Unit 4: Neurons and Cells of the Nervous System Flashcards
1
Q
Central Nervous System (CNS
A
Brain and spinal cord = integrating centers
2
Q
Peripheral Nervous System (PNS)
A
All parts of the nervous system outside of the brain and spinal cord.
3
Q
Nerves
A
bundles of axons of neurons in the PNS
4
Q
What can the PNS be divided into?
A
Sensory (Afferent) Nervous System
Motor (Efferent) Nervous System
5
Q
Sensory (Afferent) Nervous System
A
Ø Connects sensory receptors in organs to CNS.
Ø Receptors are neurons or cells that transmit signals to neurons
They detect signals/stimuli.
Ø Sensory neurons relay signal to CNS
6
Q
Motor (Efferent) Nervous System
A
Connects CNS to effectors (targets).
takes info from CNS to target cells via efferent neurons
Ø 2 divisions of the Motor Nervous System
7
Q
What are the 2 divisions of the Motor Nervous System?
A
1) Somatic Motor Nervous System
2) Autonomic Nervous System (ANS)
8
Q
Somatic Motor Nervous System
A
Effectors/targets are specifically skeletal muscle
cells
9
Q
Autonomic Nervous System (ANS)
A
Ø Effectors/targets are specifically smooth muscle cells, cardiac muscle cells, exocrine gland cells (e.g. sweat glands, sebaceous/oil glands); some endocrine gland cells (e.g. medulla of adrenal gland), and some adipose (fat) tissue
Ø The ANS is divided into two divisions
10
Q
What are the 2 divisions of the ANS?
A
a) Parasympathetic Nervous System
Ø In most cases, stimulates organs
involved in rest and digest functions
and inhibits organs involved in fight or
flight responses.
b) Sympathetic Nervous System
Ø In most cases, stimulates organs
involved in fight or flight functions.
Inhibits organs involved in rest and
digest functions.
11
Q
Enteric Nervous System
A
Ø Nervous system of the digestive tract.
Ø Can act independently from the CNS/PNS and/or can be controlled by the autonomic nervous system.
Ø Effectors are smooth muscle, exocrine glands and some endocrine glands involved in digestion.
12
Q
Describe the functions of neurons:
A
carry electrical signals and communicate
with each other and with target cell
have a high density of ion channels
(both leak channels and gated ion channels).
Ø Neurons secrete signaling molecules =
neurotransmitters and neurohormones
13
Q
Cell body
A
control/integration center
14
Q
Dendrites
A
Ø Receive incoming signals and relay to cell body.
Ø Some dendrites act as receptors for specific
stimuli
Ø e.g. thermoreceptors (temperature receptors)
and nociceptors (pain receptors) in the skin
15
Q
Axon hillock
A
region where action potentials are
initiated
16
Q
Axons
A
Relay outgoing signal (action potential) to axon
terminal.
May branch to form collaterals
17
Q
collaterals
A
which allow a
single neuron to signal to several other neuronal cells or to several target/effector cells
18
Q
nodes of Ranvier
A
Spaces in between myelinated
sections of an axon
19
Q
Axon terminal/
A
Relays signal to other neurons or to effector/
target cells
Ø Release neurotransmitter by exocytosis
20
Q
Varicosities
A
are associated with the neurons that
innervate (send axons to) effectors in the Autonomic Nervous System.
They are located along the length of
the axon near its terminal end (like beads on a string)
21
Q
Synapse
A
Space between two connecting neurons or between a neuron and an effector
22
Q
presynaptic
cell/presynaptic membrane/presynaptic axon terminal
A
Cell/membrane leading up to synapse
23
Q
postsynaptic cell membrane
A
Cell membrane of receiving cell (after the synapse
24
Q
Pseudounipolar (unipolar) neurons
A
Ø Single process off cell body (axon)
Ø Dendrites fused to axon
Ø All unipolar polar neurons are sensory (carry information from skin/sensory organs/internal organs to the spinal cord and/or brain.
