Lesson 14 Flashcards
synapse
point where an axon terminal meets the next cell
electrical synapse
direct physical contact between cells, links cytoplasm of adjacent cells
electrical synapse characteristics (4)
- locked together at gap junctions (connexons)
- allows ions to pass between cells
- produce continuous local current and action potential propagation
- found in areas of the brain, eye, ciliary ganglia (PNS), cardiac muscle cells
chemical synapse
signal transmitted across a gap by chemical neurotransmitters; cells do not touch
who discovered the synaptic cleft?
Ramon y Cajal
how did Otto Loewi demonstrate that neurons can communicate by releasing chemicals?
Flooded exposed hearts of two frogs with saline, stimulated the vagus nerve of the first frog and its heart slowed. He removed the saline from the first frog and found found it slowed heart rate of the second frog.
where do electrical synapses occur?
neurons, neuroglia, cardiac, and single-unit smooth muscle
advantage of electrical synapses
much faster, no delay for release, diffusion, and binding of neurotransmitter
disadvantages of electrical synapses
cannot integrate information
synaptic cleft
gap between presynaptic neuron and postsynaptic neuron; typically only 20 um wide
postsynaptic density
the postsynaptic neuron membrane contains a certain density of neurotransmitter receptors and ion channels
synaptic delay
since chemical synapses are not directly in contact with the postsynaptic cell and require the release of a neurotransmitter to stimulate/inhibit the target cell, there is delay of 0.2-0.5 msec
If information is being passed along a series of neurons…
the fewer synapses found between the transmission of the information means the faster response; important for reflexes
synaptic fatigue
occurs when neurotransmitters cannot recycle fast enough to meet demands of intense stimuli
two major chemical categories of neurotransmitters
- acetylcholine
- amino acids
acetylecholine
formed from acetic acid and choline
amino acids
include glycine, glutamate, aspartate, and y-aminobutyric acid (GABA)
monoamines (biogenic amines)
synthesized from amino acids; ex. serotonin and the catecholamines
catecholamines
epinephrine, norepinephrine, and dopamine
serotonin
a CNS neurotransmitter that affects attention and emotional states. decreased serotonin production may be responsible for some cases of severe chronic depression
norepinephrine (NE)
aka noradenaline; released at adrenergic synapses; has excitatory and depolarizing effects and is widely distributed in the brain and portions of the ANS
dopamine
a CNA neurotransmitter that may be excitatory or inhibitory that is used for movement control. Damage to neurons that produce dopamine involved in Parkinson’s disease and cocaine use
monoamines affect the cell _____
indirectly, through a second messenger system (G protien)
gases
nitric acid (NO) and carbon monoxide (CO) synthesized as needed rather than store in vesicles; these gases affect cells by diffusing into a cell and affecting intracellular enzymes
neuropeptides
chains of 2-40 amino acids; ex. cholecystokinin (CCK) and endorphins; indirect effect
- stores in large secretory granules
- function as hormones or neuromodulators
endorphins relieve pain perception by inhibiting the release of the neuropeptide _____ at synapses that relay pain sensations
substance P
three kind of synapses
- excitatory cholinergic synapse (ACh)
- inhibitory GABA-ergic synapse (GABA)
- excitatory adrenergic synapse (NE)
cholinergic synpase
acetylcholine is the neurotransmitter; located between all neuromuscular junctions, many CNS synapses, all neuron-to-neuron, neuromuscular synapses in the PNS
steps in acetylcholine synaptic transmission (4)
- action potential depolarizes the axon terminal, opens voltage-gated Ca2+ channels
- Ca2+ enters and causes exocytosis of the vesicles containing ACh
- ACh diffuses across cleft, binds to postsynaptic sodium receptors that the open and allow Na+ to enter the postsynaptic cell via diffusion
- entry of Na+ depolarizes the cell by generating a postsynaptic potential
_____ is reabsorbed by the synaptic terminal to form more _____ while the _____ is absorbed by the postsynaptic cell or other cells/tissues in the area
choline, ACh, acetate
GABA-ergic synapse
y-aminobutyric acid (GABA) is an amino acid neurotransmitter
steps in a GABA synapse (3)
- action potential triggers release of GABA into synaptic cleft
- GABA receptors are chloride channels
- allows Cl- to enter the cell causing hyperpolarization of the postsynaptic membrane
