The Neuron and the Synapse. The Neuron. Parts of the Neuron. Functions of the neuron:

Transcription

1 The Neuron and the Synapse The Neuron Functions of the neuron: Transmit information from one point in the body to another. Process the information in various ways (that is, compute). The neuron has a specialized structure for carrying out these functions, as shown in the next slide. Parts of the Neuron Cell Body (soma) Input, process Dendrites Input, process Axon Transmit Terminal Buttons Output 1

2 Neural Potentials (voltages) Resting Potential A steady charge maintained between action potentials Value about 70 to 90 mv Action Potential A rapid change in potential that moves from the base of the axon to its terminals Also called the neural impulse Sequence of Events Activity at the neuron s dendrites or cell body initiates an action potential at the base of the axon. Action potential races down the axon to the terminal buttons. Terminal buttons release a neurotransmitter substance into the synapses. (See next slide) The Synapse The synapse is a connection between a terminal button of one neuron and a dendrite (or the cell body) of another neuron. If the connection is between a terminal button and a muscle cell, then the connection is called a neuromuscular junction. 2

3 Parts of a Synapse Terminal button Vesicles containing neurotransmitter Synaptic gap Receptor proteins (in membrane of receiving neuron) Action at the Synapse Arrival of action potential at the terminal button causes vesicles to move toward synaptic gap. Those reaching the cleft burst open, dumping neurotransmitter into the cleft. Neurotransmitter locks onto receptor proteins, activating them. Resting potential of receiving neuron changes temporarily. Neurotransmitter is removed from receptor protiens, terminating the action. Two Types of Synapse Excitatory release of neurotransmitter tends to depolarize the receiving neuron, making it more likely to fire an action potential. Inhibitory release of neurotransmitter tends to hyperpolarize the receiving neuron, making it less likely to fire an action potential. 3

4 What Causes an Action Potential? Neurotransmitters arrive at the neuron s dendrites or cell body. Effects summate (add together). Result may be that the neuron becomes depolarized (reduced in charge). If depolarization reaches a threshold value (e.g., -50 mv), an action potential begins at the axon base. Drug Effects at the Synapse Activation drug binds to receptors, mimicking effect of natural neurotransmitter. Blocking drug binds to receptors but fails to activate them. Natural neurotransmitter cannot work. Reuptake inhibition drug interferes with termination of neurotransmitter action, increasing its effect. Neuromuscular Junction: Blocking Example Monkey in South-American jungle canopy struck by poison dart. Curare on dart enters bloodstream. Curare blocks acetylcholine (Ach) receptors in the neuromuscular junction. Monkey s muscles cannot contract; monkey falls out of tree. 4

5 Neuromuscular Junction: Activation Example Tetanus organism secretes toxin into bloodstream. Toxin enters neuromuscular junction, occupies Ach receptors. Toxin activates receptors and muscles contract to the max. (Imagine having severe cramps in every muscle!) Synapse: Reuptake Interference Example Certain neurotransmitters are inactivated by the chemical action of an enzyme called monoamine oxydase. MAO inhibitors slow down the action of this enzyme, prolonging the activity of the natural neurotransmitter. Used to treat depression. Synapse: Parkinson s Disease Neurons in the brainstem secrete the neurotransmitter dopamine at synapses in the basal ganglia, which are involved in movement. The effect is inhibitory. These neurons die off. Loss of inhibition produces excess motor activity (tremors, rigidity). 5