The Nervous System

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The Nervous System

• Chapter 44

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Nervous System Organization

• All animals must be able to respond to
  environmental stimuli
• -Sensory receptors Detect stimulus
• -Motor effectors Respond to it
• -The nervous system links the two
• -Consists of neurons and supporting cells

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Types of Neurons

• Vertebrates have
• 3 types of neurons
• -Sensory neurons
• to CNS(afferent neurons)
• -Motor neurons
• (efferent neurons)
• to effectors
• (muscles and glands)
• -Interneurons
• (association neurons)
• provide associative
• functions

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A Neuron

• Neurons have the same basic structure
• -Cell body Enlarged part containing nucleus
• -Dendrites Short, cytoplasmic extensions that
  receive stimuli
• -Axon Single, long extension that conducts
  impulses away from cell body

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A Neurons Charge

• The inside of the cell is more negatively charged
  than the outside because of
• 1. Sodium-potassium pump Brings two K into
  cell for every three Na it pumps out
• 2. Ion leakage channels Allow more K to
  diffuse out than Na to diffuse in

When a neuron is not being stimulated, it
maintains a resting Potential _at_ -70mv
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Sodium-potassium pump Helps create a Neurons
charge
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Synapses

• Synapses are intercellular junctions
• Two basic types electrical and chemical
• Electrical synapses involve direct cytoplasmic
  connections between the two cells formed by gap
  junctions
• -Relatively rare in vertebrates

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Synapses

• Chemical synapses have a synaptic cleft between
  the two cells

-End of presynaptic cell contains synaptic
vesicles packed with neurotransmitters
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Neurons Begin to Communicate _at_ Synapses

• Synapses are intercellular junctions
• -Presynaptic cell transmits action potential
• -Postsynaptic cell receives it

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More Facts About the Synapse

• Action potential triggers influx of Ca2
• -Synaptic vesicles fuse with cell membrane
• -Neurotransmitter is released by exocytosis
• -Diffuses to other side of cleft and binds to
  chemical- or ligand-gated receptor proteins
• -Neurotransmitter action is terminated by
  enzymatic cleavage or cellular uptake

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Neurotransmitters

• Acetylcholine (ACh)
• -Crosses the synapse between a motor neuron and
  a muscle fiber
• -Neuromuscular junction

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Neurotransmitters

• Acetylcholine (ACh)
• -Binds to ligand-gated receptor in the
  postsynaptic membrane
• -Produces a depolarization called an excitatory
  postsynaptic potential (EPSP)
• -Stimulates muscle contraction
• -Acetylcholinesterase (AChE) degrades ACh
• -Causes muscle relaxation

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Excitatory and Inhibitory Neurotransmitters

• Amino acids
• -Glutamate is the major excitatory
  neurotransmitter in the vertebrate CNS
• -Glycine and GABA (g-aminobutyric acid) are
  inhibitory neurotransmitters
• -Open ligand-gated channels for Cl
• -Produce a hyperpolarization called an
  inhibitory postsynaptic potential (IPSP)

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Neurotransmitters
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Nerve Impulse Transmission

• Chemically-gated or ligand-gated channels
• -Ligands are hormones or neurotransmitters

-Induce opening and cause changes in cell
membrane permeability
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Nerve Impulse Transmission

• Depolarization makes the membrane potential more
  positive, whereas a hyperpolarization makes it
  more negative
• -These small changes result in graded potentials
• -Can reinforce or negate each other
• Summation is the ability of graded potentials to
  combine

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Synaptic Integration
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Synaptic Integration

• Integration of EPSPs (depolarization) and ISPSs
  (hyperpolarization) occurs on the neuronal cell
  body
• -Small EPSPs add together to bring the membrane
  potential closer to the threshold
• -IPSPs subtract from the depolarizing effect of
  EPSPs
• -And will therefore deter the membrane
  potential from reaching threshold

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Synaptic Integration

• There are two ways that the membrane can reach
  the threshold voltage
• -Spatial summation
• -Many different dendrites produce EPSPs
• -Temporal summation
• -One dendrite produces repeated EPSPs

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Nerve Impulse Transmission
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Nerve Impulse Transmission

• Action potentials result when depolarization
  reaches the threshold potential about
• -55mV
• Remember resting potential is -70mV so Positive
  charge has to come in to get it to be -55mV!!

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Nerve Impulse Transmission

• When the threshold voltage is reached, sodium
  channels open rapidly
• -Transient influx of Na causes the membrane to
  depolarize
• In contrast, potassium channel opens slowly
• -Efflux of K repolarizes the membrane

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The intensity of a stimulus is coded by the
frequency, not amplitude, of action potentials
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Propagation of an Action Potential
-The action potential is caused by
voltage-gated ion channels -Two different
channels are used -Voltage-gated Na channels
-Voltage-gated K channels
-Positive charges due to influx of Na can
depolarize the adjacent region to threshold
-And so the next region produces its own action
potential -Meanwhile, the previous region
repolarizes back to the resting membrane potential
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Nervous System Organization

• Neurons are supported both structurally and
  functionally by cells called neuroglia
• -Schwann cells (PNS) and oligodendrocytes (CNS)
  produce myelin sheaths surrounding axons
• -In the CNS, myelinated axons form white matter
• -In the CNS dendrites/cell bodies form gray
  matter
• -In the PNS, myelinated axons are bundled to form
  nerves

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Nerve Impulse Transmission

• Two ways to increase velocity of conduction
• 1. Axon has a large diameter
• -Less resistance to current flow
• -Found primarily in invertebrates
• 2. Axon is myelinated
• -Action potential is only produced at the
  nodes of Ranvier
• -Impulse jumps from node to node
• -Saltatory conduction

