Download Chapter 12

Unit 4- The Nervous
System
Nervous system organization and
structure, Action potentials -Chapter 12
Nervous system functions

____________ function- sensory receptors detect
internal & external stimuli


____________ function- integrates sensory info



Sensory = afferent brain, spinal cord
 increase in blood acidity
 a raindrop landing on your arm
analyzing & storing some info
making decisions regarding appropriate responsesinterneurons
____________ function- responding to integration
decisions

motor = efferent  carry info out of CNS to effector
Properties of the nervous
system


Shares responsibility w/ endocrine system
in maintaining homeostasis.
Rapidly responds to stimuli


transmits nerve impulses to adjust body
processes
Responsible for perceptions, behaviors, &
memories and initiates voluntary
movements
Structure of neuron, fig 12.3

______________- nerve cell


______________ – “little trees”


neuronal process that carries nerve impulse toward cell
body
_______________–usually single, long process of a
nerve cell that propagates an nerve impulse towards
the __________________:


consists of a cell body, dendrites, and an axon.
toward other nerve cells, muscles fibers, or gland cells.
________________- contains nucleus surrounded
by cytoplasm and typical organelles



___________________- cone shaped elevation
where axon joins cell body
________________- where the impulse arises, then
impulse conducts along the axon
__________________- insulation by multiple layers
of protein and lipid




increases speed of impulse conduction
Done by Schwann cells in PNS
_____________________- a space along a
myelinated nerve fiber between Schwann cells
_________________- the peripheral, nucleated
cytoplasmic layer of the Schwann cell

AKA - sheath of Schwann
Neurons: structural fig 12.4

________________- several dendrites, one axon


_______________- one main dendrite, one axon


most neurons in brain & spinal cord
retina of the eye, inner ear, & olfactory area of brain.
________________- sensory neurons, originate
in embryo as bipolar neurons


During development dendrite and axon fuse
Both branches characteristic of structure & function of
axon- long, cylindrical processes to propogate AP

Dendrites from periphery, monitor sensory
stimuli (touch or stretching)

Axon  CNS
Neuroglia- table 12.1, fig 12.6






___________= “glue”
1/2 the volume of CNS, support cells
Smaller than neurons & 5-50x more
Can multiply and divide in mature NS
Can fill space formerly occupied by neuron
_____________- brain tumors from glial cells


Highly malignant, grow rapidly
6 types: 4 in CNS, 2 in PNS
CNS glial cells, table 12.1

__________- “star,” maintain appropriate
chemical environment for impulses






regulate [K+]
Provide nutrients for neurons
Take up excess neurotransmitters
Assist w/neuron migration during brain develop
Blood-brain barrier
_____________________- “few trees,”support network around CNS neurons


Myelin sheath in CNS
CNS- little regrowth after injury (more later)
CNS glial cells (2)

________- protect CNS cells from disease
by engulfing and invading microbes

Migrate to areas of injured nerve tissue


Clear away debris of dead cells, and may kill
healthy cells
__________________- line ventricles of
brain and central canal of spinal cord

Assist in circulation of cerebrospinal fluid
PNS glial cells, figure 12.7

_____________ - participate in
regeneration of PNS axons (neurolemma
aids in regeneration of PNS axons)

unmyelinated axons surrounds multiple axons
with single layer of plasma membrane



Not myelinated, but enclosed by the Schwann cell
myelinated axons- produces part of the myelin
sheath around a single axon
______________- support neurons in PNS
ganglia
Excitability of membrane, 12.12

Excitable cells communicate with one another by
electrical signals



Action potentials
_____________________ = voltage difference
across the membrane
________________________________ = voltage
difference between inside and outside of cell
membrane when not responding to stimulus


In many neurons and muscle fibers = -70 to –90 mV
Inside of cell negative (w.r.t outside of cell)

Current is flow of charged particles


in cells = ions are the charged particles
AP occur in neurons because:


Many different ion channels
Ion channels open and close in response to
specific stimuli


Stimulus is a change in environment strong enough to
initiate an AP
phospholipid bilayer = good insulator

Current flow thru ion channels
Ion channels

Ions move across membrane down electrochemical
gradient:





Current changes membrane potential (voltage
across the membrane)
AP travels (or propagates along cell) due to flow of
ions thru channels
Ion channels open and close due to gates


High to low concentration
Positive  negative, negativepositive
 Opposites attract
Gate= part of protein channel: shuts or opens pore
4 types of ion channels
Types of ion channel

1. ____________ – gates randomly alternate
between open and closed


2. ____________ (12.11a)- open in response
to change in membrane potential


more K+ ion leakage channels (than Na+)
generation & conduction of AP
3. _________________ – open/closes due to
mechanical stimulation

