Chapter 8: Neurons - Cellular & Network Properties Flashcards
consists of brain and spinal cord
central nervous system
Receives and processes information form sensory organs and the viscera to determine the state of the external and internal environment
central nervous system
consists of afferent and efferent neurons
peripheral nervous system
- transmit sensory and visceral info to CNS
- input
afferent neurons
- are either somatic (control skeletal muscle) or autonomic (control smooth & cardiac muscle, endocrine glands)
- output
efferent neurons
divided into sympathetic and parasympathetic systems
autonomic nervous system
excitable cells
neurons
support cells
glial cells
- cell body
- contains nucleus and most organelles
soma
reception of incoming information
dendrites
Transmits electrical impulses called action potentials
axon
Where axon originates and action potentials are initiated
axon hillock
- synaptic knob, synaptic bouton
- Releases neurotransmitter
axon terminal
what are the components of a neuron?
- soma
- dendrites
- axon
- axon hillock
- axon terminal
-have a single process called the axon -During development, the dendrite fused with the axon.
pseudounipolar neurons
have two relatively equal fibers extending off the central cell body
bipolar neurons
have
no apparent axon
anaxonic CNS internuerons
highly branched but lack
long extensions
multipolar CNS interneurons
how many dendrites does a typical multipolar efferent neuron have?
has five to seven
dendrites, each branching
four to six times
what are the forms of axonal transport?
- anterograde transport
- retrograde transport
transport from soma to axon terminal
anterograde transport
-transport from axon to soma
-microtubules & neurofilaments
-Slow: 0.5–40 mm/day
-Fast: 100–400 mm/day
~Vesicles
~kinesins
retrograde transport
what are the steps of fast axonal transport?
- peptides made on rough ER & packaged by Golgi apparatus
- fast axonal transport walks vesicles & mitochondria along microtubule network
- vesicle contents released by exocytosis
- synaptic vesicle recycling
- retrograde fast axonal transport
- old membrane components digested in lysosomes
the region where the axon terminal meets the target cell
synapse
what are presynaptic and postsynaptic cells separated by?
synaptic cleft
a chemical signal diffuses across the synapse
chemical synapse
gap junctions connect pre and postsynaptic cells (bidirectional and faster)
electric synapses
- provide structural integrity, chemical, and anatomical support of neurons
- “glue”
- 90% of cells
glial cells
what are the 2 types of glial cells of the peripheral nervous system?
- schwann cells
2. satellite cells
wrap around axon and form insulating myelin sheaths
schwann cells
- gaps in the insulation of myelin sheaths
- section of unmyelinated axon membrane
nodes of ranvier
form supportive capsules around the cell body, regulate the chemical environment
satellite cells
what are the 4 types of glial cells of the central nervous system?
- Oligodendrocytes
- Astrocytes
- Microglia
- Ependymal cells
form myelin sheath
Oligodendrocytes
take up and release chemicals, feed neurons, water-K+ balance, and part of blood-brain-barrier
astrocytes
provide immune defense by removing damaged cells and foreign invaders
microganglia
form the barrier between fluid compartments of the CNS and are a source of stem cell
ependymal cells
consists of multiple layers of cell membrane
myelin
difference in voltage between two points
potential difference (E)
difference in voltage across the plasma membrane; always given in terms of voltage inside the cell relative to voltage outside the cell
membrane potential
a relatively small change in the membrane potential produced by a stimulus that triggers the opening or closing of ion channels
graded potential
graded potentials produced in the post-synaptic cell in response to neurotransmitters binding to receptors
synaptic potential
graded potentials produced in response to a stimulus acting on a sensory receptor
receptor potential
a large, rapid change in the membrane potential produced by depolarization of an excitable cell’s plasma membrane to threshold
action potential
the membrane potential that counters the chemical forces acting to move an ion across the membrane, thereby putting the ion at………
equilibrium potential
what are the 2 factors needed to determine the resting membrane potential?
- ion concentration gradients
- membrane permeability to these ions
~ion channels
what is needed to establish the equilibrium potential?
- hypothetical cell
- ion distribution
~outside cell= sodium and chloride
~inside cell= potassium and organic anions
what is the potassium equilibrium potential?
- K+ chemical driving force= out of cell
- K+ diffuses out of cell
what happens as K+ diffuses out of the cell?
the inside of the cell becomes more negative
what pulls K+ back into the cell?
electrical driving force
- opposite in direction
- equal in magnitude
chemical and electrical driving forces
when is an ion at equilibrium?
