NERVOUS SYSTEM

1
NERVOUS SYSTEM
2
NERVOUS FUNCTIONS

• Bodys master controlling and communicating
  system
• Three functions
• Sensory input
• Gathers information from sensory receptors
• Integration
• Processes and interprets sensory input
• Motor output
• Activates effector organs to cause a response

3
ORGANIZATION

• Two Principal Parts of the System
• Central nervous system (CNS)
• Brain and spinal cord
• Integrating and command center
• Interprets sensory input
• Dictates motor responses
• Peripheral nervous system (PNS)
• Nerves extending from brain and spinal cord
• Carry impulses to and from the CNS

4
PERIPHERAL DIVISIONS

• Two Functional Subdivisions of the PNS
• Sensory division
• a.k.a. afferent division
• Nerve fibers conveying impulses to the CNS
• Somatic afferent fibers convey impulses from the
  skin, muscles, and joints
• Visceral afferent fibers convey impulses from
  visceral organs
• Motor division
• a.k.a., efferent division
• Nerve fibers conveying impulses from the CNS

5
MOTOR DIVISIONS

• Two Parts of the Motor Division
• Somatic nervous system
• a.k.a., voluntary nervous system
• Nerve fibers conducting impulses from CNS to
  skeletal muscles
• Autonomic nervous system
• a.k.a., involuntary nervous system
• Nerve fibers regulating the activity of smooth
  muscles, cardiac muscles, and glands

6
AUTONOMIC DIVISIONS

• Functional Subdivisions of the Autonomic Nervous
  System
• Sympathetic
• Mobilizes body systems during emergency
  situations
• Parasympathetic division
• Conserves energy
• Promotes non-emergency functions

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ORGANIZATION

• Summary
• Central nervous system
• Brain
• Spinal cord
• Peripheral nervous system
• Sensory division
• Motor division
• Somatic nervous system
• Autonomic nervous system
• Sympathetic division
• Parasympathetic division

8
HISTOLOGY

• Nervous system consists mainly of nervous tissue
• Highly cellular
• e.g., lt20 extracellular space in CNS
• Two principal cell types
• Neurons
• Excitable nerve cells that transmit electrical
  signals
• Supporting cells
• Smaller cells surrounding and wrapping neurons
• Neuroglia

9
NEUROGLIA

• Nerve glue
• Six types of small cells associated with neurons
• 4 in CNS
• 2 in PNS
• Most have central cell body and branching
  processes
• Several functions
• e.g., Supportive scaffolding for neurons
• e.g., Electrical isolation of neurons
• e.g., Neuron health and growth

10
CNS NEUROGLIA

• Astrocytes
• Microglia
• Ependymal cells
• Oligodendrocytes

11
CNS NEUROGLIA

• Astrocytes
• Most abundant and versatile glial cells
• Numerous processes support branching neurons
• Anchor neurons to capillary blood supply
• Guide migration of young neurons
• Facilitate nutrient delivery to neurons
• (blood ? glial cell ? neuron)
• Control chemical environment around neurons
• Uptake of K, neurotransmitters
• Communicate with astrocytes neurons
• Gap junctions, Ca2 surges

12
CNS NEUROGLIA

• Microglia
• Small ovoid cells
• Relatively long thorny processes
• Processes touch nearby neurons
• Checking vitals
• Migrate toward injured neurons
• Transform into macrophage
• Phagocytize microorganisms, debris
• (Cells of immune system cannot enter the CNS)

13
CNS NEUROGLIA

• Ependymal Cells
• Line central cavities of brain and spinal cord
• Form permeable barrier between cerebrospinal
  fluid inside these cavities and tissue fluid of
  CNS tissue
• Shapes range from squamous to columnar
• Many are ciliated
• Beating helps circulate cerebrospinal fluid
  cushioning brain and spinal cord

