Transcript Cells of the Nervous System

Types of Neurons
The Neuron
The Cell Membrane
Inside the Neuron
Myelination
Schwann
Oligodendrocyte
Astrocytes
Synaptic Transmission and
Cellular Communication
Microelectrodes
Holder
Used to record the membrane potential
•Glass Pipettes
•~1 mm tips
•Pulled using heat
•Filled with Internal
solution.
•Connected via silver
wire to an amplifier.
Cell Activity
A Chemical Process
Recorded Electrically
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Electrical activity secondary to chemical
events
Basic Unit
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Volt – a potential difference between
charges in 2 different places in space
Thus, no single thing has “voltage”

e.g., battery voltage is determined by the potential
difference between 2 terminals
Basic Concepts
Ions – charged particles
Anions – Negatively charged particles
(chloride: Cl-)
Cations – Positively charged particles (sodium:
Na+; potassium: K+)
Electrostatic Pressure: attraction and repulsion
between ions
Basic Ion Concentrations
High Sodium and Chloride outside
the cell.
High Potassium inside the cell.
Ion Distributions
•Na/K pump maintains
Na and K distributions.
Resting Potential of a Neuron = -70 mV
• The internal environment of the cell is negatively charged
in relation to the outside of the cell
Four factors interact to maintain the resting potential
Random motion
Electrostatic pressure
Properties of the cell membrane
Sodium potassium pump
Selective Permeability of
Membranes
Some ions permitted to cross more easily
than others
Neuronal membranes contain ion
channels
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Protein tubes that span the membrane
Some stay open all the time (nongated)
Some open on the occasion of an action
potential, causing a change in the permeability
of the membrane (gated)
Ion Channels
Recognize and select among specific ions
The distribution of ionic species across
the membrane depends on the particular
distribution of ion channels in the cell
membrane.
EPSPs and IPSPs
EPSPs and IPSPs are graded responses
That is, they are proportional to the intensity of the signal that elicits them
Depolarizing
Hyperpolarizing
Decremental Conduction
Faster (don’t have energy expense of
activating voltage gated channels), but lose
signal strength over distance.
Action Potential
When cells are sufficiently depolarized, voltage-gated channels open up and sodium
rushes in, depolarizing the cell to about +50 mV. The point at which the channels open
up is called THRESHOLD and represents an all or none event.
Spatial Summation
Spatially separate
synapses can
exhibit spatial
summation.
Temporal Summation
Activity from
one synapse
can exhibit
temporal
summation.
Components of an Action Potential
Once triggered, an action potential occurs and can’t be stopped. For 1-2 ms
after an action potential, it is impossible to elicit a 2nd A.P. This is called the
absolute refractory period. The absolute refractory period is followed by the
relative refractory period, where it is only possible to elicit an action potential
by applying higher than normal stimulation. The neuron is slightly
hyperpolarized during the relative refractory period.
AP’s are non-decremental
Ap’s travel at about 60 m/s
AP’s travel more slowly than
Post-Synaptic Potentials
Action Potential: Sodium Ion Movement
Saltatory Conduction
Fast (signal is carried passively between nodes)
Reliable (signal is regenerated at each node)
EM of Chemical synapse
mitochondria
Figure 5.3, Bear, 2001
Active Zone
Anatomy of a Synapse
Figure 4-18b, Sherwood, 2001
Anatomy of a Synapse
Exocytosis: Transmitter Release
Synaptic Transmission (simplified version)
When an action potential reaches the terminal button,
synaptic vesicles release neurotransmitter (NT) into
the synaptic cleft
NT diffuses across the cleft
At least three possible scenarios after this:
-NT molecules do not attach to a postsynaptic
receptor
-NT released in an area with no immediate
receptors
-NT binds to a receptor site
The latter scenario leads to change in the ionic
permeability of the postsynaptic membrane
Excitatory postsynaptic potential (EPSP)
Inhibitory postsynaptic potential (IPSP)
Summation of EPSPs and IPSPs is the main principle
of interneuronal communication
Ion Channels
Ionotropic
Metabotropic
Neurotransmitter Deactivation
3 main processes
Diffusion: neurotransmitter diffuses away from synapse,
reduces amount available for binding, adequate for cases
where precise timing not critical, diffusion most often
involved in inactivation of peptide neuromodulators.
