Anatomy of the Nervous System

How Nerve Signals Maintain
Chapter 9
The Nervous System Intro
• Every single stimulus (light, touch, feeling of
muscles moving, pressure, blinking, etc.)
involves your nervous system.
• Coordinated motions (which do not seems to
be coordinated) are controlled mostly by the
nervous system.
• nervous system + endocrine system control
the actions of the body.
The Nervous System
• Elaborate communication system
– Contains 100 billion nerve cells in the brain alone.
– Humans: memory, learning, language, etc.
Vertebrate Nervous System
• 2 main divisions:
– Central nervous system (CNS)
• Brain + spinal cord  coordinating centre
– Incoming and outgoing info.
– Peripheral nervous system (PNS)
• Info between organs of body and CNS.
– Two peripheral divisions:
» Somatic
• Skeletal muscles, bones, and skin
• Sensory somatic PNS: relay info about environment to
• Motor somatic PNS: initiate appropriate responds.
» Autonomic
• Motor nerves which control internal organs of body.
• Sympathetic
• Parasympathetic
Organization of the Nervous System
Nerve Cells
• Glial cells
– Nonconducting cells  structural support and metabolism
of nerve cells.
• Neurons
– Functional units of nervous system (conduct electricity)
• Sensory neurons
– “afferent neurons”
– Sense and relay info (stimuli) from environment to CNS
» (ex// photoreceptors, chemoreceptors, thermoreceptors, etc.)
» Ganglia: clusters of sensory receptors.
• Interneurons
– Integrate and interpret sensory and motor neurons (found mostly in the
• Motor neurons
– “efferent neurons”
– Relay info to effectors: muscles, organs, and glands (can produce a
Anatomy of a Nerve Cell (Neuron)
• All neurons contain:
– Dendrites
• Receive info from other nerve
• Conduct info towards the
neuron’s cell body
– Soma: neuronal cell body
(containing nucleus)
– Axon
• Extension of cytoplasm
• Carries nerve impulse towards
other neurons or effectors.
• Creates an “attachment” to other
• Super super thin (100 could fit
inside single human hair!)
Myelin Sheath
• Insulation
– Myelin sheaths are
formed by special glial
cells called Schwann cells
– Myelin sheaths prevents
the loss of ions from
• (important when you
understand how these
cells work!)
• Made of ‘fatty protein’
Nodes of Ranvier
• Between sections of myelin sheath
• Nerve impulses jump from one node to another 
speed up movement of nerve impulses.
– Nerve impulses move much faster along myelinated nerve
fibres than nonmyelinated ones.
(Nerve impulses move much faster along smaller diameter
• Thin membrane which surrounds the axon in
peripheral nervous system
– Promotes regeneration of damaged axons
– Severed neurons can be rejoined.
– (feeling gradually returns to your finger following
a paper cut)
White and Grey Matter
• White matter
– Nerve cells within the brain that contain
myelinated fibres and neurilemma
• Myelinated axons white in appearance
• Grey matter
– Nerve cells within the brain and spinal cord
referred to as grey matter: lack myelin sheath and
neurilemma  do not regenerate after injury
• Damage to grey matter is usually permanent.
Categories of Neurons
• Sensory Neurons: AFFERENT neurons: relay info from environment
– Located in clusters called ganglia
• Photoreceptors (eyes)
• Chemoreceptors (tongue and nose)
• Thermoreceptors (skin and hypothalamus)
• Interneurons
– Interpret sensory info and send info to outgoing motor neurons
(mostly in CNS)
• Motor neurons: EFFERENT neurons: relay info from interneurons 
effectors (produce a response)
– Muscles
– organs,
– glands
Organization of Nerves
• Most nerves are part of many neurons/axons
held together in a ‘bundle.’
Neural Circuits
• Reflex
– Involuntary and often unconscious
• Example: touching a hot stove
– How long does it take you to remove your hand?
– Nerve impulse is carried to the spinal chord 
motor neuron (skipping brain)  sensation felt
after removing hand.
Reflex arc
• Simplest nerve pathway.
• Occurs without brain coordination
• Five essential components:
– Receptor
– Sensory neuron
– Interneuron (spinal chord)
– Motor neuron
– Effector
• Page 417
1) # 1-6, 8.
2) Draw/ Label a general neuron: dendrite(s),
soma, nucleus, axon, myelin sheath, axon
terminals, direction of impulse.
3) Read page 415 article. Make brief notes.

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