Module 5 Neuroanatomy

What is the distribution of charged particles relative to the cell membrane that contribute to the membrane potential in the ECF and the ICF?

ECF: cation = sodium, anion = chloride ion

ICF: cation = potassium, anion = protein

Relative to a neuron cel membrane, describe the concentration of sodium, chloride, calcium, potassium, and intracellular proteins in the ECF and ICF, and then their relative permeability (RP).

sodium: ECF = 150, ICF = 15, RP = 1x

chloride: ECF = 150, ICF = 13, RP = 1x

calcium: ECF = 2, ICF = 0.0002, RP = ~0.5x of sodium

potassium: ECF = 5, ICF = 100, RP = 75x of sodium

intracellular proteins: ECF = 5, ICF = 0, RP = 0 (because they cannot move outside of the membrane)

Membrane potential

the separation of charges across the cell membrane

What is the net charge on the inner- and outer-face of the neuronal membrane of a cell at rest?

inner: negative charge

outer: positive charge

What is the value of the neuronal resting membrane potential (RMP)?

-70mV

How is the RMP established?

Measured by a voltmeter that sticks within the membrane. The sodium/potassium pump is what establishes this. 3 Na+ move out of the cell, and then 2 K+ moves inside of the cell. Even though a positive charge is coming back in the membrane, there is still more positive charges leaving (Na+) which contributes to why it has a negative charge.

Concentration gradient

moves ions from a higher to lower concentration of that molecule

example: if there is 100mM of K+ on the inside of a cel, with 5mM on the outside, the K+ will move to the outside of the cell where there is less of it

Electrical gradient

moves ions opposite of their charge

example: if K+ is a positive ion, when it leaves the cell to a lesser concentration, it will then move IN due to the inside of the cell being more negative in nature. It is a positive ion that is seeking a more NEGATIVE (opposite charge) environment.

Equilibrium potential

the potential of the membrane which exists when the concentration gradient is exactly counterbalanced by the electrical gradient

What is the Nersnt Equation used to determine and what is the equation is a ratio of?

Used to determine the equilibrium potential of each ion - basically when each force will act to counterbalance each other.

Essentially is a ratio of the concentration of ions out of the cell compared to within the cell.

If the membrane suddenly became completely permeable to only sodium, would sodium move into or out of the cell?

Na+ would move inside of the cell, making it more positive until the force of inward acting concentration gradient is counterbalanced by the outward acting electrical gradient. This has Na+ move outside of the cell because cells want to move opposite of their charge, and because the inside of the cell is negative to start, sodium will move towards the more negative (opposite) charge.

If the membrane suddenly became completely permeable to only potassium, would potassium move into or out of the cell?

K+ would leave the cell causing the inside to become more negative until the force of outward concentration gradient counterbalances by the inward pull of the electrical gradient. This has K+ move outside of the cell because it moves from an area of higher concentration inside of the cell to a lower concentration on the outside.

What is the Goldman Equation useful in determining?

the membrane potential any time by incorporating ALL participating ions into the equation. It is determined by: concentration of each ion in the ECR and ICF, and the membrane permeability of each ion.

How does hyperkalemia affect the neuronal membrane potential?

brings the inside of the cell more positive, which helps to bring it closer to its threshold potential, therefore making it more excitable and easier to fire

How does hypokalemia affect the neuronal membrane potential?

brings the inside of the cell more negative, which takes it further and further away from the threshold potential, therefore making it harder to fire and less excitable

What are the properties of a chemically gated (ligand) channel?

- located in the dendrites and soma

- change their shape once a ligand, or a neurotransmitter, binds to the receptor, which allows for ions to flow through

- responsible for creating graded potentials

What are the biological properties associated with graded potentials?

- allow for transient changes in the membrane potential

- occur post-synaptically in the dendrites and cell body

- generated in response to stimulation of chemical or mechanical gated ion channels (sodium, potassium, chloride)

- signals are variable, travel with decrement, and can be summed

- HAVE to reach a threshold value in order to generate an action potential

What are the post-synaptic responses of a graded potential?

- depolarization: EPSP (sodium flows into the membrane, which causes the inner-face to become more positive.

