nervous systems three main functions: 1. sensory input 2. integration 3. motor output
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Nervous Systems
Three Main Functions:
• 1. Sensory Input
• 2. Integration
• 3. Motor Output
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Figure 48.1 Overview of a vertebrate nervous system
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Two Main Parts of Nervous Systems
• Central nervous system (CNS) – brain and spinal cord – integration
• Peripheral nervous system (PNS)– network of nerves extending into different parts
of the body– carries sensory input to the CNS and motor
output away from the CNS
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Two Cell Types in Nervous Systems
• Neurons– Cells that conduct the nerve impulses
• Supporting Cells - Neuroglia
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Neuron
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Figure 48.2x Neurons
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Three Major Types of Nerve Cells
• Sensory neurons – communicate info about the external or internal
environment to the CNS
• Interneurons – integrate sensory input and motor output– makes synapses only with other neurons
• Motor neurons – convey impulses from the CNS to effector cells
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Supporting Cells - Neuroglia
• provide neurons with nutrients, remove wastes
Two important types in vertebrates– Oligodendrocytes – myelin sheath in CNS– Schwann cells -myelin sheath in PNS
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Myelin Sheath Formation
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Conduction of the Nerve Impulse
• Membrane Potential– Voltage measured across a membrane due to
differences in electrical charge– Inside of cell is negative wrt outside
• Resting potential of neuron = -70 mV
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Figure 48.6 Measuring membrane potentials
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1. Carrier in membrane binds intracellular sodium.
2. ATP phosphorylates protein with bound sodium.
3. Phosphorylation causes conformational change in protein, reducing its affinity for Na+. The Na+ then diffuses out.4. This conformation has higher
affinity for K+. Extracellular K+ binds to exposed sites.
5. Binding of potassium causes dephosphorylation of protein.
Extracellular
Intracellular
ATPADP
Pi
P+
K+
Na+
6. Dephosphorylation of protein triggers change to original conformation, with low affinity for K+. K+ diffuses into the cell, and the cycle repeats.
Pi
Pi
Pi
Sodium-Potassium Pump
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Excitable Cells
• Neurons & muscle cells
• Have gated ion channels that allow cell to change its membrane potential in response to stimuli
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Gated Ion Channels
• Some stimuli open K+ channels– K+ leaves cell– Membrane potential more negative– hyperpolarization
• Some stimuli open Na+ channels– Na+ enters cell– Membrane potential less negative– depolarization
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Gated Ion Channels
• Strength of stimuli determines how many ion channels open= graded response
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Nerve Impulse Transmission
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Action Potentials
• Occur once a threshold of depolarization is reached– -50 to –55 mV
• All or none response (not graded)– Magnitude of action potential is independent of
strength of depolarizing stimuli
• Hyperpolarization makes them less likely
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Mem
bra
ne
po
ten
tial
(m
V)
2. Rising Phase
Stimulus causes above threshold voltage
+50
0
1 2 3
–70
Time (ms)1. Resting Phase
Equilibrium between diffusion of K+ outof cell and voltage pulling K+ into cell
Voltage-gatedpotassium channel
Potassiumchannel
Voltage-gatedsodium channel
Potassium channelgate closes
Sodium channelactivation gate closes.
Inactivation gate opens.
Sodium channelactivation gate opens
3. Top curve
Maximum voltage reached
Potassiumgate opens
4. Falling Phase
Potassiumgate open
Undershoot occurs as excess potassiumdiffuses out before potassium channel closes
Equilibriumrestored
Na+ channelinactivation gate
closes
K+
Na+
Na+ channelinactivation gate
closed
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Refractory Period
• During undershoot the membrane is less likely to depolarize
• Keeps the action potential moving in one direction
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Propagation of Action Potential
• Action potential are very localized events
• DO NOT travel down membrane
• Are generated anew in a sequence along the neuron
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Cellmembrane
Cytoplasm
restingrepolarizeddepolarized
+ + + + + + + + + – – – – – – – – –
+ + + + + + + + + – – – – – – – – –
– – + + + + + + + + + – – – – – – –
+ + + + + + + – – – – – – – – – + +
+ + – – + + + + + – – + + – – – – –
+ + + + + – – + + – – – – – + + – –
+ + + + – – – + + – – – – + + + – –
+ + – – – + + + + – – + + + – – – –
+ + + + + + + – –– – – – – – – + +
– – + + + + + + + + + – – – – – – –
Na+
K+
Na+
K+
Na+
K+
Na+
K+
K+
K+
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Saltatory Conduction
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Transfer of Nerve Impulse to Next Cell
• Synapse– the gap between the synaptic terminals of an
axon and a target cell
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Transfer of Nerve Impulse to Next Cell
