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1
AP Lecture 17

• Chapter 12 Neural Tissue
• Part 2

2
IV. Ion Movements and Electrical Signals, p. 390

• While the membranes of all cells produce
  electrical signals by ion movements
  (transmembrane potential), this function is
  particularly important to neurons.

3
The main membrane processes involved in neural
activities are

• Figure 12-7
• resting potential the transmembrane potential of
  a resting cell
• graded potential a temporary localized change in
  the resting potential, caused by a stimulus
• action potential an electrical impulse (produced
  by the graded potential) that propagates along
  the surface of an axon to a synapse.
• synaptic activity the release of
  neurotransmitters at the presynaptic membrane,
  which produce graded potentials in a postsynaptic
  membrane.
• information processing the response (integration
  of stimuli) of a postsynaptic cell.

4
Fig. 12-7, p. 391
5
The Transmembrane Potential, p. 390

• Figure 12-8
• The 3 main requirements for a transmembrane
  potential (review chapter 3) are
• A concentration gradient of ions (Na, K) across
  the cell membrane
• The membrane be selectively permeable through
  membrane channels
• Passive and active transport mechanisms maintain
  a difference in charge across the membrane
  (resting potential -70 mV)

6
Fig. 12-8, p. 392
7
The Transmembrane Potential

• Passive forces acting across the membrane are
  chemical and electrical.

8
1. Chemical gradients

• Concentration gradients of ions (Na, K) across
  the membrane

9
2. Electrical gradients

• the charges of positive and negative ions are
  separated across the membrane, resulting in a
  potential difference.
• positive and negative charges attract one another
• if charges are not separated, they will move to
  eliminate potential difference, resulting in an
  electrical current
• how much current a membrane can restrict is
  called its resistance

10
Electrochemical gradient

• Figure 12-9
• the sum of chemical and electrical forces acting
  on an ion (Na, K) across a cell membrane is the
  electrochemical gradient for that ion.

11
Electrochemical gradient

• Figure 12-9
• chemical gradient of potassium tends to move
  potassium out of the cell, but the electrical
  gradient of the cell membrane opposes this
  movement (Figure 12-9a)

12
Electrochemical gradient

• Figure 12-9
• the transmembrane potential at which there is no
  net movement of a particular ion across the cell
  membrane is the equilibrium potential for that
  ion (K -90 mV, Na 66 mV).
• the electrochemical gradient is a form of
  potential energy

13
Fig. 12-9, p. 393
14
Fig. 12-9a, p. 393
15
Fig. 12-9b, p. 393
16
Fig. 12-9c, p. 393
17
Fig. 12-9d, p. 393
18
Changes in the Transmembrane Potential, p. 394

• The transmembrane potential rises or falls in
  response to temporary changes in membrane
  permeability resulting from opening or closing
  specific membrane channels.

19
Changes in the Transmembrane Potential, p. 394

• It is primarily the membranes permeability to
  sodium and potassium ions that determines
  transmembrane potential.
• Sodium and potassium channels are either passive
  or active.

20
Changes in the Transmembrane Potential, p. 394

• Passive channels (leak channels) are always open,
  but their permeability changes according to
  conditions.
• Active channels (gated channels) open and close
  in response to stimuli. At the resting potential,
  most gated channels are closed.

21
Changes in the Transmembrane Potential, p. 394

• Gated channels can be in one of 3 conditions
• closed, but capable of opening
• open (activated)
• closed, and not capable of opening (inactivated)

22
There are 3 classes of gated channels

• Figure 12-10
• chemically regulated channels
• voltage-regulated channels
• mechanically regulated channels

23
There are 3 classes of gated channels

• Figure 12-10
• chemically regulated channels
• open in response to the presence of specific
  chemicals (e.g. ACh) at a binding site
• found on the dendrites and cell body of a neuron

24
There are 3 classes of gated channels

• Figure 12-10
• voltage-regulated channels
• respond to changes in the transmembrane potential
• characteristic of excitable membrane
• found in axons of neurons, sarcolemma of skeletal
  muscle fibers, and cardiac muscle cells
• have an activation gate (opens) and an
  inactivation gate (closes)

25
There are 3 classes of gated channels

• Figure 12-10
• mechanically regulated channels
• open in response to distortion of the membrane
• found in sensory receptors for touch, pressure
  and vibration

26
Fig. 12-10, p. 395
27
Fig. 12-10a, p. 395
28
Fig. 12-10b, p. 395
29
Fig. 12-10c, p. 395
30
Key

• A transmembrane potential exists across the cell
  membrane.
• It is there because (1) the cytosol differs from
  extracellular fluid in chemical and ionic
  composition and (2) the cell membrane is
  selectively permeable.
• The transmembrane potential can change from
  moment to moment, as the cell membrane changes
  its permeability in response to chemical or
  physical stimuli.

