Chapter 3 The Biological Bases of Behavior. Neural and Hormonal Systems Module 7.
Chapter 4 Principles of Neural and Hormonal Communication
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Transcript of Chapter 4 Principles of Neural and Hormonal Communication
Chapter 4Principles of Neural and
Hormonal Communication
• Write and answer detailed questions about how membrane potentials are propagated, converted to chemical signals (neurotransmitters) at synapses, and create postsynaptic potentials. This will be measured by exams, quizzes and student generated questions in class discussions.
Outline• Graded Potentials• Action Potentials• Synapses and integration• Intracellular communication• Signal Transduction• Hormonal Communication• Nervous vs. Endocrine System
Communication is critical for the survival of the cells that compose the body.
Two major regulatory systems of the body – nervous and endocrine - communicate with the cells/tissues/organs/systems they control.
Neural communicationHormonal communication
Neural Communication• Nerve and muscle cells are excitable tissues. • Can alter their membrane permeabilities.• Permeability changes lead to membrane
potential changes. • Potential changes act as electrical signals. • Electrical signals are necessary for normal nerve
and muscle function.
Time (msec)
Mem
bra
ne
po
ten
tial
(m
V)
Hyperpolarization (increase inpotential; membrane more negative)
Depolarization (decrease in potential;membrane less negative)
Repolarization (return to resting potentialafter depolarization)
Resting potential
–90–80–70
–60–50–40–30–20–10
0+10+20
Fig. 4-1, p. 90
• Membrane electrical states– Polarization
• Any state when the membrane potential is other than 0mV– Depolarization
• Membrane becomes less polarized than at resting potential– Repolarization
• Membrane returns to resting potential after having been depolarized– Hyperpolarization
• Membrane becomes more polarized than at resting potential
Neural Communication
Voltage clamp • The technique allows an experimenter to "clamp" the cell potential at a chosen value.
• This makes it possible to measure how much ionic current crosses a cell's membrane at any given voltage.
• This is important because many of the ion channels in the membrane of a neuron are voltage gated ion channels, which open only when the membrane voltage is within a certain range.
Kenneth Cole[2] and George Marmount
https://www.youtube.com/watch?v=Wd_gKJoo25Y
Patch clamp• A patch-clamp microelectrode is a micropipette with a relatively large tip diameter. • The microelectrode is placed next to a cell, and gentle suction is applied through the
microelectrode to draw a piece of the cell membrane (the 'patch') into the microelectrode tip; the glass tip forms a high resistance 'seal' with the cell membrane.
• This can be used for studying the activity of the ion channels that are present in the patch of membrane.
• If more suction is now applied, the small patch of membrane in the electrode tip can be displaced, leaving the electrode sealed to the rest of the cell.
• This "whole-cell" mode allows very stable intracellular recording.
This technique was developed by Erwin Neher and Bert Sakmann who received the Nobel Prize in 1991.
https://www.youtube.com/watch?v=8LDO0hWWc0Q
Channels• Leak channels
– Unregulated passage of ions• Gated channels
– Voltage gated– Chemically gated– Mechanically gated– Thermally gated
• These channels create and alter membrane potentials• Two kinds of potential change
– Graded potentials• Serve as short-distance signals
– Action potentials• Serve as long-distance signals
Graded Potential• Occurs in small, specialized region of excitable cell
membranes • Magnitude of graded potential varies directly with the
magnitude of the triggering event• Die out over short distances
Fig. 4-2, p. 87
Current Flow During a Graded Potential
Graded Potentials
Examples of graded potentials:• Postsynaptic potentials• Receptor potentials• End-plate potentials• Pacemaker potentials• Slow-wave potentials
Action Potentials• Brief, rapid, large (100mV) changes in
membrane potential during which potential actually reverses
• Involves only a small portion of the total excitable cell membrane
• Do not decrease in strength as they travel from their site of initiation throughout remainder of cell membrane
Time (msec)
Slow depolarizationto threshold
1 msec
After hyperpolarization
Resting potential
Threshold potential
Action potential
Mem
bra
ne
po
ten
tial
(m
V)
+70
0
–90
–80
–70–60
–50–40–3020
–10
+10+20+30
+40+50
+60
Fig. 4-4, p. 94
(e) Open
Delayedopeningtriggeredat threshold
Activationgate
VOLTAGE-GATED POTASSIUM CHANNEL
(c) Closed and notcapable of opening(inactivated)
Rapidopeningtriggeredat threshold
(a) Closed butcapable of opening
Inactivation gate
Activation gateICF
Plasmamembrane
ECF
(d) Closed
Slowclosingtriggeredat threshold
(b) Open(activated)
VOLTAGE-GATED SODIUM CHANNEL
Fig. 4-5, p. 95
ANIMATION: Action Potential
Action Potentials
• When membrane reaches threshold potential – (-50 to-55mv)
– Voltage-gated channels in the membrane undergo conformational changes
– Flow of sodium ions into the ICF reverses the membrane potential from -70 mV to +30 mV
– Flow of potassium ions into the ECF restores the membrane potential to the resting state
Action Potentials• Additional characteristics
– Sodium channels open during depolarization by positive feedback.