- when there is no net force for it to move across the membrane
- Chemical force = negative electrical force
- Electrochemical force = 0
when is potassium at equilibrium?
When membrane potential = –90 mV
what is the sodium equilibrium potential?
- Na+ chemical driving force: into the cell
- Na+ diffuses into cell
what happens as Na+ diffuses into the cell?
the inside of the cell becomes less negative (positive)
what pushes Na+ out of the cell?
electrical driving force
what are the 2 forces acting on Na+?
- chemical: to move in
- electrical: to move out
*electrochemical force
~intially in, due to stronger chemical force
~but electrical force continues to increase
net force
when is sodium at equilibrium?
When membrane potential = +60 mV
is a neuron more permeable to potassium or sodium?
25x more permeable to potassium
what is the ion distribution of a neuron?
- outside cell: sodium and chloride
- inside cell: potassium and organic anions
what are the chemical driving forces for the resting potential of a neuron?
- K+ out of cell
- Na+ into cell
in what ratio does K+ and Na+ enter and leave the cell?
more K+ leaves the cell than Na+ enters
what is the result of more K+ leaving the cell than Na+ entering?
inside of cell becomes negative
what do the electrical forces for the resting potential of a neuron do?
- Na+ into cell
- K into cell
what do the electrical forces of the resting potential for a neuron result in?
- K+ outflow slows
- Na+ inflow speeds up
inflow of Na+ is balanced by outflow of K+
steady state develops
what is the resting membrane potential of a neuron?
-70 mV
-Predicts membrane potential that results from the contribution of all ions that can cross the membrane
Goldman-Hodgkin-Katz (GHK) Equation
determined by the combined contributions of the concentration gradient times the membrane permeability for each ion
Resting membrane potential
maintains the resting potential of a neuron
sodium pump
what is the resting membrane potential closer to?
the potassium equilibrium potential
what happens when the membrane potential is not at equilibrium for an ion?
- electrochemical force is not 0
- net force acts to move the ion across the membrane in the direction that favors its being at equilibrium
- strength of the net force increases the farther away the membrane potential is from the equilibrium potential
what are the forces on K+?
- Resting potential = –70 mV
- EK = –94 mV
- Vm is 24 mV less negative than EK
into the cell (lower)
electrical force on K+
out of the cell (higher)
chemical force on K+
net force on K+ is weak
K+ flows out of the cell, but the membrane is highly permeable to K+
what are the forces on Na+?
- Resting potential= –70 mV
- ENa = +60 mV
- Vm is 130 mV less negative than ENa
into the cell
electrical force on Na+
into the cell
chemical force on Na+
net force on Na+ is strong
Na+ flows into the cell, but the membrane has low permeability to Na+
high force, low permeability
Small Na+ leak at rest
low force, high permeability
Small K+ leak at rest
returns Na+ and K+ to maintain gradients
sodium pump
20% of the resting membrane potential is directly due to……….?
Na+/K+-ATPase
- Electrogenic: 3 Na+ out, 2 K+ in
- Net: +1 out
Na+/K+ pump
80% of resting membrane potential is indirectly due to…….?
Na+/K+-ATPase
-produces concentration gradients
~Na+= high outside, low inside
~K+=low outside, high inside
80% of resting membrane potential is indirectly due to Na+/K+-ATPase
- a difference in potential across the membrane
- membrane is polarized
membrane potential
-70 mV
resting potential
a change to a more negative value
Hyperpolarization
a change to a less negative value
Depolarization
when the membrane potential returns to resting
Repolarization
what are the components of a resting membrane potential?
- ion concentration gradients
2. membrane permeability to these ions
what are the gated channels that control ion permeability?
