2 - Neuronal transport & membrane potential Flashcards
Dendrite
- Short, bristle-like, highly branched processes
- Receive nerve input (at synapses)
- Not myelinated
Axon
• Long, thin process • Propagates nerve impulse to another neuron, muscle fibre or gland • Often myelinated • Terminates at axon terminals or synapses
Soma (cell body)
- Contains the normal cell organelles
- Main site of protein synthesis and degradation
- Has pronounced rough ER = ‘Nissl’ substance
Neuronal structure
Unique features of neurons:
• Can have very long axons, so nerve terminals are remote from the cell body, which is the main site of protein synthesis and degradation
• Have a well-defined cytoskeleton with a special type of intermediate filament (= neurofilament)
Neuronal cytoskeleton
- Actin microfilaments – form a meshwork under the cell surface
- Microtubules - help maintain structure of axon
- Microtubule-associated proteins (e.g. tau)
- Neurofilaments - Involved in motility, structural support and axonal transport
Anterograde (orthograde)
• Materials are transported from the soma to the axon terminals
Retrograde transport
Transport of materials from the axon terminals to the soma
Materials carried by Orthograde (fast)
200-400 nm/day: small vesicles, enzymes for transmitter metabolism
40 nm/day: mitochondria
Materials carried by Orthograde (slow)
1-5 nm/day: tubulin, neurofilament proteins, Tau protein
Materials carried by Retrograde
200 nm/day: Larger vesicles, nerve growth factor (NGF)
Can be hijacked by viruses and toxins (e.g. herpes, rabies, tetanus)
Neuronal signalling
- Neurons receive information at dendrites (up to 100,000 synaptic inputs/neuron) and integrate in cell body
- Information is transmitted along the axon in the form of electrochemical signals or nerve impulses (= action potentials)
- Action potentials are due to the flow of ions (Na+, K+) through specific protein channels in the membrane
- The lipid bilayer of the membrane is impermeable to these charged ions
2 types of forces that move ions across membranes
chemical + electrical
chemical force
Differences in concentration: diffusion from a region of high concentration to a region of low concentration
electrical force
Interior of cell is negatively charged so positively charged cations and retained and negative ions will be expelled
The electrochemical driving force is a combination of the chemical and electrical forces acting on any particular ion
Movement of ions across cell membranes
2 categories of ion channels that facilitate ion movement into and out of neurons
1. Channels that are gated and require a stimulus to open
• ligands, mechanical force (pressure change in membrane) or voltage (+ or -)
• specific to particular ion(s)
2. Channels that are always open and allow free movement of ions
Potassium movement
- There is a constant flow of K+ ions down their concentration gradient, from the inside of the neuron to the outside
- This movement occurs via open (or leaky) K+ channels that are situated in the membrane of the neuron
Na+/K+ ATPase pump
- The ion gradient is maintained by the continuous operation of the Na+/K+ ATPase pump
- 3 Na+ ions move from the inside of the neuron to the outside of the cell
- At the same time, 2 K+ ions are moved from outside the neuron to the inside of the cell
- At each cycle of the Na+/K+ATPase pump, the cell loses one positively charged ion from the intracellular environment
Resting membrane potential
- Because of the diffusion of K+ and the action of the Na+/K+ ATPase pump is a more positive charge outside the neuron compared to the inside of the neuron
- The difference in charge across the membrane of the neuron is referred to as polarisation
- The difference in voltage across the plasma membrane when the neuron is at rest (not firing or receiving a signal) is called the resting membrane potential
- For most neurons, the resting membrane potential is ~ -70mV
Forces that drive ion movement
• When ion channels open, the chemical gradient drives ion movement
from high concentration to low concentration
• In the absence of polarisation, diffusion would occur until chemical equilibrium was reached
• However, this does not occur because of electric forces
Electrochemical gradients of sodium
When Na+ channels open:
• chemical gradient drives ion movement into the cell (more Na+ on outside than inside)
• electrical force pulls + ions into the cell (attracted to the negative inside of cell)
• both act in the same direction = Na+ will enter the cell
Equilibrium of sodium movement
- As Na+ moves into the neuron, the charge inside the cell starts to become positive and the electrical gradient decreases, along with the chemical gradient
- Eventually, the chemical and electrical forces will be exactly in balance and there will be no nett flow through any open channels
Equilibrium potential
The equilibrium potential (E) is the membrane potential required to exactly counteract the chemical forces acting to move one particular ion across the membrane
Electrochemical gradient of potassium
When K+ channels open:
• chemical gradient drives ion movement out of the cell (less K+ on outside)
• but electrical force pulls + ions into the cell
• two forces act in opposite directions
• chemical force > electrical force, so K+ moves out of the neuron
Equilibrium of potassium movement
- As K+ moves out of the neuron, the charge inside the cell starts to become even more negative, so the electrical gradient becomes stronger
- Eventually, the chemical force that drives K+ out of the cell = the electrical force driving K+ back into the cell and there will be no nett flow of K+ ions
Nernst equation
The equilibrium potential (E) can be calculated using the Nernst equation:
E = (61/z) log (C0/ Ci)
E = equilibrium potential in millivolts (mV)
Equilibrium potential for Na+
E Na+ = +60 mV
No nett movement of Na+ ions
Equilibrium potential for K+
E k+ = -94
No nett movement of K+ ions