31
Graded Potentials, p. 396

• Graded potentials (local potentials) are changes
  in transmembrane potential that cannot spread far
  from the site of stimulation.
• Any stimulus that opens a gated channel produces
  a graded potential

32
Opening a sodium channel produces a graded
potential

• Figure 12-11
• The resting membrane is exposed to a chemical
  that opens the sodium channel. Sodium ions enter
  the cell, raising the transmembrane potential. A
  shift in transmembrane potential toward 0 mV is
  called depolarization.
• The movement of sodium ions through the channel
  produces a local current that depolarizes nearby
  parts of the cell membrane (the graded
  potential). The change in transmembrane potential
  is proportional to the stimulus.

33
Fig. 12-11, p. 397
34
Fig. 12-11 top, p. 397
35
Fig. 12-11, Step 1, p. 397
36
Fig. 12-11, Step 2, p. 327
37
graded potentials

• Figure 12-12
• When the stimulus is removed, the transmembrane
  potential returns to normal (repolarization).
• Opening a potassium channel has the opposite
  effect of opening a sodium channel (since
  positive ions move out of, instead of into the
  cell), increasing the negativity of the resting
  potential (hyperpolarization).

38
Fig. 12-12, p. 398
39
graded potentials

• Table 12-2 summarizes the basic characteristics
  of graded potentials.

40
Action Potentials, p. 398

• Action potentials are propagated changes in
  transmembrane potential that affect an entire
  excitable membrane.

41
Action Potentials, p. 398

• The stimulus that initiates an action potential
  is a graded depolarization of the axon hillock,
  large enough (10 to 15 mV) to reach the threshold
  level required to open voltage-regulated sodium
  channels (usually -60 to -55 mV).

42
Action Potentials, p. 398

• A smaller stimulus will produce only a local,
  graded depolarization.
• As long as a stimulus exceeds the threshold
  amount, the action potential is the same, no
  matter how large the stimulus may be (much like
  the pressure on the trigger of a gun). Either an
  action potential is triggered, or it is not. This
  is called the all-or-none principle.

43
An action potential is generated in 4 steps
Figure 12-13

• Depolarization to threshold.

44
An action potential is generated in 4 steps
Figure 12-13

• 2. Activation of sodium channels and rapid
  depolarization
• sodium ions rush into the cytoplasm
• the inner membrane surface changes from negative
  to positive

45
An action potential is generated in 4 steps
Figure 12-13

• 3. Inactivation of sodium channels and activation
  of potassium channels
• at 30 mV, inactivation gates of sodium channels
  close (sodium channel inactivation), and
  potassium channels open, beginning repolarization

46
An action potential is generated in 4 steps
Figure 12-13

• 4. Return to normal permeability
• potassium channels begin to close when the
  membrane reaches normal resting potential (-70
  mV)
• when potassium channels finish closing, the
  membrane is hyperpolarized to -90 mV
• transmembrane potential then returns to resting
  level, and the action potential is over
• Table 12-3 summarizes the generation of action
  potentials

47
Fig. 12-13, p. 399
48
Fig. 12-13, part 1, p. 399
49
Fig. 12-13, Step 1, p. 399
50
Fig. 12-13, Step 2, p. 399
51
Fig. 12-13, Step 3, p. 399
52
Fig. 12-13, Step 4, p. 399
53
The Refractory Period

• During the time period from the beginning of the
  action potential to the return to resting state
  (the refractory period), the membrane will not
  respond normally to additional stimuli.

54
The Refractory Period

• The refractory period is divided into
• the absolute refractory period, when sodium
  channels are either open or inactivated and no
  action potential is possible
• the relative refractory period, when the membrane
  potential is almost normal, and a very large
  stimulus can initiate an action potential.

55
The Sodium-Potassium Exchange Pump

• A stimulated neuron can produce 1000 action
  potentials per second.
• Maintaining the concentration gradients of Na
  and K over time requires the sodium potassium
  pump, which in turn requires energy in the form
  of ATP (1 ATP for each exchange of 2K for 3
  Na).
• If a cell ran out of ATP, neurons would stop
  functioning.

56
Propagation of Action Potentials

• Once an action potential has been generated in
  the axon hillock, it must be propagated along the
  entire length of the axon.
• The term propagation is used because it is a
  series of repeated actions rather than a passive
  flow.