- postsynaptic neuron is inhibited
adrenergic synpase
norepinephrine (NE) is the neurotransmitter
steps in a adrenergic synapse
- monoamines and neuropeptides bind to G-protein coupled receptors on postsynaptic membrane
- activate a second-messenger system such as cyclic AMP
- slower response that others
- advantage of enzyme amplification
to end a signal…
- presynaptic neuron stops releasing neurotransmitter
- neurotransmitter already in the synapse is cleared in various ways
neurotransmitter degradation
enzyme in synaptic cleft breaks down the neurotransmitter
acetylcholinesterase (AChE)
breaks down ACh into acetate and choline
reuptake
neurotransmitter or its broken down products are reabsorbed into the axon terminal
diffusion
neurotransmitter or its broken down products simply diffuse away from the synapse into nearby ECF
neuromodulators
chemicals secreted by neurons that have long term effects on groups of neurons
what can neuromodulators cause? (2)
- may alter the rate of neurotransmitter synthesis, release, reuptake, or breakdown
- may adjust sensitivity of postsynaptic membranes
Nitric oxide (NO)
simple neuromodulator; gas enters the postsynaptic cell and activates second messenger pathways (Ex. relaxing smooth muscle)
_____ are chains of amino acids that can act as neuromodulators
neuropeptides
_____ and _____ are neuropeptides that inhibit pain signals in the CNS (aka opioids)
enkephalins and endorphins
neural integration
the ability to process, store, and recall information and use it to mae decisions
What is a tradeoff to neural integration?
Chemical transmission involves synaptic delay that makes information travel slower that is would if there was no synapse
characteristics of neural integration
- chemical synapses allow for decision making
- brain cells are incredibly well connected, allowing for complex integration
two types of postsynaptic potentials
EPSP (excitatory postsynaptic potential)
IPSP (inhibitory postsynaptic potential)
EPSP - excitatory postsynaptic potential
voltage change from the RMP toward threshold
IPSP - inhibitory postsynaptic potential
voltage becomes more negative than it is at rest
what does an EPSP usually result from?
Na+ flowing into the cell
what does and IPSP usually result from?
Cl- entry or K+ exiting from the cell
what type of neurotransmitters cause EPSPs?
glutamate and aspartate
what type of neurotransmitter cause IPSPs?
glycine and GABA
what type of neurotransmitter can both excite and inhibit cells, depending on its type?
acetylcholine and norepinephrine
ACh _____ skeletal muscle and _____ cardiac muscle
excites, inhibits
summation
the process of adding up postsynaptic potentials and responding to their net effect
where does summation occur?
in the trigger zone
a neuron’s response depends on whether or not the _____ input is excitatory or inhibitory
net
temporal summation
a single synapse generates EPSPs so quickly that each is generated before the previous one fades
spatial summation
EPSPs from several different synapses add up to threshold at the axon hillock
presynaptic facilitation
occurs when one presynaptic neuron enhances another one; increases necessary synaptic transmisison
presynaptic inhibition
occurs when one presynaptic neuron suppresses another; reduces of halts unwanted synaptic transmission
neural coding
converting stimulus information into meaningful patterns of action potentials
labeled line code
mechanism for transmitting qualitative information; depends on what neurons fire
ex. optic nerve input labeled as light
heavy metal poisoning
chronic exposure to heavy metals can damage neuroglia and lead to demyelination which causes axons to irreversibly deteriorate (arsenic poisoning)
diphtheria
disease caused by bacterial toxin
- damages Schwann cells and destroys PNS myelin sheath
- can lead to fatal paralysis
- rare occurrence since disease vaccine is available
multiple sclerosis
autoimmune disorder caused by inflammation of nerve triggered by immune cells (immune cells probably triggered by virus of defective gene)
- demyelination disease affecting CNS axons
- vision, speech, balance, and motor coordination affected
what is multiple sclerosis treated with?
corticosteroids (anti-inflammatory agents) and interferon to slow the disease progression
Guillain-Barre syndrome
autoimmune disorder that is trigger by a virus;
- demyelination disease affecting peripheral nerves
- sensation of weakness or tingling of the legs that spreads to arms
- progression leads to paralysis (diaphragm also affected)
- most fully recover but some continue to have residual weakness