14
CNS NEUROGLIA

• Oligodendrocytes
• Fewer processes than astrocytes
• Wrap processes tightly around thicker neuron
  fibers in CNS
• Myelin sheath
• Insulating covering

15
PNS NEUROGLIA

• Satellite cells
• Schwann cells

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PNS NEUROGLIA

• Satellite cells
• Surround neuron cell bodies within ganglia
• (A ganglion is a collection of nerve cell bodies
  outside of the CNS)
• Function poorly understood

17
PNS NEUROGLIA

• Schwann cells
• a.k.a., Neurolemmocytes
• Surround and form myelin sheaths around larger
  nerve fibers of PNS
• Functionally similar to oligodendrocytes
• Vital to regeneration of peripheral nerve fibers

18
NEURONS

• a.k.a., Nerve cells
• Structural units of nervous system
• Billions are present in nervous system
• Conduct messages throughout body
• Nerve impulses
• Extreme longevity
• Can function optimally for entire lifetime
• Amitotic
• Ability to divide is lost in mature cells
• Cannot be replaced if destroyed
• Some (very few) exceptions
• e.g., stem cells present in olfactory epithelium
  can produce new neurons
• Stem cell research shows great promise in
  repairing damaged neurons
• High metabolic rate
• Require large amounts of oxygen and glucose

19
NEURONS

• Generally large, complex cells
• Structures vary, but all neurons have the same
  basic structure
• Cell body
• Slender processes extending from cell body
• Plasma membrane is site of signaling

20
NEURON CELL BODY

• Most neuron cell bodies are located in the CNS
• Protected by bones of skull or vertebral column
• Clusters of cell bodies in the CNS are termed
  nuclei
• Clusters of cell bodies in the PNS are termed
  ganglia

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NEURON CELL BODY

• a.k.a., perikaryon or soma
• 5 140 mm in diameter
• Transparent spherical nucleus
• Contains conspicuous nucleolus

22
NEURON CELL BODY

• Major biosynthetic center of neuron
• Other usual organelles present
• ER ribosomes most active and best developed in
  body
• What do they do?
• Centrioles absent
• What do centrioles do?
• Sometimes contains pigment inclusions

23
NEURON CELL BODY

• Focal point for the outgrowth of neuron processes
  during embryonic development
• Some processes receive signals
• Plasma membrane generally also acts as part of
  the receptive surface

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NEURON PROCESSES

• Extend from the neurons cell body
• CNS contains both neuron cell bodies and their
  processes
• Bundles of CNS processes are termed tracts
• PNS consists mainly of neuronal processes
• Bundles of PNS processes are termed nerves
• Two types of neuron processes
• Dendrites
• Axons

25
NEURON PROCESSES

• Typical Dendrite
• Short, tapering, diffusely branching extensions
• Generally hundreds clustering close to cell body
• Most cell body organelles also present in
  dendrites
• Main receptive / input regions
• Large surface area for receiving signals from
  other neurons
• Convey incoming messages toward cell body
• Short-distance signals are graded potentials
• Not action potentials

26
NEURON PROCESSES

• Typical Axon
• Single axon per neuron
• Axon hillock of cell body narrows to form a
  slender process of uniform diameter
• Sometimes very short
• Sometimes very long
• e.g., axons controlling big toe are 3 4 feet
  long

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NEURON PROCESSES

• Typical Axon
• Single axon may branch along length
• Axon collaterals extend from neurons at 90o
  angles
• Usually branches profusely at end
• 10,000 or more terminal branches is common
• Distal endings termed axonal terminals

28
NEURON PROCESSES

• Typical Axon
• Conducting component of neuron
• Generates nerve impulses
• Generated at axon hillock / axon junction in
  motor neurons
• Trigger zone
• Transmits nerve impulses away from cell body
• To axonal terminals

29
NEURON PROCESSES

• Typical Axon
• Axonal terminals are secretory component of
  neuron
• Sequence of events
• Signal reaches terminals
• Membranes of vesicles fuse with plasma membrane
• Axolemma
• Neurotransmitters released
• Neurotransmitters interact with either other
  neurons or effector cells
• Excite or inhibit

30
NEURON PROCESSES

• Typical Axon
• Contains most of the same organelles found in
  dendrites and cell body
• Lacks ER and Golgi apparatus
• What do these organelles do?
• Must rely on cell body to renew what?

