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Synapse and Synaptic Transmission, Practice Problems and FAQs

Synapse and Synaptic Transmission, Practice Problems and FAQs

You know that all the functions in humans like digestion, respiration, excretion etc. are well coordinated for maintenance of a healthy body. The master organ of the body called the brain controls every activity that takes place in the body. The neurons or the nerve cells are responsible for receiving the stimuli, sending electrical signals or impulses across the body and commanding action to all the body organs.

Ever wondered how this takes place? How do these tiny cells of the nervous system take charge of such an intricate process?

It is because of the array of communication that binds neurons to work together in harmony. The signals are transmitted between the brain and all different parts of the body. But how are neurons connected to each other? Did you know that most of the neurons that are lined one after the other for the transmission of impulses are not even in physical contact and are separated by a fine gap? Then how are impulses transmitted from one neuron to the next? Let us discuss that in this article.

Table of contents

  • Synapse
  • Types of synapse
  • Transmission of impulses
  • Practice Problems
  • FAQs

Synapse

We know that the nervous system of our body controls and coordinates different functions by transmitting impulses from the CNS to the various body parts and vice versa with the help of neurons. Impulses are transmitted from the axon terminals of the signal-passing neurons called the presynaptic neurons to the dendrites of the target neurons called postsynaptic neurons across junctions known as synapses.

The presynaptic and postsynaptic neuron membranes form a synapse which forms a point of contact between the two neurons. A gap may be present between these neurons which is called the synaptic cleft.

Fig: Synapse

Types of synapse

The two types of synapses are:

  • Electrical synapses
  • Chemical synapses

Electrical synapses

The membranes of presynaptic and postsynaptic neurons in electrical synapses are in close proximity to each other. The electric current from one neuron flows directly into the other neuron across electrical synapses. Impulse conduction along a single axon is extremely similar to impulse transmission between electrical synapses. Electrical synaptic impulse transmission is always faster than chemical synapse transmission.

Fig: Electrical synapse

Chemical synapses

The distance between one neuron and the next neuron is further at chemical synapses. Chemical synapses are more common than electrical synapses in the nervous system.

Fig: Chemical synapse

Transmission of impulses

Electrical synapses

In electrical synapses, the presynaptic and postsynaptic membranes of the two communicating neurons live very close at the synapse and are connected with the help of an intercellular connection called a gap junction. The gap junctions contain paired ion channels in the presynaptic and postsynaptic membranes each of which form a pore.

A neuron membrane is always in a polarised state with high potential difference across its two surfaces. The membrane potential of a polarised neuron membrane is known as the resting membrane potential and it is negative in value as the outer surface of the membrane facing the extracellular fluid is positive in charge whereas the interior of the neuron has a negative charge. This is because of the prevalence of Na+ ions on the outer side. This membrane potential is maintained with the help of Na+-K+ ion pumps.

Fig: Resting membrane potential of polarised neuron membrane

Excitation of a neuron results in reversing the resting membrane potential as the membrane becomes more permeable to Na+ and they are pumped inside, making the interior of the neuron positive and the exterior negative. At this point the neuron membrane is said to be depolarised and the membrane potential is called the action potential.

Fig: Action potential of a depolarised membrane

The depolarisation of the presynaptic neuron membrane results in a low potential difference across the membrane whereas the polarised postsynaptic membrane has high potential difference. This results in the generation of a local ionic current which is passed passively through the gap junction pores from the presynaptic membrane to the postsynaptic membrane. As the postsynaptic neuron membrane receives the current, it depolarises and the conduction of impulses continues across the length of the postsynaptic neuron.

Chemical synapses

A fluid-filled area called synaptic cleft separates the membranes of the pre- and postsynaptic neurons at a chemical synapse. The molecules involved in the transmission of impulses at these synapses are known as neurotransmitters. Because neurotransmitters are released at the presynaptic end exclusively and their receptors are only present on the postsynaptic end, synapses ensure one-way or unidirectional transmission of impulses.

Fig: Synaptic cleft

The transmission of impulses is aided by the ion channels present in the postsynaptic neuron.

Fig: Ion channels present in the postsynaptic neuron

Synaptic vesicles are found at the axon terminals. Neurotransmitters are substances that fill these vesicles.

Fig: Synaptic vesicles bearing neurotransmitters

Let us move forward to the sequence of events that occur during transmission of impulses. Firstly, the axon terminal receives an impulse (action potential). At the terminal position of axons, synaptic vesicles containing neurotransmitters are present. Synaptic vesicles migrate to the presynaptic membrane upon reception of the action potential.

Fig: Synaptic vesicles move towards the presynaptic membrane

Next, the synaptic vesicles bind to the presynaptic neuron's plasma membrane and the neurotransmitters are released in the synaptic cleft.

