Chapter 12: Neural Tissue

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Chapter 12 Neural Tissue
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Neural Tissue

• 3 of body mass
• Cellular, 20 extracellular space
• Two categories of cells
• Neurons conduct nervous impulses
• - cells that send and receive signals
• Neuroglia/glial cells nerve glue
• Supporting Cells
• Protect neurons

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Organs of the Nervous System

• Brain and spinal cord
• Sensory receptors of sense organs (eyes, ears,
  etc.)
• Nerves connect nervous system with other systems

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Nervous Systems

• Central Nervous System (CNS)
• Spinal cord, brain
• Functions
• integrate, process, coordinate sensory input and
  motor output
• Peripheral Nervous System (PNS)
• All neural tissue outside of CNS
• Functions Carry info to/from the CNS via nerves
• Nerves
• Bundle of axons (nerve fibers) with blood vessels
  and CT
• Carry sensory information and motor commands in
  PNS
• Cranial nerves brain
• Spinal nerves spinal cord

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Division of PNS

• Sensory/Afferent Division carries sensory
  information
• Sensory receptors ? CNS
• Somatic afferent division
• - From skin, skeletal muscles, and joints
• Visceral afferent division
• - From internal organs

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Division of PNS

• 2. Motor/Efferent Division carries motor
  commands
• CNS ? effectors
• Somatic Nervous System Controls skeletal muscle
  contractions
• - voluntary nervous system
• To skeletal muscles ? contractions
• Autonomic Nervous System (ANS)
• involuntary nervous system
• To smooth and cardiac muscle, glands ?
  contractions
• Sympathetic Division stimulating effect
• - fight or flight
• Parasympathetic Division relaxing effect
• rest and digest
• Tend to be Antagonistic to Each Other

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Receptors and Effectors

• Receptors
• detect changes or respond to stimuli
• neurons and specialized cells
• complex sensory organs (e.g., eyes, ears)
• Effectors
• respond to efferent signals
• cells and organs

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What would damage to the afferent division of the
PNS affect?

1. ability to learn new facts
2. ability to experience motor stimuli
3. ability to experience sensory stimuli
4. ability to remember past events

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The structure of a typical neuron, and the
function of each component.
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Histology of Nervous System

• Neuron/Nerve Cell
• Function conduct nervous impulses (message)
• Characteristics
• Extreme longevity
• Amitotic
• - Direct cell division by simple cleavage of the
  nucleus without spindle formation or appearance
  of chromosomes
• - exceptions hippocampus, olfactory receptors
• High metabolic rate need O2 and glucose

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The Structure of Neurons
Figure 121
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The Structure of Neurons

• Large soma/perikaryon(cytoplasm)
• Large nucleus, large nucleolus (rRNA)
• Many mitochondria, ribosomes, RER, Golgi
• Increase ATP, increase protein synthesis to
  produce neurotransmitters
• Nissl bodies visible RER and ribosomes, gray
• Neurofilaments internal structure
• Neurofibrils, neurotubules
• No centrioles
• 2 types of processes (cell extension)
• Dendrite
• Axon

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Regions of a Neuron

• Dendrites
• Receive info
• Carry a graded potential toward soma
• Contain same organelles as soma
• Short, branched
• End in dendritic spines

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Regions of a Neuron

• Axon
• single, long
• Carry an action potential away from soma
• Release neurotransmitters at end to signal next
  cell
• Long ones nerve fibers
• Contains
• Neurofibrils and neurotubules (abundant)
• Vesicles of neurotransmitter
• Lysosomes, mitochondria, enzymes
• No nissl bodies, no golgi (no protein synthesis
  in axon)

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Regions of a Neuron

• 2. Axon
• Connects to soma at axon hillock
• Covered in axolemma (membrane) --- Axoplasm
  (cytoplasm)
• May branch axon collaterals
• End in synaptic terminals or knobs
• May have myelin sheath proteinlipid
• Function
• Protection, Insulation, and Increase speed of
  impulse
• CNS myelin from Oligodendrocytes
• PNS myelin from Schwann cells

