2. Nucleus
Trigger
zone
1 mm
Node of Ranvier
Schwann cell
(myelin sheath)
Muscle cell
Synapses
Direction
of signal
transmission
Cell body
Dendrites
Synaptic terminals
Figure 26.4
Axon
2
5. Figure 26.5
Peripheral nervous system
Sensory input
Motor response
Sensory integration
Central nervous system
(brain and spinal cord)
Stimulus:
pain
Sensory
receptors
Dendrites
Sensory
neuron
cell body
Sensory neuron
conveys impulse
to CNS
Response:
muscle contraction
Axon
Muscle cell
Synapses
Axon
Axon
Motor neuron
conveys impulse
to muscle
Synapses
Dendrites
Dendrites
Synapses
Motor neuron cell body
Interneuron
connects
sensory neuron
to motor neuron
Interneuron
cell body
5
6. Action Potentials Convey Information
Section 26.3
The message is an
electrical impulse
called an action
potential, which
travels along a
neuron’s axon.
6
11. 1. Resting Potential
* Many negative ions trapped inside cell (proteins and other molecules)
11
12. Figure 26.6
Trigger zone
Axon
Direction of
neural impulse
Resting potential
–70 mV
–70
–50
+35
0
50
1 2 3
–70
–50
0
Sodium (Na+) Potassium (K+) Negatively
charged protein
K+ leakage
channel
(always
open)
Inside axon
Outside axonDelayed K+
channel (closed)
Na+ channel
(closed)
ATP
Membranepotential(mV)
Na+/K+ pump
(always active)
Time in milliseconds
Threshold
12
13. Threshold Potential
• Stimulation of great enough intensity; triggers
change in cell membrane permeability to Sodium
ions.
If met, action potential begins…
13
14. 2. Depolarization = Action Potential
• If threshold potential is met, voltage-gated Sodium
channels open and Sodium ions pour into cell.
Membrane Charges reverse.
14
15. Depolarization
–50 mV
–70
–50 +35 Na+ channel
(open)
ATP
Membranepotential(mV)
0
50
1 2 3
–70
–50
0
Time in milliseconds
Threshold
Figure 26.6 Contd.
*Remember that Sodium leakage channels and Potassium leakage
channels are always open.
15
16. 3. Repolarization
• As wave of Depolarization passes, this changes
membrane permeability to Potassium. Potassium
channels open and Potassium moves out of cell.
This restores the charges to their correct sides of the membrane
but not the ions are in wrong locations.
16
20. 4. Refractory Period
• The Sodium/Potassium Pump re-establishes resting
Potential.
• 3 Na+ Pumped OUT for every 2 K+ pumped IN
• Uses ATP
20
21. Return to resting potential
–70 mV
–70
–50 +35
ATP
Membranepotential(mV)
0
50
1 2 3
–70
–50
0
Time in milliseconds
Threshold
Delayed K+
channel (closed)
Na+ channel
(closed)
Figure 26.6 Contd.
As the Sodium/Potassium Pump returns the cell to Resting Potential, it is IMPOSSIBLE
to generate another Action Potential.
21
22. Dendrites
0
50
Membranepotential(mV)
Time in milliseconds
Threshold
Axon
Direction of
neural impulse
–70
–50
Na+ channel
(closed)
Na+ channel
(open)
K+ channel
(closed)
K+ channel
(open) 0 1 2 3
Depolarization:
Sodium ions enter
Repolarization:
Potassium ions exit
Resting potential
Na+ channel
(closed)
K+ channel
(closed)
Cell body
Peak of action
potential
Figure 26.23
• Which membrane proteins are
• open at each step?
• Where is Sodium at each step?
• Where is Potassium at each step?
• What is the membrane charge,
• inside and outside, at each step?
