Download Basis of Membrane Potential Action Potential Movie

Basis of Membrane Potential
Membrane pumps:
• use energy (ATP) to move ions against a concentration gradient
• Na+-K+ pump moves K+ ions to the inside of the nerve cell
– Expels 3 Na+ for every 2 K+ ions it brings in
Ion Channel:
• Channels are selective
• Channels may be gated or un-gated
• Gated channels are either voltage-gated or chemically-gated
Sudden Changes in Ion Channels
Generate Action Potentials
• Action potential (AP) is a sudden and
major change in membrane potential
– Lasts 1-2 milliseconds
– Pulse of electric charge is conducted along the
axon at speeds up to 100 meters per second
– Membrane potential changes from -60 to +50
mV
Action Potential Movie
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Role of voltage-gated ion channels in AP 1
1) Resting phase: Both sodium and potassium channels are
closed, resting state maintained
Role of voltage-gated ion channels in AP 2
2) Depolarizing phase: Acitvation gates of sodium channels
open (letting in Na+) and potassium channels remain
closed
Role of voltage-gated ion channels in AP 3
3) Repolarizing phase: Inactivation gates close sodium
channels and potassium channels open; potassium ions
leave cell and lose + ions
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Role of voltage-gated ion channels in AP 4
4) Undershoot: Both gates of sodium channels are closed but
potassium channels remain open because of slower
response.
• Action potentials are
conducted along axons
without a reduction of
signal
• Na ions spread
downstream, bringing
next region to Na+
channel threshold
Saltatory Conduction I
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Feature of vertebrate but not invertebrate axons
Schwann cells have projections which wrap around axons
AP appear to jump from node to node in myelinated axons
Saltatory APs greatly accelerate rate of nerve impulse
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Saltatory Conduction II
• The speed of nerve impulse conduction also depends on
diameter of axon -- larger diameters enable faster
conduction
• Unmyelinated axon responsible for squid escape behavior
is whopping 1 mm in diameter
Neurons, Synapses & Communication
• Synapses are junctions where one cell
influences another through the transfer of a
chemical or electrical message
• Chemical synapses: comprise most
synapses in vertebrates
• Electrical synapses: more abundant in
invertebrates -- act rapidly but don’t
integrate well
Chemical Synapse Movie
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Chemical Synapse
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AP depolarises membrane
of synaptic terminal
Triggers influx of Ca2+
Causes synaptic vesicles to
fuse w/ membrane
Release neurotransmitter
molecules which bind to
receptors of ion channels
Binding of neurotransmitter
molecules open ion
channels
Neurotransmitter is
degraded rapidly, closing
ion channel
Neurotransmitters
• Acetylcholine (Ach):
one of most common
neurotransmitters in
vertebrates
• Ach broken down by
acetylcholinesterase
• Breakdown products
are then resynthesized into
more Ach
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Organization of the Vertebrate Nervous System
Central Nervous System(CNS)
– Brain: integration; complex
behavior
– Spinal cord: integrates simple
(reflex responses)
Peripheral Nervous System
(PNS)
• Sensory (afferent):
– PNS --> CNS
• Motor: (efferent)
– CNS --> PNS
– Somatic: signals to skeletal
muscles in response to external
stimuli (largely voluntary)
– Autonomic: signals to skeletal
muscles in response to internal
stimuli (largely involuntary)
Reflex Response
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Autonomic Nervous System: Parasympathetic &
Sympathetic Divisions
• When parasympathetic (rest &
digest) & sympathetic (flight
or fight) divisions innervate
same organ, effects are often
antagonistic (opposite)
• In general, parasympathetic
division enhances activities
that gain or conserve energy
such as digestion & heart rate
• In general, sympathetic
division increases energy
consumption & prepares
individual for action by
accelerating heart rate &
metabolic rate
Parts of the Human Brain
• Hindbrain: controls
movement & balance
– Cerebellum
– Pons
– Medulla oblongata
• Midbrain: conducts
information & controls
automatic activities
essential for survival
• Forebrain:
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Thalmus
Hypothalmus
Epithalmus
Cerebrum: cerebral cortex,
white matter, basal nuclei
How Do We Determine Brain Function:
The Tragic Case of Phineas P. Gage
• 19th railroad construction
accident blew tamping
rod through brain of
Phineas P. Gage
• He survived w/ senses &
memory in tact
• Before accident, very
responsible, well
respected man
• After accident,
quarrelsome, bad
tempered, lazy, profane
and irresponsible
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Alternatives
to Ablation I:
Electrodes
Alternatives to Ablation II:
Imaging Brain Function
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Magnetic Resonance Imaging (MRI): exploits behavior of H atoms in
water; useful at distinguishing soft tissue from bone
Computed Tomography (CT): produces images in a series of thin Xray sections which can be integrated by a computer into 3D image
Positron Emission Tomography (PET): can reveal local areas of
physiological activity; involves use of minute quantities of
radioisotopes injected into bloodstream
Alternatives to ablation III: Reverse genetics
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DNA sequences (antisense
expression constructs) can be
injected into the brain and used to
temporarily turn off specific genes
Has been performed extensively
in mice but only recently in
primates (PNAS 2004)
In rhesus monkeys, DNA
antisense expression constructs
were injected into the rhinal
cortex in order block the D2 gene
(produces dopamine receptors)
In operant conditioning trials, the
technique turned “slacker
monkeys” into efficient, hardworkers by suppressing their
ability to anticipate a reward
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• Your Brain:
– Use it or lose it!
