Download Packet 6- The neuron

Lecture 6: The Neuron
Silverthorn: Chapter 8
The nervous system, in partnership with the endocrine system, coordinates the body’s actions. The functional unit of the nervous
system is the neuron, although they make up only 10% of all nervous system cells. 90% of all cells in the NS are NOT neurons…they
are glial cells. Neuron support crew (Discovered by Camillo Golgi when he accidentally dropped a brain into silver nitrate...right.)
mm Anatomy Link!
Note: This link is a REVIEW (or PREVIEW). You will not be tested on this material.
There are 5 types of glial cells, each with a unique structure and function. Astrocytes help create the
blood brain barrier and are the mama cells taking care of neurons. Microglia are specialized macrophages
(immune system phagocytes) who patrol the brain and clean up garbage. Ependymal cells are cilia-lined
epithelial cells that line cavities of the brain, producing cerebrospinal fluid and helping it circulate (using
their cilia). Oligodendrocytes produce the myelin sheaths around axons in the CNS. Schwan cells produce
myelin sheaths around axons in the PNS.
The Neuron
There are 100 billion neurons in the central nervous system (brain + spinal cord)...this is the same number of stars in a galaxy!
Draw a picture of a neuron, including all the following structures:
• Dendrites
• Cell body
• Axon hillock
• Axon
• Nodes of Ranvier
• Axon terminal (bouton, other?)
• Synapse
• Effector
A neuron transmits information through a wave of electrical signals that travels through the plasma membrane…
Electricity is simply the movement of charged particles...
QUESTION: How can we create electricity in the cell?
ANSWER: Set up a difference of charges outside and inside neuron (true of all cells), creating a membrane potential.
Membrane potential
The membrane potential represents potential energy established by a difference of electrical CHARGE between the inside of the cell
(ICF) and the outside (ECF).
1. The ECF has a slightly positive net charge
A. There is a high concentration of Na+ in the ECF (thanks to what molecule??)
B. There is also a higher concentration of Cl- in the ECF
2. The ICF has a slightly negative net charge
A. There is a high concentration of K+ in the ICF (thanks to what molecule??)
B. There is also a higher concentration of negatively charged proteins in the ICF.
3. Do you need proof? Use this voltmeter to measure the DIFFERENCES between the inside and outside charges.
4. This difference is the membrane potential
A. Resting potential is -70mV (fig 5-30)
B. This is maintained because:
C. Na+/K+ pump constantly pumps 3 +’s OUT and 2 +’s IN (leading to a net removal of positive ions)
5. When Na/K pump establishes a gradient around -70 mV:
A. K+ wants to get OUT due to the concentration gradient
B. If K+ leaves (down its conc grad), then inside becomes MORE negative, making it harder for K+ to leave again!
C. Therefore, K+ is kept IN by the electrical gradient!
6. At -70 mV, all forces BALANCE…
7. The membrane potential can be used to do work (so it represents potential energy!)
Bio 7: Human Physiology
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Spring 2014: Riggs
The Action Potential (AP)
The action potential is a rapid change in the membrane potential that spreads quickly along the cell membrane of a neuron.
A signal is transmitted when a STIMULUS triggers a neuron…and causes an ACTION POTENTIAL (aka nerve impulse). The action
potential was discovered in 1952 by Huxley and Hodgkin after studying giant squid axons.
General description of an ACTION POTENTIAL:
1. Stimulus triggers the opening of “STIMULUS-GATED Na+ channels! (These are located in neuron dendrites…why?)
A. Na+ rushes in, and membrane potential decreases: DEPOLARIZATION
2. If the depolarization reaches a threshold level (about -55mV) then VOLTAGE GATED Na+ channels open
A. More Na+ rushes in, and membrane potential moves to a peak of +30mV
3. 1 millisecond later, the INACTIVATION GATE snaps shut. This is triggered by the same voltage stimulus that opened the
gate…but this part of the change happens a fraction of a second SLOWER.
