The multicoloured edition of the textbook of Cell Biology, Genetics, Molecular Biology, Evolution and Ecology is the outcome of sincere and combined efforts of the authors and editors (namely Shishir Bhatnagar, Shubha Pradhan, Malini Kothiyal) and young but talented persons of DTP of S.Chand & Company Ltd. Their main motive remained to provide relevant coloured photographs explaining various intricate biological
topics. Multicoloured figures and photographs of this edition would help our target readers to understand and fully appreciate the very gist of the subject matter. Authors and editors have remained quite choosy and vigilant regarding relevance and authenticity of each and every illustration/picture finding its place in this textbook.
Authors earnestly hope that this multicoloured edition of the textbook of Cell Biology, Genetics, Molecular Biology, Evolution and Ecology will enhance the curiosity of our target readers to know more and more about the subject. It will arm them with latest information for facing any type of exam quite adequately.
This book is meant for students of B.Sc., B.Sc. (Hons.) and M.Sc. of biological group. Students appearing in entrance exams of C.P.M.T., I.F.S., P.C.S. and I.A.S., etc, may be immensely benefited by this book.
The multicoloured edition of the textbook of Cell Biology, Genetics, Molecular Biology, Evolution and Ecology is the outcome of sincere and combined efforts of the authors and editors (namely Shishir Bhatnagar, Shubha Pradhan, Malini Kothiyal) and young but talented persons of DTP of S.Chand & Company Ltd. Their main motive remained to provide relevant coloured photographs explaining various intricate biological
topics. Multicoloured figures and photographs of this edition would help our target readers to understand and fully appreciate the very gist of the subject matter. Authors and editors have remained quite choosy and vigilant regarding relevance and authenticity of each and every illustration/picture finding its place in this textbook.
Authors earnestly hope that this multicoloured edition of the textbook of Cell Biology, Genetics, Molecular Biology, Evolution and Ecology will enhance the curiosity of our target readers to know more and more about the subject. It will arm them with latest information for facing any type of exam quite adequately.
This book is meant for students of B.Sc., B.Sc. (Hons.) and M.Sc. of biological group. Students appearing in entrance exams of C.P.M.T., I.F.S., P.C.S. and I.A.S., etc, may be immensely benefited by this book.
Cell Biology, Genetics, Molecular Biology, Evolution and Ecology by Verma, Agarwal
Neurons receive information from sensory organs, send information to motor organs, or share information with other neurons. The process of communicating information is very similar, whether it is to another neuron or to a muscle or gland cell. However, by far the largest number of neuronal connections is with other neurons. The rest of this tutorial therefore focuses on inter-neuronal communication. The transmission of information is accomplished in two ways:
Electrically: the neuron is directly adjacent to other neurons. Small holes in each cell's membrane, called gap junctions, are juxtaposed so that as the action potential reaches the end of the axon (at the terminal boutons), the depolarization continues across the membrane to the postsynaptic neuron directly.
Chemically: there is a space (the synaptic cleft) between the axon terminus and the adjacent neuron. As the action potential reaches the end of the axon, a chemical is released that travels across the synaptic cleft to the next neuron to alter its electric potential.
With very few exceptions, mammalian organisms use chemical means to transmit information.
Synaptic vesicles contain Neurotransmitters, chemical substances which ultimately cause postsynaptic changes in the receiving neuron, is contained within the synaptic vesicles. Common neurotransmitters include:
Electrical transmission occurs by virtue of the fact that the cells are in direct contact with each other: depolarization of the presynaptic cell membrane causes a depolarization of the postsynaptic cell membrane, and the action potential is propagated further. Here transmission of information is always excitatory: the conduction of information always causes a depolarization of the adjacent cell's membrane.
Chemical transmission, albeit more complex allows for far more control, including the ability to excite or inhibit the postsynaptic cell. Here the conduction of information can cause either depolarization or hyperpolarization, depending on the nature of the chemical substance.
The sequence of events that lead to postsynaptic changes is as follows:
Ca2+ enters the presynaptic cell because its concentration is greater outside the cell than inside.
The Ca2+, by binding with calmodulin, causes vesicles filled with neurotransmitter to migrate towards the presynaptic membrane.
The vesicle merges with the presynaptic membrane.
The presynaptic membrane and vesicle now forms a continuous membrane, so that the neurotransmitter is released into the synaptic cleft. This process is called exocytosis.
The neurotransmitter diffuses through the synaptic cleft and binds with receptor channel membranes that are located in both presynaptic and postsynaptic membranes. (Are these channels voltage, chemically, or mechanically gated? Answer.)
The time period from neurotransmitter release to receptor channel binding is less than a millionth of a second.
The process is depicted in the diagram below:
Direct and Indirect Binding to Postsynaptic Receptor
There are two kinds of receptor channels: direct and indirect
Direct: the receptor channel allows ions to pass through the membrane. The neurotransmitter acts like a key which opens the ion channel. This is the fastest kind of channel (about 0.5 ms). This is called an ionotropic receptor.
Indirect: the binding of neurotransmitter to the receptor channel causes the release of a molecule, called a secondary messenger, that indirectly activates nearby ion channels. This is called a metabotropic receptor.
This process is much slower than direct receptor ion channels: from 30 ms up to 1 second.
However, this is the most common type of postsynaptic receptor channel
Postsynaptic Stimulation
Once the postsynaptic ion channel is opened, whether directly or indirectly, the effect can be either excitatory (depolarizing) or inhibitory (hyperpolarizing).
