Transcript Chapter 12: Neural Tissue

Chapter 12: Neural Tissue
1
Neural Tissue
• 3% of body mass
• Cellular, ~20% extracellular space
• Two categories of cells:
1. Neurons: conduct nervous impulses
- cells that send and receive signals
2. Neuroglia/glial cells: “nerve glue”
-
Supporting Cells
Protect neurons
2
Organs of the Nervous System
• Brain and spinal cord
• Sensory receptors of sense organs
(eyes, ears, etc.)
• Nerves connect nervous system with
other systems
3
4
Nervous Systems
1. Central Nervous System (CNS)
-
Spinal cord, brain
Functions:
-
integrate, process, coordinate sensory input and
motor output
2. Peripheral Nervous System (PNS)
- All neural tissue outside of CNS
-
Functions: Carry info to/from the CNS via nerves
Nerves:
-
Bundle of axons (nerve fibers) with blood vessels and
CT
Carry sensory information and motor commands in PNS
Cranial nerves
brain
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Spinal nerves
spinal cord
Division of PNS
1. Sensory/Afferent Division: carries sensory
information
- Sensory receptors  CNS
A. Somatic afferent division
-
From skin, skeletal muscles, and joints
B. Visceral afferent division
-
From internal organs
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Division of PNS
2. Motor/Efferent Division: carries motor
commands
- CNS  effectors
A. Somatic Nervous System: Controls skeletal muscle
contractions
-
-
“voluntary nervous system”
To skeletal muscles  contractions
B. Autonomic Nervous System (ANS)
- “involuntary nervous system”
- To smooth and cardiac muscle, glands  contractions
1. Sympathetic Division: stimulating effect
- “fight or flight”
2. Parasympathetic Division: relaxing effect
- “rest and digest”
**Tend to be Antagonistic to Each Other**
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Receptors and Effectors
• Receptors:
– detect changes or respond to stimuli
– neurons and specialized cells
– complex sensory organs (e.g., eyes, ears)
• Effectors:
– respond to efferent signals
– cells and organs
8
What would damage to the afferent
division of the PNS affect?
1.
2.
3.
4.
ability to learn new facts
ability to experience motor stimuli
ability to experience sensory stimuli
ability to remember past events
9
The structure of a typical
neuron, and the function of
each component.
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Histology of Nervous System
• Neuron/Nerve Cell
–
–
Function: conduct nervous impulses
(message)
Characteristics:
1. Extreme longevity
2. Amitotic
-
Direct cell division by simple cleavage of the nucleus
without spindle formation or appearance of chromosomes
exceptions: hippocampus, olfactory receptors
3. High metabolic rate: need O2 and glucose
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The Structure of Neurons
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Figure 12–1
The Structure of Neurons
• Large soma/perikaryon(cytoplasm)
• Large nucleus, large nucleolus (rRNA)
• Many mitochondria, ribosomes, RER, Golgi
– Increase ATP, increase protein synthesis to produce
neurotransmitters
• Nissl bodies: visible RER and ribosomes, gray
• Neurofilaments: internal structure
– Neurofibrils, neurotubules
• No centrioles
• 2 types of processes (cell extension):
1. Dendrite
2. Axon
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Regions of a Neuron
1. Dendrites:
-
Receive info
Carry a graded potential toward soma
Contain same organelles as soma
Short, branched
End in dendritic spines
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Regions of a Neuron
2. Axon:
-
single, long
Carry an action potential away from soma
Release neurotransmitters at end to signal
next cell
Long ones = “nerve fibers”
Contains:
-
Neurofibrils and neurotubules (abundant)
Vesicles of neurotransmitter
Lysosomes, mitochondria, enzymes
No nissl bodies, no golgi (no protein synthesis in
axon)
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2. Axon
Regions of a Neuron
- Connects to soma at axon hillock
- Covered in axolemma (membrane) --- Axoplasm
(cytoplasm)
- May branch: axon collaterals
- End in synaptic terminals or knobs
- May have myelin sheath: protein+lipid
- Function:
- Protection, Insulation, and Increase speed of impulse
- CNS: myelin from Oligodendrocytes
- PNS: myelin from Schwann cells
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Axoplasmic Transport
• Move materials between soma and terminal
• Large molecules synthesized in the cell body,
such as vesicles and mitochondria are unable to
move via simple diffusion
• Large molecules are transported by motor
proteins called kinesins, which walk along
neurotubule tracks to their destinations.
