Anatomy & Physiology I and II

Anatomy & Physiology Study Guide • In presynaptic inhibition, GABA release at an axoaxonic synapse and inhibits the opening of voltage-gated calcium channels in the synaptic knob. This inhibition reduces the amount of neurotransmitter released when an action potential arrives at the synaptic knob. • In presynaptic facilitation, activity at an axoaxonic synapse increases the amount of neurotransmitter releasedwhen an action potential arrives at the synaptic knob. This increase enhances and prolongs the effects of the neurotransmitter on the postsynaptic membrane. • The neurotransmitters released at a synapse have excitatory or inhibitory effects. The effect on the initial segment reflects an integration of the stimuli arriving at any moment. The frequency of generation of action potentials depends on the degree of depolarization above the threshold at the axon hillock. • Neuromodulators can alter either the rate of neurotransmitter release or the response of a postsynaptic neuron to specific neurotransmitters. Neurons may be facilitated or inhibited by extracellular chemicals other than neurotransmitters or neuromodulators. • The effect of a presynaptic neuron's activation on a postsynaptic neuron may be altered by other neurons. • The greater the degree of sustained depolarization at the axon hillock, the higher the frequency of generation of action potentials. At a frequency of about 1000 per second, the relative refractory period has been eliminated, and further depolarization will have no effect.

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