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Axoplasmic Transport

• Move materials between soma and terminal
• Large molecules synthesized in the cell body,
  such as vesicles and mitochondria are unable to
  move via simple diffusion
• Large molecules are transported by motor proteins
  called kinesins, which walk along neurotubule
  tracks to their destinations.
• Anterograde transport soma ? terminal
• neurotransmitters from soma
• Retrograde transport terminal ? soma
• Recycle breakdown products from used
  neurotransmitters
• Some viruses use retrograde transport to gain
  access to CNS (polio, herpes, rabies)

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Synapse

• Site where neuron communicates
• with another cell
• neuron or effector
• Presynaptic cell sends message
• along axon to axon terminal
• Postsynaptic cell receives message
• as neurotransmitter
• Neurotransmitter chemical, transmits signal
  from pre- to post- synaptic cell across synaptic
  cleft
• Synaptic knob small, round, when postsynaptic
  cell is neuron, synapse on dendrite or soma
• Synaptic terminal complex structure, at
  neuromuscular or neuroglandular junction

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Structural Classification of Neurons

• Anaxonic neurons
• Dendrites and axon look same
• Brain and special sense organs
• Bipolar neurons
• 1 dendrite, 1 axon
• Soma in middle
• Rare special sense organs,
• relay from receptor to neuron
• Unipolar neurons
• 1 long axon, dendrites at one end,
• soma off side (T shape)
• Most sensory neurons
• Multipolar neurons
• 2 or more dendrites
• 1 long axon
• 99 of all neurons
• Most CNS

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A tissue sample shows unipolar neurons. Are
these more likely to be sensory neurons or motor
neurons?

1. sensory neurons
2. motor neurons

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Functional Classification of Neurons

• Sensory/Afferent Neurons
• Transmit info from sensory receptors to CNS
• Mostly unipolar neurons
• Soma in peripheral sensory ganglia
• Ganglia collection of cell bodies in PNS
• Somatic Sensory Neurons
• - Receptors monitor outside conditions
• Visceral Sensory Neurons
• Receptors monitor internal conditions

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Functional Classification of Neurons

• Motor/Efferent Neurons
• Transmit commands from CNS to effectors
• Mostly multipolar neurons
• Somatic Motor Neurons
• Innervate skeletal muscle
• Innervation distribution of sensory/motor
  nerves to a specific region/organ
• Conscious control or reflexes
• Visceral/Autonomic Motor Neurons
• - Innervate effectors on smooth muscle, cardiac
  muscle, glands, and adipose

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Functional Classification of Neurons

• Interneurons/Association Neurons
• Distribute sensory info and coordinate motor
  activity
• Between sensory and motor neurons
• In brain, spinal cord, autonomic ganglia
• Most are multipolar

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The locations and functions of neuroglia.
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Neuroglia

• Neuroglia supporting cells
• Neuroglia in CNS
• Outnumber neurons 101
• Half mass of brain
• Neuroglia Cell in the CNS
• Ependymal cells
• Astrocytes
• Oligodendrocytes
• Microglia

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Neuroglia Cells of the CNS

• Ependymal Cells
• Line central canal of spinal cord and ventricles
  of the brain
• Secrete cerebrospinal fluid (CSF)
• Have cilia to circulate CSF
• CSF cushion brain, nutrient and gas exchange
• Astrocytes
• Most abundant CNS neuroglia
• Varying functions
• Blood brain barrier
• Processes wrap capillaries
• Control chemical exchange between blood and
  interstitial fluid of the brain
• Framework of CNS
• Repair damaged neural tissue
• Guide neuron development in embryo
• Control interstitial environment
• - Regulate conc. Ions, gasses, nutrients,
  neurotransmitters

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Neuroglia Cells of the CNS

• Oligodendrocytes
• Wide flat processes wrap around local axons
  myelin sheath
• 1 cell contributes myelin to many neighboring
  axons
• Lipid in membrane insulates axon for faster
  action potential conductance
• Gaps on axon between processes/myelin nodes of
  Ranvier, necessary to conduct impulse
• White, myelinated axons white matter
• Microglia
• Phagocytic
• Wander CNS
• Engulf debris, pathogens
• Important CNS defense
• No immune cells or antibodies

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Neuroglia of the CNS
Figure 124
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Neuroglia in PNS