• Anterograde transport = soma  terminal
– neurotransmitters from soma
• Retrograde transport = terminal  soma
– Recycle breakdown products from used
neurotransmitters
• Some viruses use retrograde transport to gain
access to CNS (polio, herpes, rabies)
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Synapse
• Site where neuron communicates
with another cell:
– neuron or effector
• Presynaptic cell sends message
along axon to axon terminal
• Postsynaptic cell receives message
as neurotransmitter
Neurotransmitter = chemical, transmits signal from preto post- synaptic cell across synaptic cleft
Synaptic knob = small, round, when postsynaptic cell is
neuron, synapse on dendrite or soma
Synaptic terminal = complex structure, at neuromuscular
or neuroglandular junction
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Structural Classification of Neurons
1.
Anaxonic neurons:
-
2.
Dendrites and axon look same
Brain and special sense organs
Bipolar neurons:
-
3.
1 dendrite, 1 axon
Soma in middle
Rare: special sense organs,
relay from receptor to neuron
Unipolar neurons:
-
4.
1 long axon, dendrites at one end,
soma off side (T shape)
Most sensory neurons
Multipolar neurons:
-
2 or more dendrites
1 long axon
99% of all neurons
Most CNS
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A tissue sample shows unipolar
neurons. Are these more likely to be
sensory neurons or motor neurons?
1. sensory neurons
2. motor neurons
20
Functional Classification of Neurons
1. Sensory/Afferent Neurons
-
Transmit info from sensory receptors to CNS
Mostly unipolar neurons
Soma in peripheral sensory ganglia
-
Ganglia = collection of cell bodies in PNS
A. Somatic Sensory Neurons
-
Receptors monitor outside conditions
B. Visceral Sensory Neurons
-
Receptors monitor internal conditions
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Functional Classification of Neurons
2. Motor/Efferent Neurons
- Transmit commands from CNS to effectors
- Mostly multipolar neurons
A. Somatic Motor Neurons
-
-
Innervate skeletal muscle
- Innervation = distribution of sensory/motor
nerves to a specific region/organ
Conscious control or reflexes
B. Visceral/Autonomic Motor Neurons
-
Innervate effectors on smooth muscle, cardiac
muscle, glands, and adipose
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Functional Classification of Neurons
3. Interneurons/Association Neurons
-
Distribute sensory info and coordinate
motor activity
Between sensory and motor neurons
In brain, spinal cord, autonomic ganglia
Most are multipolar
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The locations
and functions of neuroglia.
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Neuroglia
• Neuroglia = supporting cells
• Neuroglia in CNS
–
–
Outnumber neurons 10:1
Half mass of brain
• Neuroglia Cell in the CNS
1.
2.
3.
4.
Ependymal cells
Astrocytes
Oligodendrocytes
Microglia
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Neuroglia Cells of the CNS
1.
Ependymal Cells
-
2.
Line central canal of spinal cord and ventricles
of the brain
Secrete cerebrospinal fluid (CSF)
Have cilia to circulate CSF
CSF: cushion brain, nutrient and gas exchange
Astrocytes
-
Most abundant CNS neuroglia
Varying functions:
1.
2.
3.
4.
5.
Blood brain barrier
- Processes wrap capillaries
- Control chemical exchange between blood
and interstitial fluid of the brain
Framework of CNS
Repair damaged neural tissue
Guide neuron development in embryo
Control interstitial environment:
Regulate conc. Ions, gasses, nutrients,
neurotransmitters
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Neuroglia Cells of the CNS
3.
Oligodendrocytes
-
-
4.
Wide flat processes wrap around local axons =
myelin sheath
1 cell contributes myelin to many neighboring
axons
Lipid in membrane insulates axon for faster
action potential conductance
Gaps on axon between processes/myelin =
nodes of Ranvier, necessary to conduct
impulse
White, myelinated axons = “white matter”
Microglia
-
Phagocytic
Wander CNS
Engulf debris, pathogens
Important CNS defense
-
No immune cells or antibodies
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Neuroglia of the CNS
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Figure 12–4
Neuroglia in PNS
1. Satellite Cells
-
Surround somas in ganglia
Isolate PNS cells
Regulate interstitial environment of ganglia
-
Ganglia = mass of neuronal soma and dendrites
2. Schwann cells
-
Myelinate axon in PNS
Whole cells wraps axon, many layers
Neurilemma: bulge of schwann cell,
contains organelles
Nodes of Ranvier between cells
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Neuroglia in PNS
2. Schwann Cells cont.
-
Some hold bundles of unmyelinated axon
-
Vital to repair of peripheral nerve fibers after
injury
-
Guide growth to original synapse
30
Which type of neuroglia would occur in
larger than normal numbers in the
brain tissue of a person with a
CNS infection?
