to spinal cord
anaxonic neuron
many dendrites but no axon
help in visual processes
Dendrites
Axon
Multipolar neurons
Dendrites
Axon
Bipolar neurons
Dendrites
Axon
Unipolar neuron
Dendrites
Figure 12.5
Anaxonic neuron

11. Myelin
in PNS, Schwann cell spirals repeatedly around a single nerve fiber
neurilemma – thick outermost coil of myelin sheath
contains nucleus and most of its cytoplasm
external to neurilemma is basal lamina and a thin layer of fibrous connective tissue – endoneurium
in CNS – oligodendrocytes reaches out to myelinate several nerve fibers in its immediate vicinity
nerve fibers in CNS have no neurilemma or endoneurium

12. Myelin
many Schwann cells or oligodendrocytes are needed to cover one nerve fiber
myelin sheath is segmented
nodes of Ranvier – gap between segments
internodes – myelin covered segments from one gap to the next
initial segment – short section of nerve fiber between the axon hillock and the first glial cell
trigger zone – the axon hillock and the initial segment
play an important role in initiating a nerve signal

13. nodes of Ranvier and internodes
Myelin Sheath in PNS
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One schann cell covers one axon.
Schwann cell
nucleus
Axoplasm
Axolemma
Neurilemma
Figure 12.4c
Myelin sheath
(c)
nodes of Ranvier and internodes
12-13

14. Myelination in CNS Figure 12.7b
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One oligodendrocytes covers multiple axons
Oligodendrocyte
Myelin
Nerve fiber
Figure 12.7b
(b)

15. Myelination in PNS Figure 12.7a Schwann cell Axon Basal lamina
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Schwann cell
Axon
Basal lamina
Endoneurium
Nucleus
(a)
Neurilemma
Myelin sheath
Figure 12.7a

16. Electrical Potentials and Currents
electrical potential – a difference in the concentration of charged particles between one point and another
electrical current – a flow of charged particles from one point to another
in the body, currents are movement of ions, such as Na+ or K+ through gated channels in the plasma membrane
gated channels enable cells to turn electrical currents on and off
resting membrane potential (RMP) – is the charge difference across the plasma membrane of a cell
-70 mV in a resting, unstimulated neuron
negative value means there are more negatively charged particles on the inside of the membrane than on the outside

17. Resting Membrane Potential
RMP exists because of unequal electrolyte distribution between extracellular fluid (ECF) and intracellular fluid (ICF)
RMP results from the combined effect of three factors:
ions diffuse down their concentration gradient through the membrane
plasma membrane is selectively permeable and allows some ions to pass easier than others
electrical attraction of cations and anions to each other

18. Creation of Resting Membrane Potential
potassium ions (K+) found inside the cell
K+ is about 40 times as concentrated in the ICF as in the ECF
have the greatest influence on RMP
leaks out until electrical charge of cytoplasmic anions attracts it back in and equilibrium is reached and net diffusion of K+ stops
Sodium ions (Na+) found outside the cell
Na+ is about 12 times as concentrated in the ECF as in the ICF
resting membrane is much less permeable to Na+ than K+
Large anions in the ICF that keep the RMP negative.
Na+/K+ pumps out 3 Na+ for every 2 K+ it brings in
works continuously to compensate for Na+ and K+ leakage, and requires great deal of ATP
70% of the energy requirement of the nervous system

19. Ionic Basis of Resting Membrane Potential
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ECF
Na+ 145 mEq/L
K mEq/L
Figure 12.11 K+
channel
Na+
channel
Na mEq/L
K mEq/L
Large anions
that cannot
escape cell
ICF
Na+ concentrated outside of cell (ECF)
K+ concentrated inside cell (ICF)

20. Excitation of a Neuron by a Chemical Stimulus
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Dendrites
Soma
Trigger
zone
Axon
Current
ECF
Ligand
Receptor
Plasma
membrane
of dendrite
Na+
ICF
Figure 12.12

21. Action Potentials
action potential – dramatic change produced by voltage-regulated ion gates in the plasma membrane
trigger zone where action potential is generated
threshold – critical voltage to which local potentials must rise to open the voltage-regulated gates
Between -55mV and -50mV
when threshold is reached, neuron ‘fires’ and produces an action potential
more and more Na+ channels open in in the trigger zone in a positive feedback cycle creating a rapid rise in membrane voltage – spike
when rising membrane potential passes 0 mV, Na+ gates are inactivated
when all closed, the voltage peaks at about +30 mV
membrane now positive on the inside and negative on the outside
polarity reversed from RMP - depolarization
by the time the voltage peaks, the slow K+ gates are fully open
K+ repelled by the positive intracellular fluid now exit the cell
their outflow repolarizes the membrane
shifts the voltage back to negative numbers returning toward RMP
K+ gates stay open longer than the Na+ gates
slightly more K+ leaves the cell than Na+ entering
negative overshoot – hyperpolarization or afterpotential

