Transcript The Nervous System

The Nervous
System
Chapters 39 & 40
Overview
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Three overlapping functions: sensory input,
integration, and motor output
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Sensory input – the conduction of signals from sensory
receptors to integration centers of the nervous system
Integration – information from sensory receptors is
interpreted and associated with appropriate responses
from the body
Motor output – the conduction of signals from the
processing center to effector cells (muscle cells or gland
cells) that carry out the response
Parts
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Central nervous system (CNS)
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Brain and spinal cord
Responsible for integration
Peripheral nervous system (PNS)
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Nerves which carry sensory input to the CNS and
motor output away from the CNS
Neurons
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Specialized for transmitting chemical and
electrical signals from one location to another
Neurons are made up of:
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Cell body: contains cytoplasm, nucleus, organelles
Dendrites: carry information to the cell body; short,
numerous and very branched
Axons: conduct impulses away from the cell body; long
and singular
Types of Neurons
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Sensory
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Interneurons
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Convey information about external and internal
environments from sensory receptors to the CNS
Integrate sensory input and motor output;
Located within the CNS
Motor
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Convey impulses from the CNS to effector cells
Supporting Cells
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Glial cells
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Structurally reinforce, protect, insulate, and assist
neurons
Do not conduct impulses
Outnumber neurons 10-50 to 1
The Nature of Neural Signals
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Signal transmission along the length of a neuron
depends of voltages created by ionic changes
across the plasma membranes
There is a difference in ion concentrations
between the cell’s contents and the extracellular
fluid – this is the membrane potential
All cells have this membrane potential
Only neurons and muscle cells can change their
membrane potentials in response to stimuli
Action Potential
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An all-or-none change in the membrane potential
Cells which can do this (nerve and muscle) are called
excitable cells
The membrane potential of an excitable cell at rest
(unexcited state) is called a resting potential
The presence of gated ion channels in these cells permits
them to change the plasma membrane’s permeability and
alter the membrane potential in response to stimuli
An action potential is the rapid change in membrane
potential of an excitable cell, caused by stimulus-triggered
selective opening and closing of voltage-gated ion channels.
Refractory period
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A refractory period occurs after the changes in
membrane potential during which time the neuron
is insensitive to stimuli.
The action potential is all-or-none: the neuron
either fires or doesn’t fire
The nervous system distinguishes between strong
and weak stimuli based on the frequency of action
potentials generated
Action potentials travel along an axon
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A neuron is stimulated at its dendrites or
cell body, and the action potential travels
along the axon to the other end of the
neuron
Chemical or electrical communication
between cells occurs at synapses
Synapses
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Gaps between neurons
Electrical synapses:
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Allows impulses to travel quickly and without loss of signal
strength
Not very common
Chemical synapses:
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A neurotransmitter is released from the axon of one neuron
into the gap
This in turn stimulates the dendrites of the next neuron
The neurotransmitter chemical is quickly degraded by enzymes
and the components recycled
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Some neurotransmitters:
 Acetylcholine, epinephrine, dopamine
Organization of the CNS
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Provides the basis for complex behavior in
vertebrates
Spinal cord
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Carries information to and from the brain
Can integrate simple responses to some stimuli
(reflexes)
CNS…
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The Brain:
Carries out complex integration for
homeostasis, perception, movement,
intellect, and emotions
White matter – the inner region
Gray matter – the outer region (opposite
orientation in the spinal cord)
The Brain
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Brain stem
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Cerebellum
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Controls movement and balance
Thalamus and hypothalmus
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Conducts data and control automatic activities essential
for life
Regulates homeostasis
Cerebrum
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The ‘thinking’ part of the brain
CNS…
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In both:
Meninges – protective layers of connective
tissue
Cerebrospinal fluid – fills the ventricles of
the brain and the central canal of the spinal
cord; acts as a shock absorber and
circulates hormones, nutrients, and white
blood cells
The PNS
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Sensory division
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Brings information from the sensory receptors to the CNS
Motor division
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Carries signals from the CNS to effector cells
Two separate divisions:
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Somatic nervous system – carry signals to skeletal
muscles; voluntary
Autonomic nervous system – controls smooth and cardiac
muscles; involuntary