WEEK6 - module 3. Structure of the neuron, the RMP

Learning objectives

Describe the functions of sensory, motor and interneurons

Describe the organization of the nervous system,in brief

Review the different parts of a neuron and what they do

Describe the structures and functions of glial cells, in brief

Review the differences in distribution of key ions and proteins between the intracellular and extracellular compartments

Understand the distribution of charge on both sides of the cell membrane

Describe the concepts of electrical disequilibrium and chemical disequilibrium as they relate to resting membrane potentials of cells

Describe the contribution of K+ and Na+ ions towards the resting membrane potential and be able to describe how it is formed.

In which region of the cell is the charge localized ? Explain

How does the Na+-K+ pump contribute towards the resting membrane potential

Understand what the Nernst equation represents

Understand why hyperkalemia is potentially deadly

 

Nervous system organization 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Nervous Tissue

Schwann cell form the myelin sheath in PNS / Oligodendrocytes form the myelin sheath in PNS

The functions of Nissl bodies are thought to be the same as those of the rough endoplasmic reticulum in general, primarily the synthesis and segregation of proteins.

Microtubules are abundant in neurons, occupying axons and dendrites as paraxially aligned arrays. These microtubule arrays provide a structural backbone for axons and dendrites that allows them to acquire and maintain their specialized morphologies.

 

 

*What is the name for the electrical signal? What is the chemical signal?

The electrical signals are action potentials and the chemical signals are neurotransmitters.

 

*List all the glial cells that you know and their functions 

 

CNS

-Astrocyte : cling to neurons, anchor them to capillaries, determine capillary permeability, recapture neurotransmitter and mop up leaked K+.

-microglia : detect damaged tissue, migrate toward the neuron and transform into a specialised macrophage that is able to phagocyte.

-Oligodendrocyte : form myelin sheath

-Ependymal : line the central cavities of the brain and spinal cord, form a permeable barrier between the cerebospinal fluid and tissue around the cell, circulate the CSF

 

PNS

-Satellite cell : astrocytes in PNS

-Schwann cell : oligodendrocyte in PNS

 

*Where speed of neural signalling is important, axons are myelinated. Why not then have all axons myelinated?

 

Myelination provides electrical insulation to the axons and increases the velocity of action potentials through them.

Not all axons in the body are myelinated because myelination costs energy and faster transmission is not needed in short neurons.

 

 

 

 

 

 

 

 

 

 

 

 

 

RMP

mV

-Law or conservation of charge : The net amount of e is zero

-Opposite charges attract, like charges repel

-Energy is needed to seperate chargee (It is generally expensive and difficult to seperate charge)

-Conductor: The material which allows charges can move towards one another

-Insulator: The material which prevents the movement of seperate charges

 

*Ca2+

: abundunt in Blood plasma, interstitial fluid and membranous organelles

 

* Anion Protein

: Abundunt in blood plasma and intracellular space

 

*Intracellular has high density of

: K+, Phosphate, Anion Proteins, (Mg2+)

 

*Extracellular has high density of

: Na+, Ca2+, Cl-

Na+-K+ ATPase Pump (active transport) maintains the Na+-K+ gradients.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Measuring the RMP

1mV is 0.001V

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

How is the RMP formed?

 

Resting membrane potential depends on two factors.

 

1) Presence of sodium and potassium gradients across the plasma membrane.

More sodium ions outside the neuron than inside and more potassium ions inside the neuron than outside. 

(They are in electrical equilibrium but chemical disequilibrium, gradient made by Na+K+ ATPase pump)

 

2) The resting membrane potential depends on the differential permeability of the plasma membrane to sodium and potassium ions.

-Leak channels in the plasma membrane allow sodium and potassium ions to diffuse or leak down their concentration gradients.

-The membrane contains more K+ leak channels than Na+ leak channels, and the membrane is more permeable to K+ ions. If potassium is the only ion moving, the equilibrium potential would be charged at -90mV. However, Na+ leaks and enters the neuron, which slightly offsets the negative charge and raises the potential to -70mV. 

-Also the Na+-K+ pump actively transport Na+ out of the cell and K+ back in, compensating for the Na+, K+ leaks. 

-The membrane contains many more potassium leak channels than sodium leak channels.

-The membrane is much more permeable or leaky to potassium ions.

-As positively charged potassium ions leak out of the neuron the inside surface of a membrane becomes negatively charged compared to the outside surface. If potassium is the only ion moving, the potential would be stabilise at -90mV. However, positively charged sodium ions leak, enter the neuron which slightly offsets the negative charge and raises the voltmeter reading to -70mV. Sodium Potassium pump actively transport sodium out of the neuron and potassium ions back in compensating for the sodium and potassium leaks. Thus the pumps helped to maintain the resting membrane potential)

 

 

*What makes the Resting membrane potential?

First, Na+ is more abundant in the outside of the cell than inside, and K+ is more abundant in the inside of the cell than in outside. This chemical gradient is made by Na+K+ ATPase pump by active transport. They are in the electrical equilibrium but  chemical disequilibrium.

 

Secondly, Cell membrane is more permeable to K+  than Na+. 

If K+ was the only moving ion, K+ would leak out of the cell by concentration gradient. This movement generate the electrical forces to move K+ inside the cell also. When K+ reaches equilibrium of membrane potential, K+ stops to flow, and the equilibrium membrane potential would be -90mV. 

If same thing happens to Na+ , the equilibrium memebrane potential would be  +60mV .

For the real cell, which allows both K+ and Na+ to move across the cell membrane and the cell membrane is about 40 times more permeable to K+  than Na+,  movement of K+  is more active than Na+.  So the membrane potential is closer to EK (-90mV),  which is -70mV.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Self paced quiz

 

 

 

 

 

 

 

 

Summary