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Copyright © 2005 Pearson
Education, Inc., publishing as
Benjamin Cummings
Neuron structure and classification
Neurons
• Neurons = nerve cells
– Cells specialized to transmit messages
– Major regions of neurons
• Cell body – nucleus and metabolic center of the cell
• Processes – fibers that extend from the cell body
Neuron Anatomy
• Cell body
– Nissl substance –
specialized rough
endoplasmic
reticulum
– Neurofibrils –
intermediate
cytoskeleton that
maintains cell
shape
Neuron Anatomy
• Extensions outside
the cell body
– Dendrites –
conduct impulses
toward the cell
body
– Axons – conduct
impulses away
from the cell
body
Axons and Nerve Impulses
• Axons end in axonal terminals
• Axonal terminals contain vesicles with
neurotransmitters
• Axonal terminals are separated from the
next neuron by a gap
– Synaptic cleft – gap between adjacent neurons
Synapses
• Site at which neurons communicate
• Signals pass across synapse in one direction
• Presynaptic neuron
– Conducts signal toward a synapse
• Postsynaptic neuron
– Transmits electrical activity away from a synapse
Structure of a Synapses
Synapse
Two Neurons Communicating at a
Synapse
How Neurons Communicate at Synapses
Myelin Sheaths
• Segmented structures composed of the
lipoprotein myelin
• Surround the axon except at its endings
• Form an insulating layer
– Prevent leakage of electrical current
• Increase the speed of impulse conduction
Myelin Sheaths
• Formed by Schwann
cells
• Schwann cells wrap
in concentric layers
around the axon
– Cover the axon in a
tightly packed coil of
membranes
Myelin Sheaths
• Thick axons are myelinated
– Fast conduction velocity
• Thin axons are unmyelinated
– Slow conduction velocity
Nerve Fiber Coverings
• Schwann cells –
produce myelin
sheaths in jelly-roll
like fashion
• Nodes of Ranvier –
gaps in myelin sheath
along the axon
Nervous System: Functions
• Three overlapping functions
– Sensory receptors monitor changes inside and
outside the body
• Change – a stimulus
• Gathered information – sensory input
– CNS Processes and interprets sensory input
• Makes decisions – integration
– Dictates a response by activating effector organs
• Response – motor output
Neurons Classified by Function
Structural Classification of Neurons
Classification based on number of processes
– Multipolar
– Bipolar
– Unipolar (pseudounipolar)
Multipolar Neurons
Possess more than two processes
Numerous dendrites and one axon
Bipolar Neurons
Possess two processes
Rare neurons – found in
some special sensory organs
Unipolar (Pseudounipolar) Neurons
Possess one single process
Start as bipolar neurons during
development
Structural Classification of the Nervous
System
• Central nervous system (CNS)
– Brain
– Spinal cord
• Peripheral nervous system (PNS)
– Nerve outside the brain and spinal cord
Sensory Input and Motor Output
• Sensory signals picked up by sensory receptors
– Carried by afferent nerve fibers of PNS to the CNS
• Motor signals are carried away from the CNS
– Carried by efferent nerve fibers of PNS to effectors
– Innervate muscles and glands
Sensory Input and Motor Output
• Divided according to region they serve
– Somatic body region
– Visceral body region
• Results in four main subdivisions
– Somatic sensory
– Visceral sensory
– Somatic motor
– Visceral motor
Somatic Sensory
• Somatic sensory
– General somatic senses – receptors are widely
spread
• Touch, pain, vibration, pressure, and temperature
• Body sense – position and movement of body in
space
Somatic Motor
• Somatic motor
– General somatic motor – signals contraction of
skeletal muscles
• Under voluntary control
• Often called “voluntary nervous system”
Visceral Sensory
• Visceral sensory
– General visceral senses – stretch, pain,
temperature, nausea, and hunger
• Widely felt in digestive and urinary tracts, reproductive
organs
Visceral Motor
• Visceral motor
– Regulates the contraction of smooth and cardiac
muscle and gland secretion
– Makes up autonomic nervous system
– Controls function of visceral organs
– Often called “involuntary nervous system”
Types of Sensory and Motor Information
Nerve Fibers That Transmit Different Types of Signals, and Their
Physiologic Classification
• Nerve fibers come in all sizes between 0.5 and
20 micrometers in diameter-the larger the
diameter, the greater the conducting velocity.
