NERVOUS SYSTEM and SENSE ORGANS

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CHAPTER 28

• NERVOUS SYSTEM and SENSE ORGANS

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Nervous system

• The nervous system controls and coordinates
  functions throughout the body and responds to
  internal and external stimuli.

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Transmission of Nerve Impulses

• A neuron is a specialized cell for transmitting
  electrical signals called nerve impulses.
• Neurons consist of a cell body, dendrites that
  carry impulses to the cell body and an axon that
  transmits impulses to other cells

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Neuron Function

• When a message travels down the axon of a neuron,
  it is sent as an electro-chemical signal.
• This signal is self-propagating (once started, it
  continues the whole way without help).
• This response is all or nothing.
• It takes some time for the message to be sent and
  the neuron to prepare for the next message.

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Nerve Signals

• Resting Potential voltage across the plasma
  membrane
• Ion Pumps move specific ions across membrane
• Depolarization charge difference decreases
• Action Potential Strong Depolarization

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Nerve Signal Speed

• Normal nerve impulses move at 5m/sec.
• Myelin covered nerve cells move at 150M/sec. (30
  times faster) because impulse jumps from node to
  node
• The stronger the stimulus the number and
  frequency of action potentials increases

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Bridging the gap between neurons

• Synapse gap between two neurons.
• Neurotransmitters secreted by the end of the
  neuron which flow across the gap and cause the
  impulse to be continued on the other side. (Sarin
  gas!)

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Nervous System

• ORGANIZATION
• The human nervous system is divided into two main
  parts
• central nervous system (CNS)- bodys main
  processing center composed of brain and spinal
  cord
• peripheral nervous system (PNS)- branches of
  nerves that relay information to and from CNS.
• sensory neurons transmit toward CNS
• motor neurons transmit information away from CNS

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Major nervous system parts
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The Brain

• The human brain is responsible for
• overseeing the daily operations of the body
• interpreting the vast amount of information it
  receives
• maintaining homeostasis
• Makes up about 2 of body weight
• Contains approximately 100B neurons

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Brain

• The brain consists of several parts
• Cerebrum
• divided into two halves (left brain/right brain)
  7/8 of total
• gray matter outside
• white matter inside
• center of intellect, memory, language,
  consciousness
• receives and interprets sensory information
• controls motor
• functions
• Medulla regulates
• heart rate, respiration
• rate, blood pressure

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Brain

• Cerebellum - controls balance, posture
  and coordinates voluntary muscle contraction.
• Hypothalamus - center of control of homeostasis
• controls body temperature, appetite, water
  balance
• links nervous and endocrine systems

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CNS The Spinal Cord

• The spinal cord is a cable of nerve tissue
• connects the brain to the rest of the body
• gray matter inside white matter outside
• also responsible for reflexes

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Peripheral Nervous System

• PNS takes information to and from CNS
• The PNS has two types of nerves
• sensory neurons -gather information about
  environment send it to CNS
• motor neurons - receive information from CNS
  send it to muscles and glands.

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PNS

• The motor neurons of the PNS are divided further
• somatic nervous system - conducts impulses to
  skeletal muscles under our conscious control
• autonomic nervous system - regulates the
  activities of cardiac muscles, smooth muscles and
  glands (involuntary control) ANS helps maintain
  homeostasis

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Autonomic Nervous System

• Sympathetic Division Increases the activity of
  the body , fight or flight.
• Parasympathetic Division Regular function or
  calms the body after stress

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Reflexes

• Movements called reflexes are not under conscious
  control.
• reflex arc involves 2-3 neurons
• used for quick response to danger, injury (a
  protective mechanism)
• brain is not involved
• (may interpret pain,
• heat, etc. after
• the response)

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The Senses Eye

• The eye responds to light stimuli
• light passes through transparent, protective
  cornea
• amount of light entering eye is controlled by
  the colored iris (a ring of muscles)
• opening through which light enters - pupil
• lens is a flexible structure attached to ciliary
  muscles that changes lens shape to help you focus

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Vision

• retina is the light sensitive tissue at the back
  of eye
• two types of photoreceptors are found in retina
• rods - extremely sensitive to light (dim vision)
• cones - detect color (fovea area of sharpest
  vision)
• nerve impulses travel along optic nerve to brain
• image arrives at retina inverted brain rights it
• binocular vision which allows depth perception
  results from the overlap of the visual fields
  from both eyes

Cone
Rod
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Hearing and Balance

• The ear
• responsible for sensing equilibrium and sound
• divided into three sections - outer, middle,
  inner
• sound vibrations enter ear through auditory canal
• this causes tympanic membrane (eardrum) to
  vibrate
• three small bones (ossicles) transfer vibrations
  to fluid-filled cochlea
• hair cells within cochlea send impulses to the
  brain via the auditory nerve

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Ear
Eustachian tube is an opening to the throat that
enables you to equalize pressure on both sides of
the eardrum.
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Basilar Membrane
Normal
Damaged
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Balance

• Another portion of inner ear maintains balance
• semicircular canals also contain fluid and hairs
• three canals at different orientations sense
  changes in position of head as fluid movement
  stimulates hairs
• two sacs located below canals contain crystals
  that enable you to sense the pull of gravity
• together these allow you to sense your bodys
  position and maintain balance

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Taste and Smell

• Taste and smell are chemical senses
• Taste buds - embedded in surface of tongue
• four types of taste receptors - sweet, sour,
  bitter, salty allow perception of hundreds of
  tastes
• hot sensation of spicy foods detected by pain
  receptors, not chemoreceptors!

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Smell

• Olfactory receptors - located in roof of nasal
  passage
• chemorecptors that respond to chemicals in the
  gaseous state
• strongly influence our sense of taste