Vibration, pressure, tissue stretch
Types continued

4. _______________ (12.11b)– opens/closes
due to specific chemical stimulus

Neurotransmitters, hormones, specific ions


Ex. Acetylcholine: opens cation channels so Na+ and
Ca2+ can move in, K+ can move out
Ligand can:


Open or close by binding a portion of the protein
channel
Indirectly activate by signaling a G-protein (18.4)
Resting membrane potential


Figure 12.12
Small build up of negative ions inside


Separation of such electrical charges = potential
energy (in volts… usually mV)


Equal buildup of positive ions on ECM side
Potential energy- potential to move
The > difference in charge, the > the membrane
potential (voltage)


Neuron resting mem potential: -40 to-90mV (-70mV)
If cell exhibits membrane potential then is “polarized”
 Most body cells polarized, potential varies
Summary: resting mem. potential





polarized
typically around -70mV
inside negative, outside positive
higher [Na+] outside than inside
higher [K+] inside than outside

2 conditions allow maintenance of resting
membrane potential in excitable cells:

Unequal distribution of ions across the plasma
membrane


ECF rich in Na+ and ClCytosol- main ion is K+


Anions are phosphates & amino acids in proteins
Relative permeability of plasma membrane to Na+
and K+

At rest in neuron or muscle fiber, permeability to K+ is
50-100X greater than Na+ due to leak channels
Sodium (Na+)

Electrical & concentration gradients promote
Na+ inflow


Negative interior attracts cations (more Na+ECF)
Na+ leak is slow, but would eventually destroy
gradient

Na+/K+ pump counteracts the Na+ slow leak from
affecting the resting membrane potential
Graded potential



small deviation from membrane potential that
makes the membrane more or less polarized
(Na+ and Ca++ in, and K+ out)
occur in the dendrites and cell body of the
motor neuron, if reach the axon:
voltage-gated ion channels openAP
Action potential, fig 12.14-16

Sequence of rapidly occuring events that
happen in 2 phases:



Depolarizing phase- negative membrane potential
decreases toward zero & eventually becomes
positive.
Repolarizing phase- restores resting mem.
potential to –70mV
2 types of voltage-gated ion channels open
then close (Na+ gates, K+ gates)

Channels present mainly in axon & axon terminals
AP – basic sequence of events

1st: Na+ channels open

Na+ rush into cell


2nd: K+ channels open

K+ flow out


Begins depolarization phase
Begins repolarizing phase
Together these 2 phase last  1 msec
All or none principle

Depolarization must reach a certain level for an AP
to occur





Threshold – the membrane potential that must be reached
in order to trigger an AP
  -55mV in most neurons
The voltage gates will open
AP that is always the same size occurs
Analogy: hit the first domino:
 Strong or weak hit, as long as it knocks over…
All or none- action potential happens or it doesn’t (all
dominos fall or none do)
Depolarization


Threshold reached,Na+ channels open rapidly
Gradient favors Na+ inward movement


Na+ channels: 2 separate gates



Activation gate and Inactivation gate
At resting state: inactivation open, activation gate is
CLOSED


Membrane potential –55mV  = +30 mV
 Depolarized=MORE positive inside than outside
Na+ cannot move into cell thru these channels
Activated state: both gates open, Na+ in
+ feedback: as more depolarized, more open

Shortly after activation gates open,
inactivation CLOSE


Channel now: inactivated state
Less than 1 msec,  20,000 Na+ in, change mem
potential considerably


BUT, [Na+] hardly changes because of millions of Na+
present in nearby ECF
Na+/K+ pump can easily bail out Na+ to then maintain
low Na+ inside cell
Repolarization

Depolarization also opens voltage gated K+
channels



Na+ channels inactive, Na+ inflow slows
K+ channel open, K+ outflow accelerates



K+ channels open more slowly
 K+ channels open when Na+ closing
 This causes REPOLARIZATION
Membrane potential ∆ +30 to –70 mV
Inactivated Na+ channels return to resting state
If outflow K+ large enoughhyperpolarization:

Membrane more permeable to K+ than at resting (-90mV)


Subthreshold stimulus – stimulus of such weak
intensity its not strong enough to initiate AP
Refractory period – time period in which an excitable
cell cannot respond to stimulus that is usually
adequate to evoke an AP


Absolute r.pd. – time during which a 2nd AP cannot be
initiated even with very strong stimulus
 coincides with Na+ channel activation & inactivation
Relative r. pd - 2nd AP can be initiated BUT only by a larger
than normal stimulus
 voltage gated K+ channels still open after inactivated Na+
channels returned to resting state
Nerve impulse propagation

Nerve impulse must travel from trigger zone to axon
terminals:

