Structure and function of the nervous system What is the brain, and - - PDF document

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Structure and function of the nervous system What is the brain, and - - PDF document

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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

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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 Inside of body Light, sound, smell, pressure, temperature, etc. Sensory neuron Motor neuron muscle ACTION! Central Nervous System

Higher level processing (integration, coordination, memory formation, physiological changes, etc.)

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

  • ther neurons and their

fibers

  • Cell body

– contains nucleus and

  • ther 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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Synaptic transmission

  • 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

Action potential Action potential Presynaptic terminal Postsynaptic terminal Synapse

Neurobiology BIO 475/675

Synaptic transmission

  • 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 Action potential Action potential Presynaptic terminal Postsynaptic terminal Synapse

Electrical Chemical Electrical

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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
  • 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” Fig 1.8

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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

  • 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

Action potential Action potential Presynaptic terminal Postsynaptic terminal Synapse

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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
  • rganized into larger

functional circuits?

  • Types of neurons:

– sensory, motor, interneurons

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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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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