How Do Nerve Cells and Neurotransmitters Work - Essay Example

How do nerve cells and neurotransmitters work, and why is it important for psychologists to know this? The central nervous system (CNS) consisting ofthe brain, the spinal cord and the peripheral nervous system (PNS) controls every aspect of human life. The brain is a highly specialised organ which is largely made up of structural cells called glial cells and astrocytes. In addition to the structural cells, the brain contains specialised cells known as neurons that conduct electrical impulses. The average human brain contains about 200 billion neurons. About 10,000 neurons die everyday that is, we lose about 2% of neurons over our lifetime. On average, each neuron is connected to a 1000 other neurons, forming a very intricate neural network that is crucial for the functioning of the brain. Akin to a chip in the computer, the neurons play a key role in the processing activity of the brain. The electrical input or impulse received by a neuron from the 1000 or so neurons to which it is connected, generates an electrical discharge or action potential in the receiving neuron which transmits the nerve impulse to the next neuron in the network. The propagation of the action potential is aided by sodium- and potassium channels present on the neuronal membrane. The refractory period of the sodium channels during which time they are forcibly closed, ensures that the action potential is propagated in a specific direction along the axon. The axon is a projection on the dendrites which are present all around the neuron cell body. The dendrites receive signals while the axon conducts the nerve signal, that is the action potential. The myelin sheath around the axon insulates the axon while increasing the transmission rate of the signal along the axon. It plays a very important role in nerve impulse transmission: e.g., in Multiple Sclerosis, a demyelinating disease affecting bundles of axons in the brain, spinal cord and optic nerve, a lack of co-ordination and muscle control as well as difficulties with speech and vision occur as a result of loss of the myelin sheath. Neuronal communication occurs at points on the neuronal network called synapses. A synapse consists of two neurons, one transmitting the impulse (called pre-synaptic neuron) and the other receiving it (i.e., post-synaptic neuron). The communication between neurons is an electrochemical process. When an action potential reaches a synapse, pores in the cell membrane open to allow an influx of calcium ions carrying a positive charge into the pre-synaptic terminal. This results in a chemical neurotransmitter being released into the synaptic cleft (the small gap present between the two neurons). The neurotransmitter diffuses across the synaptic cleft and binds to receptors which are specialised proteins embedded in the post-synaptic membrane. As a result of this, an influx of ions occurs into the post-synaptic terminal, which is propagated along the dendrite towards the neuron cell body. Synaptic transmission can be excitatory that is, it can lead to the generation of an action potential in the post-synaptic neuron or it can be inhibitory wherein the post-synaptic action potential is greatly diminished. The former happens due to activation of the sodium channels caused by depolarisation of the neuronal membrane whereas inhibition occurs when the membrane potential is stabilised by an influx of negatively charged chloride ions that neutralises the positively charged sodium ions. In the absence of depolarization, propagation of action potential ceases. Neurotransmitters and Receptors Nerve impulses are transmitted at synapses by the release of chemicals called neurotransmitters stored in tiny sacs called vesicles. Neurotransmitters act as chemical messengers that transmit electrical signals across synapses. On receiving an action potential, the vesicles fuse with the nerve cell membrane and spill their contents into the synapse. The neurotransmitters are a diverse group of chemical compounds comprising of simple amines such as dopamine and acetylcholine, amino acids and their derivatives such as glutamate, serotonin, γ-aminobutyrate (GABA), and polypeptides such as the enkephalins. The released neurotransmitters are transported across membranes to specific locations by a class of membrane transport proteins (neurotransmitter transporters). In other words, they essentially allow the action potential to jump across the synapse to the next nerve cell. More than twenty types of transporters exist in the membranes of nerons and glia. The transported neurotransmitters then bind to receptors on the surface of the postsynaptic cell at the specified location. Majority of neurotransmitter receptors are also proteins. And each neurotransmitter has its own specific set of receptors. Therefore, in spite of the highly intricate neural network and the large number of neurotransmitters involved, the neural communication works accurately because the neurotransmitters work only at matching receptor sites. Excitatory and inhibitory synaptic transmissions are caused by different neurotransmitters and receptors. In the CNS, the amino acid L-glutamate is the major excitatory neurotransmitter whereas inhibitory synaptic transmission is brought about by GABA and glycine. The effect of the neurotransmitter is dictated by the receptor. Other neurotransmitters and their actions include acetylcholine (voluntary movement of the muscles), norepinephrine ( wakefulness or arousal), dopamine (voluntary movement and emotional arousal), and serotonin (memory, emotions, wakefulness, sleep). In the post-synaptic action, the neurotransmitters are removed from the synaptic cleft by a process called reuptake. Certain diseases can affect specific neurotransmitter pathways. Alzheimers and Parkinsons diseases are neurodegenerative disorders which are associated with a loss of neurotransmitters. Synaptic plasticity The brain displays ‘plasticity’ that is, it is constantly changing all through its lifetime. Learning abilities and memory are dependent on the ability of neurons to change the way in which they communicate with each other. This is termed synaptic plasticity because neurons communicate at the synapse. Synaptic plasticity is crucial for brain development as well as throughout life. Diseases that affect memory systems such as Alzheimers disease and dementias result from synaptic plasticity failure. Neurotransmitters have great relevance to psychology and human behaviour. They influence learning, memory, mood and sleep besides several other mental activities. Thus, they profoundly influence human mental health. Some of the neurotransmitters which play a significant role in our mental health are: acetylcholine, in excess leads to depression while its deficiency is associated with dementia; dopamine in excess is associated with schizophrenia; too little norepinephrine is linked to depression while in excess, it is associated with schizophrenia; deficiency of serotonin is linked to depression and some anxiety disorders, especially obsessive-compulsive disorder and serotonin imbalance is also implicated in schizophrenia, alcohol abuse, chronic pain, premenstrual syndrome, eating disorders, and panic attacks. Certain antidepressant medications have been formulated to increase the availability of serotonin at the receptor sites; too little GABA has been found associated with anxiety and anxiety disorders. Despite much research, it is not yet known if the change in the neurotransmitter causes the illness or it is vice versa. As seen from the discussion above, most of the mental illnesses have a background in neuron function and neurotransmitter discrepancy. Focal stimulation of the brain to elicit changes in specific neurotransmitter production would definitely be more advantageous than stimulation of the whole brain with electric current as is currently done using electroconvulsive therapy. It is, therefore, important for psychologists to be aware of the neurochemical basis of mental illnesses. References Myers, D.G., 2007. Psychology, 8th edition, Worth Publishers, 928pp. The Principles of Nerve Cell Communication. Accessed 27 April, 2009 http://pubs.niaaa.nih.gov/publications/arh21-2/107.pdf http://www.bris.ac.uk/synaptic/public/basics_ch1_3.html Accessed 26 April, 2009 Read More