Human Brain and Senses
Outline for today
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Levels of analysis
Basic structure of neurons
How neurons communicate
Basic structure of the nervous system
Levels of
analysis
Organism
Brain
Cell
Synapses
Membrane channels
Basic structure of neurons
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Dendrites: site of inputs
to neuron
„ Soma or cell body –
integration of inputs and
initiation of action
potentials
„ Axon: output pathway
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Neurons communicate with
electrical pulses (action
potentials or ‘spikes’) that travel
from the cell body down the axon.
Axon terminals: make
synapses with other
neurons by releasing
neurotransmitter
Neurons come
in a great
variety of
shapes and
sizes
Purves et al., 2001
Retinal
neurons
Cortico-spinal
neuron
How do they differ?
Dendrites and their synaptic inputs
show great variability
Cerebellar Purkinje cell
Cochlear nucleus
bushy cell
Example of aggregations of
neurons to form nuclei
Glial cells
Glial cells outnumber neurons by about 10:1!
They serve to provide support and nourishment to neurons
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Astrocytes: help to regulate extracellular ionic concentrations
Microglia: serve as garbage collectors to clean up dead tissue
Oligodendroglia: in the CNS to form myelin around axons for
electrical insulation which speeds conduction
Schwann cells: in the PNS to form myelin
How neurons communicate
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Ionic basis for resting membrane
potential
Action potentials
Synaptic transmission: chemical
Action potentials propagate
down their axon
Larger diameter axons
have less resistance to ion
flow
Speed of conduction is
faster in large diameter
axons
Saltatory conduction in myelinated
axons
Myelinated axons
conduct action
potentials up to
10X faster
Large myelinated
axons can conduct
up to 150 m/sec,
330 mph
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Action potential propagation animation
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Saltatory conduction animation
In
If K channels open:
K will follow
concentration
gradient
Inside negative
Out
In
Out
If Na channels
open:
Inside positive
Equil. potential when
elect. potential =
concentration grad.
EK = -58 mV
ENa = +58 mV
• At rest, K permeability is much higher than Na permeability
• Resting potential is near EK
Ionic basis of resting membrane potential
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The inside of neurons is about -60 mV relative
to the outside
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Cell membrane is semi-permeable
Ionic concentrations inside and outside are
different
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Sodium/potassium pump helps maintain this
imbalance
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[Na] is high outside, low inside
[K] is high inside, low outside
Much of the energy (70%) of the brain is expended here
At rest K permeability is about 40x higher than Na
Resting potential: when electrical potential
balances concentration gradient
Action potentials:
arise from changes in Na and K permeability
Out
In
Depolarization
At rest K
channels are
open
Na channels
open briefly to
produce surge
of inward Na
K channels open
again to
repolarize
Increase
Increase Na
inward Na
permeability
current
A.p. is regenerative
Properties of a.p.:
• ‘All-or-none’, not graded in
amplitude
• Membrane potential must
be depolarized above a
threshold of about -55 mV
Synaptic transmission at a chemical
synapse
Action potential in presynaptic
terminal triggers release of
neurotransmitters from synaptic
vesicles
Neurotransmitter diffuses
across synaptic cleft to bind
with receptors for transmitter
Activation of receptors opens
ion channels leading to
excitatory (EPSP) or inhibitory
post-synaptic potentials (IPSP)
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Synaptic transmission animation
Synaptic vesicles come in two flavors
Neurotransmitters
Small, clear
vesicles
(specific)
Dense core
Excitatory
Inhibitory
Acetylcholine (Ach)
Glycine
Glutamate
GABA
Catecholamines (epi-,
norepi-, dopamine)
vesicles
(modulatory) Serotonin
Histamine
Peptides (many!!)
Peptides
Neurotransmitters
Small, clear
vesicles
(specific)
Dense core
Excitatory
Inhibitory
Acetylcholine (Ach)
Glycine
Glutamate
GABA
Catecholamines (epi-,
norepi-, dopamine)
vesicles
(modulatory) Serotonin
Histamine
Peptides (many!!)
Peptides
Bear 5.13 and 5.14
Excitatory
depolarize
(glutamate)
Inhibitory
hyperpolarize
(GABA)
Bear et al., 2001
Synaptic vesicles
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Small, clear vesicles
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Associated with pre- (release sites) and
post-synaptic specializations
Fast-acting
Dense core vesicles
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Released into extracellular space
Slow, modulatory effect
Cell body integrates inputs
EPSPs can summate
spatially or temporally
Bear et al., 2001
Action potentials: neurons communicate
with other neurons by electrical pulses
• They are more or less identical
• The membrane potential must be depolarized above a
threshold of about -55 mV
• They are ‘all-or-none’, not graded in amplitude
• They are followed by an absolute and relative refractory
period
Divisions of the nervous system
Cytoarchitectonic maps of the
cerebral cortex
Korbinian Brodmann
(1868-1918)
Levels of
analysis
Organism
Brain
Cell
Synapses
Membrane channels
Anatomical reference terms
281 MSC