Download nervous tissue organization neurons neuroglia action potentials

nervous tissue
organization
neurons
neuroglia
action potentials
nerve conduction
excitation or inhibition
integration
memory
skeletal
So motor
involuntary reflexes
V motor
sym
para
P motor
Brain
CNS
excites
calms
PNS
SpCord
P sensory
SO = somatic
V = visceral
S sen
skin, muscle, bone, joints
V sen
thoracic, abdominal, urinary
P = peripheral
nerve cells (neuron)
excitability = stimuli result in an electrical impulse
conductivity = impulse conducted to other cells
secretory = neurotransmitter released to excite next cell
sensory = afferent (in to PNS or CNS)
motor = efferent (out to effectors)
interneurons = (go to neurons in CNS)
neuroglia (supporting cells) majority of cells
oligodendrocytes= wraps about nerves to form myelin sheath, insulates
nerve fiber from extracellular fluid
ependymal = cubed, line internal cavities of CNS, makes CSF
microglia = small macrophages, dead cells, foreign matter, germs
astrocytes = the most, support, form blood-brain barrier, regulate blood
flow, glucose to lactate, secrete NGF, integration, recycle K &
neurotransmitters, form scar tissue, cover all but synaptic areas
Schwann cells = neurilemmocytes = envelop PNS, make myelin in PNS,
aids regeneration
satellite cells = envelop soma in ganglia of PNS, reg chem env of neurons
neuron types
multipolar = more than one dendrite into soma, one axon
bipolar = one dendrite into soma, one axon
unipolar = no dendrites touch soma, one axon
anaxonic = more than one dendrite into soma, no axon, integration
different types of neurons have
different rates of axonal transport speed
antegrade & retrograde fast transport = 20-400 mm/day= express
slow axonal transport (antegrade) = 0.5 -10 mm/day=stop & go
molecular motors use
antegrade away from soma uses kinesin
retrograde towards soma uses dynein
Myelin (fatty protein)
wraps about neurons and increases conduction speed
in CNS made by oligodendrocytes
in PNS made by Schwann cells
many wraps of thin myelin with a thick outer neurilemma
CNS has no neurilemma or endoneurium
larger fibers have faster conduction speeds
regeneration after damage/cut
- damage – followed by clean up, soma hypertrophies
- distal ends degenerate
– axon stump develops multiple processes
– Schwann cells, basal lamina, & neurilemma form a
regeneration tube + NGF & adhesion proteins
– tube guides sprout to original target
– connection reestablished & soma shrinks to normal size
BUT- mistakes, wrong muscle, no muscle, die,
1-2 years for complete regeneration
you pick up a hot spoon
you perceive the heat
you know that you want to drop the spoon
you open your fingers and let go
we do this for many stimuli every day
so how does this action or reaction come about?
it starts with a nerve cell at rest
having a resting membrane potential
the interior of a nerve cell and
the exterior of the cell have
different electrical properties
this difference is brought about by
the concentrations of Na and K
inside (ICF) and outside of the cell (ECF)
brought about by the selective permeability
of the cell membrane
-cell membrane is very permeable to K
- at equilibrium K is 40 times as concentrated in ICF as ECF
- at equilibrium Na is 12 times as concentrated in ECF as ICF
- the difference in concentrations and the Na/K pump
causes a RMP of -70mV
- Na/K pump uses 70% of the energy needs of the nervous system
diffusion, selective permeability, and ion concentration
result in the electrical differences across the membrane
which allows for nerve conduction to take place
stimulation of a neuron causes
Na flows into ICF (into the cell)
RMP shifts from negative to positive
which generates a local potential
that travels towards the trigger zone(axon hillock)
local potential is graded according to strength of stimulus
if an action potential is not generated
does not reach threshold
it degrades as it spreads
and the RMP returns to -70 mV ICF
If enough units
of muscles or dendrites
are excited
the local potentials (LP)
can build upon each other
and spread to the trigger zone
and initiate a
self propagating action potential
action potential
- local potential to axon hillock
– depolarizes membrane – if voltage reaches -55 mV
- Na gates open quickly – Na enters cell – voltage rises to +35 to +50
– K channels open slowly – Na channels close – K channels fully open
– K leaves cell – repolarization as K leaves – hyperpolarization by K
– Na diffuses back into cell – repolarization to -70 mV
– in CNS astrocytes remove K from ECF
refractory period
a period of time during which it is impossible to stimulate that area of
a neuron to generate a second action potential
absolute refractory = even if strong stimulus cannot evoke an AP
relative refractory = needs a strong stimulus to evoke an AP
nerve conduction
action potential (AP) moves along axon