1.
2.
3.
4.
astrocytes
microglial cells
ependymal cells
oligodendrocytes
31
Neural Responses to Injuries
32
Figure 12–6 (1 of 2)
Neural Responses to Injuries
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Figure 12–6 (2 of 2)
KEY CONCEPT
• Neurons perform all communication,
information processing, and control
functions of the nervous system
• Neuroglia preserve physical and
biochemical structure of neural tissue,
and are essential to survival and
function of neurons
34
How the resting potential is
created and maintained.
35
5 Main Membrane
Processes in Neural Activities
36
Figure 12–7 (Navigator)
Neurophysiology
• Neurons: conduct electrical impulse
– Requires transmembrane potential = electrical difference
across the cell membrane
– Cells: positive charge outside (pump cations out) and
negative charge inside (protein)
Voltage = measure of potential energy generated by
separation of opposite charges
Current = flow of electrical charges (ions)
Cell can produce current (nervous impulse) when ions move
to eliminate the potential difference (volts) across the
membrane
Resistance = Restricts ion movement (current)
• High resistance = low current
• Membrane has resistance, restricts ion flow/current
37
Neurophysiology
• Ohm’s Law: current = voltage ÷ resistance
• Current is highest when voltage is high and
resistance is low
• Cell voltage set at -70mV but membrane
resistance can be altered to create current
• Membrane resistance depends on
permeability to ions: open or closed ion
channels
• Cell must always have some resistance or
ions would equalize, voltage = zero
– No current generated = no nervous impulse
38
Membrane Ion Channels
•
•
1.
2.
Allow ion movement (alter resistance)
Each channel is specific to one ion type
Passive Channels (leaky channels)
Active Channels
A. Chemically regulated/ligand-gated
B. Voltage regulated channels
C. Mechanically regulated channels
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Membrane Ion Channels
1. Passive Channels (leaky channels)
-
Resting Potential
Always open, free flow
Sets resting membrane potential at -70mV
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Active Channels: Gated Channels
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Figure 12–10
Membrane Ion Channels
2. Active Channels
- open/close in response to signal
A. Chemically regulated/ligand-gated
-
Open in response to chemical binding
Located on any cell membrane
-
Dendrites and soma
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Membrane Ion Channels
2. Active Channels
B. Voltage regulated channels
- open/close in response to shift in
transmembrane potential
- excitable membrane only: conduct action
potentials
- axolemma, sarcolemma
43
Membrane Ion Channels
2. Active Channels
C. Mechanically Regulated Channels
- Open in response to membrane distortion
- On dendrites of sensory neurons for:
- touch, pressure, vibration
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Membrane Ion Channels
• When channel opens, ions flow along
electrochemical gradient:
– Diffusion (high conc. to low)
– Electrical attraction/repulsion
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Sodium-Potassium Pump
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Sodium-Potassium Pump
•
Uses ATP to move 3 Na+ out and 2 K+ in
–
•
•
•
70% of neurons use ATP for this
Runs anytime the cell is not conducting an impulse
Creates high [K+] inside and high [Na+] outside
When Na+ cell opens
–
Na+ flows into cell:
1. Favored by diffusion gradient
2. Favored by electrical gradient
Open channel = decr. Resistance = incr. ion flow/current
•
When K+ channel opens
–
K+ flows out of cell:
1. Favored by diffusion gradient only
2. Electrical gradient repels K+ exit
- Thus less current than Na+
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• Channels open = resistance low = ions move
until equilibrium potential: depends on
– Diffusion gradient
– Electrical gradient
• Equilibrium Potential
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Electrical vs. Chemical Gradients
• The electrical gradient opposes the chemical
gradient
– K+ inside and outside of the cell are attracted to
the negative charges on the inside of the cell
membrane, and repelled by the positive charges
on the outside of the cell membrane
• indicated in white on the next slide
– Chemical gradient is strong enough to overpower
the electrical gradient, but this weakens the
force driving K+ out of the cell
• Net driving force indicated in grey on the next slide
– The Electrochemical Gradient
49
Electrochemical Gradients
50
Figure 12–9c, d
Summary: Resting Potential
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Table 12-1
