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Nervous System
Chapter 31
Neurons and Nerve Impulses
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Neurons Conduct Electrical Signals
Neuron:
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Specialized cells that transmit information throughout the
body.
Enable important functions
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Movement
Perception
Thought
Emotion
Learning
Structure of Neurons
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Dendrites:
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Extend from the cell body
The antennae of the neuron
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Axon:
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Long extension of the cytoplasm that conducts
nerve impulses
Nerves:
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Receives information from other cells
Bundle of neurons that contain axons of many
different neurons
Some neurons are insulated with myelin
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Fatty outer layer which insulates the axon
Allows for faster nerve impulses
Neuron Function
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Depends on electrical activity
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Membrane potential:
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Measured in volts (V)
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The difference in electrical charge across the cell membrane
Depends on the movement of ions into and out of a cell
Ions diffuse across a neuron’s cell membrane through a special
protein called
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Voltage-gated ion channels
Resting Potential
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When a neuron is not conducting a nerve impulse the
neuron is at rest.
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Resting potential:
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The membrane potential of a neuron at rest.
-70 millivolts (mV)
The inside of the cell is neg. charged compared to the outside of the
cell.
Action Potential
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Changes occur in the cell membrane when a neuron is
conducting a nerve impulse is called an..
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Action potential:
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Local reversal of polarity inside the neuron
Moves down an axon like a flame burns down a match.
Action potential events:
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1. At the resting potential Na channels are closed and K
channels are opened.
2. During an action potential, Na channels open, causing Na
ions to move into the axon.
Neurons Use Neurotransmitters to
Communicate
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When a nerve impulse arrives at axon terminals, the
impulse is then transmitted to other cells
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Synapse:
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Synaptic cleft:
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The junction at which neuron meets another cell
Tiny gap between receiving cell and axon terminal
Transmission of a nerve impulse uses electrical energy. It
then converts to chemical energy and then back to
electrical energy.
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Neurotransmitters:
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Produced by neurons and stored inside vesicles
Signal molecule that transmits nerve impulses across the synaptic
cleft.
Release of Neurotransmitters
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Neurotransmitters are released
from a presynaptic neuron and
diffuse across the synaptic cleft,
stimulating a postsynaptic cell.
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They either excite or inhibit
Structure of the Nervous System
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Central nervous system (CNS):
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Consists of the brain and spinal
cord
Peripheral nervous system (PNS):
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Contains sensory and motor
neurons
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Sensory neurons:
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Send information from sense organs
Ex. Skin to CNS
Motor neurons:
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Send commands from the CNS to
muscles and other organs
Brain
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Brain:
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Body’s main processing center
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Cerebrum:
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Cerebellum:
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The largest part of the brain
Used for learning, memory, perception,
and intellectual function
Located in the posterior base of the
brain
Regulates balance, posture, and
movement
Brain Stem:
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Base of the brain
Leads to the spinal cord
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Thalamus:
 Critical site of sensory processing
Hypothalamus:
 Regulates vital homeostatic functions
like breathing
Spinal Cord
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Spinal Cord:
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Dense cable of nervous tissue that runs
through the vertebral column
Links the brain to the PNS
Functions in reflexes
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Reflex:
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Sudden, involuntary movement of muscles in
response to a stimulus
Interneurons:
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Within the spinal cord that links neutrons
together
Peripheral Nervous System
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Branches throughout the body
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Somatic nervous system
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Stimulate skeletal muscles under
conscious control
Spinal reflexes are involuntary
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Self-protective motor response
Extremely rapid
Ex. Knee-jerk reflex
Autonomic nervous system
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Regulate smooth muscles not under
conscious control
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Ex. Heart rate and blood flow
Two divisions
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Parasympathetic: Breathe when you
sleep
Sympathetic: “fight or flight”
Sensory Systems
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Sensory systems enable perception
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Sensory receptors:
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Specialized neurons that detect sensory stimuli and convert it to electrical signals,
in the form of nerve impulses, that are interpreted by the brain.
Specific areas of the brain control different regions and functions of the body
Processing information
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Large % of neurons in the cerebral cortex are responsible for processing
incoming sensory information from the sense organs.
Eyes
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Eyes detect light
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Light enters through the pupil and is
focused on the retina.
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Lining on the back inner surface of the
eye that consists of photoreceptors and
neurons.
Contains two types of photoreceptors
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Rods:
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Cones:
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Respond best to dim light
Respond best to bright light and enable
color vision
Optic Nerve:
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Carries information from the retina to
the brain
Ears
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Ears detect sound and help maintain
equilibrium
Ears convert the energy in sound waves
to electrical signals that can be
interpreted by the brain.
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Cochlea:
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Coiled in the shape of a snail shell
Contains mechanoreceptors called hair cells
that vibrate when waves enter.
Semicircular canals:
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Fluid-filled chambers in the inner ear that
contain hair cells that bend according to the
magnitude and direction of the fluid’s
movement.
Signals generated by hair cells enable the
brain to determine orientation and position
of the head.
Taste and Smell
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Related to chemical senses
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Chemoreceptors for taste cells
detect 4 basic chemical substances
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Sugars
Acids
Alkaloids
Salts
Chemoreceptors for odor
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Olfactory receptors
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Located in the roof of the nasal passage
Drugs and the Nervous System
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Psychoactive drugs:
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Drugs that alter the functioning of the central nervous system
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Ex. Alcohol, nicotine, cocaine, and heroine
Drug addiction
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Involved changes in neuron function
Stimulant:
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A drug that generally increases the activity of the central nervous
system
Depressant:
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A drug that generally decreases the activity of the central nervous
system.