• Satellite Cells
• Surround somas in ganglia
• Isolate PNS cells
• Regulate interstitial environment of ganglia
• Ganglia mass of neuronal soma and dendrites
• Schwann cells
• Myelinate axon in PNS
• Whole cells wraps axon, many layers
• Neurilemma bulge of schwann cell,
• contains organelles
• Nodes of Ranvier between cells

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Neuroglia in PNS

• Schwann Cells cont.
• Some hold bundles of unmyelinated axon
• Vital to repair of peripheral nerve fibers after
  injury
• Guide growth to original synapse

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Which type of neuroglia would occur in larger
than normal numbers in the brain tissue of a
person with a CNS infection?

1. astrocytes
2. microglial cells
3. ependymal cells
4. oligodendrocytes

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Neural Responses to Injuries
Figure 126 (1 of 2)
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Neural Responses to Injuries
Figure 126 (2 of 2)
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KEY CONCEPT

• Neurons perform all communication, information
  processing, and control functions of the nervous
  system
• Neuroglia preserve physical and biochemical
  structure of neural tissue, and are essential to
  survival and function of neurons

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How the resting potential is created and
maintained.
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5 Main Membrane Processes in Neural Activities
Figure 127 (Navigator)
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Neurophysiology

• Neurons conduct electrical impulse
• Requires transmembrane potential electrical
  difference across the cell membrane
• Cells positive charge outside (pump cations out)
  and negative charge inside (protein)
• Voltage measure of potential energy generated
  by separation of opposite charges
• Current flow of electrical charges (ions)
• Cell can produce current (nervous impulse) when
  ions move to eliminate the potential difference
  (volts) across the membrane
• Resistance Restricts ion movement (current)
• High resistance low current
• Membrane has resistance, restricts ion
  flow/current

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Neurophysiology

• Ohms Law current voltage resistance
• Current is highest when
• Voltage is High Resistance is Low
• Cell voltage set at -70mV
• membrane resistance can be altered to create
  current
• Membrane resistance depends on permeability to
  ions
• open or closed ion channels
• Cell must always have some resistance or ions
  would equalize, voltage zero
• No current generated no nervous impulse

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Membrane Ion Channels

• Allow ion movement (alter resistance)
• Each channel is specific to one ion type
• Passive Channels (leaky channels)
• Active Channels
• Chemically regulated/ligand-gated
• Voltage regulated channels
• Mechanically regulated channels

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Membrane Ion Channels

• Passive Channels (leaky channels)
• - Resting Potential
• Always open, free flow
• Sets resting membrane potential at -70mV

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Active Channels Gated Channels
Figure 1210
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Membrane Ion Channels

• Active Channels
• open/close in response to signal
• Chemically regulated/ligand-gated
• Open in response to chemical binding
• Located on any cell membrane
• Dendrites and soma

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Membrane Ion Channels

• 2. Active Channels
• B. Voltage regulated channels
• - open/close in response to shift in
• transmembrane potential
• - excitable membrane only conduct action
• potentials
• - axolemma, sarcolemma

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Membrane Ion Channels

• 2. Active Channels
• C. Mechanically Regulated Channels
• - Open in response to membrane distortion
• - On dendrites of sensory neurons for
• - touch, pressure, vibration

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Membrane Ion Channels

• When channel opens, ions flow along
  electrochemical gradient
• Diffusion (high conc. to low)
• Electrical attraction/repulsion

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Sodium-Potassium Pump
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Sodium-Potassium Pump

• Uses ATP to move 3 Na out and 2 K in
• 70 of neurons use ATP for this
• Runs anytime the cell is not conducting an
  impulse
• Creates high K inside and high Na outside
• When Na channel opens
• Na flows into cell
• Favored by diffusion gradient
• Favored by electrical gradient
• Open channel decr. Resistance incr. ion
  flow/current
• When K channel opens
• K flows out of cell
• Favored by diffusion gradient only
• Electrical gradient repels K exit
• - Thus less current than Na

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• Channels open resistance low ions move until
  equilibrium potential depends on
• Diffusion gradient
• Electrical gradient
• Equilibrium Potential

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Electrical vs. Chemical Gradients

