Transcript Neuro 1 - Somerset Academy

Neuro I
Or:
What makes me do that Voodoo that I
Do so Well!
Neurons and More Neurons
The root of it all…...
The Brain

Responsible for all
behavior
 Sensation
– Sensory (Afferent)
Neurons

Movement
– Motor (Efferent)
Neurons

Integration of info
– Interneurons
The Brain

Donald Hebb
 Proposed that the
brain is not merely a
mass of tissue
– but a highly integrated
series of structures that
perform specific
functions

cell assemblies
Cell Assemblies

Groups of connected
neurons that perform
certain functions
Cell Assemblies: The Neuron

A specialized cell that receives, processes
and/or transmits information
– Modulatory Characteristics
Modulatory Characteristics

Depolarize

– Make a neighbor more
likely to be active

– Make it less receptive
to a signal (NT)
Hyperpolarize
– Make a neighbor less
likely to fire
Change the dynamics
of a receptor

Affect synthesis,
movement and release
of NT to another
neuron

Moduation
Neuronal Structure
Spinal Motor Neuron
Variations on a Theme
Basket Cell
(Cerebellum)
Golgi Type II
(Cortex)
Sensory Neurons
Bipolar
(Vision)
Unipolar
(Pain/Touch)
Neuronal Structure
Spinal Motor Neuron
Soma

Contains the nucleus and machinery
– Life Processes
Neuronal Structure: Dendrites
Spinal Motor Neuron
Dendrites (Tree)

Highly Aborized
 Receive “messages” from other neurons
– Some have dendritic “spines”
 Input sites
– Separated from neighbor by a synapse (space)

Caveat: They can transmit signals as well
Dendritic Spines
Neuronal Structure: Axon
Spinal Motor Neuron
The Axon

Tube-like structure
– Micrometers to meters
– Covered by the
“Myelin Sheath”
Axon
The Axon

Tube-like structure
– Carries a signal from
the soma to the
terminal buttons

Axon
Signal = Action
Potential (AP)
(electrical/chemical
event)
Myelin Sheath
Myelin Sheath

Surrounds many (but not all) axons
 Formed by Oligodendrocytes (CNS) and
Schwann Cells (PNS)
 There are gaps between adjacent cells
– Several micrometers
– Called “Nodes of Ranvier”
– Internode region
Neuronal Structure: Terminal
Buttons
Spinal Motor Neuron
Terminal Buttons

Found at the end of the axon
– When an AP reaches the terminal
 Release chemical into the synapse
– Neurotransmitter (NT)
Neurotransmitters

This Info can be excitatory or inhibitory to
a neighboring neuron
Cell Assemblies
Signaling in the Neuron
Electrical Potentials

Most work done with the Giant Squid Axon
– Neurons work by electrical and chemical
activity
Electrical Potential

Inside is more
negative than the
outside


-70 mv
Membrane resting
potential
Ions

Molecules that have given up or taken on an
electron
– Gives the molecule a charge
– Some move more readily across the membrane
then others

Dependent on circumstances
Ion Distribution
Ion Concentrations
ION
INSIDE
OUTSIDE
RATIO
K+
400
10
40:1
Na++
50
460
1:9
Cl-
40
540
1:13
A-
400
------
------
Ca++
0.4
10
1:25
The number is not as important as the ratio
Ion Concentration

More positive charge on the outside then on
the inside of the neuron
The Active Neuron
The Action Potential (AP)
Its hard to know what’s going on
 Difficult to isolate ions

– Everything is occurring at once
– The charge is changing
 Impacts ion movement
Reaching Threshold

Excitatory Input (Depolarization)
– Causes the influx of positive ions (Na+) into
the cell by opening Na+ channels

Voltage gated channels
– Great variety in threshold level
– If enough positive charge comes in
 The threshold is reached
– More NA+ channels open
– Making the cell more positive
– All or none
Caveat

Takes many excitatory inputs to reach
thresholds
– Temporal summation
– Spatial summation
Repolarization

After time
– The Na+ channels automatically close
– K+ channels begin to open
 K+ leaves the cell carrying with it the positive
charge
– Repolarization
Overshoot

Too much K+ leaves causing the cell to be
hyperpolarized
Back to Resting State

The Na+/K+ pump restores the normal ion
concentrations and distributions
Axonal Conduction

This measurement takes place at one point
on the giant squid axon
– The signal must travel distances to reach its
destination
Signal Decrement
Weak depolarization = loss of signal
AP Propagation
Strong depolarization = strong signal
Neuronal Structure
AXON HILLOCK
Spinal Motor Neuron
Axon Hillock

Has a high concentration of low threshold
Na+ Channels
– Very sensitive to changes in ion movement
– Activation results in a autocataclysmic response
 All Or none
Neuronal Structure
AXON HILLOCK
Spinal Motor Neuron
Myelin Sheath

Act as an insulator
– Prevents things from moving in and out of the
cell

Including Ions
Oligodendrocytes
Nodes of Ranvier
Nodes of Ranvier

Gaps in the sheath
 High concentration of Na+ channels
– Reenergizes the signal so it can reach the axon
terminal
Neuron: Axon Terminal
Axon Terminal: Synaptic
Vesicles
Synaptic Transmission
Cell Assemblies
Synaptic Transmission:
Caveat
In conclusion:

Neurons are good.
 They excite or inhibit.
 They produce 1 neurotransmitter (in
mammals).
 Transmission is essential.
 Neuromodulators can change everything
(more on that later)