31
NEURON PROCESSES

• Typical Axon
• Rely on cell body for some molecules
• Rely on efficient transport mechanisms for
  delivery
• Anterograde movement toward axonal terminals
• e.g., Mitochondria, membrane components,
  neurotransmitters or enzymes required for
  neurotransmitter synthesis, etc.
• Retrograde movement toward cell body
• e.g., Organelles being returned for recycling

32
NEURON PROCESSES

• Typical Axon
• Some viruses and bacterial toxins use retrograde
  transport to reach the cell body
• e.g., poliovirus, rabies virus, herpes simplex
  viruses, tetanus toxin, etc.
• Such viruses can be used as vehicles for the
  therapeutic delivery of engineered DNA
• Gene therapy

33
MYELIN SHEATH

• Whitish, fatty covering of many nerve fibers
• Particularly those long are large in diameter
• Protects and electrically insulates fibers
• Increases speed of nerve impulse transmission
• Some axons and all dendrites are unmyelinated

34
MYELIN SHEATH

• In PNS, myelin sheaths formed by Schwann cells
• Continually wrap around nerve
• Cytoplasm gradually squeezed from intracellular
  space
• Result is many concentric layers of plasma
  membrane surrounding the axon
• These plasma membranes contain little protein
• Some proteins present interlock adjacent
  membranes
• Thickness depends on number of wrappings
• Nucleus and most of cytoplasm exist as a bulge
  external to the myelin sheath
• Neurilemma

35
MYELIN SHEATH

• Adjacent Schwann cells on axon do not touch each
  other
• Gaps in sheath occur at regular intervals
• Nodes of Ranvier
• a.k.a., Neurofibril nodes
• Axon collaterals can emerge at these nodes

36
MYELIN SHEATH

• CNS contains both myelinated and unmyelinated
  axons
• Those long are large in diameter are typically
  myelinated
• Oligodendrocytes, not Schwann cells, form CNS
  myelin sheaths
• Oligodendrocytes possess numerous processes that
  can coil around numerous (up to 60) axons at once
  
• CNS myelin sheaths lack a neurilemma

37
MYELIN SHEATH

• White matter
• Regions of the brain and spinal cord containing
  dense collections of myelinated fibers
• Gray matter
• Regions of the brain and spinal cord containing
  mostly nerve cell bodies and unmyelinated fibers

38
NEURON CLASSIFICATION

• Structural classification based upon number of
  processes
• Multipolar neurons
• Bipolar neurons
• Unipolar neurons
• Functional classification based upon direction
  nerve impulse travels
• Sensory (afferent) neurons
• Motor (efferent) neurons
• Interneurons (association neurons)

39
NEURON CLASSIFICATION

• Structural Classification
• Multipolar neurons
• Three or more processes
• Most common neuron type in humans
• (gt 99 of neurons)
• Bipolar neurons
• Two processes axon and dendrite
• Found only in some special sense organs
• e.g., retina of eye
• Act as receptor cells

• Unipolar neurons
• Single short process
• Pseudounipolar neurons
• Originate as bipolar neurons
• Two processes converge and fuse
• Process divides into proximal and distal branches
• Distal process often associated with a sensory
  receptor
• Peripheral process
• Central process enters CNS
• Most are sensory neurons in PNS

40
NEURON CLASSIFICATION

• Functional Classification
• Sensory (afferent) neurons
• Transmit impulses toward CNS
• From sensory receptors or internal organs
• Most are unipolar
• Cell bodies are located outside CNS
• Motor (efferent) neurons
• Carry impulses away from CNS
• Toward effector organs
• Multipolar
• Cell bodies generally located in the CNS