• Environmental
complexity and
diversity of social
experience
influence brain
development in
rats and mice
Meditation alters brain structure
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Brain imaging shows meditation
increased thickness in cortical
regions related to sensory,
auditory and visual perception,
as well as the automatic
monitoring of heart rate or
breathing, for example.
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Most of the brain regions
changed through meditation are
in the right hemisphere, which is
essential for sustaining attention
(anger processed in left brain)
PREFRONTAL CORTEX SYSTEM
Functions
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attention span
perseverance
planning
judgment
impulse control
organization
self-monitoring and supervision
problem solving
critical thinking
forward thinking
learning from experience and mistakes
ability to feel and express emotions
influences the limbic system
empathy
internal supervision
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MRI scans compress 15 yr brain development (ages 5-20).
Red indicates more gray matter, blue less gray matter.
Gray matter wanes in a back to front wave as brain matures and neural connections are pruned.
Areas performing more basic functions mature earlier;
Areas for higher-order functions (emotion, self-control) mature later.
The pre-frontal cortex, which handles reasoning and other "executive" functions, emerged late
in evolution, and is among the last to mature.
Frontal lobe injury – problems with:
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planning
time
persistence
diligence
inhibiting behavior
distracted easily
Prefrontal Cortex & Criminal
Behavior
• 41 murderers pleading NGRI and
41 controls.
• Each group had 6 schizophrenics.
(Raine et al. 1997)
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Prefrontal Cortex & Criminal
Behavior cont.
• PET scans showed murderers had lower glucose
metabolism in the bilateral and medial PFC
• Reduced prefrontal activity has been observed
to be associated with severe violence.
The Brain, Emotions & Decision
Making
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Antonio Damasio: Looking for
Spinosa
Recognized importance of
emotion in decision making from
patients whose emotional centers
had been damaged by strokes,
accidents, or tumors.
The change took place in the
realm of day-to-day decision
making, not in the realm of
knowledge and skills. They could
speak perfectly well. They could
deal with the logic of a problem.
They could learn new things.”
Nonetheless, the lives of these
tumor or stroke victims fell apart.
Their marriages dissolved, and
their careers were reduced to a
series of odd jobs and disability
checks.
Transorbital Lobotomy
• “Ice pick” Lobotomy
about to be performed
by Dr. Walter
Freeman
• Freeman believed that
mental illness was
related to overactive
emotions, and that by
cutting the brain he
cut away these
feelings.
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The Essential Difference
by Simon Baron-Cohen
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Male & female brains & the truth about autism
Autism: A lifelong, neurological disorder appearing before the age of
three yeas that involves language and communication deficits, and
withdrawal from social contacts; pathological “self-absorption”
Incidence of autism in males is more than 4 times higher than in
females
Baron-Cohen’s thesis:
– the female brain is predominantly hard-wired for empathy
– the male brain is predominantly hard-wired for understanding and
building sysems
– sex differences result from the effects of testosterone acting on
brain development during fetal development
– Autism: the extreme male brain
Evolution of the human brain:
comparative genomics
• ASPM (Abnormal spindle-like microcephaly
associated) gene shows an excess of nonsynonymous over synonymous substitutions in
ape lineages leading to humans
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