• The electrical gradient opposes the chemical
  gradient
• K inside and outside of the cell are attracted
  to the negative charges on the inside of the cell
  membrane, and repelled by the positive charges on
  the outside of the cell membrane
• indicated in white on the next slide
• Chemical gradient is strong enough to overpower
  the electrical gradient, but this weakens the
  force driving K out of the cell
• Net driving force indicated in grey on the next
  slide
• The Electrochemical Gradient

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Electrochemical Gradients
Figure 129c, d
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Summary Resting Potential
Table 12-1
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Changes in Transmembrane Potential

• Transmembrane potential rises or falls
• in response to temporary changes in membrane
  permeability
• resulting from opening or closing specific
  membrane channels
• Membrane permeability to Na and K determines
  transmembrane potential
• Sodium and potassium channels are either passive
  or active

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Graded Potentials The Resting State

• Opening sodium channel produces a current which
  causes graded potential

Figure 1211 (Navigator)
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Graded Potential

• Graded potential
• localized shift in transmembrane potential due to
  movement of charges into/out of cell
• Na channel opens Na flows in
• depolarization (cell less negative)
• K channel opens K flows out
• hyperpolarization (cell more negative)

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Graded Potentials

• Occur on dendrites and somas
• Can be depolarizing or hyperpolarizing
• Amount of depolarization or hyperpolarization
  depends on the intensity of stimulus
• Incr. channels open Incr. voltage change
• Passive spread from site of stimulation over
  short distance
• Effect on membrane potential decreases with
  distance from stimulation site
• Repolarization occurs as soon as stimulus is
  removed
• Leaky channels and Na/K pump reset resting
  potential

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Graded Potentials

• Localized change in transmembrane potential, not
  nervous impulse (message)
• If big enough depolarization
• Action potential ? nervous impulse ? transmission
  to next cell

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Graded Potentials Step 1

• Resting membrane exposed to chemical
• Sodium channel opens
• Sodium ions enter the cell
• Transmembrane potential rises
• Depolarization occurs
• A shift in transmembrane potential toward 0 mV

Figure 1211 (Step 1)
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Graded Potentials Step 2

• Movement of Na through channel
• Produces local current
• Depolarizes nearby cell membrane (graded
  potential)
• Change in potential is proportional to stimulus

Figure 1211 (Step 2)
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Characteristics of Graded Potentials
Table 12-2
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Action Potential

• Occur on excitable membranes only
• Axolemma, sarcolemma
• Always depolarizing
• Must depolarize to threshold (-55mV) before
  action potential begins
• Voltage gated channels on excitable membrane open
  at threshold to propagate action potential
• all-or-none
• All stimuli that exceed threshold will produce
  identical action potentials
• Action potential at one site depolarizes adjacent
  sites to threshold
• Propagated across entire membrane surface without
  decrease in strength

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Generating the Action Potential
Figure 1213 (Navigator)
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Generation of an Action Potential

• Depolarization to threshold
• A graded potential depolarizes local membrane and
  flows toward the axons
• If threshold is met (-55mV) at the hillock, an
  action potential will be triggered
• Activation of sodium channels and rapid
  depolarization
• At threshold (-55mV) , voltage-regulated sodium
  channels on the excitable membrane open
• Na flows into the cell depolarizing it
• The transmembrane potential rapidly changes from
  -55mV to 30 mV

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Generation of an Action Potential

• 3. Inactivation of sodium channels and
  activation of potassium channels
• At 30mV Na channels close and K channels open
• K flows out of the cell repolarizing it
• 4. Return to normal permeability
• At -70mV K channels begin to close
• The cell hyperpolarizes to -90mV until all
  channels finish closing
• Leak channels restore the resting membrane
  potential to -70mV

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Table 12-3
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Generation of an Action Potential

• Restimulation only when Na channels closed
• Influx of Na necessary for action potential
• Absolute Refractory Period
• Threshold (-55mV) to 30mV, Na channels open,
  membrane cannot respond to additional stimulus
• Relative Refractory Period
• 30mV to -70mV (return to resting potential)
• Na channels closed, membrane capable of second
  action potential but requires larger/longer
  stimulus (threshold elevated)
• Cell has ions for thousands of action potentials
• Eventually must run Sodium-Potassium pump (burn
  ATP) to reset high K inside and high Na
  outside

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How would a chemical that blocks the sodium
channels in neuron cell membranes affect a
neurons ability to depolarize?

