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Synapse-definition and Types | Medical Science Optional Notes for UPSC PDF Download

There are different types of synapses according to the synaptic structures: 

  1. Axodendritic synapses: signaling between axons and dendrites 
  2. Axoaxonic synapses: signaling between axons
  3. Axosomatic synapses: signaling between axons and the cell body of neurons 
  4. Dendrodendritic synapses: signaling between dendrites

Synapse-definition and Types | Medical Science Optional Notes for UPSC

Steps of synaptic transmission

Synapse-definition and Types | Medical Science Optional Notes for UPSC

Postsynaptic Potentials

  1. Excitatory Postsynaptic Potential (EPSP):

    • Effect: Stimulates firing and propagation of the action potential.
    • Mechanism: Result of increased Na+ influx into the cell.
    • Examples:
      • Neuromuscular junction.
      • Nicotinic synapses (e.g., autonomic ganglia).
      • NMDA synapses (e.g., glutamate and aspartate neurotransmitters).
  2. Explanation:

    • EPSPs depolarize the postsynaptic membrane, bringing it closer to the threshold for initiating an action potential.
    • Commonly involves the opening of ligand-gated channels that allow Na+ ions to flow into the cell.
  3. Inhibitory Postsynaptic Potential (IPSP):

    • Effect: Decreases firing and propagation of the action potential.
    • Mechanism: Result of increased influx of Cl- into the cell.
    • Examples:
      • GABAergic synapses.
      • Glycine synapses (e.g., Renshaw cells in the spinal cord).

Explanation:

  • IPSPs hyperpolarize the postsynaptic membrane, moving it away from the threshold for initiating an action potential.
  • Typically involves the opening of ligand-gated channels that allow Cl- ions to flow into the cell.

Synapse-definition and Types | Medical Science Optional Notes for UPSC

The minimal period of time required for all these events to take place, even when large numbers of excitatory synapses are stimulated simultaneously, is about 0.5 millisecond, which is called the synaptic delay.

Significance 
Neurophysiologists can measure the minimal delay time between an input volley of impulses into a pool of neurons and the consequent output volley. From the measure of delay time, one can then estimate the number of series neurons in the circuit.
Because the minimum time for transmission across one synapse is 0.5 ms, it is also possible to determine whether a given reflex pathway is monosynaptic or polysynaptic (contains more than one synapse) by measuring the synaptic delay.

Question for Synapse-definition and Types
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Which type of synapse involves signaling between axons and dendrites?
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Myasthenia Gravis Overview

  1. Definition:

    • MG is an autoimmune condition characterized by antibodies hindering neuromuscular transmission, leading to skeletal muscle weakness.
  2. Autoimmune Attack:

    • Autoantibodies develop against nicotinic acetylcholine receptors at the neuromuscular junction in skeletal muscles.
  3. Clinical Hallmark:

    • The main feature is variable muscle weakness intensifying with activity and easing during rest.
  4. Triggers/Worsening Factors:

    • Exacerbations can be prompted or aggravated by warm weather, surgery, immunization, emotional stress, and menstruation.
  5. Diagnosis:

    • Diagnosis involves a positive Anti-AChR radioimmunoassay, repetitive nerve stimulation indicating decrement, single-fiber electromyography showing blocking and jitter, and an edrophonium test.
  6. Treatment:

    • Management includes anticholinesterase (pyridostigmine) and options like thymectomy, plasmapheresis, or IVIG.

Lambert-Eaton Syndrome Overview

  1. Autoimmune Mechanism:

    • Muscle weakness results from an autoimmune attack on voltage-gated Ca2+ channels in nerve endings at the neuromuscular junction.
  2. Impact on Ca2+ Influx:

    • Autoimmune attack decreases the normal Ca2+ influx, impacting acetylcholine release.
  3. Affected Muscles:

    • Primarily affects proximal muscles of the lower extremities, leading to a waddling gait and difficulty raising the arms.
  4. Association with Small Cell Carcinoma:

    • Lambert-Eaton syndrome is often associated with small cell carcinoma (40%).
  5. Comparison with Myasthenia Gravis:

    • In Lambert-Eaton syndrome, repetitive stimulation of the motor nerve enhances Ca2+ accumulation in the nerve terminal, increasing acetylcholine release and muscle strength.
    • This contrasts with myasthenia gravis, where symptoms worsen with repetitive stimulation.
  6. Treatment:

    • Treatment involves medications such as 3,4-diaminopyridine and pyridostigmine, along with therapeutic options like intravenous immunoglobulin (IVIG) and plasmapheresis.

