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Neurobiology BIO 475/675
Structure and function of the
nervous system
What is the brain, and how can we
figure out how it works?
Neurobiology BIO 475/675
A preview of neurobiology
• Chapter 1
• Case study: the patellar reflex
– the reflex
– anatomy
– neural signaling
• the action potential
– synaptic transmission
• synapse
• neurotransmitters: release and reception
• Chapter 2
• Organization and evolution of nervous systems
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Neurobiology BIO 475/675
Nervous system: basic function
Outside
Light, sound,
smell, pressure,
temperature, etc.
Inside of body
Sensory
neuron
Central
Nervous
System
Motor
neuron
ACTION!
Higher level processing
(integration, coordination,
memory formation,
physiological changes, etc.)
muscle
Neurobiology BIO 475/675
The patellar reflex
Fig 1.1
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Neurobiology BIO 475/675
Anatomy of patellar reflex
• Reflex loop
–
–
–
–
–
sensory terminal in muscle activated by stretch
Afferent signal: sent along sensory fiber into spinal cord
connection to motor neuron
Efferent signal: sent out of spinal cord along motor nerve fiber
nerve activates contraction of muscle
Fig 1.2
Neurobiology BIO 475/675
Two-neuron reflex
• Sensory neuron
– stretch-activated fiber
terminal in muscle
– important for sensing
contraction state of
muscles, and for posture
– cell body in dorsal root
ganglion
– terminal branches contact
motor neurons
• Motor neuron
– cell body in spinal cord
– extends fiber out to muscle
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Neurobiology BIO 475/675
A neuron
• A single motor neuron
– embedded in matrix of
other neurons and their
fibers
• Cell body
– contains nucleus and
other cell organelles
• Two kinds of neurites
– dendrites: input, highly
branched
– single axon: output
– in case of motor neuron,
axon extends out to
contact muscle
Neurobiology BIO 475/675
Signaling by neurons
• Signal within a neuron: the action potential
– electrical signal, spread by ionic changes in cell
– efficiently transmitted along very long axons
• Signal between neurons: the synapse
– synapse is point of contact (tiny gap) between cells
– specialized to promote and regulate transmission of
signal from one cell to another
– chemical signal is passed: neurotransmitter
– neuron-muscle synapse = neuromuscular junction
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Neurobiology BIO 475/675
Measuring the
action potential
• Insert voltage probe into
sensory nerve fiber
– microelectrode is a hollow
glass tube
– filled with saline solution
– hooked up to voltage meter
• The membrane potential
– common to all cells
– define: resting membrane
potential
• Stimulation causes Action
Potential:
– rapid depolarization, then
quickly resets
Neurobiology BIO 475/675
Behavior of the action potential
• Transmits signal along the
nerve fiber
• Neuron fires
• First fires at terminal, then
signal is propagated along
fiber
– evidence for propagation
• Speed: 50+ meters per sec
– So, what time from toe to spinal
cord, and back to toe?
• What questions does this
raise?
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Neurobiology BIO 475/675
Synaptic transmission
Action
potential
Action
potential
Postsynaptic
terminal
Presynaptic
terminal
Synapse
• Presynaptic terminal: vesicles store neurotransmitter
• Action potential depolarizes presynaptic terminal
– triggers fusion of synaptic vesicles to membrane; release of neurotransmitter
• Neurotransmitter binds to receptors on postsynaptic membrane
• Receptors depolarize postsynaptic terminal
• Action potential carries signal to new target cells
Neurobiology BIO 475/675
Synaptic transmission
Action
potential
Action
potential
Postsynaptic
terminal
Presynaptic
terminal
Synapse
Electrical
Chemical
Electrical
• Important point of regulation
– Electrical signal is all-or-none
– But chemical signal can be modulated
• Pharmacology: drugs modify synaptic transmission
– increase or decrease transmission of signals between neurons
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Neurobiology BIO 475/675
Acetylcholine, ACh
• Several lines of evidence: motor neurons use
acetylcholine as a neurotransmitter
• Synthetic enzyme: choline acetyltransferase
• Degradative enzyme: acetylcholine esterase
Neurobiology BIO 475/675
Evidence for neurotransmitter
• How can we determine if a
neurotransmitter is used in a particular
neural pathway?