57
Propagation of Action Potentials

• Action potentials travel along axons by
• continuous propagation (unmyelinated axons)
• saltatory propagation (myelinated axons)

58
Action potentials

• Figure 12-14
• Action potentials move along an unmyelinated axon
  by continuous propagation, in which the moving
  action potential affects one segment of the axon
  at a time
• The action potential in segment 1 depolarizes the
  membrane to 30 mV.
• A local current depolarizes the 2nd segment to
  threshold.
• The 2nd segment develops an action potential, and
  the 1st segment enters the refractory period.
• A local current depolarizes the next segment to
  threshold, and the cycle repeats, propagating the
  action potential along the axon in 1 direction
  only, at a speed of about 1 meter/sec.

59
Fig. 12-14, Part 1, p. 401
60
Fig. 12-14, Step 1, p. 401
61
Fig. 12-14, Step 2, p. 401
62
Fig. 12-14, Step 3, p. 401
63
Fig. 12-14, Step 4, p. 401
64
Action potentials

• Figure 12-15
• An action potential moves along a myelinated axon
  by saltatory propagation, which is faster and
  uses less energy than continuous propagation.
  (Continuous propagation cannot occur in
  myelinated axons because of the insulating effect
  of the myelin.) In saltatory propagation, the
  local current produced by the action potential
  jumps from node to node along the axon, so
  depolarization occurs only at the nodes.
• Table 12-4 reviews the key differences between
  graded potentials and action potentials.

65
Fig. 12-15 top, p. 403
66
Fig. 12-15, Steps 12, p. 403
67
Fig. 12-15, Step 34, p. 403
68
Axon Diameter and Propagation Speed

• Since ion movement is related to the cytoplasm,
  the diameter of an axon affects the speed of an
  action potential -- the larger the diameter, the
  lower the resistance.

69
Axon Diameter and Propagation Speed

• There are 3 groups of axons, depending on
  diameter, myelination and speed of their action
  potentials
• Type A fibers
• myelinated
• large diameter
• high speed (140 m/sec)
• Type B fibers
• myelinated
• medium diameter
• medium speed (18 m/sec)
• Type C fibers
• unmyelinated
• small diameter
• slow speed (1 m/sec)

70
Type A fibers

• Type A fibers carry the most time-sensitive
  information to and from the CNS (senses of
  position, balance and touch and motor impulses
  to skeletal muscles). All other signals (sensory
  information, peripheral effectors, and
  involuntary muscle and gland controls) travel by
  Type B and Type C fibers.

71
Key

• Information travels within the nervous system
  primarily in the form of propagated electrical
  signals known as action potentials.
• The most important information -- including
  vision and balance sensations, and the motor
  commands to skeletal muscles -- is carried by
  large-diameter myelinated axons.

72
V. Synaptic Activity, p. 404

• To be effective, action potentials (also called
  nerve impulses)
• must be transmitted across a synapse, from a
  presynaptic neuron to a postsynaptic neuron or
  other type of postsynaptic cell.

73
There are 2 types of synapses

• electrical synapses in which there is direct
  physical contact between cells, and
• chemical synapses in which the signal is
  transmitted across a gap by a chemical
  neurotransmitter.

74
Electrical Synapses

• Electrical synapses are locked together at gap
  junctions held together by connexons which allow
  ions to pass between the cells, producing a
  continuous local current and action potential
  propagation (Figure 4-2).
• Electrical synapses are found in some areas of
  the brain, eye, and ciliary ganglia.

75
Chemical Synapses

• Most synapses between neurons and all synapses
  between neurons and other types of cells are
  chemical synapses.
• At a chemical synapse, the cells are not in
  direct contact. An arriving action potential may
  or may not be propagated to the postsynaptic
  cell, depending on the amount of neurotransmitter
  released and the sensitivity of the postsynaptic
  cell.

76
Chemical Synapses

• There are 2 classes of neurotransmitters, based
  on their effects on postsynaptic membranes
• Excitatory neurotransmitters cause depolarization
  and promote action potentials.
• Inhibitory neurotransmitters cause
  hyperpolarization and suppress action potentials.

77
Chemical Synapses

• However, the effect of a neurotransmitter on a
  postsynaptic membrane depends on the properties
  of the receptor, not on the nature of the
  neurotransmitter.
• For example, the neurotransmitter acetylcholine
  (ACh) usually promotes action potentials, but
  inhibits neuromuscular junctions in the heart.

