Introduction to the Nervous System

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Introduction to the Nervous System

• The nervous system is one of the body systems
  that many organisms use to maintain homeostasis.
• The human nervous system sends electrochemical
  messages throughout the body to help it respond
  to its environment.
• The basic unit of the nervous system is a special
  kind of cell known as a neuron.

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Structure of a Neuron
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Structure of a Neuron

• cell body
• nucleus
• dendrites
• axon
• Schwann cell nucleus

• myelin sheath
• node of Ranvier
• Schwann cell
• terminal branches
• synaptic knobs

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Impulses Along a Neuron

• Dendrites receive the nerve impulse and carry it
  toward the cell body, which contains the nucleus.
• The axon carries the impulse from the cell body
  toward the synaptic knobs where it will be
  transferred to other neurons.

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Myelinated vs. Unmyelinated

• Myelinated neurons carry nerve impulses faster
  than unmyelinated ones because the impulse jumps
  from one node of Ranvier to the next, instead of
  traveling the whole length of the axon.

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Impulses from PNS to CNS

• Specialized receptors (light, sound, taste,
  touch) react to a stimulus and generate nerve
  impulses in the sensory neurons around them.
• The sensory neurons carry the impulse to the
  spinal cord and then to the brain where
  interneurons interpret the sensory information.

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Impulses from PNS to CNS, contd

• The interneurons send out impulses to motor
  neurons which elicit a response by an effector
  (muscle or gland).

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Reflex Arcs

• The simplest neural pathway is called a reflex
  arc and it does not involve the brain.
• The pathway is receptor, sensory neuron,
  interneuron in spinal cord, motor neuron,
  effector.

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Reflex Arc
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Reflex Arc, contd

• The fact that the decision is made in the
  spinal cord saves the time that it would take the
  nerve impulse to travel through the many circuits
  of the brain.
• Reflexes are often so fast they are involuntary.
• Reflexes are protective mechanisms that are
  important in maintaining homeostasis.