Medical brain notes 31

  The Synapse The axon ends with numerous small knob-like swellings, the  axon terminals  or  boutons . Together with the apposed membrane of the next neuron, the bouton forms the  syn-apse  where excitation is transmitted fromone neuron to another. The synapse consists of the  presynaptic com-ponent  ( bouton ) ( AB1 ) with the presynapticmembrane  ( BC2 ), the  synaptic cleft  ( B3 ), andthe postsynaptic component  with the  postsyn-aptic membrane  ( BC4 ) of the next neuron.The bouton is free of neurofilaments and neurotubules but contains mitochondria and small, mainly clear vesicles  ( BC5 ) which are clustered near the presynaptic mem-brane ( active zone ). The synaptic cleft con-tains filamentous material and communi-cates with the extracellular space. The pre-synaptic and postsynaptic membranes ex-hibit dense  zones of apposition , which re-semble those found at various cell junctions ( zonulae ...

  Types of Synapses There are numerous variations on the simple form of synapses. The synaptic con-tact between parallel axons and dendrites is called  parallel contact or  bouton en passant  ( A1 ). Many dendrites have thornlike projec-tions (spines) that form a  spinous synapse  ( A2 ) with the bouton. On the apical den-drites of some pyramidal cells, the terminal swelling of the axon encloses the entire spine, which may be relatively large and branched, bearing numerous synaptic con-tacts ( complex synapse ) ( B ). Several axons and dendrites can join to form  glomerulus-like complexes  in which the different synap-tic elements are closely intertwined. They probably affect each other in terms of fine-tuning (modulating) the transmission of impulses. Each brain division has characteristic forms of synapses.  Gray type I and II synapses  are predominantly found in the cerebral cortex,  glomerulus-like complexes  are found in t...

  Neurotransmitters Transmission of impulses at the chemical synapses is mediated by neurotransmitters. The most widely distributed transmitter substances in the nervous system are  acetylcholine  (ACh),  glutamate ,  gamma-aminobutyric acid  (GABA), and  glycine .Glutamate is the most common excitatory transmitter, GABA is a transmitter of the in-hibitory synapses in the brain, and glycine is an inhibitory transmitter in the spinal cord. The  catecholamines norepinephrine  (NE) and  dopamine  (DA) also act as transmitters, and so does  serotonin  (5-HT). Many  neu-ropeptides  act not only as hormones in thebloodstream but also as transmitters in the synapses (e.g., neurotensin, cholecys-tokinin, somatostatin). The transmitters are produced in the peri-karyon and stored in the vesicles of the axon terminals. Often only the enzymes required for transmitter synthesis are produced in the perikaryon, while the transm...

  Axonal Transport The transmitter substances or their synthesizing enzymes are produced in the perikaryon and must be transported to the axon terminal. The microtubules of the neuron,  neurotubules  ( D1 ), play a key role in thistransport mechanism. If they are destroyed by applying the mitotic poison  colchicine , the  intra-axonal transport  stops. This rapidtransport of material is energy-dependent and takes place in vesicles that are moved along the microtubules by  motor proteins . The retrograde transport (in the direction of the cell body and toward the minus end of the microtubules) is mediated by  dynein  ( D2 ), while the anterograde transport (in the direction of the axon terminal and toward the plus end of the microtubules) is mediated by  kinesin  ( D3 ). The transporting vesicles are endowed with several motor proteins, the ATP-binding heads of which interact with the surface of the microtubule in an alternating and ...

  Transmitter Receptors There are two categories of neurotransmitter receptors:  ligand-gated ion channels  and  transmitter receptors  coupled to an intra-cellular guanosine triphosphate-(GTP-) binding protein ( G protein ). Ligand-gated Ion Channels Ligand-gated ion channels consist of differ-ent subunits ( A1 ) that are inserted into the cell membrane ( A2 ). Binding of the neu-rotransmitter to the specific receptor causes the channel to become permeable to certain ions ( B ). Excitatory amino acid receptors. Recep-tors for the excitatory transmitter  glutamate  are classified according to the synthetic li-gands binding to them. There are three types of  glutamate-gated ion channels : the AMPA (aminohydroxymethylisoxazole-propionic acid) receptor ( C3 ), the NMDA ( N -methyl- D -aspartate) receptor ( C4 ), and the kainate receptor. Binding to the AMPA re-ceptor causes an influx of sodium ions, thus leading to depolarization of the cell. Simi-la...

  Synaptic Transmission The synaptic transmission is essentially characterized by three processes:   1            Conversion of the  action potential  arriv-ing at the axon terminal into a chemical signal. Depolarization results in the opening of calcium channels ( C5 ) and in the  influx of calcium , which, mediated by certain proteins, causes fusion of synaptic vesicles ( C6 ) with the presynaptic mem-brane and  release of the transmitter  into the cleft ( C7 ). 2            The released transmitter binds to  specificreceptors . 3            In the case of ligand-gated ion channels, this results in their opening for certain ions. In the case of glutamatergic recep-tors, the influx of Na +  or Ca 2 +  causes depolarization of the postsynaptic mem-brane ( excitatory postsynaptic potential ,...

  Neuronal Systems Groups of neurons that have the same transmitter and the axons of which often form dense fiber bundles are described according to their transmitter substance as  cholinergic,noradrenergic, dopaminergic, serotoninergic, GABAergic , or  peptidergic systems . The im-pulse can be transmitted to neurons of the same type or to neurons with a different neurotransmitter. In the sympathetic nervous system, for example, the neurons in the spinal cord are cholinergic; however, transmission in the peripheral ganglia switches to  noradrenergic  neurons . Noradrenergic, dopaminergic, and sero-toninergic neurons are located in the  brainstem . Noradrenergic neurons form the  locus caeruleus  ( A1 )  and cell groups in the lateral part of the  reticular formation  of the medulla oblongataand the pons; their fibers project to the hy-pothalamus, to the limbic system, diffusely into the neocortex, and to the anterior horn and latera...