Summation, Neurotransmitters, Neuromodulation & Neuronal Circuits

Neural integration

Process by which all excitatory and inhibitory input from presynaptic neurons is combined to produce one cumulative effect on the postsynaptic neuron.

Summation

Adding the effects of several postsynaptic potentials to influence the membrane potential at the trigger zone.

Temporal summation

Summation caused by repeated neurotransmitter release from a single presynaptic neuron; postsynaptic potentials occur rapidly enough to add together.

Spatial summation

Summation caused by simultaneous neurotransmitter release from multiple presynaptic neurons onto one postsynaptic neuron.

EPSPs and IPSPs in summation

EPSPs increase the likelihood of reaching threshold, while IPSPs reduce it; both can combine during temporal and spatial summation.

Three ways synaptic transmission is terminated

Diffusion and absorption, degradation in the synaptic cleft, and reuptake into the presynaptic neuron.

Diffusion and absorption

Neurotransmitter diffuses away from the synaptic cleft and may be absorbed by a neuron or astrocyte and returned to the presynaptic neuron.

Degradation in the synaptic cleft

Enzymes in the synaptic cleft break down certain neurotransmitters; components may then be taken up by the neuron and reused.

Reuptake

Neurotransmitter is transported back into the presynaptic neuron by proteins in the axolemma and is then repackaged or degraded.

Neurotransmitter effects

Most neurotransmitters can have both excitatory and inhibitory effects depending on the receptor they bind to, and a neurotransmitter may have several receptor types.

Ionotropic receptor

Receptor that is part of a ligand-gated ion channel; directly controls ion movement and produces rapid but short-lived effects.

Metabotropic receptor

Receptor connected to intracellular G-proteins; activates a cascade of reactions involving second messengers that can open or close ion channels.

Ionotropic vs. metabotropic receptors

Ionotropic = direct, rapid, and short-lived; metabotropic = uses G-proteins and second messengers, producing slower but usually longer-lasting and more varied effects.

Second messenger

Intracellular signaling molecule, such as cyclic AMP (cAMP), produced during a G-protein-mediated cascade that can influence ion channels and cellular activity.

Acetylcholine (ACh)

Neurotransmitter used at the neuromuscular junction, in the brain and spinal cord, and in the ANS; generally excitatory but inhibitory at some PNS synapses.

Acetylcholinesterase (AChE)

Enzyme in the synaptic cleft that breaks acetylcholine into acetic acid and choline; choline can be taken back into the neuron and reused.

Biogenic amines (monoamines)

Group of neurotransmitters widely used by the CNS and PNS with diverse roles in homeostasis and cognition; include catecholamines, serotonin, and histamine.

Catecholamines

Biogenic amines synthesized from tyrosine; include norepinephrine, epinephrine, and dopamine.

Norepinephrine (noradrenaline)

Catecholamine used by the ANS to influence heart rate, blood pressure, and digestion and by the brainstem to regulate attention, feeding, and the sleep/wake cycle.

Epinephrine (adrenaline)

Catecholamine used by the ANS with effects similar to norepinephrine; more widely used as a hormone by the endocrine system.

Dopamine

CNS catecholamine involved in coordinating movement, emotion, and motivation.

Serotonin

Biogenic amine synthesized from tryptophan in the brainstem; involved in mood, emotions, attention, cognition, motor and feeding behaviors, and daily rhythms.

Histamine

Biogenic amine synthesized from histidine; involved in arousal and attention in the CNS and allergic responses outside the CNS.

Glutamate

Most important excitatory neurotransmitter in the brain; generally produces EPSPs by opening Na+ or Ca2+ channels.

GABA (gamma-aminobutyric acid)

Major inhibitory neurotransmitter, especially in the brain; induces IPSPs by opening ligand-gated Cl- channels.

Glycine

Major inhibitory neurotransmitter used most widely in the spinal cord; induces IPSPs by opening ligand-gated Cl- channels.

Major excitatory vs. inhibitory amino acid neurotransmitters

Glutamate is the major excitatory neurotransmitter of the brain; GABA and glycine are major inhibitory neurotransmitters, with GABA especially important in the brain and glycine in the spinal cord.

Neuropeptides

Neurotransmitters synthesized in the cell body that produce a wide variety of effects; examples include substance P, opioids, and neuropeptide Y.

Substance P

Neuropeptide released from sensory afferent fibers that transmit pain and temperature sensations; also released by neurons in the brain, spinal cord, and gut.

Opioids

Group of more than 20 neuropeptides, including endorphins, enkephalins, and dynorphins, that produce pain relief and act as nervous system depressants.

Neuropeptide Y

Neuropeptide involved in feeding behavior and may help mediate hunger or the feeling of fullness.

Neuromodulation

Alteration of synaptic transmission by neurotransmitters in ways other than directly exciting or inhibiting the postsynaptic neuron.

Neuromodulators

Neurotransmitters that bind metabotropic receptors and alter processes such as neurotransmitter release, postsynaptic sensitivity, or gene transcription; their effects may last minutes to days.

Neurohormones

Hormones that modulate the activity of neurons.

Neuronal pool

Group of interneurons within the CNS that performs a common function defined by the synaptic connections of the group.

Input neurons

Neurons that initiate the series of signals in a neuronal pool and branch to multiple neurons; their signals may generate EPSPs or IPSPs.

Neuronal circuit

Pattern of synaptic connections between neuronal pools; each pool receives input and produces output that travels to additional pools.

Diverging circuit

Circuit in which one input axon branches to contact multiple postsynaptic neurons, which may then branch to even more neurons.

Functions of diverging circuits

Allow one neuron to communicate with multiple parts of the brain or body; may amplify a signal to many muscle fibers or send information through multiple tracts to different brain areas.

Converging circuit

Circuit in which axons from multiple input neurons converge onto a single postsynaptic neuron, allowing spatial summation.

Functions of converging circuits

Important in skeletal muscle control and in nervous system responses to sensory stimuli.

Inhibitory negative feedback mechanism

Mechanism that prevents excessive CNS excitation by using neuromodulators to inhibit input neurons or postsynaptic neurons when they become overly excited.

Synaptic fatigue

Progressive weakening of synaptic transmission during prolonged, intense excitation due to decreased neurotransmitter sensitivity and downregulation of receptors.