presynaptic neuron

Examples of presynaptic neuron in the following topics:

• The Synapse

  • The neuron conducting impulses towards the synapse is called presynaptic neuron.
  • At a synapse , the presynaptic neuron sends information and postsynaptic neuron receives the information.
  • Most neurons, function as both as presynaptic and postsynaptic neurons.
  • In a chemical synapse, the plasma membrane of the signal-passing neuron (the presynaptic neuron) comes into close apposition with the membrane of the target (postsynaptic) cell.
  • The presynaptic neuron contains a chemical called a neurotransmitter that is packaged into synaptic vesicles clustered beneath the membrane in the axon terminal.

• Peripheral Motor Endings

  • A neuromuscular junction is the synapse or junction of the axon terminal of a motor neuron with the motor end plate, as shown in Figures 1 and 2.
  • Upon the arrival of an action potential at the presynaptic neuron terminal, voltage-dependent calcium channels open and Ca2+ ions flow from the extracellular fluid into the presynaptic neuron's cytosol.
  • This influx of Ca2+ causes neurotransmitter-containing vesicles to dock and fuse to the presynaptic neuron's cell membrane, which results in the emptying of the vesicle's contents (acetylcholine) into the synaptic cleft; this process is known as exocytosis.
  • The affects of myasthenia gravis illustrate the importance of effective and functioning neuromuscular junctions for communication between neurons and muscles to allow contraction and relaxation of muscle fibers.
  • Detailed view of a neuromuscular junction: 1) Presynaptic terminal; 2) Sarcolemma; 3) Synaptic vesicle; 4) Nicotinic acetylcholine receptor; 5) Mitochondrion.

• Synaptic Plasticity

  • However, when the postsynaptic neuron is depolarized by multiple presynaptic inputs in quick succession (either from one neuron or multiple neurons), the magnesium ions are forced out and Ca2+ ions pass into the postsynaptic cell.
  • The insertion of additional AMPA receptors strengthens the synapse so that the postsynaptic neuron is more likely to fire in response to presynaptic neurotransmitter release.
  • With the decrease in AMPA receptors in the membrane, the postsynaptic neuron is less responsive to the glutamate released from the presynaptic neuron.
  • LTD occurs when few glutamate molecules bind to NMDA receptors at a synapse (due to a low firing rate of the presynaptic neuron).
  • This makes the postsynaptic neuron less responsive to glutamate released from the presynaptic neuron.

• Signal Summation

  • Signal summation occurs when impulses add together to reach the threshold of excitation to fire a neuron.
  • Each neuron connects with numerous other neurons, often receiving multiple impulses from them.
  • Sometimes, a single excitatory postsynaptic potential (EPSP) is strong enough to induce an action potential in the postsynaptic neuron, but often multiple presynaptic inputs must create EPSPs around the same time for the postsynaptic neuron to be sufficiently depolarized to fire an action potential.
  • One neuron often has input from many presynaptic neurons, whether excitatory or inhibitory; therefore, inhibitory postsynaptic potentials (IPSPs) can cancel out EPSPs and vice versa.
  • A single neuron can receive both excitatory and inhibitory inputs from multiple neurons.

• Stages of the Action Potential

  • A neuron affects other neurons by releasing a neurotransmitter that binds to chemical receptors.
  • The effect upon the postsynaptic (receiving) neuron is determined not by the presynaptic (sending) neuron or by the neurotransmitter itself, but by the type of receptor that is activated.
  • However, in order for a presynaptic neuron to release a neurotransmitter to the next neuron in the chain, it must go through a series of changes in electric potential.
  • The action potential is a rapid change in polarity that moves along the nerve fiber from neuron to neuron.
  • Reuptake refers to the reabsorption of a neurotransmitter by a presynaptic (sending) neuron after it has performed its function of transmitting a neural impulse.

• Autonomic Ganglia

  • The axons of dorsal root ganglion neurons are known as afferents.
  • The first neuron in this pathway is referred to as the preganglionic or presynaptic neuron.
  • This second neuron is referred to as the postganglionic or postsynaptic neuron.
  • The axons of presynaptic parasympathetic neurons are usually long.
  • The pathways of the ciliary ganglion include sympathetic neurons (red), parasympathetic neurons (green), and sensory neurons (blue).

• Postganglionic Neurons

  • At the synapses within the ganglia, the preganglionic neurons release acetylcholine, a neurotransmitter that activates nicotinic acetylcholine receptors on postganglionic neurons.
  • The postganglionic neurons of sweat glands release acetylcholine for the activation of muscarinic receptors.
  • Presynaptic nerves' axons terminate in either the paravertebral ganglia or prevertebral ganglia.
  • Because paravertebral and prevertebral ganglia are relatively close to the spinal cord, presynaptic neurons are generally much shorter than their postsynaptic counterparts, which must extend throughout the body to reach their destinations.
  • The axons of presynaptic parasympathetic neurons are usually long.

• Mechanics of the Action Potential

  • The synapse is the junction where neurons trade information.
  • The membrane of a neuron is normally at rest with established concentrations of sodium ions (Na+) and potassium ions (K+) on either side.
  • The response in the postsynaptic neuron is generally smaller in amplitude than the source.
  • The amount of attenuation of the signal is due to the membrane resistance of the presynaptic and postsynaptic neurons.
  • This image shows electric impulses traveling between neurons; the inset shows a chemical reaction occurring at the synapse.

• Synaptic Transmission

  • Neurotransmission at a chemical synapse begins with the arrival of an action potential at the presynaptic axon terminal.
  • Na+ ions enter the cell, further depolarizing the presynaptic membrane.
  • Fusion of a vesicle with the presynaptic membrane causes neurotransmitters to be released into the synaptic cleft.
  • First, reuptake by astrocytes or presynaptic terminal where the neurotransmitter is stored or destroyed by enzymes.
  • When the presynaptic membrane is depolarized, voltage-gated Ca2+ channels open and allow Ca2+ to enter the cell.

• Neurotransmitters

  • Neurotransmitters are chemicals that transmit signals from a neuron across a synapse to a target cell.
  • A neuron has a negative charge inside the cell membrane relative to the outside of the cell membrane; when stimulation occurs and the neuron reaches the threshold of excitement this polarity is reversed.
  • This allows the signal to pass through the neuron.
  • This illustration shows the process of reuptake, in which  leftover neurotransmitters are returned to vesicles in the presynaptic cell.
  • Explain the role of neurotransmitters in the communication process between neurons