78
Chemical Synapses

• Figure 12-16
• Synapses that release acetylcholine are
  cholinergic synapses.
• ACH is released at
• all neuromuscular junctions involving skeletal
  muscle fibers
• many synapses in the CNS
• all neuron-to-neuron synapses in the PNS
• all neuromuscular and neuroglandular junctions
  within the parasympathetic division of the ANS

79
Events at a Cholinergic Synapse

• Figure 12-16
• An action potential arrives and depolarizes the
  synaptic knob.
• Extracellular calcium ions enter the synaptic
  knob, triggering the exocytosis of ACh.
• ACH binds to receptors and depolarizes the
  postsynaptic membrane.
• AChE breaks down ACh into molecules of acetate
  and choline
• Table 12-5 summarizes the events that occur at a
  cholinergic synapse

80
Fig. 12-16, Step 1, p. 406
81
Fig. 12-16, Step 2, p. 406
82
Fig. 12-16, Step 3, p. 406
83
Fig. 12-16, Step 4, p. 406
84
Synaptic Delay

• A 0.2 - 0.5 msec synaptic delay occurs between
  the arrival of the action potential at the
  synaptic knob and the effect on the postsynaptic
  membrane -- a long time relative to the speed of
  the action potential along the axon.
• The fewer synapses involved in relaying a
  message, the faster the response.
• Reflexes may involve only one synapse because
  they are important to survival.

85
Synaptic Fatigue

• When a neurotransmitter cannot be recycled fast
  enough to meet the demand of an intense stimulus,
  synaptic fatigue occurs.
• The synapse becomes inactive until ACh is
  replenished.

86
The Activities of Other Neurotransmitters, p. 408

• At least 50 neurotransmitters have been
  identified so far, including certain amino acids,
  peptides, prostaglandins, ATP, and some dissolved
  gases.

87
The Activities of Other Neurotransmitters, p. 408

• Other than acetylcholine, some of the most
  important neurotransmitters are
• norepinephrine (NE)
• dopamine
• serotonin
• gamma aminobutyric acid (GABA)

88
The Activities of Other Neurotransmitters, p. 408

• norepinephrine (NE)
• found in the brain and portions of the ANS
• effect is excitatory and depolarizing
• synapses that release NE are called adrenergic
  synapses

89
The Activities of Other Neurotransmitters, p. 408

• - dopamine
• a CNS neurotransmitter
• may be excitatory or inhibitory
• involved in Parkinsons disease and cocaine use

90
The Activities of Other Neurotransmitters, p. 408

• - serotonin
• a CNS neurotransmitter
• affects attention and emotional states

91
The Activities of Other Neurotransmitters, p. 408

• gamma aminobutyric acid (GABA)
• has an inhibitory effect
• its functions in the CNS are not well understood

92
Key

• At a chemical synapse, a synaptic terminal
  releases a neurotransmitter that binds to the
  postsynaptic cell membrane.
• The result is a temporary, localized change in
  the permeability or function of the postsynaptic
  cell.
• This change may have broader effects on the cell,
  depending on the nature and number of stimulated
  receptors.
• Many drugs affect the nervous system by
  stimulating receptors that otherwise respond only
  to neurotransmitters.
• These drugs can have complex effects on
  perception, motor control and emotional states.

93
Neuromodulators, p. 408

• In addition to neurotransmitters, synaptic knobs
  may release other chemicals, called
  neuromodulators.
• (There may be little functional difference
  between neurotransmitters and neuromodulators.)

94
Neuromodulators, p. 408

• Some characteristics of neuromodulators are
• Their effects are long-term and slow to appear.
• They trigger responses that involve multiple
  steps and intermediary compounds.
• They may affect the presynaptic membrane,
  postsynaptic membrane, or both.
• They can be released alone or with a
  neurotransmitter.

95
Neuromodulators, p. 408

• Neuropeptides are neuromodulators that act by
  binding to receptors and activating enzymes.

96
Neuromodulators, p. 408

• Opioids are neuromodulators that bind to the same
  receptors as opium or morphine, and probably
  function to relieve pain. There are 4 classes of
  opioids in the CNS
• 1. endorphins
• 2. enkephalins
• 3. endomorphins
• 4. dynorphins
• Table 12-6 lists the major neurotransmitters and
  neuromodulators of the brain and spinal cord, and
  their primary effects.

97
How Neurotransmitters and Neuromodulators Work,
p. 409

• There are 3 groups of neurotransmitters and
  neuromodulators
• 1. Compounds that have a direct effect on
  membrane potential
• 2. Compounds that have an indirect effect on
  membrane potential
• 3. Lipid soluble gases that affect the inside of
  the cell

98
Fig. 12-17, p. 409
99
Fig. 12-17a, p. 409
100
Fig. 12-17b, p. 409
101
Fig. 12-17c, p. 409
102
VI. Information Processing by Individual Neurons,
p. 412

• A single neuron has many dendrites and can
  receive many neurotransmitter messages at the
  same time -- some excitatory, some inhibitory.
• The net effect on the axon hillock, which
  determines whether or not an action potential
  will be produced, is the simplest level of
  information processing.