Changes in
Transmembrane Potential
• Transmembrane potential rises or falls:
– in response to temporary changes in
membrane permeability
– resulting from opening or closing specific
membrane channels
+
+
• Membrane permeability to Na and K
determines transmembrane potential
• Sodium and potassium channels are
either passive or active
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Graded Potentials:
The Resting State
• Opening sodium channel produces a
current which causes graded potential
53
Figure 12–11 (Navigator)
Graded Potential
• Graded potential:
– localized shift in transmembrane potential
due to movement of charges into/out of cell
• Na+ channel opens = Na+ flows in
– depolarization (cell less negative)
• K+ channel opens = K+ flows out
– hyperpolarization (cell more negative)
54
Graded Potentials
• Occur on any membrane: dendrites and somas
• Can be depolarizing or hyperpolarizing
• Amount of depolarization or hyperpolarization
depends on the intensity of stimulus
– Incr. channels open = Incr. voltage change
• Passive spread from site of stimulation over short
distance
• Effect on membrane potential decreases with
distance from stimulation site
• Repolarization occurs as soon as stimulus is
removed
– Leaky channels and Na+/K+ pump reset resting potential
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Graded Potentials
• Localized change in transmembrane
potential, not nervous impulse (message)
• If big enough depolarization
– Action potential = nervous impulse =
transmission to next cell
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Graded Potentials: Step 1
• Resting membrane
exposed to chemical
• Sodium channel opens
• Sodium ions enter the
cell
• Transmembrane
potential rises
• Depolarization occurs
– A shift in
transmembrane
potential toward 0
mV
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Figure 12–11 (Step 1)
Graded Potentials: Step 2
• Movement of Na+
through channel
• Produces local current
• Depolarizes nearby cell
membrane (graded
potential)
• Change in potential is
proportional to
stimulus
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Figure 12–11 (Step 2)
Characteristics of Graded Potentials
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Table 12-2
Action Potential
• Occur on excitable membranes only
– Axolemma, sarcolemma
• Always depolarizing
• Must depolarize to threshold (-55mV) before action
potential begins
– Voltage gated channels on excitable membrane open at
threshold to propagate action potential
• “all-or-none”
– All stimuli that exceed threshold will produce identical
action potentials
• Action potential at one site depolarizes adjacent
sites to threshold
• Propagated across entire membrane surface without
decrease in strength
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Generating the Action Potential
61
Figure 12–13 (Navigator)
Generation of an Action Potential
1. Depolarization to threshold:
-
A graded potential depolarizes local membrane and
flows toward the axons
If threshold is met (-55mV) at the hillock, an action
potential will be triggered
2. Activation of sodium channels and rapid
depolarization:
-
At threshold (-55mV) , voltage-regulated sodium
channels on the excitable membrane open
Na+ flows into the cell depolarizing it
The transmembrane potential rapidly changes from 55mV to +30 mV
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Generation of an Action Potential
3. Inactivation of sodium channels and
activation of potassium channels:
-
At +30mV Na+ channels close and K+ channels
open
K+ flows out of the cell repolarizing it
4. Return to normal permeability:
-
At -70mV K+ channels begin to close
The cell hyperpolarizes to -90mV until all channels
finish closing
Leak channels restore the resting membrane
potential to -70mV
63
Generation of an Action Potential
• Restimulation only when Na+ channels closed:
– Influx of Na+ necessary for action potential
• Absolute Refractory Period:
– Threshold (-55mV) to +30mV, Na+ channels open,
membrane cannot respond to additional stimulus
• Relative Refractory Period:
– +30mV to -70mV (return to resting potential)
– Na+ channels closed, membrane capable of second action
potential but requires larger/longer stimulus (threshold
elevated)
• Cell has ions for thousands of action potentials
• Eventually must run Sodium-Potassium pump (burn
ATP) to reset high [K+] inside and high [Na+] outside
– Death = no ATP, but stored ions can generate action
potentials for awhile
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65
Table 12-3
How would a chemical that blocks the
sodium channels in neuron cell
membranes affect a neuron’s ability
to depolarize?