Ø E.g. thermoreceptors and nociceptors (pain receptors) are
the dendrites of unipolar neurons whose axons run through
nerves to the spinal cord/brain
25
Bipolar neurons
Ø Two processes off cell body – axon and dendrite
Ø Most are sensory neurons associated with special senses
Ø E.g. bipolar cells of the retina of the eye for vision help to transmit signals from photoreceptors (rods/cones) in the eye.
26
Anaxonic neurons
Ø No apparent axon. Appears as cell body with dendrites.
Ø Interneurons in CNS. Most connect or modulate sensory neurons.
Ø E.g. amacrine cells in the retina of the eye (affect the output of
bipolar cells in retina)
27
Multipolar neurons
Single axon off cell body. Many dendrites off cell body.
Ø Includes interneurons in the CNS. Some transmit signals between
brain regions, others connect spinal cord to brain, others connect
some sensory neurons to motor neurons (especially in spinal cord).
Ø Also includes all motor neurons (both somatic and autonomic neurons) in the PNS.
Ø Most common type of neuron structure.
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Sensory (afferent) neurons
From receptors to CNS.
Ø Present in sensory nerves and mixed nerves (mixed nerves contain axons of both sensory and motor neurons and most nerves are mixed)
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Interneurons
Ø Within CNS. Connect different regions of the brain and connect brain to spinal cord.
Ø Form tracts (bundles of axons of interneurons).
Ø Most common type of the three functional types
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Motor neurons
Ø From CNS to effectors.
Ø Present in motor nerves and mixed nerves.
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Schwann cells
Ø Form myelin sheath that acts to electrically insulate axons in the PNS (like the plastic wrapping around an electrical wire).
Ø Each Schwann cell wraps around a single axon, and each axon has multiple Schwann cells
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Satellite Cells
Ø Similar to Schwann cells, but membrane not wrapped around axon
Ø Support and protect cell bodies of neurons found in PNS ganglia (ganglion = collection of neuron cell bodies in peripheral nervous system - equivalent in CNS is called a nucleus – e.g. cranial nerve nuclei)
33
Oligodendrocytes
Ø Form myelin sheath that acts to electrically insulate axons in the CNS
Ø Unlike Schwann cells, one oligodendrocyte can form myelin on multiple axons.
34
Astrocytes
Ø Many processes (contact capillaries and axons) – gives them a “star” shaped appearance (astro=star).
Ø Form part of blood brain barrier – they cover capillaries and regulate transport of material from blood into the ISF that surrounds neural tissue in brain.
Ø Take up K+, water, and neurotransmitters
Ø Secrete neurotrophic factors (peptides and small proteins that promote growth and maturation and survival of neurons)
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Microglia
Ø Act as immune cells (macrophages) of CNS (white blood cells cannot cross the blood brain barrier).
Ø Scavenge pathogens and remove dead/damaged
cells
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Ependymal cells
Ciliated cells found lining the ventricles in the brain (fluid filled spaces of the brain) and also the central canal of spinal cord.
Ø Assists with the formation and circulation of
cerebrospinal fluid.
37
Describe the relative ion concentrations
a. Large negatively charged organic ions inside of cells that are not able to diffuse out.
b. K+ = high concentration inside cells (ICF) due to action of Na+/K+ ATPase pump.
c. Na+ = high concentration outside cells (ECF) due to action of Na+/K+ ATPase pump
d. Cl- = high concentration outside of the cell (ECF) because repelled by large negatively charged organic ions inside of cell (like charges repel one another).
e. Ca++ = high concentration outside of cells (ECF) due to transporters in cell membrane (e.g. Ca++ ATPase pumps). Also low in ICF due to Ca++ pumps in endoplasmic reticulum (removes Ca++ from cytosol).
38
Non-gated channels
Ø Also called leakage channels – because they allow a constant leak of the ion they are specific for down it’s concentration gradient.
Ø Always open (like a doorway into/out of the cell with no door on it).
Ø specific for particular ions
Ø There are more K+ non-gated (leakage) channels in cell membranes than there are Na+ non-gated channels. So cells are more permeable to K+ at rest
(important factor that establishes resting membrane
potential in neurons).
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Gated ion channels
Not involved in “resting” cells.
Ø Present in neurons and muscle cells.
Ø Open in response to a stimulus.