• Interneurons
• a.k.a., association neurons
• Lie between motor and sensory neurons in neural
  pathways
• Shuttle signals through CNS pathways where
  integration occurs
• gt 99 of neurons in body
• Most are multipolar
• Most are confined within the CNS

41
NEUROPHYSIOLOGY

• Neurons are highly irritable
• Responsive to stimuli
• Response to stimulus is action potential
• Electrical impulse carried along length of axon
• Always the same regardless of stimulus
• The underlying functional feature of the nervous
  system

42
ELECTRICITY

• Voltage (V)
• Measure of potential energy
• Measured between two points
• Potential difference or simply potential
• Measured in volts or millivolts
• Current (I)
• Flow of electrical charge from one point to
  another
• Can be used to do work
• Amount of charge moved depends on voltage
  resistance
• Resistance (R)
• Hindrance to charge flow
• Provided by substances through which the current
  must pass

43
ELECTRICITY

• Ohms Law
• Current Voltage / Resistance
• I V / R
• voltage current resistance
• V I R

44
ELECTRICITY

• Electrical currents involve the flow of ions
  across membranes
• Resistance to current flow is provided by the
  plasma membrane
• Movement of ions across the plasma membrane is
  regulated by membrane ion channels

45
ION CHANNELS

• Plasma membranes contain various ion channels
• Passive channels (leakage channels)
• Always open
• Active channels (gated channels)
• Ligand-gated channels
• Open when specific chemical binds
• Voltage-gated channels
• Open and close in response to membrane potential
• Mechanically-gated channels
• Open in response to physical deformation of
  receptor
• e.g., touch and pressure receptors

46
ION CHANNELS

• Channels are specific as to what type of ions are
  allowed to pass
• e.g., K channels allow only K to pass
• Ions moving through open channels follow their
  electrochemical gradients
• Electrical current is generated
• Voltage changes across the membrane

47
MEMBRANE POTENTIALS

• A voltage exists across the plasma membrane
• Due to separation of oppositely charged ions
• Potential difference in a resting membrane is
  termed its resting membrane potential
• -70 mV in a resting neuron
• Membrane is polarized

48
MEMBRANE POTENTIALS

• Resting potential exists across the membrane
• Majority of Na outside of cell Why?
• Majority of K inside of cell Why?
• Resting membrane
• Only slightly permeable to Na
• 75 times more permeable to K
• How do these ions cross the membrane?

49
MEMBRANE POTENTIALS

• Neurons use changes in membrane potentials as
  signals
• Used to receive, integrate, and send signals
• Changes in membrane potentials produced by
• Anything changing membrane permeability to ions
• Anything altering ion concentrations
• Two types of signals
• Graded potentials
• Short-distance signals
• Action potentials
• Long-distance signals

50
MEMBRANE POTENTIALS

• Changes in membrane potentials are caused by
  three events
• Depolarization
• Inside of membrane becomes less negative
• Nerve impulses more likely to be produced
• Repolarization
• Membrane returns to resting membrane potential
• Hyperpolarization
• Inside of membrane becomes more negative than the
  resting potential
• Nerve impulses less likely to be produced

51
MEMBRANE POTENTIALS

• Graded Potentials
• Short-lived local changes in membrane potential
• Either depolarizations or hyperpolarizations
• Cause current flows that decrease in magnitude
  with distance
• Magnitude of potential dependent upon stimulus
  strength
• Stronger stimulus ? larger voltage change
• Larger voltage change ? farther current flows

52
MEMBRANE POTENTIALS

• Graded Potentials
• Triggered by change in neurons environment
• Change causes gated ion channels to open
• Small area of neurons plasma membrane becomes
  depolarized (by this stimulus)
• Current flows on both sides of the membrane
• moves toward and vise versa