1. It would enhance depolarization.
2. It would inhibit depolarization
completely.
3. It would slow depolarization.
4. It would have no effect on
depolarization.
66
What effect would decreasing the
concentration of extracellular potassium
ions have on the transmembrane potential
of a neuron?
1. repolarization
2. hypopolarization
3. decreased transmembrane
potential
4. hyperpolarization
67
Propagation of Action Potential
• Once generated must be transmitted
along the length of the axon hillock
to terminal
• Speed depends on:
1. Degree of myelination
2. Axon diameter
68
2 Methods of Propagating
Action Potentials
1. Continuous propagation:
–
unmyelinated axons
2. Saltatory propagation:
–
myelinated axons
69
Propagation of Action Potential
1. Myelination
A. Continuous Propagation:
-
Unmyelinated axons
Whole membrane depolarizes and repolarizes
sequentially hillock to terminal
Only forward movement
- Membrane behind always in absolute refractory
period
70
Continuous Propagation
• Of action potentials along an
unmyelinated axon
• Affects 1 segment of axon at a time
71
Figure 12–14
Continuous Propagation: Step 1
• Action potential in segment 1
• Depolarizes membrane to +30 mV
72
Figure 12–14 (Step 1)
Continuous Propagation: Step 2
• Local current
• Depolarizes second segment to
threshold
73
Figure 12–14 (Step 2)
Continuous Propagation: Step 3
• Second segment develops action
potential
• First segment enters refractory period
74
Figure 12–14 (Step 3)
Continuous Propagation: Step 4
• Local current depolarizes next segment
• Cycle repeats
• Action potential travels in 1 direction
(1 m/sec)
75
Figure 12–14 (Step 4)
Propagation of Action Potential
1. Myelination
A. Saltatory Propagation:
-
Myelinated axons
Depolarization only on exposed membrane at
nodes
Myelin insulates covered membrane from ion flow
Action potential jumps from node to node
- Faster and requires less energy to reset
76
Saltatory Propagation
• Of action potential along myelinated
axon
77
Figure 12–15
Saltatory Propagation
78
Figure 12–15 (Steps 1, 2)
Saltatory Propagation
79
Figure 12–15 (Steps 3, 4)
Graded Potentials and Action
Potentials
80
Table 12–4
Axon Diameter
and Propagation Speed
• Ion movement is related to cytoplasm
concentration
• Axon diameter affects action potential
speed
• The larger diameter, the lower the
resistance
81
Propagation of Action Potentials
2. Axon Diameter
-
–
Larger axon  less resistance  easier ion
flow  faster action potential
Axons are classified by:
• Diameter, myelination, speed of action potentials
–
Three types of axons:
• Type A, Type B, and Type C fibers
82
Axon Diameter
1. Type A Fibers
-
4-20µm diameter
Myelinated (saltatory propagation)
Action potential 140m/sec
Carry somatic motor and somatic sensory info
2. Type B Fibers
-
2-4µm diameter
Myelinated (saltatory propagation)
Action potential 18m/sec
Carry autonomic motor and visceral sensory info
3. Type C Fibers
-
< 2µm diameter
Unmyelinated (continuous propagation)
Action potential 1m/sec
Carry autonomic motor and visceral sensory info
83
KEY CONCEPT
• “Information” travels within the
nervous system as propagated
electrical signals (action potentials)
• The most important information
(vision, balance, motor commands) is
carried by large-diameter myelinated
axons
84
Myelination
• Requires space, metabolically expensive
• Only important fibers large and
myelinated
• Occurs in early childhood
• Results in improved coordination
• Multiple Sclerosis:
– Genetic disorder, myelin on neurons in PNS
destroyed  numbness, paralysis
85
Synapse
• Synapse
–
Junction between transmitting neuron
(presynaptic cell) and receiving cell
(postsynaptic cell), where nerve impulse
moves from one cell to the next
• Two types:
1. Electrical Synapse:
-
Direct contact via gap junctions
Ion flow directly from pre to post cell
Less common synapse
In brain (conscious perception)
2. Chemical Synapse:
- Most common
86
2. Chemical Synapse
-
Most common
Pre and post cells separated by synaptic cleft
Presynaptic neuron releases neurotransmitter to trigger effect on
post synaptic cell
Dynamic: facilitate or inhibit transmission, depends on
neurotransmitter
1. Excititory Neurotransmitters =
- Depolarization (shift from resting potential toward 0 mV)
- Propagate Action Potential
2. Inhibitory Neurotransmitters =
- Hyperpolarization (shift from resting potential to -80 mV)
- Suppress Action Potential
Propagation across chemical synapse always slow but
87
allow variability
The events that occur
at a chemical synapse.
