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The Nervous System
Chapters 39 & 40
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Overview Three overlapping functions: sensory input,
integration, and motor output 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
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Parts Central nervous system (CNS)
Brain and spinal cord Responsible for integration
Peripheral nervous system (PNS) Nerves which carry sensory input to the CNS
and motor output away from the CNS
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Neurons Specialized for transmitting chemical and
electrical signals from one location to another Neurons are made up of:
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
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Types of Neurons Sensory
Convey information about external and internal environments from sensory receptors to the CNS
Interneurons Integrate sensory input and motor output;
Located within the CNS
Motor Convey impulses from the CNS to effector cells
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Supporting Cells Glial cells
Structurally reinforce, protect, insulate, and assist neurons
Do not conduct impulses Outnumber neurons 10-50 to 1
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The Nature of Neural Signals 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
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Action Potential 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.
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Refractory period 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
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Action potentials travel along an axon
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
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Synapses Gaps between neurons Electrical synapses:
Allows impulses to travel quickly and without loss of signal strength
Not very common Chemical synapses:
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 Some neurotransmitters:
Acetylcholine, epinephrine, dopamine
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Organization of the CNS Provides the basis for complex behavior in
vertebrates Spinal cord
Carries information to and from the brain Can integrate simple responses to some stimuli
(reflexes)
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CNS… 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)
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The Brain Brain stem
Conducts data and control automatic activities essential for life
Cerebellum Controls movement and balance
Thalamus and hypothalmus Regulates homeostasis
Cerebrum The ‘thinking’ part of the brain
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CNS… 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
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The PNS Sensory division
Brings information from the sensory receptors to the CNS
Motor division Carries signals from the CNS to effector cells Two separate divisions:
Somatic nervous system – carry signals to skeletal muscles; voluntary
Autonomic nervous system – controls smooth and cardiac muscles; involuntary
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