Unit 4: Neurons and Cells of the Nervous System

Question 1

Central Nervous System (CNS

Answer 1

Brain and spinal cord = integrating centers

Question 2

Peripheral Nervous System (PNS)

Answer 2

All parts of the nervous system outside of the brain and spinal cord.

Question 3

Nerves

Answer 3

bundles of axons of neurons in the PNS

Question 4

What can the PNS be divided into?

Answer 4

Sensory (Afferent) Nervous System
Motor (Efferent) Nervous System

Question 5

Sensory (Afferent) Nervous System

Answer 5

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

Question 6

Motor (Efferent) Nervous System

Answer 6

Connects CNS to effectors (targets).
takes info from CNS to target cells via efferent neurons

Ø 2 divisions of the Motor Nervous System

Question 7

What are the 2 divisions of the Motor Nervous System?

Answer 7

1) Somatic Motor Nervous System
2) Autonomic Nervous System (ANS)

Question 8

Somatic Motor Nervous System

Answer 8

Effectors/targets are specifically skeletal muscle
cells

Question 9

Autonomic Nervous System (ANS)

Answer 9

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

Question 10

What are the 2 divisions of the ANS?

Answer 10

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.

Question 11

Enteric Nervous System

Answer 11

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.

Question 12

Describe the functions of neurons:

Answer 12

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

Question 13

Cell body

Answer 13

control/integration center

Question 14

Dendrites

Answer 14

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

Question 15

Axon hillock

Answer 15

region where action potentials are
initiated

Question 16

Axons

Answer 16

Relay outgoing signal (action potential) to axon
terminal.
May branch to form collaterals

Question 17

collaterals

Answer 17

which allow a
single neuron to signal to several other neuronal cells or to several target/effector cells

Question 18

nodes of Ranvier

Answer 18

Spaces in between myelinated
sections of an axon

Question 19

Axon terminal/

Answer 19

Relays signal to other neurons or to effector/
target cells

Ø Release neurotransmitter by exocytosis

Question 20

Varicosities

Answer 20

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)

Question 21

Synapse

Answer 21

Space between two connecting neurons or between a neuron and an effector

Question 22

presynaptic
cell/presynaptic membrane/presynaptic axon terminal

Answer 22

Cell/membrane leading up to synapse

Question 23

postsynaptic cell membrane

Answer 23

Cell membrane of receiving cell (after the synapse

Question 24

Pseudounipolar (unipolar) neurons

Answer 24

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

Question 25

Bipolar neurons

Answer 25

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.

Question 26

Anaxonic neurons

Answer 26

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)

Question 27

Multipolar neurons

Answer 27

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.

Question 28

Sensory (afferent) neurons

Answer 28

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)

Question 29

Interneurons

Answer 29

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

Question 30

Motor neurons

Answer 30

Ø From CNS to effectors.
Ø Present in motor nerves and mixed nerves.

Question 31

Schwann cells

Answer 31

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

Question 32

Satellite Cells

Answer 32

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)

Question 33

Oligodendrocytes

Answer 33

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.

Question 34

Astrocytes

Answer 34

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)

Question 35

Microglia

Answer 35

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

Question 36

Ependymal cells

Answer 36

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.

Question 37

Describe the relative ion concentrations

Answer 37

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).

Question 38

Non-gated channels

Answer 38

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).

Question 39

Gated ion channels

Answer 39

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.

Question 40

Electrochemical gradients

Answer 40

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-)

Question 41

Hyperpolarization –

Answer 41

cell becomes more negative than resting
membrane potential (e.g. from -70mV to -75mv, or -70mV to -90mV)

Question 42

Depolarization

Answer 42

cell becomes less negative (more positive) than
resting (e.g. from -70mV to -65 mV; -70 to -55mV, -70mV to +30 mV

Question 43

Repolarization

Answer 43

return to resting potential (e.g. from -90mV to -70 mV; from +30mV to -70mV)

Question 44

What are the different electric potential in the cell?

Answer 44

membrane potential
resting membrane potential
equilibrium potential

Question 45

Resting Membrane potentials

Answer 45

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;

Question 46

Equilibrium potential (E ion)

Answer 46

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+

Question 47

List the factors establishing RMP:

Answer 47

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

Question 48

Describe the role of the sodium-potassium ATPase pump in maintaining the resting membrane potential

Answer 48

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.

Question 49

Explain how the Nernst and GHK equations show how ions affect the resting membrane potential

Question 50

Graded Potentials (GPs)

Answer 50

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)

Question 51

Properties of Graded Potentials (GPs)

Answer 51

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

Question 52

Temporal summation

Answer 52

signals come to same spot over
time

Question 53

Spatial summation

Answer 53

multiple signals over different areas
of cell body/dendrites arrive at the same time

Question 54

Action potentials (APs)

Answer 54

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).

Question 55

Properties of Action Potentials (APs)

Answer 55

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.

Question 56

Absolute Refractory Period

Answer 56

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).

Question 57

Relative Refractory Period

Answer 57

. 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

Question 58

Continuous conduction

Answer 58

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

Question 59

Saltatory conduction

Answer 59

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

Question 60

What does the rate of propagation depend on?

Answer 60

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.

Question 61

synaptic cleft

Answer 61

Presynaptic membrane comes into close contact with postsynaptic
membrane, but the two cells are physically separated by a small space
called the

Question 62

Define a synapse

Answer 62

site of transmission of signals from one neuron to the next, or from a neuron to the effector

Question 63

explain the difference between an electrical synapse and a chemical synapse

Answer 63

electric- formed by gap junctions, electrical signals pass directly between the 2 cells

Question 64

Diagram and describe mechanisms of electrical synaptic transmission.

Answer 64

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

Question 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.

Answer 65

. 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)

Question 66

Describe the different mechanisms (e.g., reuptake, enzymatic breakdown, diffusion) by which neurotransmitter activity at a synapse can be terminated

Answer 66

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

Question 67

Excitatory Postsynaptic Potentials (EPSPs

Answer 67

Depolarizing
caused by opening of Na+ channels, Ca++ channels;
closure of K+ channels

Question 68

Inhibitory Postsynaptic Potential (IPSPs)

Answer 68

Hyperpolarizing
caused by opening of Cl- channels, opening of K+ channels (so that more are open than at rest); closure of Na+ channels

Question 69

Divergent pathways

Answer 69

one presynaptic neuron synapses with
multiple postsynaptic neuron

Question 70

Convergent pathways

Answer 70

many
presynaptic neurons synapse onto a
smaller and smaller number of
postsynaptic neurons.

Question 71

Spatial summation

Answer 71

two or more neurons
simultaneously fire and stimulate different areas on
the same postsynaptic neuron. The graded potentials
can summate to reach threshold potential

Question 72

Temporal summation

Answer 72

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.

Question 73

Postsynaptic Inhibition

Answer 73

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

Question 74

Long-Term Potentiation

Answer 74

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.