53
MEMBRANE POTENTIALS

• Graded Potentials
• Inside cell ions move away from depolarized
  area
• Outside cell ions move toward depolarized area
• ( and ions switch places)
• Membrane is leaky
• Most of the charge is quickly lost through
  membrane
• Current dies out after traveling a short distance

54
MEMBRANE POTENTIALS

• Graded Potentials
• Act as signals over very short distances
• Important in initiating action potentials

55
MEMBRANE POTENTIALS

• Action Potentials
• Principal means by which neurons communicate
• Brief reversal of membrane potential
• Total amplitude of 100 mV (-70 ? 30)
• Depolarization followed by repolarization, then
  brief period of hyperpolarization
• Time for entire event is only a few milliseconds
• Events in generation and transmission of an
  action potential identical between neurons and
  skeletal muscle cells

56
ACTION POTENTIALS
57
ACTION POTENTIALS

• Not all local depolarizations produce action
  potentials
• Depolarization must reach threshold values
• Brief, weak stimuli produce subthreshold
  depolarizations that are not translated into
  nerve impulses
• Stronger threshold stimuli produce depolarizing
  events

58
ACTION POTENTIALS

• Action potential is all-or-nothing phenomenon
• Happens completely or doesnt happen
• Independent of stimulus strength once generated
• Strong stimuli generate more impulses of the same
  strength per unit time
• Intensity is determined by number of impulses per
  unit time

59
ACTION POTENTIALS

• Refractory Periods
• Neuron cannot respond to a second stimulus while
  the Na channels are still open from previous
  stimulus
• This period of time is termed the absolute
  refractory period
• Relative refractory period follows the absolute
  refractory period
• Repolarization is occurring
• Threshold for impulse generation is elevated
• Only strong stimuli can generate impulses

60
ACTION POTENTIALS

• Conduction Velocities
• Conduction velocities of neurons vary widely
• Rate of impulse propagation dependent upon
• Axon diameter
• Larger axons conduct impulses faster
• Degree of myelination
• Myelin sheath dramatically increases rate of
  propagation
• Myelin acts as an insulator to prevent almost all
  leakage from axon

61
ACTION POTENTIALS

• Multiple Sclerosis (MS)
• Autoimmune disease mainly affecting young adults
• Myelin sheaths in CNS are gradually destroyed
• Interferes with impulse conduction
• Visual disturbances, muscle control problems,
  speech disturbances, etc.
• Some modern treatments showing some promise in
  delaying problems

62
NERVE FIBERS

• Classified based on
• Diameter
• Degree of myelination
• Conduction speed

63
NERVE FIBER CLASSIFICATION

• Group A fibers
• Largest diameter
• Thick myelin sheaths
• Conduct impulses at high speeds (gt 300 mph)
• Mostly somatic sensory ad motor fibers serving
  skin, skeletal muscles, and joints
• Group B fibers
• Intermediate diameter
• Lightly myelinated
• Transmit impulses at moderate speeds (40 mph)
• Group C fibers
• Smallest diameter
• Unmyelinated
• Transmit impulses comparatively slowly (2 mph or
  less)

64
ION CHANNELS

• Various chemicals block nerve impulses
• e.g., alcohol, sedatives, anesthetics, etc.
• Mechanisms differ, but all reduce membrane
  permeability to Na
• No Na entry ? no action potential
• Neurons also impaired by cold or continuous
  pressure
• Blood supply interrupted
• O2 delivery compromised

65
SYNAPSE

• Junction mediating information transfer from one
  neuron to another neuron or an effector cell
• Axodendritic synapses
• Axonal endings ? dendrites of second neuron
• Axosomatic synapses
• Axonal endings ? cell body of neuron
• Presynaptic neuron
• Conducts impulses toward the synapse
• Postsynaptic neuron
• Transmits impulse away from the synapse

