Anatomy & Physiology

Anatomy & Physiology Study Guide whereas inhibitory neurotransmitters cause hyperpolarization and suppress the generation of action potentials. • The effect of a neurotransmitter on the postsynaptic membrane depends on the properties of the receptor, not on the nature of the neurotransmitter. • Cholinergic synapses release the neurotransmitter acetylcholine (ACh). Communication moves from the presynaptic neuron to the postsynaptic neuron across a synaptic cleft. A synaptic delay occurs because calcium influx and the release of the neurotransmitter takes an appreciable length of time. Neurotransmitters and neuromodulators have various functions: • Adrenergic synapses release norepinephrine (NE), also called noradrenaline. Other important neurotransmitters include dopamine, serotonin, and gamma aminobutyric acid (GABA). • Neuromodulators influence the postsynaptic cell's response to neurotransmitters. • Neurotransmitters can have a direct or indirect effect on membrane potential, or they can exert their effects via lipid-soluble gases that diffuse across the plasma membrane. Information processing by individual neurons involves integrating excitatory and inhibitory stimuli: • Excitatory and inhibitory stimuli are integrated through interactions between postsynaptic potentials. This interaction is the simplest level of information processing in the nervous system. • A depolarization caused by a neurotransmitter is an excitatory postsynaptic potential (EPSP). Individual EPSPs can combine through summation, which can be either temporal (occurring at a single synapse when a second EPSP arrives before the effects of the first have disappeared) or spatial (resulting from the cumulative effects of multiple synapses at various locations). • Hyperpolarization of the postsynaptic membrane is an inhibitory postsynaptic potential (IPSP). • The most important determinants of neural activity are EPSP–IPSP interactions. • 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. Achieve Page 158 of 368 ©2018