Neuronal transmission I Flashcards
What is the structure of a neuron?
Dendrites - recipient of information from other neurons. Large receptive field.
Soma - contains the machinery that controls processing in the cell & integrates information coming from the dendrites.
Axon - carries info (AP) from the soma to the terminal boutons & other cells. Axons can branch to contact multiple neurons.
Terminal boutons - found at the end of the axon, location of the synapse, communication point with other neurons.
What is a neuronal membrane?
Boundary of soma, dendrites, axon & terminal boutons.
Separates the extracellular environment from the intracellular environment. What surrounds the neuron is the membrane. It keeps certain substances out, this is how specific information can reach the neuron. Separates internal and external environments.
Most chemicals in our body are hydrophillic, and this membrane stops most of these substances from coming through.
Certain proteins reside in the membrane called receptors that detect chemical messengers. The transmembrane proteins respond to these messages, change confirmation, and then send signals to inside the cells, called metabotropic receptors. Then you have the donut like structures called ion channels, these have a pore that allow certain ions to pass through.
What properties of the neuron are important for within neuron communication?
Membrane potential: electrical charge across the membrane. At rest: difference between the inside & outside of the neuron is apprize. 65-70 mV. At rest: inside is more negatively charged than the outside (inside of the membrane is -65 to -70mV).
What causes there to be a membrane potential?
Molecules move from an area of high concentration to an area of low concentration. Particles with a similar charge repel, particles with an opposite charge attract. Eventually a point is reached when the diffusion force=electrostatic force. ‘equilibrium potential’ (outward movement=inward movement)
- Initial concentration difference important->High conc.=large eq. pot. (vice versa)
- Very few ions need to move to achieve this. What’s really important for an equilibrium potential is the initial concentration difference. If the difference is high you need a large electrical potential difference (and vice versa).
What is the resting membrane potential?
-65-70mV. High concentration of Na+ outside the neuron, high concentration of K+ inside neuron. At rest, more K+ channels open than Na+ channels. Na+ move into the neuron and K+ out of neuron due to diffusion. As more K+ than Na+ can diffuse, the membrane comes to rest near the K+ equilibrium potential (-85mV). At rest, there are 40x more K than Na channels.
What is the Nernst Equation?
The equilibrium potential can be calculated for any ion using this equation. Takes into account body temperature. Need to know only the change of the ion and its concentration inside and outside the cell.
What is the sodium-potassium pump?
Maintains the ionic concentration gradients (Na+ and K+) across the membrane and therefore the resting membrane potential. This pump takes sodium thats inside the cell and uses a molecule called ATP, which takes the sodium from the inside to bring it outside, and brings potassium from outside and brings it inside. ATP is broken down to release energy which forces the ions to move against their concentration gradient.
What is an action potential?
Nerve impulse. Allows communication within the neuron, along the axon. Generated at the axon hillock. Generated either by the summation of converging inputs from the dendrites or by electrical stimulation (experimentally).
What is hyper polarisation and depolarisation?
Hyperpolarisation = membrane potential is more -ve than resting membrane potential. Depolarisation = membrane potential more +ve than resting membrane potential.
What is conductance?
Small depolarisation to a neuron. Following a small stimulation there is a small degree of depolarisation that decays along the length of the neuron.
This is known as decremental conductance.
What are voltage gated channels?
Channels are ‘voltage-gated’ – opened when the membrane becomes depolarised.
Different degrees of depolarisation open the channels.
Look at properties using voltage clamp experiments.
Positive membrane potentials (very high degrees of depolarisation) results in an inactivation of the Na+ channels.
What are voltage clamp experiments?
Inject a current into the axon to create a steady membrane potential. Can record the membrane current: product of what ions are moving across the membrane. Transient inward current
followed by a slow outward current. Sodium current: fast
Movement of ions into the neuron through the Na+ channels. Potassium current: slow
Movement of ions out of the neuron through the K+ channels.
What are the steps of an action potential?
- At the RMP, the majority of the channels are closed.
- Small depolarisation opens a few Na+ channels. Na+ begins to move into the neurone (diffusion & electrostatic forces) leading to further depolarisation.
- If the stimulation is large enough (above the action potential threshold) the majority of the Na+ channels open and more Na+ moves into the neurone (diffusion & electrostatic forces), again leading to further depolarisation.
- As the neurone continues to depolarise, some K+ channels are opened. Allowing K+ to leave the neurone (diffusion).
- At positive potentials the Na+ channels become deactivated (refractory) so that no Na+ can pass through. The remaining K+ channels open and K+ continues to leave the neuron driven by both diffusion and electrostatic forcers (as inside the neuron is now positive).
- K+ continue to leave the neuron (diffusion). The membrane potential decreases and becomes negative. This is known as repolarisation.
- The K+ channels begin to close and Na+ channels return to their closed normal state. The membrane potential drops to below the resting membrane potential due to a few remaining open K+ channels and the high concentration of K+ outside of the neurone. This is known as hyperpolarisation. During this time another AP is difficult to elicit.
- The final K+ channels close and external K+ is diffused away. The membrane potential returns to the resting membrane potential. Na+/K+
pumps work like mad to restore resting membrane potential. Brain uses a lot of energy to restore back to original resting membrane state.