88
The Effect of a Neurotransmitter
• On a postsynaptic membrane:
– depends on the receptor
– not on the neurotransmitter
• e.g., acetylcholine (ACh):
– usually promotes action potentials
– but inhibits cardiac neuromuscular
junctions
89
Synaptic Activity
90
Figure 12–16 (Navigator)
Cholinergic Synapses
• Any synapse that releases ACh:
– all neuromuscular junctions with skeletal
muscle fibers
– many synapses in CNS
– all neuron-to-neuron synapses in PNS
– all neuromuscular and neuroglandular
junctions of ANS parasympathetic division
91
Events at a Cholinergic
Synapse: Step 1
• Action potential arrives, depolarizes
synaptic knob
92
Figure 12–16 (Step 1)
Events at a Cholinergic
Synapse: Step 2
• Calcium ions enter synaptic knob,
trigger exocytosis of ACh
93
Figure 12–16 (Step 2)
Events at a Cholinergic
Synapse: Step 3
• ACh binds to receptors, depolarizes
postsynaptic membrane
94
Figure 12–16 (Step 3)
Events at a Cholinergic
Synapse: Step 4
• AChE breaks ACh into acetate and
choline
95
Figure 12–16 (Step 4)
Events at a Cholinergic Synapse
96
Table 12-5
What effect would blocking voltageregulated calcium channels at a
cholinergic synapse have on synaptic
communication?
1. Communication would cease.
2. Communication would be
enhanced.
3. Communication would be
misdirected.
4. Communication would continue
as before.
97
Neurotransmitter Mechanism of Action
1. Direct effect on membrane potential
-
Open or close ion channels upon binding to
the post synaptic cell
Provides a rapid response
E.g. Ach (cholinergic synapse)
98
Neurotransmitter Mechanism of Action
2. Indirect effect on membrane potential
-
Binds a receptor that activates a G protein in
the post synaptic cell
Active G protein activates a second
messenger
-
-
cAMP, cGMP, diacylglyceride, Ca++
The second messenger opens ion channels or
activates enzymes
Provides slower but longer lasting effects
E.g. Norepinephrine (Adrenergic synapse)
99
Neurotransmitter Mechanism of Action
2. Indirect effect on membrane potential
-
Example of indirect action:
1.
2.
3.
4.
5.
Neurotransmitter binds receptor
Receptor activates G protein
G Protein activates adenylate cyclase
Adenylate cyclase converts ATP into cyclic AMP
cAMP opens ion channels
100
Post Synaptic Potential
• Graded potential caused by a
neurotransmitter due to opening or
closing of ion channels on post synaptic
cell membrane
• Two types:
1. Excititory Post Synaptic Potential (EPSP)
-
Causes depolarization
2. Inhibitory Post Synaptic Potential (IPSP)
-
Causes hyperpolarization
Inhibits postsynaptic cell
- Need larger stimulus to reach threshold
101
Post Synaptic Potential
•
•
Multiple EPSPs needed to trigger action
potential in post cell axon
EPSP summation:
– Temporal and Spatial Summation
1. Temporal Summation
-
Single synapse fires repeatedly
-
-
String of EPSPs in one spot
Each EPSP depolarizes more until threshold
reached at hillock
102
Post Synaptic Potential
• EPSP summation:
– Temporal and Spatial Summation
1. Spatial Summation
- Multiple synapses fire stimultaneously
- Collective depolarization reaches threshold
103
EPSP/IPSP Interactions
104
Figure 12–19
Post Synaptic Potential
• Facilitated = Depolarized
– Brought closer to threshold by some sort of stimulus
– Less stimulus now required to reach threshold
– E.g. Caffeine
• Post Synaptic Potentiation:
– Repeat stimulation of the same synapse conditions
synapse, pre cell more easily stimulated, allowing the
post cell to reach the threshold (repetition)
• Most nervous system activities results from
interplay of EPSPs and IPSPs to promote differing
degrees of facilitation or inhibition to allow
constant fine tuning of response
• Neuromodulators:
– Chemicals that influence synthesis, release, or
degradation of neurotransmitters thus altering normal
