The Nervous System and the Senses by pnx67864

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									    The Nervous System and the
             Senses

                 Chapter 24




 Neurons are the functional unit of
       the nervous system
• To understand how neurons work it is
  important to understand their anatomy.




                   Anatomy of a neuron

                   1. Synaptic terminals: Bring
                   chemical signals from other neurons.
                    2. Dendrites: Receive signals
                   from other neurons.
                   3. Cell body: Integrates signals;
                   coordinates metabolic activities.
     Fig. 24-1
                   4. Axon Hillock: Neuron action
                   potential initiated here.
                   5. Axon: Transmits the action
                   potential.
                   6. Myelin: Insulates axon, speeds
                   Action potential conduction.
                   7. Synaptic terminals: Transmits
                   signals to other neurons.
                   8. Dendrites: (of other neurons)




                                                          1
                   How Do Nerve Cells Work?
                   1. The cell body receives signals from other
                      neurons through its dendrites. That signal is
                      processed within the cell body.
      Fig. 24-1
                   2. The cell body then initiates an action potential
                       at the axon hillock. The action potential then
                       travels down the axon.

                   3. The action potentials signal the release of
                       neurotransmitters from at the synaptic
                       terminals.




 How do nerve cells produce and
        transmit signals?
• The electrical charge inside a neuron
  changes during an action potential.
• A neurons electrical charge is the result of
  a voltage difference across the plasma
  membrane.
• When a neuron unstimulated and inactive,
  its potential is called the resting potential.




Electrical events in a neuron during
         an action potential




                       Fig. 24-2




                                                                         2
 Neurons Communicate at Synapses


                            • Signals that either excite or inhibit
                            the next neuron are produced at
                            synapses.

     Fig. 24-3              • Excitatory and inhibitory signals are
                            integrated in the cell body.

                            • The nervous system uses many
                            neurotransmitters to produce excitatory
                            and inhibitory signals at the synapse.




   How does the nervous system
      process information?
The intensity of a stimulus is coded by the
 frequency of signals/action potentials.



                    Fig. 24-4




  How is the human nervous system
             organized?




                    Fig. 24-5




                                                                      3
          How is the human nervous system
                     organized?
    There are 2 subdivisions of the nervous system:
    1. The central nervous system – made up of the brain
        and spinal cord.
    2. The peripheral nervous system – all nerves and
        neurons outside of the brain and spinal cord.

    The peripheral nervous system links the central nervous
        system to the body and is sub-divided into 2 systems:
         1.    The somatic system - controls voluntary movement.
         2.    The autonomic system - controls involuntary actions.




                                  Reflex Archs

 Reflex pathways include neurons
 in the spinal cord.
This simple reflex circuit begins with
a sensory neuron that has pain-
sensitive endings in the skin and a
long axon leading to the spinal cord.                       Fig. 24-7
This sensory neuron stimulates an
association neuron in the spinal cord,
which in turn stimulates a motor neuron
in the cord, causing withdrawal of the
injured body part. An association neuron
also sends the signal to the brain, informing
it of the painful event.




      The Brain Consists of Many Parts



                                                Fig. 24-8




Each region of the brain performs a specific task.




                                                                        4
Each region of the brain performs a
          specific task.
• The hindbrain controls breathing, sleep, and
  movement.
• The midbrain filters input.
• The forebrain gives us our emotions, thoughts,
  and perceptions.
  – The thalamus relays signals to other parts of the
    brain.
  – The limbic system produces emotions.
  – The cerebral cortex produces thoughts and
    perceptions.




         The Cerebral Cortex



                         Fig. 24-9




Each side of the brain processes different information and
                      controls things.




  What are sensory receptors?
• Sensory receptors relay information from
  the outside world to the brain.



                         Table 24-2




                                                             5
             How is sound sensed?
             The ear captures and transmits information about
             movement, gravity and sound.




                                 Fig. 24-11




             How is sound sensed?
• Sound is converted into
electrical signals (action
potentials) in the cochlea.

• Hair cells are deflected by
the tectorial membrane as                         Fig. 24-11
sound waves travel through
the cochlea.

• As the hair cells are
deflected, action potentials
are sent to the brain via the
auditory nerve.




   • Loud sounds can
      damage hair cells
      leading to cell death
      and subsequent
      deafness.
                                                      Fig. 24-12

   • Once hair cells have
     died, they do not
     regenerate.




                                                                   6
              How is light sensed?
       The eye collects and focuses light.




                                   Fig. 24-13




The adjustable lens can focus on                Light striking the retina is captured
distant and nearby objects.                     by photoreceptors.




       Focusing in the human eye



                                   Fig. 24-14




              How is light sensed?
• Rods and cones differ in distribution and
  light sensitivity.
• Rods detect light levels but are unable to
  detect colors.
• Cones detect color. There are 3 types of
  cone cells. Those that detect red, blue and
  green.




                                                                                        7
      How are chemicals sensed?
      •   Both smell and taste are chemical senses.
      •   The ability to smell arises from olfactory receptors.




                                      Fig. 24-16




      How are chemicals sensed?
 Taste receptors are clustered on the tongue.

                                       The human tongue is covered with papillae,
                                       bumps in which taste buds are embedded.

                                       Small papillae are located on the front two-
                                       thirds of the tongue; larger ones with more
              Fig. 24-17               taste buds are far in the back.

                                       Each taste bud consists of supporting cells
                                       surrounding taste receptors, whose
                                       microvilli have protein receptors that bind
                                       tasty molecules, producing an action
                                       potential.




      How are chemicals sensed?
      Pain is a specialized chemical sense.


An injury, such as a tack stab, damages
both cells and blood vessels.

The damaged cells and vessels release                    Fig. 24-18
molecules that activate pain receptor
neurons.

The pain receptor neurons then send
information to the brain via action
potentials.




                                                                                      8
  Other special senses in nature

Echolocation is a
special sense that
some animals have
that allow them to
detect ultrasonic                               Fig. 24-19
sounds which
bounce off of nearby
objects. These series
of sounds produce an
auditory image of
nearby surroundings.




  Other special senses in nature
Detecting electrical fields. Some fish that live in dark water, locate
nearby objects by producing an electrical field and then sensing
distortions in the field caused by the objects.




                                Fig. 24-20




                     Vocabulary List
                   Know the following terms
     Synaptic terminals                      Forebrain
     Dendrites                               Thalamus
     Cell body                               Limbic system
     Axon Hillock                            Cerebral cortex
     Axon                                    Sensory receptors
     Myelin                                  Hair cells
     Synaptic terminals                      tectorial membrane
     Action potential                        Lens
     Resting potential                       Retina
     Excitatory                              Photoreceptors (rods and
     Inhibitory                              cones)
     The central nervous system              Chemical senses
     The peripheral nervous system           Papillae
     The somatic system                      Supporting cells
     The autonomic system                    Microvilli
     Reflex circuit                          Protein receptors
     Hindbrain                               Echolocation
     Midbrain




                                                                         9

								
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