– When the sodium channels become inactive, the channels for potassium open. This repolarizes the membrane.
– As the action potential develops at one point in the plasma membrane, it regenerates an identical action potential at the next point in the membrane.
– Therefore, it travels along the plasma membrane undiminished.
Fig. 4-7a, p. 96
Na+ channel closes and is inactivated(activation gate still open; inactivationgate closes)
Na+ channelopens andis activated(activationgate opens;inactivationgate alreadyopen)
K+ channel opens(activation gate opens)
K+ voltage-gated channel closed(activation gate closed)
Me
mb
ran
e p
ote
nti
al
(mV
)
Na+ voltage-gated channel closed(activation gate closed; inactivation gate open)
Time (msec)
Depolarizingtriggering event
Resting potential
Threshold potential
K+ channelcloses(activationgate closes)
Na+ channelreset to closedbut capableof opening(activationgate closes;inactivationgate opens)
K+ o
ut →
falling
ph
ase
Na
+ in
→ r
isin
g p
has
e
ICF
ECF
11
22
33
44
55
66
77
88
Action Potentials
The Na+/K+ pump gradually restores the concentration gradients disrupted by action potentials.
• Sodium is pumped into the ECF• Potassium is pumped into the ICF• Refractory period keeps the action potential going in one
direction and limits the Ap frequency• All or none• Frequency and line coding
Table 4-1 p101
Action Potentials• The Na+–K+ pump gradually restores the ions
that moved during the action potential.• After an impulse has occurred in a patch of
membrane, the membrane enters its refractory period.
• It is impossible to re-stimulate the patch of membrane until it has recovered from its refractory period.
Refractory Periods• Absolute refractory period- period of time
when a patch of membrane cannot be re-stimulated no matter how strong the stimulus.
• Relative refractory period- period of time during which a patch of membrane can only be re-stimulated by a stronger than normal stimulus.
• Refractory periods ensure the one-way propagation of action potentials.
Fig. 4-10, p. 101
“Backward” currentflow does not reexcitepreviously active areabecause this area isin its refractory period
Direction of propagationof action potential
“Forward” current flow excitesnew inactive area
New adjacent inactive areainto which depolarizationis spreading; will soon reachthreshold
New active areaat peak of actionpotential
Previous activearea returned toresting potential
Neuron• Once initiated, action potentials are conducted
throughout a nerve fiber• Action potentials are propagated from the axon
hillock to the axon terminals• Basic parts of neuron (nerve cell)
– Cell body
– Dendrites
– Axon
Neuron
Neuron• Cell body
– Houses the nucleus and organelles
• Dendrites – Project from cell body and increase surface area
available for receiving signals from other nerve cells
– Signal toward the cell body
Dendrite and cell body serve as the neurons input zone.