Botulinum Toxins Overview

  1. Toxin Family:

    • Botulinum toxin comprises a family of seven neurotoxins, with primary relevance to humans for toxins A, B, and E.
  2. Target Proteins:

    • Botulinum toxins A and E cleave synaptosome-associated protein (SNAP-25), a crucial presynaptic membrane protein.
    • Botulinum toxin B targets synaptobrevin, a vesicle-associated membrane protein (VAMP).
  3. Mechanism:

    • These toxins interfere with the fusion of synaptic vesicles containing acetylcholine to the terminal membrane, a critical step in transmitter release.
  4. Effect on Neuromuscular Junction:

    • By blocking acetylcholine release at the neuromuscular junction, botulinum toxins induce flaccid paralysis.
  5. Symptoms:

    • Clinical manifestations include ptosis, diplopia, dysarthria, dysphonia, and dysphagia.

Understanding the actions of botulinum toxins on synaptic proteins provides insight into their role in causing flaccid paralysis and associated symptoms.

Synapse-definition and Types | Medical Science Optional Notes for UPSC

Tetanus Toxin Overview

  • Tetanus toxin, produced by Clostridium tetani, irreversibly binds to the presynaptic membrane at the neuromuscular junction.
  • The toxin utilizes retrograde axonal transport to travel from the neuromuscular junction to the cell body of the motor neuron located in the spinal cord.

Question for Synapse-definition and Types
Try yourself:
What is the main difference between Lambert-Eaton syndrome and myasthenia gravis?
View Solution

Nerve  muscle physiology-Repeats

Synapse and NMJ
Q1: Describe the sequence of events during impulse transmission of neuromuscular junction. How can neuromuscular junction be blocked (1996)
Q2: Describe the sequence of events in neuromuscular transmission. What happens in myasthenia gravis (2005)?
Q3: Discuss in short the structure and sequence of events at neuromuscular junction during nerve impulse transmission. Add a note on myasthenia gravis. (2011)
Q4: Give the sequence of events that occurs during transmission of nerve impulse through neuromuscular junction. (2015)
Q5: Define synapse; enumerate the steps of synaptic transmission. What is the significance of Synaptic delay? (2013)

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FAQs on Synapse-definition and Types - Medical Science Optional Notes for UPSC

1. What is synaptic transmission?
Ans. Synaptic transmission is the process by which information is transferred from one neuron to another at a synapse. It involves the release of neurotransmitters from the presynaptic neuron, which then bind to receptors on the postsynaptic neuron, leading to a change in its membrane potential.
2. What are postsynaptic potentials?
Ans. Postsynaptic potentials are changes in the membrane potential of the postsynaptic neuron that occur in response to the binding of neurotransmitters from the presynaptic neuron. These potentials can be either excitatory, depolarizing the postsynaptic neuron and increasing the likelihood of an action potential, or inhibitory, hyperpolarizing the postsynaptic neuron and decreasing the likelihood of an action potential.
3. What is Myasthenia Gravis?
Ans. Myasthenia Gravis is an autoimmune disorder characterized by muscle weakness and fatigue. It occurs when the immune system mistakenly attacks the neurotransmitter receptors, specifically the acetylcholine receptors, at the neuromuscular junction. This leads to a decrease in the effectiveness of synaptic transmission and muscle weakness.
4. What is Lambert-Eaton Syndrome?
Ans. Lambert-Eaton Syndrome is a rare autoimmune disorder that affects the neuromuscular junction. It is caused by antibodies targeting the voltage-gated calcium channels in the presynaptic membrane. This results in a decrease in the release of neurotransmitters, particularly acetylcholine, leading to muscle weakness and other symptoms.
5. What are Botulinum Toxins?
Ans. Botulinum toxins are a group of neurotoxins produced by the bacterium Clostridium botulinum. These toxins block the release of acetylcholine, a neurotransmitter involved in synaptic transmission, from the presynaptic neuron. This causes muscle paralysis and is often used in medical and cosmetic treatments, such as Botox injections.
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