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NEURONS C O M M U N I C AT E W I T H OTHER CELLS AT SYNAPSES 34.3 NEURONS COMMUNICATE WITH OTHER CELLS AT SYNAPSES • Neurons communicate with other neurons or target cells at synapses. • Chemical synapse: a very narrow space between cells (synaptic cleft) that an action potential cannot cross – When an action potential arrives at the end of the presynaptic cell, a neurotransmitter is released that diffuses across the space. CHEMICAL SYNAPSES ARE MOST COMMON, BUT ELECTRICAL SYNAPSES ALSO EXIST • Neurotransmitters diffuse across the synaptic cleft very rapidly (short distance). • They bind to receptors on the postsynaptic cell membrane, which generates another action potential or other change. • Neurotransmitters are quickly removed from the cleft—to end signal transmission—by enzymatic breakdown, uptake by other neurons or glial cells, or reuptake by the presynaptic cell. ELECTRICAL SYNAPSE • Electrical synapse: cells are joined by gap junctions where the cytoplasm is continuous; signals cross with essentially no delay – They occur where very fast, invariant signal transmission is needed, such as neurons that control escape swimming in some fish. – Also occur where many cells must be stimulated to act together, such as fish electric organs. VERTEBRATE NEUROMUSCULAR JUNCTION IS A MODEL CHEMICAL SYNAPSE • Neuromuscular junctions: chemical synapses between motor neurons and skeletal muscle cells. • The axon of the presynaptic cell branches close to the muscle cell, creating several axon terminals (boutons) that synapse with the muscle cell. NEUROMUSCULAR JUNCTIONS • An action potential causes voltage-gated Ca+ channels to open in the presynaptic membrane, allowing Ca+ to flow in. • This induces release of the neurotransmitter acetylcholine (ACh): – ACh is stored in vesicles that fuse with the cell membrane to release ACh into the cleft by exocytosis. CONCEPT 34.3 NEURONS COMMUNICATE WITH OTHER CELLS AT SYNAPSES • ACh diffuses across the cleft and binds to receptors on the postsynaptic cell. • These receptors allow Na+ and K+ to flow through, and the increase in Na+ depolarizes the membrane. • If it reaches threshold, more Na+ voltage-gated channels are activated and an action potential is generated. • Synaptic Transmission • Neurons and Synapses • Put some Ach into it! MANY NEUROTRANSMITTERS ARE KNOWN • Three categories of neurotransmitters: • Amino acids—glutamate, glycine, and γ-aminobutyric acid (GABA) – Biogenic amines include acetylcholine, dopamine, norepinephrine, and serotonin – A variety of peptides (strings of amino acids) MANY NEUROTRANSMITTERS ARE KNOWN • In the brain, a postsynaptic neuron may have chemical synapses with hundreds or thousands of presynaptic neurons, which may use different neurotransmitters. • Receptors for a given neurotransmitter on the postsynaptic cell may be of different types with different actions. • This complexity in synapse function helps explain the complexity of brain function. SYNAPSES CAN BE FAST OR SLOW DEPENDING ON THE NATURE OF RECEPTORS • Two broad classes of receptors are recognized, they are fast or slow • Neurotransmitter receptors: • Ionotropic receptors are ligand-gated ion channels—cause changes in ion movement; response is fast and short-lived. • Metabotropic receptors are G protein-linked receptors that produce second messengers that induce signaling cascades; responses are slower and longer-lived. Khan Academy Video FAST SYNAPSES PRODUCE POSTSYNAPTIC POTENTIALS THAT SUM TO DETERMINE ACTION POTENTIAL PRODUCTION • Excitatory synapses produce graded membrane depolarizations called excitatory postsynaptic potentials (EPSPs); shift membrane potential towards threshold. • Inhibitory synapses shift membrane potential away from threshold; produce graded membrane hyperpolarizations called inhibitory postsynaptic potentials (IPSPs). FAST SYNAPSES PRODUCE POSTSYNAPTIC POTENTIALS THAT SUM TO DETERMINE ACTION POTENTIAL PRODUCTION • Each EPSP or IPSP is usually less than 1 mV, and disappears in 10–20 milliseconds. • They are graded potentials, typically produced at synapses on dendrites and the cell body. • They affect membrane potential at the axon hillock, where action potentials are generated. • Summation of the graded potentials is both temporal (must be present at the same time), and spatial. FAST SYNAPSES PRODUCE POSTSYNAPTIC POTENTIALS THAT SUM TO DETERMINE ACTION POTENTIAL PRODUCTION • The postsynaptic cell sums the excitatory and inhibitory input. • Summation determines whether the postsynaptic cell produces action potentials. • If the sum of EPSPs and IPSPs at the axon hillock is great enough to reach threshold, an action potential is produced. SYNAPTIC PLASTICITY • Synaptic plasticity: synapses in an individual can undergo long-term changes in functional properties and physical shape during the individual’s lifetime. • This may be one of the major mechanisms of learning. – Experiences at one time in life produce long-term changes in synapses, so that future experiences are processed by the nervous system in altered ways. SYNAPTIC PLASTICITY • Sea hares (mollusks) pull their gills inside when certain parts of the body are touched: • They withdraw their gills more vigorously if they have previously been exposed to a noxious agent (sensitization). • The synapses between the sensory neurons and the motor neurons for gill withdrawal are functionally strengthened—more neurotransmitter is released per impulse. • The postsynaptic cell is thus excited to a greater degree. SYNAPTIC PLASTICITY • In mammals, the hippocampus is associated with spatial learning and memory formation. • In studies of mice brains, when a circuit is repeatedly stimulated, the postsynaptic structures physically grow and the synapses strengthen functionally. • The postsynaptic receptor molecules increase, increasing response. • Synaptic plasticity has been shown to depend on second messengers, altered protein synthesis, and altered gene transcription. SYNAPTIC PLASTICITY • In studies of mice brains, when a circuit is repeatedly stimulated, the postsynaptic structures physically grow and the synapses strengthen functionally. • The postsynaptic receptor molecules increase, increasing response. • Synaptic plasticity has been shown to depend on second messengers, altered protein synthesis, and altered gene transcription. Synaptic Plasticity 1 Brain Repair - TedEd