Neuron• Axon
– Nerve fiber– Single, elongated tubular extension that conducts action
potentials away from the cell body– Conducting zone of the neuron– Collaterals
• Side branches of axon
– Axon hillock• First portion of the axon plus the region of the cell body fro m
which the axon leaves• Neuron’s trigger zone
– Axon terminals• Release chemical messengers that simultaneously influence
other cells with which they come into close association• Output zone of the neuron
Action Potentials
• Two types of propagation– Contiguous conduction
• Conduction in unmyelinated fibers
• Action potential spreads along every portion of the membrane
– Saltatory conduction • Rapid conduction in myelinated fibers
• Impulse jumps over sections of the fiber covered with insulating myelin
Fig. 4-9b, p. 100
Remainder of axonstill at resting potential
New adjacentinactive area into which depolarization is spreading; will soon reach threshold
Adjacent area that was brought tothreshold by local current flow; nowactive at peak of action potential
Previous activearea returnedto restingpotential; nolonger active; inrefractory period
Contiguous Conduction
Fig. 4-9a, p. 100
Direction of propagation of action potential
Gradedpotential> threshold
Local current flow that depolarizesadjacent inactive area from restingpotential to threshold potential
Remainder of axonstill at resting potential
Adjacent inactive area into which depolarization is spreading; will soon reach threshold
Active area atpeak of actionpotential
Contiguous Conduction
Fig. 4-12a, p. 103(a) Myelinated fiber
Plasmamembrane
Axon of neuron
Nodes of Ranvier Myelin sheath
Myelinsheath
Axon
Saltatory Conduction
Fig. 4-12d, p. 103
Node ofRanvier
Axon
Voltage-gatedNa+ and K+channels
Myelinsheath
Saltatory Conduction• Propagates action potential faster than
contiguous conduction because action potential does not have to be regenerated at myelinated section
• Myelinated fibers conduct impulses about 50 times faster than unmyelinated fibers of comparable size
• Myelin – Primarily composed of lipids
– Formed by oligodendrocytes in CNS
– Formed by Schwann cells in PNS
Direction of propagationof action potential
Adjacent inactive nodeinto which depolarizationis spreading; will soonreach threshold
Remainder of nodesstill at resting potential
Active node at peakof action potential
Local current flow thatdepolarizes adjacent inactivenode from resting to threshold
Fig. 4-13, p. 104
Regeneration of Nerve Fibers• Regeneration of nerve fibers depends on its
location• Schwann cells in PNS guide the regeneration of
cut axons• Fibers in CNS myelinated by oligodendrocytes
do not have regenerative ability– Oligodendrocytes inhibit regeneration of cut
central axons
Synapses• Junction between two neurons• Primary means by which one neuron directly interacts
with another neuron (muscle cells or glands as well)• Anatomy of a synapse
– Presynaptic neuron – conducts action potential toward synapse
– Synaptic knob – contains synaptic vesicles
– Synaptic vesicles – stores neurotransmitter (carries signal across a synapse)
– Postsynaptic neuron – neuron whose action potentials are propagated away from the synapse
– Synaptic cleft – space between the presynaptic and postsynaptic neurons
Synaptic knob(presynapticaxon terminal)
Chemically gatedreceptor-channelfor Na+, K+, or Cl–
Neuro-transmittermolecule
Voltage-gatedCa2+ channel Ca2+
Postsynaptic neuron
Subsynapticmembrane
Synapticcleft
Synapticvesicle
Axon ofpresynapticneuron
Receptor forneurotransmitter
1
2
3
4
5
3
4
5
Fig. 4-15, p. 108
Axon terminalof presynapticneuron
Synapticvesicles
Synapticcleft
Dendrite ofpostsynapticneuron
Fig. 4-15, p. 108
Neurotransmitters• Vary from synapse to synapse• Same neurotransmitter is always released at a particular
synapse• Quickly removed from the synaptic cleft
SynapsesSignal at synapse either excites or inhibits the postsynaptic
neuron• Two types of synapses
– Excitatory synapses– Inhibitory synapses
• If binding of NT opens Na+ and K+ channels the result is a small depolarization called an excitatory post-synaptic potential (EPSP).
• EPSP brings the cell closer to threshold. • If binding of NT opens either K+ or Cl– channels the result
is a small hyperpolarization called an inhibitory post-synaptic potential (IPSP).
• IPSP means cell less likely to reach threshold.
Cell body of postsynapticneuron
Axon hillock
Synaptic Summation
• Multiple EPSP and IPSP’s from numerous synapses converge on one neuron.
