Lecture Quiz 7 Flashcards
Define what a synapse is
a junction that mediates information transfer between neurons or between a neuron and an effector cell
What are electrical synapses?
cells that are closely bounded and electrical stimulus is passed directly from one cell to another
fast but rare in NS
Where are electrical synapses mostly found?
embryonic nervous system
cardiac muscle
What are chemical neurons?
not in direct contact
use chemical substance (neurotransmitter) to convey stimulus from one cell to another
What are the three types of chemical synapses and what do they connect?
axodendritic - axon to dendrite
axosomatic - axon to neuron body
axoaxonix - axon to axon
How does direction of impulse define the neuron?
neurons conducting impulses towards the synapse are presynaptic
neurons conducting impulses away from the synapse are postsynaptic
What do presynaptic neurons contain?
presynaptic membrane with voltage-gated Ca2+ channels
synaptic knob where vesicles of neurotransmitter are found
How do neurotransmitters end up in the axon terminal?
most neurotransmitters are made in the cell body of a presynaptic neuron and then transported down the axon via microtubules to the axon terminal
this process is called axonal transport
What is the synaptic cleft?
a small space between the neurons
what is found in the postsynaptic neuron?
postsynaptic membrane (body or dendrite) has receptors to neurotransmitter the receptors are connected to ion channels ligand-gated channels are found here (ligand = neurotransmitter)
What do synapses provide?
the transduction of the electrical stimulus from cell to cell
Briefly describe the signal transduction between presynaptic and postsynaptic neurons
presynaptic neuron generates action potential that propagates along the axon to the synaptic knob
electrical impulse is converted into chemical and transmitted to the postsynaptic neuron
What happens when the action potential arrives at the synaptic knob?
a change in membrane polarity causes voltage-gated Ca2+ channels to open
What happens at synapse after Ca2+ channels open?
Ca2+ enters the cell down the concentration gradient and stimulates exocytosis of the neurotrasmitter vesicles into the synaptic cleft
done through facilitated diffusion which does not require ATP
What happens when the neurotransmitter is released from the synaptic knob?
neurotransmitter travels across the synaptic cleft and binds to receptors on the postsynaptic membrane
activation of the receptors causes chemically gated channels to open
chemical transmission requires diffusion
What is the rate-limiting step of neural transmission?
when the neurotransmitter is released and binds to postsynaptic membrane
What happens after ion channels open on postsynaptic membrane?
this allows ions to flow through the membrane
this generates a graded potential (postsynaptic potential) which is also an electrical impulse
What happens if the neurotransmitter activates sodium channels?
Na inflow results in depolarization on the postsynaptic membrane
excitatory postsynaptic potential (EPSP) results
neurotransmitter is considered excitatory neurotransmitter
What happens if the neurotransmitter activates potassium or chloride channels?
K+ outflow or Cl- inflow results in hyperpolarization on the postsynaptic membrane
inhibitory postsynaptic potential (IPSP) results
neurotransmitter is considered inhibitory
Describe postsynaptic potentials
graded potentials
decremental, vary in amplitude and duration, may be summed
What is temporal summation?
results from one presynaptic neuron firing repeatedly
What is spatial summation?
results from several presynaptic neurons firing simultaneously
How does the transmission of the impulse in the synapse end?
neurotransmitter is removed from the synaptic cleft
What is the first way neurotransmitter is removed from the synaptic cleft?
reuptake by the presynaptic membrane: the whole neurotransmitter molecule is taken back into the axon terminal that released it
this is commonly found with norepinephrine, dopamine, and serotonin
What is the second way neurotransmitter is removed from the synaptic cleft?
diffusion of the synapse
glial cells may facilitate this process
What is the third way neurotransmitter is removed from the synapse?
degradation by special enzymes
a specific enzyme changes the structure of the neurotransmitter so it is not recognized by the receptor
ex: cholinesterase breaks down Ach -> acetate + choline
presynaptic terminal reuptakes choline for Ach synthesis
What type of neurotransmitter is acetylcholine and where is it produced in the neuron?
excitatory or inhibitory
main skeletal muscle excitatory neurotransmitter
produced in axon terminal
Acetyl CoA + choline -> Ach + CoA
Where is acetylcholine found?