• Identify presence of synthetic enzyme
Fig 1.8
• Technique: antibody labeling
– immunocytochemistry
– immunofluorescence
• Isolate antibody molecule that
specifically binds to choline
acetyltransferase protein
– cut sections of brain
– wash with solution of antibody
– visualize with color reaction: Fig 1.8
• Neurons of a type are often clustered
together
– termed “nucleus”
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Neurobiology BIO 475/675
Neurotransmitter degradation
• Neurotransmitter signal needs to be short, so…
• Neurotransmitters need to be broken down quickly
– several different mechanisms for this
• Degradative enzyme: acetylcholine esterase
– AChE, or Achase
• AChE is present in neuromuscular junction
Fluorescent label of
motor axon terminal
Pattern of AChE
enzyme activity
Neurobiology BIO 475/675
Neurotransmitter degradation
• Neurotransmitters need to be broken down
quickly
• Genetic defect leads to paralysis in humans
• Congenital endplate acetylcholine esterase
deficiency
– failure to produce functional AChE at neuromuscular
junction
– ACh released, but not broken down
– So, one signal from motor neurons, but multiple
muscle contractions
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Neurobiology BIO 475/675
What senses neurotransmitter?
• Acetylcholine receptor:
– AChR, Fig 1.10
• Membrane structure:
– ions cannot cross membrane
• ACh receptor: AChR
– ligand-gated ion channel
– ACh diffuses to postsynaptic membrane
– ACh binds, channel opens, lets positive ions cross
membrane
– depolarizes postsynaptic cell membrane
• Triggers action potential in muscle fiber,
leads to muscle contraction
• AChR present in synapse:
– fluorescent-tagged molecule that binds to AChR
– alpha bungarotoxin, from snake venom
Neurobiology BIO 475/675
Synaptic transmission
Action
potential
Action
potential
Postsynaptic
terminal
Presynaptic
terminal
Synapse
• Presynaptic terminal: vesicles store neurotransmitter
• Action potential depolarizes presynaptic terminal
– triggers fusion of synaptic vesicles to membrane; release of neurotransmitter
• Neurotransmitter binds to receptors on postsynaptic membrane
• Receptors depolarize postsynaptic terminal
• Action potential carries signal to new target cells
9
Neurobiology BIO 475/675
Anatomy of patellar reflex
• Fig 1.2
Neurobiology BIO 475/675
Systems
neurobiology
• Neurons are part of larger
neural circuits or systems
• Example: patellar reflex can
be consciously repressed
• How are groups of neurons
organized into larger
functional circuits?
• Types of neurons:
– sensory, motor, interneurons
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Neurobiology BIO 475/675
Organization of nervous systems
• Evolution of nervous systems
– electrical signaling important for
single cells
– nerve nets
– bilateral symmetry
• central nervous system
evolved
– central vs. peripheral
• neurons became more
specialized
• cephalization
– head nervous system
bigger, more complex,
more interneurons
Neurobiology BIO 475/675
Hierarchical organization of NS
• Head
– primary responsibility to initiate action
– integration of sensory information
– coordination of motor
• Vertebrates
– have specialized brain that regulates spinal cord
• chicken
• Brain subdivisions
–
–
–
–
forebrain
midbrain
hindbrain
spinal cord
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Neurobiology BIO 475/675
Brain subdivisions
• Various brain regions become specialized in
different vertebrates
• Rodents: nocturnal mammals
– relatively large olfactory bulbs
– primary reception and processing of olfactory signals
• Humans
– Forebrain, specifically cerebral cortex, is hugely
expanded
– complex folds allow very large surface area
– increased number of neurons
Neurobiology BIO 475/675
Brain subdivisions
• Make table of main subdivisions and functions
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Neurobiology BIO 475/675
Overall human CNS anatomy
Fig 2.5
Neurobiology BIO 475/675
A preview of neurobiology
• Chapter 1
• Case study: the patellar reflex
– the reflex
– anatomy
– neural signaling
• the action potential
– synaptic transmission
• synapse
• neurotransmitters: release and reception
• Chapter 2
• Organization and evolution of nervous systems
13