Electrical Signaling:
1. Describe the importance of internal signaling for responding to the environment.
★ Adaptation and survival
○ Plants
– Phototropism: (growth toward light by increasing auxin)
– Stomatal closure: (reduces water loss in drought by increasing
ABA)
– Defense mechanisms: (can use chemical/toxic
compounds/proteins)
○ Animals
– Homeostasis: (insulin regulation (in pancreas) in response to
high glucose= lowers blood sugar)
– Immune system: (infection when pathogens invade= body
releases cytokines which signals immune cells to migrate to site
of infection)
– Stress response: (fight or flight (ANS) releases epinephrine (from
adrenal glands)= increases heart rate, redirects blood flow to
muscles, enhances alertness)
– (Endocrine) Behavioral response: (honey foraging behavior of
bees or seasonal migration of birds)
2. Contrast the relative concentrations of sodium and potassium inside and outside
cells and how this affects their movement through ion channels.
★ Concentrations needed for causing action potential - depolarization,
repolarization, hyperpolarization
○ Inside of cell is already negative (neuron = -70mV)
○ When a presynaptic neuron gets excited or stimulated, graded potentials
move up the cell body
■ Basically little ligand gated channels that open when you reach
the threshold, last one smaller than the next - decay with distance
■ But if you combine a bunch of them, they can turn into an action
potential (ESPS)
○ Sodium Potassium pump: responsible for maintaining the concentration
gradients of Na⁺ and K⁺ in the body (USED FOR APs AND MUSCLE
CONTRACTION) by pumping 3 Na⁺ ions out of the cell and 2 K⁺ ions
into the cell against their concentration gradients using energy from ATP
hydrolysis
○ Na⁺ concentration: The pump ensures that Na⁺ is kept low inside the cell
and high outside the cell.
○ K⁺ concentration: The pump keeps K⁺ high inside the cell and low
outside the cell. (slow after reaching threshold)
,3. Describe how voltage-gated channels function in action potentials.
★ Strength over long distances “all or
nothing”
★ Very fast (rising to falling phase happens
within 1ms)
★ Larger diameters & myelinated axons
increase velocity of nerve impulses
○ Myelin allows for saltatory
conduction of Na+
★ Responds to changes in membrane
potential (electrical)= produces action
potentials (rising, falling, undershoot)
○ THRESHOLD STIMULUS (E1
Excitatory (depolarizing) stimulus)
- Action potential firing rate
increases as depolarizing
current increases
- Received by a neuron=
opens Na+ channels
○ REACHES THRESHOLD POTENTIAL (~55mV)
- Opens voltage gated channels
- Causes influx of Na+ (depolarization) *RISING PHASE*
- Membrane Potential= +35mV to +40mV *PEAK OF AP*
○ Once action potential peaks…
- Na+ channels inactivate
- K+ channels slowly open (leaving the cell) (repolarizing)
*FALLING PHASE*
○ Returning back to resting potential
- K+ channels close (slowly to allow more K+ to exit the cell than
needed) *UNDERSHOOT PHASE*
★ REFRACTORY PERIOD: limits action potentials
○ Absolute refractory period: Na+ channels inactivate & can NOT open
○ Relative refractory period: harder to reach threshold for action potential
4. Describe how action potentials propagate down axons and the importance of
myelin in this process.
★ Can only propagate in ONE DIRECTION only
★ Release NT after they get to the synapse (space between dendrites where NT are
released)
, ★ 1) Action Potential is generated: neuron must be stimulated enough to cause voltage
gated channels in cell membrane to open. Na+ ions rush INTO CELL making cell more
+ (depolarization) - rapid voltage change 1ms
○ Membrane potential reaches threshold - and AP is generated leading to
more and more Na+ (like a positive feedback loop)
★ Propagation of AP actually going down:
○ depolarization from the action potential in one part of the axon causes
adjacent parts to also depolarize.
■ bc the positive charges from the Na⁺ ions spread out along the
inside of the membrane, basically contaminating it
○ As one segment reaches the next, the AP is initiated - SELF
PROPAGATING
○ Next, voltage-gated potassium channels open, allowing K⁺ ions to flow
out of the cell, which helps (repolarize) the membrane back to its resting
potential — because the previous segment is temporarily in a refractory
period, ensures that AP goes in 1 direction
○ Usually overshoots (hyperpolarization) causing it to be just a bit too
negative but comes back in the refractory period
★ Importance of Myelin:
○ Allows for rapid transmission of electrical signals
○ In unmyelinated axons, conduction velocity is much slower because the
action potential must propagate continuously along the axon
○ myelinated axons can conduct at much higher speeds over longer
distances
○ increases the speed and efficiency of action potential propagation
through saltatory conduction, where the action potential jumps between
the nodes of Ranvier
○ conserves energy by limiting ion exchange to the nodes, reducing the
workload on the sodium-potassium pump
5. Distinguish between electrical and chemical synapses & 6. Describe how
neurotransmitters are released at axon terminals.
★ Electrical
○ Direct cytoplasmic
connections through GAP
JUNCTIONS (between
pre/postsynaptic neurons)
○ Passes action potentials,
membrane changes, etc.
○ Common in invertebrates
★ Chemical ( common in vertebrates)
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