- mechanically gated
- chemically gated
- voltage gated
varies from one channel type to another
threshold voltage
small changes in membrane potential that occur when ion channels open or close in response to a stimulus
graded potentials
such as neurotransmitter molecules binding to receptors on the dendrite or cell body of a neuron
chemical stimuli
such as touch or light acting on a sensory receptor
sensory stimuli
varies according to the strength of the stimuli
The magnitude of the change in membrane potential
occur in the membranes of excitable tissue (nerve or muscle) in response to graded potentials that reach threshold
action potentials
Once initiated, it is capable of being propagated long distance along the length of an axon without any decrease in strength
action potential
lose strength as they move through the cell due to current leak and cytoplasmic resistance
graded potentials
if strong enough, graded potentials reach the……?
the trigger zone in the axon hillock and initial segment
the ability to fire an action potential
Cell’s excitability
depolarizing
excitatory
hyperpolarize
inhibitory
- a graded potential starts above threshold (T) at its initiation point but decreases in strength as it travels through the cell body
- at the trigger zone, it is below threshold and, therefore, does not initiate an action potential
subthreshold graded potential
a stronger stimulus at the same point on the cell body creates a graded potential that is still above threshold by the time it reaches the trigger zone, so an action potential results
suprathreshold graded potential
when is an action potential triggered?
if the membrane threshold at the axon hillock is depolarized to threshold
what happens if the potential is below threshold?
no action potential will occur
adding effects of graded potentials
summation
- are graded potentials
- can be summed
IPSPs and EPSPs
what are the types of summation?
- temporal
2. spatial
- type of summation
- one synapse through time
temporal
- type of summation
- several synapses at the same time
spatial
what is the degree of depolarization at the axon hillock signaled by?
frequency of action potentials
- affects depolarization
- influences frequency of action potentials
summation
More action potentials result in more neurotransmitter released, which leads to……?
a greater IPSP or EPSP in the next neuron
The degree of depolarization at the axon hillock is signaled by the frequency of action potentials
frequency coding
The regulation of communication across a synapse
presynaptic modulation
what are the two modulatory synapses?
- presynaptic facilitation
- presynaptic inhibition
the high-speed movement of a action potential along an axon
conduction
how do action potentials move down an axon?
Not a single action potential (AP) that moves down the axon but rather many replenished AP’s
what do axons travelling down an axon form?
a wave of electrical signal at constant amplitude
also known as nerve spikes
action potentials
the principle that the strength by which a nerve or muscle fiber responds to a stimulus is independent of the strength of the stimulus. If that stimulus exceeds the threshold potential, the nerve or muscle fiber will give a complete response; otherwise, there is no response
all-or-none law
what happens during the conduction of an action potential?
- each section of the membrane is a different phase of the action potential
- a wave of electrical current passes down the axon
- each section of the axon is experiencing a different phase of the action potential
what are the steps of an action potential?
- resting membrane potential
- depolarizing stimulus
- Membrane depolarizes to threshold.
Voltage-gated Na+ and K+
channels begin to open. - rapid Na+ entry depolarizes cell
- K+ moves from cell to extracellular fluid
- K+ channels remain open and
additional K+ leaves cell, hyperpolarizing it. - Voltage-gated K+ channels close,
less K+ leaks out of the cell. - Cell returns to resting ion permeability
and resting membrane potential.
How do you stop depolarization and this positive feedback loop?
Voltage-gated sodium channels have two gates
what are the two voltage-gated sodium channels?
- activation gates
- inactivation gates
- responsible for opening sodium channels during the depolarization phase of AP
- closes the channel at the resting membrane potential
- Na+ enters the cell
activation gates
- (0.5 msec delay)responsible for the closing of sodium channels during the repolarization phase of AP
- Na+ entry stops
inactivation gates
what happens during repolarization caused by K+ leaving the cell?
the activation and inactivation gates reset to their original positions
- due to Na+ gates resetting
- Potential delay of 1-2 msec. between action potentials independent of intensity of trigger
- no stimulus can trigger another action potential
absolute refractory period
prevents backward conduction
refractory period
-follows an absolute refractory period
-only a larger-than-normal stimulus can initiate a new action
potential
relative refractory period
-spreads along adjacent sections of axon by local current flow
positive charge
-causes new section of the membrane to depolarize
local current flow
what are the phases of an action potential?
- rising phase
- peak
- falling phase
what happens during the conduction of an action potential?
1. A graded potential above threshold reaches the trigger zone 2. Voltage-gated Na+ channels open, and Na+ enters the axon. 3. Positive charge flows into adjacent sections of the axon by local current flow. 4. Local current flow from the active region causes new sections of the membrane to depolarize. 5. The refractory period prevents backward conduction. Loss of K+ from the cytoplasm repolarizes the membrane.
what is the speed of an action potential in a neuron influenced by?
- diameter of axon
- resistance of axon membrane to ion leakage out of the cell
how does the diameter of an axon affect the speed of an action potential?
larger axons are faster
how does the resistance of axon membrane to ion leakage out of the cell affect the speed of an action potential?