Propagation or conduction = ability to conduct AP along the
p.m.
 Na+ ions flow in  depolarization  opens Na+ channels in
adjacent segments of membrane
 Nerve impulse self-propagates along the membrane (like
row of dominos)
 Nerve is in refractory behind the leading edge of impulse,
so normally the impulse moves in one direction.
Saltatory conduction, fig 12.16



Saltatory = leaping
Propagation of AP along the exposed parts of a
myelinated axon.
AP appears at successive Nodes of Ranvier


Uneven distribution of voltage-gated channels


Seems to leap
Myelin sheath few there along the myelinated portion
and many at the node
Current flows thru ECF surrounding sheath & thru
cytosol inside axon until reaches next node

Ionic flow continues down myelinated axon
Saltatory conduction (2)

Consequences:

Leaping conduction


Impulse leaps from one area of axolemma to the next
Smaller number of channels in general because
only opening channels at the nodes  is more
energetically efficient

Only a small area of axolemma has to depolarize and
repolarize
Signal transmission at synapse

Synapse- functional junction






between 2 neurons, neuron & effector
Can be chemical or electrical
 Both differ structurally and functionally
allow info to be communicated, filtered and integrated
Synapses can change
 To allow learning
 Diseases and neurological disorders can result
 Sites of action for theraputic & addictive chemicals
Presynaptic- sending message
Postsynaptic- receiving message
Electrical synapses

AP conduct directly between adjacents cells at gap
junctions:



tunnels to allow ion flow
visercal smooth muscle, cardiac muscle, developing
embryo, some in CNS
Advantages to electrical:


Faster communication than chemical
 Pass directly from pre to postsynaptic cell
Synchronization of activity of a group of neurons or muscle
fibers
 In unison due to connection by gap junctions
 Heart beat, coordination of smooth muscle contraction in GIperistalsis
Chemical synapse

Occurs since p.m. of pre & post synaptic cell are not
touching

Synaptic cleft- space between, filled w/ interstitial fluid
 Nerve impulses cannot conduct across
 Presynaptic releases neurotransmitter:





Diffuses across
Binds receptor on p.m. of postsynaptic neuron
Postsynaptic potential is produced
Presynaptic converts electrical signal to chemical
signal
Synaptic delay – about 0.5 msec
Typical chemical synapse, 12.17


Impulse arrives at synaptic end bulb
Depolarizing phase opens voltage-gated Ca2+
channels present at end bulb


 [Ca2+] inside presynaptic signals to trigger
exocytosis of synaptic vesicles



 [Ca2+] in ECF, Ca2+ flows inward
Vesicle merge w/ neuron plasma membrane
Neurotranmitters released into synaptic cleft
 Each vesicle several thousand neurotransmitters
NT diffuse cleft & bind to postsynaptic receptors
Excitatory postsynaptic potential



EPSP
Depolarizing postsynaptic potential
Often result of opening of cation channels




Na+ (this inflow being greater than the following)
K+
Ca2+
Single EPSP not always cause an impulse BUT
makes cell more excitable b/c partially depolarized
Inhibitory postsynaptic potential




IPSP
Hyperpolarization of postsynaptic membrane
Generation of AP more difficult than usual
because membrane potential is more
negative than at resting
Often result of opening ligand-gated channels:


Cl- (diffuse in)
K+ (diffuse out)
Nerve regeneration, fig 12.20

Plasticity: change based on experiences


New dendrites, new proteins, new synapses
Limited regeneration (replicate or repair)


PNS: dendrite & myelinated axon can be repaired if:
 Cell body intact, schwann cell active, there is slow scar
tissue formation
 Neurolemma remains though part axon & sheath
deteriorated
CNS: little or no repair
 Inhibitory influence of neuroglia




Oligodendrocytes have no neurolemma like schwann
CNS myelin is a factor in inhibiting regeneration
Scar tissue due to rapid astrocyte proliferation creates barrier
Adults: absence of growth stimulating cues (unlike fetus)
Neural circuits, fig 12.19



Complicated networks
CNS contains billions
Functional group- processes specific kind of info


Simple series circuit= presynaptic neuron stimulated
only one postsynaptic neuron, 2nd stimulates one other,
and so on
Diverging circuit= presynaptic neuron synapses w/
several postsynaptic
 Also stimulate several cells along the circuit
 Amplify signal

Figure 12.19 continued…



Converging circuit= several presynaptic neurons
synapse w/ a single postsynaptic
 Receiving input from several diff. Sources
 Motor neuron receiving info from many areas of the
brain
Reverberating circuit= stimulate presynaptic 
postsynaptic to send a series of nerve impulses.
 Inhibitory neuron may turn off after time
 Breathing, coordinated muscular activities, waking,
short term memory
Parallel after-discharge circuit= single presynaptic cell
stimulates group of neurons each which synapse with
common postsynaptic
 Precise activities such as math calculation