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Physiology 7: Neurons

7 of 11 of my Physiology Unit. Image Credits: Biology (Campbell) 9th edition, copyright Pearson 2011, & The InternetProvided under the terms of a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. By David Knuffke
by

Andrew Huff

on 26 February 2013

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Transcript of Physiology 7: Neurons

Weak point of nervous system
any substance that affects neurotransmitters or mimics them affects nerve function
gases: nitrous oxide, carbon monoxide
mood altering drugs:
stimulants
amphetamines, caffeine, nicotine
depressants
quaaludes, barbiturates
hallucinogenic drugs: LSD, peyote
SSRIs: Prozac, Zoloft, Paxil
poisons
Neurons Nucleus! How is a neuron's structure related to its function?

Why are neurons an “animals only” phenomenon? Big Questions Characteristics of a nervous system

fast
accurate
reset quickly Structure fits function
many entry points for signal
one path out
transmits signal
Most specialized cell in animals
Longest cell
blue whale neuron: 10-30 meters
giraffe neuron: 5 meters
human neuron: 1-2 meters
Fun facts about neurons Think dominoes!
1. start the signal-
knock down line of dominoes by tipping 1st one
2. propagate the signal-
do dominoes move down the line? no, just a wave through them!
3. re-set the system-
before you can do it again, have to set up dominoes again
 The Neuron has a similar system
protein channels are set up
1. once the first one is opened, the rest open in succession
2. a “wave” action travels along neuron
3. have to re-set channels so neuron can react again Cells live in a sea of charged ions

more concentrated within the cell
anions (negative)
K+
charged amino acids (aa-)

more concentrated in the extracellular fluid
cations (positive)
Na+
Cl-
Opposite charges on opposite sides of cell membrane

membrane is polarized ("membrane potential")
negative inside; positive outside
charge gradient
stored energy (like a battery) 1. Wave: nerve impulse travels down neuron

Change in charge opens next Na+ gates down the line
“voltage-gated” channels
Na+ ions continue to diffuse into cell
“wave” moves down neuron = action potential 3. Re-set: 2nd wave travels down neuron

K+ channels open more slowly than Na+ channels
K+ ions diffuse out of cell
charges reverse back at that point
negative inside; positive outside Combined waves travel down neuron
wave of opening ion channels moves down neuron
signal moves in one direction     
flow of K+ out of cell stops activation of Na+ channels in wrong direction
Action potential generation:
wave = nerve impulse
brain to fingertips in milliseconds! 2. After firing a neuron has to re-set itself

Na+ needs to move back out
K+ needs to move back in
both are moving against concentration gradients
need a pump!! We have one! 1. Resting potential

2. Stimulus reaches threshold potential

3. Rising (Depolarization):
Na+ channels open; K+ channels closed

4. Falling:
Na+ channels close;
K+ channels open

5. Undershoot:
K+ channels close slowly, reset charge gradient Axon coated with Schwann cells insulates axon, speeds signal

Signal hops from node to node

Saltatory conduction:
Myelinated vs. Unmyelinated
150 m/sec vs. 5 m/sec (330 mph vs. 11 mph) Impulse has to jump the synapse!
junction between neurons
has to move quickly from one cell to next
Events at synapse
action potential depolarizes membrane
opens Ca++ channels
neurotransmitter vesicles fuse with membrane
release neurotransmitter to synapse (diffusion)
neurotransmitter binds with protein receptor
ion-gated channels open
neurotransmitter degraded or reabsorbed Post-synaptic neuron
triggers nerve impulse in next nerve cell
chemical signal opens ion-gated channels
Na+ diffuses into cell
K+ diffuses out of cell
switch back to voltage-gated channel
Acetylcholine
transmit signals to skeletal muscle
Epinephrine (adrenaline) & norepinephrine stimulate release
fight-or-flight response

Dopamine
widespread in brain
affects sleep, mood, attention & learning
lack of dopamine in brain associated with Parkinson’s disease
excessive dopamine linked to schizophrenia

Serotonin
widespread in brain
affects sleep, mood, attention & learning Compare the regulatory structures and functions of the nervous and endocrine systems

Diagram the processes by which nervous signals are transmitted by and between neurons.

Label all parts of a neuron

Explain the causes and effects of major disruptions to neuron function Make Sure You Can The need for fast,
long-distance messages Dendrites Neurons Do These Things! sensory neuron Interneuron Motor Neuron Signal moves from black to red Glial cells help neurons neurons stained with "Brainbow" flourescence Incoming signals enter here.
Each dendrite is connected to another neuron at a synapse Cell Body Axon Hillock Myelin Sheath Schwann Cell Node of Ranvier Nerve Terminals Axon How membrane potential is measured Lots of pump proteins and ion channels Maintain concentrations of ions

Reset concentrations following nerve signal

Different kinds of "gates" control the openings to the channels:
mechanical gates: Opened by force
voltage gates: Opened by voltage
ligand gates: Opened by ligand binding Summary Action Potential A nerve signal
Electrical (ion exchange)
Binary (all or nothing) Action Potential Propagation Incoming Impulses Come in 2 Flavors Excitatory or Inhibitory An "All or None" Signal Neural membrane polarity is constantly fluctuating around the resting potential.

For an action potential to be generated, the membrane must be depolarized to a "threshold potential". Action Potential Generation 1. Neuron is at rest:
All gated channels are closed

2. Neuron Depolarizes:
Na+ gated channels begin to open.

3. Rising Phase of AP:
Na+ gated channels all open.

4. Falling Phase of AP:
Na+ gates shut. K+ gates open

5. Undershoot:
K+ gates begin to close. Na/K
pump restores resting potential "Na+ in, K+ out" Action potential travel by jumping ("saltations") between Schwann cells. Multiple Sclerosis:
Results from De-mylenation of motor neurons Layers of Myelin insulate neurons.
Speeds Transmission of AP's Saltatory Conduction Major Neurotransmitters Snake venom (green) blocking the active site of acetylcholinesterase Cone Snail Venom Blocks Na+ channels on post synaptic neurons Why are axons so long?
Why have synapses at all?
How do “mind altering drugs” work?caffeine, alcohol, nicotine, marijuana…
Do plants have a nervous system?
Do they need one? Any Questions?
Yes! Complexity Emerges! So Cool! Contains all organelles for the neuron. Signals move from Dendrites to cell body to axon to nerve terminals
Where an Action Potential begins (if it's going to). The Action Potential moves through the Nodes of Ranvier by saltatory conduction The start of the axon. Modes of action of some of the major venoms
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