13) Nerve cells and nerve impulses Flashcards
What is found in most multicellular animal species?
Nervous Tissue
Sponges don’t have Ns but have SJs
Neurons/Synaptic Junctions
Jellyfish nervous structure
diffuse net nerves, no CNS
Bilaterians nervous structure
Brain and nerve cord = CNS
Cephalisation
Nervous tissue concentrated toward one end of an organism
CNS
brain and spinal cord
PNS
Peripheral NS= nerves from sense organs to the CNS
Nervous system microstructure
Nerve cells (neurons)/ Glia cells
Cajal & Golgi (Nobel Prize)
found neurons are separable (small gap between the tips of one neurons fibres and the next neuron)
The NS consists of individual neurons
Neurons
nerve cells
how many neurons in cerebellum?
70 billion
how many neurons in cerebral cortex
12-15 billion
how many neurons in spinal cord
1 billion
glia cells
glial cells
neuroglia
smaller than neurons
more frequent (x1.2)
three most common glia cells
oligodendrocytes (76%)
astrocytes (17%)
microglia (6%)
glia cell functions (6)
1- provide structure - surround neurons and hold them in place
2 - insulate nerve cells with myelin sheaths
3 - supply nutrients and oxygen to neuron
4 - removal of dead neuronal tissue & immune defence of the CNS
5 - provide scaffolding for neurons to migrate to destination during development
6 - modulate neurotransmission in the synapses
astrocyte function
provide structure by surrounding neurons and holding them in place
supply nutrients and oxygen to neuron
oligodendrocytes
CNS
Schwann cells
PNS
phagocyte function
microglia - immune defence of the CNS
radial glia
provide scaffolding for neurons to migrate to destination during development
name three glial cell tumours
glioblastoma
astrocytoma
oligodendroglioma
neurons def
cells in the NS that specialise in performing information- processing tasks
neuron structure
body cell (soma) and fibres (dendrites and axon)
soma
cell body
contains nucleus and ‘machinery’
cell nucleus
contains genetic info of cell organized as DNA molecules
cell ‘machinery’
mitochondria
ribosomes
endoplasmic reticulum
golgi apparatus
nissl bodies
mitochondria
performs metabolic activities, extracts energy from nutrients
ribosomes
protein production
endoplasmic reticulum
transports proteins to other locations
dendrites
branching fibres receiving information from other neurons
greater surface area, more info receivable
where are synapses located on a dendrite
the surface - some have spines which enlarge the surface on which the synapses are placed
axon
thin fibre - transmits information to other neurons - axon can be 1m long
axon structure
distal part - branches
these swell at tip and form presynaptic terminal (end bulb or bouton)
some have boutons along the fibre - not just at the end
bouton(s)
end bulbs (swollen sections) of axon fibres - can be along as well as on the end
myelin sheath
insulating layer - speeds up electrical transmission
why are neurons diverse shapes
specialisation
three major types of neuron
afferent
efferent
interneurons
afferent
sensory neurons - Afferent:Arrive
carry info from receptors
efferent
Efferent:Exit
carry signals away to the effectors muscles or glands
interneurons
connect other neurons
neuronal activity
resting potential and neurons’ excitation
Resting polarization
the membrane of a neuron maintains an electrical gradient (the difference in electrical charge) between the inside and the outside of the cell
resting potential
the electrical potential inside the neuron is slightly lower than outside (average - 70mV)
potential change (two stages)
transmission of info through neuron is realized by the change of potential
1) transmission of info from the exterior through dendrites to the cell body
2) transmission from cell body through the axon out of the neuron
excitation of a neuron
transmission of info within neuron is not a simple passive act
at level of a neuron, there’s a mechanism of analysis deciding whether to pass/block a message
Stages of excitation (2)
1) Dendrites
Many post synaptic potentials (strong/weak changes in electrical potential) moving towards the centre
Potentials from dendrites sum up
2) Cells body
If sum is strong enough - the neuron fires
(action potential, always the same strength)
2 types of synapses
excitatory/ inhibitory synapses
excitatory synapse
excitatory post synaptic potential. EPSP is a change in polarization moving along the dendrites towards a cell body
EPSP - positive - provokes polarization (decreases polarization)
inhibitory synapse
IPSP change in polarization moving along dendrites to cell body
IPSP - negative - provokes hyperpolarization (increases polarization)
what happens to postsynaptic potentials along the membrane
they become smaller and may vanish before the reaching body cell
summation
post synaptic potentials sum up when meet other post synaptic potentials or if followed by others
two types of summation
over space/ over time
over space summation
from different dendrites
over time
from the same dendrites
what happens to potentials in the cells body
the free potential is formed and moves toward the proximal part of the axon
what happens if the free potential is low (doesn’t reach threshold level)
it dies
if free potential is high (reaches potential level)?
provokes a sudden and massive electric excitation at the proximal part of the axon: the action potential (spike)
process of action potential
moves along the axon without any loss in its parameters to reach the presynaptic membrane where it produces the release of chemical substance
all or none principle (action potential)
-amplitude of action potential is independent of amount of current which produced it - large current doesn’t equal large AP
-amplitude is constant for given axon (+30/40mV)
- they either occur fully or not occur
propagation of action potential
first AP - on the axon hillock
action potentials move down axon towards another cell using saltatory conduction
saltatory conduction
APs hop along the axon recurring at successive nodes of ranvier -> fast propagation
myelin prevents any charge leakage through the axon
Multiple Sclerosis MS
neurological condition affecting 10k ppl in uk
symptoms - vision problems/ fatigue/ difficulties with walking
cause - demyelination of axons in the brain and spinal cord
SUMMARY
two main cells in NS - glia/neurons
Neurons - perform info processing tasks
Neuron basic structure - soma + dendrites + axon
Neurons resting potential = -70mV
stimulated neuron: if depolarisation exceeds threshold, neuron fires an action potential of a constant size