66
SYNAPSE TYPES

• Electrical Synapses
• Less common than chemical synapses
• Correspond to gap junctions found elsewhere
• Cytoplasm of adjacent neurons connected through
  protein channels
• Ions flow directly between neurons
• Neurons are electrically coupled
• Transmission across synapse is very rapid

67
SYNAPSE TYPES

• Chemical Synapses
• Specialized for release reception of
  neurotransmitters
• Two parts
• Axonal terminal of presynaptic neuron
• Contains numerous synaptic vesicles filled with
  neurotransmitter molecules
• Neurotransmitter receptor region
• Present on dendrite or cell body of postsynaptic
  neuron
• Separated by synaptic cleft
• Remember this stuff in muscles?

68
SYNAPSE

• Nerve impulse reaches axonal terminal
• Voltage-gated Ca2 channels open in axon
• Ca2 enters presynaptic neuron
• Neurotransmitter is released via exocytosis
• Vesicles fuse with axonal membrane
• Neurotransmitter binds to postsynaptic receptors
• Ion channels open in postsynaptic membrane
• Result is excitation or inhibition

69
SYNAPSE

• Binding of neurotransmitter to its receptor is
  reversible
• Permeability affected as long as neurotransmitter
  is bound to its receptor
• Neurotransmitters do not persist in the synaptic
  cleft
• Degraded by enzymes associated with postsynaptic
  membrane
• Reuptake by astrocytes or presynaptic terminal
• Diffusion of neurotransmitters away from synapse

70
SYNAPSE

• Transmission of impulses along axon can be very
  fast
• Up to 300 mph (150 m/s)
• Transmission of a signal across a synapse is slow
  in comparison
• Leads to synaptic delay
• 0.3 0 5.0 milliseconds
• Rate-limiting step of neural transmission
• Transmission along multisynaptic pathways is
  slower than along pathways with fewer synapses

71
SYNAPSE

• Postsynaptic Potentials
• Many receptors present on postsynaptic membranes
  open ion channels
• Ligand-gated channels
• Electrical signal converted to chemical signal
  converted to electrical signal
• Graded potential is produced
• Magnitude is dependent upon amount of
  neurotransmitter released
• Action potential may be produced
• Either excitatory or inhibitory

72
SYNAPSE

• Excitatory Synapses
• Neurotransmitter binding causes depolarization
• Single type of channel opens in membrane
• Na and K simultaneously diffuse through the
  membrane in opposite directions
• Na influx exceeds K efflux
• Net depolarization occurs
• Local graded depolarization events formed
• Excitatory postsynaptic potential (EPSP)
• May trigger an action potential at axon hillock
• Voltage-gated channels at hillock open, etc.

73
SYNAPSE

• Inhibitory Synapses
• Neurotransmitter binding reduces a postsynaptic
  neurons ability to generate an action potential
• Increased permeability to K and Cl-, not Na
• Postsynaptic neuron becomes less likely to fire
• Inhibitory postsynaptic potential (IPSP)

74
SYNAPSE

• Summation
• A single ESPS cannot induce an action potential
• Requires multiple axonal termini firing in
  concert
• Hundreds or thousands of EPSPs act together
• Summation
• Two types of summation
• Temporal summation
• One or more neurons transmit in rapid succession
• Spatial summation
• Simultaneous stimulation by numerous termini from
  one or more neurons
• (Both EPSPs and IPSPs summate)

75
SYNAPSE

• Synaptic Potentiation
• Repeated or continuous use of a synapse enhances
  presynaptic neurons ability to excite
• Larger postsynaptic potentials produced
• Synaptic potentiation
• Greater Ca inside presynaptic terminals
• More neurotransmitter released
• Larger EPSPs produced

76
SYNAPSE

• Presynaptic Inhibition
• Release of excitatory neurotransmitter can be
  inhibited by activity of another neuron
• Less neurotransmitter released and bound

