Nervous System: General Principles

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Nervous System General Principles

• Anatomy Physiology 1
• Tony Serino. Ph.D.
• Biology Dept.
• Misericordia University

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Nervous System

• Controls and/or modifies all other systems
• Rapid response time
• Usually short duration

Lecture Outline

• General anatomy and physiology of neurons
• CNS (Central Nervous System)
• PNS (Peripheral Nervous System)

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Functional Areas
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Divisions of the Nervous System
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Nervous Tissue

• Non-excitable Tissue (Supportive cells)
• Neuroglia present in CNS
• Schwann and Satellite cells present in PNS
• Neurons (excitable tissue)
• Initiate and conduct electrical signals (action
  potentials)

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Neuroglia (glial cells)
Phagocytic,protective

• Form BBB
• Regulate microenvironment
• Pass on nutrients get rid of waste

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Neuroglia

• Line cavities
• Create CSF

Secrete myelin in CNS
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PNS Supportive Cells

• Schwann cells secrete myelin in PNS
• Satellite cells surround neuron cell bodies in
  PNS

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Neuron Anatomy
Axonal terminal Nerve ending Synaptic
boutons Synaptic knobs
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Functional Zones of a Neuron
Receptor Zone
Initial segment of Axon(trigger zone)
Nerve endings
Axon
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Internal Cell Body Structures
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Myelination

• In PNS, a Schwann cell wraps and individual
  segment of a single axon
• In the CNS, an oligodendrocyte performs the same
  function but can attach to more than one axon

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Node of Ranvier gaps in myelin sheath
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Types of Neurons

• Anatomical classification
• Based on number of process projecting from cell
  body
• Functional Classification
• Based on location of neuron and direction of
  information flow

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General Terms

• Ganglia vs. Nuclei
• Areas of densely packed nerve cell bodies
• Ganglia are usually found in PNS
• Nuclei are found in CNS
• Nerve vs. nerve fiber
• A nerve is a dissectible structure containing
  hundreds of axons
• A nerve fiber is a single axon
• CT sheaths covering peripheral nerves

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Nerve CT sheaths
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Synapses

• Areas where neurons communicate with other cells
• Can be chemical (with neurotransmitters) or
  electrical (gap junctions)

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Anatomy of Synapse (chemical)
Neurotransmission ends when NT diffuses
away, re-absorbed by presynaptic neuron, or NT
metabolized (degraded) by enzymes in cleft
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Neurotransmission signal transduction
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Neurotransmission

• Electrical signal (action potential (AP))
  descends axon to synaptic knob (nerve end)
• Depolarization opens Ca channels to open in
  presynaptic membrane
• Triggers a number of synaptic vesicles to fuse
  with outer membrane
• Dumps neurotransmitter (NT) into synaptic cleft
• NT diffuses across cleft and binds to receptor on
  postsynaptic membrane
• This leads to channels opening on postsynaptic
  membrane changing the membranes potential

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Types of Anatomical Synapses
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Membrane Potentials

• Produced by the unequal distribution of ions
  across a selectively permeable membrane
• The inside of the cell is called negative by
  convention
• The intensity of the ion difference is expressed
  as voltage (measured in millivolts (mV))

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Measuring Membrane Potentials
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Resting Membrane Potential
Parameters necessary to create a resting membrane
potential

• A semi-permeable membrane
• Distribution of ions across membrane
• Presence of large non-diffusible anions in
  interior
• Na-K pump (3 Na out for every 2 K in)

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Gated Channel Proteins

• Opening gate allows ions to travel into or out of
  the cell thereby changing the membrane potential
• Can be controlled chemically or electrically

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Chemically Gated Channel Protein
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Voltage (electrically) Gated Channel Protein
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Graded Potentials

• Transient
• Decremental
• Most due to chemically gated channels opening
• Can be summated
• May be excitatory or inhibitory

Depolarization
Inside of cell becomes less negative
Will only trigger AP if the threshold of the
neuron is reached.
Hyperpolarization
Inside of cell becomes more negative
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Graded potentials magnitude vary with stimulus
strength
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Summation

• Temporal a single axon fires repeatedly
• Spatial two or more axons fire simultaneously

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Typical Receptor Zone Activity
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Action Potentials

• Wave-like, massive depolarization
• Propagated down entire length of axon or muscle
  cell membrane
• All or none
• No summation possible
• Due to opening of voltage gated channels and
  corresponding positive feedback cycle established
• 1. Foot graded potentials
• 2. Uplimb fast depolarization
• 3. Downlimb fast repolarization
• 4. After Hyperpolarization overshoot due to ion
  distribution

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Events in Membrane during the AP
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Refractory Periods
Foot
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AP propagation in unmyelinated axons
The depolarization event triggersdepolarization
in the next area of theaxon membrane followed
by repolarization. In this way the AP appears to
move in a wave-like fashion over an unmyelinated
axon membrane.
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AP propagation in myelinated axons
The AP appears to jump from node to node
(saltatory conduction)the myelin sheath
eliminates the need to depolarize the entire
membrane.
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Axonal Transport

• Anterograde towards synapse flow of synaptic
  vesicles, mitochondria, etc.
• Retrograde towards CB recycled membrane
  vesicles, neuromodulators, etc.

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Regeneration of Nerve Fibers

• Damage to nerve tissue is serious because mature
  neurons are post-mitotic cells
• If the soma of a damaged nerve remains intact,
  damage may be repaired
• Regeneration involves coordinated activity among
  Schwann cells, Neurons and WBCs or microglia
• remove debris
• form regeneration tube and secrete growth factors
• regenerate damaged part

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Response to Injury

• Anterograde degeneration with some retrograde
  phagocytic cells (from Schwann cells, microglia
  or monocytes) remove fragments of axon and myelin
  sheath
• Cell body swells, nucleus moves peripherally
• Loss of Nissl substance (chromatolysis)
• In the PNS, some Schwann cells remain and form a
  tubular structure distal to injury if gap or
  scarring is not great axon regeneration may occur
  with growth down tube
• In the CNS, glial scar tissue seems to prevent
  regeneration

If contact with tube is not established then no
regeneration and a traumatic neuroma forms
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Drug Intervention Possibilities

1. Increase leakage and breakdown of NT from
   vesicles
2. Agonize NT release
3. Block NT release
4. Inhibit NT synthesis
5. Block NT uptake
6. Block degradative enzymes in cleft
7. Bind to post-synaptic receptor
8. Stimulate or inhibit second messengers in
   post-synaptic cell