Surrounding the neuron is the membrane, which is composed of protein and two layers of lipid (fat). The membrane potential is the electrical charge across the membrane and when cells are not firing in addition to having a 70mV difference between the inside and outside of the membrane, this is known as the resting potential. Electrically-charged particles are known as ions and like all cells; neurons maintain different concentrations of particular ions across their cell membranes. Therefore the existence of the resting potential is due to the different concentrations on either side of the membrane. The ions related to the membrane consist of sodium (Na+) and chlorine (Cl-) outside of the cell and potassium (K+) as well as organic anions inside of the cell. Forces on the ions are exerted through electrical charges and concentration gradients across the membrane. This causes sodium ions to enter the cell. Whilst the forces of voltage and concentration are balanced for the potassium and chlorine; the organic anions are too large to pass through the membrane. In order to maintain the resting potential, sodium ions are actively transported. The sodium-potassium pump pumps out positively charged sodium ions and on top of this, positively charged potassium ions are being pumped in. This leads to a high concentration of sodium ions present on the outside of the neuron, whilst there is a high concentration of potassium ions on the inside. The neuronal membrane also consists of channels, which make up pores in the membrane. These pores are selectively permeable to particular ions, thus sodium channels allow sodium ions through the membrane whilst potassium channels allow potassium ions through. During the resting potential, the potassium channel is more permeable to potassium ions than the sodium channel is to sodium ions. The membrane is said to be polarized when the membrane has a negative charge on the inside, when compared to the outside. This is because more positively charged ions flow out of the neuron than flow in.
The action potential is a raid depolarization of the membrane and begins at the axon hillock, which then quickly passes along the axon. To allow subsequent firing, the membrane is promptly repolarized. The action potential begins with a partial depolarization and when this reaches the activation threshold, voltage-gated sodium ion channels open, allowing positively charged sodium ions to flood into the neuron, making the inside of the cell depolarized as it is positively charged; the membrane potential changes from -70mV to +40mV. Once the sodium ion channels close, they become refractory, ensuring that the action potential is propagated in a specific direction along the axon. From this, depolarization triggers potassium channels to open, which allows potassium ions to exit the cell. Therefore there is an initial influx of sodium ions, causing a large depolarization; which is when the excitation threshold is reached, followed by a rapid efflux of potassium ions from the neuron, initiating a repolarization. This is then followed by a brief hyperpolarization. To resume the resting potential, potassium channels close and the sodium ion channels are reset through repolarization. From this, ions diffuse away from the area, allowing the membrane to be ready to ‘fire’ again. In order for information processing to be effective and efficient, it is important that this process and stage is efficient and does not take too long.
Passive conduction of the action potential ensures that the neighboring membrane will depolarize, thus the action potential will move down and along the axon. However, the disadvantage of this is that this process (of transmission via continuous action potentials) can be relatively slow and energy consuming, making it less effective for information processing.
Axons are often surrounded by myelin, composed of fatty membrane cells called oligodendroglia and Schwann cells, which wraps around the axon serving as an insulator. This prevents the dissipation of the depolarization wave. Regions amongst the axon which are myelinated are electrically charged, and as opposed to moving across the membrane, the electrical charge moves along the axon. Action potentials only take place in regions which are not myelinated. These regions are known as the nodes of Ranvier, therefore the sodium and potassium ion channels, pumps and other factors associated with action potential transmissions are concentrated at these sites. This is known as salutatory conduction.
Neurons communicate with each other at synapses through a process called synaptic transmission. This consists of information being transmitted from the presynaptic (sending) neuron to the postsynaptic (receiving) cell. Information is comprised of chemical neurotransmitters, which crosses the synapse, from the terminal to the dendrite or soma.
When an action potential reaches a synapse, synaptic vesicles, which contain neurotransmitter, gather together at the presynaptic membrane. From this, the action potential instigates the voltage-gated calcium (Ca2+) channels to open, leading the pores in the cell membrane to open allowing calcium ions (positively charged calcium atoms) into the pre-synaptic terminal. Due to this, chemical neurotransmitters are released into a small gap between the two cells, known as the synaptic cleft. The neurotransmitter diffuses across the synaptic cleft and interacts with receptors which are embedded in the post-synaptic membrane. These receptors are ion channels which allow particular types of ions to pass through a pore within their structure. Due to the interaction with the neurotransmitter, the pore is opened allowing ions into the post-synaptic terminal. The postsynaptic cell membrane can become either depolarized or hyperpolarized and this depends on which type of ion channel opens. Ions tend to follow the concentration gradient of high to low, whilst the electrostatic gradient follows the opposite charge.
Neurotransmission can either be excitatory, where the action potential becomes more likely as the opening of ion channels leads to depolarization, or inhibitory, where the occurrence of an action potential is less likely due to the opening of ion channels leading to hyperpolarization. The result of synaptic transmission, on whether it will be excitatory or inhibitory, depends on the type of neurotransmitter used and the ion channel receptors they interact with.
Finally, with the use of neural communication, effective information processing is achieved through a number of factors. Firstly, because information transmission via continuous action potentials can be time-consuming, making it less effective, salutatory conduction, which is provided through myelination, ensures a faster and more efficient approach of conducting the action potential. This is because the myelin sheath allows the action potential to jump and transmit more efficiently from one node (of Ranvier) to another, leading to a more rapid rate of transmission. In addition, fast and efficient information transmission is also achieved as the synapse is very narrow, thus allowing for effective information processing.
BIBLIOGRAPHY
Carlson, Neil R. Physiology of Behaviour, Eighth Edition. Pearson, 2004
Kalat, James W. Biological Psychology, Eighth Edition. Thompson, 2004
