SYNAPSES AND SYNAPTIC TRANSMISSION A synapse is the

SYNAPSES AND SYNAPTIC TRANSMISSION • A synapse is the functional connection between a neuron and a second cell. In the CNS, this other cell is also a neuron. • In the PNS, the other cell may be either a neuron or an effector cell within a muscle or gland. • Although the physiology of neuron-neuron synapses and neuron-muscle synapses is similar, the latter synapses are often called myoneural, or neuromuscular, junctions. • Neuron-neuron synapses usually involve a connection between the axon of one neuron and the dendrites, cell body, or axon of a second neuron.

• These are called, respectively, axodendritic, axosomatic, and axoaxonic synapses. • In almost all synapses, transmission is in one direction only—from the axon of the first (or presynaptic) neuron to the second (or postsynaptic) neuron. • Most commonly, the synapse occurs between the axon of the presynaptic neuron and the dendrites or cell body of the postsynaptic neuron.

Types of Synapses in the nervous system can be divided into two main types; chemical and electrical synapse CHEMICAL SYNAPSES • In a chemical synapse, the first neuron (presynaptic neuron) secretes a chemical substance called a neurotransmitter at the synapse, which then act on receptor proteins in the membrane of the postsynaptic neuron to excite the neuron, to inhibit it or to modify its sensitivity in some other way. • Almost all the synapses in the central nervous system of human beings are chemical synapses. • Chemical synapses always transmit signals in one direction. • The importance of one-way conduction mechanism is that it allows signals to be directed towards specific goals. • The synaptic delay at chemical synapses is about 0. 5 millisecond.

• CHEMICAL SYNAPSES

ELECTRICAL SYNAPSES • Electrical synapses are also present in the central nervous system of animals, from invertebrate to mammals. • At an electrical synapse, a change in the membrane potential of one cell is transmitted to the other cell by direct flow of current. • Because current flows directly between the two cells that make an electrical synapse, there is no synaptic delay. • Electrical synapses allow conduction in both directions. • Cells that form electrical synapses typically are joined by gap junctions. • At gap junctions, the plasma membranes of the coupled cells are very close (< 3 nm) and the connections are aligned to form channels. • The channels allow the passage of water soluble molecules up to molecular weights 1200 -1500 from one cell to the other. • These channels are the pathways for electrical current flow between the coupled cells. • Electrical synapses are particularly useful in reflex pathways in which rapid transmission between cells is necessary or when synchronous response of a number of neurons is required.

• There are many non-neuronal cells that are coupled by gap junctions; examples are hepatocytes, myocardial cells, intestinal smooth muscles cells and the epithelial cells of the lens.

• ELECTRICAL SYNAPSES

Release of transmitter at presynaptic terminals • The chemical transmitter is released from the synaptic knobs when action potentials pass along the axon to the nerve endings. • When the action potential depolarizes the terminal, large numbers of calcium ions, along with sodium ions that cause most of the action potential, flow into the terminal. • The membranes of the vesicles fuse with the nerve cell membrane and the area of fusion breaks down, releasing the neurotransmitter it contains by the process of exocytosis. • Calcium ions trigger this process after action potential has opened the voltage-gated calcium ion channels in the synaptic knob.

• As calcium ions enter the ending, exocytosis is increased. • The action of calcium ions is terminated by its rapid sequestration within the nerve ending. • The quantity of transmitter substance that is released into the synaptic cleft is directly related to the number of calcium ions that enter the terminal.

Criteria for a Neurotransmitter Substance Four basic criteria must be met in order for any chemical to be classified as a neurotransmitter • Nerves must have the required enzymes to synthesize the chemical, and the chemical must be present in the nerves. • Stimulation of the nerves must cause the release of the transmitter chemical. • Following its release, the chemical must react with a specific receptor on the membrane of the postsynaptic neuron or other cell, and thereby cause one or more specific biologic effects. • There must be local mechanisms present that can rapidly turn off the biologic effects of the chemical.

NEUROMUSCULAR TRANSMISSION • When a nerve impulse reaches the neuromuscular junction, some of the vesicles in the axon endings rupture and acetylcholine is released. • Acetylcholine diffuses across the synaptic cleft and combines with a receptor in the muscle membrane. • The acetylcholine increases the permeability of the muscle fibre membrane to Na+ ions. • Sodium ions enter inside the muscle fibre causing depolarization. • When the depolarization reaches the firing level, it produces an action potential. • The action potential is propagated on both sides of the motor end plate along the membrane of the muscle fibre.

• Once the action potential is produced, the secreted acetylcholine is rapidly hydrolyzed by cholinestrase to the inactive acetic acid and choline. • The rapid removal of the acetylcholine prevents reexcitation after the muscle fibre has recovered from the first action potential. Properties of neuromuscular transmission • It is unidirectional • There is a delay of about 0. 5 millisecond • It is easily fatigued • It can be blocked by drugs such as curare • Effects of ions.