Transcript 9.5-9.9 Notes

9.5-9.9 Notes
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cell Membrane Set-up
A.
B.
A cell membrane is usually
polarized.
-excess of negative charges on
the inside of the membrane
-sodium and potassium ions
polarization is important to the
conduction of nerve impulses.
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Distribution of Ions
1. More sodium outside and more potassium
inside.
2. Cytoplasm has a high amount of negative
ions and proteins that can’t leave the cell. Inside
is more negative than outside.
3. Channels in membranes are selective.
1. Potassium has easier time leaving than sodium
2. Potassium is impt in membrane polarization
3. Calcium has the hardest time of all.
4. Sodium-potassium pump-ATP is used to
pump sodium and potassium ions across the
membrane
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Resting Potential
• Active Transport allows for potassium to
move out faster than sodium moving in.
• This separation of charge, or potential
difference, is called the resting potential.
• Cell (inside) is at -70 millivolts
Things happen to the membrane:
• When a neurotransmitter is picked up, it
will stimulate the membrane.
• An action potential can occur:
– Repolarization
– Depolarization
– http://bcs.whfreeman.com/thelifewire/content/c
hp44/4401s.swf
Depolarization
• Inside of the membrane becomes less
negative (more positive)-increase in volts
• Sodium channels open and potassium
channels close
• Stimulation must reach a certain point
(threshold) before the neuron continues on.
Repolarization
• As depolarization takes place, potassium
channels open and sodium closes, which is
repolarization.
• Inside of the membrane becomes negative
• Refractory period-time when the neuron
cannot respond to a second stimulus (time
when the graph dips below resting
potential)
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Action Potential
• At threshold potential, membrane
permeability to sodium suddenly changes in
the region of stimulation.
• As sodium channels open, sodium ions rush
in, and the membrane potential changes and
becomes depolarized.
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• At the same time, potassium channels open to
allow potassium ions to leave the cell, the
membrane becomes repolarized, and resting
potential is reestablished.
-This rapid sequence of events is the
action potential.
• The active transport mechanism then works to
maintain the original concentrations of sodium and
potassium ions.
• Hyperpolarization-a deep below the -70 mv occurs
because gates are closed and chlorine comes into
the cell. So the cell negativity drops more.
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Nerve Impulse
•
A nerve impulse is conducted as
action potential is reached at the
trigger zone and spreads by a local
current flowing down the fiber, and
adjacent areas of the membrane
reach action potential.
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Impulse Conduction
MYELINATED VS UNMYELINATED
• Unmyelinated fibers conduct impulses over their
entire membrane surface.
• Myelinated fibers conduct impulses from node of
Ranvier to node of Ranvier, a phenomenon called
saltatory conduction.
– Saltatory conduction is many times faster than
conduction on unmyelinated neurons.
•
http://www.blackwellpublishing.com/matthews/actionp.html
AXON DIAMETER
• The greater the diameter, the faster it travels.
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
All-or-None Response
• If a nerve fiber responds at all to a stimulus, it
responds completely by conducting an impulse
(all-or-none response).
• Greater intensity of stimulation triggers more
impulses per second, not stronger impulses.
• Complete depolarization must take place for a
neuron to travel along the axon
• How does an action potential move from one
neuron to the next?
• http://www.execulink.com/~ekimmel/mixed_flash
.htm
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The Synapse
http://outreach.mcb.harvard.edu/animations/synaptic.swf
• The junction between two communicating neurons
is called a synapse
• There exists a synaptic cleft between them across
which the impulse must be conveyed.
• Presynaptic neuron-carries pulse
• Postsynaptic neuron-receives it
• When the impulse reaches the synaptic knobs
(also known as axon terminal), neurotransmitters
must be released from synaptic vesicles into the
synaptic cleft.
• The nts are released because of the influx of
calcium.
• Recycled, deactivated, or attach to receptors
Label on your noteguide.
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Excitatory and Inhibitory Actions
• Neurotransmitters that increase postsynaptic
membrane permeability to sodium ions may
trigger impulses and are excitatory.
– Acetylcholine, epinephrine, norepinephrine,
serotonin, amino acids
• Decrease membrane permeability to sodium
ions, reducing the chance that it will reach
threshold, and are inhibitory.
– GABA, dopamine
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Impulse Processing
•
•
How impulses are processed is dependent upon
how neurons are organized in the brain and
spinal cord.
Neuronal Pools
– Neurons within the CNS are organized into neuronal
pools with varying numbers of cells.
– Each pool receives input from afferent nerves and
processes the information according to the special
characteristics of the pool.
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Facilitation
• A particular neuron of a pool may receive
excitatory or inhibitory stimulation; if the
net effect is excitatory but subthreshold, the
neuron becomes more excitable to incoming
stimulation (a condition called facilitation).
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Convergence
• A single neuron within a pool may receive
impulses from two or more fibers
(convergence), which makes it possible for
the neuron to summate impulses from
different sources.
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Divergence
• Impulses leaving a neuron in a pool may be passed
onto several output fibers (divergence), a pattern
that serves to amplify an impulse.
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Reflex Arc
• Sensory neuron---interneuron in spinal
cord---motor neuron
• Autonomic
• Homeostasis and survival (withdraw reflex)