GABA (also known as <em>gamma-Aminobutyric acid</em>) is a substance of great relevance in the Central Nervous System since it is the most important inhibitory neurotransmitter. GABA is released from presynaptic neurons to reduce the excitability of postsynaptic neurons, inhibiting them.
GABA inhibits the neurons because, when this neurotransmitter acts as a ligand in the GABAa receptors of the postsynaptic neurons, chloride channels open, allowing the flow of chloride ions. <u>Chloride is highly concentrated in the extracellular fluid, while it is poorly concentrated in the inside of the cell - this generates a concentration gradient</u>. When the chloride channels open, this ion will enter the cell to even out the concentrations between the two fluids. The inhibitory effect occurs because chloride is an anion, meaning that is negatively-charged and is entering a fluid that is already very negative in comparison to the extracellular fluid - this <u>makes the intracellular fluid more and more negative</u>, and this phenomenon is called <u>hyperpolarization</u>. To fire action potentials as excitatory neurons do, the cell needs to depolarize (become less negative), not hyperpolarize. This is why the opening of chloride channels is crucial to reducing the excitability of a neuron and why GABA works as an inhibitory neurotransmitter.
The main difference between the organisms are the uses of the appendage and the size and number of certain bones. The human arm is used for a variety of functions while the lizard's and the cat's are used for walking an climbing, the whale's for swimming, the frog's for support and the bird's and the bat's for flying.
For you to understand the process we can say that the weak will die. If the animal or organism can't get food or find any it is naturally "selected" to die.So a Cuticle could have evolved like that. Hope tgis is useful
Darwin would be happy that his theory of evolution is correct. Stating that biological evolution is through natural selection of inherited traits that enable a species to survive, compete and reproduce.