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
Nervous System Organization
4
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

5
PERIPHERAL DIVISIONS

• Two Functional Subdivisions of the PNS
• Sensory division
• 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
• , efferent division
• Nerve fibers conveying impulses from the CNS

6
ORGANIZATION
7
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

8
NEURONS

• 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

9
Neurons
Dendrites of another neuron
Axon of anotherneuron
10
Collins I4 lines

• Based on the diagram, what do you think each part
  does to receive and pass along an impulse toward
  the brain

11
Agenda11/3/11 Day 1

• Take more notes
• HW- vocab

12
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

13
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

14
NEURON CELL BODY

• Major biosynthetic (control) center of neuron
• Other usual organelles present except CENTRIOLES
• -Why?
• What do centrioles do?

15
NEURON PROCESSES

• Extend from the neurons cell body
• Two types of neuron processes
• Dendrites
• Axons

16
NEURON PROCESSES

• Typical Dendrite
• Short, slender, branching extensions of cell body
• 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

17
NEURON PROCESSES

• Typical Axon
• Single axon per neuron
• The axon forms from the narrowing of the cell
  body. The region between the large cell body and
  the axon is the axon hillock
• Sometimes very short
• Sometimes very long
• e.g., axons controlling big toe are 3 4 feet
  long

18
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

19
NEURON PROCESSES

• Typical Axon
• Conducting component of neuron
• Generates nerve impulse
• Transmits nerve impulses away from cell body
  towards the
• axonal terminals

20
NEURON PROCESSES

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

21
VocabularyEither in flash card form OR in list

• CNS
• PNS
• Neuron
• Stimulus
• Afferent division
• Efferent division
• neuroglia

• Amitotic
• Dendrite
• Cell body
• Axon
• Axon terminal
• Ganglia
• Nuclei (in terms of clusters)

22
Collins I2 lines

• What is the difference between the PNS and the
  CNS?

23
Agenda11/4/11 -- Day 2

• Remember quiz 11/9
• Take notes
• Complete labeling and coloring of neuroglia
• HW-complete ALL vocab terms

24
MYELIN SHEATH

• Whitish, fatty covering the axons of many neurons
• Protects and electrically insulates fibers
• Increases speed of nerve impulse transmission
• Some axons and all dendrites are unmyelinated

25
MYELIN SHEATH

• In PNS, Schwann cells Continually wrap around the
  axon of a neuron
• Result is many concentric layers of plasma
  membrane surrounding the axon
• Thickness depends on number of wrappings
• Nucleus and most of cytoplasm exist as a bulge
  external to the myelin sheath

26
Myelin sheath and schwann cells
Node of Ranvier
Schwann Cells
27
MYELIN SHEATH

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

28
MYELIN SHEATH

• In CNS, there are both myelinated and
  unmyelinated axons
• Oligodendrocytes, not Schwann cells, form CNS
  myelin sheaths
• Numerous processes that can coil around numerous
  (up to 60) axons at once

29
NEUROGLIA

• Nerve glue
• Six types of small cells associated with neurons
• 4 in CNS
• 2 in PNS
• Several functions
• Supportive scaffolding for neurons
• Electrical isolation of neurons
• Neuron health and growth

30
CNS NEUROGLIA

• Astrocytes
• Microglia
• Ependymal cells
• Oligodendrocytes

31
CNS NEUROGLIA

• Astrocytes
• Anchor neurons to capillary blood supply
• Facilitate nutrient delivery to neurons
• (blood ? astrocyte ? neuron)

32
CNS NEUROGLIA

• Microglia
• Small ovoid cells thorny looking
• Transform into macrophage
• Phagocytize microorganisms, debris
• (Cells of immune system cannot enter the CNS)

33
CNS NEUROGLIA

• Oligodendrocytes
• Wrap processes tightly around thicker neuron
  fibers in CNS
• Makes Myelin sheath
• Insulating covering

34
CNS NEUROGLIA

• Ependymal Cells
• Line central cavities of brain and spinal cord
• Many are ciliated
• Beating helps circulate cerebrospinal fluid
  cushioning brain and spinal cord

35
PNS NEUROGLIA

• Schwann cells
• Surround and form myelin sheaths around larger
  neurons of PNS
• Functionally similar to oligodendrocytes

36
PNS NEUROGLIA

• Satellite cells
• Surround cell bodies of PNS ganglia

37
HW- Vocab Terms

• Myelin sheath
• Schwann cells
• Nodes of ranvier
• Oligodendrocytes
• Neuroglea
• Astrocyte
• Microglia
• Ependymal cell
• Satalite cell

38
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

39
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)

40
NEURON CLASSIFICATION

• Unipolar neurons
• Single short process
• 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

• 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

41
Classification of neurons by shape
42
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

43
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

44
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

45
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

46
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

47
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

48
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

49
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

50
MEMBRANE POTENTIALS

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

51
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

52
ACTION POTENTIALS
53
ACTION POTENTIALS

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

54
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

55
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

56
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

57
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

58
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