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Nerve Impulse Transmission
Action potential is only produced at the nodes
of Ranvier
Insulation allows for charge, accumulated at a
distance, to be detected by neighbor-Nachannels
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Drug Addiction

• Prolonged exposure to a stimulus may cause cells
  to lose the ability to respond to it
• -This process is called habituation
• -The cell decreases the number of receptors
  because there is an abundance of
  neurotransmitters

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Drug Addiction

• Cocaine affects neurons in the brains pleasure
  pathways (limbic system)
• -Binds dopamine transporters and prevents the
  reuptake of dopamine
• -Dopamine survives longer in the synapse and
  fires pleasure pathways more and more
• -Prolonged exposure triggers the limbic system
  neurons to reduce receptor numbers
• -The cocaine user is now addicted

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Drug Addiction

• Nicotine binds directly to a specific receptor on
  postsynaptic neurons of the brain
• -Brain adjusts to prolonged exposure by turning
  down the volume in two ways
• 1. Making fewer nicotine receptors
• 2. Altering the sensitivity to stimulation by
  neurotransmitters

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Nervous System Organization

• The CNS consists of the brain and spinal cord
• The Peripheral Nervous System (PNS) consists of
  sensory and motor neurons
• -Somatic NS stimulates skeletal muscles
• -Autonomic NS stimulates smooth and cardiac
  muscles, as well as glands
• -Sympathetic and parasympathetic NS
• -Counterbalance each other

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CNS
Brain and Spinal Cord
Motor Pathways
Sensory Pathways
Sensory neurons registering external stimuli
Sensory neurons registering external stimuli
PNS
Somatic nervous system (voluntary)
Autonomic nervous system (involuntary)
Sympathetic nervous system "fight or flight"
Parasympathetic nervous system "rest and repose"
central nervous system (CNS)
peripheral nervous system (PNS)
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Vertebrate Brains

• All vertebrate brains have three basic divisions
• -Hindbrain or rhombencephalon
• -Midbrain or mesencephalon
• -Forebrain or prosencephalon
• In fishes,
• -Hindbrain Largest portion
• -Midbrain Processes visual information
• -Forebrain Processes olfactory information

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Vertebrate Brains

• The relative sizes of different brain regions
  have changed as vertebrates evolved
• Forebrain became the dominant feature...Neuronal
  correlate to learning, association, emotions.

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Compare Brain Diagrams
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Vertebrate Brains

• Forebrain is composed of 2 elements
• -Diencephalon
• -Thalamus Integration and relay center
• -Hypothalamus Participates in basic drives
  emotions controls pituitary gland
• -Telencephalon (end brain)
• -Devoted largely to associative activity
• -ALSO Called the CEREBRUM in mammals

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Cerebral Cortex is the outer layer of the Cerebrum

• Cerebral cortex
• -Contains about 10 of all neurons in brain
• -Highly convoluted surface
• -Increases threefold the surface area of the
  human brain
• -Divided into three regions, each with a
  specific function

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Cerebral Cortex 3 Regions
Increase in brain size in mammals reflects the
great enlargement of the cerebrum

• Cerebral cortex

1Primary motor cortex Movement
control 2Primary somatosensory cortex Sensory
control 3Association cortex Higher mental
functions Basal ganglia -Aggregates of neuron
cell bodies -Form islands of grey matter within
the cerebrums white matter

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Sensory/Motor Homunculus
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Complex Functions of the Brain

• Memory
• -Appears dispersed across the brain
• -Short-term memory is stored in the form of
  transient neural excitations
• -Long-term memory appears to involve structural
  changes in neural connections

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Complex Functions of the Brain

• Alzheimer disease is a condition where memory and
  thought become dysfunctional
• -Two causes have been proposed
• 1. Nerve cells are killed from the outside in
• -External protein b-amyloid
• 2. Nerve cells are killed from the inside out
• -Internal proteins tau (t)

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Spinal Cord

• The spinal cord is a cable of neurons extending
  from the brain down through the backbone

-Enclosed and protected by the vertebral column
and the meninges -It serves as the bodys
information highway -Relays messages between
the body and the brain
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The Peripheral Nervous System

• The PNS consists of nerves and ganglia

-Nerves are bundles of axons bound by connective
tissue
-Ganglia are aggregates of neuron cell bodies
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The Peripheral Nervous System

• Sensory neurons
• -Axons enter the dorsal surface of the spinal
  cord and form dorsal root of spinal nerve
• -Cell bodies are grouped outside the spinal cord
  in dorsal root ganglia
• Motor neurons
• -Axons leave from the ventral surface and form
  ventral root of spinal nerve
• -Cell bodies are located in the spinal cord

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Cutaneous Spinal Reflex (in Somatic
Nervous System) Use SAME DAVE Acronym
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The Peripheral Nervous System

• PNS is divided into the
• Somatic System
• Autonomic System
• .

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Somatic System

• In GENERAL, The Somatic System is responsible for
  skeletal muscle voluntary movements reflexes.

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The Autonomic Nervous System

• Composed of the sympathetic and parasympathetic
  divisions, plus the medulla oblongata
• In both, efferent motor pathway has 2 neurons
• -Preganglionic neuron exits the CNS and
  synapses at an autonomic ganglion
• -Postganglionic neuron exits the ganglion and
  regulates visceral effectors
• -Smooth or cardiac muscle or glands

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The Autonomic Nervous System
Efferent Ganglia outside of spinal cord
General Model Seen In Both Sympathetic
Parasympathetic Division