Stimulus may be:
i. A change in membrane potential (voltage) –
voltage gated channels
ii. A chemical (ligand, like a neurotransmitter) =
chemically gated channels. May involve GPCR
pathways or ion channel receptors.
iii. Temperature = thermally gated channels
iv. Mechanical deformation of membrane =
mechanically gated channels.
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Electrochemical gradients
Ø a combination of an electrical gradient (gradient of charges) and a chemical gradient (concentration gradient)
Ø ions move passively down their chemical concentration gradient (from areas of high to low concentration) or actively against their concentration gradient (from low to
high concentrations)
Ø Movement of ions creates electrical gradients = separation of charges across the membrane. ICF in contact with inner surface of cell membrane is more negatively charged than outer (ECF) surface.
Ø Opposite charges attract; like charges repel. Creates a driving force for positively charged ions into the cell and repels negatively charged ions (like Cl-)
41
Hyperpolarization –
cell becomes more negative than resting
membrane potential (e.g. from -70mV to -75mv, or -70mV to -90mV)
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Depolarization
cell becomes less negative (more positive) than
resting (e.g. from -70mV to -65 mV; -70 to -55mV, -70mV to +30 mV
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Repolarization
return to resting potential (e.g. from -90mV to -70 mV; from +30mV to -70mV)
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What are the different electric potential in the cell?
membrane potential
resting membrane potential
equilibrium potential
45
Resting Membrane potentials
Ø Steady state balance between active transport of ions and leakage of ions
through channels in an unstimulated (i.e. resting) cell.
Ø For most cells, RMP is between –20 mV and -90 mV.
Ø For ”average” neuron = -70mV;
46
Equilibrium potential (E ion)
Ø = the membrane potential (electrical gradient) that opposes the concentration gradient
Ø E.g. K+ diffuses out of the cell through K+ leak channels down its
concentration gradient. As K+ moves down its concentration gradient,
the cell loses positive charge and the ICF at inner surface becomes
more negatively charged. Eventually the negative charge inside of the
cell will start to attract the positively charged K+ ions, and slow their
movement out of the cell. The membrane potential (voltage) at which
the electrical gradient and concentration gradient are balanced is the
equilibrium potential for K+
47
List the factors establishing RMP:
1. Na+/K+ ATPase pump
2. Large anions (negatively charged ions) inside the cell that cannot cross membrane.
3. Most importantly – neuronal cells are more permeable to K+ than to Na+, therefore K+ is major determinant of RMP
48
Describe the role of the sodium-potassium ATPase pump in maintaining the resting membrane potential
not a channel, but an active
transport carrier protein.
Øbreaks down 1 ATP and uses energy to pump 3 Na+ out of the
cell and 2 K+ into the cell.
ØEffects:
a. Maintains the concentration gradients of K+ and Na+
b. Contributes a few millivolts to RMP (since it pumps more positive charges out than in, creating a loss of positive charge
from inside the cell (-3 + (+2)) = -1.
49
Explain how the Nernst and GHK equations show how ions affect the resting membrane potential
50
Graded Potentials (GPs)
are small changes in
membrane potential in response to a stimulus.
Can cause inner cell membrane to become:
1. More positive than RMP = depolarization (e.g. -78 mV to -70 mV).
Ø Depolarization can be caused by: opening Na+ channels (Na+ entry), opening Ca++ channels (Ca++ entry), closing K+ channels (traps K+ inside cell)
2. More negative than RMP = hyperpolarization (e.g -78 mV to -85 mV).
Ø Hyperpolarization can be caused by: opening K+ channels (K+ exit), opening Cl- channels (Cl- entry)
51
Properties of Graded Potentials (GPs)
1. Small subthreshold changes in membrane potential.
2. Usually occur on dendrites and cell body
3. No minimum threshold required to initiate them.
4. Involve mechanically, chemically, or voltage gated channels
5. Size or amplitude of GP is directly proportional to the size of the stimulus - large stimulus causes large graded
potential, small stimulus causes small graded potential.
6. Spread through cells due to local current flow
(electrochemical gradient – down concentration and electrical gradients).