103
Postsynaptic Potentials, p. 412

• The graded potentials that develop in a
  postsynaptic cell in response to
  neurotransmitters are called postsynaptic
  potentials. The 2 major types of postsynaptic
  potentials are
• excitatory postsynaptic potential (EPSP)
• a graded depolarization of the postsynaptic
  membrane
• inhibitory postsynaptic potential (IPSP)
• a graded hyperpolarization of the postsynaptic
  membrane

104
Summation

• Figure 12-18
• A single EPSP is not enough to trigger an action
  potential. EPSPs (and IPSPs) must combine through
  a process called summation. If the resulting
  depolarization is large enough, an action
  potential is triggered.

105
Fig. 12-18, p. 413
106
Fig. 12-18a, p. 413
107
Fig. 12-18b, p. 413
108
Facilitation

• As EPSPs accumulate, raising the transmembrane
  potential closer to threshold, the neuron becomes
  facilitated, making it easier for even a small
  stimulus to trigger an action potential.

109
Neuromodulators and hormones

• Figure 12-19
• Neuromodulators and hormones can change a
  membranes sensitivity to excitatory or
  inhibitory neurotransmitters, shifting the
  balance between EPSPs and IPSPs.

110
Fig. 12-19, p. 414
111
Presynaptic Inhibition and Presynaptic
Facilitation, p. 414

• Figure 12-20
• Synapses may be found not only between cell
  bodies and dendrites, but also between the axons
  of 2 neurons (an axoaxonal synapse).
• An axoaxonal synapse at a synaptic knob may
  either decrease the amount of neurotransmitter
  released by the presynaptic membrane (presynaptic
  inhibition) or increase the amount of
  neurotransmitter released by the presynaptic
  membrane (presynaptic facilitation).

112
Fig. 12-20, p. 415
113
Fig. 12-20a, p. 415
114
Fig. 12-20b, p. 415
115
The Rate of Generation of Action Potentials, p.
415

• The information received by a postsynaptic cell
  is often interpreted only in terms of the
  frequency or rate of action potentials received.
• The frequency of action potentials in turn
  depends on the degree of depolarization above
  threshold.
• Holding the membrane above threshold for an
  extended period of time has the same effect as
  applying a second, larger stimulus, reducing the
  relative refractory period.
• Table 12-7 summarizes the basic principles of
  information processing.

116
Key

• In the nervous system, the changes in
  transmembrane potential that determine whether or
  not action potentials are generated represent the
  simplest form of information processing.

117
SUMMARY

• In chapter 12 we learned
• - about neural tissue and its basic functional
  unit, the neuron
• - the anatomical divisions of the nervous system
• - central nervous system and peripheral nervous
  system
• - nerves and axons
• - the functional divisions of the nervous system
• - the afferent division and its receptors
• - the efferent division and its effectors
• - the somatic and autonomic nervous systems
• - the structure of neurons
• - organelles of the neuron
• (neurofilaments, neurotubules, neurofibrils)
• - structures of the axon
• (axon hillock, initial segment, axoplasm)
• - the synapse and neurotransmitters

118
SUMMARY

• - the classification of neurons
• - structural classifications
• (anaxonic, bipolar, unipolar and multipolar)
• - functional classifications
• (sensory, motor and interneurons)
• - the 4 types of neuroglia
• - ependymal, astrocytes, oligodendrocytes,
  microglia
• - about ganglia and neurons of the PNS
• - satellite cells, Schwann cells
• - about repair of neurons in the PNS (Wallerian
  regeneration)
• - the transmembrane potential
• - electrochemical gradient
• - passive and active channels
• - gated channels
• - chemically regulated, voltage-regulated, and
  mechanically regulated
• - graded potentials
• - depolarization and hyperpolarization
• - action potentials
• - threshold

119
SUMMARY

• - the transmission of a nerve impulse across a
  synapse
• - presynaptic and postsynaptic neurons
• - electrical and chemical synapses
• - excitatory and inhibitory neurotransmitters
• - cholinergic synapses (ACh)
• - other neurotransmitters (NE, dopamine,
  seratonin, GABA)
• - neuromodulators
• - direct, indirect and lipid-soluble gases
• - information processing
• - integration of postsynaptic potentials
• - EPSPs and IPSPs
• - spatial and temporal summation
• - presynaptic inhibition and facilitation
• - the rate of generation of action potentials