Question

How do things get from the neuronal cell body to the presynaptic terminal, and vice versa? Describe / explain / discuss any molecules and/or cellular structures involved

Answer 1

How do things get from the neuronal cell body to the presynaptic terminal?

Neurons talk to each other across synapses.

When an action potential reaches the presynaptic terminal, it causes neurotransmitter to be released from the neuron into the synaptic cleft, a 20–40nm gap between the presynaptic axon terminal and the postsynaptic dendrite (often a spine)

At chemical synapses, impulses are transmitted by the release of neurotransmitters from the axon terminal of the presynaptic cell into the synaptic cleft. ... Multiple cytosolic proteins including synapsin recruit synaptic vesicles to the active zone of the plasma membrane adjacent to the synaptic cleft.

Normally, the inside of the cell is more negative than the outside; neuroscientists say that the inside is around -70 mV with respect to the outside, or that the cell’s resting membrane potential is -70 mV.

Action potentials are the fundamental units of communication between neurons and occur when the sum total of all of the excitatory and inhibitory inputs makes the neuron’s membrane potential reach around -50 mV (see diagram), a value called the action potential threshold.

How do things get from presynaptic terminal to neuronal cell body?

There is a small gap between the axon terminal of the presynaptic neuron and the membrane of the postsynaptic cell, and this gap is called the synaptic cleft. ... When an action potential, or nerve impulse, arrives at the axon terminal, it activates voltage-gated calcium channels in the cell membrane.

Messages travel along a single neuron as electrical impulses, but messages between neurons travel differently. The transfer of information from neuron to neuron takes place through the release of chemical substances into the space between the axon and the dendrites.

Molecules or cellular structures involved in the neuronal cell body and presynaptic terminal:

Numerous small molecules synthesized in the cytosol of axon terminals function as neurotransmitters at various chemical synapses. The “classic” neurotransmitters are stored in synaptic vesicles, uniformly sized organelles, 40 – 50 nm in diameter. With the exception of acetylcholine, the classic neurotransmitters depicted in alamino acids or derivatives of amino acids. Nucleotides such as ATP and the corresponding nucleosides, which lack phosphate groups, also function as neurotransmitters. Each neuron generally produces just one type of classic neurotransmitter. Following their exocytosis from synaptic vesicles into the synaptic cleft, neurotransmitters bind to specific receptors on the plasma membrane of a postsynaptic cell, causing a change in its permeability to ions.

Influx of Ca2+ Triggers Release of Neurotransmitters:
The exocytosis of neurotransmitters from synaptic vesicles involves vesicle-targeting and fusion events similar to those that occur at many points in the secretory pathway Indeed the same types of proteins — including T-SNARE and V-SNAREs, α, β, and γ SNAP proteins, and NSF — participate in both systems. However, exocytosis of neurotransmitters at chemical synapses differs from other secretory pathways in two critical ways: (a) Secretion is tightly coupled to arrival of the action potential at the axon terminus, and (b) synaptic vesicles are recycled locally after fusion with the plasma membrane, a process that takes less than one minute.

Depolarization of the plasma membrane cannot, by itself, cause synaptic vesicles to fuse with the plasma membrane. In order to trigger vesicle fusion, an action potential must be converted, or transduced, into a chemical signal — namely, a localized rise in the cytosolic Ca2+ concentration. The transducers of the electric signals are voltage-gated Ca2+ channels localized to the region of the plasma membrane adjacent to the synaptic vesicles. The membrane depolarization due to arrival of an action potential opens these channels, permitting an influx of Ca2+ ions into the cytosol from the extracellular medium. The amount of Ca2+ that enters an axon terminal through voltage-gated Ca2+ channels is sufficient to raise the level of Ca2+ in the region of the cytosol near the synaptic vesicles from <0.1 μM, characteristic of the resting state, to 1 – 100 μM. As we describe below, the Ca2+ ions bind to proteins that connect the synaptic vesicle with the plasma membrane, inducing membrane fusion and thus exocytosis of the neurotransmitter. The extra Ca2+ ions are rapidly pumped out of the cell by Ca2+ ATPases, lowering the cytosolic Ca2+ level and preparing the terminal to respond again to an action potential.

The presence of voltage-gated Ca2+ channels in axon terminals has been demonstrated in neurons treated with drugs that block Na+ channels and thus prevent conduction of action potentials. When the membrane of axon terminals in such treated cells is artificially depolarized, an influx of Ca2+ ions into the neurons occurs and exocytosis is triggered. Patch-clamping experiments show that voltage-gated Ca2+ channels, like voltage-gated Na+ channels, open transiently upon depolarization of the membrane.

Structures involved in the neuronal cell body and post synaptic terminal:

The primary components of the neuron are the soma (cell body), the axon (a long slender projection that conducts electrical impulses away from the cell body), dendrites (tree-like structures that receive messages from other neurons), and synapses (specialized junctions between neurons).

Embedded within the neuronal cytoplasm are the organelles common to other cells, the nucleus, nucleolus, endoplasmic reticulum, Golgi apparatus, mitochondria, ribosomes, lysosomes, endosomes, and peroxisomes.

Neurons (nerve cells) have three parts that carry out the functions of communication and integration: dendrites, axons, and axon terminals. They have a fourth part the cell body or soma, which carries out the basic life processes of neurons. The figure at the right shows a "typical" neuron.

Inside the axon terminal of a sending cell are many synaptic vesicles. These are membrane-bound spheres filled with neurotransmitter molecules. There is a small gap between the axon terminal of the presynaptic neuron and the membrane of the postsynaptic cell, and this gap is called the synaptic cleft.

Types of Postsynaptic Potentials (PSP) - Excitatory (EPSP) ...
Excitatory Postsynaptic Potential. aka EPSP. ...
Inhibitory Postsynaptic Potential. aka IPSP. ...
Postsynaptic Potential. ...
EPSP & IPSP. ...
Axon Hillock. ...
EPSP/IPSP Summation Types. ...
Spatial Summation.