7. Strength dissipates (dies out) as they spread away from the point of stimulus – like ripples on a pond
Ø Signal degrades due to membrane being leaky to ions and
to the electrical resistance of cytoplasm
Small graded potentials can be added together to form larger ones = summation
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Temporal summation
signals come to same spot over
time
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Spatial summation
multiple signals over different areas
of cell body/dendrites arrive at the same time
54
Action potentials (APs)
are large changes in membrane potential that propagate along an axon with no change in intensity (constant amplitude).
ØInitiated at the trigger zone (e.g. axon hillock of multipolar and bipolar neurons; just past dendrites of unipolar neurons).
55
Properties of Action Potentials (APs)
1. Wave of depolarization.
2. All or none – there is maximal depolarization if threshold is reached (i.e. AP occurs), and there is no depolarization if threshold is not reached (i.e. AP will not occur).
3. Fast – lasts only msecs.
4. Large amplitude – from rest (-70 mV) to peak
(+30mV) = 100 mV
5. Has a refractory period
a. Absolute refractory period (prevents AP summation)
b. Relative refractory period
6. No summation.
56
Absolute Refractory Period
a. Prevents summation of APs
b. NO AP can be generated, regardless of stimulus size (excitability is 0).
c. Results from:
i. All Na+ voltage gated channel activation gates
being open (depolarization phase); or
ii. All Na+ voltage gated channel inactivation
gates being closed (cannot reopen until
activation and inactivation gates are re-set).
57
Relative Refractory Period
. Period when AP can be generated but only by a stronger than normal stimulus.
b. Some inactivation and activation gates are reset and can be opened again, but....
c. K+ voltage gated channels are still open (since
they are also slow to close) and the cell is
hyperpolarized, which makes it more difficult for the cell to reach threshold.
Ø E.g. the cell is at -90mV and needs to reach
-55mV vs being at -70 and needing to reach
-55. A larger change in membrane potential
is needed in the former and this larger
change is more difficult to achieve without a
much stronger than normal stimulus.
d. Excitability increases throughout this phase
58
Continuous conduction
AP must be continuously reproduced at
every point along the axon (as such is often slower than saltatory
conductions).
b. Relies on local current flow
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Saltatory conduction
AP occurs only at the nodes of Ranvier
(exposed areas of axon in between Schwann cells or oligodendrocytes)
Relies on local current flow
b. Conduction is faster as electrical charges are insulated (less
leakage out of the membrane in areas with myelin
60
What does the rate of propagation depend on?
a. Myelination – unmyelinated axons are slower than myelinated axons
b. Fiber (Axon) diameter - larger diameter has faster propagation because there is less resistance to ion flow (current). – like a two lane highway vs a 4 lane highway – the 4
lane highway will move more cars in the same amount of time.
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synaptic cleft
Presynaptic membrane comes into close contact with postsynaptic
membrane, but the two cells are physically separated by a small space
called the
62
Define a synapse
site of transmission of signals from one neuron to the next, or from a neuron to the effector
63
explain the difference between an electrical synapse and a chemical synapse
electric- formed by gap junctions, electrical signals pass directly between the 2 cells
64
Diagram and describe mechanisms of electrical synaptic transmission.
a. Formed by gap junctions that align and connect the presynaptic cell to the
postsynaptic cell.
Ø Recall that gap junctions are formed from 2 connexons – each connexon consists of 6
connexin proteins. The connexon from one cell must line up with the connexon of
another cell in order to make a channel for exchange of substances like ions.
b. Electrical signals (e.g. the Na+ ions from an action potential) pass directly between the two cells
Ø Small molecules like ATP, cAMP and some second messengers can also diffuse
through gap junctions)
c. Uncommon, but present in some neurons
Ø E.g. hormone secreting neurons in the hypothalamus
Ø more common in cardiac and smooth muscle.
d. Electrical signal can move in both directions.
e. Very fast transmission (short synaptic delay of 0.2ms).
f. Allows a group of cells to fire action potentials almost simultaneously
Ø E.g. in the hypothalamus this facilitates a burst of hormone secretion into the blood
65
Diagram and describe the events of chemical synaptic transmission in proper chronological order including the events that trigger the release of the neurotransmitter by synaptic vesicles to the possible effects of the neurotransmitter on the postsynaptic cell.
