BIOLOGY

Biology

Biology

(a)

Neurons are cells specialized to allow our brains to communicate with the rest of the body. The resting potential prepares neurons to send the messages. Before it is stimulated to transmit an electrical impulse, the membrane of a neuron has a slight polarization. The fluid inside the axon has a negative charge relative to the extra-cellular fluid. This polarized state is known as the resting potential. It prepares a neuron for the propagation of an action potential or nerve impulse. The cell membrane of a neuron has charged particles (ions) on either side. When the neuron is at rest (not actively transmitting an impulse), the outside of the membrane has a net positive charge because it has more positive ions than negative ones. The inside has a net negative charge, because it has more negative ions than positive ones. A membrane potential is simply a voltage that exists across a membrane due to this unequal distribution of charges. When a neuron is at rest, the resting potential is about -70 millivolts (mV), indicating that the inside is negative relative to the outside. The specific ions that are responsible for the charge difference are positive sodium and potassium ions and negatively-charged protein molecules and chloride ions.



(b)

Activation of a cell affects its behaviour by altering these permeabilities. Another important consideration for trans-membrane ion movement is the fact that the ionic composition inside the cell differs greatly from that outside the cell. Consequently, concentration gradients exist for all permeable ions that contribute to the net ion movement or flux. The principle whereby ions flow from regions of high to low concentration is called diffusion. One consequence of this ion flow is the tendency for ions to accumulate at the inner and outer membrane surfaces, a process by which an electric field is established within the membrane.

(c)

A fragile balance between excitation and inhibition maintains the normal functioning of the CNS. The dominant inhibitory neurotransmitter of the mammalian brain is GABA, which acts mainly through GABAA and GABAB receptors. Small changes in GABA-mediated inhibition can alter neuronal excitability profoundly and, therefore, a wide range of compounds that clearly modify GABAA-receptor function are used clinically as anaesthetics or for the treatment of various nervous system disorders.

(d)

One of the most important features of chemical synapses is that they are the site of action for the majority of psychoactive drugs. Synapses are affected by drugs such as curare, strychnine, cocaine, morphine, alcohol, LSD, and countless others. These drugs have different effects on synaptic function, and often are restricted to synapses that use a specific neurotransmitter. For example, curare is a poison which stops acetylcholine from depolarising the post-synaptic membrane, causing paralysis. Strychnine blocks the inhibitory effects of the neurotransmitter glycine, which causes the body to pick up and react to weaker and previously ignored stimuli, resulting in uncontrollable muscle spasms.

(e)

i

Computations in cortical circuits are mediated by synaptic interactions between excitatory and inhibitory neurons, and yet we know little about their activity ...