1. It would enhance depolarization.
2. It would inhibit depolarization completely.
3. It would slow depolarization.
4. It would have no effect on depolarization.

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What effect would decreasing the concentration of
extracellular potassium ions have on the
transmembrane potential of a neuron?

1. repolarization
2. hypopolarization
3. decreased transmembrane potential
4. hyperpolarization

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Electrochemical Gradients
Figure 129c, d
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Propagation of Action Potential

• Once generated must be transmitted along the
  length of the axon hillock to terminal
• Speed depends on
• Degree of myelination
• Axon diameter

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2 Methods of Propagating Action Potentials

• Continuous propagation
• unmyelinated axons
• Saltatory propagation
• myelinated axons

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Propagation of Action Potential

• Myelination
• Continuous Propagation
• Unmyelinated axons
• Whole membrane depolarizes and repolarizes
  sequentially hillock to terminal
• Only forward movement
• Membrane behind always in absolute refractory
  period

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Continuous Propagation

• Of action potentials along an unmyelinated axon
• Affects 1 segment of axon at a time

Figure 1214
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Continuous Propagation Step 1

• Action potential in segment 1
• Depolarizes membrane to 30 mV

Figure 1214 (Step 1)
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Continuous Propagation Step 2

• Local current
• Depolarizes second segment to threshold

Figure 1214 (Step 2)
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Continuous Propagation Step 3

• Second segment develops action potential
• First segment enters refractory period

Figure 1214 (Step 3)
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Continuous Propagation Step 4

• Local current depolarizes next segment
• Cycle repeats
• Action potential travels in 1 direction (1
  m/sec)

Figure 1214 (Step 4)
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Propagation of Action Potential

• Myelination
• Saltatory Propagation
• Myelinated axons
• Depolarization only on exposed membrane at nodes
• Myelin insulates covered membrane from ion flow
• Action potential jumps from node to node
• Faster and requires less energy to reset

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Saltatory Propagation

• Of action potential along myelinated axon

Figure 1215
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Saltatory Propagation
Figure 1215 (Steps 1, 2)
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Saltatory Propagation
Figure 1215 (Steps 3, 4)
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Graded Potentials and Action Potentials
Table 124
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Axon Diameter and Propagation Speed

• Ion movement is related to cytoplasm
  concentration
• Axon diameter affects action potential speed
• The larger diameter, the lower the resistance

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Propagation of Action Potentials

• Axon Diameter
• Larger axon ? less resistance ? easier ion flow ?
  faster action potential
• Axons are classified by
• Diameter, myelination, speed of action potentials
• Three types of axons
• Type A, Type B, and Type C fibers

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Axon Diameter

• Type A Fibers
• - 4-20µm diameter
• Myelinated (saltatory propagation)
• Action potential 140m/sec
• Carry somatic motor and somatic sensory info
• Type B Fibers
• 2-4µm diameter
• Myelinated (saltatory propagation)
• Action potential 18m/sec
• Carry autonomic motor and visceral sensory info
• Type C Fibers
• lt 2µm diameter
• Unmyelinated (continuous propagation)
• Action potential 1m/sec
• Carry autonomic motor and visceral sensory info

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KEY CONCEPT

• Information travels within the nervous system
  as propagated electrical signals (action
  potentials)
• The most important information (vision, balance,
  motor commands) is carried by large-diameter
  myelinated axons

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Myelination

• Requires space, metabolically expensive
• Only important fibers large and myelinated
• Occurs in early childhood
• Results in improved coordination
• Multiple Sclerosis
• Genetic disorder, myelin on neurons in PNS
  destroyed ? numbness, paralysis

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Synapse

• Synapse
• Junction between transmitting neuron (presynaptic
  cell) and receiving cell (postsynaptic cell)
• Two types
• Electrical Synapse
• Direct contact via gap junctions
• Ion flow directly from pre to post cell
• Less common synapse
• In brain (conscious perception)
• Chemical Synapse
• - Most common

