Neural Control and the Senses

1
Neural Control and the Senses

• Chapter 30

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Neurons

• Basic units of communication in nearly all
  nervous systems
• Monitor information in and around the body and
  issue commands for responsive actions

3
In Pursuit of Ecstasy

• MDMA is amphetamine-like drug
• Interferes with removal of chemical messenger
  (serotonin) used by nervous system
• Memory loss and depression are common side
  effects
• In rare cases it causes seizures, hypothermia,
  and even death

4
Neurons

• Communication units of the nervous system
• Sensory neurons
• Interneurons
• Motor neurons

5
Neuroglia

• Make up more than half the volume of the
  vertebrate nervous system
• A variety of cells that metabolically assist,
  structurally support, and protect the neurons

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Structure of a Neuron
dendrites
INPUT ZONE
cell body
axon
OUPUT ZONE
TRIGGER ZONE
CONDUCTING ZONE
axon endings
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How Ions Move across Membrane
interstitial fluid
cytoplasm
Na/K pump
passive transporters with open channels
passive transporters with voltage-sensitive gated
channels
active transporters
lipid bilayer of neuron membrane
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Ion Concentrations in a Resting Neuron

• Potassium (K)
• Higher inside than outside
• Sodium (Na)
• Higher outside than inside

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Action Potential

• A brief reversal in membrane potential
• Voltage change causes voltage-gated channels in
  the membrane to open
• Inside of neuron briefly becomes more positive
  than outside

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Action Potential
Na
1
2
Na
Na
K
K
K
K
K
K
K
Na
Na
Na
Na
3
4
Na
Na
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Positive Feedback
more Na ions flow into the neuron
neuron becomes more positive inside
more gated channels for Na open
12
All or Nothing

• All action potentials are the same size
• If stimulation is below threshold level, no
  action potential occurs
• If it is above threshold level, cell is always
  depolarized to the same level

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Repolarization

• Once action potential peak is reached, Na gates
  close and K gates open
• Movement of K out of cell
• The inside of the cell once again becomes more
  negative than the outside

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Recording of Action Potential
action potential
20
0
-20
Membrane potential (millivolts)
threshold
-40
resting membrane potential
-70
0
2
3
5
1
4
Time (milliseconds)
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Propagation of Action Potentials

• An action potential in one part of an axon brings
  a neighboring region to threshold
• Action potential occurs in one patch of membrane
  after another

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Chemical Synapse
plasma membrane of axon ending of presynaptic cell

• Gap between the terminal ending of an axon and
  the input zone of another cell

plasma membrane of postsynaptic cell
synaptic vesicle
synaptic cleft
membrane receptor
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Synaptic Transmission

• Action potential in axon ending of presynaptic
  cell causes voltage-gated calcium channels to
  open
• Flow of calcium into presynaptic cell causes
  release of neurotransmitter into synaptic cleft

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Synaptic Transmission

• Neurotransmitter diffuses across cleft and binds
  to receptors on membrane of postsynaptic cell
• Binding of neurotransmitter to receptors opens
  ion channels in the membrane of postsynaptic cell

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Ion Gates Open
neurotransmitter
ions
receptor for neurotransmitter
gated channel protein
20
Synaptic Integration

• Many signals reach a neuron at the same time
• Signals may suppress or reinforce one another
• Whether or not an action potential occurs depends
  on the sum of the signals the neuron is receiving

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Cleaning Up

• Presence of neurotransmitter in synaptic cleft is
  normally brief
• Molecules diffuse away, are reabsorbed, or broken
  down
• Drugs, such as ecstasy and cocaine, interfere
  with the reuptake of neurotransmitters

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Information Flow
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Nerve
axon
myelin sheath

• A bundle of axons enclosed within a connective
  tissue sheath

many neurons inside a connective tissue sheath
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Myelin Sheath

• Sheath blocks ion movements
• Action potential must jump from node to node
• Greatly enhances speed of transmission

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Multiple Sclerosis

• A condition in which nerve fibers lose their
  myelin
• Slows conduction
• Symptoms include visual problems, numbness,
  muscle weakness, and fatigue

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Reflexes

• Automatic movements made in response to stimuli
• In the simplest reflex arcs, sensory neurons
  synapse directly on motor neurons
• Most reflexes involve an interneuron

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Stretch Reflex
STIMULUS Biceps stretches.
sensory neuron
motor neuron
RESPONSE Biceps contracts.
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Drugs and Addiction

• A drug is a substance introduced into the body to
  provoke a specific physiological response
• In addiction, a drug assumes an essential
  biochemical role in the body

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Types of Drugs

• Stimulants
• Depressants, hypnotics
• Analgesics
• Psychedelics, hallucinogens

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Sensory Receptors

• Convert the energy of a stimulus into action
  potentials

Mechanoreceptors Thermoreceptors Pain
receptors
Chemoreceptors Osmoreceptors Photoreceptors
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Assessing a Stimulus

• Action potentials dont vary in amplitude
• Brain tells nature of stimulus by
• Particular pathway that carries the signal
• Frequency of action potentials along an axon
• Number of axons recruited

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Somatic Sensations

• Touch
• Pressure
• Temperature
• Pain
• Motion
• Position

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Receptors in Skin

• Free nerve ending
• Ruffini ending
• Pacinian corpuscle
• Bulb of Krause
• Meissners corpuscle

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Smell

• A special sense
• Olfactory receptors
• Receptor axons lead to olfactory lobe

olfactory bulb
receptor cell
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Taste

• A special sense
• Chemoreceptors
• Five primary sensations
• sweet, sour, salty, bitter, and umami

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Balance and Equilibrium

• Organs of equilibrium are located in the inner ear

semicircular canals
utricle
saccule
vestibular apparatus
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Properties of Sound

• Ear detects pressure waves
• Amplitude of waves corresponds to perceived
  loudness
• Frequency of waves (number per second)
  corresponds to perceived pitch

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Anatomy of Human Ear
stirrup
auditory nerve
anvil
hammer
auditory canal
eardrum
cochlea
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Sound Reception

• Sound waves make the eardrum vibrate
• Vibrations are transmitted to the bones of the
  middle ear
• The stirrup transmits force to the oval window of
  the fluid-filled cochlea

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Sound Reception

• Movement of oval window causes waves in the fluid
  inside cochlear ducts

oval window (behind stirrup)
scala vestibuli
eardrum
round window
scala tympani
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Sound Reception
hair cells in organ of Corti
lumen of cochlear duct
tectorial membrane
basilar membrane
to auditory nerve
lumen of scala tympani
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Vision

• Sensitivity to light does not equal vision
• Vision requires two components
• Eyes
• Capacity for image formation in the brain

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Human Eye
sclera
retina
choroid
iris
fovea
optic disk
lens
pupil
cornea
part of optic nerve
aqueous humor
ciliary muscle
vitreous body
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Pattern of Stimulation

• Light rays pass through lens and converge on
  retina at back of eye
• The image that forms on the retina is upside down
  and reversed right to left compared with the
  stimulus
• Brain accounts for this during processing