- myelinated axons are much faster
- saltatory condution between nodes of ranvier
the propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials
- Action potentials appear to jump from one node of Ranvier to the next
- Only the nodes have voltage-gated Na+ channels.
saltatory conduction
reduce or block conduction when current leaks out of the previously insulated regions between the nodes
demyelinating diseases
how can chemical factors alter action potentials?
-Binding of chemicals (neurotoxins, anesthetics) to Na+, K+, or Ca+ can alter the conduction of action potentials.
~blocking votage gated Na+ channels
-procaine hydrochloride (novacaine)
-tetrdotoxin (fugu toxin)
-K+ concentrations can also cause abnormal electrical activity in the nervous system
~hyperkalemia vs hypokalemia
increased blood K+ concentration, brings the membrane closer to the threshold. Now a stimulus that would normally be subthreshold can trigger an action potential.
hyperkalemia
decreased blood K+ concentration, hyperpolarizes the membrane and makes the neuron less likely to fire an action potential in response to a stimulus that would normally be above the threshold.
hypokalemia
what is the type of signal in a graded potential?
input signal
where do graded potentials occur?
dendrites and cell body
what are the types of gated ion channels in neurons?
- mechanically gated
- chemically gated
- voltage gated
what are the ions involved in graded potential?
Na+, K+, Ca^2+
what is the signal type of graded potentials?
- depolarizing (Na+)
- hyperpolarizing (Cl-)
what is the strength of the signal of a graded potential?
-depends on the initial stimulus, but it can be summed
what initiates the signal of a graded potential?
entry of ions through gated channels
what are the unique characteristics of a graded potential?
- no minimum level required to initiate
- two signals coming together in time will sum
- initial stimulus strength is indicated by frequency of a series of action potentials
what type of signal is an action potential?
regenerating conduction signal
where does an action potential occur?
trigger zone through axon
what are the types of gated channels involved in action potentials?
voltage gated channels
what are the ions involved in action potentials?
Na+ and K+
what type of signal is an action potential?
depolarizing
what is the strength of a signal of an action potential?
all or none phenomenon; cannot be summed
what initiates the signal of an action potential?
above-threshold graded potential at the trigger zone opens ion channels
what are unique characteristics of an action potential?
- threshold stimulus required to initiate
- refractory period; two signals too close together in time cannot sum
what do neurons communicate at?
synapses
pass electrical signals through gap junctions (cytoplasm continuous)
electrical synapses
- signal can be bi-directional
- synchronizes the activity of a network of cells
- cardiac, smooth muscle, glial cells, CNS
electrical synapses
- use neurotransmitters that cross synaptic clefts
- Make up majority of synapse in the nervous system
chemical synapses
- act at short distances
- released by a neuron
- diffuse across a narrow cleft to effect postsynaptic cell
neurotransmitters
- act over long distances
- released by neurons into the circulation
- exerts its effects on distant peripheral targets
neurohormones
what are the two types of neurocrine receptors?
- receptor channels (ionotropic receptors)
2. metabotropic receptors
- mediate rapid response
- alter ion flow across membranes
receptor channels (ionotropic receptors)
- G protein-mediated receptors
- mediate slower responses due to 2nd messenger system
- some open or close ion channels
metabotropic reeceptors
all neurotransmitters except……….., bind to specific receptors
nitric oxide
allows the same neurotransmitter to have different effects in different tissues
when receptors have multiple subtypes
either mimic or inhibit activity by binding to receptors
agonist and antagonist molecules
what are the seven classes of neurocrines by structure?
- Acetylcholine (Ach)- derived from Acetyl CoA
- Amines (biogenic amines)- derived from a single amino acid
- amino acids
- peptides
- purines
- gases - NO, CO, H2S
- lipids
Neurons that secrete Acetylcholine and receptors that bind it
cholinergic
what is the site of release of acetylcholine?
- CNS and ANS synapses
- neuromuscular junctions
what is the effect of acetylcholine?
- Excitatory in the CNS and neuromuscular junctions
- Inhibitory or excitatory in ANS synapses
what is the agonist of acetylcholine?
nicotine
what is the antagonist of acetylcholine?
curare (plant derived)
how is acetylcholine synthesized?
