Neurons & Neurotransmission
|We have all experienced the reflex reaction when we accidentally touch something hot. But did you know how fast the nerve impulse that conducts this response is? It is estimated that the speed of a neural impulse can reach up to 120 meters/second or 268 miles/hour! That is much faster than the top speed of any of our cars, unless one of us is lucky enough to own a supercar like the one below. Impressive isn’t it?|
Look at the speed at which this Bugatti Veyron Super Sport takes off! It's similar to the speed of a neural impulse.
|Records of studying the nervous system dates back to Egyptian manuscripts from 1700 BC. However, major breakthrough discoveries were made in the field of Neuroscience in the late 19th century, and finally in 1906, Golgi and Ramón y Cajal shared the Nobel Prize in Physiology or Medicine in recognition of their work on the structure of the nervous system1. A neuron, or a nerve cell, is one of the two main components of the central nervous system (glial cells are the other type). Unlike other cells in the body, a neuron is a specialized cell (with great powers!) that is capable of conducting electrical and chemical signals in a process known as neurotransmission. A typical neuron is divided into three parts: the soma or cell body, dendrites and axon. While there are as many as 10,000 different types of neurons in the human brain, they can be broadly classified into three types: motor neurons (for conveying motor information), sensory neurons (for transmitting sensory information), and interneurons (which convey information between different types of neurons).|
|Synapses are the junctions where neurons pass signals to other neurons, muscle cells, or gland cells. Synaptic signals from other neurons can be received either by the soma, dendrite or axon, whereas signals to other neurons are transmitted only by the axon. The typical synaptic cleft (cell-to-cell distance) can vary between 3.5 nm to 40 nm (compare that to the thickness of your hair which is more than 250 times wider!). Thus the nervous system is wired by numerous neurons interconnected by synapses in a super-complicated neuronal circuit. It is estimated that the human brain alone contains around one hundred billion neurons and one hundred trillion synapses2!
|There are two main types of synapses, chemical and electrical. Electrical synapse is an electrical connection between neurons through structures known as gap junctions. Gap junctions are intercellular channels that allow ions and small molecules to pass directly from one cell to the other3. On the other hand, in a chemical synapse, signals are transmitted via release of neurotransmitters (that are packed in synaptic vesicles in the presynaptic neuron4) into the synaptic cleft. Binding of the neurotransmitters to receptors in the postsynaptic membrane causes activation of this cell through the post synaptic density (a protein dense signaling apparatus). Depending on the type of effect the neurotransmitter has on the postsynaptic cell, a chemical synapse can either be excitatory or inhibitory.|
Neurotransmission at a chemical synapse.
|The fundamental process that triggers the release of neurotransmitters is the action potential, which is an electrochemical wave that travels along the axon of a neuron when there is a change in the membrane potential5. Neurons maintain a steady resting voltage gradient (typically from –70 to –80 millivolts) across their membranes via ion channels and ion pumps. In response to an external stimulus, when enough voltage-dependent positive ion channels such as sodium or calcium channels are activated, they allow a net inward sodium or calcium current which depolarizes the cell to a threshold potential above the resting potential and this triggers the neuron to fire. Eventually, the sodium/calcium channels inactivate and the potassium/chloride channels are activated which results in an outward current of positively charged potassium ions/inward current of negatively charged chloride ions causing the neuron to repolarize and hyperpolarize and ultimately return to the resting state.|
|Neurons and their complex neuronal circuitry, as we all know, allow us to perceive, comprehend, communicate and respond to the world around us. It helps us to learn and make new memories besides helping the body perform essential functions such as breathing and digestion. Thus any disorders or damage to these cells can cause detrimental effects such as those observed in patients with Alzheimer’s and Parkinson’s diseases, which I will talk about in one of my next blogs. Until then, please let us know if you know any other interesting facts about neurons and neurotransmission by e-mailing us at firstname.lastname@example.org|
|Contributed by Mohar Chattopadhyay, PhD.|