77
SYNAPSE

• Neuromodulation
• Presynaptic event effecting postsynaptic activity
• Can occur when neurotransmitter acts via slow
  changes in target cell metabolism
• Can occur when chemicals other than
  neurotransmitters modify neuronal activity
• Neuromodulators can influence
• Synthesis, release, degradation, or reuptake of
  neurotransmitters
• Sensitivity of postsynaptic membrane

78
NEUROTRANSMITTERS

• Facilitate communication by neurons
• Neurotransmitter release or destruction can be
  enhanced or inhibited
• How can synaptic transmission be affected?
• Enhanced or inhibited neurotransmitter release
• Enhanced or inhibited neurotransmitter
  degradation
• Blocked receptors or postsynaptic membrane

79
NEUROTRANSMITTERS

• More than fifty neurotransmitters identified
• Most neurons make two or more
• Can be released singly or together
• Classification by Structure
• Acetylcholine (ACh)
• Biogenic amines
• Amino acids
• Peptides
• ATP
• Dissolved gases

• Classification by Function
• Excitatory/Inhibitory
• Direct/Indirect

80
NEUROTRANSMITTERS

• Two main types of neurotransmitter receptors
• Channel-linked receptors
• Mediate fast synaptic transmission
• G protein-linked receptors
• Mediate slow synaptic responses

81
RECEPTORS

• Channel-Linked Receptors
• Ligand-gated ion channels
• Composed of rosettes of several protein
  subunits surrounding central pore
• Ligand binds to subunit(s) ? subunit shape
  changes ? central channel opened

82
RECEPTORS

• G Protein-Linked Receptors
• Indirect, slow, and prolonged response
• Neurotransmitter binding activates G protein
• Intracellular enzymatic activity activated
• Second messenger(s) formed inside the cell
• Cellular response

83
NEURAL INTEGRATION

• Neurons function in groups, not singly
• These various components must interact
• Multiple levels of neural integration

84
NEURONAL POOLS

• Neurons in CNS are organized into pools
• Functional groups
• Integrate incoming information
• Forward processed information

85
NEURONAL POOLS

• Simple Neuronal Pool
• Incoming fiber branches profusely upon entering
  pool
• EPSPs induced in multiple postsynaptic neurons
• EPSPs exceed threshold in some neurons
• Mainly those with multiple synaptic contacts
• EPSPs do not exceed threshold in some neurons
• Mainly those with fewer synaptic contacts
• Some close to threshold
• Facilitated zone

86
TYPES OF CIRCUITS

• Patterns of synaptic connections in neuronal
  pools are called circuits
• Determine neuronal pools functional capabilities
• Four basic circuit patterns
• Diverging circuits
• Converging circuits
• Reverberating (oscillating) circuits
• Parallel after-discharge circuits

87
TYPES OF CIRCUITS

• Diverging (Amplifying) Circuit
• One incoming fiber triggers responses in
  ever-increasing numbers of neurons
• Common in both sensory and motor systems

88
TYPES OF CIRCUITS

• Converging Circuits
• Pool receives inputs from several neurons
• Circuit has funneling effect
• Common in sensory and motor systems

89
TYPES OF CIRCUITS

• Reverberating (Oscillating) Circuits
• Incoming signal travels through chain of neurons
• Each neuron makes synapses with neurons upstream
  in the pathway
• Involved in rhythmic activities (e.g., breathing)

90
TYPES OF CIRCUITS

• Parallel After-Discharge Circuits
• Incoming fiber stimulated parallel neuron arrays
• Parallel arrays ultimately stimulate a common
  output cell
• Create prolonged burst of impulses
• Involved in complex mental processing

91
PROCESSING PATTERNS

• Serial input processing
• Input travels along one pathway to a specific
  destination
• All-or-nothing function of system
• e.g., reflexes
• Parallel input processing
• Inputs are segregated into multiple pathways
• Integrated in different CNS regions
• Different circuits do different things with input
• Not repetitious