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Nerve Impulse, Receptors, Synapse and Neurotransmitters

Electrical signals pass along dendrites to generate an action potential, which is actually a nerve impulse.

Nerve Impulse

Electrical signals pass along dendrites to generate an action potential, which is actually a nerve impulse. Cells experience action potentials when ions move into and out of them. Na+ and K+ ions are specifically involved. Na+ and K+ channels and the sodium-potassium pump transport them into and out of the cell. The conductors along which nerve impulses are transmitted are active and electronic. A synapse is responsible for transmitting internal signals between the cells. The membrane resistance of nerve conductors is relatively higher and the axial resistance is relatively low. An electrical synaptic connection is found in escape reflexes, the heart, and in vertebrates' retinas. Whenever it is important to respond quickly and accurately, their primary objective is to do so. The membranes of the two cells are flooded with ionic currents when an action potential reaches the stage of such a synapse.

Mechanism

Axons or nerve fibres are shaped like cylinders, in which axoplasm fills the interior and axolemmas cover the exterior. ECF is used to immerse the nerve fibres. There are ionic forms of the solution found in axoplasm and extracellular fluids. Na + ions neutralize the negative charge of chloride ions external to the axon. Protein molecules negatively charged are neutralized by potassium ions in the axoplasm. Neurons have an inner membrane that is -ve, and an outer membrane that is +ve. This difference would represent the resting potential. A membrane would be polarized if there was a difference in charge between 70 and 90 millivolts. Potassium sodium pumps are useful for maintaining the equilibrium potential. Axon membrane is attached to the pump. Axoplasm and ECF will now pump potassium ions to one another.

A stimulus applied to the membrane of a nerve fibre causes the sodium-potassium pump to stop working. An electrical, chemical or mechanical stimulus could be applied. Consequently, potassium ions rush outside the membrane while sodium ions rush inside, resulting in a net charge on the outside and a net charge on the inside. Action potentials occur when nerve fibres become depolarized or when they are in an action potential state. In the action potential, the membranes transmit nerve impulses. Nerve impulses are action potentials that travel along membranes. The voltage of nerve impulses is approximately +30 mV. Upon completion of the action potential, the sodium-potassium pump begins to operate. Repolarization will ultimately result in a resting potential for the axon membrane.

In reverse order, the process occurs now. During the reversal of an action potential, a process that occurred during an action potential is reversed. The rushing of potassium ions is happening here and the rushing of sodium ions is happening outside. A refractory period would prevent the impulse from being transmitted through the nerve fibre. It is saltatory propagation that occurs with white fibres. Thus, impulses jump from nerve node to nerve node and increase as nerve impulse speed increases. Compared to non-medullated nerve fibers, they are around twenty times faster. Fibre diameter determines how fast nerve impulses are transmitted. Mammals send twenty-meter-per-second nerve impulses, while frogs send 30-meter-per-second nerve impulses.

Receptors

A receptor is a protein that binds to a specific molecule. A ligand is the molecule to which it binds. A ligand may be a molecule such as a mineral, a protein, hormone, or neurotransmitter produced by an organism. Several ligand-binding sites are present on the receptor protein. The receptor's conformation is altered by this binding. The shape change slightly changes the function of the protein. As a result of the conformational change, the receptor can become an enzyme, which can combine or separate molecules actively.

A change in one protein can also activate changes in other proteins, eventually transferring information to the rest of the cell. Signals like these might be metabolic regulation messages, or they might be sensory signals. A receptor's ability to hold on to its ligand is known as its binding affinity. The receptor will lose its attraction, release the ligand, change shape, and stop sending messages. Depending on the affinities between receptors and ligands, the turnover occurs rapidly. Besides ligand-binding sites, other molecules are also capable of attaching to receptors. A natural ligand is an agonist if the molecule mimics its actions. The chemical compounds in many drugs, including prescription and illegal ones, act as agonists to molecules like endorphins that produce feelings of satisfaction. Natural ligands tend to have a lower affinity for receptors than synthetic ligands. Due to this, some drugs and painkillers cause tolerance because they stay attached to the receptor longer. If so, many nerves are already blocked, increased dosage is required to elicit the same number of firings.

Types of receptors

The mammalian body has literally thousands of types of receptors. There is far too much variety in receptors to list each one individually, but they can be categorized very broadly. The process of "cellular signalling" relies almost entirely on receptors and ligands to generate signals and responses. They include membrane receptor proteins that receive ligands and activate other sequences within the cell, as well as receptors within the immune system that specifically target protein and molecule invaders.

Synapse

An synapse is a junction between neuronal cells that permits communication between them. A chemical messenger called a neurotransmitter transmits messages from the presynaptic neuron (neuron that transmits the message) to the postsynaptic neuron (neuron that receives the message). Therefore, a synapse is a junction in the nervous system that helps in transmitting chemical or electrical signals known as nerve impulses. Chemical synapse or electrical synapse refers to a synapse depending on the type of signal transmitted via it.

Types

Depending on the types of signals it transmits, a synapse can either be chemical or electrical. Even though a chemically excitable neuron generates electrical impulses as a result of voltage gradients over their membranes, the signal transmitted by their synapse is either chemical or electrical, depending on the type of stimulation. Here is a brief description of the two synapses:

Chemical synapse - A chemical synapse is formed when neurotransmitters, or chemical messengers, are released by presynaptic neurons. Postsynaptic neuronal membrane receptors bind to the neurotransmitters released in the synaptic cleft. The signal is further transmitted or inhibited through secondary messengers triggered by these neurotransmitters. Neuronal signaling of this type is valuable and essential for sending signals through complex and long routes.

Electrical synapse - As voltage changes occur between the presynaptic cell and postsynaptic cell, electrical current is transmitted through special channels known as gap junctions of an electrical junction, which is present on the membrane of the cells involved. Electrical synapses allow the transmission of signals between cells at very high speed.

Neurotransmitters

Our bodies are filled with chemical messengers known as neurotransmitters. Cells in the peripheral nervous system and target cells transmit signals through them. Depending on the type of target cells, they may be in muscles, glands, or nerves.

Among the many functions the brain regulates with neurotransmitters are:
  • Digestion
  • Sleep cycle
  • Appetite
  • Mood
  • Breathing
  • Heart rate

Muscle movement

Several organs, mental functions, and physical functions are controlled by the nervous system. A neurotransmitter, also known as a neuron, plays a critical role in this system. Firing nerve impulses occurs in nerve cells. To do so, neurotransmitters are released, which are chemicals that carry signals between cells. Neurotransmitters carry messages by passing between cells and attaching to those cells' receptors. Several neurotransmitters attach to different receptors, such as dopamine molecules attaching to dopamine receptors. Their interaction with the receptors triggers the cell to respond. Upon delivering their messages, neurotransmitters are broken down or recycled by the body.

Types

Different types of neurotransmitters act in different ways:
  • Excitatory neurotransmitter - Target cells are stimulated to act by neurotransmitters.
  • An inhibitory neurotransmitter reduces the likelihood that the target cell will act. These neurotransmitters can sometimes have a relaxing effect.
  • Several neurons can be sent messages by modulatory neurotransmitters at the same time. Neurotransmitters can also communicate with each other.
Depending on the receptor they are connected to, neurotransmitters can carry out a variety of functions.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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