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• 2. Chemical Synapse
• Most common
• Pre and post cells separated by synaptic cleft
• Presynaptic neuron releases neurotransmitter to
  trigger effect on post synaptic cell
• Dynamic facilitate or inhibit transmission,
  depends on neurotransmitter
• Excititory Neurotransmitters
• Depolarization (shift from resting potential
  toward 0 mV)
• Propagate Action Potential
• Inhibitory Neurotransmitters
• Hyperpolarization (shift from resting potential
  to -80 mV)
• Suppress Action Potential
• Propagation across chemical synapse always slow
  but allow variability

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The events that occur at a chemical synapse.
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The Effect of a Neurotransmitter

• On a postsynaptic membrane
• depends on the receptor
• not on the neurotransmitter
• e.g., acetylcholine (ACh)
• usually promotes action potentials
• but inhibits cardiac neuromuscular junctions

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Synaptic Activity
Figure 1216 (Navigator)
92
Cholinergic Synapses

• Any synapse that releases ACh
• all neuromuscular junctions
• many synapses in CNS
• all neuron-to-neuron synapses in PNS
• all neuromuscular and neuroglandular junctions of
  ANS parasympathetic division

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Events at a Cholinergic Synapse Step 1

• Action potential arrives, depolarizes synaptic
  knob

Figure 1216 (Step 1)
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Events at a Cholinergic Synapse Step 2

• Calcium ions enter synaptic knob, trigger
  exocytosis of ACh

Figure 1216 (Step 2)
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Events at a Cholinergic Synapse Step 3

• ACh binds to receptors, depolarizes postsynaptic
  membrane

Figure 1216 (Step 3)
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Events at a Cholinergic Synapse Step 4

• AChE breaks ACh into acetate and choline

Figure 1216 (Step 4)
97
Events at a Cholinergic Synapse
Table 12-5
98
What effect would blocking voltage-regulated
calcium channels at a cholinergic synapse have on
synaptic communication?

1. Communication would cease.
2. Communication would be enhanced.
3. Communication would be misdirected.
4. Communication would continue as before.

99
Neurotransmitter Mechanism of Action

• Direct effect on membrane potential
• Open or close ion channels upon binding to the
  post synaptic cell
• Provides a rapid response
• E.g. Ach (cholinergic synapse)

100
Neurotransmitter Mechanism of Action

• Indirect effect on membrane potential
• Binds a receptor that activates a G protein in
  the post synaptic cell
• Active G protein activates a second messenger
• cAMP, cGMP, diacylglyceride, Ca
• The second messenger opens ion channels or
  activates enzymes
• Provides slower but longer lasting effects
• E.g. Norepinephrine (Adrenergic synapse)

101
Neurotransmitter Mechanism of Action

• Indirect effect on membrane potential
• Example of indirect action
• Neurotransmitter binds receptor
• Receptor activates G protein
• G Protein activates adenylate cyclase
• Adenylate cyclase converts ATP into cyclic AMP
• cAMP opens ion channels

102
ATP vs. cyclic AMP
103
Post Synaptic Potential

• Graded potential caused by a neurotransmitter due
  to opening or closing of ion channels on post
  synaptic cell membrane
• Two types
• Excitatory Post Synaptic Potential (EPSP)
• - Causes depolarization
• Inhibitory Post Synaptic Potential (IPSP)
• Causes hyperpolarization
• Inhibits postsynaptic cell

104
EPSP/IPSP Interactions
Figure 1219
105
Post Synaptic Potential

• Multiple EPSPs needed to trigger action potential
  in post cell axon
• EPSP summation
• Temporal and Spatial Summation
• Temporal Summation
• Single synapse fires repeatedly
• Each EPSP depolarizes more until threshold
  reached at hillock

106
Post Synaptic Potential

• EPSP summation
• Temporal and Spatial Summation
• Spatial Summation
• Multiple synapses fire stimultaneously
• Collective depolarization reaches threshold

