By Mohamed Hany Mohamed Zanaty

University of AlexandriaFaculty of Medicine

Chemical synapses are specialized junctions

through which neurons signal to each other and to

non-neuronal cells such as those in muscles or

glands. Chemical synapses allow neurons to form

circuits within the central nervous system. They

are crucial to the biological computations that

underlie perception and thought. They allow the

nervous system to connect to and control other

systems of the body.ِ

Synapses are functional connections between neurons, or between neurons and other types of cells.A typical neuron gives rise to several thousand synapses, although there are some types that make far fewer. Most synapses connect axons to dendrites,but there are also other types of connections, including axon-to-cell-body, axon-to-axon,and dendrite-to-dendrite. Synapses are generally too small to be recognizable using a light microscope except as points where the membranes of two cells appear to touch, but their cellular elements can be visualized clearly using an electron microscope.

neuropil is a region between neuronal cell bodies in the gray matter of the brain and spinal cord (i.e. the central nervous system). It consists of a dense tangle of axon terminals, dendrites and glial cell processes. It is where synaptic connections are formed between branches of axons and dendrites.

White matter, which is mostly composed of axons and glial cells, is generally not considered to be a part of the neuropil

On a coarse scale, nervous tissue (ignoring blood vessels etc.) is composed of the cell bodies of neurons and glial cells and their processes or protrusions. For neurons, these are dendrites, dendritic spines and axons. Dendrites collect input from other neurons, which is processed by the neuron (in both its dendrites and its cell body) and propagated to other cells via axons, which act as long-distance cables. At the end of an axon synapses are formed, serving as chemical junctions to other cells.

synaptic vesicles or neurotransmitter vesicles store various neurotransmitters that are released at the synapse. The release is regulated by a voltage-dependent calcium channel. Vesicles are essential for propagating nerve impulses between neurons and are constantly recreated by the cell. The area in the axon which holds groups of vesicles is a bouton. Up to 130 vesicles can be released per bouton over a ten minute period of stimulation at 0.2 Hz

Synaptic vesicles are made of a lipid bilayer in which transport proteins specific to each type of neurotransmitter are inserted. Neurotransmitters are moved from the cell's cytoplasm into the vesicles by vesicular transporters that rely on active transport mechanisms involving an exchange of protons (H+ ions). The necessary proton gradient is created by V-ATPase, which breaks down ATP for energy. Vesicular glutamate transporters, for example, sequester glutamate into vesicles by this process.

Vesicle pools in the nerve terminal are grouped into three pools: the

readily releasable pool, the recycling pool and the reserve pool These pools are distinguished by their function and position in the nerve terminal. The readily releasable pool are docked to the cell membrane, making these the first group of vesicles to be released on stimulation. The readily releasable pool is small and is quickly exhausted. The recycling pool is proximate to the cell membrane, and tend to be cycled at moderate stimulation, so that the rate of vesicle release is the same, or lower than, the rate of vesicle formation. This pool is larger than the readily releasable pool, but it takes longer to become mobilised. The reserve pool constitutes the vast majority of vesicles in the nerve terminal, but it is not clear that vesicles in this pool are released under normal conditions. Under experimental conditions, this pool is mobilised by intense stimulation, and might only occur once the other two pools are exhausted.

neuromuscular junction (NMJ) is the synapse or junction of the axon terminal of a motoneuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract. In vertebrates, the signal passes through the neuromuscular junction via the neurotransmitter acetylcholine.

Electron micrograph showing a cross section through the neuromuscular junction. T is the axon terminal, M is the muscle fiber. The arrow shows junctional folds with basal lamina. Postsynaptic densities are visible on the tips between the folds. Scale is 0.3 µm

Neuromuscular junction (global view)1.Axon2.Synaptical junction3.Muscle fiber4.Myofibrils

Neuromuscular junction (closer view)1.presynaptic terminal2.sarcolemma3.synaptic vesicles4.Acetylcholine receptors5.mitchondrion

electrical synapse is a mechanical and electrically conductive link between two abutting neuron cells that is formed at a narrow gap between the pre- and postsynaptic cells known as a gap junction. At gap junctions, such cells approach within about 3.5 nm of each other (Kandel et al. 2000), a much shorter distance than the 20 to 40 nm distance that separates cells at chemical synapse (Hormuzdi et al. 2004). In organisms, electrical synapse-based systems co-exist with chemical synapses.

Compared to chemical synapses, electrical synapses conduct nerve impulses faster, but unlike chemical synapses they do not have gain (the signal in the post synaptic neuron is always smaller than that of the originating neuron). Electrical synapses are often found in neural systems that require the fastest possible response, such as defensive reflexes. An important characteristic of electrical synapses is that most of the time, they are bidirectional, i.e. they allow impulse transmission in either direction.[1] However, some gap junctions do allow for communication in only one direction

StructureEach gap junction (aka nexus junction) contains

numerous gap junction channels which cross the membranes of both cells (Gibson et al., 2004). With a lumen diameter of about 1.2 to 2.0 nm (Bennet and Zukin, 2004; Hormuzdi et al., 2004), the pore of a gap junction channel is wide enough to allow ions and even medium sized molecules like signaling molecules to flow from one cell to the next (Kandel et al., 2000, p. 178-180; Hormuzdi et al., 2004), thereby connecting the two cells' cytoplasm. Thus when the voltage of one cell changes, ions may move through from one cell to the next, carrying positive charge with them and depolarizing the postsynaptic cell.

An interneuron (also called relay neuron, association neuron or local circuit neuron) is a multipolar neuron which connects afferent neurons and efferent neurons in neural pathways. Like motor neurons, interneuron cell bodies are always located in the central nervous system (CNS)

Renshaw cells are inhibitory interneurons found in the gray matter of the spinal cord, and are associated in two ways with an alpha motor neuron.

They receive an excitatory collateral from the alpha neuron's axon as they emerge from the motor root, and are thus "kept informed" of how vigorously that neuron is firing.

Next, they send their own inhibitory axon to synapse with the cell body of the initial alpha neuron.

In this way, Renshaw cell inhibition represents a negative feedback mechanism. A Renshaw cell may be supplied by more than one alpha motor neuron collaterals and it may synapse on multiple motor neurons.