05. Synapses

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synapse1 In 1897, The English neurophysiologist Charles Sherrington [ 74 ] found that the rate of nerve impulses from the cortex to the members was lower than the spread at a single nerve fiber. He then concludes that there must be some kind of interruptions that slow the transmission of impulses. Thus he brought the concept of Synapse (from the Greek syn meaning together haptein means touch or grasp, that is to say, free) [ 52 ].

Indeed, neurons communicate with each other through synapses each. On a single neuron can be found from 1,000 to 10,000 synapses [ 109 ] (about 300,000 at the cerebellar Purkinje cells [ 57 ]). Multiply this number by 100 billion neurons to get an idea on the number of communication within the nervous system!

1. Classification [ 39 , 71 ]:

Synapses can be classified according to their location, structure function or the nature of the neurotransmitter released at their level.

1.1. Depending on the location [ 4 ]

Axon terminals are in contact with the dendrite (axo-dendritic synapse), the cell body (axo-somatic synapse) or even end on an axon (axo-axonal synapse).

1.2. According to their nature:

synapse2 There are two broad categories of synapses [ 41 ]:

  • Electrical synapses [ 42 ] which are communicating junctions between certain neurons, they play an important role during development and often turn after chemical synapses. In adults they are limited to a few regions of the brain.
  • Chemical synapses [ 52 ] are by far the most common, the signal circulates secretion of chemical mediators called: neurotransmitters or neurotransmitters. These neurotransmitters may have an excitatory effect (acetylcholine - glutamate) or inhibitory (GABA). A neuron can secrete more than one type of neurotransmitter [ 38 , 39 , 41 ].

1.3. According to the post-synaptic cell:

Synapses can bind neurons with other neurons or effector cells [2]: glandular (junction neuro-glandular) or muscle (neuromuscular junction).

synapse3 2. Anatomy of a synapse:

A synapse consists of three parts [ 5 ]: a presynaptic terminal region that corresponds to the button of the presynaptic axon, a post-synaptic region (the area next to the terminal button), although these two regions very close are always separated by a space called (the synaptic cleft).

The terminal button contains synaptic vesicles filled with neurotransmitters, and several mitochondria (energy source). The postsynaptic part contains no synaptic vesicles making the unidirectional propagation of the signal level.

The part contains postsynaptic receptors usually ductal type which will open in response to the action of released neurotransmitters.

3. Process:

When a train of action potentials (potentials of succession) arrives at the button terminal, it causes the opening of voltage-gated calcium channels. Calcium massively then penetrates inside the cell and stimulates a cascade of chemical reactions [ 39 , 57 ] the fusion of synaptic vesicles with the plasma membrane. An average of 300 synaptic vesicles are released with each action potential [ 57 ]. There is more action potentials, the number of released vesicles increases.

The neurotransmitters diffuse to the postsynaptic region to activate their receivers are then rapidly removed [ 57 ] or by diffusion outside the synaptic cleft (they will be captured by gliocytes) is degraded by a specific enzyme and reabsorbed by the button terminal to produce other neurotransmitters (Recapture - Reuptake [ 100 ]).

4. Post-synaptic potentials:

4.1. Excitatory postsynaptic potential:

In a promoting synapse, a neurotransmitter causes sodium channels to open the sodium to enter the interior of the cell and creating a local depolarization called (post-synaptic potential or exciter EMPP [ 4 , 39 , 41 ]).

It rarely causes an action potential in the dendrites or cell body of the view that these two areas are very poor voltage-gated sodium channels. It is therefore a measuring potential whose amplitude decreases with time and the distance between the enhancer and the synapse emergence cone (area extremely rich in voltage-gated sodium channels and usual place of initiation potential action).

4.2. Postsynaptic potential inhibitor:

Inhibitory synapses, the neurotransmitter (eg GABA) causes in the region postsynaptic a channel opening of chlorine (which will penetrate inside the cell) or potassium (which will leave the cell).

In these two cases there is a local hyperpolarization of the plasma membrane called (postsynaptic potential inhibitor PSIP or [ 4 ]), the hyperpolarization will broadcast in the same way that the emergence of cone to EMPP where it will make it harder to produce an action potential.

Inhibitory synapses are often located near the emergence cone, it is at this level that their inhibitory action can be most effective. synapse4

5. Integration:

In real time, it is rare that a single stimulus can give rise to a PA. The neuron receives more stimuli to both. The processing thereof is performed at the site by the axon spatiotemporal summation different potentials collected [area 38 , 39 , 54 ].

In spatial summation [ 1 ]: If the addition of excitatory and inhibitory potentials from different synapses and arriving together cone emergence is greater than a threshold value it will trigger an action potential, otherwise it will be ignored .

In temporal summation [ 52 ] While many excitatory potentials are close in time, they add up and can also reach the depolarization threshold and result in an action potential.

The cone emergence will then play the role of integrator nervous [ 96 ] who will decide, according to different potentials collected at his level, he will trigger an action potential or not.