107
Post Synaptic Potential

• Facilitated Depolarized
• Brought closer to threshold by some sort of
  stimulus
• Less stimulus now required to reach threshold
• E.g. Caffeine
• Neuromodulators
• Chemicals that influence synthesis, release, or
  degradation of neurotransmitters thus altering
  normal response of the synapse
• Most nervous system activities results from
  interplay of EPSPs and IPSPs
• promotes differing degrees of facilitation or
  inhibition to allow constant fine tuning of
  response

108
Common Neurotransmitters

• Acetycholine cholinergic synapses
• Excititory
• Direct effect
• Skeletal neuromuscular junctions, many CNS
  synapses, all neuron to neuron PNS, all
  parasympathetic ANS
• Norepinephrine adrenergic synapses
• Excititory
• Second messengers
• Many brain synapses, all sympathetic ANS effector
  junctions

109
Common Neurotransmitters

• Dopamine
• Excititory or inhibitory
• Second messengers
• Many brain synapses
• Cocaine inhibits removal high
• Parkinsons disease damage neurons ticks,
  jitters
• Serotonin
• Inhibitory
• Direct or second messenger
• Brain stem for emotion
• Anti-depression/anti-anxiety drugs block uptake
• Gamma aminobytyric acid (GABA)
• Inhibitory
• Direct effect
• Brain anxiety control, motor coordination
• Alcohol augments effects loss of coordination

110
Presynaptic Facilitation

• Activity at Axoaxonal synapse
• Increases the amount of neurotransmitter released
  when an action potential arrives at the synaptic
  knob.
• This increase enhances and the neurotransmitters
  effect on the Postsynaptic membrane

Figure 1220a
111
Presynaptic Inhibition

• Activity at Axoaxonal synapse
• GABA inhibits the opening of voltage-regulated
  calcium channels in the synaptic knob.
• Results in a reduced amount of neurotransmitters
  released when an action potential arrives there
• Thus, reducing the effects of synaptic activity
  on the postsynaptic membrane

Figure 1220a
112
Factors that Disrupt Neural Function

• ph normal 7.4
• At pH 7.8 ? spontaneous action potentials
  convulsions
• At pH 7.0 ? no action potentials unresponsive
• Ion concentration
• - High extracellular K ? depolarize membrane
  death
• Temperature normal 37C
• higher neurons more excitable ? Fever
  hallucinations
• Lower neurons non-responsive ? Hypothermia
  lethargy, confusion
• Nutrients
• neurons no reserves, use a lot of ATP
• Require constant and abundant glucose
• Glucose only
• Oxygen
• Aerobic respiration only for ATP
• No ATP neuron damage/death

113
SUMMARY

• Neural tissue and the neuron
• Anatomical divisions of the nervous system
• Central and peripheral nervous systems
• Nerves and axons
• Functional divisions of the nervous system
• Afferent division and receptors and Efferent
  division and effectors
• Somatic and autonomic nervous systems
• Structure of neurons
• organelles of neuron neurofilaments,
  neurotubules, neurofibrils
• structures of axon axon hillock, initial
  segment, axoplasm
• synapse and neurotransmitters
• Classification of neurons
• structural classifications anaxonic, bipolar,
  unipolar, and multipolar
• functional classifications sensory, motor, and
  interneurons

114
SUMMARY

• 4 types of neuroglia
• ependymal, astrocytes, and oligodendrocytes,
  microglia
• Ganglia and neurons of PNS
• satellite cells, Schwann cells
• Repair of neurons in the PNS
• Transmembrane potential
• electrochemical gradient
• passive and active channels
• Gated channels
• chemically regulated, voltage-regulated,
  mechanically regulated
• Action potentials
• threshold
• refractory period
• continuous and saltatory propagation
• 3 types of axons (A, B, and C fibers)

115
SUMMARY

• Transmission of nerve impulses across a synapse
• presynaptic and postsynaptic neurons
• electrical and chemical synapses
• excitatory and inhibitory neurotransmitters
• cholinergic synapses (ACh)
• other neurotransmitters (NE, dopamine, seratonin,
  GABA)
• Graded potentials
• depolarization and hyperpolarization
• Neuromodulators
• direct, indirect, and lipid-soluble gases
• Rate of generation of action potentials
  Information processing
• integration of postsynaptic potentials
• EPSPs and IPSPs
• spatial and temporal summation
• presynaptic inhibition and facilitation