- hyperpolarization: IPSP (potassium leaves the cell, which causes the inner-face to become more negative)

How does the strength of the EPSP change as it spreads from its point of origin?

as it spreads from its point of origin, the strength will decrease over time. the sodium ion will have a higher concentration once it gets into the cell, but after it begins to diffuse further away from this point, it will lose its strength

What is temporal summation?

ONE axon terminal sums several EPSPs in a way where they occur very close together in time because of rapid, successful firing of a single, pre-synaptic neuron

What is spatial summation?

the summation of EPSPs originating simultaneously from SEVERAL different pre-synaptic inputs (IPSPs can also occur!)

What are the properties of an action potential?

- transient, non-graded changes in membrane potential

- occur in axons

- created in response to stimulation of voltage gated ion channels such as sodium and potassium channels

- requires a threshold to be generated, and once it has been generated, it will GO. All or none principle --> if doesn't reach threshold it won't go, but if it does, it's going and can't be stopped.

- magnitude of depolarization is constant, but frequency can change

- if weak signal used to generate action potential, then a low frequency occurs

- if a strong signal used to generate action potential, then a strong frequency occurs

What are the six events involved in generating an action potential?

1. RMP is maintained at -70mV. All voltage gated ion channels are closed

2. EPSPs cause depolarization of axon to reach the threshold point --> about -55 to -60mV

3. now since at threshold, rapid depolarization occurs until the cell reaches +30mV as voltage gated sodium channels open and it fluxes in

4. Na+ channels close and K+ channels open. Repolarization begins as K+ leaves and the membrane becomes more negative.

5. during the process of repolarization, Na+ channels recharge and then K+ channels begin to close. may have some hyperpolarization that occurs where the K+ channels are open for too long and end up dropping the potential lower than the RMP

6. return to normal RMP when the Na+/K+ pump restores ion distribution

What is the absolute refractory period?

period where another action potential cannot be generated, no matter the size of it. Occurs in 1msec. Na+ channels are inactivated. Prevents summation of signals. Maximum frequency = 1kHz

What is the relative refractory period?

2nd EPSP can create an action potential if it is strong enough to reach the threshold

What is an orthodromic and antidromic action potential?

orthodromic = travels from the cell body towards the axon terminal

antidromic = travels away from the axon terminal and towards the cell body

What direction does an AP travel if in the middle of an axon?

it will go in both directions

What is continuous conduction?

- occurs with unmyelinated axons

- axon has voltage gated channels located along the entire length of the axon. every region needs to have a molecular event in order to open these, and even though it is fast, it can still take some time for this to occur for multipel channels along the axon

What is saltatory conduction?

- occurs with myelinated axons

- myelinated regions don't have any voltage gated channels, but the nodes of Ranvier do. these nodes travel along as usual, but get the opportunity to skip over the internodes where myelin is present

What happens when you inhibit voltage-gated sodium channels in a sensory and motor axon?

sensory = local anesthetics and general anesthics; local works by decreasing nerve membrane permeability to sodium, thereby increasing the threshold. interferes with binding to a deactivated voltage sensor region. if you block this, decrease AP generation. general works by causing a reduction in conduction of AP in all neurons

motor = neurotoxins that are sodium-channel blockers

What is the nomenclature for sensory axons based on degree of myelination?

I = large, heavily myelinated

II and III = smaller, less myelinated

IV = small, unmyelinated

What is the nomenclature for sensory axons based on conduction velocity of the action potential?

A = fast

B = intermediate

C = slow

What are examples of class I / A alpha fibers?

sensory = muscle spindles, golgi tendon organs

motor = motor neurons to skeletal muscle alpha-motor neurons

What are examples of class II /A beta fibers?

sensory = muscle spindles, hearing and vestibular function, fine touch, proprioceptors

What are examples of class III / A delta fibers?

sensory = temperature, crude touch, sharp pain

motor = motor nerves to muscle spindles, gamma motor neurons

What are examples of class B fibers?

autonomics --> white rami communicans

What are examples of class IV / C fibers?

sensory = tickle, itch, dull pain

autonomics = gray rami communicans