Chapter 12: Nervous Tissue I

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Chapter 12 Nervous Tissue I
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Objectives

• Provide nervous system overview.
• Present histology of nervous tissue.
• Develop an understanding of how electrical
  signals in neurons are generated and propagated.
• Signal transmission at electrical and chemical
  synapses.
• Identify neural circuits.

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Functions of the nervous system

• Sensory functiondetect internal and external
  stimuli.
• Afferent neurons carry sensory information to the
  brain.
• Integrative functionintegration of sensory
  information that leads to analysis, storage, and
  decisions.
• Interneurons bring information together from
  different areas of brain.
• Motor functionprovide a motor or secretory
  response to sensory information.
• Efferent neurons carry information from the brain
  and spinal cord to effectors such as muscles,
  organs, and glands.

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Nervous system organization

• Central nervous system (CNS)
• Brain and spinal cord
• Peripheral nervous system (PNS)
• All nervous tissue outside of brain and spinal
  cord.

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CNS

• Processes incoming sensory information
• It is the source of thoughts, emotions and
  memories.
• It is where most somatic muscular activity
  originates.
• It regulates seretions from glandular tissue.

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PNS divisions

• Somatic nervous system
• Includes sensory neurons
• Includes motor neurons that conduct information
  to skeletal muscle.
• Autonomic nervous system
• Includes sensory neurons that convey information
  from visceral organs.
• Includes motor neurons that conduct information
  to smooth muscle, cardiac muscle and glands.

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ANS motor response divisions

• Sympathetic division
• Supports fight or flight responses
• Parasympathetic division
• Supports rest and digest activities

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Enteric nervous system

• An involuntary nervous system that is the brain
  of the gut.
• Consists of 100 million neurons found in the
  enteric plexus that extend throughout the entire
  gut.
• Sensory neurons monitor chemical conditions in
  the gut.
• Motor neurons govern contraction of gut smooth
  muscle.
• ENS functions independently of the ANS and CNS
  but does communicate with the CNS and receives
  communication from the CNS via the ANS.

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Divisions of the Nervous system
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Nervous Tissue Histology

• Neurons are electrically excitable.
• Neurons are diverse in sizefrom lt1mm to ones
  that extend over meter in length.
• Nerve impulses travel from 1 to 280 mph.
• Neuroglia
• Support nourish and protect neurons.
• Maintain the neuronal environment through
  homeostatic mechanisms

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Parts of the neuron

• Cell body (perikaryon, somata)
• Nucleus
• Lysosomes
• Mitochondria
• Golgi complex
• Nissl bodiesrough endoplasmic reticulum
• Cytoskeleton
• Neurofibrils
• Microtubules

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Parts of the neuron

• Nerve fibersextensions from the cell body
• Dendritesreceive input from other neurons
• Axonspropagate nerve impulses

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Parts of a neuron

• Axons
• Axon hillock
• Initial segment
• Trigger zone
• Axon terminal
• Synaptic end bulb
• varicosities

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Neuronal structural diversity
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Neuronal structural diversity
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Neuronal Structural diversity
Purkinje cell
Motor neuron
Pyramidal cell
Visual neuron
Mechanosensory interneuron
From Introduction to Neurobiology Reichert
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Neuroglia

• Make up half the volume of the CNS.
• They are smaller than neurons.
• They are 5 to 50 times more numerous than
  neurons.
• Undergo division.
• Do not generate action potentials BUT do generate
  waves of electrical activity.
• Will multiply to fill in spaces formerly occupied
  by neurons.

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CNS Neuroglia Astrocytes

• Astrocyteslargest of the glial cells
• Protoplasmic astrocytes
• Fibrous astrocytes
• Astrocyte function
• Support
• Maintain blood brain barrier
• Regulate growth
• Maintain chemical environment
• Influence the formation of synapses

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CNS Neuroglia

• Oligodendrocytes
• Smaller and form fewer processes
• Form and maintain the myelin sheath
• Microgliafunction as phagocytes.
• Ependymal cells
• Possess microvilli and cilia and line the
  ventricles of brain and spinal canal.
• Monitor spinal fluid and assist in its
  circulation
• Also function in maintaining blood-brain barrier.

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PNS Neuroglia

• Schwann cellsmyelinates a single axon or a group
  of axons.
• Satellite cellssurround the cell bodies of
  ganglia.

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Formation of Myelin sheath

• Each Schwann cell wraps around 1mm of axonal
  length (forms nodes of Ranvier).
• The Schwann cell wraps around the axon up to 100
  times to form the myelin sheath.
• The outer portion is nucleated cytoplasm
• The inner layer is the neurolemma.
• Each Schwann cell wraps one axonal segment
  between two nodes.

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Myelination
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Myelination
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Myelination
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Myelination in CNS

• Oligodendrocytes extend 15 broad flat processes
  that wrap CNS axons
• No neurolemma
• Fewer nodes of Ranvier.
• There is no ability to regenerate damaged axons.
• Myelin increases from birth to maturity.

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Gray and White matter

• Whitemyelinated axons
• Graycell bodies, dendrites, unmyelinated axons,
  axon terminals and neuroglia.