CNS - basal nuclei and motor cortex
PNS - efferent somatic NS, parasympathetic NS (entirely), parts of sympathetic (preganglionic)
What happens if a patient has low Ach?
basal nuclei are destroyed
Alzheimer’s disease
What are catecholamines?
chemically related group of neurotransmitters
synthesized from a common precursor, amino acid tyrosine
Describe norepinephrine
excites or inhibits
reuptaken then degraded by monoamine oxidase (MAO) in the presynaptic neuron
Where is norepinephrine found?
CNS
PNS - sympathetic nervous system between the neuron and effector organ
What do most antidepressants use and why?
monoamine oxidase (MAO) slows down the destruction of epinephrine
Describe dopamine
excites but also inhibits
in the brain, it affects mood, sleep, attention and learning
What do abnormal levels of dopamine do to a patient?
low - parkinson’s
high - schizophrenia
Describe serotonin
indolamine
synthesized from tryptophan
mostly inhibitory
important transmitter in control of sleep
What happens to the body when LSD is taken?
binds to serotonin receptors
blocks inhibitory effect
leads to hallucinations
Describe GABA
gamma amino butyric acid
main inhibitory neurotransmitter in CNS
What happens when there is a loss of GABA in the cortex?
hyperactivity in the neurons such that is seen in epilepsy
Describe glutamate
main excitatory neurotransmitter in CNS
Describe endorphins
polypeptides
“natural pain killers” released in the brain
stimulate the opioid receptors in the CNS eliciting a pain-killing effect
produced in large amounts during labor
What is a neuronal pool?
a functional circuit of the CNS
each pool integrates incoming information from receptors or other neuronal pools and relays the information to other areas
How is the strength of a stimulus received in a neuronal pool?
it is assessed by the nervous system based on neural recruitment (what neurons are firing and how many neurons are firing) and the rate of action potentials
Describe how an input fiber enters a neuronal pool
it branches and synapses with numerous neurons
What happens in the discharge zone of a neuronal pool?
neurons form multiple synapses with the presynaptic neuron
all EPSPs will summate and generate action potential
a neuron will fire even when it receives a weak stimulus from the presynaptic neuron
What happens in the facilitated zone of a neuronal pool?
neurons form single or few synapses with presynaptic neuron
action potential will result only after multiple stimulations from the presynaptic neuron
Define neurotransmitter
special chemicals that transmit impulse between two neurons or neuron and effector cell
What is neural integration?
different parts of nervous systems have to be working together
they are integrated into one system
Describe diverging (amplifying) circuits
common in sensory (afferent) and motor (efferent) pathways
incoming fiber triggers a response from several neurons and the signal is amplified
they are characterized by an incoming fiber that triggers responses in an ever-increasing number of fibers along the circuit
Describe converging circuits
common in sensory and motor pathways
characterized by the reception of input from many sources
funnels the sources to a given circuit, resulting in strong stimulation or inhibition
Describe reverberating (oscillating) circuits
input signal travels through a chain of neurons, each sending axons to the following and previous neurons
signals travel through the circuit over and over again
involved in the control of rhythmic cycles (sleep-wake, breathing)
Describe parallel after-discharge circuits
involved in complex activities
characterized by the stimulation of several neurons parallel to the stimulating neuron
input from one neuron is delivered to several neurons simultaneously
stimulate single output neuron
signal not delivered simultaneously providing multiple stimulations to the output neuron
involved in complex mental processing
What is serial processing?
exemplified by spinal reflexes
involves sequential stimulation of the neurons in a circuit
response is predictable
ex: reflex arc
What is parallel processing?
results in inputs stimulating many pathways simultaneously
vital to higher level mental functioning
not always predictable
ex: diff people have diff rxns to snakes
What do exocrine glands do (brief)?
produce and release substances via ducts onto the surface or the organ’s lumen
What do autocrine glands do (brief)?
produce chemicals that are released into the extracellular space and affect the same cells that produced them
What do paracrine cells do?
affect cells in the immediate proximity of the secreting cells
What do endocrine glands do?
ductless
produce hormones and release them into the blood stream
hormones affect target cells located elsewhere in the body
What are the first and second most controlling systems in the human body?