22. Action Potentials Figure 12.13a
only a thin layer of the cytoplasm next to the cell membrane is affected
in reality, very few ions are involved
action potential is often called a spike – happens so fast
characteristics of action potential versus a local potential
follows an all-or-none law
if threshold is reached, neuron fires at its maximum voltage
if threshold is not reached it does not fire
nondecremental - do not get weaker with distance
irreversible - once started goes to completion and can not be stopped
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+35 3 5
Depolarization
Repolarization
Action
potential mV
Threshold 2
–55
Local
potential 1 7
–70
Resting membrane
potential 6
Hyperpolarization
Time
(a)
Figure 12.13a

23. Sodium and Potassium Gates
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. K+
Na+ K+
gate
Na+
gate 35 35 mV mV 1
Na+ and K+ gates closed 2
Na+ gates open, Na+
enters cell, K+ gates
beginning to open
–70
–70
Resting membrane
potential
Depolarization begins
Figure 12.14 35 35 mV mV 3
Na+ gates closed, K+ gates
fully open, K+ leaves cell 4
Na+ gates closed,
K+ gates closing
–70
–70
Depolarization ends,
repolarization begins
Repolarization complete

24. The Refractory Period Figure 12.15
during an action potential and for a few milliseconds after, it is difficult or impossible to stimulate that region of a neuron to fire again.
refractory period – the period of resistance to stimulation
two phases of the refractory period
absolute refractory period
no stimulus of any strength will trigger AP
relative refractory period
only especially strong stimulus will trigger new AP
refractory period is occurring only at a small patch of the neuron’s membrane at one time
other parts of the neuron can be stimulated while the small part is in refractory period
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Absolute
refractory
period
Relative
refractory
period
+35 mV
Threshold
–55
Resting membrane
potential
–70
Time
Figure 12.15

25. Nerve Signal Conduction Unmyelinated Fibers
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
unmyelinated fiber has voltage-regulated ion gates along its entire length and Na+ and K+ gates open and close producing a new action potential all the way down the axon. This slows down the signal being sent.
Dendrites
Cell body
Axon
Signal
– – –
Action potential
in progress
– – – – – – – – – – – – – – –
Refractory
membrane
– – – – – – – – – – – – – – –
Excitable
membrane
– – –
– – –
– – – – – – – – – – – – – – –
– – – – – – – – – – – – – – –
– – –
– – – + +
Figure 12.16
– – – – – – – – – – – – – – –
– – – – – – – – – – – – – – –
– – – + +

26. Saltatory Conduction Myelinated Fibers
voltage-gated channels needed for APs
fast Na+ diffusion occurs between nodes
signal weakens under myelin sheath, but still strong enough to stimulate an action potential at next node
saltatory conduction – the nerve signal seems to jump from node to node increasing speed of signal being sent.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 12.17a
Na+ inflow at node
generates action potential
(slow but nondecremental)
Na+ diffuses along inside
of axolemma to next node
(fast but decremental)
Excitation of voltage-
regulated gates will
generate next action
potential here
(a)

27. Saltatory Conduction
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
+ +
– –
+ +
+ +
+ +
+ +
– –
+ +
– –
– –
– –
– –
– –
+ +
– –
– –
– –
– –
+ +
– –
+ +
+ +
+ +
+ +
+ +
+ +
– –
+ +
+ +
+ +
– –
– –
+ +
– –
– –
– –
– –
– –
+ +
– –
– –
– –
+ +
+ +
– –
+ +
+ +
+ +
+ +
+ +
+ +
– –
+ +
+ +
– –
– –
– –
+ +
– –
– –
– –
– –
– –
+ +
– –
– –
+ +
+ +
+ +
– –
+ +
+ +
Action potential
in progress
Refractory
membrane
Excitable
membrane
(b)
Figure 12.17b
much faster than conduction in unmyelinated fibers

28. Synapses
a nerve signal can go no further when it reaches the end of the axon
triggers the release of a neurotransmitter
stimulates a new wave of electrical activity in the next cell across the synapse
electrical synapses do exist
advantage of quick transmission
no delay for release and binding of neurotransmitter
Gap junctions in cardiac and smooth muscle and some neurons
disadvantage is they cannot integrate information and make decisions
ability reserved for chemical synapses in which neurons communicate by releasing neurotransmitters
synapse between two neurons
1st neuron in the signal path is the presynaptic neuron
releases neurotransmitter
2nd neuron is postsynaptic neuron
responds to neurotransmitter
presynaptic neuron may synapse with a dendrite, soma, or axon of postsynaptic neuron
neuron can have an enormous number of synapses
in cerebellum of brain, one neuron can have as many as 100,000 synapses

29. Synaptic Relationships Between Neurons
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Soma
Synapse
Axon
Presynaptic
neuron
Direction of
signal
transmission
Postsynaptic
neuron
(a)
Axodendritic synapse
Axosomatic
synapse
Figure 12.18
Axoaxonic synapse
(b)

30. © Omikron/Science Source/Photo Researchers, Inc
Synaptic Knobs
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Axon of
presynaptic
neuron
Synaptic
knob
Soma of
postsynaptic
neuron
© Omikron/Science Source/Photo Researchers, Inc
Figure 12.19

31. Structure of a Chemical Synapse
synaptic knob of presynaptic neuron contains synaptic vesicles containing neurotransmitter
ready to release neurotransmitter on demand
postsynaptic neuron membrane contains proteins that function as receptors and ligand-regulated ion gates