. Action Potential (AP) a rrives at axon terminal of presynaptic cell
ii. Change in membrane potential due to AP causes voltage gated Ca++ channels to open
iii. Ca++ diffuses into the cell down its electrochemical gradient (recall high [Ca++] in ECF, low in ICF and Ca++ attracted to negatively
charged interior of cell, so when you open a doorway for Ca++, it moves into the cell).
iv. Ca++ triggers exocytosis of neurotransmitter into synaptic cleft
v. Neurotransmitter diffuses across synaptic cleft and binds to receptors on postsynaptic membrane.
Ø Some receptors are chemically gated ion channel receptors, some are GPCR.
vi. Postsynaptic response depends on the type of receptor
Ø Synaptic delay is ~2ms (time it takes for neurotransmitter to cross
the cleft and stimulate response in postsynaptic cell)
66
Describe the different mechanisms (e.g., reuptake, enzymatic breakdown, diffusion) by which neurotransmitter activity at a synapse can be terminated
Ø Results from termination (removal) of the neurotransmitter from the synaptic cleft.
Ø Neurotransmitter is removed by:
i. Enzymatic breakdown
ii. Transport back into the axon terminal by active transport where
it can be recycled and repackaged back into vesicles.
iii. Diffusion away from the synapse.
iv. Taken up into the postsynaptic cell by endocytosis.
Ø E.g. The neurotransmitter acetylcholine (ACh)
i. ACh is broken down on the postsynaptic membrane by the
enzyme acetylcholinesterase (AChE) into acetic acid and choline.
ii. Choline is transported back into presynaptic axon terminal by
cotransport (secondary active transport) with Na+
iii. Choline is recycled to make more acetylcholine
67
Excitatory Postsynaptic Potentials (EPSPs
Depolarizing
caused by opening of Na+ channels, Ca++ channels;
closure of K+ channels
68
Inhibitory Postsynaptic Potential (IPSPs)
Hyperpolarizing
caused by opening of Cl- channels, opening of K+ channels (so that more are open than at rest); closure of Na+ channels
69
Divergent pathways
one presynaptic neuron synapses with
multiple postsynaptic neuron
70
Convergent pathways
many
presynaptic neurons synapse onto a
smaller and smaller number of
postsynaptic neurons.
71
Spatial summation
two or more neurons
simultaneously fire and stimulate different areas on
the same postsynaptic neuron. The graded potentials
can summate to reach threshold potential
72
Temporal summation
a single neuron sends multiple
signals over time to the same postsynaptic cell.
Graded potentials overlap on the postsynaptic cell and
summate to reach threshold potential.
73
Postsynaptic Inhibition
A type of spatial summation in which
the graded potentials on the
postsynaptic neuron are a
combination of EPSPs and IPSPs, and
in which the IPSPs prevent threshold
from being reached.
Ø Summation of all EPSPs and IPSPs is
overall below threshold (-55mV) so
no action potential is initiated on the
postsynaptic cell
74
Long-Term Potentiation
1. Release of glutamate from the presynaptic neuron.
2. Glutamate binds to the AMPA and NMDA
receptors.
3. The AMPA receptor channel opens first, and allows Na+ to enter the cell down its electrochemical gradient, creating an EPSP in the postsynaptic cell.
4. Depolarization of the postsynaptic cell causes
Mg++ that is blocking the NMDA receptor channel pore to be ejected. This allows the NMDA channel to open.
5. Ca++ ions enter the cell through the open NMDA channel. CA++ is also released from intracellular stores (Endoplasmic reticulum stores) as a result of activation of a metabotropic (GPCR) pathway.
6. Large influx of Ca++ ions into cytosol triggers
second messenger pathways that result in:
a. Phosphorylation of AMPA receptors, which
increases their affinity for glutamate.
b. Insertion of more AMPA receptors into the
membrane
ØBoth a) and b) make the membrane more
sensitive to glutamate since there are now
more receptors for it, and those receptors
more readily bind glutamate.
c. Release of a paracrine (nitric oxide) from the
postsynaptic cell that acts on the presynaptic
cell and causes it to enhance glutamate release.