unmyelinated fiber = has voltage gated channels along total length
AP reaches axon hillock
AP moves distally along axon
AP is self propagating
AP moves at up to 2 m/sec
Myelinated nerve fiber
- incoming stimulus generates action potential
- AP travels down axon by saltatory conduction
- AP deteriorates as it travels down axon
- AP are reenergized at each node
- AP occurs only at nodes
- AP does not jump over nodes
- fast conduction up to 120 m/sec (about 400 ft/sec)
it was less than 100 years ago that Otto Loewi
showed that communication between nerves and nerves
and nerves and muscles was chemical not electrical
thus was born the study of synapses and neurotransmitters
synapses
presynaptic may contact
a dendrite, a soma, or postsynaptic axon
a postsynaptic becomes presynaptic
to the next order of neurons
scientists have discovered
more than 100 neurotransmitters
synaptic transmission
- AP reaches synaptic knob – Ca gated channels open
– Ca enters synaptic knob – exocytosis of NT from knob
– empty vesicles recycled and refilled with NT
– ACh diffuses across synaptic cleft
– binds to postsynaptic receptors which open Na channels
– Na rushes in and depolarizes postsynaptic cell
– if potential change is strong enough it reaches axon hillock of cell
– causes postsynaptic cell to fire an AP
this is an example of an excitatory synaptic event
as an inhibitory event Ach would cause hyperpolarization
adrenergic NE synapse
presynapse releases NE – binds to postsynaptic membrane
– G protein activated – G protein binds to adenyl cyclase
- (ATP becomes cAMP)
– cAMP binds to membrane and opens receptor which allows
ions in to depolarize cell – or – activates cytoplasmic
enzymes – or – activates genetic transcription
slower than ACh but NE activity accompanied
by amplification can be ↑ or ↓
neurotransmitters
acetylcholine
amino acids = glycine↓, glutamate↑, aspartate↑, GABA↓
monoamines = epinepherine, norepinepherine, dopa, catecholamines
histamines, serotonin, ATP
neuropeptides = cholecystokinin, sub P, enkephalins, endorphins
neuromodulating hormones = long term effectors = NO, dopa,
serotonin, histamine
ACh and NE can be ↑ or ↓ depending on situation
If neurotransmitter remains in synaptic cleft
it causes excess stimulation
of postsynaptic cell the result is
too much activation or
too much inhibition
neurotransmitters are reabsorbed by
astrocytes and presynaptic cell
and enzymes
there is a reason that neurons have many dendrites
and many terminal arborizations
your CNS and PNS do not operate on a 1 to 1 basis
each neuron receives input from more than 1 neuron
and send their action potentials to more than 1 neuron
the result of an AP is therefore the result of
many inputs some excitatory and some inhibitory
or it is the result of neural integration
10,000 inputs to get 1 output
it is estimated that a spinal neuron has
8,000 synapses on dendrites
2,000 synapses on soma
cerebellum estimates are 100,000 inputs
why
information processing (integration)
here are a bunch of terms to know
summation = Ex + In results in net effect, takes place in trigger zone
temporal sum=rapid input from 1 causes LPs to reach threshold and AP
spatial sum = input from many add LPs to threshold and AP produced
facilitation = 1 neuron makes another more likely to fire
presynaptic inhibition = 1 neuron makes another less likely to fire
divergence = one stimulus causes AP in many neurons
convergence = stimuli from many neurons affect one neuron
recruitment = as stimulus↑ affects more neurons
neural coding = convert synapses into meaningful pattern of Aps
neural pool = all of the + and – used to determine an effect
labelled line code = info to brain recognized as coming from a specific stimulus
memory
something we talk about, prognosticate about
but really do not understand
even though we have identified
areas of the brain which affect memory
we have identified different areas of the brain which are
involved with different types or stages of memory but we still
do not understand how an electrical potential travelling down
a neuron results in a visual image, train of thoughts, smell,
emotion
our ability to recall information or a task
memory trace (engram) = new or existing synapses have been
modified to make info more easily remembered
synaptic plasticity = the ability of a synapse to change
synaptic potentiation = ability to make transmission easier
immediate = able to hold for a few seconds
short term = remember for a few sec to hours, then forgotten
working = stored in brain & can be recalled by new input, facilitated
synapses which are quiet and can be reactivated
post tetanic potentiation = easily excited due to recent use
long term = info that you retain for a lifetime
declarative = memory of events you can describe with words
procedural = memory of how to do motor or repetitive skills