• These signals can cause different changes in the postsynaptic neuron– Cancellation
– Spatial summation
– Temporal summation
Synaptic Summation
Inhibitorypresynaptic input
Postsynapticcell
Excitatorypresynaptic inputs
Membranepotentialrecorded
Po
sts
yn
ap
tic
me
mb
ran
e p
ote
nti
al
(mV
)
ThresholdpotentialRestingpotential
(d) EPSP-IPSPcancellation
Time (msec)
–70
(a) Nosummation
(b) Temporalsummation
(c) Spatialsummation
+30
0–50
Fig. 4-17, p. 111
Fig. 4-19, p. 109
Presynapticinputs
Postsynapticneuron
Convergence of input(one cell is influencedby many others)
Presynapticinputs
Divergence of output(one cell influencesmany others)
Postsynapticneurons
Arrows indicate direction in which information is being conveyed. Fig. 4-20, p. 111
Synaptic Drug Interactions• Possible drug actions
– Altering the synthesis, axonal transport, storage, or release of a neurotransmitter
– Modifying neurotransmitter interaction with the postsynaptic receptor
– Influencing neurotransmitter reuptake or destruction
– Replacing a deficient neurotransmitter with a substitute transmitter
Examples of drugs that alter synaptic transmission• Cocaine
– Blocks reuptake of neurotransmitter dopamine at presynaptic terminals
• Strychnine– Competes with inhibitory neurotransmitter glycine
at postsynaptic receptor site
• Tetanus toxin– Prevents release of inhibitory neurotransmitter
GABA, affecting skeletal muscles
(a) Gap junctions
Small molecules and ions
(b) Transient direct linkup of cells’ surface markers
(c) Paracrine secretion
Local target cell
Paracrine
Secreting cell
Neurotransmitter(d) Neurotransmitter secretion
Secreting cell(neuron)
Electrical signal
Local target cell
(e) Hormonal secretion
Nontarget cell(no receptors)
Distant target cell
Hormone
Secreting cell(endocrine cell)
Blood
Distant target cell
Nontarget cell(no receptors)
Secreting cell(neuron)
Electrical signal
Neurohormone Blood
DIRECT INTERCELLULAR COMMUNICATION
INDIRECT INTERCELLULAR COMMUNICATION VIA EXTRACELLULAR CHEMICAL MESSENGERS
(f) Neurohormone secretionFig. 4-20, p. 117
Chemical Messengers
Four types:• paracrines (local chemical messengers) • neurotransmitters (very short-range chemical messengers released by neurons)
• hormones (long-range chemical messengers secreted into the blood by endocrine glands)
• neurohormones (long-range chemical messengers secreted into blood by neurons)
Hormones• Endocrinology
– Study of homeostatic activities accomplished by hormones
• Two distinct groups of hormones based on their solubility properties– Hydrophilic hormones (Proteins, peptides)
• Highly water soluble
• Low lipid solubility
– Lipophilic hormones (Steroids)• High lipid solubility
• Poorly soluble in water
Chemical Messengers• Extracellular chemical messengers bring about
cell responses primarily by signal transduction– Process by which incoming signals are conveyed
to target cell’s interior• Binding of extracellular messenger (first
messenger) to matching receptor brings about desired intracellular response by either– Opening or closing channels– Activating second-messenger systems
• Activated by first messenger• Relays message to intracellular proteins that carry out
dictated response
Fig. 4-23, p. 116
Fig. 4-24, p. 118
Fig. 4-25, p. 119
ANIMATION: Signal Transduction
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ANIMATION: Response Pathways Activated by G-Protein-Coupled Receptors
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Nucleus
Hormoneresponseelement
Gene
DNA
mRNA
Newprotein
Cellular response
DNA-bindingsite (active)
Portionthat bindshormone
Steroidhormone
Plasmaproteincarrier
Blood vessel
Steroidhormonereceptor Portion
that bindsto DNA
ECF
Cytoplasm
PlasmamembranePlasmamembrane
1
2
3
4
6
9
8
7
5
Fig. 4-28, p. 128
Comparison of Nervous System and Endocrine System