Excitatory Postsynaptic Potentials (EPSP)
Excitatory ion channels are permeable to Na+ and K+
Because of the electrical and concentration gradient, more Na+ moves into the cell than K+
The inside of the cell becomes more positive, hence causing a local depolarization
If enough depolarization occurs (for example, because the neurotransmitter released caused nearby ion channels to open), an action potential is generated
Inhibitory Postsynaptic Potentials (IPSP)
Inhibitory ion channels are permeable to Cl- and K+
Because of the concentration gradient (not electrical), Cl- moves into the cell and K+ moves out of the cell
The inside of the cell thus becomes more negative, hence causing a local hyperpolarization
The hyperpolarization will make it more difficult for the cell membrane potential to reach threshold, thereby making it less likely that an action potential will be generated
Summation
Depending on the kind of neurotransmitter released, the effect can be either excitatory or inhibitory
The local excitatory depolarizations or inhibitory hyperpolarizations are graded (passive) potentials and therefore can summate or cause additive changes to the post-synaptic membrane potential. This process is known as summation
Spatial summation occurs when multiple synapses in nearby locations are stimulated simultaneously
Temporal summation occurs when the same channel is repeatedly opened (for example, because the presynaptic cell receives many impulses in a row), thereby altering the membrane potential further before it has the time to return to normal
Although receptor ion channels are all chemically gated, enough depolarization past threshold can cause nearby voltage gated channels to open. An action potential would then be generated
Neurotransmitter Deactivation
If neurotransmitters were continually in the synaptic cleft, the postsynaptic channels would be continually stimulated and the membrane potential would not be able to become stable. There are three ways in which neurotransmitter is deactivated:
Degradation: Enzymes located in the synaptic cleft break down the neurotransmitter into a substance which has no effect on the receptor channel
Reuptake: The neurotransmitter can reenter the presynaptic cell through channels in the membrane.
Autoreceptors: Receptors for a particular neurotransmitter are located on the presynaptic membrane that act like a thermostat. When there is too much neurotransmitter released in the synapse, it decreases the release of further neurotransmitter when the action potential arrives at the presynaptic membrane. It may accomplish this by decreasing the number of Ca2+ channels that open when the next action potential arrives at the presynaptic terminal
Neurotransmitters
A molecule is considered a neurotransmitter if it meets the following criteria:
Derived from secretory proteins formed in the cell body
They are first processed in the endoplasmic reticulum (ER) and are moved to the Golgi apparatus before being secreted as large vesicles and transported down the axon in preparation for exocytosis
More than 50 peptides have been isolated in nerve cells. For example,
Substance P and enkephalins: Active during inflammation and pain transmission in the PNS
Endorphins: Endogenous opiates which cause euphoria, suppress pain, or regulate responses to stress
Are either excitatory or inhibitory, and can also act as neuromodulators, affecting the amount of neurotransmitter released
Some form part of the neuroendocrine system by functioning both as hormones and neurotransmitters
As neurotransmitters, each one of these molecules undergo a similar life cycle:
Inactivation: The neurotransmitter is degradedeither by being broken down enzymatically, or reused by active reuptake in which case the cycle begins again
Drugs can affect any of the stages in the "life-cycle" of a neurotransmitter.
Drugs that bind with receptors on the post-synaptic (and sometimes pre-synaptic) membrane fall into two groups:
Agonists: Bind to receptors and simulate or enhance a neurotransmitter's actions (i.e., opening ion channels and causing EPSPs or IPSPs).
Antagonists: Have the opposite effect of agonists by blocking the receptors and inactivating it (usually by taking up the space but without specifically causing the opening of the channel or the operation of the secondary messenger). The neurotransmitter's effect is nullified or diminished.
The table below lists some common drugs, they action in the brain and their observable behavior:
Drug
Action (Brain)
Behavior
Nicotine
Acetylcholine receptor agonist
Smokers: relaxation, alertness, reduced desire for food. Non-smokers: Nausea, vomiting, cramps, and diarrhea.
Alcohol
1. Reduces flow of Ca2+ into cells 2. GABA agonist 3. Increases number of binding sites for glutamate 4. Interferes with some secondary messenger systems
Low doses effect is excitatory. Moderate to high doses effect is inhibitory.
The multicoloured edition of the textbook of Cell Biology, Genetics, Molecular Biology, Evolution and Ecology is the outcome of sincere and combined efforts of the authors and editors (namely Shishir Bhatnagar, Shubha Pradhan, Malini Kothiyal) and young but talented persons of DTP of S.Chand & Company Ltd. Their main motive remained to provide relevant coloured photographs explaining various intricate biological
topics. Multicoloured figures and photographs of this edition would help our target readers to understand and fully appreciate the very gist of the subject matter. Authors and editors have remained quite choosy and vigilant regarding relevance and authenticity of each and every illustration/picture finding its place in this textbook.
Authors earnestly hope that this multicoloured edition of the textbook of Cell Biology, Genetics, Molecular Biology, Evolution and Ecology will enhance the curiosity of our target readers to know more and more about the subject. It will arm them with latest information for facing any type of exam quite adequately.
This book is meant for students of B.Sc., B.Sc. (Hons.) and M.Sc. of biological group. Students appearing in entrance exams of C.P.M.T., I.F.S., P.C.S. and I.A.S., etc, may be immensely benefited by this book.
Cell Biology, Genetics, Molecular Biology, Evolution and Ecology by Verma, Agarwal