• Electrical synapses– Gap junctions allow ion currents to continue
• Chemical synapses– More common– Electrical impulses must be changed to a
chemical signal that crosses the synapse
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Synapses
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Neurotransmitters
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Effects of Cocaine
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Receptor proteinDrug
moleculeSynapse
1. Reuptake of neuro- transmitter by transporter at a normal synapse.
2. Drug molecules block transporter and cause overstimulation of the postsynaptic membrane.
3. Neuron adjusts to overstimulation by decreasing the number of receptors.
4. Decreased number of receptors make the synapse less sensitive when the drug is removed.
Neurotransmitter
Transporter protein
Transporterprotein
Dopamine
CocaineReceptorprotein
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Integration of multiple synaptic inputs
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Summation of postsynaptic potentials
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Human
CerebrumCerebellum
Spinal cord
Cervicalnerves
Thoracicnerves
Lumbarnerves
Femoralnerve
SciaticnerveTibialnerve
Sacralnerves
Nervenet
Nerve cordsAssociativeneurons
BrainGiant axon
Mollusk
Echinoderm
Central nervoussystem
Peripheralnerves
Brain
Ventralnerve cords
Radialnerve
Nerveribs
Cnidarian
Flatworm
Earthworm
Arthropod
Diversity of Nervous Systems
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PN
SC
NS
Brain and Spinal Cord
Sympathetic nervoussystem
"fight or flight"
Parasympathetic nervoussystem
"rest and repose"
Somatic nervoussystem
(voluntary)
Sensory neuronsregistering external
stimuli
Autonomic nervoussystem
(involuntary)
Sensory Pathways Motor Pathways
central nervous system (CNS)peripheral nervous system (PNS)
Sensory neuronsregistering external
stimuli
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Constrict
Secrete saliva
Dilate
Stop secretion
Dilate bronchioles
Speed up heartbeat
Increase secretion
Empty colon
Increase motility
Empty bladder
Slow down heartbeat
Constrict bronchioles
Sympatheticganglionchain
StomachSecrete adrenaline
Decrease secretion
Decrease motility
Retain colon contentsDelay emptying
Adrenal gland
Bladder
Small intestine
Large intestine
Spinal cord
ParasympatheticSympathetic
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Vertebrate Central Nervous System
• Spinal Cord– Receives info from skin & muscles– Sends out motor commands for movement &
response
• Brain– More complex integration– Homeostasis, perception, movement, emotion,
learning
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Vertebrate Central Nervous System
• White matter– Internal part of brain & external part of spinal
cord– Myelinated axons
• Gray matter– Cell bodies of neurons
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Figure 48.16x Spinal cord
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Vertebrate Central Nervous System
• Cerebrospinal Fluid– Fills central canal of spinal cord and ventricles
of brain– Shock absorption
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Functions of Spinal Cord
• Carrying information to and from the brain
• Integration of simple responses– Reflexes
• Unconscious programmed response to stimuli
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Quadricepsmuscle
(effector)
Spinal cord
Dorsal rootganglion
Graymatter
Whitematter
Monosynapticsynapse
Sensoryneuro
Nerve fiberStretch receptor(muscle spindle)
Skeletalmuscle
Stimulus
Response
Motor neuron
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The knee-jerk reflex
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Evolution of Vertebrate Brain
• Evolved from a set of three bulges at the anterior end of spinal cord– Forebrain (cerebrum)– Midbrain (optic lobe)– Hindbrain (cerebellum & medulla oblongata)
• Regions have been further subdivided structurally and functionally
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Vertebrate BrainsOlfactory
bulbCerebrumThalamusOptic
tectumCerebellumSpinalcord
Medullaoblongata
PituitaryHypothalamus
Optic chiasm
Forebrain(Prosencephalon)
Midbrain(Mesencephalon)
Hindbrain(Rhombencephalon)
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Vertebrate BrainsThe relative sizes of different brain regions have
changed as vertebrates evolved
-Forebrain became the dominant feature