• The range of conducting velocities is between
0.5 and 120 m/sec
Physiologic classifications and functions of nerve fibers
(Erlanger and Gasser)

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Lec6

  • 1. Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Neuron structure and classification
  • 2. Neurons • Neurons = nerve cells – Cells specialized to transmit messages – Major regions of neurons • Cell body – nucleus and metabolic center of the cell • Processes – fibers that extend from the cell body
  • 3. Neuron Anatomy • Cell body – Nissl substance – specialized rough endoplasmic reticulum – Neurofibrils – intermediate cytoskeleton that maintains cell shape
  • 4. Neuron Anatomy • Extensions outside the cell body – Dendrites – conduct impulses toward the cell body – Axons – conduct impulses away from the cell body
  • 5. Axons and Nerve Impulses • Axons end in axonal terminals • Axonal terminals contain vesicles with neurotransmitters • Axonal terminals are separated from the next neuron by a gap – Synaptic cleft – gap between adjacent neurons
  • 6. Synapses • Site at which neurons communicate • Signals pass across synapse in one direction • Presynaptic neuron – Conducts signal toward a synapse • Postsynaptic neuron – Transmits electrical activity away from a synapse
  • 7. Structure of a Synapses Synapse
  • 10. Myelin Sheaths • Segmented structures composed of the lipoprotein myelin • Surround the axon except at its endings • Form an insulating layer – Prevent leakage of electrical current • Increase the speed of impulse conduction
  • 11. Myelin Sheaths • Formed by Schwann cells • Schwann cells wrap in concentric layers around the axon – Cover the axon in a tightly packed coil of membranes
  • 12. Myelin Sheaths • Thick axons are myelinated – Fast conduction velocity • Thin axons are unmyelinated – Slow conduction velocity
  • 13. Nerve Fiber Coverings • Schwann cells – produce myelin sheaths in jelly-roll like fashion • Nodes of Ranvier – gaps in myelin sheath along the axon
  • 14. Nervous System: Functions • Three overlapping functions – Sensory receptors monitor changes inside and outside the body • Change – a stimulus • Gathered information – sensory input – CNS Processes and interprets sensory input • Makes decisions – integration – Dictates a response by activating effector organs • Response – motor output
  • 16. Structural Classification of Neurons Classification based on number of processes – Multipolar – Bipolar – Unipolar (pseudounipolar)
  • 17. Multipolar Neurons Possess more than two processes Numerous dendrites and one axon
  • 18. Bipolar Neurons Possess two processes Rare neurons – found in some special sensory organs
  • 19. Unipolar (Pseudounipolar) Neurons Possess one single process Start as bipolar neurons during development
  • 20. Structural Classification of the Nervous System • Central nervous system (CNS) – Brain – Spinal cord • Peripheral nervous system (PNS) – Nerve outside the brain and spinal cord
  • 21. Sensory Input and Motor Output • Sensory signals picked up by sensory receptors – Carried by afferent nerve fibers of PNS to the CNS • Motor signals are carried away from the CNS – Carried by efferent nerve fibers of PNS to effectors – Innervate muscles and glands
  • 22. Sensory Input and Motor Output • Divided according to region they serve – Somatic body region – Visceral body region • Results in four main subdivisions – Somatic sensory – Visceral sensory – Somatic motor – Visceral motor
  • 23. Somatic Sensory • Somatic sensory – General somatic senses – receptors are widely spread • Touch, pain, vibration, pressure, and temperature • Body sense – position and movement of body in space
  • 24. Somatic Motor • Somatic motor – General somatic motor – signals contraction of skeletal muscles • Under voluntary control • Often called “voluntary nervous system”
  • 25. Visceral Sensory • Visceral sensory – General visceral senses – stretch, pain, temperature, nausea, and hunger • Widely felt in digestive and urinary tracts, reproductive organs
  • 26. Visceral Motor • Visceral motor – Regulates the contraction of smooth and cardiac muscle and gland secretion – Makes up autonomic nervous system – Controls function of visceral organs – Often called “involuntary nervous system”
  • 27. Types of Sensory and Motor Information
  • 28. Nerve Fibers That Transmit Different Types of Signals, and Their Physiologic Classification • Nerve fibers come in all sizes between 0.5 and 20 micrometers in diameter-the larger the diameter, the greater the conducting velocity. • The range of conducting velocities is between 0.5 and 120 m/sec
  • 29. Physiologic classifications and functions of nerve fibers (Erlanger and Gasser)