Inactivation by Enzymatic degradation: enzyme
degrades neurotransmitter directly. Most common is
acetylcholinesterase that degrades acetylcholine; also
monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT) that degrade the monoamines
Reuptake: Most common. Neurotransmitter is taken back
up into the presynaptic terminal after being released.
7 Steps in Neurotransmission
Classical criterion for neurotransmitter
Must be synthesized in the neuron
When an action potential occurs it must be
released in sufficient quantity to produce an
effect on the post-synapatic cell
Should be able to experimentally duplicate the
action on the post-synapatic cell
Some mechanism exists to end the interaction
between the chemical and the post-synaptic cell
Hormones
Classical definition: Substances that are released from
the tissue in which they are synthesized and then travel
via blood to other organs whose activities they influence.
Actions of hormones tend to be slower and much longer
lasting than actions of NTs
Neurohormones are also called neuroactive peptides
and are synthesized in hypothalamus and transported to
pituitary gland that releases them: e.g. oxytocin
(regulates smooth muscle contraction), vasopressin
(regulates water balance)
We now know that neurohormones can be released at
the synapse alone or in conjunction with NTs and can
produce NT-like effects (i.e. rapid communication)
Neurotransmitters are generally classified
according to molecular size
Small molecule neurotransmitters
• Amino acids (glutamate, GABA, aspartate, glycine)
• Monoamines (dopamine, norepinephrine, epinephrine, serotonin)
• Soluble gases (nitric acid, carbon monoxide)
• Acetylcholine (NT at neuromuscular synapses)
Large molecule neurotransmitters
• Peptides
Classes of Neurotransmitters
(4)
(4)
(2)
Not released, diffuse through cell walls
(1)
I
(50+)
I
Distribution of Neurotransmitters
•
Acetylcholine (ACh)
In the CNS, involved in motor function, attention,
learning and memory
•
Serotonin (5-HT)
Plays a major role in the sleep-wake cycle
Low levels associated with severe depression
•
Norepinephrine (NE):
Involved in mood, memory, motor behavior, depression,
and anxiety
•
Dopamine (DA)
Crucial to our ability to move efficiently and effectively,
implicated in motivation, mood, perception
•
Amino Acids
• GABA: Most common inhibitory neurotransmitter
• Glutamate: Most widespread excitatory neurotransmitter
• Glycine: inhibitory NT important in spinal cord and brain
stem
Making Catecholamines in 4 easy steps
Dopamine
Norepinephrine
a.k.a Noradrenaline
Epinephrine
a.k.a Adrenaline
Phenylethanolamine N-methyltransferase
Making serotonin
Tryptophan
Enzyme tryptophan hydroxylase makes
5-hydroxytryptophan
Enzyme 5HT decarboxylase makes
5-hydroxytryptamine (5-HT)
Monoamine oxidase breaks down 5-HT
GABA
Found almost exclusively in brain
Glutamic acid
Enzyme glutamic acid decarboxylase
makes
Gamma amino butyric acid (GABA)
Neuropeptides
Large molecule NTs
Can be released into circulation and act at a distant site,
or can be confined to synapse
Synthesized at soma and transported to release sites
(NTs are synthesized in synaptic terminal)
Have slow postsynaptic effects and actions are
terminated b y diffusion or extracellular degradation
Do not require point to point synaptic connections to
produce actions
Co-released with classical NTs.
Egs: Substance P, Neurotensin, thyrotropin releasing
hormone (TRH), oxytocin, vasopressin, met-enkephalin,
prolactin