22
23. Figure 26.8
Movement of Impulse Axon
Trigger zone
(high density of
Na+ channels)
Myelin
Nodes of Ranvier
(high density of
Na+ channels)
Axon
0.4 µm
TEM
(false color)
Na+ diffues
to next node
Na+ diffues
to next node
Myelin
sheath
Role of Myelin
• Because of high concentration of voltage-gated Sodium channels at the Nodes,
Action Potential appears to “jump” from node to node.
• Impulses travel 100x faster on myelinated vs. unmyelinated neurons.
(Ex. Multiple sclerosis)
23
25. Figure 26.9
Mitochondrion
Presynaptic
neuron
Postsynaptic
neuron
Synaptic
cleft
Synaptic
terminal
TEM
(false color)
1 µm
Synaptic
vesicle
Action
potential
Neural impulse arrives at
synaptic terminal.
Depolarization causes calcium
ions to enter synaptic terminal.
Calcium influx causes vesicles
loaded with neurotransmitters
to fuse with presynaptic
neuron’s membrane.
Neurotransmitters are
released into synaptic cleft.
Neurotransmitters bind to
ion channels in postsynaptic
neuron’s membrane,
stimulating the channels to
open.
Axon of
presynaptic
neuron
New graded
potential generated
SEM
(false color) 5 µmSynaptic
terminals
Cell body of postsynaptic neuron
Synaptic vesicle containing
neurotransmitters
Ca2+
Mitochondrion
Synaptic cleft
Ion
channels
Cell body of
postsynaptic neuron
Neurotransmitter
New graded
potential generated
Na+
25
*Certain Neurotransmitters specific to
certain ion channels on certain cell types.
26. Disorders Associated with Neurotransmitter Imbalances
Condition Imbalance of Neurotransmitter in Brain Symptoms
Alzheimer disease
Epilepsy
Huntington disease
Hypersomnia
Insomnia
Myasthenia gravis
Schizophrenia
Deficient acetylcholine (caused by death of
acetylcholine-producing cells)
Excess GABA leads to excess
norepinephrine and dopamine
Deficient GABA
Excess serotonin
Deficient serotonin
Deficient receptors for acetylcholine at synapse between
motor neuron and muscle cell
Deficient dopamineParkinson disease
Deficient GABA leads to excess dopamine
Memory loss, depression, disorientation, dementia,
hallucinations, death
Seizures, loss of consciousness
Uncontrollable movements, dementia, behavioral and
personality changes, death
Excessive sleeping
Tremors of hands, slowed movements, muscle rigidity
Inappropriate emotional responses, hallucinations
Inability to sleep
Progressive muscle weakness
Table 26.1
26
Overstimulation or under stimulation?
27. White matter
Gray matter
Sensory neuron
Interneuron
Motor neuron
Spinal cord
To brain Synapses
Cell body of
sensory neuron
receives impulse
Cell body of
motor neuron
receives impulse
Axon of
motor neuron
Effector
Muscle
stimulated
to contract,
withdrawing
the hand
Pain
receptors in
skin stimulated
Sensory neuron
excited
Figure 26.13
27
28. Nondepressed individual
Abundant serotonin in synaptic cleft
Sending
neuron
Serotonin
reuptake
protein
Serotonin
receptor
Neuro-
transmitter
(serotonin)
Synaptic
cleft
Receiving neuron
Depressed individual, untreated
Too much reuptake; insuffi•cient
serotonin in synaptic cleft
Drug
blocks
serotonin
reuptake SSRI
drug
Depressed individual, treated with SSRI
Reuptake blocked; abundant serotonin in
synaptic cleft
Figure 26.A
Drugs and Neurotransmitters
28
Under stimulation or over stimulation?
Paralysis or convulsions (tetany)?
29. Nervous system
includes cells called is divided into
Neurons
Neuroglia
Peripheral
nervous
system
Central
nervous
system
support
of three
types
transmit
Sensory
pathways
Motor
pathways
Brain
Spinal
cord
Sensory
Motor
Interneuron
Action potentials
trigger release of
Neurotransmitters
at
Synapses
includes includes
Figure 26.24
29