A. The INACTIVATION GATE will NOT reopen until the membrane potential returns to resting levels, at which point the gate
returns to its original conformation.
B. This creates an ALL OR NONE RESPONSE…once the AP is generated, the nature of the sodium channel ensures it will hit
a peak then recover.
C. Membrane potential no longer becomes more positive…
4. Finally the voltage gated K+ channels open (they were also stimulated at first, but they open more slowly than the Na+ gates, so
they don’t start to act until the peak is achieved)
A. K+ rushes out of the cell, bringing the membrane potential back to a negative value (the membrane repolarizes).
5. Sometimes, open K+ gates allow too much K+ out…hyperpolarizes the cell
A. -90 mV means K+ can’t leave…(through leaky channels) and in fact, it leaks IN through that channel!
B. At the same time, Na is being pumped OUT, by the Na/K pump.
Other landmarks on the action potential…
1. Absolute Refractory period: The neuron CANNOT be stimulated because it is firing.
2. Relative Refractory Period: The neuron resists restimulation because it is reestablishing the resting potential, but it can fire with
a big enough stimulation…
3. The frequency that a neuron fires determines the STRENGTH of the stimulus. More stimuli = more frequent firing = more
NT…
4. MYELIN: Actually increases the SPEED of the Action Potential
A. Fewer gates to open to keep AP going…
B. Saltatory conduction
Synaptic Transmission
An action potential cannot cross the space between the neuron’s axon terminal and the effector/postsynaptic neuron, which is what the
synapse IS. So there must be a way to pass the information along.
1. Electrical Synapse:
A. Use GAP JUNCTIONS to pass messages from one cell to another
B. Found in heart and eye…
2. Chemical Synapse: Utilizes NEUROTRANSMITTER as a ONE-WAY message
A. Anatomy: (Fig 8-20)
Transmission of the AP across a chemical synapse
1.
2.
3.
4.
ACTION POTENTIAL reaches the axon terminal of the presynaptic neuron
Voltage-gated Ca++ channels open in the axon terminal…
Ca++ rushes IN
Increased Ca++ concentration in the axon terminal causes synaptic vesicles filled with neurotransmitter to migrate to the plasma
membrane and release neurotransmitter (exocytosis)
5. NT diffuses across synaptic cleft and binds with receptors on the postsynaptic cell…
A. Can produce a postsynaptic potential in a neuron…How would it do this? (Na+ and K+ channels open, just like normal…)
B. Can inhibit a postsynaptic potential in a nerve cell by opening K and Cl channels instead… (this creates a very NEGATIVE
inside, leading to hyperpolarization)
C. Can also bind with a receptor and induce a second messenger effect!
6. NT is then removed from the synapse, very quickly, providing control
A. Some NT is taken back into the cell and reused
B. Some is broken down by enzymes in the synapse
C. Some diffuses away and is absorbed by glial cells
Bio 7: Human Physiology
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Spring 2014: Riggs
External Brain 6: The Neuron
Study Guide Questions
1. What are glial cells? Give examples of different types of glial cells, and know their functions.
2. Know the structure and function of the neuron. (Including Myelin!)
3. What is electricity?
4. What is “membrane potential”? How is it established? How is it maintained? How is it MEASURED?
5. Know all the steps involved in the generation of an action potential.
6. Understand absolute and relative refractory periods.
7. Describe the process of synaptic transmission.
Calculating Membrane Potentials
8. Assuming a membrane potential of -70mV, what is the actual charge of the extracellular fluid IF the intracellular fluid has a
net charge of 49 mV?
9. What is the charge in the intracellular fluid if the extracellular fluid has a charge of 143 mV?
10.Imagine REMOVING potassium ions from the inside of the cell described in Q2. What will happen to the membrane potential
in this scenario?
11. Imagine ADDING sodium ions to the inside of the cell described in Q2. What will happen to the membrane potential in this
scenario?
12.Imagine ADDING chloride ions to the inside of the cell described in Q2. What will happen to the membrane potential in this
scenario?
Bio 7: Human Physiology
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Spring 2014: Riggs