Basic physiology of nerve cells Flashcards
How are action potentials generated?
The presynaptic neuron, can be affected by various stimuli on the dendrites and cell body (the soma). These areas, however, do NOT usually generate action potentials. Instead, they produce graded responses that alter the membrane potential. Some depolarise it and some hyperpolarise it. These affects spread, but not in an all-or-nothing way. They meet at the axon hillock. If the hillock reaches threshold, then THIS is where the AP is generated.
What types of neurons are there?
- ESPS (excitatory post-synaptic potentials
- ISPS (inhibitory post-synaptic potentials)
What do ESPS do?
- they will cause the membrane potential to depolarise
What do ISPS do?
- cause the membrane potential to hyper polarise
What it is called when ESPS or ISPS fire together?
- spatial summation, depending on whether its ESPS or ISPS the membrane potential will depolarise, hyperpolarise or stay the same if an ESPS and ISPS fire together as they will cancel each other out
Where does summation occur?
the axon hillock
How are action potentials propagated?
- axons are wrapped in Schwann cells that are made up of myelin. Between these cells are gaps where the axon membrane touches the extracellular fluid, these are called the nodes of Ranvier
Why does an AP travel faster down a myelinated axon than an unmyelinated axon?
- The AP will jump from node to node and miss the myelin sheath which acts as an electrical insulator. So it travels at 150m/s rather than 0.5-10m/s. Charges leak out if they don’t have Schwann cells
What controls an AP’s speed?
- the action potential will travel down a neutron at different speed according to the axons diameter, the wider the axon the faster the AP.
give some examples of AP and their speed
Motor neurons/proprioceptors = muscle spindles, Golgi tendon organs touch= Mechanical touch / pressure Pain (fast) temp pain (slow) Temp itch
Why is an AP changed into a chemical signal?
An electrical synapse acts directly and quickly, as there are gap junctions that allow the AP to simply continue from one cell to the next. This signalling does not, however, allow for much variation in the post-synaptic response.
A chemical synapse is more nuanced. Small molecules known as neurotransmitters are released into the synaptic cleft (a thin area between the two cells). As these bind, unbind, and rebind to the receptors on the postsynaptic membrane, they may trigger an AP, and may even retrigger further APs if they stay bound or re-bind.
How do voltage- activated Ca2+ channels work?
Action potential arrives at synaptic terminal
• Depolarisation opens voltage-activated Ca2+ channels
• [Ca2+] inside is very low
• [Ca2+] outside is much higher
• Large inward concentration and electrical gradients cause calcium ions to flow inward
How are neurotransmitters released?
Neurotransmitter uptake
• Translocation (mobilisation) by increasing Ca2+ dependent phosphorylation of v-SNARE proteins
• Vesicle associates with docking region of plasma membrane and is then primed through action of synaptic t-SNARE proteins followed by Ca2+stimulated fission of vesicle with plasma membrane
How are neurotransmitters produced?
- Small neurotransmitters (e.g. acetylcholine and noradrenaline) are synthesized in synaptic bulb and packaged into vesicles
- Large (protein) neurotransmitters are synthesized in the cell body and transported inside vesicles down axon to the synapse
- When you consider that motor neurons extend from the CNS to the muscle in question, this can be a very long distance!
What is criteria for a neurotransmitter?
- The chemical must be produced within a neuron.
• The chemical must be found within a neuron.
• When a neuron is stimulated (depolarized), a neuron must release the chemical
• When a chemical is released (or applied), it must act on a post-synaptic receptor and cause a biological effect.
• After a chemical is released, it must be inactivated. Inactivation can be through a reuptake mechanism or by an enzyme that stops the action of the chemical.
