Physiology: Lecture 3 Flashcards
CASE #3: A 36-yr-old woman is complaining of blurry vision, numbness, tingling, and weakness in her legs. Her medical history is unremarkable aside from a upper respiratory illness 1 month prior. Blood cultures and metabolic panel are drawn, results negative. Between visits, the numbness became progressively worse, leading to a fall. On neurological examination, the patient was alert and oriented, pupils were equal, round and relative to light. Dysmetria was noted for finger-to-nose and rapid alternating movements bilaterally. Her cranial nerve exam was intact. Sensation was intact on upper extremities but diminished on lower extremities and abdomen. Her gait was ataxic, deep tendon reflex preset and symmetrical in upper extremities, but absent in knees and ankles. The patient's laboratory studies did not reveal any infectious process. Eye exam reveals papilledema. Cerebrospinal fluid (CSF) studies reveal elevated protein count of 200 mg/dL and a negative cell count. A) Diagnosis? B) What caused the papilledema? C) What process is affected?
A) Guillain-Barre Syndrome (demyelinating disease, which is common)
B) High protein = papilledema
C) Propagation of action potential
**Any time you see protein in the CSF, think about hydrocephalus (water on brain) because protein crossed the membrane, bringing in more H2O, causing higher pressure in the brain.
In what direction does the action potential propagate?
From the brain to the muscle; AP is initiated in the cell body
Name of period that keeps signals from moving backwards
Refractory period
How long can the distance be between the axon hillock and the synapse? What does the AP have to maintain to be able to elicit the response?
- can be meters in length
- AP has to maintain strength to elicit a response
Can single AP’s carry information along a neuron?
They are usually not sufficient to carry information along a neuron
Is propagation of the action potential positive or negative feedback?
Positive feedback
How are AP’s propagated?
- Initial AP gets new AP’s in adjacent membrane
- bi-directional if able (only travels in one direction)
- Each new AP is full strength and follows the same steps and propagates itself
- Allows for full strength AP’s to travel along length of neuron
**Each and every AP made looks exactly like AP behind it. Physiologically, moves in 1 direction.
Speed at which AP’s are conducted away from the initiation site
Conduction velocity
What is the conduction velocity dependent upon?
The time and length constants
How fast the membrane can depolarize to 63% of max AP voltage
“How quickly you change charge across the membrane”
Time constant
How far depolarization travels before falling below 63% of max
“How far charge moves before it drops below 63%”
Length constant
What is the time constant dependent on? What’s the only thing that changes?
- number of open channels (Rm) - more open channels, lower resistance, more ion flow, faster depolarization [how resistant movement of ion is…how many channels you have]
- how well the membrane holds a charge (Cm) - less charge in membrane, more is transferred inside, faster depolarization [capacitance = how much charge gets tied up in the membrane]
- Only thing that changes = resistance
What is the length constant dependent on?
- number of open channels (Rm) - high resistance, less open channels, fewer ions leaking out, depolarization can travel further
- internal resistance (Ri) - how easy current can flow inside the cell; cytoplasm is better conductor than membrane; thicker nerves have more cytoplasm than thinner nerves (because they have a larger diameter with more room for more cytoplasm)
What makes up conduction velocity?
Time constant + length constant
What does increasing the diameter of the nerve do to the internal resistance?
- lowers Ri (internal resistance)
- -Increases area of cytoplasm (easy to pass current) to membrane (hard to pass current)
- -Higher length constant means further/faster conduction
How do anatomical constraints limit the size of the nerves?
Thick nerves conduct impulses faster than smaller nerves
QUESTION:
Which of the following set of conditions would give you the most rapid conduction?
A) low time constant, low length constant
B) low time constant, high length constant
C) high time constant, low length constant
D) high time constant, high length constant
B) low time constant, high length constant
- Want time to not be a factor
- Want long length because we want the conduction to move really far before it has to start a new AP
Layers of glial cells wrapped around axons, evenly distributed along the axon with small spaces between; provides insulation for the axon, and makes the AP ‘jump’ between the spaces
Myelination
Spaces between the myelination
Nodes of Ranvier
What is the action called when the AP jumps between the Nodes of Ranvier?
Saltatory conduction
Shortens the length of the neuron because it only has to depolarize (jump) at little points with the myelination
What does myelin do in regards to ion flow?
-Myelin wraps around the axon, not allowing any ion flux (flow) in the area it surrounds = Increases Rm (membrane resistance) under the sheath to increase the length constant
-Allows for ion channels to be enriched in the nodes = Decreases Rm (membrane resistance) at the node to decrease the time constant
=Essentially shortens the nerve (increases length constant; decreases time constant)
Where are all the ion channels crammed into throughout the nerve?
Into the Nodes of Ranvier (myelination doesn’t allow for any ion flow)
Compare myelination vs. non-myelination
Myelination is two orders of magnitude thinner, with much more information getting into the destination than the non-myelinated species. (Ex: cat is better than squid by a lot)
QUESTION: Which nerve is likely the most heavy myelinated? A) Interneuron in CNS B) Motor neuron C) Sensory neuron from the gut D) Olfactory neuron
B) Motor neuron = distance is long and don’t have time to think about it (touching hot stove)
[A) distance is short; C) how food is digesting..who cares?; D) very short anatomical distance]