- nervous
2. endocrin
Where are only endocrine glands found?
pituitary thyroid parathyroid adrenal pineal thymus
Where are end/exocrine glands found?
pancreas
ovaries
testes
Where are neural/endocrine glands found?
hypothalamus
What organs have a small endocrine component in the body?
adipose tissue intestinal wall stomach kidneys heart
What are hormones?
the agents which are released into the blood and elicit specific effects on cells which possess special receptors to recognize the hormone
specific cells are known as target cells
How are hormones found in the blood?
low concentration
the transduction of the signal to the target cells is amplified at every step
At what rate are hormones produced?
not constant, but rather in a pulsatile manner
How long do effects last on target cells?
prolonged
can be first observed in seconds to days
What does hormone metabolism include?
clearance by the liver and kidneys
intracellular degradation
Describe amino acid and protein hormones
produced from amino acids and are usually hydrophillic (exception: thyroid hormone)
examples: insulin, glucagon, growth hormone, thyroxin, epinephrine
Describe steroid hormones
synthesized from cholesterol
hydrophobic
examples: glucocorticoids, mineralocorticoids, sex hormones
these require a transporter
Describe eicosanoid hormones
synthesized from arachidonic acid
act only as local hormones (para/autocrine)
ex: leukotrienes, prostaglandins, thromboxanes
How does polarity affect hormone-cell interaction?
Cell membrane is hydrophobic
if hormone is hydrophilic, such as amino acid hormones, a hydrophilic target cell on the surface is necessary to attract these hormones
What do hormones interact with on target cells?
cell receptor
very specific and have a high affinity to the hormone
What are the five different ways hormonal receptor stimulation can change normal processes in target cells?
change in membrane permeability stimulation of protein synthesis activation/deactivation of enzymes change in the rate of secretion change in the rate of cell division
What are the two major types of hormone receptors?
surface receptors
cytoplasmic/nuclear receptors
Describe the tyrosine kinase hormonal mechanism of action
receptor is an integral protein
tyrosine kinase is the intracellular domain of the receptor
hormone binds to the extracellular part
conformation changes cause activation (phosphorylation) of tyrosine kinase
tyrosine kinase activates intracellular enzymes
ex: insulin
What are some of the major properties of hormones?
low concentrations signal amplification pulsatile secretion have lag times prolonged effects transport metabolism - clearance by liver and kidneys
Describe the mechanism of action via 2nd messenger
hormone binds to the receptor on the outer surface of the target cells
receptors conformation change activates relay protein (g-protein)
g-protein stimulates effector protein, adenylate cyclase
adenylate cyclase converts ATP into cyclic AMP which acts as a 2nd messenger
cAMP activates intracellular enzymes that trigger a cascade of reactions in the cytoplasm
Describe the mechanism of action via 3rd messenger
hormone binds to receptor
conformation changes relay protein (g-protein)
g-protein stimulates effector protein, phospholipase C
phospholipase splits PIP2 into DAG and IP3, both act as 2nd messengers and activate intracellular enzymes
IP3 triggers release of Ca2+ from endoplasmic reticulum
Ca2+ (3rd messenger) also activates intracellular enzymes
Describe the gene activation mechanism aka stimulation of intracellular receptors
hormones are hydrophobic and can diffuse into the cell, where they bind to intracellular receptors
hormone-receptor complex enters the nucleus and binds to the promoter protein on the DNA
gene is activated and transcription (mRNA synthesis) begins
mRNA travels to cytoplasm where its used to make a protein encoded by activated gene - translation
What is significant about thyroid hormone?
It is the only hydrophobic amino acid hormone
also binds to receptors in the nucleus as opposed to the cytoplasm like other hormones
The physiologic effects of hormones on the target cells depend largely on what two factors?
sensitivity and number of receptors on the target cells
hormone concentration in the blood and extracellular fluid
How is the concentration of hormones regulated by rate of production?
hormonal stimulation - secretion of hormones in direct response to changing blood levels
neural stimulation - nervous system modifies the stimulation of endocrine glands and their negative feedback mechanisms
endocrine stimulation - endocrine glands can be stimulated by the hormones produced by other endocrine glands
What is negative feedback?
seen when the output of a pathway inhibits input to the pathway (most common)
what is positive feedback?
when the output of a pathway stimulates the input to the pathway
How is rate of delivery of hormones controlled?
amino acid-derived hormones are easily dissolved in the plasm and do not require a carrier
steroid hormones are transported via binding protein
rate of delivery can be regulated via vascular system (vasoconstriction/dilation) and availability of carrier proteins
What can a decreased number of hormone receptors do to cells?
influences the effects that hormones exhibit on the target cell
How are hormones regulated by rate of elimination?
hormones are excreted by the liver and kidneys as well as destroyed by intracellular enzymes in the target cells