response of the synapse
105
Common Neurotransmitters
1. Acetycholine: cholinergic synapses
-
Excititory
Direct effect
Skeletal neuromuscular junctions, many CNS
synapses, all neuron to neuron PNS, all
parasympathetic ANS
2. Norepinephrine – adrenergic synapses
-
Excititory
Second messengers
Many brain synapses, all sympathetic ANS effector
junctions
106
Common Neurotransmitters
3. Dopamine
-
Excititory or inhibitory
Second messengers
Many brain synapses
-
Cocaine: inhibits removal = ‘high’
Parkinson’s disease: damage neurons = ticks, jitters
4. Serotonin
-
Inhibitory
Direct or second messenger
Brain stem for emotion
-
Anti-depression/anti-anxiety drugs block uptake
5. Gamma aminobytyric acid (GABA)
-
Inhibitory
Direct effect
Brain: anxiety control, motor coordination
-
Alcohol: augments effects = loss of coordination
107
Presynaptic Facilitation
• Activity at an
axoaxonal synapse
increases the amount
of neurotransmitter
released when an
action potential arrives
at the synaptic knob.
• This increase enhances
and prolongs the
neurotransmitter’s
effect on the
Postsynaptic membrane
108
Figure 12–20a
Presynaptic Inhibition
• Activity at an axoaxonal
synapse via the release of
GABA inhibits the opening
of voltage-regulated
calcium channels in the
synaptic knob.
• Results in a reduced
amount of
neurotransmitters
released when an action
potential arrives there
• Thus, reducing the effects
of synaptic activity on the
postsynaptic membrane
109
Figure 12–20a
Factors that Disrupt Neural Function
1.
ph: normal = 7.4
-
2.
At pH 7.8  spontaneous action potentials = convulsions
At pH 7.0  no action potentials = unresponsive
Ion concentration
-
3.
High extracellular [K+]  depolarize membrane = death, cardiac
arrest
Temperature: normal = 37°C
-
4.
higher: neurons more excitable  Fever = hallucinations
Lower: neurons non-responsive  Hypothermia = lethargy, confusion
Nutrients
-
5.
neurons: no reserves, use a lot of ATP
Require constant and abundant glucose
Glucose only
Oxygen
-
Aerobic respiration only for ATP
No ATP = neuron damage/death
110
SUMMARY
•
•
•
•
•
•
Neural tissue and the neuron
Anatomical divisions of the nervous system
Central and peripheral nervous systems
Nerves and axons
Functional divisions of the nervous system
Afferent division and receptors and Efferent division and
effectors
• Somatic and autonomic nervous systems
• Structure of neurons:
– organelles of neuron: neurofilaments, neurotubules, neurofibrils
– structures of axon: axon hillock, initial segment, axoplasm
– synapse and neurotransmitters
• Classification of neurons:
– structural classifications: anaxonic, bipolar, unipolar, and multipolar
– functional classifications: sensory, motor, and interneurons 111
SUMMARY
• 4 types of neuroglia:
– ependymal, astrocytes, and oligodendrocytes, microglia
• Ganglia and neurons of PNS:
– satellite cells, Schwann cells
• Repair of neurons in the PNS
• Transmembrane potential:
– electrochemical gradient
– passive and active channels
• Gated channels:
– chemically regulated, voltage-regulated, mechanically regulated
• Action potentials:
–
–
–
–
threshold
refractory period
continuous and saltatory propagation
3 types of axons (A, B, and C fibers)
112
SUMMARY
• Transmission of nerve impulses across a synapse:
–
–
–
–
–
presynaptic and postsynaptic neurons
electrical and chemical synapses
excitatory and inhibitory neurotransmitters
cholinergic synapses (ACh)
other neurotransmitters (NE, dopamine, seratonin, GABA)
• Graded potentials:
– depolarization and hyperpolarization
• Neuromodulators:
– direct, indirect, and lipid-soluble gases
• Rate of generation of action potentials Information
processing:
–
–
–
–
integration of postsynaptic potentials
EPSPs and IPSPs
spatial and temporal summation
presynaptic inhibition and facilitation
113