PL3020 Neurophysiology

Gordon ReidDept of Physiology

Western Gateway Building, UCCE-mail g.reid@ucc.ie

Purves, Augustine et al“Neuroscience”

5th editionSinauer 2011

Textbooks

Textbooks

Nicholls et al“From Neuron to Brain”

4th editionSinauer 2001

Kandel et al“Principles of Neural

Science”4th edition

McGraw-Hill 2000

Textbooks

Online resources

•Teaching website:http://epu.ucc.ie/greid/PL3020/•Bookmark this link – everything will be available there on one page

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The diversity of neuroscience

Levels of organisation:•Molecular neuroscience•Cellular neurophysiology•Developmental neuroscience, plasticity and repair•Systems,behavioural and cognitive neuroscience•Neurobiology of disease

…or looked at in terms of questions:•What are the signals?•What are the connections?•How do the signals relate to behaviour/perception?•How are signals and connections modified by experience?

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This course

Levels of organisation:•Molecular neuroscience•Cellular neurophysiology•Developmental neuroscience, plasticity and repair•Systems,behavioural and cognitive neuroscience•Neurobiology of disease

…or looked at in terms of questions:•What are the signals?•What are the connections?•How do the signals relate to behaviour/perception?•How are signals and connections modified by experience?

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The neuron(e): development of our understanding

•Schleiden, Schwann: Cells as the fundamental unit of life (1830s)•Easy to see in some tissues•But not initially accepted for the nervous system: cells couldn’t be seen microscopically

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The neurone: development of our understanding

•Example: a section of liver tissue•Cells are obvious

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The neurone: development of our understanding

•Contrast with a section of spinal cord•Appears as a continuous mesh; no obvious discrete cells

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The neurone: development of our understanding

•Golgi staining method (Camillo Golgi): - ~1% of neurones stained - stains them completely•Used by Santiago Ramon y Cajal (early 1900s) to explore the nervous system•Made clear the existence of neurones•Cajal and Golgi shared the 1906 Nobel Prize for this work

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The neurone: Golgi stained sections

Hippocampal pyramidal neurones

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The neurone: Golgi stained sections

Hippocampal pyramidal neurones

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Many neuronal morphologies

Golgi stain revealed all sorts of shapes of neurone…do they have anything in common?

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Some guiding (simplifying) principles

•Information flows only one way: neurones have input and output zones

•Rapid neuronal signalling is electrical

•A neurone’s function is defined by its connections

•Electrical signals are the same in all neurones regardless of function

(except for sensory receptors)

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Unity within diversity: 4 zones

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Input zone

•In most cases, the sum of all synaptic inputs

•Exception: sensory neurones, where the input zone transduces the stimulus into an electrical signal 17

The sum of all synaptic inputs…what it really looks like

•Each neurone can have from 10,000 - 100,000 synaptic inputs

•In some regions, essentially the whole neurone surface is covered

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Integrative (“trigger”) zone

•This is the point where the neurone “decides” whether to fire an action potential in response to its input(s)

•In many neurones this zone is the “axon hillock”; in sensory neurones the first node of Ranvier

•They have this in common: a high density of Na+ channels - necessary for action potentials to be initiated 19

Conductile zone

•The axon of the neurone

•Function: conduction of an action potential unaltered from integrative zone to output zone

•May be very short - almost nonexistent

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Why use action

potentials?

•Without action potentials, voltage falls off quickly with distance

•This is OK in a short interneurone

•Over longer distances we need a digital code

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Squid axon: the first recorded action potential (1939)

Turtle retinal ganglion cell

•The signal is the same, regardless of what it signifies

•“All or nothing”: if threshold is exceeded, AP is initiated

Action potentials as a digital code

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•How do you signal intensity with an all-or-nothing signal?

•The key is to code frequency

Action potentials as a digital code

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The axon doesn’t think...

•The axon delivers the sum of the neurone’s inputs to the output zone, as a frequency-coded message

•George Bishop, axonologist:“The axon doesn’t think, it only ax”- its job is just to faithfully transmit what is given to it

•The axon was the first part of the neurone to begin to be understood: the first part we will look at too

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Output zone

•Output always involves secretion of neurotransmitter

•Usually at a synapse onto neurone or muscle cell

•May secrete direct into circulation

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Bringing it all together

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A simple neural circuit

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What the connections really look like

Simple textbook diagram

Reality:•Thousands of connections•Divergence and convergence

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Divergence and convergence

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Monosynaptic stretch reflex

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Reading for today’s lecture:

•Purves et al chapter 1•Nicholls et al chapter 1•Kandel et al chapter 2

Next lecture: How the resting potential is generated

Reading for next lecture:•Purves et al chapter 2•Nicholls et al chapter 5•Kandel et al chapter 7