Nerve Impulse and Synapse

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What is Nerve Impulse?

“Nerve impulse is a wave of electrochemical changes, which travels along the length of the neuron including chemical reactions and motion of ions across the cell membrane”.

An electrical potential is a measure of the capability to do electrical work. It represents a kind of saving energy that is manifested during the separation of charges across a barrier.

When it comes to neurons, the charges are positive and negative ions, and the charge separating barrier is the plasma membrane. The electrical potential that exists throughout a cell membrane is called the membrane potential.

Electrical-potential

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Resting Membrane Potential

A typical neuron at rest is more positive electrically outside than inside the cell membrane. This net difference in charge between the inner and the outer surface area of a non-conducting neuron is called the resting membrane potential.

Threshold-stimulus

Factors associate with Resting Membrane Potential

The significant factors which are involved in resting membrane potential are:

1.Salt and potassium ions

Of the many kinds of ions present in the nerve cell and the surrounding fluid, sodium (Na+) and potassium (K+) ions are the most essential. Sodium ions are tenfold higher in concentration outside than inside the membrane surface area, whereas potassium ions are twenty times more concentrated inside than outside. All the nerve cells have very active sodium and potassium pumps situated in their cell membranes.

Driven by the splitting of ATP, these pumps transport Na+ out and K+ into the cell, both against their particular concentration gradients. For every two K+ that are actively carried inward, three Na+ are pumped out. So inside becomes more negative than the outside of the cell membrane of neurons.

2.Negative organic ions

The large negative organic ions (such as proteins, organic acids, etc) are a lot more inside the membrane than outside, where they are only in minimal concentration. This makes the inside of the neuron membrane more negative.

3.Leakage of K+ ions from neurons

The cell membrane is practically impenetrable to all ions other than K+. Considering that the membrane is somewhat permeable to K+, a few of it leakages out of the cell. The loss of this positive ion from the neuron by diffusion accounts for more negative’ charges within than outside the cell membrane of the neuron.

4.Conduction of nerve impulse

There is no conduction of nerve impulses.

5.Membrane Potential

A membrane potential of 0.07 volts or -70 mV exists.

Initiation of Nerve Impulse

Threshold stimulus: Under normal conditions, a nerve impulse is started by a proper stimulus (called threshold stimulus) applied at one end of the neuron and it results in an impressive localized change in the resting membrane potential.

Active Membrane Potential

“When a neuron is stimulated, the cell membrane at the point of stimulation undergoes a momentary reversal in charge (dark color) called an action potential. Perhaps for a millisecond, the inside of the membrane becomes positive relative to the outside”.

Threshold stimulus results in a remarkable localized change in resting membrane potential. It disappears for a brief moment and is changed by a new potential called action or active membrane potential which is in the form of impulse.

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During this state, the inner membrane surface area becomes more positive than the outside. This change is so short (for perhaps a millisecond) that just a portion of the neuron remains in the active membrane potential state.

Factors Responsible for Active Potential
1.Na+ and K+ ions movement

The passage of nerve impulse is associated with an increase in permeability of Na+ ions moving inwards disturbing the potential momentarily, making the inside more positive than outside. Neurophysiologists think that the increased permeability is because of the opening of specific pores in the membrane, described as “sodium gates”. When these gates open, sodium ions rush into the neuron by diffusion. Some K+ moves out.

Na+-and-K+-ions

2.Charges are reversed

The inner side of the cell membrane has an excess of positive ions (thus positive charges) at its internal surface area, and the outer surface area becomes more negative.

3.Passage of nerve impulse

Throughout active membrane potential, the neuron conducts the impulse in the form of the nerve impulses.

4.Membrane potential

Active membrane potential of +0.05 volts (+50 mv) exists.

5.Traveling of impulse

These changes occur along the length of the neuron till the impulse reaches synapse.

Restoration of Resting Membrane Potential

Shortly after passage of the impulse, the resting membrane potential is brought back by the movement of a small number of ions particularly K+ moving out. This neuron now is ready to conduct another impulse.

Saltatory Impulse:

It may be added that in myelinated neurons the impulse leaps from node to node (node of Ranvier). This is called saltatory impulse.

Do you know???