32. Structure of a Chemical Synapse
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Microtubules
of cytoskeleton
Axon of presynaptic neuron
Mitochondria
Postsynaptic neuron
Synaptic knob
Synaptic vesicles
containing neurotransmitter
Synaptic cleft
Neurotransmitter
receptor
Figure 12.20
Neurotransmitter
release
Postsynaptic neuron
presynaptic neurons have synaptic vesicles with neurotransmitter and postsynaptic have receptors and ligand-regulated ion channels

33. Neurotransmitters and Related Messengers
more than 100 neurotransmitters have been identified
fall into four major categories according to chemical composition
acetylcholine
in a class by itself
formed from acetic acid and choline
amino acid neurotransmitters
include glycine, glutamate, aspartate, and -aminobutyric acid (GABA)
monoamines
synthesized from amino acids by removal of the –COOH group
retaining the –NH2 (amino) group
major monoamines are:
epinephrine, norepinephrine, dopamine (catecholamines)
histamine and serotonin
neuropeptides

34. Function of Neurotransmitters at Synapse
they are synthesized by the presynaptic neuron
they are released in response to stimulation
they bind to specific receptors on the postsynaptic cell
they alter the physiology of that cell

35. Effects of Neurotransmitters
a given neurotransmitter does not have the same effect everywhere in the body
multiple receptor types exist for a particular neurotransmitter
14 receptor types for serotonin
receptor governs the effect the neurotransmitter has on the target cell

36. Synaptic Transmission
neurotransmitters are diverse in their action
some excitatory
some inhibitory
some the effect depends on what kind of receptor the postsynaptic cell has
some open ligand-regulated ion gates
some act through second-messenger systems
three kinds of synapses with different modes of action
excitatory cholinergic synapse
inhibitory GABA-ergic synapse
excitatory adrenergic synapse
synaptic delay – time from the arrival of a signal at the axon terminal of a presynaptic cell to the beginning of an action potential in the postsynaptic cell
0.5 msec for all the complex sequence of events to occur

37. Excitatory Cholinergic Synapse
cholinergic synapse – employs acetylcholine (ACh) as its neurotransmitter
ACh excites some postsynaptic cells
skeletal muscle
inhibits others
describing excitatory action
nerve signal approaching the synapse, opens the voltage-regulated calcium gates in synaptic knob
Ca2+ enters the knob
triggers exocytosis of synaptic vesicles releasing ACh
empty vesicles drop back into the cytoplasm to be refilled with ACh
reserve pool of synaptic vesicles move to the active sites and release their ACh
ACh diffuses across the synaptic cleft
binds to ligand-regulated gates on the postsynaptic neuron
gates open
allowing Na+ to enter cell and K+ to leave
pass in opposite directions through same gate
as Na+ enters the cell it spreads out along the inside of the plasma membrane and depolarizes it producing a local potential called the postsynaptic potential
if it is strong enough and persistent enough
it opens voltage-regulated ion gates in the trigger zone
causing the postsynaptic neuron to fire

38. Excitatory Cholinergic Synapse
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Presynaptic neuron
Presynaptic neuron 3
Ca2+ 1 2
ACh
Na+ – – – – 4 – – – + + + + + + +
Figure 12.22 5 K+
Postsynaptic neuron

39. Cessation of the Signal
mechanisms to turn off stimulation to keep postsynaptic neuron from firing indefinitely
neurotransmitter molecule binds to its receptor for only 1 msec or so
then dissociates from it
if presynaptic cell continues to release neurotransmitter
one molecule is quickly replaced by another and the neuron is restimulated
stop adding neurotransmitter and get rid of that which is already there
stop signals in the presynaptic nerve fiber
getting rid of neurotransmitter by:
diffusion
neurotransmitter escapes the synapse into the nearby ECF
astrocytes in CNS absorb it and return it to neurons
reuptake
synaptic knob reabsorbs amino acids and monoamines by endocytosis
break neurotransmitters down with monoamine oxidase (MAO) enzyme
some antidepressant drugs work by inhibiting MAO
degradation in the synaptic cleft
enzyme acetylcholinesterase (AChE) in synaptic cleft degrades ACh into acetate and choline
choline reabsorbed by synaptic knob

40. Kinds of Neural Circuits
diverging circuit
one nerve fiber branches and synapses with several postsynaptic cells
one neuron may produce output through hundreds of neurons
converging circuit
input from many different nerve fibers can be funneled to one neuron or neural pool
opposite of diverging circuit
reverberating circuits
neurons stimulate each other in linear sequence but one cell restimulates the first cell to start the process all over
diaphragm and intercostal muscles
parallel after-discharge circuits
input neuron diverges to stimulate several chains of neurons
each chain has a different number of synapses
eventually they all reconverge on a single output neuron
after-discharge – continued firing after the stimulus stops

41. Neural Circuits Figure 12.30 Diverging Converging Input Output Output
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Diverging
Converging
Input
Output
Output
Input
Reverberating
Parallel after-discharge
Figure 12.30
Input
Output
Input
Output