- Acetyl CoA + choline —> acetylcholine + CoA
- synthesized in axon terminal
- Choline acetyl transferase (CAT) = enzyme for synthesis
how is acetylcholine broken down?
- Acetylcholine —> acetate + choline
- occurs in synaptic cleft
- Acetylcholinesterase (AChE) = enzyme of degradation
what are the cholinergic receptors of acetylcholine?
- nicotinic
- muscarinic
- receptor of acetylcholine
- on skeletal muscle, in autonomic division of PNS and CNS
- monovalent cation channels –> Na+ & K+
nicotinic
- receptor of acetylcholine
- in CNS and autonomic parasympathetic division of the PNS
- G protein coupled receptors
muscarinic
- derived from tryptophan
- Involved in regulating sleep, alertness, thermoregulation, and mood
amines (serotonin)
what is the site of release of amines (serotonin)?
CNS synapses
what is the effect of serotonin?
- inhibitory and excitatory
- serotonergic receptors
- used to treat depression
- block serotonin symporters
SSRIs (prozac & zoloft)
- derived from tyrosine
- Highly concentrated in substantia nigra control of skeletal muscles, involved in elevation of mood, motivation, and reward
amines (dopamine)
what is the site of release of dopamine?
Selected CNS synapses, some ANS synapses
what is the effect of dopamine?
inhibitory and excitatory
blocks dopamine transporters, which increases levels in synaptic cleft
cocaine
- derived from tyrosine
- involved in dreaming, waking, and mood, excited cardiac muscle, excite or inhibit smooth muscle and glands
- Adrenergic Receptors (alpha and beta)
amines (norepinephrine/epinephrine)
what is the site of release of norepinephrine/epinephrine?
Selected CNS synapses, some ANS synapses
what is the effect of norepinephrine/epinephrine?
inhibitory and excitatory
what are some important amino acids?
- glutamate
- glycine
- GABA (ƴ-aminobutyric acid)
accounts for about 75% of all excitatory synaptic transmission in the brain, involved in learning and memory
glutamate
- inhibitory brain, most common inhibitory neurotransmitter in the spinal cord
- enhances the excitatory effect of glutamate
glycine
most common inhibitory neurotransmitter in the brain
GABA(ƴ-aminobutyric acid)
peptide neurotransmitters involved in pain
substance P and opioid peptides
AMP and ATP bind to receptors in the CNS
purine neurotransmitters
gas neurotransmitters that diffuse in the cells
NO, CO, H2S
Eicosanoids, endogenous ligands for cannabinoid receptors
lipid neurotransmitters
what happens when an action potential reaches an axon terminal?
opens voltage-gated Ca+ channels due to depolarization
once the Ca+ channels are open what happens?
Ca+ enters axon terminal and initiates exocytosis of synaptic vesicles containing neurotransmitter
-H+ dependent antiporters. Exchange H+ for neurotransmitters in vesicle
-rapid removal or inactivation in the synaptic cleft
termination of neurotransmitter activity
what happens during the rapid removal or inactivation in the synaptic cleft?
- diffusion out of the synaptic cleft into the ECF
- enzymatic breakdown
- Intact or broken down neurotransmitters are taken up by presynaptic axon terminal or glial cells
determines how much neurotransmitter is released
frequency of action potentials
releases little neurotransmitter
weak stimulus
causes more action potentials and releases more neurotransmitter
strong stimulus
what is involved in the integration of neural information transfer?
- divergent and convergent (branching) pathways at synapses
- synaptic plasticity
a change of activity at the synapses (i.e. enhance or decrease activity)
synaptic plasticity
one presynaptic neuron branches to affect a larger number of postsynaptic neurons
divergent pathway
many presynaptic neurons provide input to influence a smaller number of postsynaptic neurons
convergent pathway
nearly covered with synapses providing input from other neurons.
cell body of a somatic motor neuron
Which of the following glial cells become part of the blood-brain barrier?
astrocytes
If the ECF K+ concentration increases from 3 mM to 5 mM, what happens to the resting membrane potential of cells?
it becomes less negative
A neuron under the influence of a neurotransmitter that opens K+ channels will……
- become hyperpolarized
- be less likely to fire an action potential
What effect does hyperkalemia, an increase in plasma K+ concentration, have on the resting membrane potential of neurons?
- less K+ leaves the cell so membrane potential becomes less negative.
- Shifts it closer to threshold so the neuron is more likely to depolarize.