The normal speed of nerve impulse in human beings is 100 meters per second but the maximum speed recorded is 120 meters per second.
Synapse
Definition of Synapse

Successive neurons are so arranged that the axon endings of one nerve cell are linked to the dendrites of the next neuron. There is no cytoplasmic connection between the two nerve cells and tiny spaces are left in between them. Each of these contact points is referred to as a synapse.

Definition-of-Synapse

A single nerve cell might form synapses with many incoming fibers of different nerve cells. A nerve impulse is passed from one nerve cell to the other through the synapse, however, a single impulse does not always get across the synapse. It may take two or three impulses getting here in quick succession or maybe all at once from two or more fibers to start an impulse in the next neuron.

Neurotransmitter

The action potential can not jump from one nerve cell to the next in line; rather the message is transferred through synapse in the form of a chemical messenger called a neurotransmitter.

Neurotransmitters are chemicals that are released at the axon ending of the neurons, at the synapse.

Numerous different kinds of neurotransmitters are known. These are:

  • acetylcholine, 2. adrenaline, 3. norepinephrine, 4. serotonin, and 5. dopamine.
  • Acetylcholine is the main transmitter for synapses that lie outside the central nervous system.
  • Others are primarily involved in synaptic transmission within the brain and spinal cord.

Neurotransmitter

Nerve Impulse through Synapse

When an impulse reaches a synaptic knob, synaptic vesicles within fuse with the presynaptic membrane, causing the release of neurotransmitter particles into the synaptic cleft. The neurotransmitter molecules bind to the receptors, on the postsynaptic membrane, setting off an action potential in the postsynaptic neuron, by causing changes in its permeability to specific ions.

Multiple Choice Questions (MCQs)

  1. What is a nerve impulse?
    • a) A chemical reaction in neurons
    • b) A wave of electrochemical changes along a neuron
    • c) The synthesis of neurotransmitters
    • d) A mechanical movement within neurons
    • Answer: b
  2. What is the resting membrane potential?
    • a) The maximum charge inside a neuron
    • b) The potential during an action potential
    • c) The net charge difference between inner and outer neuron surfaces at rest
    • d) The charge when a neuron is stimulated
    • Answer: c
  3. What is the role of sodium (Na+) and potassium (K+) ions in resting membrane potential?
    • a) Sodium ions move into the neuron
    • b) Potassium ions move out of the neuron
    • c) Both a and b
    • d) None of the above
    • Answer: c
  4. What is the active membrane potential?
    • a) The potential during an action potential
    • b) The potential at rest
    • c) The potential during impulse restoration
    • d) The potential during resting membrane state
    • Answer: a
  5. What is responsible for the active membrane potential?
    • a) Movement of Na+ and K+ ions
    • b) Leakage of K+ ions
    • c) Reversal of charges
    • d) Conduction of nerve impulses
    • Answer: a
  6. What occurs during the restoration of resting membrane potential?
    • a) Movement of K+ ions out of the neuron
    • b) Movement of Na+ ions into the neuron
    • c) Both a and b
    • d) Reversal of charges
    • Answer: a
  7. What is a synapse?
    • a) A cytoplasmic connection between neurons
    • b) Tiny spaces within a neuron
    • c) The point where axon endings of one neuron connect to dendrites of the next
    • d) A chemical messenger
    • Answer: c
  8. What is a neurotransmitter?
    • a) A cytoplasmic substance in neurons
    • b) A mechanical transmitter at synapses
    • c) A chemical messenger released at the synapse
    • d) The potential during an action potential
    • Answer: c
  9. Which neurotransmitter is the main transmitter for synapses outside the central nervous system?
    • a) Adrenaline
    • b) Norepinephrine
    • c) Acetylcholine
    • d) Serotonin
    • Answer: c
  10. How is a nerve impulse transferred through a synapse?
    • a) Through cytoplasmic connections
    • b) Mechanically
    • c) Chemically via neurotransmitters
    • d) Electrically
    • Answer: c
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Depolarization is caused by

  1. K+ influx
  2. Na+ influx
  3. Na+ efflux
  4. All of these

Answer: B

At ATPase Na+ – K+ pump the no. of binding sites for Na+

  1. 1
  2. 2
  3. 3
  4. 4

Answer: C

Under resting condition inner part of the cell membrane of neuron is:

  1. Negatively charged
  2. Positively charged
  3. Neutral
  4. Variable

Answer: A

Maximum speed of nerve impulse in humans is

  1. 100 m/sec
  2. 120 km/sec
  3. 100 km/sec
  4. 120 m/sec

Answer: D

A nerve impulse is passed from one to another neuron by

  1. Jumping movements
  2. Neurotransmitters
  3. Hormones
  4. All

Answer: B

 

Frequently Asked Questions (FAQs):

  1. What is a nerve impulse?
    • A nerve impulse is a wave of electrochemical changes that travels along the length of a neuron, involving chemical reactions and motion of ions across the cell membrane.
  2. What is resting membrane potential?
    • Resting membrane potential is the net difference in charge between the inner and outer surfaces of a non-conducting neuron at rest, where the neuron is more positively charged outside than inside.
  3. What are the factors associated with resting membrane potential?
    • The factors include:
      • Salt and potassium ions
      • Negative organic ions
      • Leakage of K+ ions from neurons
      • Absence of conduction of nerve impulses
      • Membrane potential of -70 mV
  4. How is a nerve impulse initiated?
    • A nerve impulse is initiated by a proper stimulus (threshold stimulus) applied at one end of the neuron, resulting in a localized change in resting membrane potential.
  5. What is active membrane potential?
    • Active membrane potential occurs when a neuron is stimulated, causing a momentary reversal in charge, known as an action potential. The inner membrane becomes briefly more positive than the outside.
  6. What are the factors responsible for active membrane potential?
    • Factors include:
      • Movement of Na+ and K+ ions
      • Reversal of charges
      • Passage of nerve impulse
      • Membrane potential of +50 mV
      • Traveling of impulse along the neuron
  7. How is resting membrane potential restored after an impulse?
    • The resting membrane potential is restored by the movement of K+ ions out of the neuron shortly after the passage of the impulse.
  8. What is a synapse?
    • A synapse is a contact point between successive neurons, where the axon endings of one neuron connect to the dendrites of the next, with no cytoplasmic connection between them.
  9. What is a neurotransmitter?
    • A neurotransmitter is a chemical messenger released at the synapse, facilitating the transfer of a nerve impulse from one neuron to another.
  10. Name some neurotransmitters and their functions.
    • Acetylcholine: Main transmitter for synapses outside the central nervous system.
    • Adrenaline, norepinephrine, serotonin, and dopamine: Primarily involved in synaptic transmission within the brain and spinal cord.
  11. How does a nerve impulse travel through a synapse?
    • When an impulse reaches a synaptic knob, synaptic vesicles fuse with the presynaptic membrane, releasing neurotransmitter particles into the synaptic cleft. These molecules bind to receptors on the postsynaptic membrane, triggering an action potential in the postsynaptic neuron.
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Summary:

The tutorial on Nerve Impulse and Synapse explains the processes governing the transmission of electrochemical signals along neurons. Key components include:

  • Nerve Impulse: Described as a wave of electrochemical changes, the nerve impulse involves motion of ions and chemical reactions across the cell membrane.
  • Resting Membrane Potential: A typical neuron at rest exhibits a net positive charge outside, creating a resting membrane potential.
  • Initiation of Nerve Impulse: Triggered by a threshold stimulus, a localized change in resting membrane potential initiates the nerve impulse.
  • Active Membrane Potential: Stimulated neurons undergo a momentary reversal in charge, resulting in active membrane potential or action potential.
  • Factors Responsible for Active Potential: Involves movement of Na+ and K+ ions, reversal of charges, passage of nerve impulse, and changes in membrane potential.
  • Restoration of Resting Membrane Potential: After impulse passage, the resting membrane potential is restored through the movement of ions, particularly K+.
  • Synapse: Successive neurons connect at synapses, allowing the transfer of nerve impulses through chemical messengers called neurotransmitters.
  • Neurotransmitters: Chemicals like acetylcholine, adrenaline, norepinephrine, serotonin, and dopamine facilitate synaptic transmission.
  • Nerve Impulse through Synapse: Synaptic vesicles release neurotransmitters into the synaptic cleft, triggering an action potential in the postsynaptic neuron.

The tutorial provides a comprehensive understanding of the electrochemical events underlying nerve impulses and synaptic transmission, offering insights into the intricate workings of the nervous system.

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