12.5 - Synapses

synapse

point where a nerve endings meets the next cell in line

presynaptic neuron

neuron that releases a neurotransmitter into a synapse

axodendritic synapse

synapse between a presynaptic neuron and dendrite

axosomatic synapse

synapse between a presynaptic neuron and cell body

axoaxonic synapse

synapse between a presynaptic neuron and the axon of a postsynaptic neuron

postsynaptic neuron

neuron that responds to the neurotransmitter sent by the presynaptic neuron

electrical synapses

synapses where adjacent cells are joined by gap junctions and ions diffuse directly from one cell into the next

electrical synapses (advantage)

quick transmission because there is no delay for the release and binding of neurotransmitter

electrical synapses (function)

important in synchronizing the activity of local suites of neurons in certain regions of the brain

electrical synapses (disadvantage)

cannot integrate information and make decisions

chemical synapses

synapses in which neurons communicate by neurotransmitters

chemical synapses (function)

serve as the site of learning and memory, target of many prescription drugs, and site of action of drugs of addiction

synaptic cleft

gap between two neurons

cell-adhesion molecules (CAMs)

proteins in the membrane of a neuron that reach into the synaptic cleft and link to the CAMs of the other cell

cell-adhesion molecules (function)

prevent the two neurons from falling apart at the synapse and hold them in proper alignment for the release and receipt of the neurotransmitter

synaptic vesicles

bubbles in the presynaptic axon terminal that store neurotransmitters

active zone

portion of the presynaptic plasma membrane at which some synaptic vesicles dock, ready to release neurotransmitter on demand

postsynaptic density

area of the receiving neuron with a high concentration of neurotransmitter receptors and ion channels, especially on its dendrites and cell body

retrograde signaling

backward signaling of dendrites to presynaptic neurons

neurotransmitters

molecules that are synthesized by a neuron, released when a nerve signal reaches an axon terminal, and have a specific effect on a receiving cell’s physiology

six categories of neurotransmitters

acetycholine, amino acid neurotransmitters, monoamines, purines, gases, and neuropeptides

acetylcholine

neurotransmitter formed from acetic acid and choline

amino acid neurotransmitters (example)

glycine, glutamate, aspartate, and gamma-aminobutyric acid (GABA)

monoamines (biogenic amines)

neurotransmitters synthesized from amino acids by removal of the COOH group

monoamines (example)

epinephrine, norepinephrine, dopamine, serotonin, and histamine

catecholamines

subclass of monoamines including epinephrine, norepinephrine, and dopamine

purines (example)

adenosine and adenosine triphosphate (ATP)

gases

inorganic exceptions to the usual definition of neurotransmitters that are synthesized as needed and diffuse out of the axon terminal into the receiving neuron

gases (example)

nitric oxide (NO) and carbon monoxide (CO)

neuropeptides

chains of 2 ro 40 amino acids stored in secretory granules that can also function as hormones or neuromodulators

neuropeptides (example)

cholecystokinin (CCK) and endorphins

gut-brain peptides

neuropeptides produced by neurons and the digestive tract

cholinergic synapse

synapse that employs ACh as its neurotransmitter

first step of an excitatory cholinergic synapse

The arrival of a nerve signal at the axon terminal opens voltage-gated calcium channels.

second step of an excitatory cholinergic synapse

Ca²⁺ enters the terminal and triggers exocytosis of the synaptic vesicles, releasing ACh.

third step of an excitatory cholinergic synapse

Empty vesicles drop back into the cytoplasm to be refilled with ACh, while synaptic vesicles in the reserve pool move to the active sites and release their ACh.

fourth step of an excitatory cholinergic synapse

ACh diffuses across the synaptic cleft and binds to ligand-gated channels on the postsynaptic neuron. These channels open, allowing Na⁺ to enter the cell and K⁺ to leave.

fifth step of an excitatory cholinergic synapse

As Na⁺ enters, it spreads out along the inside of the plasma membrane and depolarizes it, producing a local voltage shift called the postsynaptic potential. If the postsynaptic potential is strong and persistent enough, it opens voltage-gated ion channels in the trigger zone and causes the postsynaptic neuron to fire.

GABA-ergic synapses

synapses that employ GABA as their neurotransmitter

inhibitory GABA-ergic synapses (process)

similar to excitatory cholinergic synapses, except using Cl^- to make the inside even more negative than the resting membrane potential, therefore inhibiting the neuron

adrenergic synapse

synapse that employs norepinephrine (NE) as its neurotransmitter

first step of an excitatory adrenergic synapse

The unstimulated NE receptor is bound to a G protein.

second step of an excitatory andrenergic synapse

The binding of NE to the receptor causes the G protein to dissociate from it.

third step of an adrenergic synapse

The G protein binds to adenylate cyclase and activates this enzyme, which converts ATP to cyclic AMP (cAMP).

fourth step of an excitatory adrenergic synapse

cAMP can induce several alternative effects in the cell.

fifth step of an excitatory adrenergic synapse

One effect is to produce an internal chemical that binds it to a ligand-gated ion channel from the inside, opening the channel and depolarizing the cell.

sixth step of an excitatory adrenergic synapse

Another effect is for cAMP to activate preexisting cytoplasmic enzymes, which can lead to diverse metabolic changes.

seventh step of an excitatory adrenergic synapse

Yet another effect is for cAMP to induce genetic transcription, so that the cell produces new enzymes leading to diverse metabolic effects.

adrenergic synapses (advantage)

signal amplification

synaptic delay

time from the arrival of a signal at the axon terminal of a presynaptic cell to the beginning of an action potential in the postsynaptic cell

three methods of cessating the signal

neurotransmitter degradation, reuptake, and diffusion

neurotransmitter degradation

an enzyme in the synaptic cleft breaks the neurotransmitter down into fragments that have no stimulatory effect on the postsynaptic cell

reuptake

a neurotransmitter or its breakdown products are reabsorbed by transport proteins in the axon terminal, removing them from the synapse and ending their stimulatory effect

monoamine oxidase (MAO)

enzyme that reabsorbs and breaks down amino acid and monamine neurotransmitters in the axon terminal

diffusion

neurotransmitters or their breakdown products simply diffuse away from the synapse into the nearby extracellular fluid

neuromodulators

chemical signals secreted by neurons that have long-term effects on entire groups of neurons

nitric oxide (NO)

diffuses readily into a postsynaptic cell and activates second-messenger pathways with effects such as relaxing smooth muscle

enkephalins and endorphins

inhibit spinal neurons from transmitting pain signals into the brain

neuromodulators (example)

nitric oxide, enkephalins, endorphins, hormones, dopamine, serotonin, and histamine