Fig: Release of neurotransmitters into the synaptic cleft

The neurotransmitters bind to specific receptors on the postsynaptic membrane. Ion channels are opened in the postsynaptic membrane when released neurotransmitters bind to their appropriate receptors. Ions can enter the postsynaptic neuron through the open channels.

Fig: Binding of neurotransmitters to receptors

In the postsynaptic neuron, these ions cause a new action potential that can be either excitatory or inhibitory. The conduction of impulses takes place by the movement of action potential across the cell. Charges are reversed as sodium ions move inside and potassium ions flow outward. This causes the neuron membrane at this region of the neuron to be depolarised. The action potential that arises from this depolarization causes the nerve impulse to travel along the length of the axon.

Fig: Depolarisation of postsynaptic membrane

Practice Problems

Q1. Which of the following statements is incorrect regarding the electrical synapses?

A. Transmission of impulses is slower compared to chemical synapses.
B. Impulse transmission is similar to impulse conduction along a single axon.
C. Electric current flows directly from one neuron into the other neuron.
D. Membranes of presynaptic and postsynaptic neurons are in close proximity.

Solution: The membranes of presynaptic and postsynaptic neurons together form the synapse. There are two types of synapses; electrical and chemical synapses. Chemical synapses transmit impulses more slowly because they rely on the release of neurotransmitters from synaptic vesicles to relay their signal, whereas electrical synapses transfer impulses by electrical signals that can flow right across the synapse in a way similar to impulse conduction along a single axon. This is possible because membranes of presynaptic and postsynaptic neurons are in close proximity. Hence, option a is correct.

Q2. Based on the transmission of impulses, rearrange the following in the correct sequence.

  1. Entry of ions to the postsynaptic neuron.
  2. Release of neurotransmitter in the synaptic cleft.
  3. Binding of neurotransmitters to appropriate receptors.
  4. Binding of the plasma membrane of the presynaptic neuron with synaptic vesicles.
  5. Migration of synaptic vesicles to the presynaptic membrane.
    A. III→II→I→IV→V
    B. IV→II→III→I→V
    C. V→IV→II→III→I
    D. I→V→III→IV→II

Solution: At the synapse, neurotransmitters take part in the transmission of impulses. Synaptic vesicles carrying neurotransmitters are located at the terminal location of axons. Synaptic vesicles move to the presynaptic membrane when the action potential of an impulse arrives at the axon terminal. The membrane of the synaptic vesicle and the presynaptic neuron fuse or bind together after which the neurotransmitters are released in the synaptic cleft. The released neurotransmitters bind to the appropriate receptors on the postsynaptic membrane which open the ion channels in the postsynaptic membrane. Through these open channels, the ions move into the postsynaptic neuron where they generate a new action potential. Hence, option c is correct.

Q3. Define neurotransmitters. State their function.
Answer:
A neurotransmitter is a biochemical messenger secreted by the axon terminals of a neuron. They are chemicals that carry messages from neurons to muscles across neuromuscular junctions or between neurons across a synapse.

Q4. From where are the neurotransmitters released?
Answer:
At the synapse, impulses are passed from presynaptic to postsynaptic neurons. When a nerve impulse reaches the presynaptic axon terminal, membrane-bound sacs or synaptic vesicles travel toward the presynaptic membrane, where they fuse with the membrane and release a chemical substance known as a neurotransmitter into the synaptic cleft.

FAQs

Q1. What happens if the synapses are damaged or absent?
Answer:
The sites of contact between neurons where information is transmitted from one neuron to the next are referred to as synapses. The central nervous system would be constantly bombarded with impulses without synapses, resulting in central nervous system exhaustion. The reactions would be delayed, and the backward flow of impulses would result in a lack of coordination. The pathogenesis of Alzheimer's disease (AD) is based on synapse damage and loss, which results in decreased cognitive performance.

Q2. Are electrical impulses bidirectional?
Answer:
The nature of electrical synapses is that they are bidirectional. When a presynaptic action potential travels to the postsynaptic cell, the postsynaptic cell's membrane resting potential also travels to the presynaptic cell.

Q3. Are enzymes responsible for the breakdown of neurotransmitters?
Answer:
Neurotransmitters can be broken down by the action of enzymes. The breakdown of the neurotransmitter acetylcholine into its component parts, acetate and choline, by the enzyme acetylcholinesterase (AChE) is an important example of this process.

Q4. Can neurotransmitters be inhibitory?
Answer:
Neurotransmitters such as gamma amino-butyric acid (GABA), glycine, serotonin, etc are inhibitory in nature and prevent the transmission of impulses from one neuron to the next.

 

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